Elevated Water

Part of Energy Storage & Batteries

Pumped hydro — lifting water to height during surplus power, releasing it through a turbine when power is needed — is the most efficient and durable large-scale energy storage method.

Why This Matters

Pumped hydroelectric storage is the dominant grid-scale energy storage technology in the modern world, accounting for more than 90% of installed storage capacity globally. The principle is elegantly simple: excess electrical energy pumps water uphill; when energy is needed, the water flows back down through a turbine. The storage medium is free, inexhaustible, and lasts indefinitely without degradation.

For a rebuilding civilization, this principle scales from the massive (mountain reservoirs) down to the practical (a rooftop water tank, a hillside cistern, or a millpond). A 10,000-liter tank elevated 30 meters stores approximately 800 Wh of potential energy — comparable to a small battery bank, using only water and gravity.

Unlike chemical batteries, elevated water storage has zero degradation. The same cistern that stores energy today stores energy 100 years from now with identical efficiency, assuming the structure remains intact. This multi-generational durability makes it uniquely valuable for permanent community infrastructure.

Energy Storage Calculation

Potential energy: E = mgh, where m = mass in kg, g = 9.81 m/s², h = height difference in meters.

For water: 1 liter = 1 kg, so E = V × 1000 × g × h joules (V in cubic meters).

Practical examples:

• 10,000 liters (10 m³) elevated 10 m: E = 10,000 × 9.81 × 10 = 981,000 J = 272 Wh
• 10,000 liters elevated 30 m: E = 2,943,000 J = 817 Wh
• 100,000 liters elevated 30 m: 8.17 kWh — comparable to a large lead-acid bank

Power output: Power = flow rate × head × efficiency × g. At 10 liters/second through 30 m head with 75% turbine/generator efficiency: P = 0.01 × 1000 × 30 × 9.81 × 0.75 = 2,207 W ≈ 2.2 kW.

The energy density is low (water is heavy and heights are limited), but the scale can be enormous and the infrastructure lasts indefinitely.

Site Selection and Reservoir Design

Height difference (head): Every additional meter of head increases storage capacity proportionally. Natural topography — hillsides, cliffs, elevated terrain — dramatically reduces the work of creating height difference. Identify the highest practical point within your available land.

Upper reservoir design:

• Earthen pond lined with clay or bentonite (impervious natural clay): lowest cost
• Concrete tank on elevated ground or platform: most compact
• Natural lake at elevation, dammed if needed

Lower reservoir or source:

• Natural stream, river, or lake works well — the pump draws from this source
• If no natural water source exists at the lower elevation, a second lined pond is needed

Seepage and liner: Upper reservoirs must minimize seepage losses. Compact clay layers 30+ cm thick provide adequate sealing for earth ponds. Line any sand or gravel subsoil with 3–5 overlapping layers of clay. Bentonite powder spread on the pond bottom swells on wetting to seal small voids.

Freeboard: Leave 1–2 meters of wall height above normal full level for wave action and overflow safety.

Pump Selection and Installation

During charging (filling): An electric pump driven by your generator or turbine lifts water to the upper reservoir.

Centrifugal pumps (most common): Work well for moderate heads and large volumes. Efficiency 60–75%. Easy to build — cast impeller, simple shaft seal. Cannot self-prime — must be primed initially.

Piston pumps (positive displacement): Better for high head, lower volume. Self-priming. Can be driven by wind or water mechanical power directly (no electricity needed for charging). Less efficient per unit power but more versatile.

Pump head requirement: Must exceed the static head (height) plus pipe friction losses. Calculate pipe friction using Hazen-Williams: for 100 m of 50 mm pipe at 2 liters/second, friction loss ≈ 4–6 m. Size pump for static head + 20% margin.

Check valve: Essential — prevents water from flowing back through the pump when it stops. Place at pump outlet. A simple flap valve (rubber over an opening) works well.

Turbine and Generator Integration

When generating: Water flows from upper to lower reservoir through a pipe (penstock), driving a turbine connected to a generator.

Pelton wheel (impulse turbine, best for high head/low flow): Works from 10–500+ m head. Jet of water strikes bucket-shaped buckets on the wheel rim. Easiest to fabricate — buckets can be cast iron or machined steel; wheel and nozzle from basic metalworking.

Crossflow (Banki-Michell) turbine (moderate head, moderate flow): Rectangular water jet crosses through the turbine twice. Excellent for 3–50 m heads. Can be fabricated from steel plate and standard pipe components.

Generator connection: Turbine shaft drives an AC generator or DC dynamo directly, with a speed-increasing belt/gear drive if turbine RPM is lower than generator design RPM.

Governing (speed control): As water flows through the turbine, speed varies with load. A simple mechanical governor (fly-balls on the turbine shaft) adjusts a valve to maintain constant flow and RPM. Without governing, voltage and frequency fluctuate with load.

System Efficiency

Pumped hydro round-trip efficiency: pump efficiency × turbine efficiency × generator efficiency. Typical: 0.70 × 0.75 × 0.85 = 0.45, or 45% overall. For every 2 units of energy used to pump water up, you recover approximately 1 unit on discharge.

This seems poor compared to lead-acid batteries (70–80% efficiency) but the storage medium (water) costs nothing, degrades nothing, and lasts indefinitely. Over a 50-year system lifetime, the lifecycle economics are unmatched.