Updated September 2014

Highlights

  • Rooftop solar power can meet up to 20% of your dairy farm’s electricity requirement in India
  • If your farm 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

Overview

Dairy farming is a sector in which expensive machines that consume a great deal of power are used. High levels of diesel consumption for power generation are a concern, as are the exhaust fumes.

Rooftop solar power presents several advantages to dairy farm facing the above issues. Several constraints will, however need to be considered when introducing solar power into energy mix of a dairy farm. Both aspects 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-effectiveRooftop 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). 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 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 dairy farm 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
  • 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 loads 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
  • 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 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 dairy farm include

  • Water Heaters
  • Vacuum pumps
  • Cooling System/Refrigeration
  • Pressure Washers
  • Equipment with Motors/Generators
  • Ventilation system
  • Feeding equipment
  • Security system
  • Lighting

In a dairy farm, many of the loads require high starting currents. These, such as cooling system; motors; and generators, are usually the heavier loads, accounting for more than 70% of the total energy consumed. The cooling system/refrigeration needs to be operated 24/7 for preserving products and it is the critical load in the dairy industry.

As many of the loads are likely to consume a great deal of power, the usage of loads needs to be taken into account and load sizing must be performed while designing the solar plant. Moreover, loads which require heavy starting current may require the use of another source of power in addition to solar.

Depending on the temperature required, solar water heaters may be a better substitute for heating water than electrical water heaters powered by electricity from Solar PV.

Our Recommendation

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

Alternative 1 – With clear separation of loads

Where it is possible for some loads (typically the lighter loads) in the dairy farm to be isolated and fed using a dedicated feeder, the following architecture can be used

  • Powering some critical loads –When combined with a battery bank, a rooftop solar plant can be used to reliably support some critical loads; which critical loads can be decided in consultation with the dairy farm’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 limiting the battery bank to about an hour of backup
  • Hybrid inverter – A rooftop solar PV system that utilises a hybrid inverterwill 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
    • 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 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 requires 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.

  • Load Scheduling –Some machines consume large amounts of energy; usage of such machinery can be scheduled to work during the daytime, if possible, to take advantage of solar generation.

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
  • As solar output varies with meteorological conditions we recommend using solar power only in conjunction with another source of power, such as EB or diesel generator, to ensure that the critical loads are not affected
    • As batteries are quite expensive, we recommend limiting the backup duration to an hour, subject to specific needs at your site
  • 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