Energy payback time and life-cycle conversion efficiency of solar energy park in Indian conditions

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1 *Corresponding author: Energy payback time and life-cycle conversion efficiency of solar energy park in Indian conditions... Prabhakant and G.N. Tiwari * Center for Energy Studies, Indian Institute of Technology Delhi, Haus Khas, New Delhi , India... Abstract This paper presents an analysis of energy payback time and life-cycle conversion efficiency earning by solar energy park (SEP) at I.I.T Delhi, New Delhi, India. There are a number of hybrid photovoltaic thermal (PV/T) systems with stand-alone photovoltaic (SAPV) power supply in the SEP. The analysis is based on experimental and theoretical annual performance of each system. efficiency analysis considering the embodied energy of various systems in the park has been performed for life cycles of 30, 40 and 50 years. It is found that the cost of power generated by the SAPV system decreases with increase in the life cycle of the system. The energy payback time of the park is 2.52 years and years for thermal and electrical energies. The return on capital decreases marginally with increase in life cycle, however it increases marginally if embodied energy is taken into account. Keywords: Photovoltaic thermal hybrid solar systems; return on capital of solar systems Received: 27 March 2008; revised: 22 April 2009; accepted: 26 May INTRODUCTION India is potentially one of the largest markets for solar energy in the world. The estimated potential of power generation through solar photovoltaic system is about 20 MW/km 2 in India. The Government of India is planning to electrify villages by the year 2012 through renewable energy systems, especially by solar PV systems [1]. In India the cost of electricity generated by Solar PV cells comes to E0.122/KWh. Globally the capital cost of installing a Solar PV system comes to E /kWh [2]. Figure 1 shows that the cost of installation of solar PV system has been reduced substantially and is expected to reduce further by 50% in the next years [3]. Both the capital cost as well as the cost of electricity generated is likely to go down substantially with the economy of scale, advancement in technology and by considering carbon credits earned by such PV plants (as per Kyoto Protocol). Stand-alone photovoltaic (SAPV) systems are better-suited for Indian conditions. These systems do not require sophisticated grid synchronization equipments and systems. The electricity generated is directly used in running the electrical loads and balance electricity is stored in battery banks. These batteries along with the inverter (a device to convert direct current into AC) are used to run the electrical loads during night/off sunshine period. There are no moving parts, so the system s lifetime is very long and it is virtually maintenance free. The operating cost is negligible as there is no need for fuel; it is fully automatic and suitable for unmanned applications [4]. Carbon credit trading (emission trading) is an administrative approach used to control pollution by providing economic incentives for achieving reductions in the emissions of pollutants. In the year 2005, 375 million tons of carbon dioxide equivalent (tco 2 e) were transacted at a value of E3.31 billion with an average price of E In the first 3 months of 2006, average reported price of tco 2 ewase16.72/ton. European and Japanese Companies were the major buyers and China was the major seller of the carbon credits in [5]. While calculating value of carbon credit has been taken at E15/credit (1 carbon credit ¼ 1 ton of CO 2 equivalent emission). Evaluation of carbon credits earned by energy security in India has been studied by Prabhakant and Tiwari [6]. Prabhakant and Tiwari [7] have also computed carbon credits earned by the SAPV systems and have done return on capital analysis without taking into account the effect of embodied energy. In this paper the energy payback time and life-cycle cost analysis of solar SAPV system has been carried out by considering the effect of carbon credit earned and the embodied International Journal of Low-Carbon Technologies 2009, 4, # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org doi: /ijlct/ctp020 Advance Access Publication 6 July

2 Energy payback time and life-cycle conversion efficiency energy of the SAPV, life-cycle conversion efficiency analysis of the SEP has also been done. 2 PROBLEM IDENTIFICATION Carbon credits earned by a system are directly deductible from the running cost. Carbon credits earned by solar energy park (SEP) will mitigate the higher cost of electricity produced by photovoltaic systems. In this paper an attempt has been made on: (i) Variation of insolation across the country, improvement in life cycle, reduction in capital cost with improvement in technology has been considered while estimating the cost of electricity produced. (ii) The economic life of the SAPV system has been taken as 30, 40 and 50 years for calculating return on capital. (iii) Energy payback time and life-cycle conversion efficiency of SAPV system and the SEP have been evaluated. 3 SOLAR ENERGY PARK Solar Energy Park (SEP) is located in the campus of I.I.T. New Delhi ( N, E). It is spread over an area of 23 m 42 m. It has a built-up area (mud house) of 11 m 13 m. The classification of different systems installed in SEP is shown in Figure 2. The SEP has following systems: 3.1 Mud house Mud house is a six room building, having built with traditional building material, generally used to build houses in Indian villages. As mud forms 70% of the building material hence the name mud house. 3.2 Photovoltaic systems The stand-alone photovoltaic system (SAPV) consists of two arrays of PV modules. First array is of CEL make and has 32 modules; second array has 34 modules and is of SIEMENS make. SAPV modules are connected with a battery bank and inverter. SAPV supplies the entire electrical requirement of SEP. 3.3 Hybrid greenhouse dryer It is a hybrid photovoltaic integrated roof type even span greenhouse dryer. It is used for drying crop produce. The dryer can operate in forced as well as natural circulation mode. Figure 1. Capital cost of stand-alone photovoltaic systems over time [3]. Figure 2. Different systems installed in solar energy park. International Journal of Low-Carbon Technologies 2009, 4,

3 Prabhakant and G.N. Tiwari 3.4 Hybrid greenhouse dryer for cultivation The hybrid greenhouse has facilities of various heating and cooling technique such as roof vent, forced mode by fans, evaporative cooling and earth air heat exchanger. All the electrical gadgets/instruments are operated by electricity produced by PV modules integrated with it. 3.5 Passive solar still There are five single slope and three double slope solar stills in the SEP. Arrangement for rain water harvesting in double slope solar still can be made to enhance the annual yield. 3.6 Active solar still There is an active solar still having two flat plate collectors connected in series. The PV module is integrated at the bottom of the first collector to generate electricity to operate the water pump. The water pump circulates the hot water between the collectors and solar still. 3.7 Hybrid photovoltaic/thermal air heater There are two PV modules fitted to the wooden air duct. Air flows inside the duct by a DC fan. It is well known that the efficiency of the SAPV system reduces with an increase in ambient temperature. This arrangement takes care of increased ambient temperature. Efficiency of the PV modules is about 12%, which means about 88% of the solar insolation is lost to ambient conditions. By this arrangement we can extract thermal energy in addition to the electrical energy from SAPV. 3.8 Hybrid photovoltaic/thermal water heater It is a hybrid photovoltaic/thermal water heater with water storage capacity with a DC pump for forced mode. 3.9 Water pump Photovoltaic-operated submersible water pump of 0.35 kw capacity has been installed in the SEP to supply water round the clock. Detailed description of all the systems installed at SEP is given in Prabhakant and Tiwari [7]. 4 ASSUMPTIONS (i) Power output of the SAPV system varies with variation in solar intensity. Prakash et al. [8] have computed the efficiency of the solar PV modules on a typical day (11 September 2006) used in the SEP at 6.08% for CEL module and 12.52% for SIEMENS module. Although the efficiency of the cells will vary with variation in ambient temperature, inclination of PV modules, for the purpose of calculation it has been taken as the average efficiency of the PV modules installed in the SEP. (ii) Peak solar insolation in Delhi varies from 650 W/m 2 during November to December to 1450 W/m 2 during April to May [9]. In this paper, the average annual insolation at Delhi has been taken as 750 W/m 2 for 12 h/day. (iii) There are 300 clear days in a year in Delhi. (iv) Average value of carbon credit is E15. 5 NUMERICAL COMPUTATIONS The effect of solar intensity, number of clear days, carbon credit earned by PV/T and SAPV system on national level, return on capital cost and effect on initial cost have been described in Prabhakant and Tiwari [7]. 5.1 Life-cycle cost analysis Taking these values and the procedure given in Prabhakant and Tiwari [7] one gets: Power produced by the SAPV system per day ¼ 30 kwh Electrical energy produced (E) by the SAPV system per annum E ¼ ¼ 9000 kwh ¼ 9.0 MWh The electrical cost/kwh ¼ E0.149/kWh (for 30 years life cycle) CO 2 emission reduction by SAPV plant per annum ¼ E (@ E15/credit) CO 2 credit earned by SEP ¼ E ¼ E on the basis of energy ¼ E ¼ E37.5 on the basis of exergy (i) For life cycle ¼ 30 years The electrical cost/kwh ¼ E0.149/kWh (for 30 years life cycle) Return on capital ¼ 4.4% (ii) For life cycle ¼ 40 years The electricity cost/kwh from SAPV ¼ E0.134/kWh Return on capital ¼ ¼ 4.28% (iii) For life cycle ¼ 50 years The electricity cost/kwh from SAPV ¼ E/kWh Return on capital ¼ ¼ 4.20%. Variation in power produced, carbon credits earned and return on capital with variation in solar intensity and number of clear days in a year (by SAPV) has been computed as suggested by Prabhakant and Tiwari [6] and is given in Table Embodied energy Embodied energy analysis shows the total energy consumed during the manufacture of the product. It includes material production energy, the transportation energy, the solar cell/module fabrication energy, the human energy, installation energy, maintenance energy and finally the disposal/salvage energy. The detailed list of embodied energy is given in Table 2 and Figure Energy payback time Total embodied energy of the park (E ebd ) ¼ kwh (Table 2) Embodied energy of SAPV system ¼ kwh (Table 2) 184 International Journal of Low-Carbon Technologies 2009, 4,

4 Energy payback time and life-cycle conversion efficiency Table 1. Variation in power produced, carbon credits earned and return on capital with variation in solar intensity and number of clear days in a year (by SAPV) computation as suggested by [6]. S. No. Insolation (W/m 2 ) No of clear days in a year Power produced (MWh/annum) Carbon credits earned/annum (E) (life ¼ 30 years); capital cost ¼ E5432/kWp (life ¼ 40 years); capital cost ¼ E5432/kWp) (life ¼ 50 years); capital cost ¼ E5432/kWp) (life ¼ 30 years); capital cost ¼ E3500/ kwp Table 2. Embodied energy of different systems installed in Solar Energy Park. S. No Systems Embodied energy (kwh) 1 Mud house [7] 2 HPVTWC [10] 3 HPVTG [11] 4 HPVTGD [12] 5 ASD 4012 [13] 6 HPVTAC [14] 7 PSD [13] 8 SAPV [10] Total HPVTG, hybrid greenhouse dryer; HPVTGD, hybrid greenhouse dryer for cultivation; HPVTAC, hybrid photovoltaic/thermal air heater; HPVTWC, hybrid PV/T water heater; ASD, active solar still; PSD, passive solar still; SAPV, stand-alone photovoltaic system. Annualized embodied energy of the SAPV is given by: Annualized embodied energy ¼ Total embodied energy of the system Life of the plant in years ¼ 24435:2=30kWh/year ¼ 814:5kWh/year ðfor 30 years life cycleþ Energy payback time is defined as the number of years required to recover embodied energy which is given by Totalembodiedenergy of thesystem Energy payback time¼ Energy produced by the system per annum ¼ :89 ð2þ ðtable2þ ¼2:52 years Energy payback time for electrical energy ðincluding exergyþ Total embodied energy of the system ¼ Electrical energy produced by the system per annum ¼ : ¼17:21years ð1þ Figure 3. Embodied energy of the systems installed at solar energy park. This indicates that the invested energy (E ebd ) in the solar park is recovered within 2.52 years if thermal energy gain is considered, however energy payback time increases to years if only electrical energy is considered. 5.4 Effect of embodied energy on return on capital Net electrical energy produced (E NET ) can be defined as the electrical energy produced/year 2 annualized embodied energy of the SAPV system [7] E NET ¼ :5 ¼ 8185:5 kwh=yr Substituting this value for E NET in Equation (3), the return on capital (R C ) of the SAPV system can be evaluated as: 818:55 þ 125:85 R C ¼ ¼ 0:0405 ¼ 4:05% ðfor30years life cycleþ ð4þ ¼ 4:15% ðfor40years life cycleþ ¼ 4:2% ðfor50years life cycleþ 5.5 efficiency This is the net energy productivity of the system with respect to the solar energy (insolation) over the life time of the system ð3þ International Journal of Low-Carbon Technologies 2009, 4,

5 Prabhakant and G.N. Tiwari Table 3. efficiency and energy production factor of stand-alone photovoltaic (SAPV) and solar energy park (SEP). S. No. Life cycle (years) efficiency of SAPV system (%) efficiency of SEP (Thermal) (%) efficiency SEP (electrical) (%) Energy production factor of SAPV system Energy production factor of SEP (Thermal) Energy production factor of SEP (Electrical) given by: ðe n Eebd FðtÞ ¼ E sol n where n is the life cycle and E sol, total solar energy falling on the SAPV system. Values of F (t) for different life cycles have been given in Table Energy production factor It is used to predict the overall performance of the system. This is defined as the ratio of the output energy and the input energy, which predicts the overall performance of the system. Energy production factor (EPF) is defined as: x ¼ E n E obd Values of x for different life cycles have been given in Table 3. 6 CONCLUSION On the basis of the experimental and theoretical analysis of embodied energy and life-cycle analysis of the hybrid solar systems and SAPV systems installed at SEP, IIT Delhi, New Delhi, India, the following conclusions have been drawn: (i) The overall annual thermal and exergy energy for SEP is kwh ( MWh) and 2692 kwh (2.692 MWh). (ii) One such SEP is built in each village having a population of more than 1000; then the total carbon credits earned by such systems will be E2.09 million and the value of total electricity generated by such systems will be E11.5 million. (iii) The energy payback time of the park is 2.25 years for thermal energy and is years for electrical energy. (iv) There is a marginal reduction in return on capital of the SAPV system with increase in life cycles. The return on capital reduces from 4.4% to 4.23% with 20 years increase in the life of the system. (v) The return on capital increases with life cycle of the system if embodied energy is taken into account. The ð5þ ð6þ return on capital increases from 4.05% to 4.2% with 20 years increase in the life of the SAPV system. (vi) Life-cycle energy efficiency of SAPV as well as SEP increases marginally with increase in life cycle, whereas the EPF for both SAPV and SEP increases substantially by about 60% with increase in life cycle. REFERENCES [1] Anon. Solar Photovoltaic Potential and Prospects. CII Godrej GBC Publication, (January 2008, date last accessed). [2] Renewable energy outlook, World Energy Outlook 2008, International Energy Agency, 2008: [3] Imhoff J, Rodrigues GF, Bulow AR, Gules R, Pinheiro JR, Hey HL. A stand-alone photovoltaic system based on DC-DC converters in a multistring configuration, Proc. of International RIO-6, Nov 2006, Rio de Janeiro, Brazil. [4] Autonomous or standalone photovoltaic systems, Seners Energy Systems. (August 2008, date last accessed). [5] Capoor K, State and trends of the carbon market Report of International Emission Trading Association, Washington, May [6] Prabhakant, Tiwari GN. Evaluation of carbon credits earned by energy security in India. Int J Low-Carbon Technol, 2009;4: [7] Prabhakant, Tiwari GN. Evaluation of carbon credits earned by solar energy park in Indian conditions. Open Energy Fuel J 2008;1: [8] Prakash O, Chel A, Tiwari GN. Solar Radiation and Day Lighting (SOLARIS 2007). In: Dube SK, Muneer T, Tiwari GN (eds), Proceedings of the Third International Conference, New Delhi, India, 8 10 February Anamaya Publishers, New Delhi, India, 2008, Vol. 2, pp [9] Bansal NK, Minke G. Climatic Zones and Rural Housing in India Scientific Series of the International Bureau. Kernforschungsanlage jü lich GmbH, [10] Dubey S, Tiwari GN. Life cycle cost analysis and carbon credit earned by hybrid PV/T solar water heater for Delhi climatic conditions. Open Environ J, 2008;2: [11] Barnwal P, Tiwari A. Performance analysis of a hybrid photovoltaic thermal (PV/T) integrated greenhouse air heater and dryer. Int J Agric Res 2008;3: [12] Sujata N, Tiwari A. Performance evaluation of a hybrid photovoltaic thermal (PV/T) greenhouse system. Int J Agric Res, 2007;2: [13] Tiwari GN, Tiwari AK. Solar Distillation Practice for Water Desalination Systems (1st ed.). Anamaya Publishers, New Delhi, India, [14] Huang BJ, Lin TH, Hung WC, Sun FS. Performance evaluation of solar photovoltaic/thermal systems. Solar Energy 2001;70: International Journal of Low-Carbon Technologies 2009, 4,