A rooftop solar power plant is a photovoltaic system that has its electricity-generating solar panels mounted on the rooftop of a residential or commercial building or structure. Solar power plants have many advantages, but one major drawback is that it requires more space. One cannot provide fertile agricultural or forest land for this purpose. Only arid lands and desert areas can be utilised for harnessing solar power. But there are large buildings, on top of which solar power plants can be installed. For rooftop solar plant, space required is about 10 square metre per kW of installed capacity.
Rooftop mounted solar systems are small compared to ground-mounted photovoltaic power stations with capacities in the megawatt range. Rooftop PV systems on residential building typically, feature a capacity of about 5 to 20 kilowatts (kW), while those mounted on commercial building often reach 100 kW or more.
Installation
The urban environment provides a large amount of empty rooftop space and hence can be used for installation of solar panels. Solar insolation in rooftops depend on time of the year, latitude of the place, weather conditions, roof slope, roof aspect and shading from adjacent buildings and vegetation.
The various components of such a system include photovoltaic modules, mounting systems, cables, inverters and other electrical accessories.
Grid-connected and Off-grid Rooftop Solar System
The rooftop solar power plant can be connected to the nearest electricity grid or it can directly feed to the consumer. In a grid connected rooftop photovoltaic power station, the generated electricity can sometimes be sold to the servicing electric utility for use elsewhere in the grid. This arrangement provides payback for the investment of the installer. Many consumers from across the world are switching to this mechanism owing to the revenue yielded. A public utility regulatory commission usually sets the rate that the utility pays for this electricity.
Advantages
With rooftop solar plant, the power bills will be massively reduced because we will need less electricity from the grid. The cost of electricity from the grid is expected to rise in the coming years, so our cost savings should also rise. With a linear power warranty of 20 to 25 years, we can expect long-term cost savings from avoiding the cost of expensive electricity from the grid.
Installers have the right to feed solar electricity into the public grid and hence, receive a reasonable premium tariff.
Rooftop solar system helps in reducing the generation capacity of fossil fuel power plants thereby, reducing carbon emissions to the atmosphere. It also reduces the burden of transmission and distribution system as it is being generated and utilised at the load centre. 1 kWh generated at load centre can save generation of 1.5 kWh by fossil fuel-based plants.
Disadvantages
An electrical power system containing a 10 per cent contribution from PV stations would require a 2.5 per cent increase in load frequency control (LFC) capacity over a conventional system. The capacity of the grid should be very high to tackle this problem.
Hybrid Systems
A rooftop photovoltaic power station (either on-grid or off-grid) can be used in conjunction with other power components like diesel generators, wind turbines, batteries etc. These solar hybrid systems may be capable of providing a continuous source of power.
Rooftop Solar Projects in India
The Indian solar PV market has seen significant growth rising from 40 MW to more than 26,000 MW in the last eight years. As on 13th February 2019, the installed capacity of rooftop solar power plants in India is 1443.74 MW. The estimated realistic market potential for rooftop solar PV in urban settlements of India is about 124 GW. It may be noted that the current total installed power generation capacity is 350 GW. Thus, rooftop solar PV can play an important role in providing energy security and in multiple utilisation of land, a scarce resource.
Jawaharlal Nehru National Solar Mission (JNNSM), aims to install 100 GW of solar capacity in the country by 2022, of which 40 GW is expected to be achieved through decentralised and rooftop solar projects.
World’s largest single rooftop solar power plant of 11.5 MW capacity was inaugurated in Beas near Amritsar in Punjab in May 2016. It is spread at a single rooftop stretch of 42 acres at Dera Baba Jaimal. In addition to single largest rooftop solar power plant, seven rooftop solar power plants of 8 MW capacity were also inaugurated in Beas Dera campus making this place the highest single campus generating solar power of 19.5 MW at multiple rooftops in the country.
State-owned gas utility GAIL has commissioned India’s second largest rooftop solar power plant in Uttar Pradesh. It is 5.76 MW solar plant installed at GAIL’s petrochemical complex at Pata in UP. The plant installed on the rooftops of warehouses covers a total area of 65,000 square metre.
The installed capacity of ground mounted and rooftop solar power plants in different states of India are given in the Table – 1.
Policy for Installation of Rooftop Solar System
In India, different states have formulated policy to attract the consumers and investors for installation of rooftop solar power plants. However, major decisions are similar. For rooftop solar plants, minimum capacity is fixed at 1 kW and there is no maximum capacity limit. The condition is that the installed capacity of the rooftop solar plant will be limited to 100 per cent of the sanctioned load of the owner from the local electricity distribution company. If the electricity generated exceeds 90 per cent of the electricity consumed at the end of the settlement period, no payment shall be made by the distribution licensee for the additional amount of generation and shall not be carried over to the next settlement period.
The billing is done by net metering method. Net metering is a billing system that allows rooftop owners having solar system installed at their rooftop to sell any excess electricity generated from solar system to local electricity utility. The total bill is calculated with the units of electricity imported subtracted by the number of units exported by the consumer with the condition that the export cannot be more than 90 per cent of the import.
Another condition is that cumulative capacity of all solar systems installed in an area shall not exceed 75 per cent of distribution transformer capacity in that area.
Ownership Options
Rooftop solar power plants can be installed either by self-ownership (CAPEX model) or third-party ownership (OPEX model).
CAPEX Model
In this model, the entire investment comes from the owner of the building (power consumer). The owner generally hires a solar EPC company who provides turnkey installation of entire solar power system and hand over assets to the owner. The EPC company also does annual operation and maintenance (O&M) of the plant on mutually agreed cost per annum.
OPEX Model
In the OPEX model, an investor or project developer (sometimes called Renewable Energy Service Company – RESCO) invests the total capital amount and also does the annual operation and maintenance of the plant. The owner (consumer) pays for the energy consumed or supplied to the developer. Both owner and developer sign a long-term power purchase agreement (PPA) for an agreed tenure & tariff.
Conclusion
India has committed to Paris Agreement to generate 30 per cent of its total electricity from renewable sources by 2030. Rooftop solar system will help it to some extent. It is also expected that the distributed generation (rooftop solar PV) at the consumer end will compensate to the acute power shortage in several states.
Mayadhar Swain
Director
School of Electrical Engineering,
KIIT Deemed-to-be University, Bhubaneswar
