Updated September 2014

Highlights

  • Rooftop solar power can meet 15-20% of a Shopping Mall’s electricity requirements in India
  • If your shopping mall consumes a lot of diesel for power generation, rooftop solar can abate up to 20% 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

Overview

Shopping malls are large energy consumers in cities: they function for 12 hours a day or more, are typically fully air conditioned, and feature a great deal of lighting even during daytime. With abundant rooftop space both in the mall buildings and in parking areas they are ideal candidates for solar power. There are some constraints, however, in using solar power that should also be considered; both are discussed 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 cost at Rs. 18/kWh (or more). Depending on your tariff, solar may even be cheaper than EB power. Additionally, your energy cost is now fixed for the next 25 years, unlike diesel and EB 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 mall may be constrained by lack of sufficient shadow-free rooftop space. 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 on your roof may be sufficient to meet only part of your electrical load
    • When estimating available rooftop space, alternative uses for the roof, such as billboards or mobile phone towers, should also be considered as these will not only occupy space on the roof but also cast a shadow that will limit the space where solar panels can be installed
  • 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 applications 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 will require other sources of power, or power from batteries charged through solar
  • Load-shedding timings – This could be an issue in some shopping malls as a great deal of electricity is consumed only in evening hours: if most of your load-shedding occurs 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. Building aesthetics/access should be considered when creating this space.

Typical Load

As the rooftop space may not be sufficient to support the entire electrical load of your facility 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 shopping mall include

  • Lighting
    • Inside the mall (stores and common areas)
    • Decorative lighting
    • Outdoor lighting
    • Basement parking lighting
  • Air Conditioning
  • Elevators and Escalators
  • Computers
  • Play area
  • Food court
    • Refrigeration units
    • Ovens and stoves
  • Equipment in beauty salons
  • CCTV
  • Other electrical equipment

Air conditioning is usually the heaviest load; lighting, air conditioning and elevators together accounts for nearly 80% of the energy consumed.

Our Recommendation

The wide variety of electrical loads and timing of loads gives us different options to be considered when designing a rooftop solar system for shopping malls. Solar Mango has framed two different usage scenarios for malls, and recommends

Alternative 1 – With clear separation of critical loads

Where it is possible for some critical lighter loads (such as security systems or basement parking lighting) to be isolated and fed using a dedicated feeder, we suggest

  • Powering some critical loads – When combined with a battery bank, a solar plant can be used to reliably support some critical loads; which critical loads can be decided in consultation with the mall’s infrastructure team. In the event of a power failure these loads will be supported by solar power and the battery bank will compensate for any deficiency in solar power at that time
  • Battery backup – The battery bank will be charged by solar power, and will support critical loads for short durations during the daytime when solar output may be reduced. Night time support is also possible, depending on the size of the battery bank
    • 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 considering both current and future cost and space requirements before deciding on the size of the battery bank
  • Hybrid inverter – A rooftop solar PV system that utilises a hybrid inverter will allow the solar plant to integrate with a diesel generator. Here the rooftop solar plant serves to reduce diesel bills by supporting part of the load. Hybrid inverters also feature charge controllers to regulate battery charging
    • 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 rooftop solar plant

Alternative 2 – Without clear separation of loads

If it is not possible to isolate critical loads through a dedicated feeder, the solar power will have to be fed along with the diesel generator/EB power to the entire facility. Such a solar plant will contribute to savings in diesel, but support to specific loads in the event of a power failure will not be provided.

This configuration also require a hybrid inverter, but batteries will not be required. It should be noted that in the event of a power failure, if the DG is shut down the solar plant will also shut down.

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.

Takeaways
  • Solar power costs Rs. 4.5-5/kWh or less, compared to Rs. 18/kWh or more for diesel power
    • Cost of solar power is fixed for the next 25 years, whereas diesel cost keeps increasing
  • Rooftop space may be constrained by alternative uses, such as billboards or mobile phone towers
  • As your rooftop space may not permit a solar plant large enough to power your entire load, your solar plant may need to integrate with your diesel generator
    • Integrating a solar plant with a diesel generator requires careful design and sizing of your solar plant with respect to your diesel generator
  • Solar power combined with a battery backup can be used to ensure that critical loads are kept running even during load shedding
    • Critical loads that can be powered by solar include basement parking lighting and security systems