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I’ve decided to start my primer on renewable power technology with one of the oldest forms of renewable energy: hydropower. Water wheels have been used for centuries to run mills, providing mechanical power directly to heavy machinery through rotating shafts.

The leap from mechanical power to hydroelectricity was a small one, once the relationship between electricity and magnetism was understood. It’s estimated that around 20% of the world’s electricity comes from hydroelectric generators. So how does a hydroelectric generator work?

Hydroelectricity simplified

In its simplest form, moving water turns a wheel. The wheel is connected by a shaft or belt to a generator, which is really just a fancy term for a loop of conductive wire in the field of a strong magnet. As the loop of wire rotates, the magnetic field pushes the electrons in the loop of wire first in one direction and then the other. And viola, you have alternating current! Easy right?

This is a dramatic simplification of the process, to be sure. Modern generators have a lot more copper wire than one simple loop, and there are usually multiple magnets with special housings to concentrate the magnetic field in the windings where it will have the strongest effect. There are also control loops with speed sensors, current and voltage detectors, and countless other gadgets designed to insure that the electricity coming out of the generator conforms to the standards of the local utility grid. This is all so that people like you and I can plug in our laptops and our televisions without fear of them blowing up.

Storage projects

As I said at the beginning of this post, we’ve been using hydropower for centuries. And we’ve come a long way from the water wheel. Most utility scale hydroelectric projects use dams to impound water in a reservoir, which is why they are sometimes called storage projects. Large scale hydro turbines are driven by the high head pressures built up behind these enormous dams. Note the representative size of the tiny human figure in the picture to the left. Hydroelectricity generators this size have capacities numbering in the hundreds of megawatts. They can be dispatched quickly in response to spikes in demand: just open the control gates to let water from the dam into the turbine. Since they store large volumes of water in their dams, their output is relatively insensitive to river level changes caused by storms or run-off. But they are sensitive to extended precipitation irregularities, like year long droughts.

Pumped storage projects

To understand the difference between traditional storage and pumped storage hydroelectric systems, you first have to know a little about how the electric utility grid functions. At any given time, there are energy producers and energy consumers attached to the grid. When your laptop is plugged into the outlet in your home, it is a consumer of electricity, or a load. When a generator is connected to the other end of the grid, it is (usually) a producer of electricity, or a source.

The goal of the electric utility companies that own and operate the grid is to keep producers and consumers in balance at all times. It’s a real juggling act. On hot summer afternoons when people come home from work and crank up their air conditioning, there is a spike in demand. The utility grid must be able to respond to this sudden surge by turning on more generators. Or they might buy energy from other regions that have more than they need. Likewise, when all the lights and appliances are turned off after everyone goes to bed, there is a drop in demand. Then the electric utilities have to idle the generators they brought online to respond to the surge. On top of these relatively predictable daily and seasonal variations in demand, there are also short duration spikes. They can be caused by sudden local weather changes, damage to transmission systems, fluctuation in generator production, you name it. It’s enough to have a utility manager pulling their hair out.

Though the cost of generating electricity doesn’t vary throughout the day, the price that an energy producer can sell their power for does. It goes up during peak demand and drops during low demand. This is what people are referring to when they talk about peak power, or peaking capacity plants. Since hydroelectric generators can be turned on or off fairly quickly in response to surges in demand, they make very good peaking plants. Hydroelectric power stations are arguably the most dispatchable of the renewable energy technologies available.

Pumped storage projects are specialized hydroelectric peaking plants. In addition to the standard reservoir above the turbines, a pumped storage project will have a second storage reservoir or a lake below the turbines. When electricity prices are low, like at night, electricity is taken from the grid to pump water from the lower reservoir into the upper one. When prices are high again, the upper reservoir drains into the lower reservoir through the turbines, supplying electricity back to the grid.

Of course you don’t get something for nothing. Pumped storage projects are net energy consumers: It takes more energy to pump the water uphill than you get from letting it flow back down again. But utility companies can use pumped storage projects to displace peak demand. And if you combine pumped storage with a non-dispatchable renewable energy source like wind, it can be a winning combination.

Run-of-river projects

At the other end of the spectrum you can find so called “run-of-river” hydro. Small scale installations, like the sketch at left from Electravent, don’t require construction of dams that restrict water flow. The capacities for systems like this are closer to a few hundred watts, enough to power household lights, a radio or a television. Since they don’t have the storage inherent in dammed hydro systems, their output is tied to how much water is flowing in the river at any given time. But for locations with reliable water levels, run of river hydro can make clean renewable electrification of remote locations very affordable. Run-of-river projects also avoid most of the environmental impacts that dams can have on river habitat, including water temperature, sediment flow, water temperature, dissolved oxygen levels, gas saturation. They tend to have a smaller greenhouse gas footprint as well, given the high greenhouse gas intensity of the concrete used in most large dams.

Between these two ends of the spectrum, you can find hydroelectric systems of all shapes and sizes. Water power had been around almost as long as the wheel, and looks like it will remain a part of our world energy solutions for a long time to come. What goes around, comes around, after all.

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