Case Study #8: Photovoltaic Water Pumping for Livestock
Due to recent regulations for water protection, a farm near Farmville needs to fence off a slough where its livestock used to drink. Now the farmer will need to pump water out of the slough into a storage tank and watering trough. The farm needs to supply water to 200 cows.
Initially the farm was considering extending a power line to the slough to run the pump. But when the farm found that grid extension would cost $15,000, it decided to search for cheaper options.
One option the farm would like to investigate is an off-grid photovoltaic system powering a pump with a DC motor. The DC motor and an electronic device that boosted the current available from the photovoltaic modules in order to start the motor would together cost around $700 more than the AC pump motor that could be used if a power line were installed. Also, because the sun cannot be counted on to shine brightly every day, a larger water storage tank, capable of storing three days of water for the cows, will be used. Storage capacity is estimated to cost $800 per thousand gallons.
The photovoltaic modules, mounting rack fixed at a near optimal angle, and wiring would cost around $5000/kW installed; the high price reflects the small size of the system.
The vertical distance from the water level of the slough to the top of the storage tank would be around 30 feet. The pumping system will not be used in December and January.
1) About how much water do 200 cows require per day? How large a storage tank will be needed?
2) What is the correlation between when the pump runs and when there is bright sunshine? (Hint: Pick the “intermittent resource-load correlation” accordingly.)
3) What is the optimal tilt angle for a south-facing photovoltaic array powering a load that must operate from February through November? (Hint: enter the parameters for the battery and then pick a photovoltaic array capacity that delivers roughly 80% of the electricity required by the load. Then adjust the tilt angle until the electricity delivered to the load is maximized.)
4) What size photovoltaic array is required? Do you think that it is worthwhile to oversize the array?
5) Is the photovoltaic option cheaper than the grid extension option? If so, by how much?
Additional questions if you finish early:
6) Why is a battery not needed for this off-grid power system? If battery capacity costs $200/kWh, might it be cheaper to use a battery rather than a larger storage tank?
7) Still have time? Compare the photovoltaic system with gasoline powered pump. Make sure that you attribute costs for time spent in operation and maintenance.
Getting stuck? Try this:
- Open the “Water Pumping” tool in the “Tools” tab and select “Method 2” within the tool.
- For daily water use per unit in the Water Pumping tool, consult RETScreen help.
- RETScreen’s Water Pumping tool bases its power requirement calculation on the total head, so it does not matter how the 30 feet of head is divided into suction head, discharge head, and drawdown.
- Select “Percent of month used” to indicate that the system operates February through November.
- Even though a battery may not be required, for the purpose of sizing the photovoltaic array, indicate to RETScreen that a battery with 3 days of autonomy will be used. You can choose 24 V for the battery voltage and efficiencies according to RETScreen help; just make sure that the battery capacity is equal to RETScreen’s suggested battery capacity (i.e., use a formula to equate the two).