Energy and water
Generating electricity takes water, and moving and treating water takes electricity. The two systems constrain each other in ways that only become visible in a heatwave.
Thermal power generation is the largest industrial water withdrawer in most developed economies, and the water sector is frequently a municipality’s largest single electricity consumer. Each depends on the other, and both are stressed by the same conditions.
Water for energy
A thermal power station — coal, gas, nuclear or biomass — converts heat into electricity and must reject the waste heat somewhere. That somewhere is almost always water.
| Row | Once-through | Recirculating tower | Dry cooling |
|---|---|---|---|
| Withdrawal | Very high | Roughly 2% of once-through | Negligible |
| Consumption | About 1% of withdrawal | About 70–80% of withdrawal | Negligible |
| Discharge | Nearly all, several degrees warmer | Small blowdown stream, concentrated | None |
| Main constraint | Thermal discharge limits in hot weather | Water availability | Efficiency and capital cost penalty |
| Ecological impact | Entrainment of organisms; thermal plume | Lower, but consumes more | Minimal |
When it binds
In a heatwave, river water is warmer and river flow is lower. Environmental permits limit how warm a discharge may be and by how much it may raise the receiving water. Both constraints tighten at exactly the moment electricity demand peaks for cooling.
France has curtailed nuclear output during several summers for precisely this reason. It is the clearest demonstration that water availability is an energy security question, not only an environmental one.
Energy for water
| Process | Energy (kWh/m³) |
|---|---|
| Conventional surface water treatment | 0.2 – 0.4 |
| Groundwater abstraction and treatment | 0.3 – 0.6 |
| Distribution pumping | 0.2 – 0.5 |
| Wastewater treatment, activated sludge | 0.3 – 0.6 |
| Brackish water reverse osmosis | 0.5 – 1.5 |
| Water reuse to potable standard | 1.0 – 2.5 |
| Seawater reverse osmosis | 3.0 – 4.0 |
| Long-distance transfer, per 100 m of lift | ~0.3 |
Indicative ranges. Lift dominates in hilly terrain, and a long uphill transfer can use more energy than desalinating the water in the first place.
The last row is the one most often overlooked. Desalination produces water at the coast; getting it inland and uphill can cost more energy than producing it. That is why coastal cities desalinate and inland ones generally do not.
Sources
- International Energy Agency — Water–energy nexus analysis. Public but restricted · Publisher terms — public access, reuse not clearly granted
- United States Geological Survey — Estimated Use of Water in the United States. Open · US Government work — public domain
- International Renewable Energy Agency / academic literature — Desalination capacity and energy intensity assessments. Open — attribution required · CC BY 4.0