Analytical and experimental investigations of recyclic double pass photovoltaic thermal system

ABSTRACT

Hybrid photovoltaic-thermal (PV/T) system consisting of a double-pass solar air heater under external recycling has been proposed for the combined generation of electrical and thermal power from the same area. One of the main concerns with PV modules is their poor efficiency due to high temperatures caused by extreme sun radiation. One of the most prevalent approaches for reducing the temperature of PV modules and improving performance is to use hybrid photovoltaic thermal (PV/T) systems. The heat generated by the PV panel is conveyed via working fluids such as air which is further utilized for other applications such as space heating. The present research explored the theoretical and experimental investigation of the proposed system for its thermal, electrical, and combined efficiency. The investigated double pass photovoltaic thermal system (PV/T) under recycling operation mainly consists of a glass cover, a PV module that is located below the glass, a back plate, and insulating material that is placed below the back plate. The theoretical model describes heat transfer characteristics and the divergence in the temperatures of the anticipated photovoltaic thermal system. Furthermore, under identical geometric and operational flow conditions, analytical results are compared with experimental results and showed that good agreement is achieved in the acceptable range. The performances of the proposed system that includes the thermal, electrical, and combined efficiency was measured for the mass flow rate ranging from 0.03 to 0.15 kg/sec, recycle ratio 0.3–1.8, irradiation 300–1000 W/m², packing factors 0.4–0.95, and varying channel depth ratio ranging from 1.0 to 6.0. The maximum growth in net electrical power of the current design is 43.32% more than the previous design with external recycling at a mass flow rate of 0.15 kg/sec, recycle ratio of 0.9, and at depth ratio of 3.

KEYWORDS:

• Double pass solar air heater
• photovoltaic thermal system
• recycle ratio
• depth ratio
• packing factor

Disclosure statement

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

Future scope and recommendation

The primary study included in this research is the influence of recycling on the performance of a photovoltaic thermal system. However, techno-economic evaluations are not included in the investigation and can be performed in future research. Further, the CFD investigation of the proposed recycling type PV/T system for different airflow patterns needs to be carried out to see the velocity and temperature profile diagram.

Nomenclature

${\mathrm{A}}_{\mathrm{c}}$ Photovoltaic module area (m²)

${\mathrm{C}}_{\mathrm{P}}$ Fluid’s specific heat (kJ/kg.K)

d₁ Duct’s depth among glass and collector plate (m)

d₂ Duct’s depth among the back and collector plate (m)

${\mathrm{d}}_{\mathrm{h}1}$ Upper duct’s characteristic length (m)

${\mathrm{d}}_{\mathrm{h}2}$ Lower duct’s characteristic length (m)

${\mathrm{h}}_{\mathrm{c}}$ Convection heat transfer coefficient (W/m²K)

D Depth ratio

${\mathrm{h}}_{\mathrm{r}}$ Radiation heat transfer coefficient (W/m²K)

I Solar radiation which is incident on PV module (W/m²)

${\mathrm{k}}_{\mathrm{f}}$ The conductivity of the fluid (W/m.K)

L PV module’s span (m)

$\dot{m}$ Mass flow rate of air (kg/s)

P Packing factor

P_E Electrical power (W)

${\mathrm{T}}_{\mathrm{P}{\mathrm{V}}_{\mathrm{m}}}$ PV module’s mean temperature (^oC)

$\mathrm{W}$ Photovoltaic collector plate’s width(m)

Greek Letters

$\alpha$ Absorptivity

$\tau$ Transmissivity

$\epsilon$ Emissivity

${ }_{th}$ Thermal efficiency

${ }_{PV/T}$ Photovoltaic thermal efficiency

${ }_{op}$ Nominal efficiency

$\sigma$ Stefan’s Boltzmann constant

Subscript

$a$ Ambient air

$bp$ Backplate

$bp{f}_2$ Among working fluid at a lower duct and backplate

$f_1$ Airflow at the upper channel

$f_2$ Airflow at the lower channel

$g$ Upper glass

$gs$ Among sky and glass

$gw$ Among wind and upper thin glass

$g{f}_1$ Fluid at upper duct and glass

$gpv$ Among collector plate and thin upper glass

$pv$ Photovoltaic cells

$pv{f}_1$ Among fluid at PV module and upper duct

$p{f}_2$ Among fluid at PV module and lower duct

pvbp Among back and collector plate

s Sky