Techno - Economic Analysis of Rooftop PV System

Electrical energy at household levels is mainly used for end uses such as lighting, heating and pumping of water. The household electricity demand is currently being met by the grid connected system which has its own limitations (Raghavan Shubha. V, et. al., 2011, P: 3180). Many households in the state depend on battery-inverter or diesel generator as a backup system which increases the household expenditure. Also, over 400 million people do not have access to electricity (13 villages in Karnataka) in the country. Hence the decentralized rooftop solar PV systems at individual household level could be the technically feasible solution as it can meet the demand of the household and meeting the increasing demands of rural (remote area) electrification (Ramachandra, Krishnadas Gautham and Jain Rishab, 2012a, P: 3179).
Monthly average electricity consumption of a household in Karnataka (Uttara Kannada district) ranges from 50 kWh to 100 kWh. The average roof area of an urban household is about 1,200 square feet (120 m²) and that of rural household is 2,000 to 2,500 square feet (200-250 m²). The portion of this rooftop is sufficient to harvest electrical energy using solar photovoltaic (SPV) system. Table 3 gives the area of PV cell required to generate electric energy at varied efficiencies (like 4, 8, 12 and 16%). Rooftop SPV is a standalone or an off grid system and hence do not face any uncertainty such as grid interventions and hence it would be more reliable. The system uses a part of rooftop area for installing PV modules which will be less than 5% of the total roof area. Though the initial cost of such systems is high, it has a payback period of 5 to 7 years and has a life span of more than 20 years (Jain Abhishek, 2012, http://www.bijlibachao.in/Solar/roof-top-solar-pv-system-project-for-home-and-office.html).

Table 3: Rooftop area (m²) required for installing SPV1

¹ Ministry of New and Renewable Energy (GoI)

For example, to generate 1,000 watts of electricity from 12% of efficiency, 100 m² roof area is required. But this does not give the output for 24 hours a day and all throughout the year. The electricity generated (kWh) from the PV system depends on the panel efficiency and the availability of solar insolation in a location. The factor that defines this output is called CUF (Capacity Utility Factor). For India, it is typically taken as 19% and the energy generated is:
                                        Energy Generated Annually (in kWh) = System Size in KW * CUF * 365 * 24  --------------------------- (7)
A typical 1 kW capacity solar system will generate 1,600-1,700 kWh of electricity per year. (It may vary according to the location and PV technology used.) This means electrical energy generated per month from rooftop PV system ranges from 130 to 140 kWh (consumption in household is 50-100 kWh). Roof area required for 1KW output PV system ranges from 300 m² (η=4%) to 80 m² (η=16%) (Installing and Maintaining a Home Solar Electric System, 2012, http://energy.gov/energysaver/articles/installing-and-maintaining-home-solar-electric-system).

Roof area required to meet the monthly demand of a household is estimated for different PV technologies is given in Table 4 considering the average solar insolation of 5 kWh/m²/day. Area calculated is the actual area of PV module to be installed to meet the demand on rooftop

Table 4: Fraction of rooftop area required for various PV cell technologies

6.1 Economic Analysis

Table 5 gives the total installation cost of a typical rooftop PV system with the generation cost and payback period. Solar PV module of 1 kWp with overall system efficiency of 10% is considered for the calculations (Lacchini Corrado, João Carlos and Santos Dos, 2011, P: 183). The costs estimated include all the system components such as battery, wiring and mounting equipment (does not include inverter and backup unit).

Table 5: Unit cost and payback period for SPV system1,2

1. Alan Goodrich, Ted James and Michael Woodhouse, 2011. Residential, Commercial, and Utility-Scale Photovoltaic (PV) System Prices in the United States: Current Drivers and Cost-Reduction Opportunities. Technical report by NREL, NREL/TP-6A20-53347, February 2012
2.  Alice Solar City 2011. Rooftop solar photovoltaic (PV) system, part of the Australian Government’s Solar Cities Initiative

Photovoltaic cells directly convert solar radiations into electric power due to the process called photo electric effect. When sun light falls on the surface of PV cell, free electrons are emitted from the cell which flows through the external circuit and delivers the power. The output power is unidirectional or DC power. Normally rated output power is measured in peak Watt (W_p) in standard test condition (STC) which is the product of short circuit current (I_sc) and open circuit voltage (V_sc). The actual output of the panel may vary according to the location and insolation over the year (seasonal variation). The mean DC output power in Indian climatic conditions ranges from 1,600 to 1,700 kWh per year per kWp (Vardimon Ran, 2011, P: 592).
The other important aspect which affects the output of the panes is the efficiency which is calculated by measuring the net output of the PV of unit square metre area. The efficiency varies for different materials depending on purity of the silicon and manufacturing technology. For crystalline Silicon, efficiency ranges from 12 to 16% and maximum efficiency achieved is more than 40%. Amorphous silicon, Cadmium Telluride (CdTe) and CIGS (Copper indium gallium selenide) solar cells have lower efficiency of 5-7%, 8-11% and 8-11% respectively (International Finance Corporation (IFC), A Member of World Bank Group, 2011). Cost of the rooftop PV solar system varies from 1.5 to 1.8 lakhs per KWp installed capacity. Cost may also depend on the other parameters like efficiency, capacity, type of PV cell technology, type of mounting and the geography. Table 6 gives the cost comparison of different power plant on installed capacity basis.

Table 6: Installed plant capacity cost comparison^1,2,3 (Cost/MW)

1. Nuclear Fissionary, http://nuclearfissionary.com/2010/04/02/comparing-energy-costs-of-nuclear-coal-gas-wind-and-solar/
2. http://aglasem.com/resources/reports/pdf/SOLAR%20VS%20NUCLEAR%20VS%20WIND%20ENERGY.pdf   
3.  http://openaccesslibrary.org/images/HAR224_Adesh_Sharma.pdf

The installation cost of solar PV and rooftop PV are comparable to other technologies and has a payback period of 5 to 7 years. Moreover solar PV system has very less maintenance cost and minimal issues of waste disposal. Also, rooftop solar PV uses the roof space with no landuse restrictions (Lacchini Corrado, João Carlos and Santos Dos, 2011, P: 185).
Thermal power plants are the base load plants (coal or gas based) which supply the larger loads of the country. These plants are centralized plants normally located close to raw material (coal) available places or near to load centres. Such plants may not be installed as decentralized plants for a community or household level. Nuclear and hydroelectric plants are also centralized plants, installed capacity ranges from few hundreds of MW to several thousands. Due to the waste disposal and recitation constraints nuclear power plants are located far away from load centres and cannot be installed in decentralized manner for community level. Hydroelectric plants are the biggest plants which need large area for dam construction to provide suitable head. But small hydroelectric plants (less than 50 MW) can be constructed to supply a small load centres (community level).  Compared to these, solar PV and wind turbine (or hybrid) generation plants can be used as both centralized as well as decentralized to supply community and household level demand. An off grid system may be lower capacity (few hundreds of watts to few KW) which is capable to meet the demand of household or a community demand. Rooftop solar PV systems are the latest development which can meet the household demand and also can supply to the grid. Building-integrated photovoltaic (BIPV) is the upcoming technology in which PV panels are integrated with building materials. (Ramachandra and Dabrase Pramod. S, 2000, P: 15).

6.1 Comparative analysis of Generation cost (Cost per MWh) of different power plants:

Generation cost includes the cost of installation of plant (capital cost), operation and maintenance cost (O&M), cost of the raw materials and other expenses. This cost also includes the life time valuation of a plant to the present value. Table 7 gives the generating cost comparison of different power plants (2010) based on the average of 14 countries including three non OECD countries (International Energy Agency (IEA) Nuclear Energy Agency (NEA), Organization for Economic Co-operation and Development, 2010).

Table 7: Generation cost comparison of different power plants

1 Solar PV (rooftop) system in Germany,
2 Solar thermal system in United States

7.0 Environmental aspect:

On an average, generation of 1,000 KWh of electricity from solar radiations reduces emissions by about 83.6 kg of sulfur dioxide, 2.25 kg of nitrogen oxides and about 635 kg of carbon dioxide. During its 20 years of clean energy production solar rooftop system can reduce tons of poisonous gas emissions to the climate (The National Renewable Energy Laboratory PV FAQs for: U.S. Department of Energy Office of Energy Efficiency and Renewable Energy, 2004).