Micro-hydro for a hill hamlet: turbine choice with no battery bank
A grama panchayat in Idukki wants to electrify a 40-household hamlet from a hill stream. The survey gives a usable head of 40 m between the intake and the turbine house and a lean-season flow of 0.25 m³/s; at peak monsoon the flow rises roughly fourfold. The evening load is estimated at 55 kW, and the stream runs lowest exactly when demand peaks. The penstock is 180 m of 200 mm HDPE, for which the supplier quotes a friction loss equal to 12% of the gross head at design flow. Three turbines can be sourced locally: a Pelton wheel (80% efficient, poor behaviour at part flow), a cross-flow turbine (70% efficient, tolerates wide flow variation, repairable in a village workshop), and a Francis runner (85% efficient but sensitive to silt). Take g = 10 m/s² and water density as 1000 kg/m³. The panchayat can fund one turbine and no battery bank.
1. Calculate the gross hydraulic power, the power available after penstock loss, and the electrical output for each of the three turbines at design flow. Does the scheme meet the 55 kW peak?
Read the worked answer
Gross hydraulic power
$$P_{gross} = \rho g h Q = 1000 \times 10 \times 40 \times 0.25 = 100{,}000\ \text{W} = 100\ \text{kW}$$
After penstock loss
Friction loss = 12% of 40 m = 4.8 m, so net head = 35.2 m.
$$P_{net} = 1000 \times 10 \times 35.2 \times 0.25 = 88\ \text{kW}$$
At the generator terminals
- Pelton: 0.80 × 88 = 70.4 kW
- Cross-flow: 0.70 × 88 = 61.6 kW
- Francis: 0.85 × 88 = 74.8 kW
All three clear 55 kW at design flow, but the margins differ sharply: 15.4 kW (Pelton), 19.8 kW (Francis) and only 6.6 kW (cross-flow). Generator and transmission losses of 5–10% are not yet included; at 8% the cross-flow falls to about 56.7 kW, essentially no margin at all. So the honest answer is 'yes on paper, marginally for the cross-flow, and only if lean-season flow holds at 0.25 m³/s'. Firm capacity of a run-of-river scheme is set by the lean season, not the monsoon.
2. Which turbine would you specify, and how would you handle lean-season flow, monsoon floods and silt?
Read the worked answer
Head (40 m) and design flow (0.25 m³/s) sit in the Francis/cross-flow band; a Pelton is usually reserved for heads well above 60 m, and its poor part-flow behaviour is a real liability because flow varies fourfold between seasons.
The deciding factor is not peak efficiency but maintainability and silt. Idukki streams carry heavy silt in the monsoon, and a Francis runner erodes and loses efficiency, needing specialist repair that a village workshop cannot do. A cross-flow runner can be replaced with a locally fabricated unit, so a cross-flow is the defensible choice provided the 61.6 kW output is protected by demand management.
Mitigations:
- Settling basin and trash rack ahead of the forebay to drop silt and debris before they reach the runner.
- Spillway / overflow weir at the intake so the fourfold monsoon flow bypasses the penstock instead of bursting it.
- Electronic load controller with a dump load (a community water heater or an irrigation pump) to absorb surplus power, since without a battery there is nowhere else for the energy to go.
- Part-flow operation: the cross-flow stays usable over a wide flow range, which matters when the lean season stretches on.
3. Lean flow and the evening peak coincide. Without storage, how do you keep supply and demand matched, and what would you tell the panchayat about extending the line to more households?
Read the worked answer
A battery-less micro-hydro system is a use-it-or-lose-it source: whatever the stream delivers at that instant must be consumed, dumped or spilled.
Matching tactics
- Load controller with dump load: when household load falls below generation, the excess is diverted to a water heater or a pump filling an overhead tank. This keeps voltage and frequency stable and stops the turbine overspeeding.
- Staggered high-power loads: rice mills, pumps and welding sets are scheduled rather than run at will, because each is a large fraction of a 55–60 kW system.
- Managed evening peak: lighting and fans first; heavy loads moved to daylight hours when the panchayat office and mill can use them.
On extending the line
Compute the temptation first: monsoon flow ≈ 1 m³/s gives roughly 4 × 88 ≈ 350 kW of hydraulic power, though friction loss rises with the square of flow and reduces net head. That number is real but available for only part of the year. The firm capacity is the lean-season figure (~56–62 kW). Adding households on the strength of monsoon output means the system fails every dry season. Expansion beyond the lean-season capacity should only be approved together with storage (a pondage or battery) or with strictly interruptible loads.