Water Power

Part of Generators and Motors

Converting flowing water to reliable electrical generation: site assessment, turbine selection, penstock design, and practical micro-hydroelectric systems.

Why This Matters

Hydroelectric power is the most reliable, controllable, and long-lived form of renewable energy. Unlike wind or solar, a river at appropriate head runs continuously regardless of weather, season, or time of day. A well-designed micro-hydro system can run for 50 years with only bearing replacements and occasional cleaning. For a rebuilding civilization, any community near a suitable stream should prioritize hydro as its primary electrical generation source.

The engineering is well-understood and teachable. Unlike large-scale hydro, which requires major civil works, micro-hydro (under 100 kW) can be built with hand tools, local materials, and a relatively small team. The power available — even from a small mountain stream — can be substantial compared to what a community needs in its early rebuilding phase.

Assessing a Hydro Site

Two numbers determine available power: head (the vertical height the water falls from intake to turbine) and flow (the volume of water per unit time).

Available power: P = ρ × g × Q × H × η, where ρ is water density (1000 kg/m³), g is gravitational acceleration (9.81 m/s²), Q is flow in m³/s, H is head in meters, and η is overall system efficiency (typically 0.5–0.75 for micro-hydro). Simplified: P(kW) ≈ Q(L/s) × H(m) × η / 102.

Example: a stream with 10 L/s flow (a moderate trickle) and 20 m of head, at 60% efficiency: P = 10 × 20 × 0.60 / 102 = 1.2 kW. Modest but useful — enough for LED lighting, battery charging, and small tools. The same stream with 50 m head: 3 kW. With 100 L/s (a substantial stream): 12 kW.

Measuring head: use a surveying level and staff to measure the vertical height between the proposed intake and turbine location. Or use a barometer or GPS altimeter for approximate measurement. Even rough measurements (±10%) are sufficient for initial planning.

Measuring flow: the bucket method works for small streams — divert the stream into a known bucket and time how long to fill. For larger flows, measure stream cross-section and velocity (float a stick and time it over a known distance; multiply average velocity × cross-section area × 0.85 for the non-uniform velocity profile). Seasonal variation is important: measure during the dry season if possible, or get local knowledge about minimum dry-season flow.

Turbine Types and Their Applications

The turbine type depends primarily on head. Different turbines are optimized for different head and flow combinations.

Pelton wheel (high head, lower flow — above 30 m): one or more jets of water strike cup-shaped buckets on the wheel periphery. The jet converts potential energy to kinetic energy; the buckets convert kinetic energy to rotation. Pelton wheels operate in air (not submerged) and can be made from machined steel or even cast from aluminum or iron. Efficiency reaches 85–92% for well-made units. Self-governing to some extent: as load and speed decrease, the deflector plate shifts to reduce jet flow.

Turgo wheel (medium head, 10–50 m): a simpler version of the Pelton where the jet enters at an angle and passes through the plane of the wheel. Cheaper to make, slightly lower efficiency, but handles larger flows per wheel diameter.

Crossflow (Banki-Michell) turbine (low to medium head, 2–30 m, variable flow): a cylindrical turbine where water enters across the full width of the blades and passes through twice before exiting. Less efficient (70–85%) but handles large flow variations well, making it good for streams that vary significantly in flow. Can be fabricated from flat steel plate and standard pipe — no machined buckets required.

Overshot waterwheel (2–6 m head, very low flow): water delivered over the top of the wheel fills buckets, and the weight of water drives rotation. Oldest design; achievable with hand tools and wood construction. Low speed (5–15 RPM) requires significant step-up gearing. Efficiency 60–75% for good designs. Best when flow is limited and head modest.

Penstock Design

The penstock is the pipe that carries water from the intake (diversion weir) to the turbine. It must carry the full flow volume and resist the static water pressure without leaking or bursting.

Penstock pressure: the static pressure at the bottom equals ρ × g × H. For 50 m of head, pressure is 1000 × 9.81 × 50 = 490,500 Pa ≈ 5 bar (73 psi). Standard steel pipe (schedule 40) is rated well above this pressure. PVC pipe is acceptable for heads below 20 m in small systems but becomes heavy-walled and expensive at higher pressures. Bamboo reinforced with wire is a historical solution for lower-pressure applications (under 10 m).

Pipe diameter: size the penstock so water velocity in the pipe is 1–2 m/s under full flow. Higher velocities cause excessive friction losses. Calculate: diameter (m) = √(4 × Q / (π × v)), where Q is flow in m³/s and v is velocity in m/s. For 50 L/s flow at 1.5 m/s: d = √(4 × 0.05 / (π × 1.5)) = 0.206 m, so use 200 mm diameter pipe.

Head losses in the penstock: friction reduces effective head. Calculate friction loss using the Darcy-Weisbach equation or look up for standard pipe diameters. Keep friction losses below 10% of gross head — if the penstock is eating more than 10% in friction, the pipe diameter is too small.

Surge pressure (water hammer): when a valve closes quickly, the moving water column decelerates suddenly and pressure spikes. This surge can be several times the static pressure, cracking pipes and valves. Prevent by designing the control valve to close slowly (over 5–10 seconds) or by installing a pressure relief valve or surge tank near the turbine inlet.

Generator Coupling for Hydro

Small hydro generators can be directly coupled to high-speed turbines (Pelton wheels) or belt/gear coupled to low-speed turbines (crossflow, waterwheels).

For Pelton wheels designed for high-speed operation (600–1500 RPM), a direct-coupled synchronous generator at the same speed is the simplest and most reliable arrangement. The generator shaft and turbine shaft are joined by a flanged coupling, with a misalignment tolerance of 0.1 mm or less.

For low-speed turbines, step-up gearing or belt drive is needed. A well-designed belt drive with V-belts is practical for step-up ratios up to 5:1 in a single stage, up to 15:1 in two stages. Total transmission efficiency 90–95% for V-belts. Size the belt and pulleys for 125–150% of the rated power transmission to handle transient load peaks.

Voltage regulation: with a constant-speed waterwheel and a properly governed turbine, a synchronous generator will produce stable voltage and frequency with minimal additional regulation. An automatic voltage regulator (AVR) can fine-tune excitation to hold terminal voltage within ±1–2%, but even manual field current adjustment works well for steady loads. Battery storage downstream the generator handles any residual fluctuations.

Intake Works and Filtration

A reliable micro-hydro system needs a properly designed intake that keeps debris, silt, and sand out of the turbine. Sand and grit will abrade Pelton buckets and journal bearings within weeks if not excluded.

The diversion weir diverts a fraction of stream flow into the intake channel. It should have an overflow at the design flow level — excess water simply flows over the weir crest and continues downstream.

The settling tank (forebay) is a widened pool just before the penstock inlet where water velocity slows and suspended silt settles out. Size it for at least 20 seconds of retention time: volume = 20 × Q. A regular cleaning drain allows accumulated silt to be flushed without disassembling the system.

A trash rack (bar screen) at the forebay inlet stops leaves, sticks, and debris from entering the penstock. Space bars 20–30 mm apart for most turbines. A finer screen (5–10 mm) may be needed for Pelton turbines with small nozzles. Plan for daily or weekly cleaning of the trash rack — it accumulates debris quickly.