Updated February 2015

 

Highlights

• Rooftop solar power can meet up to 25% of a college’s electricity requirements
• If your institution consumes a lot of diesel for power generation, rooftop solar can abate up to 15% of your diesel bills, subject to timing of load shedding
• 100 SF of shade-free rooftop space can provide 4 kWh of solar power per day, on average
• Students gain practical, hands on knowledge of solar plants
• Your college will be seen as a trendsetter for early solar adoption

Overview

Colleges are ideally suited for rooftop solar power plants – typically, they have large roof area; a large proportion of their electrical load is during the daytime; and they have very little air conditioning load.

Countering these advantages, there are several constraints as well such as significant reduction in load during vacations. These are discussed in greater detail below.

Advantages

• Energy security – Rooftop solar plants can deliver power during load-shedding, ensuring that critical loads are always running
  • Not all solar plant configurations can deliver power during load-shedding. More details here
• Cost-effective – Rooftop solar power has a levelised cost of Rs. 4.5-5/kWh (or less), considerably lower than diesel power at Rs. 18/kWh (or more). Additionally, your energy cost is now fixed for the next 25 years, unlike diesel power which keeps increasing
• Reliable – A solar power plant has no moving parts, ensuring reliable power over 25 years
• Minimal maintenance – A solar plant requires very little maintenance from the energy consumer
• Flexible configurations – Solar panels can be installed on different kinds of roofs, including covered parking areas, as long as the structure can bear the weight of the panels. They are also highly scalable, with rooftop plants ranging in capacity from less than 1 kW to more than 1 MW

Constraints

• Rooftop space – The capacity of the solar plant that can be installed in a college may be constrained by lack of sufficient shadow-free rooftop space. While colleges typically have large areas of roof space, we will need to ensure that they are free from shadows including shadows from any structures that have been erected on the roof for other purposes.
  • Roof requirements are discussed in detail here; a rule of thumb is that you will need about 100 SF of shade-free roof area for 1 kW of solar panels. Insufficient roof area will mean that the capacity of the solar plant that can be installed on your roof may be sufficient to meet only part of your electrical load
  • When estimating the available roof area any alternate uses of the roof, including seasonal usage, should be considered
• Infirm power – Solar power is dependent on the sun shining, and output varies depending on meteorological conditions e.g., passing clouds can temporarily reduce the solar plant’s output. Therefore solar power for critical equipment should be used in conjunction with another source of power
• Daylight power – Solar power is only available when the sun shines. Therefore night time applications (such as for hostels) will require other sources of power, or power from batteries charged through solar
• Load-shedding timings – If your college experiences load shedding primarily at night, solar power may not help in reducing your diesel consumption as it is available only during the day
• Inverter weight – The DC power output from the solar panels needs to be converted to AC via an inverter which can be very heavy: a 100 kW inverter will weigh about 1,000 Kgs but occupy only a few square feet of space. If the construction cannot support this weight the inverter may need to be placed on the ground floor, with appropriate cables chosen to compensate for energy loss

Typical Load

As the rooftop space may not be sufficient to support the entire electrical load of your college with solar, it becomes necessary to estimate the different kinds of loads to identify loads that can be/need to be supported by solar.

Electrical loads are estimated by calculating the wattage or amperage of electrical equipment in use (as shown here), which can be further classified as light loads and heavier loads, with solar being used to support the light loads.

Typical electrical loads in a college include

• Lighting and fans
• Computer labs
• College office equipment
• Water heaters

Colleges with hostels may have other loads, such as laundry facilities and water pumping, to consider.

When estimating the load to be supported by solar it is important to consider the reduction in load during vacations as solar power generated at this time may go to waste if the plant is sized for peak load during working days.

If your college is located in a state that allows net metering, your solar plant can still be sized for peak loads during working days as excess power generated on holidays can be monetised based on the state’s policy.

Our Recommendation

Based on the unique needs and constraints faced by colleges, Solar Mango recommends

Alternative 1 – Grid-interactive configuration where solar power is supplied in conjunction with EB or diesel power.

• Hybrid inverter – A rooftop solar PV system that utilises a hybrid inverter allows the solar plant to integrate with a diesel generator in addition to the utility grid. Here the solar plant serves to reduce diesel bills by supporting part of the overall load
  • Integrating a rooftop solar plant with a diesel generator involves several challenges (discussed here) that need to be overcome with careful design and sizing of the solar plant
  • Load sizing – Due to the reduction in load during vacation time, we recommend careful sizing of the solar plant to ensure that solar power does not go to waste as the solar plant will generate power whenever there is sunlight even if the power is not required – and unlike running a diesel generator, costs are not saved by turning off a solar PV plant as the bulk of the investment is made at the time of installation

Alternative 2 – A battery-backed grid-interactive solar plant that only supplies specific loads, but can support these loads without interruption even at night

• Hybrid inverter – In addition to the other functionality described above, hybrid inverters also incorporate charge controllers to regulate battery charging
• Battery backup – A battery bank can be used which will be charged by solar power, and will support critical loads for short durations during the daytime when solar output may be reduced; and at night time during power failure. Lighting, fans, computers, projectors, and other teaching aids can form part of the critical load to be supported by solar+batteries
  • Battery sizing decides the duration of battery backup available. Batteries add significantly to the cost of the project, need to be replaced every few years, require maintenance, and impose weight and space requirements. Therefore we recommend limiting the battery bank to about an hour of backup
• Powering critical loads – When combined with a battery bank, a solar plant can be used to reliably support critical loads. Implementing such a solution would require that the critical loads be fed through a dedicated circuit

For either alternative, we recommend net metering to ensure that excess solar generation, if any, is monetised.

When considering loads to be supported by solar, we recommend evaluating solar water heaters (which work on solar thermal technology) to heat or pre-heat water as this is usually more cost-effective than heating water with electricity from solar PV.

 

Cost of a Rooftop Solar Plant

The cost of a rooftop solar plant is discussed in detail here (including incentives and subsidies) and returns from substituting diesel with solar are discussed here. As a rule of thumb, a 1 kW solar plant that generates 4 kWh of solar power per day (on average) will cost Rs. 1 lakh (without considering subsidies, including installation charges but excluding batteries).

Batteries can add about 30% or more to the cost of the plant, depending on the extent of battery backup required.

Prominent Installations

• India
  • IIT – Mumbai (1 MW)
  • Sriram Engineering College – Chennai (100 kW)
  • Mar Baselios College of Engineering and Technology – Thiruvananthapuram (100 kW)
• International
  • Arizona State University – Arizona, USA (2 MW)
  • Derby College – Derbyshire, UK (58 kW)