Sizing a solar pump for boro rice and deciding whether to keep diesel backup
A farmer in Barpeta has 1.2 ha of boro rice and irrigates from a 10 m deep tube well with a 5 hp diesel pump. Diesel costs ₹92/L and the pump runs 6 h/day for 90 days, consuming 1.6 L/h. A vendor offers a 3 kWp solar photovoltaic pump set with a 3 hp submersible pump, 22 m total head, and a controller with no battery. The farmer's crop water requirement is 6 mm/day, and the pump delivers 12 m³/h at 22 m head. The panel receives about 4.5 peak sun hours per day; system efficiency is 75%. The capital cost is ₹2.8 lakh, with a 30% subsidy. The farmer wants to know whether the array is adequately sized, and whether he should keep the diesel pump as backup.
1. Check whether the 3 kWp array can deliver the daily water requirement of 6 mm over 1.2 ha. Show the hydraulic energy and PV energy calculations.
Read the worked answer
Daily volume required = 6 mm × 12,000 m² = 0.006 × 12,000 = 72 m³/day. At 12 m³/h, the pump must run 6 hours, matching the existing schedule.
Hydraulic energy:
- E = ρ g V H = 1,000 × 9.81 × 72 × 22 = 15.54 MJ.
- Convert to kWh: 15.54 / 3.6 = 4.32 kWh of useful hydraulic energy.
At a pump–motor efficiency of about 50%, electrical energy needed = 4.32 / 0.50 = 8.64 kWh/day.
PV output:
- 3 kWp × 4.5 peak sun hours × 0.75 system efficiency = 10.1 kWh/day.
The array is adequate on an average day, but only by about 17%. That margin disappears with dust, high module temperature, cable loss, pump wear or a cloudy day. A 4 kWp array would give about 13.5 kWh/day and a safer margin. The 3 hp pump draws about 2.2 kW, so the array is not grossly undersized, but I would recommend 3.5–4 kWp and a controller with dry-run protection.
2. Calculate the annual diesel cost and the simple payback of the solar pump after subsidy, assuming 90 days of operation.
Read the worked answer
Diesel use = 1.6 L/h × 6 h/day × 90 days = 864 L/year.
Annual diesel cost = 864 × ₹92 = ₹79,488/year.
Solar capital cost after 30% subsidy = ₹2,80,000 × 0.70 = ₹1,96,000.
Simple payback = 1,96,000 / 79,488 = 2.47 years, about 2.5 years.
This ignores the diesel pump's maintenance, oil changes, and the value of labour for fetching diesel, so the true payback is somewhat shorter. It also ignores the solar system's inverter/controller replacement after perhaps 8–10 years and module cleaning. Even with those costs, the solar pump is financially attractive.
A more complete comparison should subtract the resale or standby value of the diesel pump and include the cost of a small shed or fence for the controller. If the farmer borrows money at 9% interest, the annualised capital charge is roughly ₹30,000–35,000, still well below the diesel bill.
3. Should the farmer retain the diesel pump as backup? State an operating rule for cloudy spells and for the first week after transplanting.
Read the worked answer
Yes, retain the diesel pump, at least for the first two seasons. A battery-less solar pump has no stored energy, and Barpeta's boro season can have 2–4 consecutive cloudy days. Rice is most sensitive at transplanting and panicle initiation; a short water deficit at those stages can reduce yield more than the diesel saved.
Operating rule:
- On a clear day, pump in the morning and early afternoon, prioritizing the nursery and the newest transplanted plot. Stop when the field water depth reaches about 5 cm; do not over-irrigate.
- If the forecast shows one cloudy day, reduce irrigation to the most sensitive plot and use the diesel pump only if the solar pump cannot deliver at least 40 m³ that day.
- If cloudy weather lasts two or more days, run the diesel pump for 3–4 hours on the critical plot, then return to solar.
- Keep the diesel pump fuelled, with a spare filter, and test it monthly. A backup that will not start is not a backup.
After one season, record actual solar output, cloudy days and diesel use. If the record shows fewer than three cloudy spells, the farmer can consider selling the diesel pump; until then, keep it.