A review on thermal performance and heat transfer augmentation in solar air heater

ABSTRACT

Development in solar air collectors have increased because of the renewable and non-polluting sources of energy. The present study focused on artificial roughness, vortex generators, thermal storage units, etc., which are employed to improve the performance of Solar Air Heaters (SAHs). Thermal efficiency of Double Pass Solar Air Heater (DPSAH) improved from 10% to 15% as compared to the Single Pass Solar Air Heater (SPSAH) and improves further using an integrated absorber with a heat storage unit. About 12% and 15% improvement in thermal and thermo-hydraulic efficiency was achieved using a rough absorber. Integrated absorbers with Composite Phase Change Materials (CPCMs) based heat storage is one of the new signs of progress to optimize the thermal efficiency of SAHs.

KEYWORDS:

• Solar air heater
• artificial roughness
• heat transfer enhancement
• thermal performance factor
• thermal energy storage
• phase change materials

Nomenclature
SAH	=	Solar Air Heater
TES	=	Thermal Energy Storage
SHS	=	Sensible Heat Storage
PW	=	Paraffin Wax
St	=	Stanton number
CFD	=	Computational Fluid Dynamics
EG	=	Expended Graphite
e/Dh	=	Roughness height
$Q_u$	=	Useful heat gain (W)
$I_T$	=	Total Solar Irradiation (W/m2)
$A_p$	=	Area of absorber plate (m2)
$A_c$	=	The cross-sectional area of the rectangular duct (m2)
H	=	Height of duct
L	=	Length of test section (m)
$D_h$	=	Hydraulic diameter (m)
$\mathrm{\Delta }P$	=	Pressure drops through the test section (N/m2)
$P_{\mathrm{f}\mathrm{a}\mathrm{n}}$	=	Power to drive the fan (W)
$k_{\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Thermal conductivity of air (W/mK)
$C_p$	=	Specific heat capacity of air (kJ/kg.K)
$\dot{m}$	=	The mass flow rate of air (kg/s)
h	=	Convective heat transfer coefficient (W/ m2K)
$v_{\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Air velocity (m/s)
$T_{fi}$	=	Inlet air temperature (K)
$q_t^{\mathrm{\prime }\mathrm{\prime }}$	=	Total heat flux
Ut	=	Total heat loss factor
$T_{fo}$	=	Outlet air temperature (K)
$T_{fm}$	=	Fluid mean temperature (K)
$T_{pm}$	=	Plate means temperature (K)
Re	=	Reynolds number
Nu	=	Nusselt number
$N{u}_s$	=	Nusselt number for smooth duct
f	=	Friction factor
$f_s$	=	Friction factor for smooth duct
TPF	=	Thermal Performance Factor
PCM	=	Phase Change Material
DPSAH	=	Double Pass Solar Air Heater
LHS	=	Latent Heat Storage
j	=	Colburn j-factor
SPSAH	=	Single Pass Solar Air Heater
HT	=	Heat Transfer
ANN	=	Artificial Neural Network
p/e	=	Roughness pitch
HTF	=	Heat Transfer Fluid
hc	=	Convective HT coefficient
hr	=	Radiative HT coefficient
VGs	=	Vortex Generators

Subscript
i	=	Inlet
o	=	Outlet
fi	=	Fluid inlet
fo	=	Fluid outlet
u	=	useful
T	=	Total
h	=	hydraulic
th	=	Thermal
s	=	smooth
a	=	ambient
ig and og	=	Inner and outer glass
pm	=	Plate mean
fm	=	Fluid mean
air	=	Working fluid (Air)

Greek Symbols
$\mu$	=	Dynamic viscosity of air (kg/ms)
${\rho }_{\mathrm{a}\mathrm{i}\mathrm{r}}$	=	The density of air (kg/m3)
${\eta }_{\mathrm{t}\mathrm{h}}$	=	Thermal efficiency of SAH
${\eta }_h$	=	Thermohydraulic efficiency

1. Introduction

Global warming caused by carbon dioxide (CO2) has become a depressing problem and degrades the environment continuously, it needs action. The efficient use of renewable energy resources such as wind/solar energy is considered a promising solution to global warming, especially solar energy. The total amount of solar radiation incident in the earth's upper atmosphere is about $174\times {10}^{15}$ W. The Earth's energy budget according to NASA has been reported (Tian and Zhao 2013). The intensity of this incident radiation is affected by the atmosphere and clouds which reduces this intensity by 6%, 20% by reflection, and 16%, 3% by absorption respectively when it reaches the Earth's surface. About 51% ($89\times {10}^{15}$ W) of incoming solar radiation reaches the Earth's surface. The solar energy available on Earth is still extremely valuable, but the fundamental problem is tenuity and its irregularity (Smil 1991). To overcome or minimise this problem, solar energy needs to be collected with higher efficiency and stored for later application.

In 1973, when the oil crisis arises then to fulfill the gap between demand and supply of energy it focused on the research of alternative sources of conventional fuels. Out of the total energy required on the earth for different processes, about 81% is taken from conventional sources of energy such as fossil fuels, coal, petroleum, natural gas, and nuclear and thermal power plants to meet the demand. Air pollution is a result of the burning of traditional fuels and wood in an agriculturally oriented nation. So there is a need to design an energy-efficient, environmentally friendly, and economical air heating system which can supply hot air for heating/cooling purposes for a healthy interior environment and industrial use (Singh and Singh 2018a, 2018b). Conventional resources are continuously depleting because of global industrialisation and population growth, hence the increase in energy demand. These conventional sources have degraded the environment and are harmful to human beings too (Kalogirou 2004; Omer 2014). Initially, in the year 1990, the contribution of renewable sources of energy was only 0.42% out of the total required energy for the different processes, and increase up to 2% by the year 2018 (Goel et al. 2021). By taking into consideration of environmental safety and eco-friendly energy sources solar energy is best preferred first over another non-conventional source of energy. Solar energy offers many advantages over other renewable sources such as being almost available everywhere over the seasons, non-polluting, reliable, etc.

Solar air heating is one of the oldest and simple techniques to collect the incident solar radiation on the absorber surface in the form of thermal energy or heat. Heat collected on the absorber is then transferred to the process air which is flowing inside the Solar Air Heaters (SAHs) duct. The amount of heat carried by the process air can be directly used for heating/cooling purposes or stored in a storage medium for later uses according to the requirement. In this type of solar collector or absorber, the amount of heat loses to the environment increases as the temperature of the absorber plate increases. The temperature of the absorber plate inversely depends upon the flow rate and offers low thermal efficiency due to losses (Hernández and Quiñonez 2018). SAHs are simple in design structure and economical have low installation and maintenance costs and are applicable for various drying/heating/cooling applications. SAHs are used to preheat the air to dry off agriculture crops in harvest season (freeze-drying, microwave drying, solar drying, spray drying, etc.), industrial process heat, space heating, textiles, marine products, water desalination, etc. SAHs are used to heat traditional bathrooms without using any burning fuel. SAHs have been extensively employed in a variety of residential and industrial applications. However, the most recent studies show that the thermal efficiency of SAHs is still poor. Some heat losses are there which are responsible for the low efficiency including side losses, bottom losses, and mainly top reflective, radiative, and convective losses. Out of the total solar radiation (100%) incident on the top surface of the glass, only 60% is utilised by SAHs for heating purposes and the rest part of the total radiation about 40% remains unutilised or wasted as heat (Kalita).

Various methods were used to minimise these losses and to improve the overall performance of SAHs. Even porous and non-porous absorbers are made to soak up as much solar energy as possible and to minimise heat losses through leakages and convection. The air stream flows unhindered through the absorber plate in a simple flat plate air collector is known as the non-porous absorber. The porous bed air heater has matrix material arranged to remove the back side of the absorber. The design modification of flow passage (single pass/double pass/multiple passes), and modification of absorber surface (artificial roughness and extended surfaces) are more efficient methods to optimise the thermal performance of SAHs. These techniques are used to optimise the heat transfer and friction characteristics of SAHs. Although these techniques are best suited, still there is a problem related to the performance of SAHs on cloudy and rainy days when there is no solar radiation or fluctuation in radiation intensity. To overcome this problem and to make efficient working of SAHs even when intermittency of radiation, an integrated absorber with a heat storage unit is one of the new progress. SAHs without storage units are classified as non-porous and porous absorbers, while air heaters with storage units are subdivided into Sensible Heat Storage (SHS) and Latent Heat Storage (LHS) systems. Various thermal energy storage systems such as SHS and LHS (PCMs-based) storage systems and methods to enhance the thermal conductivity of PCMs are briefly described in this study. Flat plate solar collectors are classified into two categories such as SAHs without thermal storage and SAHs with thermal storage as explained in Figure 1.

Figure 1. Classification of flat plate solar air collectors.

This review article tries to describe the heat transfer augmentation techniques to enhance the thermal performance of SAHs. The performance of SAHs is determined in terms of heat transfer capacity and friction characteristics between absorber and process air. The main objective of this study is to obtain a suitable method for SAHs to get the optimum thermal efficiency with minimum friction losses and suitable Composite Phase Change Materials (CPCMs) for integrated absorber thermal energy storage systems. To encourage additional innovations for the betterment of sustainable development, this review paper offers a glimpse into the recent critical results, responsible for thermal efficiency augmentation of flat plate solar air collectors. Although the optimum research has already been done in this direction, still there are some lacking associated with the performance of SAHs. In this article, we are trying to collect sufficient information that benefits new researchers and scholars to create the best-suited design, modification in the flow passage, HT augmentation methods with newly developed composite phase change materials CPCMs based storage techniques. The combination of SHS and LHS may be the best research method to improve the thermal efficiency of the system and need more exposure.

2. Fundamental of SAHs

An absorber plate, collector surface, and air blower are three of the most common components of SAHs, which are all interconnected. The solar energy incident on the absorber's surface is absorbed in the form of heat energy. This absorbed heat is then transferred to the process air and increases the temperature of the air. A rectangular collecting box with an absorber plate is installed inside it, this is the basic design of SAHs. To minimise the top losses the collector box is covered with a single or multiple layers of transparent glass covers. Air is blown/sucked above or below the absorber plate using a blower as shown in Figure 2.

Figure 2. Schematic diagram of single-pass flat plate SAHs. (a) Flow over the absorber. (b) Flow below the absorber.

Solar air heater performs low efficiency because of the low convective heat transfer coefficient between two heat-exchanging mediums which limited the applications of SAHs. The fins or baffles are normally introduced into the SAH duct to enhance the heat transfer from the hot absorber surface to the process air. Other methods, such as increasing micro-level surface roughness, dimple, perforation, and matrix mesh over the absorber plate also used to overcome this problem and enhance the rate of heat transfer between heat exchange surface and flowing air. Solar collectors may be classified into two categories: one is the non-concentrating type (for low-temperature applications) another is concentrating type (for high-temperature applications) (De Winter 1991).

3. Performance evaluating parameters

Several things are affecting how well a SAH works. The performance of a solar air heating system depends upon the following parameters such as convective heat transfer coefficient (h) between the heat exchange surface and the process air, Nusselt number (Nu), friction factor (f), thermal performance factor (TPF) and thermal efficiency ($\eta$), etc. To reduce the complexity of calculations and to get better results following assumptions can be used.

1. Heat transfer takes place in steady-state conditions.

2. There are no leakage or heat loss to the environment.

3. Change in the temperature of working fluid allows only flow direction.

4. Thermophysical properties of working fluid do not change throughout the process.

Different performance characteristics are needed to analyse to get the better effectiveness of a solar air heater, which are briefly explained in the following section:

3.1. Heat transfer characteristics

The amount of useful heat gain (${\mathrm{Q}}_{\mathrm{u}}$), the mass flow rate of air ($\dot{\mathrm{m}}$), convective heat transfer coefficient (h), and the thermal efficiency of flat plate SAHs are calculated by Equations (1), (2), (3), and (4) respectively as follows: (1) ${\mathrm{Q}}_{\mathrm{u}}=\dot{\mathrm{m}}{\mathrm{C}}_{\mathrm{p}}\left({\mathrm{T}}_{\mathrm{f}\mathrm{o}}-{\mathrm{T}}_{\mathrm{f}\mathrm{i}}\right)$(1) (2) $\dot{\mathrm{m}}={\rho }_{\mathrm{a}\mathrm{i}\mathrm{r}}{\mathrm{A}}_{\mathrm{c}}{v}_{\mathrm{a}\mathrm{i}\mathrm{r}}$(2)

(3) $\mathrm{h}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{u}}}{{\mathrm{A}}_{\mathrm{p}}\left({\mathrm{T}}_{\mathrm{p}\mathrm{m}}-{\mathrm{T}}_{\mathrm{f}\mathrm{m}}\right)}}$(3) (4) ${\eta }_{\mathrm{t}\mathrm{h}}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{u}}}{{\mathrm{I}}_{\mathrm{T}}{\mathrm{A}}_{\mathrm{p}}}={\displaystyle \frac{\dot{\mathrm{m}}{\mathrm{C}}_{\mathrm{p}}\left({\mathrm{T}}_{\mathrm{f}\mathrm{o}}-{\mathrm{T}}_{\mathrm{f}\mathrm{i}}\right)}{{\mathrm{I}}_{\mathrm{T}}{\mathrm{A}}_{\mathrm{p}}}}}$(4) where ${\mathrm{C}}_{\mathrm{p}}$, ${\rho }_{\mathrm{a}\mathrm{i}\mathrm{r}}$, and $v_{\mathrm{a}\mathrm{i}\mathrm{r}}$ are the specific heat capacity, density, and velocity of working fluid (air). ${\mathrm{A}}_{\mathrm{c}}$ and ${\mathrm{A}}_{\mathrm{p}}$ are the cross-sectional area of the duct and area of the absorber plate respectively and total irradiation is ${\mathrm{I}}_{\mathrm{T}}$. The temperature of the air at the inlet, the outlet is ${\mathrm{T}}_{\mathrm{f}\mathrm{i}}$, ${\mathrm{T}}_{\mathrm{f}\mathrm{o}}$ and mean temperatures of fluid and plate is ${\mathrm{T}}_{\mathrm{f}\mathrm{m}}\left(={\displaystyle \frac{{\mathrm{T}}_{\mathrm{f}\mathrm{i}}+{\mathrm{T}}_{\mathrm{f}0}}{2}}\right)$, ${\mathrm{T}}_{\mathrm{p}\mathrm{m}}$ respectively.

3.2. Hydraulic characteristics

Reynolds number (Re) and Nusselt number (Nu) can be calculated by Equations (5) and (6) respectively. The amount of pressure drops ($\mathrm{\Delta }\mathrm{P}$) also affects the performance of SAHs, it can be measured by the Darcy-Wiesbach formula as expressed in Equation (7), and the friction factor calculated by the measured value of pressure drop across the test section by Equation (8). Stanton number and Colburn i-factor addressed by Equations (9) and (10) respectively, and mathematically given as follows: (5) ${\mathrm{R}}_{\mathrm{e}}={\displaystyle \frac{{\rho }_{\mathrm{a}\mathrm{i}\mathrm{r}}\ast {\mathrm{D}}_{\mathrm{h}}\ast {\mathrm{v}}_{\mathrm{a}\mathrm{i}\mathrm{r}}}{\mu },\mathrm{a}\mathrm{n}\mathrm{d}{\mathrm{R}}_{\mathrm{e}}={\displaystyle \frac{4\ast {\mathrm{r}}_{\mathrm{h}}\ast {\mathrm{G}}_o}{\mu }\left(\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{e}\mathrm{d}\mathrm{b}\mathrm{e}\mathrm{d}\mathrm{S}\mathrm{A}\mathrm{H}\right)}}$(5) (6) ${\mathrm{N}}_{\mathrm{u}}={\displaystyle \frac{\mathrm{h}\ast {\mathrm{D}}_{\mathrm{h}}}{{\mathrm{k}}_{\mathrm{a}\mathrm{i}\mathrm{r}}}}$(6) (7) $\mathrm{\Delta }\mathrm{P}={\displaystyle \frac{2\ast \mathrm{f}\ast {\rho }_{\mathrm{a}\mathrm{i}\mathrm{r}}{\ast \mathrm{L}\ast \mathrm{v}}_{\mathrm{a}\mathrm{i}\mathrm{r}}^2}{{\mathrm{D}}_{\mathrm{h}}}}$(7) (8) $f={\displaystyle \frac{24}{Re}\left(\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{l}\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{r}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}\right)\mathrm{a}\mathrm{n}\mathrm{d}f=0.079R{e}^{-0.25}\left(\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}\right)}$(8) (9) ${\mathrm{S}}_t={\displaystyle \frac{Nu}{Re\ast Pr}}$(9) (10) $\mathrm{j}={\displaystyle \frac{Nu}{Re\ast P{r}^{0.33}}}$(10)

In these equations the hydraulic diameter of rectangular duct (${\mathrm{D}}_{\mathrm{h}}$) and Prandtl number (Pr) can be calculated by ${\mathrm{D}}_{\mathrm{h}}={\displaystyle \frac{2\ast L\ast H}{\left(L+H\right)}}$, and $Pr={\displaystyle \frac{\mu \ast C_p}{k}}$, where L, and H are the length and height test sections, the dynamic viscosity of air is $\mu$, and the specific heat capacity of air is $C_p$, and the thermal conductivity of air is k.

3.3. Thermohydraulic characteristics

The thermohydraulic efficiency (${\eta }_{\mathrm{h}}$) is defined as the ratio between the total output to the total irradiation mathematically given in Equation (11) and the Thermal Performance Factor (TPF) or thermohydraulic efficiency (${\eta }_{\mathrm{h}}$) can be defined as the Nusselt number ratio to the friction factor ratio while pumping power is constant and mathematically expressed as in Equation (12). (11) ${\eta }_{\mathrm{h}}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{u}}-{\mathrm{P}}_{\mathrm{f}\mathrm{a}\mathrm{n}}}{{\mathrm{I}}_{\mathrm{T}}{\mathrm{A}}_{\mathrm{p}}}}$(11) (12) $\mathrm{T}\mathrm{P}\mathrm{F}={\displaystyle \frac{\left(\frac{Nu}{\mathrm{N}{\mathrm{u}}_{\mathrm{s}}}\right)}{{\left(\frac{\mathrm{f}}{{\mathrm{f}}_{\mathrm{s}}}\right)}^{\frac{1}{3}}}}$(12) where Nu, $\mathrm{N}{\mathrm{u}}_{\mathrm{s}}$ and f, ${\mathrm{f}}_{\mathrm{s}}$ are the Nusselt numbers and friction factors for roughed and smooth ducts respectively. Nu/Nus is the ratio of the enhancement in the heat transfer when the geometry is used to heat transfer when the geometry is not used in between the heat exchange surface and working fluid. Nu/Nus also is known as the Nusselt number enhancement ratio. The friction factor is inversely proportional to the square of the fluid velocity. A higher value of fluid velocity gives a higher Reynolds number which reduces the friction factor. The friction factor plays an important role in a heat exchanger and directly affects the performance of the heat exchanger. For better performance of the heat exchanger, the friction factor should be as less as possible.

4. Enhancement of thermal performance in SAHs

To overcome the problem of low thermal efficiency associated with SAHs a lot of work has been done. Various heat transfer augmentation methods were developed to obtain better thermal performance of SAHs. In this section various performance augmentation techniques such as optimisation of incident solar irradiation on absorber surface, thermal losses associated with SAHs and reduction techniques, modification in the path of flowing fluid (single/double/multiple passes), and modification on absorber surface (roughness) are explained. Surface modification on absorber plates including artificial roughness (protrusions, arc, and other various continuous and discrete geometry shapes, etc.) and extended surfaces such as fins, baffles, etc. discussed in this section. The modifications on the surface of the absorber plate play an important role in enhancing the thermal and thermo-hydraulic performance of solar air heating systems. Different types of extended surfaces (fins) with their geometrical effects and hydraulic performance of a SAH are presented. By reducing the thermal losses, we can also enhance the output of SAHs. These augmentation methods are further divided into sub-groups as shown in Figure 3 and some are described in this section which are more effective techniques to augment the thermal efficiency of SAHs. Table 1 includes some review articles based on augmentation techniques and methods, currently used in flat plate SAHs with and without an energy storage system.

Figure 3. Type of thermal performance enhancement techniques available for SAHs.

Table 1. List of latest review articles published in the field of SAHs with and without thermal energy storage (TES) systems.

Authors/years	Focused area for review	Category	Conclusions
Mund, Rathore, and Sahoo (2021)	Recent advancement in design and modifications to improve the thermal efficiency of SAHs has been reviewed. The impact of the impingement of air on the system's efficiency was also discussed.	SAHs With and without TES	All modifications of SAHs maintain the maximum thermal efficiency of about 80% with the increase in friction factor. Storage system integrated with any augmentation methods was found to be less effective and transferred the absorbed heat to the flowing air in fluctuating solar radiation. Jet impinging provides better thermal performance and needs more exposure in the future.
Ghritlahre et al. (2022)	Reviewed the energy and exergy work associated with various enhancement techniques employed in SAHs and suggested some future work for further research in this field.	SAHs With and without TES	To optimise the design and to find out the limitations of the theoretical and actual effectiveness of SAHs, the exergy analysis is the best-suited method. Various SAHs show a range of their energy and exergy efficiency of about 2.05–82% and 0.01–60.97% respectively. Exergy efficiency for DPSAHs with different modified absorbers offered about 60.97% at 0.025 kg/s air flow rate.
Sharma, Pitchumani, and Chauhan (2022)	Latent Heat Storage (LHS) based integrated and non-integrated absorber SAHs have been reviewed with their performance comparison.	SAHs with LHS - based TES	PW as PCM with porous media and added heat transfer surfaces significantly improves the charging or discharging rate of PCM. Except for shell and tube-type LHS systems, all other configurations give the same output.
Khimsuriya et al. (2022)	Nusselt number (Nu), friction factor (f), and thermohydraulic performance characteristics have been estimated for 11 roughness geometries using Microsoft excel 2021, and a comparison has been done to choose the best roughness.	SAHs without TES	The friction factor has been improved with heat transfer using various roughness geometries. Jet impingement with the conical ring as roughness offers maximum Nu = 315.88 at Reynolds number (Re = 24,000) over all other roughness geometries Maximum f = 0.44 was offered by delta wing with perforation among all other roughness at Re = 2000. A maximum TPF of about 11.93 was achieved for a conical disc with helical corrugations at Re = 2000.
Vengadesan and Senthil (2020)	Reviewed the various enhancement techniques such as the design of collectors, the number of passages, porous type absorber, airflow by jet impingement, TES, and PV/T systems.	SAHs with and without TES	The most important factors of roughness geometries are p/e and e/Dh, increasing these parameters also enhances Nu, f, and TPF. The electrical efficiency of a hybrid PV/T collector with PCM and fins improves by 9% and 12%, respectively, while the thermal efficiency improves by 5% when using PCM alone. Layers of PCM affect heat absorption.
Nidhul et al. (2021)	Different V-shaped rib with various configuration has been reviewed and a 3-D CFD investigation was done to understand the flow characteristics.	SAHs with TES	Multiple ribs without and with gaps improve exergetic efficiency by 12% and 31.6% respectively as compared to the simple V-ribs SAHs. Thermal efficiency improves for multiple V-rib with and without gap was found at about 9.5% and 5.7% respectively.
Sharma et al. (2020)	Various PCMs, their characteristics, and their applicability in SAHs have been reviewed with heat transfer augmentation methods employed for PCMS.	SAHs with PCMs-based TES	The melting temperature of organic PCMs generally varies from 30°C to 60°C and offers low thermal conductivity in comparison to inorganic PCMs. To determine how well various encapsulation geometries and dispersions of conductive particles in PCMs work more research is needed.
Kumar and Layek (2022)	Different variables and geometries of artificial roughness with the correlations between Nu and f have been reviewed.	SAHs without TES	The Nu increases with an increase in Re for all types of roughness geometries. Optimum thermohydraulic efficiency was obtained for V-shaped ribs with spacing.
Singh Yadav, Prakash Shukla, and Singh Bhadoria (2022)	The review was focused only on modern CFD approaches employed in ribbed absorber SAHs to optimise the heat transfer and flow properties.	SAHs without TES	The use of nanofluids has the potential to provide a significant advancement in heat transfer. The combination of two or more roughness geometries is also helpful to thermal performance.
Yadav et al. (2022)	Various roughness-based SAH were reviewed to evaluate friction and heat transfer characteristics.	SAHs without TES	A lot of correlations were made between Nu and f for various roughness under different parameter settings.
Kumar Dwivedi and Choudhary (2022)	A review was done with modern rib geometries used in SAHs for performance augmentation.	SAHs without TES	Factors such as absorber plate composition, roughness type, light intensity, ambient conditions, and the size of the duct affect the performance of SAHs.
Phu and Luan (2020)	The comparison of energy and exergy analysis has been done for roughened SAHs.	SAHs without TES	All 11 (selected) roughness elements have exergy efficiencies below 2%. The performance factor varies from 0.5–2 for selected geometries.
Verma, Saxena, and Varshney (2020)	Various wire screen matrix-packed bed SAHs configurations were surveyed. Porosity strongly affects the thermo-hydraulic behaviour of SAHs.	Matrix absorber type SAHs without TES	Counter-flow DPSAHs give superior thermohydraulic performance at higher porosity and lower airflow rates, while parallel-flow DPSAHs follow the opposite trend. A matrix with porosity of 0.937 and 0.887 give a thermal efficiency of about 68% and 42% respectively at a mass flow rate of 0.023 kg/s.
Bharadwaj, Sharma, and Mausam (2021)	Designs of alternative coarse texture employed to increase the heat transmission in SAHs were surveyed with Nu and f correlations.	SAHs without TES	Coarse texture parameters such as e/Dh, p/e, and $\alpha$ are 0.03, 8–10, and 60° respectively providing the maximum Heat Transfer (HT) and friction factor (f) approximately for all designs. Multiple V-rib was the best design with spacing
Gupta, Karmveer, and Alam (2021)	Summarised the current and modern coarse texture on the absorber plate to augment the heat transfer in SAHs.	SAHs without TES	The HT rate is significantly increased when roughness turbulators are staggered instead of the straight line. Delta-winglet improves HT without substantially raising the f.
Arunkumar, Vasudeva Karanth, and Kumar (2020b)	Surveyed the effect of turbulators with varying shapes used for enhancing the thermal performance of SAHs.	SAHs without TES	An extensive numerical simulation of SAHs is required to predict how they will function for short- and long-term performance.
Kumar et al. (2020)	Reviewed various performance augmentation methods for different designs of SAHs.	SAHs without TES	Multi-V-shaped rib roughness with spacing was found to be the most effective geometry for Re from 2000 to 20,000 in terms of TPF.
Pachori, Choudhary, and Sheorey (2022)	Packed bed-type TES methods used in single-pass and double-pass SAHs have been surveyed, and detailed explanations of SHS and LHS materials were done.	SAHs with packed beds and TES	In the comparison of SHS and LHS systems, the thermochemical storage has a higher storage capacity per unit volume with lower heat dissipation. Integrated SHS and LHS system is growing technology to boost efficiency and requires more attention.
Naveenkumar et al. (2022)	Numerous Phase Change Materials (PCMs) and their applicability in TES-based SAHs were discussed.	SAHs with PCM-based TES	In organic PCMs like paraffin phase transition takes place at specific temperatures. Expanded Graphite (EG) encapsulation is used to augment the thermal conductivity of PCMs.
Ali and Deshmukh (2019)	The application of PCM-based TES and the characteristics of good and bad PCMs were discussed.	SAHs with PCM-based TES	PCM-based TES are used in verities of applications such as air and water heating, greenhouse, space heating/cooling, RAC, solar cooking, etc.
Jain et al. (2021)	The effect of experimentally investigated V-shaped rib as roughness on the thermal performance of SAHs was surveyed.	SAHs without TES	The optimised value of multiple V-ribs such as altitude, pitch, spacing, staggering distance, and angle of attack, etc., improve HT with minimum f.
Ghritlahre, Chandrakar, and Ahmad (2021)	Reviewed the Artificial Neural Network (ANN) techniques used for performance prediction of SAHs.	SAHs without TES	ANN is found to be the best method to calculate the performance of SAHs over other techniques. ANN takes less time and does not require any programming for any solution.
Olivkar et al. (2022)	The impact of various SHS materials used to enhance the thermal performance of SAHs has been reviewed.	SAHs with SHS-based TES	Generally, SHS materials are employed above the absorber. Iron chips offer the highest thermal efficiency over other metal chips for natural convection. From the economic point of view, the pebble stone bed is the best SHS material.

To improve the thermal and thermohydraulic efficiency of solar air heating systems numerous research was conducted in this field such as roughness elements, jet impingement, baffles and fins, porous media, recycling, and flow passage modifications, etc. (Chamarthi and Singh 2021; Arunkumar, Kumar, and Karanth 2020a). Performance improvement also depends on the solar energy absorbing capacity of an absorber plate, and the different absorbing materials used (Abdullah, El-Samadony, and Omara 2017; Al-damook and Khalil 2017; Şevik and Abuşka 2019). The adjustment of absorber plate and duct arrangement has been studied by many researchers to enhance heat transfer rate. Thermal heat storage and nano-fluids have lately been investigated to improve heat transmission in a variety of applications including SAHs. The thermal conductivity of process air using nanofluid also improves the convective heat transfer coefficient (Ahmad et al. 2017; Kiliç, Menlik, and Sözen 2018). Different phase change materials are also used to store the thermal energy and hence improve in thermal performance SAHs (Alkilani et al. 2011; Zhou et al. 2019).

4.1. Augmentation of solar irradiation

The efficiency of a SAH depends upon the amount of radiation collected by the absorber plate. A flat plate collector's output may be boosted by adding reflectors which expands the collecting area for incident solar radiation. Reflectors are used to increase the flux density and concentration ratios can reach temperatures as high as possible about 180°C (Mu et al. 2019; Jin et al. 2020). Some reflectors are used to optimise the amount of incident solar radiation collected by the collectors. Vanes are used to guide the input of the heating system and spread heat equally to the entire surface of the collector to improve the thermal efficiency of SAHs (Abdullah et al. 2020; Kabeel et al. 2017a; Kabeel, Khairat Dawood, and Shehata 2017b). An external mirror was used to control and change the direction of incident radiation to make the collector plate flexible (Garg et al. 1991). Sidewalls contain secondary V-grooved reflectors. These reflectors increased the temperature up to 100°C even in the winter season without a tracking system (Jin et al. 2020). The performance of DPSAHs using reflectors is higher than that of a single-pass solar air heater (SPSAHs) (Mandal and Ghosh 2020). For optimum utilisation of daytime solar radiation, SAHs are typically oriented southward in the northern hemisphere and northward in the southern hemisphere with a slope proportionate to the latitude (Depaiwa, Chompookham, and Promvonge 2010). The thermal efficiency of SAHs increased by using a booster mirror as the increase in concentration of solar radiation on absorber plates. An increment of 40% in the radiation increases thermal performance from 90% to 130% (Prasad and Sah 2014). To increase efficiency (ElGamal et al. 2021) designed an integrated solar tracking system with SAHs. A large boost in SAHs performance may be achieved by increasing the rate of useful heat gain. Prior research focused on a variety of approaches to raising radiation intensity including the use of high absorptivity materials for the absorber plate (Rhee and Edwards 1981) and the use of focusing mirrors to boost flux density (Jamal-Abad, Saedodin, and Aminy 2016).

4.2. Modification in the path of flowing fluid

A single air pass was used to remove heat from the collector plate when SAHs were originally invented. Due to the large thermal losses to the surrounding environment, SPSAHs have poor thermal performance. The most important component that affects the performance of SAHs is the type of air pass arrangement (double pass/multiple passes). The various flow arrangements for flowing fluid are discussed as follows.

4.2.1. Type of passage for airflow

Air heater efficiency may also be improved by adjusting the flow path within the absorber duct. The absorber-to-air heat transfer efficiency is improved as the number of passages increases as a result increase in the contact area for the fluid. It may be divided into different categories like double Pass (a) reversed flow from top to bottom, (b) reversed flow from bottom to top, (c) parallel flow, (d) Counterflow, (e) reversed flow with recycling and (f) multiple pass SAHs are shown in Figure 4. Bhargava, Garg, and Sharma (1973) and Persad and Satcunanathan (1983) came up with the idea of DPSAHs to minimise the top heat losses to the environment. In normal DPSAHs the efficiency is 10% to 15% better as compared to the single-pass (Alam and Kim 2017). Reduction in top losses and enhancement in thermal efficiency as compared to the single-pass SAH has been analysed (Ho et al. 2012; Ho, Yeh, and Chen 2011). Hu et al. (2019) analysed the flow reorganisation for performance improvements and revealed that the radiation losses and the emission by convection are minimised by using double or triple-pass SAHs over SPSAHs for the small flow rate of air. The temperature of the inside air improved as extra time was provided for staying the air in the duct (Hu and Zhang 2019). Triple-pass SAH enhances the thermal efficiency from 6% to 14% and 3% to 6% as compared to the single-pass and double-pass SAHs respectively (Velmurugan and Kalaivanan 2015). The efficiency of DPSAHs is 10–15% more compared to the SPSAHs (Hassan and Abo-Elfadl 2018). The efficiency of DPSAH increased by increasing the rate of mass flow and using fins. To optimise the performance of flat plate SAHs several configurations of multiple passes SAHs have been investigated (Garg, Sharma, and Bhargava 1985).

Figure 4. Schematic diagram of various configurations of double pass SAHs. (a) Reversed to flow from top to bottom. (b) Reversed flow from bottom to top. (c) Parallel flow. (d) Counterflow. (e) Reversed flow with recycling. (f) Multiple pass SAH.

4.2.2 Solid and porous type turbulators

Turbulators were used in a variety of designs to increase heat transmission. However, as heat transfer technology employment increases the amount of pumping power needed. The thermal and hydraulic efficiency of SAHs must thus be studied. They are further subdivided into subgroups as extended surface area (fins), using ribs and baffles, and airflow ducts with porous materials. To increase the number of SAHs pathways leading to more favourable outcomes baffles are used. Thermal resistance created by the laminar sub-layer decrease by increasing the intensity of turbulence using roughness. As a result, of thinner laminar sub-layer. However, the height of the roughness should be within the limit that is 1% to 5% of the hydraulic diameter of the duct and after crossing this limit roughness is called baffles (Kumar, Saini, and Saini 2014). Baffles are used to rearrange the flow pattern inside the chamber (Romdhane 2007). Baffles serve the following purposes: extend time duration and proper mixing of circulating air with no vortex which minimises the heat losses and offers full turbulence flow to enhance heat transfer. According to the type of baffles used in a solar air heater the collector may be classified into two main parts such as baffles with perforation and without perforation. Perforation is allowed to pass through the holes only while it passed air around the baffles in case of baffles without perforation. Baffles with perforation are affected by their perforation index, while the baffles without perforation are affected by the spacing and shape of baffles. The larger size of perforation creates uniformity of Nusselt number to enhance the thermal performance of SAHs (Shin and Kwak 2008). The spacing between the baffles affects the performance of the solar collector (Romdhane 2007). The small gap between the successive baffles enhances the turbulence of flowing fluid and also increases the pressure drop through the test section (Slama, Bouabdallah, and Mora 1996). Perforated baffles inside the inlet section enhance the thermal efficiency of a rectangular duct solar heater (Wei et al. 2017). Results revealed Nusselt number enhanced from 79% to 169%, friction factor increased by 2.98–8.02 times as compared to the smooth section for fully perforated baffles. Half-perforated type baffles were best suited to fully perforated baffles (Karwa and Maheshwari 2009). For the relative height of the perforated V-shaped, the Nusselt number increases by 33%, and the friction factor was reduced by 32%. Thermohydraulic efficiency improved by 50% over solid baffles without perforation (Alam, Saini, and Saini 2014). The very best internal baffles have been shown to increase thermal efficiency (Hu et al. 2013) in contrast to inclined baffles, longitudinal and transverse, and transverse baffles alone (Romdhane 2007; Luan and Phu 2020; Mahmood 2017). It was noticed that the thermal efficiency of SAHs improves when the amount of airflow increases. In the case of longitudinal and transverse baffles combination more than 25 m³/hr/m² air volume flow rates showed better results. To get a clearer picture of how these baffles affect the performance of a SAH there is need for more extensive investigation on shape, location, and orientation of these baffles in fluid flow study.

The efficiency of the solar collector is affected by the different configurations of fins and baffles together. The decreasing order of collector efficiency was obtained as (1) using fins and baffles, (2) using only fins, and (3) using baffles only. This order follows a reverse pattern for aspect ratio with combined baffles and ribs (Yeh, Ho, and Lin 2000). Fins and baffles are generally used to create the secondary flow inside the flow channels of SAHs. The significant improvement in the performance of the solar air heating system is achieved because of secondary flow which enhances the mixing of primary fluid with viscous sub-layers. This secondary flow formation also has a negative impact including an increase in friction because of higher pressure drops hence an increase in required pumping power. Maximum thermal efficiency was obtained using various baffles as listed in Table 2.

Table 2. Optimal geometry of baffles with maximum thermal efficiency.

Type of fins and baffles used	Volume flow rate (m)	Optimum % efficiency (η)	Ref.
Longitudinal and transverse baffles	40.51	77	Romdhane 2007
Internal baffles	77.9	81.6	Hu et al. 2013
Longitudinal and transverse baffles	56.28	63.67	Luan and Phu 2020
Transverse baffles	25.95	61.5	Mahmood 2017

4.2.3. Vortex generators (VGs)

To improve the performance of SAHs the VGs are placed in an arrangement that promotes turbulence and swirls flow within the air duct. Additionally, VGs help to prevent the formation of laminar sub-layers on the surface of an absorber. The whirlwind that happened inside the air duct due to the VGs placed may be classified into two categories one is transverse and another is longitudinal. The transverse vortex axis is parallel to the main axis and the longitudinal vortex is perpendicular to the main flow. The transverse vortex is less effective than the longitudinal vortex. Type of V-baffle (Tamna et al. 2014), longitudinal (Valencia 2020), punched winglet type (Gupta et al. 2020), tear-drop (Lu and Zhai 2018), and delta-shaped (Bekele, Mishra, and Dutta 2013), etc., VGs have been investigated. The most effective VGs are delta-shaped, in this case, the TPF value is 2.15 at Re of 10,000 was obtained. The TPF of VGs initially rises with Re but after achieving a maximum value diminished excluding the V-baffle. Reynolds numbers under 5000 are best served by this class of VGs. The variance in TPF may be seen when comparing the different shapes of VGs in the SAHs given in Table 3.

Table 3. Optimum TPF of different VGs used in a SAHs.

Type of vortex generators (VGs) used	Reynolds numbers	Thermal performance factor (TPF)	Ref.
With delta-shaped	9700	2.15	Bekele, Mishra, and Dutta 2013
V-baffles (single)	4550	1.83	Tamna et al. 2014
With teardrop-shaped	1520	1.23	Lu and Zhai 2018
Longitudinal	2000	1.52	Valencia 2020
Punched winglet	9000	1.77	Gupta et al. 2020

4.2.4. Modification of the form of the absorber plate

To promote heat transmission from the absorber various alterations were made to the absorber surface. These surfaces are known as corrugated surfaces. Adding corrugations to the absorber plate increases heat transmission and improves the thermal efficiency of SAHs. The locally heated area may emerge in the duct flow SAH (particularly in the triangular duct) because of stationary sections of the duct. These heated areas do not contribute to the transmission of heat but the rate of heat loss from these locations is increased. As compared to other configurations the researchers altered the geometry of the duct and the result demonstrates that it has a superior thermohydraulic efficiency than any other condition examined (Kumar et al. 2018a; Kumar, Kumar, and Goel 2018b; Kumar et al. 2020a; Kumar, Mahanta, and Kalita 2019d). To reduce eddies at sharp corners triangular ducts with rounded edges were used instead of the sharp-corner triangular duct.

4.3 Calculation for thermal losses and their reduction methods associated with SAHs

Generally, SAHs have poor thermal efficiency due to thermal energy losses such as top, bottom, and side losses. The bottom and side losses are assumed to be negligible as compared to the top losses or minimise by employing strong insulation. Top thermal losses consist of reflection (8%) losses from glass cover, radiation (8%), convection (10%), and absorption (10%) losses from the absorber. Radiation (4%) losses also take place from the back side of the absorber. Only about 60% of total (100%) irradiation is used to generate heat by a solar air heater. About 40% of the total radiation is lost as heat and cannot be used in any way (Kalita). To optimise the performance of SAHs reduction in top losses must be focused on and try to minimise. Those who are working on the design and simulation of flat plate SAHs would do well to learn the value of the top heat loss factor. The ambient temperature, emittance properties of the collector, and thermal conductivity of air are the responsible factors for top thermal losses. The air gap between two successive glass covers and collector inclination are the other influencing parameters. The amount of heat flux for single-glazed SAHs between absorber and cover, glass cover, and surroundings are given in Equations (13) and (14) respectively. The value of inner glass temperature (T_ig) was obtained by solving the Equations (11) and (12). (13) $\dot{q^{\mathrm{\prime }\mathrm{\prime }}=\left[\left(h_{rpig}+h_{cpig}\right)\ast \left(T_{pm}-T_{ig}\right)\right]}$(13) (14) $\dot{q^{\mathrm{\prime }\mathrm{\prime }}=\left[\left(h_{riga}+h_{ciga}\right)\ast \left(T_{ig}-T_a\right)\right]}$(14) The radiative and convective heat transfer coefficients between the absorber and inner glass cover are $h_{rpig}$, $h_{cpig}$, and between the inner glass to the surrounding environment is $h_{riga}$, $h_{ciga}$ respectively. The equations of heat balance for double-glazed cover SAHs are mathematically expressed as in Equations (15)–(17). For double glazed the values of inner glass temperature (T_ig) and outer glass temperature (T_og) were obtained by solving Equations (15) and (17) (Samdarshi and Mullick 1990). The clear vision of convective and radiative coefficients associated with single and double-glazed SAHs is shown in Figure 5. (15) $\dot{q^{\mathrm{\prime }\mathrm{\prime }}=\left[\left(h_{rpig}+h_{cpig}\right)\ast \left(T_{pm}-T_{ig}\right)\right]}$(15) (16) $\dot{q^{\mathrm{\prime }\mathrm{\prime }}=\left[\left(h_{riog}+h_{ciog}\right)\ast \left(T_{ig}-T_{og}\right)\right]}$(16) (17) $\dot{q^{\mathrm{\prime }\mathrm{\prime }}=\left[\left(h_{roga}+h_{coga}\right)\ast \left(T_{og}-T_a\right)\right]}$(17) where ($h_{riog}$) and ($h_{roga}$) are the radiative heat transfer coefficients between inner to outer glass and outer glass to surrounding environment respectively, similarly, ($h_{ciog}$) and ($h_{coga}$) are the convective heat transfer coefficients between inner to outer glass and outer glass to surrounding respectively. The amount of total heat loss factor ($U_t$) by convection and radiation, mathematically expressed as in Equation (18). (18) $U_t={\displaystyle \frac{q_t^{\mathrm{\prime }\mathrm{\prime }}}{\left(T_{pm}-T_a\right)}}$(18) where ($T_{pm}$) and ($T_a$) represents the plate mean and surrounding temperature respectively and ($q_t^{\mathrm{\prime }\mathrm{\prime }}$) is total heat flux.

Figure 5. Heat transfer coefficients for single and double glass cover SAHs (Samdarshi and Mullick 1990). (a) Single-glazed SAH. (b) Double-glazed SAH.

Few strategies are used to decrease the thermal losses by reducing the radiative, and convective losses and by providing a vacuum in between two or more glass covers. Two or three glass covers are generally used in collectors that operate at quite high temperatures to minimise these losses from the top of the collectors. The surface treatment of a glass cover helps to reduce the reflectance. Various convective and radiative heat transfer coefficients are involved in single and double-glazed SAHs shown in Figure 5 (Samdarshi and Mullick 1990). Efficiencies in the range of 10% to 15% higher might be attained (Hernández and Quiñonez 2013). Several scholars have reported parametric studies on the distance between two coverages. To minimise heat loss by convection and radiation, they recommended optimal gap space between glass coverings. Reduction in heat losses was obtained by 34% using a lot of gas between the gap space (Sekhar et al. 2009). In a partly evacuated environment, a 10% drop in pressure will reduce losses by 85%. With the addition of a selective surface and moderate vacuum, it has been shown that the collector may operate at 150 °C with daily energy-collecting efficiency of more than 40%. To maximise the amount of absorbed heat by reducing the ambient losses, different types of honeycomb structures were also used (Abdullah, Abou-Ziyan, and Ghoneim 2003; Cadafalch and Cònsul 2014; Egolf et al. 2018).

Analysis of exergy loss and entropy creation in a heat exchanger because of the transfer of heat between hot and cold media and friction is an important part of determining the SAHs performance based on the II-law of thermodynamics (Kumar et al. 2021; Yadav et al. 2014). The physical, kinetic, potential, and chemical are the main exergies associated with thermal systems mathematically expresses as in Equation (19). (19) $E_{xT}={}_T^{Phy}{E}_x+{}_T^K{E}_x+{}_T^P{E}_x+{}_T^{Ch}{E}_x$(19) where ${}_T^{Phy}{E}_x,{}_T^K{E}_x,{}_T^P{E}_x,and{}_T^{Ch}{E}_x$ are the physical, kinetic, potential, and chemical components of total exergy ($E_{xT}$) respectively. It is assumed that except for physical components of exergy all other components are negligible, and the balancing equation expresses as Equation (20) (20) $E_{xw}=E_{xout}-E_{xin}+E_{xloss}$(20)

where $E_{xin}$, $E_{xout}$ and $E_{xloss}$ represent the flow of exergy at the inlet, outlet, and exergy loss expressed in Equations (21), (22) and (23) respectively as follows: (21) $E_{xin}=m\ast \left[\left(h_{in}-h_o\right)-T_o\ast \left(S_{in}-S_o\right)\right]$(21) (22) $E_{xout}=m\ast \left[\left(h_{out}-h_o\right)-T_o\ast \left(S_{out}-S_o\right)\right]$(22) (23) $E_{xloss}=T_o\ast S_{gen}$(23)

The point ‘o’ indicates the reference temperature. Enthalpy and entropy at the inlet, at the outlet, and at the reference temperature are represented by $h_{in}$, $S_{in}$, $h_{out}$, $S_{out}$ and $h_o$, $S_o$ respectively. The entropy of generation and reference temperature is $S_{gen}$ and $T_o$. When any vortex generators are placed in the path of flowing fluid then the efficiency of exergy (€) and loss of energy ($\mathrm{e}$) are calculated by Equations (24) and (25) respectively (Zheng et al. 2016). (24) $\text{€}={\displaystyle \frac{E_{xout}-E_{xin}}{E_{xloss}}}$(24) (25) $\mathrm{e}={\displaystyle \frac{E_{xloss}}{m\ast \left(E_{xin}-E_{xout}\right)}}$(25)

4.4. Enhancement of heat transfer

Enhancement of heat transfer is always a major challenge and key factor in the designing of SAHs. Various inserts such as twisted tapes, wire coils, vortex rings, etc., are used to enhance the heat transfer rate. Inserts create the swirl flow in the path of working fluid which disturbs the actual boundary layers resulting in an increase in the effective surface in the existing systems. Longer time contact between the absorber plate and process air enhances the thermal performance of SAHs the double-pass or multi-pass solar air heaters have been designed (Alam and Kim 2017). Further modifications also have been carried out to improve the thermal performance of double pass or multiple passes SAHs with the direction of flow (parallel flow, counterflow and recycle flow), different rates of mass flow, and positioning of the two channels, etc., The recycling process enhances the performance of a double-pass solar air heater (Ravi and Saini 2016). The cost of DPSAHs increases with an increase in the material requirement but the coefficient of heat transfer increases by increasing the heat transfer area. The thermal performance of SAHs may be improved by incorporating new approaches such as employing PCMs, a mixture of nanoparticles in PCMs, etc. There is a lot of research has done to enhance the thermal performance of a flat plate solar air collector using HT augmentation techniques. To improve the mixing of fluid it is necessary to disturb the flow pattern inside the collector chamber. For moderate and higher range temperature applications, the active method over the passive method and the combination of two methods are used. The active method requires an external power source and includes mechanical aids. Passive method includes roughness on the collector surface, vortex generators, increase in effective length of the test section using double or multiple pass ducts, placing fins and baffles, etc. All these methods improve performance either by increasing the surface area for heat transfer or by generating localised turbulence in the flow field. With the employment of these approaches the pressure drop is simultaneously increased with heat transmission thus it is vital to utilise suitable techniques based on the application which yields a higher thermal performance at small pressure drops through the duct.

4.4.1. Artificial roughness

To enhance the thermal efficiency of a solar air heater the surface roughness idea has been introduced by Joule first time to improve the convective heat transfer coefficient between the solid surface and process air. The formation of boundary layer thickness on the solid plate depends upon the roughness of the plate and the velocity of air on it. Reduction in the boundary layer thickness increases the heat exchange rate between the plate and the air. In the laminar flow regimes, there is a formation of a laminar sub-layer at the contact surface. This laminar sub-layer offers thermal resistance and reduces the convective heat transfer coefficient (h) between the absorber plate and process air. The absorber plate is frequently coated with an artificial roughness to disrupt the laminar sub-layer to create turbulence in SAHs. Surface roughness enhances the rate of heat transfer and offers some resistance in the path of flowing air. The type of roughness includes ribs, dimples, projections, etc. Metal grit ribs type geometry enhances the Nusselt number and friction factor up to twice and thrice respectively for the range of 4000 < Re < 17,000. Nusselt number improves up to 3.8 times at an arc angle of α/90 (Karmare and Tikekar 2007).

Turbulators are used in the SAHs as rib-roughness in a variety of shapes and sizes. These include to be semi-circular (Kumar et al. 2017a), round (Kumar et al. 2018a), square (Singh and Singh 2018; Singh and Singh 2018; Kumar 2019), rectangle (Kumar, Prajapati, and Samir 2017b), single arc-shaped (Kumar and Saini 2009), more than one arc-shaped (Kumar and Goel 2021; Kumar, Goel, and Kumar 2020b; Kumar, Goel, and Kumar 2020c), discrete multi-arc (Kumar 2019; Kumar et al. 2019a; Kumar, Kumar, and Goel 2019b; Kumar, Kumar, and Goel 2019c), W-shaped (Lanjewar and Bhagoria 2011; Lanjewar, Bhagoria, and Sarviya 2011), V-shaped (Momin 2002), rib-groove combination (Jaurker 2006), protrusions (Prakash and Saini 2018; Kumar et al. 2020d) and in addition to the shape like hemispherical cavities (Goel et al. 2021b), etc. Various scholars have considered this and analysed it to determine the thermohydraulic and thermal performance of SAHs. The Thermal Performance Factor (TPF) of SAH is not exclusively dependent on these roughness types. It is also impacted by the change in the shape and size of roughness components and the cross-sectional shape of air ducts like triangular and rectangular passageways (Kumar, Kharub, and Kumar 2020e). The greatest TPF is determined for the multiple V-shaped roughnesses at a maximum value of 4.16 at a Reynolds number of 4000 among all the investigated roughness shapes or patterns. Roughness elements that are made up of several V-shaped roughnesses work well in most of the area but at higher Reynolds numbers (Re) and discrete multi-arc ribs exhibit the highest TPF about 3.84. Roughness geometries with arcs have a lower friction factor which makes them ideal for use at higher Re. For all Re values, the combination of spherical and slanted rib protrusion is the most effective. At higher Re, the TPF value was found to be near 1 for the combination of transverse and inclined wires which means it is a less suitable roughness geometry. SAH with a Re of 7500 exhibits the best TPF value of 2.01 in V-ribbed. Dimple-shaped roughness is more suitable for low Re and TPF reduces with Re rise. There has been little research on fluid flow behaviour in the triangular channel, but no combinations of geometries have been discovered yet in this type of channel. Tables 4 and 5 contain the list of roughness geometries based on literature that offers maximum TPF.

Table 4. Various roughness geometries and their output performance.

Roughness geometry	Optimised parameters and findings	Ref.
Transverse wire ribs	Re = 12,000 (500–50,000) P/e = 10 e/D = 0.027 Correlation: Not reported (Nu/Nus) max = 2.38, (f/fs) max = 2.08	Prasad and Saini 1988
Inclined continuous ribs	Re = 14,000 (4000–18,000) e/D = 0.033 α = 600 Correlation: Reported η/ηs=1.25	Gupta, Solanki, and Saini 1997
Expanded metal mesh	Re = 9500 (1900–13,000) e/D =0.039 S/e =15.62 L/e =71.87 Correlation: Reported Nu/Nus =4 f/fs = 5	Saini and Saini 1997
Transverse wire ribs	Re = 15,000 (5000–20,000) P/e =10 e/D = 0.022 Correlation: Not reported Nu/Nus = 2.08 f/fs =1.8	Verma and Prasad 2000
Chamfered ribs	Re = 8000 (3000–20,000) P/e = 7.25 e/D = 0.0265 φ = 150 Correlation: Reported St/Sts = 2 and 4	Karwa, Solanki, and Saini 2001
V-Shaped ribs	Re = 15,000 (5000–20,000) e/D = 0.034 α = 600 Correlation: Reported Nu/Nus = 2.3 f/fs = 2.83	Momin 2002
Ribs in a wedge shape	Re = (3000–18,000) P/e = 7.57 e/Dh = 0.015–0.033 W/H = 5 Φ = 100 Nu/Nus = 2.4 f/fs = 5.3	Bhagoria, Saini, and Solanki 2002
V-ribs with different arrangements	Re = 120,000 (2500–18,000) e/D = 0.05 W/H = 7.42 Correlation: Not reported Nu/Nus = 2.47 f/fs = 3.65	Karwa 2003
90° broken transverse ribs	Re = 9000 (3000–12,000) P = 20 Correlation: Not reported Nu/Nus = 1.4	Sahu and Bhagoria 2005
Ribs combined with the groove	Re = 18,000 (3000–21,000) P/e =6 e/D = 0.0363 g/P = 0.4 Correlation: Reported Nu/Nus = 2.7 f/fs = 3.6	Jaurker 2006
Inclined continuous ribs with the gap	Re = 17,000 (3000–18,000) P/e =10 e/D = 0.0337 W/H = 5.87 g/e = 1 α = 600 Correlation: Reported Nu/Nus = 2.95 f/fs = 2.87	Aharwal, Gandhi, and Saini 2008
Arc shaped ribs	Re = 16,000 (2000–17,000) P/e =10 e/D = 0.0422 W/H = 12 α/90 = 0.333 Correlation: Reported Nu/Nus = 3.8 and f/fs =1.75	Saini and Saini 2008
Dimple shaped ribs	Re = 12,000 (2000–12,000) P/e =10 e/D = 0.026 Correlation: Reported	Saini and Verma 2008
Inclined and transverse rib combination	Re = 12,000 (2000–14,000) P/e =8 e/D = 6 W/H = 10 Correlation: Reported	Varun and Singal 2008
Metal grits	Re = 15,000 (4000–17,000) P/e =17.5 e/D = 0.044 l/s = 1.72 Correlation: Reported Nu/Nus = 1.87 f/fs = 2.13	Karmare and Tikekar 2009
U-shaped turbulators	Re = 12,000 (3800–18,000) P/e = 6.67 e/D = 0.0398 α = 900 Correlation: Reported Nu/Nus = 2.82 f/fs = 3.72	Bopche and Tandale 2009
Multi V-shaped ribs	Re = 15,000 (2000–20,000) P/e = 8 e/D = - W/w = 6 α = 600 Correlation: Reported Nu/Nus = 6 f/fs = 5	Hans, Saini, and Saini 2010
Obstacles	Tp =0.12, 0.7, and 1.2 cm Triangular obstacles (5 cm*5 cm) Rectangular obstacles (10 cm *10 cm) Leaf type (5 cm*5 cm)	Akpinar and Koçyiĝit 2010
W-shaped ribs	Re = (2300–14,000) P/e =10 e/D = 0.0337 α = 600 Correlation: Reported Nu/Nus = 2.36 f/fs = 2.01	Lanjewar and Bhagoria 2011
Discrete V-down ribs	Re = (3000–15,000) P/e =8 e/D = 0.043 g/e = 1 d/w = 0.65 α = 600 Correlation: Reported Nu/Nus = 3.04 f/fs = 3.11	Singh, Chander, and Saini 2011
Multi-V-shaped rib with the gap	Re = 18,000 (2000–20,000) P/e = 10 e/D = 0.043 W/w = 6 g/e = 1 d/w = 0.69 Correlation: Reported Nu/Nus = 6.74 f/fs = 6.37	Kumar, Saini, and Saini 2013
Dimple in arc-shaped	Re = (3600–18,100) P/e = 12 e/D = 0.03 α = 600 Correlation: Reported Nu/Nus = 2.89 f/fs = 2.93	Yadav et al. 2013
Triangular shaped ribs	Re = 15,000 (3800–18,000) P/e = 7.14 e/D = 0.042 Correlation: Not reported Nu/Nus = 3.07 f/fs = 3.35	Yadav and Bhagoria 2014a
Square shaped ribs	Re = 12,000 (3000–18,000) P/e = 10.71 e/D = 0.042 Correlation: Not reported Nu/Nus = 2.86 f/fs = 3.84	Yadav and Bhagoria 2014b
Multiple arc rib ribs	Re = 22,000 (2200–22,000) P/e = 8 e/D = 0.045 α = 600 Correlation: Reported Nu/Nus = 5.07 f/fs = 3.71	Singh et al. 2014
Multi V-shaped ribs	Re = (8000–20,000) P/e = 3 e/D = - W/w = 6 α = 450 Correlation: Not reported THPPmax = 1.93	Jin et al. 2015
Non-uniform saw tooth-shaped and trapezoidal ribs	Re = 6000 (3000–15,000) P/e = 8 e/D = 0.043 Correlation: Not reported Nu/Nus = 1.78 f/fs = 2.49	Singh et al. 2015
Semi-ellipse-shaped ribs	Re = (6000–18,000) P/e = 3.5 α = 750 Correlation: Not reported Nu/Nus = 2.05 f/fs = 6.93	Alam and Kim 2017
Multi-gap V-down with staggered ribs	Re = 12,000 (4000–12,000) P/e = 12 e/D = 0.044 W/e = 4.5 g/e = 1 α = 600 Correlation: Reported Nu/Nus = 3.34 f/fs = 3.38	Deo, Chander, and Saini 2016
Multiple arc rib with the gap	Re = 2100–21,000 P/e = 8 e/D = 0.044 W/w = 5 d/w = 0.65 g/e = 1 α = 600 Correlation: Reported Nu/Nus = 5.85 f/fs = 4.96	Pandey and Bajpai 2016
Reversed L-shaped ribs	Re = 15,000 (3800–18,000) P/e = 7.14 e/D = 0.042 Correlation: Reported Nu/Nus = 2.82 f/fs = 2.43	Gawande et al. 2016
V-ribs, Dimple, protrusion, and grooves in V-pattern	Re = (5000–20,000) P/e = 10 e/D = 0.04 e/dp = 0.5 g/p = 0.6 Correlation: Not reported	Kumar and Kim 2016
Broken arc ribs	Re = 12,000 (2000–16,000) P/e = 10 e/D = 0.043 d/w = 0.65 g/e = 1 α = 300 Correlation: Reported Nu/Nus = 2.63 f/fs = 2.44	Hans, Gill, and Singh 2017
S-shaped ribs	Re = (2400–20,000) P/e = 8 e/D = 0.43 W/w = 3 α = 600 Correlation: Reported	Kumar et al. 2017a
Non-uniform saw tooth-shaped ribs	Re = 9000 (3000–15,000) P/e = 16 e/D = 0.043 θ = 450 Correlation: Not reported Nu/Nus = 2.18 f/fs = 3.34	Singh and Singh 2017
Turbulators with spherical shape	Re = (4000–25,000) P/D = 3 D = 25 mm α = 450 Correlation: - Maximum thermal efficiency increased by 23.4%	Manjunath, Karanth, and Sharma 2017
Conical shaped turbulators	P = 16, 32, and 48 mm H = 2, 3, and 4 mm Conical pins with staggered arrangements show more effective results. Nu/Nus = 6.67	Gilani et al. 2017
Delta wing-type vortex generators combined with dimples	Nu/Nus = 2.76 f/fs = 2.75	Luo et al. 2017
Square wave transverse ribs	Re = 12,000 (3000–15,000) P/e = 10 e/D = 0.043 Correlation: Not reported Nu/Nus = 2.14 f/fs = 3.55	Singh and Singh 2018
Twisted ribs	Re = (3500–21,000) P/e = 6–10 y/e = 3–7 Correlation: Nu/Nus = 2.46 f/fs = 1.78	Kumar and Layek 2019
Multiple gap V-down ribs with turbulence paramotors	Re = - P/e = 10 e/D = 0.04 α = 450 Correlation:	Misra et al. 2020

Table 5. Maximum TPF is obtained from various optimal roughness geometry.

Type of roughness used	Reynolds number	Thermal performance factor (TPF)	Ref.
Ribs of W-shaped	6100	1.98	Lanjewar and Bhagoria 2011; Lanjewar, Bhagoria, and Sarviya 2011
Ribs of spherical and inclined shaped	19,900	3.64	Prakash and Saini 2018
Ribs of Discrete multiple arc-shaped	1900	-	Kumar 2019; Kumar et al. 2019a; Kumar, Kumar, and Goel 2019b; Kumar, Kumar, and Goel 2019c
Ribs of the square, rectangular, circular, and semi-circular shaped	18,700	2.75	Kumar and Goel 2021
Hemispherical cavity	2160	3.43	Goel et al. 2021b

4.4.2. Extended surface/fins

The extended surfaces are used to enhance the rate of heat transfer between the heat exchange surface and process air. Extended surfaces include fins of various shapes such as semi-cylindrical, triangular, rectangular, transverse, longitudinal, wavy, louvered, herringbone, corrugated, pin type, combinations of different fins with baffles and, wire mesh, etc. (Ho, Yeh, and Chen 2011). Extended surfaces include fins, baffles, tabulators, etc. used for surface variation to offer a larger surface area for heat transfer (Sahu et al. 2019). The thermal performance of SAHs can be enhanced by adding the heat transfer surface area in the flow channel. Fins enhanced the convective heat transfer coefficient which reduces the heat loss because of radiation and is responsible to obtain the high thermal efficiency of SAHs (Hu et al. 2013). Fins offer an extra enormous area to transfer the heat as compared to the simple SAHs for the same geometry and dimensions and hence improve the performance of the solar air heating system. Fins along the longitudinal direction also help to control the movement of flowing air and turbulence created by these fins when attached along the lateral direction (Daliran and Ajabshirchi 2018; Hachemi 1995; Moummi et al. 2004). The efficiency of a collector depends upon the number and length of fins. Fin-type SAHs are best suited as compared to the other collectors with high performance (Garg et al. 1991). V-shape grooves on the surface of the absorber plate increase the amount of incident solar energy and collect the radiation reflected many times from the edges of the V- groove (Karim and Hawlader 2006). The maximum efficiency obtained was about 71% to 83% for finned type absorber plates at a mass flow rate of 0.09 kg/s (Fudholi et al. 2013a; Fudholi et al. 2013b). Various collectors with and without tabulators show that the daily efficiency reached the maximum value of 68% for the mass flow rate of 0.05 kg/s (Abdullah et al. 2018). The details of various fins used in different research works and the effect of these fins on the performance of solar air heaters are given in Tables 6 and 7 shows the optimal geometry of fins with maximum thermal efficiency.

Table 6. Details of different fins used in previous research work.

Geometrical view of used fins	Used parameters	Key findings	Ref.
Rectangular fin	q = 960 W/m^2, ${\mathrm{T}}_{\mathrm{i}}$ = 33°C, N = 25, L = 4, 5 and 6 m, H = 10, 15 and 20 cm. Fixed $\dot{\mathrm{m}}$ = 100 kg/hrm^2.	Efficiency improved from 45% to 61% for different depths of duct and fin length. Higher outlet temperature. The maximum amount of irradiation incident on the absorber trapped by fins and hence improve the thermal efficiency.	Garg et al. 1991
Combination of fin and baffles	N = 0, 5 and 7, Space between baffles = 0.005, 0.1, 0.2 and 0.4, width of baffles = 0.01, 0.03 and 0.05, Mass flow rate = 0.01, 0.03 and 0.05 kg/s.	Improvement in the temperature of outlet air. Useful heat gain, outlet temperature, and efficiency directly depend upon the fins number and width of baffles.	Mohammadi and Sabzpooshani 2013
Offset strip fins	Fin thickness = 0.4 mm, and length, height, and spacing of fins are 0.01 mm to 0. 3 mm. Required power = 20 W.	$\mathrm{q}=600$ W/m^2, ${\mathrm{T}}_{\mathrm{i}}=14{}^{\circ }$, Incident angle = $0{}^{\circ }\le \theta \le 30{}^{\circ }$, ${\eta }_{\mathrm{o}\mathrm{p}\mathrm{t}}=37.8\text{\%}$, airflow volume = 100 m3/s. cost-effective method	Yang et al. 2014
Hemispherical fin	Pitch = 120 mm, thickness = 5 mm and mass flow rate = 0.012 and 0.016 kg/s.	At 0.016 kg/s rate of mass flow, the maximum thermal efficiency was obtained 51.5% and 43.94% with fins and without fins respectively.	Chabane, Moummi, and Benramache 2014
Rectangular longitudinal fin double passes SAH.	Height of fin = 5 and 8 cm, Fins = 45, 50 and 55, mass flow rate = 0.02–0.1 kg/s.	The area for heat transfer was enhanced by 66% using this geometry and 20% of heat transfer has been improved for this area.	Fakoor Pakdaman et al. 2011; Naphon 2005
Transverse fin with wire mesh	Fin height and thickness = 7.2 and 0.3 cm, division = 4. $\mathrm{m}=0.011-0.032$ kg/s,	Thermal efficiency directly depends upon the mass flow rate of air. The maximum efficiency was obtained at 7.2 cm height of fin used 55% and 62.5% for single and double pass SAH respectively.	Mahmood, Aldabbagh, and Egelioglu 2015
Rectangular AND triangular	With single and double glass covers. 100$\le$Re$\le$24,000, Critical parameters like Recr=13,500, Lcr=2 m, dcr = 0.95 m. Fan speed = 12,000 rpm	Tin metal sheets and double glass covers offer maximum efficiency at a low Reynolds number, while Single glass covers at a high Reynolds number. A large number of fins are best suited for Re < Recr and worst for reverse. The heat transfer rate and power required for the fan were reduced for an increase in duct depth.	Bahrehmand, Ameri, and Gholampour 2015
Offset finned absorber type	Collector height = 0.02 m, duct height = 0.06 m, offset distance = 0.018–0.058 m, offset space = 0.01–0.05 m, fin length = 0.003 m.$\mathrm{m}=0.0139-0.083\mathrm{K}\mathrm{g}/\mathrm{s}$.	A 67.38% improvement was obtained in thermohydraulic efficiency at a low flow rate. ${\eta }_{\mathrm{t}\mathrm{h},}{\eta }_{\mathrm{h}}$ improved by 114.1% and 112.65% with a reduction in fins spacing and with an increase in height.${\eta }_{\mathrm{h}}$ initially increase with flow rate and after reaching a certain value (0.028 kg/s) sharply reduced for further increase in flow rate.	Rai, Chand, and Sharma 2017
Herringbone corrugated fin	A=1.5 cm, $\lambda =7\mathrm{c}\mathrm{m}$, ${\mathrm{F}}_{\mathrm{p}}=2.5\mathrm{c}\mathrm{m}$, $\theta =30{}^{\circ }$, $\mathrm{m}=0.02-0.09\mathrm{K}\mathrm{g}/\mathrm{s}$.	At 2.5 cm pitch and 0.05 kg/s rate of mass flow, the maximum thermal efficiency was obtained about 68.96%.	Kumar and Chand 2017
Winglet vortex generator	$3500\le$Re$\le$16,000, Attack angle $\alpha$ = 30^0, 45^0, 60^0, 75^0 and 90^0. Edge ratio (c/a) = 0, 0.3, 0.6 and 1.	At attack angle, α=600 and edge ratio=1 heat transfer and friction factor enhance by 4.7 and 25.19 times as compared to smooth tube. Nu and f increased with edge ratio and attack angle from 300 to 600 and then decrease with an increase in attack angle. Maximum TEF from 1.72–2.2 was obtained at edge ratio=0 and 300 attack angle.	Chamoli et al. 2018
Rectangular, triangular, and elliptical fins	Fins = 46, Spacing = 2 cm, rib height = 28 mm for triangular fin and 27 mm, pitch = 25 mm.	The efficiency of rectangular-finned SAH obtained was about 12.5% and 5.5% greater than that of elliptical and triangular-finned SAH respectively. Rectangular fins offer better thermal performance at a low mass flow rate.	Hosseini, Ramiar, and Ranjbar 2018
Wavy finned	Amplitude of fin = 0–2.5 cm, Wavelength = 0–20, Mass flow rate = .0138–0.834 kg/s, spacing = 1 cm.	At lowest (0.0138 kg/s) and highest (0.0834 kg/s) rate of flow maximum efficiency was obtained upto 35.83% and 17.56% respectively.	Priyam and Chand 2016; Priyam and Chand 2018
Helicoidal spring-type fin	${\displaystyle \frac{{\mathrm{p}}_{\mathrm{s}}}{\mathrm{H}}=0.1-0.1667}$, ${\displaystyle \frac{\mathrm{d}}{\mathrm{H}}0.0533-0.1067}$, ${\displaystyle \frac{\mathrm{D}}{\mathrm{W}}0.02-0.12}$.	At a low Reynolds number, the thermohydraulic performance factor improved by 1.268 times for a 0.06 diameter ratio.	Arunkumar, Kumar, and Karanth 2020a
Inclined fin with the inline and staggered arrangement.	$3000\le \mathrm{R}\mathrm{e}\le 20,000$, Inclination of fin $30{}^{\circ }\le \theta \le 90{}^{\circ }$, $0.007\mathrm{m}\le \mathrm{L}\le 0.028\mathrm{m}$, $0.007\mathrm{m}\le \mathrm{H}\le 0.014\mathrm{m}$, $0.029\mathrm{m}\le {\mathrm{D}}_{\mathrm{h}}\le 0.057\mathrm{m}$,	$\eta =79\text{\%}$, The efficiency improvement was attained at about 13% for double-pass SAH as compared to the single-pass SAH for similar design parameters.	Singh 2019
Cylindrical fin with porous aluminium wire mesh	Fin length = 182 mm, fins = 30, outer diameter and thickness of fin = 30 and 0.3 mm	At 5 m/s flow velocity, a good rate of heat transfer and low friction factor has been obtained for the wavy-shaped plate with an arc comparatively without an arc.	Khatri et al. 2020
Fins and baffles with arc-type ribs	$2900\le \mathrm{R}\mathrm{e}\le 17,000$, ${\mathrm{W}}_{\mathrm{B}}=0.005\mathrm{m}-0.015\mathrm{m}$, ${\mathrm{L}}_{\mathrm{B}}=0.2-0.4\mathrm{m}$, N = 2–10	N=8, ${\mathrm{L}}_{\mathrm{B}}=0.2\mathrm{m}$, and ${\mathrm{W}}_{\mathrm{B}}=0.015\mathrm{m}$ are optimise parameters which offers maximise exergy efficiency about5.2% for m=0.012 kg/s.	Saravanakumar, Somasundaram, and Matheswaran 2020

Table 7. Optimal geometry of fins with maximum thermal efficiency.

Type of fins used	Volume flow rate (m)	Optimum % Efficiency (η)	Ref.
Wavy fins	0.05	75	Priyam and Chand 2016
Longitudinal fins	0.1	63	Naphon 2005
Fin with offset strip	0.05	49.8	Yang et al. 2014
Fins in a corrugated herringbone shape	0.06	72.8	Kumar and Chand 2017
Fins in helicoidal spring shape	–	1.26 (TPF)	Arunkumar, Kumar, and Karanth 2020a

5. Thermal energy storage systems used in SAHs

Due to the intermittency or fluctuating nature of solar radiation in cloudy or rainy days the supply of hot air is interrupted. Thus, a storage system is often required to store the thermal energy when the collected amount is more than the required amount and supply when it is needed. TES-integrated SAHs are the most effective ways to store solar thermal energy for heating purposes. Thermal energy storage systems such as Sensible Heat Storage (SHS) and Latent Heat Storage (LHS) plays important role in SAHs to store energy for later use. Sensible heat storage refers to the heating of liquid or solid without change in its phase, how much heat a material can store is a function of its temperature change. Latent heat storage refers to the heating/melting it undergoes the change of shape and the quantity of energy stored is determined by both the mass and the latent heat of fusion of the material Based on previous research LHS (PCM-based) is far better as compared to the SHS for long time storage. The low thermal conductivity of phase change materials (PCMs) is only the major drawback of LHS systems. Different methods are presently available to enhance the thermal conductivity of PCMs. The addition of more conductive materials with base material enhances the thermal conductivity of PCMs. The newly obtained material is known as Composite Phase Change Material (CPCM). This section also has a brief introduction to CPCMs and thermal conductivity enhancement methods of PCMs with various non-integrated and integrated storage systems. In general, the qualities of a good energy storage system include: (1) high heat storage density, (2) temperature stability during storage, (3) high energy efficiency during storage, (4) low system cost, and good reliability. To select the suitable method and design of storage unit for a particular application in SAHs following points must be considered:

1. Operating temperature range

2. Storage capacity

3. Minimum heat losses associated with storage system

4. The economic aspect of the storage system

Both energy conversion and energy storage are crucial in today's world. Energy storage is essential for both energy conservation and the improvement of energy conversion systems’ reliability and efficiency. For a solar air heating system to provide constant output energy storage is essential. By establishing an effective energy storage system it is possible to lessen the mismatch between energy demand and supply (Chamoli et al. 2012). In solar air heating systems, thermal energy storage may extend the supply of hot air. SAHs with heat storage units have a higher efficiency than conventional heaters even though thermal storage media have few disadvantages. The kind of energy storage medium required for a specific application is determined by operational characteristics such as mass flow rate, the beginning temperature of process air, and output air temperature (Farid et al. 2004). A rock bed is typically employed in air-based systems, whereas liquid-based systems use water as a heat storage medium. Various thermal energy storage (TES) used in air heating systems are briefly explained in this section. The main classification of TES systems used in SAHs is as follows:

5.1. Non-integrated thermal energy storage systems

The storage unit is provided external to the collector chamber and pipes are connected to complete the circuit for heat storage this kind of storage system is known as a non-integrated storage system. Heat storage materials are chosen by evaluating their thermophysical qualities concerning the desired operating temperature. Solar air heaters with heat storage units were described in detail (Saxena and Goel 2013). The design and construction of SAH have been done and tested to heat the different gasses. Bottom-extending longitudinal fins and heat-storage materials were used to improve heat transmission in DPSAH and the maximum thermal efficiency obtained was about 69% for this heating system (Pramanik et al. 2017). Built a solar dryer with a separate heat storage system utilising phase change materials (PCMs). Energy and economic study of sensible and latent heat storage materials in solar heating and cooling systems have been done. A heat storage tank was linked with water to the solar field to store the energy and in the second configuration, the water tank is loaded with PCMs (Noro, Lazzarin, and Busato 2014). Energy loss throughout travel is the principal disadvantage of solar air heaters with external heat storage units. To compensate for these losses, the system must be well-insulated along its full route length which increases the unit's original cost, maintenance costs, and total size of the system (Alkilani et al. 2011).

5.2. Integrated thermal energy storage systems

The integrated storage unit solves the various issues encountered with non-integrated storage systems such as heat loss during transport, low storage efficiency, excessive price, etc. SAHs integrated with TES used to keep the temperature of the exhaust air higher for a longer period when solar radiation is intermittent. Heat storage systems using both desert and granular carbon under free and forced convection are compared with the SAHs without a storage unit (Saxena, Srivastava, and Tirth 2015). A packed bed LHS (capsules carrying storage material) method was used to create a TES-integrated solar air heating system. This SAH offers 45% daily efficiency and the temperature of PCM maintain at 17°C during discharge time (Bouadila et al. 2013). An experimental investigation of a solar air heating system with and without an incorporated heat storage unit has been done. The temperature of the exhaust air is heavily influenced by operational factors such as the mass flow rate of air and the space between heat storage materials (Aboul-Enein et al. 2000). The packed bed latent heat storage unit is made up of spherical PCM capsules that were tested in an experiment (Bouadila et al. 2014). Variations in mass flow and global solar radiation periods were studied that how the heat storage unit charged and discharged. Solar air drier with TES technology has been developed for drying copra (Mohanraj and Chandrasekar 2008). A SAH with and without a TES system was examined and compared with each other. It is found that the SAH with TES was more efficient and capable to maintain the output temperature of air higher for a longer period. The LHS system consists of paraffin wax as PCM gives the best results when the storage medium is placed beneath the absorber plate. SAH with porous media absorbers was found 30% to 35% more efficient as compared the SAH without porous media (Ramani, Gupta, and Kumar 2010). An experimental study on a SAH with a single and double-pass configuration with an integrated steel wire mesh absorber panel shows that the effectiveness of the double-pass system with a packed bed has increased from 7% to 19.4% as compared to the single-pass setup (Omojaro and Aldabbagh 2010). Thermal storage integrated solar air heater that used paraffin wax with aluminium powder as a PCM shows that storage material provides a longer freezing time roughly 8 h (Alkilani et al. 2009). An active V-Trough flat plate SAH with a heat storage unit has been investigated experimentally. The experimental results revealed that when paraffin wax serves as the PCM the storage unit offers 10% better thermal efficiency compared to SAH without a storage unit (Eswaramoorthy 2016).

5.2.1. Packed bed type storage system

In this type of storage system, heating or cooling takes place when air is flowing through the bed. Liquid media storage is restricted by freezing and vaporising problems at low and high temperatures respectively. But solid media storage has no such restrictions. Loosely packed solid materials such as rock, sand, and concrete pebbles and metals form the bed of a packed bed storage unit. In comparison to open sun drying crop drying took less time using the SAHs with a pebble bed storage unit. According to the research, the standard capacity for a solar collector using pebbles was found about 0.25m³ volume per unit area (Saxena, Tirth, and Srivastava 2014). A solar air heater with rock bed storage was examined to optimise the operating parameters and design. The cross-sectional area, rock bed size, airflow rate, and void percentage were all considered for this optimisation study. For a rock bed thermal storage system with DPSAH, the optimal charging time was found about eight hours (Choudhury, Chauhan, and Garg 1995). Solar air heaters with a rock bed storage unit may lower the same amount of moisture content in the air in only two sunny days and one night in between (Chauhan, Choudhury, and Garg 1996).

5.2.2. Phase change material (PCMs) based storage systems

Phase change materials (PCMs) can store or release a significant amount of heat as they transition from solidification to gasification or liquefaction. Because the phase-transition enthalpy of PCMs is typically substantially higher and has higher storage density than sensible heat storage. The temperature variation of PCMs during phase transition varies from 100⁰ C to 897⁰ C, while their latent heat capacities vary from 124 kJ/kg to 560 kJ/kg. In PCMs the storage of heat occurs without significant temperature changes in the storage medium. Due to this reason latent heat storage is therefore perfect for applications that require exact operating temperatures. Different varieties of paraffin wax are extensively utilised in solar latent heat storage devices. When comparing the amount of heat storage material needed for water storage and SHS the size and cost of solar systems utilising water as a thermal storage medium would rise (Alva et al. 2017). Leakage of PCMs, pressure control, compatibility, and increased cost are the drawbacks of employing PCMs as thermal storage. Low thermal conductivity (about 0.2–0.7 W/mK) is also the major problem associated with PCM-based storage systems. To enhance the thermal conductivity of PCMs there is a need for the employment of relative heat transfer enhancement techniques (Zhao, Lu, and Tian 2010). Equation (26) provide the latent thermal storage system's capacity (Lane 1983; Sharma et al. 2009). (26) $Q=\underset{T_i}{\overset{T_m}{\int }}m{C}_{p}dT+m{b}_m\mathrm{\Delta }{h}_m+\underset{T_m}{\overset{T_f}{\int }}m{C}_{p}dT$(26) Latent heat is used to store heat in the phase change heat storage system and is released when the substance freezes. Solar space heating and crop drying are only two examples of PCMs that have had their usefulness for solar energy applications that have been studied. The melting point, latent heat of fusion, volume change during phase change, and the quantity of super-cooling or super-heating of the phase shift all play a key influence in the selection of these materials. Phase transition materials for LHS were extensively studied such as paraffin, nonparaffin, fatty acids, salt hydrates, and organic, and inorganic eutectics, etc.(Sharma et al. 2009). Experimental observation revealed that the source temperature is about 26°C higher as compared to the transition temperature and offers more effective charging and discharging (Zhou et al. 2015). Aluminium powder and graphite-PCM as composite materials are employed to enhance the thermal conductivity of PCMs (Marín et al. 2005). For off-sunshine hours the PCM-based storage with a V-corrugated heater offers higher output air temperature according to the findings (Kabeel et al. 2016). The PCMs can retain roughly 0.2 kJ/g of thermal energy at temperatures lower than the crystallisation point for about 10 h (Han, Li, and Grossman 2017). The design of latent heat storage integrated single pass and double pass SAHs are shown in Figure 6(a,b) respectively. Details of some sensible heat storage integrated SAHs are listed in Table 8.

Figure 6. Latent heat storage integrated solar air heater. (a) Single Pass SAH (Hassan 1995). (b) Double Pass SAH (Morrison and Abdel-Khalik 1978).

Table 8. Details of latent heat storage integrated SAHs.

Type of SAHs	Configurations for energy storage	Key findings
Single Pass (Hassan 1995)	Absorber above the copper tube filled with TES materials (sensible/latent), Figure 5(a)	The daily efficiency obtained about 27–63% for 430°C and 510°C melting temperatures of PCM
Single Pass (Bouadila et al. 2013)	LHS with spherical capsules	Average thermal efficiency reached up to 45%, and 170°C temperature during discharge
Single Pass (Aboul-Enein et al. 2000)	Angled SAH with and without a storage unit	Rate of mass flow and space between storing materials influence the output temperature
Single Pass (Bouadila et al. 2014)	Packed bed storage with PCM-filled spherical shells	Energy efficiency = 45% and exergy efficiency = 25%
Double Pass (Alkilani et al. 2011)	Paraffin wax as PCM	Maximum freezing time of about 8 h achieved at 0.05 kg/s
Double Pass (Sivarathinamoorthy and Sureshkannan 2021)	(a) Without fins and storage, (b) with fins and without storage, and (c) with fins and storage unit	Maximum radiation reached about 849 W/m^2, and output temperature inversely depends on the flow rate for all three configurations
Double Pass (Shrivastava, Yadav, and Shrivastava 2021)	Proposed a new design to maximise heat gain	Thermal efficiency improved from 49% to 61% during maximum radiation and reduced from 35% to 29% with an increase in flow rate
Double Pass (Morrison and Abdel-Khalik 1978)	Simulation with sodium sulfate and paraffin wax as PCMs, Figure 5(b)	Achieved suitable size for absorber and storage unit
Double Pass (Shrivastava, Yadav, and Shrivastava 2021)	Paraffin wax with spongy foam of Al	The phase-changing temperature reached from 150°C to 290°C. Work continuously for 1.5 h to 2.5 h after the sunset

5.2.2.1 Cross-matrix absorber SAHs with and without PCMs

The absorber is the main component of any solar air heating device and affects the thermal performance of SAHs. Various design modification of absorber for SAHs has been discussed in this review article. A newly designed absorber called matrix thermal absorber offers the highest thermal efficiency over simple SAHs. The cross-matrix absorber integrated with PCMs may be the new progress to design a SAH with higher thermal performance. The thermal performance of these types of absorbers depends upon porosity, size of the matrix, area density, and thermophysical characteristics of matrix materials (Razak et al. 2016). The simple design of thermal matrix absorber integrated single pass and double pass SAHs are shown in Figure 7(a and b) respectively and various types of thermal matrix absorbers with their material type are listed in Table 9.

Figure 7. Cross matrix-absorber SAH without PCMs. (a) SPSAH with matrix-absorber. (b) DPSAH with matrix-absorber (Ramani, Gupta, and Kumar 2010).

Table 9. List of various matrix absorber designs with matrix material.

Design of matrix – absorber	Matrix materials	Ref
Parallel sheet type	Copper galvanised and wire cloth	Kolb, Winter, and Viskanta 1999
Wire type	Steel	Omojaro and Aldabbagh 2010; Nowzari, Aldabbagh, and Egelioglu 2014
Fins integrated wire matrix	Steel	El-khawajah, Aldabbagh, and Egelioglu 2011
Cross-type and square-shaped hollow strip	Aluminium	Azmi et al. 2013
di-metallic type	Stainless steel with Aluminium	Razak et al. 2015
Composite type square shaped hollow strip	Wooden strip with mild steel	Madhlopa, Jones, and Kalenga Saka 2002
Wire type	Low carbon steel	Velmurugan and Ramesh 2011
Wire type with frictionless plate	GI sheet	Lalji, Sarviya, and Bhagoria 2006; Lalji, Sarviya, and Bhagoria 2011

Based on previous research the matrix absorber was found best suited, so to further improvement in the thermal efficiency of DPSAH using cross-matrix absorbers with and without PCMs was investigated. The efficiency of DPSAH for the present matrix absorber with PCM improved by 17% as compared to without PCM and the exergy efficiency obtained about 23% and 15% with wand without PCMs respectively (Sharol et al. 2022). Figure 8(a,b) represent the single-pass and double-pass SAH with cross-matrix absorber respectively and Table 10 represented the various design of PCM-based storage systems integrated with DPSAH.

Figure 8. Design of DPSAH using cross-matrix absorber with PCM (Sharol et al. 2022).

Table 10. Results obtained from DPSAH integrated PCMs with various designs of the storage unit.

Storage methods	PCMs (melting point)	Key findings
Cylindrical tube with fins (Sajawal et al. 2019)	RT18HC and RT44HC (17–19°C and 41–44°C)	To = 65°C and ${\eta }_{\mathrm{t}\mathrm{h}}$ = 71%
Encapsulated sphere (Sudhakar and Cheralathan 2021)	PW (68°C)	Maximum efficiency achieved at an inclined position
Capsules in the rectangular form (Salih, Jalil, and Najim 2020)	PW-RT43 (43°C)	At 0.01 kg/s flow rate, optimum efficiency is achieved about 63.53%
Tube of cylindrical shape (Kalash, Shijer, and Habeeb 2020)	PW (58°C)	Thermal conductivity improved by plunging the Al tube in PCM
Rectangular box (absorber with cylindrical baffles) (Ghiami and Ghiami 2018)	PW (52–60°C)	To = 48°C, 26.78% energy and 20.47% exergy efficiency was obtained.
Cylindrical and rectangular-shaped capsule (Raj, Srinivas, and Jayaraj 2019)	PW (58–60°C)	Maximum efficiency attained up to 53% and 54.3% for rectangular and cylindrical design
Box with rectangular shape (Baig and Ali 2019)	PW (15–29°C)	${\eta }_{\mathrm{t}\mathrm{h}}$ = 97% (optimum value)
Honeycomb type (Abuşka, Şevik, and Kayapunar 2019)	RT54HC (54°C)	Charging/discharging improved by 45 min with ${\eta }_{\mathrm{t}\mathrm{h}}$ = 31.6% (optimum value)

5.2.2.2 Enhancement in the thermal conductivity of PCMs

PCMs are used in a wide range of fields due to their versatility as either endothermic or exothermic at the constant phase transition temperature. After reviewing different heat storage methods integrated with solar air heating systems it was obtained that Latent Heat Thermal Energy Storage (LHTES) has more benefits than Sensible Heat Thermal Energy Storage (SHTES) and is employed in several applications. The fact that PCMs like paraffin and fatty acids have such a low heat conductivity is one of the main disadvantages of LHTES. The low thermal conductivity of PCMs reduces the rate of heat transmission through them. To increase the rate of heat, transfer the heat storage material's conductivity must be increased along with the surface area used for heat transmission. To boost the thermal conductivity of PCMs mainly carbon and metal-based materials are used. PCMs made of carbon and metals have more favourable thermal properties and future application potential. Fin with varying shapes was used at the interface of the storage unit to enhance heat transport as shown in Figure 9 (Mahdi, Lohrasbi, and Nsofor 2019). PCM-integrated storage units offer the highest solar thermal efficiency. Many researchers are focusing on integrated solar power generation and thermal storage units, to minimise the required space, cost, and thermal losses and maximise the ability to store the heat for further use in intermittent solar radiation.

Figure 9. Various types of fins used to enhance the rate of heat transfer in LHTES systems (Mahdi, Lohrasbi, and Nsofor 2019). (a) Longitudinal fin. (b) Pin fin. (c) Spiral fin.

This section mainly focused on metal-based materials to enhance the thermal conductivity of PCMs. Newly obtained materials with higher conductivity are known as composite phase change materials (CPCMs). Many efforts have been made to increase the thermal conductivity of PCMs by using additives that already have high thermal conductivity. Substances based on carbon and those based on metals make up the two main categories of additives and details are given in Figure 10. Expanded graphite, carbon fibre, carbon nanotubes, and graphene are all examples of carbon-based materials used in PCMs to improve their thermal conductivity. The electric and thermal properties of metals are very good. Adding metal foam, nanoparticles of metals, metal matrix, and metal oxides and their nanoparticles to PCMs is one of the simple ways to increase their thermal conductivity (Xu, Zhang, and Fang 2022). This method is simple and economic to prepare CPCMs for low and medium temperature ranges. The CPCM was prepared from paraffin using copper foam with a mass fraction of 65 wt.% and 40 PPI density of pore and found the increased thermal conductivity of CPCMs about 3 W/mk instead of 0.3 W/mk (Zheng et al. 2020). The ice crystal template method was used to prepare graphene aerogel and poured in paraffin with Ag particles to enhance the thermal conductivity of paraffin (Zhang et al. 2020). By adding the 50 wt.% of Al₂O₃ in paraffin and maximum thermal conductivity was obtained at about 1.27W/mK (Zhao et al. 2021). Increase in thermal conductivity of paraffin as PCMs by adding metal-based materials listed in Table 11.

Figure 10. Methods to enhance the thermal conductivity of PCMs (Xu, Zhang, and Fang 2022).

Table 11. Increase in thermal conductivity of paraffin as PCM by adding metal-based materials.

PCMs	Additive metals	Thermal conductivity of PCMs (W/mK)	Thermal conductivity of CPCMs (W/mK)	Increment	Ref.
Paraffin	Copper foam	0.3	3	10 times (900%)	Zheng et al. 2020
Paraffin	Ag-particle	0.26	1.2	4.6 times (361.5%)	Zhang et al. 2020
Paraffin	Al-powder	0.254	1.27	5 times (400%)	Zhao et al. 2021
Paraffin	Charcoal with nanowires of (SiC)	0.2	0.78	3.9 times (290%)	Chen et al. 2022

6. Conclusion and future suggestions

A lot of research work was done to resolve the major drawbacks which is the low thermal efficiency of SAHs. Various heat transfer augmentation techniques such as modification in the path of flowing air, absorber design modification (roughness), and integrated absorber with a heat storage unit (PCMs-based) have been discussed for a solar air heating system in the present study. Based on the study some conclusions are obtained as follows:

1. The design of the airflow passages provided in SAHs has a strong influence on its performance therefore the shape of the passage must be selected properly. In contrast to SPSAHs, the efficiency of the DPSAH is higher. The counterflow DPSAH with recycling is best suited for higher performance. The thermal and Thermo-hydraulic efficiency increased from 12% to 15% and 2.45–3.26 times respectively with increasing the roughness of the absorber plate. The thermo-hydraulic efficiency of DPSAH enhances up to 50% over conventional type heaters using baffles as compared to the other HT-enhancing techniques.

2. Surface modification is the oldest technique to enhance the heat transfer rate. As per the previous study pith ratio (p/e), height ratio (e/Dh), and geometry of roughness are the effective parameters to enhance the performance of SAHs. The spherical and inclined rib protrusions offer the best thermohydraulic performance characteristics within the range of Re about 15,000. For higher Re, multiple V-shaped geometries with spacing provide higher thermohydraulic efficiency. Fin's height and shape play an important role in enhancing the rate of heat transfer. Out of many used fins the longitudinal fins for $\dot{m}$ > 0.06 kg/s and wavy fins for $\dot{m}$ < 0.05 kg/s are best suited for the better thermo-hydraulic performance of a solar air heating device. Corrugated herringbone type and wavy fins offered maximum thermal efficiency of about 72.8% and 75% out of all other types of fins. HT rate is greatly increased when turbulators are scattered rather than linear fashion. Delta-winglet improves HT without a minimum increase in friction factor. Various SAHs show a range of their energy and exergy efficiency of about 2.05%–82% and 0.01%–60.97% respectively with artificial roughness.

3. Energy storage techniques not only improve efficiency but also improves the reliability of the system. SAHs having separate energy storage unit offers low performance as compared to the SAHs having integrated thermal storage units because of heat loss during transportation of the storage medium. Input air temperature and mass flow rate of air directly influence the charge and discharge time. PCMs with high latent heat storage ability offer higher thermal efficiency. PCMs integrated storage systems used in air/water heating, greenhouse, space heating/cooling, RAC, solar cooking, etc. An integrated steel wire mesh type packed bed absorber enhanced the effectiveness of DPSAH from 7% to 19.4% as compared to SPSAH. Paraffin wax as PCM for the integrated LHS system offers 10% better thermal efficiency as compared to the system without thermal storage.

4. CPCMs with higher thermal conductivity can be achieved easily by adding high-conductive metal in various forms such as metal foam, metal powder, metal matrix, etc., with PCMs. Nano-enhanced PCMs are another possibility for increased thermal conductivity of PCMs.

The use of various heat transfer enhancement approaches leads to an increase in the requirement of high pumping power which lowers the output performance of a solar air heating system. The requirement of the large pumping power is not beneficial in turbulent flow regimes at a high Reynolds number. The percentage of porosity is the only key factor in augmenting the overall performance of a porous bed-type solar air heater. In the future there are many avenues to explore in the research of DPSAHs as suggested below:

1. It is possible to use varied roughness on both sides of the absorber plate and heat storage unit to analyse the performance of DPSAHs. The use of perforated fins and barriers in the path of flowing fluid is the future work for DPSAHs. To improve the efficiency of SAHs the round baffles would be the best choice and exergy analysis can be performed as well for future study.

2. The combination of SHS and LHS with artificial roughness may be the best research method to improve the thermal efficiency of DPSAHs which need more exposure. SHS materials like sodium chloride, graphite, lead, copper, or their mixtures can be evaluated for use in solar air heaters, as can a wide variety of other materials including sand, pebble stone, gravel, granite, or oils that do not react with metals.

3. More research is necessary to fully understand how CPCMs adjust to their surroundings. It is important to study the properties of CPCMs after long use. The thermal conductivity of PCMs at high-temperature should be investigated in addition to the low temperature. Future research needs to focus on expanding the potential of PCMs for various applications.

4. Future research is also directed to the dual application (air and water heating simultaneously) of SAHs which may be more economical as compared to any individual heating/cooling system. The combination of two or more techniques such as double pass with recycle, absorber modification and thermal storage, etc., may also be used simultaneously for heat transfer enhancement this is known as the hybrid technique. To evaluate the impact of hybrid techniques on the thermal performance characteristics it is required to examine the two or more techniques together in a single work.

Disclosure statement

No potential conflict of interest was reported by the author(s).