Performance evaluation of solar photovoltaic electricity-generating systems: an Indian perspective

ABSTRACT

The present work seeks to assess the sustainability of different solar photovoltaic (SPV) electricity-generating systems based upon energy, environment and economics. The sustainability indicators evaluated for energy, environment and economics are electrical output, life-cycle greenhouse gas (GHG) emissions and life-cycle cost of electricity generated per kilowatt hour. The selected SPV-based electricity generation technologies for sustainability evaluation are amorphous, monocrystalline and polycrystalline at different locations and tilt angles across India. For SPV systems, most of the emissions are the result of electricity use during manufacturing. In these cases, an average grid mix for the region of manufacture is typically used to calculate energy use and emissions. Based upon these three indicators, a figure of merit (FM) has been proposed. The results proposed that polycrystalline gives the maximum electrical output, minimum GHG emission, minimum cost and maximum FM at a radiation level of 6 kWh/m²/day with latitude and tilt angle of 34° and 35°, respectively. This work will be helpful to users of solar energy, academicians, researchers and other concerned persons, in understanding the importance, severity and benefits obtained by the application and implementation of the SPV electricity-generating systems.

KEYWORDS:

• Greenhouse gas
• emission
• SPV
• figure of merit
• renewable energy

1. Introduction

Sustainable development is the development that meets the needs (energy, food, etc.) of the present without compromising the ability of future generations to meet their own needs. Sustainable energy systems can be defined as those systems that can provide energy services to the present generation while ensuring that similar levels of energy services can be provided for future generations. The present fossil-based electricity generation systems are not a sustainable source of electricity generation (MNRE 2009; Tiwari et al. 2017). Due to limited fossil fuel resources, these systems will not be able to deliver electricity (at affordable rates) in the future (Sims, Rogner, and Gregory 2003).

It is clear that the present fossil fuel-driven energy systems are unsustainable in nature due to the finite fossil fuel reserves and the environmental impacts associated with these systems (Ansari et al. 2013). It is, therefore, important to find a sustainable solution in the energy (electricity) sector and at the same time meet the increasing demand generated by the growing economy (Varun and Singal 2007). Renewable energy sources such as solar, biomass and small hydro can contribute to build up a more sustainable energy system (Muneer, Muhammad, and Saima 2005) in countries like India. Thereby, solar photovoltaic (SPV) electricity-generating systems can be used as a source of power generation which may be sustainable in terms of environment and economics (Luthra et al. 2015a). The sustainability indicators evaluated for energy, environment and economics are electrical output, life-cycle greenhouse gas (GHG) emissions and life-cycle cost of electricity generated per kilowatt hour.

India has the potential for renewable energy, as it has different climatic conditions (rainfall, humidity, temperature, etc.) across its parts (Luthra et al. 2015b). Thus, based on the different climatic conditions, the country may be divided into numerous climatic zones (Minke 1988). In this study, the different zones considered are hot and dry (Jodhpur), warm and humid (Mumbai), moderate (Bangalore), cold and cloudy (Ladakh) and composite (New Delhi).

The performance of SPV electricity-generating systems is highly influenced by climatic conditions and its angle of tilt with the horizontal. This is because of the fact that both the climatic condition and tilt angle change the solar energy reaching the surface of the solar system. Among the various kinds of SPV systems, amorphous, monocrystalline and polycrystalline are the commonly used systems (Stutenbaeumer and Mesfin 1999).

The objective of the present paper is to evaluate the figure of merit (FM) and based upon this FM, sustainability of SPV-based electricity generation technologies has been estimated. Analysis has been performed at different climatic zones of India at different tilt angles. Three different technologies of SPV systems were studied and compared with respect to their performance and suitability as per the different characteristics. There was a need to compare their performances with common parameters, so an FM was proposed and implemented.

This paper is organised as follows: the next section presents the sustainability indicators for SPV electricity-generating systems; thereafter, the FM has been developed and analysed for different SPV electricity-generating systems in Section 3. Section 4 analyses the comparative evaluation of SPV electricity-generating systems, while Section 5 proposes the optimal performance parameters for the said systems. Discussion of research findings with the concluding remarks is presented in Section 6, followed by the limitations of the study and future research directions.

2. Sustainability indicators for SPV technologies

Sustainability evaluation for three different SPV electricity generation systems based upon three different indicators has been carried out. These three indicators are electrical output, life-cycle GHG emissions and life-cycle cost of electricity generated per kilowatt hour. The SPV systems have better results for GHG emissions, which has been studied by Sherwani and Usmani (2010). The above said indicators are explained in the following subsections.

2.1. Electrical output

Electrical output from the plant is calculated for all five different locations, ϕ = 28° (New Delhi), 18° (Mumbai), 12° (Bangalore), 34° (Ladakh) and 26° (Jodhpur), four different tilt angles, β = 15°, 25°, 35° and 45°, and three different cells, amorphous, monocrystalline and polycrystalline. Three different values of Hg, i.e. 4, 5 and 6 kWh/m²/day, are considered. The efficiency of amorphous, monocrystalline and polycrystalline is taken as 0.07, 0.107 and 0.137 respectively, with temperature correction factor as 0.7 and inverter efficiency to be 0.9.

2.2 GHG emissions

Total life-cycle GHG emissions (g-CO_2eq) have been generally estimated according to the full operational life cycle of each system from the commissioning of the plant to full operation of the system (cradle to grave) (Sherwani and Usmani 2011). These emissions are found to vary widely within each technology. For the estimation of GHG emissions for the present study, life time of the projects is considered to be 30 years. Estimation of GHG emissions is given as

(1) $\begin{array}{rl}& GHGemissions=\\ & \frac{TotalC{O}_{2}emissionsthroughoutitslifecycle(g-C{O}_{2eq})}{Annualpowergeneration(kW{h}_e/year)\times lifetime(year)}\end{array}$(1)

2.3 Life-cycle cost of electricity generation

Considering the life-cycle approach for estimating the cost of electricity generation through SPV system, we have considered different parts of the life cycle. These parts include cost, installation, operations and maintenance and disposal (Asjad, Kulkarni, and Gandhi 2013, 2014). This is necessary when we are considering a holistic approach. Varun, Prakash, and Bhat (2010) have calculated different costs of electricity generation. Thus, we observe that there is a wide range of difference in the total  cost when the electricity through solar is generated.

It is clear that the SPV system has a wider range of cost for electricity generation. This is due to the different technologies of solar cell, location-specific variables, solar radiation intensity and electricity cost of manufacturing of cells. Thus, the cost of electricity generation (in Rs/kWh_e) may be estimated through the following equation:

(2) $\begin{array}{rl}& Costofelectricitygeneration=\\ & \frac{Annualisedexpensesofthesystem(Rs/year)}{Annualelectricitygenerationbythesystem(kW{h}_e/year)}\end{array}$(2)

The sustainable indicator has been proposed, which is a function of tilt angles, thereby, to study and analyse the different cell materials, radiation levels and locations, respectively.

3. Figure of merit

The comparison of different SPV systems when done through their performance is shown by the FM (Varun, Prakash, and Bhat 2010). This gives us the net energy requirement or gross carbon emission from the systems. Through this study, we have proposed the FM for SPV electricity-generating systems. This proposal is based on different sustainability indicators. These indicators are taken for a single platform and are given equal weightage. In his research, the sustainability indicators are electrical output, GHG emissions and cost per unit electricity generated with equal weightage of 0.33. When various technologies are considered they need to be ranked. In this research work, ranking is done from 1 to 10, where 1 denotes minimum and 10 denotes maximum value. Table 1 shows the different criteria for assigning relative ranks. This is based upon the three sustainability indicators. We have given higher rank for higher range of electrical output and further higher rank to lower value of GHG emissions and costs. The equation has been developed to estimate the FM and is given below.

(3) $\begin{array}{l}\text{FM}=\text{Relative}{\text{rank}}_{\text{electricaloutput}}\times \text{Relative}{\text{rank}}_{\text{GHG}\text{emisions}}\\ \times \text{Relative}{\text{rank}}_{\text{Cost}}\end{array}$(3)

Table 1. Criteria for assigning the relative rank.

S. No.	Range of electrical output (kWhe)	Relative rank for electrical output	Range of GHG emissions	Relative rank for GHG emissions	Range of cost	Relative rank for cost
1.	>2,25,000	10	0–25	10	0–5	10
2.	2,00000–2,25,000	9	25–50	9	5–7.5	9
3.	1,75,000–2,00000	8	50–75	8	7.5–10	8
4.	1,50,000–1,75,000	7	75–100	7	10–12.5	7
5.	1,25,000–1,50,000	6	100–125	6	12.5–15	6
6.	1,00000–1,25,000	5	125–150	5	15–17.5	5
7.	75,000–100,000	4	150–175	4	17.5–20	4
8.	50,000–75,000	3	175–200	3	20–25	3
9.	25,000–50,000	2	200–250	2	25–30	2
10.	0–25,000	1	> 250	1	>30	1

It may be noted that for calculating FM for different cell technologies, i.e. amorphous, monocrystalline and polycrystalline, has been taken into account. These indicators include location, tilt angle, radiation, type of cell technology and electrical output.

We have taken readings for five different locations in India (ϕ = 28°, 18°, 12°, 34° and 26°) and for a given location, four different tilt angles β = 15°, 25°, 35° and 45° have been considered. For electrical output, global radiation Hg is taken as 4, 5, and 6 kWh/m²/day. The average electricity generation efficiency is taken as 0.40 and corresponding to the electrical output a relative rank is assigned. GHG emissions in kg-CO₂/kWh_e are calculated for emissions of energy mix per kilowatt hour taken to be 600. Relative rank for life-cycle cost of standalone system is assigned for one autonomy day (AD), battery life (BL) of six years and interest rate to be 5%.

Based upon the proposed FM, three SPV electricity generation systems (amorphous, monocrystalline and polycrystalline) have been analysed for five different locations and four different tilt angles. The criteria for assigning the relative rank are given in Table 1. The relative rank for maximum electrical output has been assigned 10 and the minimum electrical output has been assigned relative rank 1. The relative rank for GHG emissions is assigned as 10 for minimum GHG emissions and 1 for the maximum GHG emissions, because the lower the value of emissions the more it is desirable. Similarly, the relative rank is assigned 10 for minimum cost and 1 for maximum cost as lower cost is desirable.

While corresponding to every location that is at different latitudes, the electrical output varies with the varying radiation level. The FM is assigned to the parameters electrical output, GHG emissions and cost; it is not assigned to a place.

Based on the above discussion, an FM has been estimated for one city each from different climatic zones in Indian perspective (New Delhi, Mumbai, Bangalore, Ladakh and Jodhpur) (Sukhatme and Sukhatme 1996).

From the above estimation, the range of values for FM for different locations is summarised and illustrated in Table 2.

Table 2. Range of figure of merit for a radiation level of 4–6 kWh/m²/day.

S. No.	Location	Cell type	Figure of merit
1	Delhi (ϕ = 28°)	Amorphous	126–256
		Monocrystalline	120–336
		Polycrystalline	210–512
2	Mumbai (ϕ = 18°)	Amorphous	126–224
		Monocrystalline	120–336
		Polycrystalline	210–448
3	Bangalore (ϕ = 12°)	Amorphous	60–224
		Monocrystalline	90–280
		Polycrystalline	144–448
4	Ladakh (ϕ = 34°)	Amorphous	126–256
		Monocrystalline	120–336
		Polycrystalline	210–512
5	Jodhpur (ϕ = 26°)	Amorphous	84–224
		Monocrystalline	120–336
		Polycrystalline	210–448

For Delhi (ϕ = 28°), amorphous with radiation level in the range of 4–6 kWh/m²/day, the FM varies from 126 to 256. For monocrystalline cell, the FM varies from 120 to 336 and for polycrystalline it varies from 210 to 512.

For Mumbai (ϕ = 18°), amorphous with radiation level in the range of 4–6 kWh/m²/day, the FM varies from 126 to 224. For monocrystalline cell, the FM varies from 120 to 336 and for polycrystalline it varies from 210 to 448.

For Bangalore (ϕ = 12°), amorphous with radiation level in the range of 4–6 kWh/m²/day, the FM varies from 60 to 224. For monocrystalline cell, the FM varies from 90 to 280 and for polycrystalline it varies from 144 to 448.

For Ladakh (ϕ = 34°), amorphous with radiation level in the range of 4–6 kWh/m²/day, the FM varies from 126 to 256. For monocrystalline cell, the FM varies from 120 to 336 and for polycrystalline it varies from 210 to 512.

For Jodhpur (ϕ = 26°), amorphous with radiation level in the range of 4–6 kWh/m²/day, the FM varies from 84 to 224. For monocrystalline cell, the FM varies from 120 to 336 and for polycrystalline it varies from 210 to 448.

Table 2 shows the estimated values of FM. For every location, an FM is defined corresponding to four different tilt angles, three different radiations and all the three technologies, namely amorphous, monocrystalline and polycrystalline, are considered.

The next section deals with comparative evaluation of performance and grading for different SPV electricity-generating systems.

4. Comparative evaluation of SPV electricity-generating systems

The proposed FM for three SPV electricity generation systems (amorphous, monocrystalline and polycrystalline) has been analysed for five different locations and four different tilt angles, and is illustrated in Table 3. The values in Table 3 present the maximum electrical output of different locations at optimum tilt angles. At the climatic zone of Bangalore (i.e. ϕ = 12°) and polycrystalline cell, for the maximum electrical output along with minimum GHG emission, the optimum tilt angle is 15° at which the cost of generation is Rs 7.54/kWh_e and the FM is 448.

Table 3. Performance evaluation and grading of solar photovoltaic electricity-generating systems.

Radiation level (kWh/m^2/day)	Cell	ϕ (°)	β (°)	Electrical output (kWh)	GHG (kg-CO2/kWhe)	Cost (Rs/kWhe)	FM
6	Polycrystalline	12	15	159,257	0.0666	7.54	448
		18	25	164,266	0.0657	7.54	448
		26	35	175,561	0.0641	7.54	448
		28	35	179,671	0.0634	7.54	512
		34	35	185,386	0.0627	7.54	512
	Monocrystalline	12	15	124,383	0.0835	8.18	280
		18	25	128,266	0.0824	8.18	336
		26	35	137,117	0.0802	8.18	336
		28	35	140,327	0.0795	8.18	336
		34	35	144,791	0.0785	8.18	336
	Amorphous	12	15	81,372	0.0777	7.61	224
		18	25	83,931	0.0761	7.61	224
		26	35	89,702	0.073	7.61	224
		28	35	91,802	0.0716	7.61	256
		34	35	94,723	0.0701	7.61	256
5	Polycrystalline	12	15	132,513	0.08	9.06	336
		18	15	133,943	0.0806	9.24	336
		26	35	145,253	0.0774	9.11	336
		28	35	148,541	0.0767	9.12	336
		34	35	153,113	0.0759	9.13	392
	Monocrystalline	12	15	103,496	0.1003	9.83	240
		18	15	104,613	0.1011	10.03	210
		26	35	113,445	0.097	9.89	280
		28	35	116,013	0.0961	9.9	280
		34	35	119,584	0.0951	9.91	280
	Amorphous	12	15	67,707	0.0934	9.14	168
		18	25	69,692	0.0917	9.16	168
		26	35	74,216	0.1173	9.17	144
		28	35	75,876	0.0866	9.20	224
		34	35	78,233	0.0849	9.21	224
4	Polycrystalline	12	15	105,770	0.1003	11.35	210
		18	25	108,528	0.0995	11.41	245
		26	35	114,999	0.0978	11.51	245
		28	35	117,410	0.0971	11.54	245
		34	35	120,839	0.0962	11.56	245
	Monocrystalline	12	15	82,608	0.1257	12.32	140
		18	25	84,763	0.1248	12.38	168
		26	35	89,817	0.1225	12.49	168
		28	35	91,700	0.1216	12.52	144
		34	35	94,378	0.1205	12.55	144
	Amorphous	12	15	54,043	0.1171	11.46	126
		18	25	55,452	0.1152	11.52	126
		26	25	58,758	0.1481	11.58	105
		28	35	59,990	0.1096	11.65	126
		34	35	61,742	0.1076	11.67	126

Figure 1 shows the unit cost of electricity generation for different locations at a radiation level of 6 kWh/m²/day, while Figure 2 represents the electrical output for different locations at a radiation level of 6 kWh/m²/day and Figure 3 demonstrates the GHG emission for different locations at a radiation level of 6 kWh/m²/day. From these graphs, it may be concluded that cost of electricity generation and GHG emission are minimum for polycrystalline, whereas electrical output is also maximum in the case of polycrystalline cell. The same can be concluded for other cases of radiation levels (i.e. 4 and 5 kWh/m²/day) also.

Figure 1. Unit cost of electricity generation for different locations at 6 kWh/m²/day.

Figure 2. Electrical output for different locations at 6 kWh/m²/day.

Figure 3. GHG emission for different locations at 6 kWh/m²/day.

5. Optimal performance evaluation of SPV electricity-generating systems

The major performance parameters selected to evaluate the optimal performance of the SPV electricity-generating systems in terms of electricity output, GHG emission, cost and FM are cell material, radiation level, latitude and tilt angle. Tables 4, 5 and 6 reveal that for any cell material (i.e. polycrystalline, monocrystalline and amorphous) at any radiation level, the optimum latitude and tilt angle are 34° and 35°, respectively, that give maximum electrical output, minimum GHG emission, minimum cost and maximum FM.

Table 4. Performance parameters for polycrystalline cell under different radiation levels, latitudes and tilt angles.

S. No.	Radiation (kWh/m^2/day)	Latitude (°)	Tilt angle (°)	Electrical output (kWh)	GHG (kg-CO2/kWhe)	Cost (Rs/kWhe)	FM
1	6	34	35	185,386	0.0627	7.54	512
2	5	34	35	153,113	0.0759	9.13	392
3	4	34	35	120,839	0.0962	11.56	245

Table 5. Performance parameters for monocrystalline cell under different radiation levels, latitudes and tilt angles.

S. No.	Radiation (kWh/m^2/day)	Latitude (°)	Tilt angle (°)	Electrical output (kWh)	GHG (kg-CO2/kWhe)	Cost (Rs/kWhe)	FM
1	6	34	35	144,791	0.0785	8.18	336
2	5	34	35	119,584	0.0951	9.91	280
3	4	34	35	94,378	0.1205	12.55	144

Table 6. Performance parameters for amorphous cell under different radiation levels, latitudes and tilt angles.

S. No.	Radiation (kWh/m^2/day)	Latitude (°)	Tilt angle (°)	Electrical output (kWh)	GHG (kg-CO2/kWhe)	Cost (Rs/kWhe)	FM
1	6	34	35	94,723	0.0701	7.61	256
2	5	34	35	78,233	0.0849	9.21	224
3	4	34	35	61,742	0.1076	11.67	126

Table 7 states that for maximum electrical output, minimum GHG emission, minimum cost and maximum FM, the polycrystalline material outperformed at a radiation level of 6 kWh/m²/day with latitude and tilt angle of 34° and 35°, respectively.

Table 7. Optimum performance parameters for different radiation levels, cells, latitudes and tilt angles.

S. No.	Radiation (kWh/m^2/day)	Cell	Latitude (°)	Tilt angle (°)	Electrical output (kWh)	GHG (kg-CO2/kWhe)	Cost (Rs/kWhe)	FM
1	6	Polycrystalline	34	35	185,386	0.0627	7.54	512

6. Conclusion

A higher value of FM represents a better SPV electricity generation system and vice versa. This FM for better SPV electricity generation system can provide a more rational choice of electricity generation sources for energy planners, compared to the case where only one of the sustainability parameters is considered.

It may be concluded that polycrystalline gives the maximum electrical output, minimum GHG emission, minimum cost and maximum FM at a radiation level of 6 kWh/m²/day with latitude and tilt angle of 34° and 35°, respectively. Since the place where the current research has been carried out, i.e. India, is a nation having different climatic conditions and is broadly being classified into five different climatic zones, the present three technologies explored may not find equal applicability in all climatic zones. Therefore, the research work has been carried out to ascertain the utility of the specific technology corresponding to different climatic zones and is able to find the optimum solution.

7. Limitations of the study and future research directions

In this research, we have taken GHG emission as the only environmental parameter while there could be many more parameters, e.g. land use may also be included for more exhaustive evaluation of FM. However, GHG emissions were considered because they contribute to a very serious problem of global warming.

New technologies have been developed and with very high demand, the economy of state for solar cell production may reduce the cost of cell. Further interest rate of 5% is too small as subsidy is being provided and in its absence this interest rate may increase.

Besides FM, the other ways to identify the better PV cell may be probabilistic models, forecasting methods, physical phenomena of degradation, and so forth.

The study is limited to only five locations which may be carried out for other climatic zones and tilt angles for attaining the maximum benefits from the SPV electricity generation system.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Ahmad Faizan Sherwani

Ahmad Faizan Sherwani is currently an Associate Professor from the Department of Mechanical Engineering, Jamia Millia Islamia, New Delhi. He did his B.Tech(Mechanical Engineering from ZHCET, AMU, Aligarh. India in the year 2000 while subsequently to which he completed his M.Tech(Thermal Sciences from ZHCET, AMU, Aligarh, India in the year 2003.He has more than 15 years of teaching experience at various levels and different colleges affiliated to Uttar Pradesh Technical University, Lucknow, Sharda university and Delhi Technological university(formerly Delhi College of Engineering),New Delhi, India. His research area includes solar energy, thermal sciences, IC engine, etc. He has published several research articles at various national and international journals and conferences of repute.

Mohammad Asjad

Mohammad Asjad is currently an Assistant Professor from the Department of Mechanical Engineering, Jamia Millia Islamia, New Delhi. He graduated in Mechanical Engineering from the Z.H.C.E.T (AMU, Aligarh) and later obtained his Master’s in Industrial and Production Engineering from the Z.H.C.E.T (AMU, Aligarh) and securing first position in both of them. Subsequently, he completed his PhD from the Indian Institute ofTechnology Delhi (IITD) under the supervision of Professor O.P. Gandhi and Professor M.S. Kulkarni. He also receipts the scholarships during Master’s and research at A.M.U. and IIT Delhi, respectively. His research interests are strategic asset management, reliability engineering, maintenance management, manufacturing operations planning, life cycle costing, soft computing and applied artificial intelligence. He has published several research articles at various national and international journals and conferences of repute. He is also on the panel of the editorial board and reviewer for various reputed international journals and conferences.

Abid Haleem

Abid Haleem is a Professor from the Department of Mechanical Engineering, Jamia Millia Islamia, New Delhi, India. He received his PhD from the Indian Institute of Technology Delhi. He graduated in Mechanical Engineering from the Z.H.C.E.T (AMU, Aligarh) and later obtained his Master’s in Industrial and Production Engineering from the Z.H.C.E.T (AMU, Aligarh).  He has more than 31 years of teaching experience and has more than 100 publications to his credit in national/international journals and conferences. His special interest includes Educational Management, Supply chain management and Industrial Engineering. Their current project is 'Additive manufacturing, it's understanding through 3DScanning and Polyjet printing' and "Management of Halal Supply Chain '.