A Solar Pump for an Ahmednagar School Garden
A school in Ahmednagar wants a solar pump for its 0.2 ha kitchen garden. The water level in the well is 9 m below ground and water must be lifted a further 3 m to a tank on a platform, so the total head is 12 m. The garden needs 6,000 litres a day in the dry season. The supplier quotes a 1 kW panel and three 150 Ah batteries, ₹95,000 in all. The school has ₹60,000 and the head teacher does not want a second round of fundraising. A 0.5 HP (373 W) pump with overall efficiency 40% is available, as is a 1 HP (746 W) pump with the same efficiency. A 5,000-litre tank on a 3 m platform costs ₹18,000. Ahmednagar gets roughly 4.5 hours of usable sun a day, and the monsoon brings 8-10 overcast days a month. The nearest grid connection is 400 m away; the electricity bill would be about ₹700 a month.
1. Estimate the daily hydraulic energy needed and the panel size that would supply it. Is the 1 kW panel justified?
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Hydraulic energy = mass × g × head = 6,000 kg × 9.81 × 12 m = 706,320 J, which is about 0.196 kWh a day. That is the energy actually given to the water.
The pump is only 40% efficient, so the electrical energy drawn is 0.196 ÷ 0.40 ≈ 0.49 kWh a day.
Panel sizing: with 4.5 usable sun hours and a derating factor of about 0.75 for heat, dust and wiring losses, each kWp delivers about 4.5 × 0.75 ≈ 3.4 kWh a day. Needed capacity is therefore roughly 0.49 ÷ 3.4 ≈ 0.15 kWp, say 200 Wp with a margin for cloudy days.
So the 1 kW panel is five to six times larger than the duty requires. The extra capacity is wasted whenever the tank is full and the pump switches off.
Check the pump too. A 1 HP pump at 40% gives about 298 W of hydraulic power, so flow = 298 ÷ (1000 × 9.81 × 12) ≈ 2.5 litres a second, filling the 6,000-litre need in about 40 minutes. A 0.5 HP pump takes about 80 minutes, which is still well within a sunny day. The 0.5 HP pump matches the duty.
2. The supplier insists batteries are essential. Explain why they are a poor choice here, and what should be done instead.
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Batteries are needed when energy must be delivered at a time or place where the panel cannot reach. Here it can: the pump runs during daylight, and the water itself is the store.
A tank is a far better battery. It costs a fraction of a battery bank, loses nothing to charging and discharging, has no cycle limit, and needs no replacement after four or five years. Storing 5,000 litres on a 3 m platform gives gravity pressure for drip irrigation and needs no electricity at all when the garden is watered, in the evening or on a cloudy morning.
Batteries also add a failure point that the school cannot maintain: deep discharge, water topping-up and disposal of lead are all new problems for a school with no technician. The correct design is direct-coupled panels, a pump, and a tank sized to bridge one or two cloudy days; the monsoon's 8-10 overcast days matter less because the garden needs least water then.
3. A parent says the grid at ₹700 a month is cheaper. How would you answer with numbers and with non-numeric reasons?
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Numbers first. ₹700 a month is ₹8,400 a year, recurring forever, with a tariff that will rise. The solar option that actually fits the duty - a 0.5 HP pump, about 0.5 kWp of panel and the ₹18,000 tank - can be delivered within the ₹60,000 budget, while the quoted ₹95,000 system is both over budget and oversized. So the real comparison is not ₹95,000 versus ₹700 a month but roughly ₹55,000 versus a recurring bill, giving a payback near six to seven years on today's tariff and less after any tariff rise.
Non-numeric reasons matter more for a school. The 400 m extension is a capital cost and a right-of-way problem that the ₹700 figure ignores; supply in rural Ahmednagar is not uninterrupted, and a pump that fails in the week the saplings need water defeats the purpose. Solar also matches the demand pattern: the garden needs most water on the sunniest days, which is exactly when the panel is most productive.