Water Energy
Water always flows downhill, and on this way it gives off energy. The trick is to capture it before it vanishes into the sea.
Hydropower is the most reliable of the renewable energies and at the same time the oldest that still supplies us with electricity today. It too is at its core disguised solar energy: the sun evaporates water from seas and lakes, it rises as a cloud and rains down over the mountains. From there it flows back into the valley, and precisely in this gradient lies the energy we use.
A bit of history
Even the Romans had water wheels drive mills and saws. Throughout the Middle Ages the water wheel on every stream was the power machine par excellence: it ground grain, hammered iron and drove bellows. When the electric generator was invented in the 19th century, it was natural to hook it up to water. In 1878 the first small hydropower plant lit a country house in England, and soon after the great dams arose that supplied whole regions with electricity.1
How it works and what forms there are
The principle is always the same: flowing or falling water turns a turbine that drives a generator. The greater the drop height and the more water, the more power.
In practice there are several forms. Run-of-river power plants lie in the river and use the steady current, they deliver electricity evenly around the clock. Storage power plants dam a lake behind a barrier and let the water shoot through the turbines on demand, which makes them flexibly controllable. Added to these are the tidal power plants on the coast, but their energy comes from the moon, not the sun, and therefore belongs in its own chapter.
Water as a battery: pumped storage
A particularly clever use is pumped storage. It does not primarily generate energy, it stores it. If there is too much electricity in the grid, for instance at midday under a blazing sun, the plant pumps water into a higher-lying basin. When electricity later becomes scarce, it is let down again through the turbines. Thus surplus electricity turns into dammed-up height and back, with astonishingly little loss.
That makes pumped storage one of the most important answers to the great problem of sun and wind: it stores the energy for the hours in which it is dark or windless. So far they are by far the largest electricity stores we have, far larger than any battery.
Advantages
Hydropower is reliable, long-lived and in operation free of CO₂. A well-built hydropower plant runs for over a hundred years. Unlike sun and wind, it can be planned and controlled: the reservoir is a filled store of energy that you open when you need it. In countries like Norway or Austria it carries the largest part of the electricity supply.
Disadvantages
The price for this is often a deep intervention in nature. A large dam floods whole valleys, drives people from their villages and cuts the river apart, so that fish can no longer migrate and fertile silt no longer reaches the lowlands. The Chinese Three Gorges Dam, for example, forced over a million people to move. Such projects show: "renewable" does not automatically mean "harmless."
Added to this is a plain limit: the good sites are in many countries long since built up. You cannot create new rivers and mountains at will, and climate change is making some rivers dry up increasingly in summer.
The critical look
Hydropower is a prime case of the fact that there is no such thing as the one clean energy. A small run-of-river plant that respects a river is something quite different from a mega-dam that sacrifices an ecosystem and the home of thousands. Both trade under the same green label.
Whoever looks closely therefore asks about the concrete case: what is being flooded here, and who is asked? Strikingly often, the burdens of such large projects are borne by the poorest and most powerless, while the electricity flows to distant cities and factories. Energy and justice are connected here too. The water flows down to the valley by itself; who profits from it and who has to give way for it is, by contrast, a thoroughly human decision.