Watch a shallow puddle on a sunny day, and it can disappear right before your eyes. Sunlight hitting the surface gives the water molecules a little burst of energy – enough for some of them to break the bonds with their neighbors – and they venture forth in a new, gaseous form. About a quarter of all solar energy that hits the Earth is used up giving water molecules that little bump, mainly in the oceans. That means water vapor in the atmosphere represents an enormous store of converted solar energy.
Almost all of that energy is released into the air, mainly as heat, as the water condenses into clouds. But a tiny fraction remains. Clouds eventually fall as rain (or snow), of course, watering our crops, ruining our picnics, filling our lakes and swelling our rivers. The weight of that water is potential energy. Sounds all very abstract – evaporating water, molecules being lifted into the sky, converted solar and potential energy. How does it all add up to power?
The Hoover Dam, an immense structure built in the 1930’s – partly as a cure for the last big economic hangover – stand less than an hour’s drive from the razzle-dazzle that is Las Vegas. Straddling the Colorado River on the Arizona-Nevada border, the plaques that adorn it stand as testament to a time when engineers – not bankers – were almost heroic figures in the economy. Peering over and down the slowly curving 726-foot-high (220-metre) concrete wall on top of the dam, you can’t help but feel a sense of awe – and vertigo. Far below, powerful currents come and go, as gates controlling the flow of water open and close.
The Hoover Dam’s job? To capture a few of those previously evaporated water molecules as they make their way back to the ocean, interrupting their flow just long enough to convert some of their movement into electricity. Built at a time when engineers used slide-rules, not computers, and a pencil and paper stood in for design software, the dam continues to produce enough power for more than two million homes. Those evaporated molecules sure can add up.
Hydropower has been used for thousands of years, starting with powered irrigation systems, as well as for milling and other industrial processes. Waterwheels appear in ancient Middle Eastern texts and were also used by the Roman Empire, and in ancient China and Greece. The United Kingdom alone once had more than 20,000 waterwheels turning in streams and rivers. As time went by, hydropower began to be used almost exclusively to produce electricity, and by the beginning of the 20th century, hydro was responsible for the bulk of electrical production. But as fossil fuels and then nuclear came online, its portion dropped to what it is today – somewhere around 20%.
The Elora Mill, a quaint inn near Toronto, Canada, went through that evolution, as did countless other mills that used to dot our landscape. The mill was built in the 19th century, on the banks of the Grand River, and was one of the area’s great industrial grist mills. As the world changed, so too did the Elora Mill, and that same water flow now produces electric power, both for the Inn and – when the lights are low – to sell back to the local grid. Inside the mill’s bar, patrons can watch a little power meter on the wall that proudly displays the excess power pumped back to the grid.
Hydro is now the modern workhorse of renewable energy, responsible for 90% of worldwide renewable energy production. Waterwheels have been replaced by highly efficient turbines, and massive dams are built to store and generate quantities of power so vast, they make a nuclear plant look small by comparison. Pumped hydro, where water is pumped uphill to large reservoirs for later use, is the only currently viable option for utility-scale grid storage. Without hydropower, our energy landscape would look very different.
Today, it’s the big and bold (and sometimes famous) sites that produce the lion’s share of hydroelectricity. Some places – like Niagara Falls and Churchill Falls in Labrador, Canada – take advantage of the same landscape that creates such dramatic scenes of falling water. Other places create artificial falls by building a dam, like the Grand Coulee dam in Washington State and the controversial Three Gorges Dam in China.
China has, by far, the most installed hydroelectric capacity and is responsible for four of the five largest projects under construction, followed by Brazil, Canada and the US. Canada gets more than half of its electrical power from hydro, and Norway – though it has relatively small production levels – generates almost all of its electrical power from hydro. The upside of hydropower is well known. Once the plant has been built, you have a reliable, clean and cheap source of power.
It’s uncertain how much more energy could be produced by these sorts of massive projects. There is certainly more energy to be had – it’s estimated that only one-fifth of the hydropower that is technically feasible has been harnessed but big dams make big lakes, displace people and flood what might otherwise be scenes of great natural beauty. Public opposition to large-scale hydro will probably be the strongest constraint on growth.
Expansion of hydro energy will likely happen with smaller and more subtle projects that don’t change a river’s flow. “Run-of-river” hydro is a way of generating power while following the natural flow of a river – no flooding required. There’s also a more far out technology on the horizon, where a well-controlled meeting of river and sea can yield some surprising results.
Most visitors to Niagara Falls have no idea that more than half of the mighty Niagara River’s flow never makes it over the dramatic cliffs that are the Falls. Instead, it’s diverted from upriver, flowing underground to power plants on both the American and Canadian sides of the border. At night, when the tourists aren’t watching, up to three-quarters of the river’s flow is diverted for power production.
The Americans were the first to produce hydropower from the Niagara River, back in 1881. By 1896, that power was being transmitted as far as Buffalo, New York. Canadians soon followed suit, building the Sir Adam Beck I generating plant in 1922, with a second, Beck II, operational by 1954. Today, a giant underground boring machine – nicknamed Big Becky – is drilling a new tunnel under the city that will be more than six miles (10 kilometres) long and 46 feet (14 metres) wide, and will feed more water to Beck II. Together, the two sides produce enough power for four million homes, and both have reservoirs for storage. That means they can pump water uphill when power demand is low, store it and release it when it’s high.
How does it work? The basic idea behind all hydropower is quite simple: The weight of a column of water builds up pressure at the bottom (that’s why your ears hurt when you dive to the bottom of a pool). That pressure is exerted on a turbine blade, 2 which spins a generator to make electricity. The potential energy of the water – its mass that can fall – converts to kinetic energy as it is captured by the turbine wheel and passed to the generator.
Modern turbines are very efficient, turning up to 95% of the water’s potential energy into movement, so the water flows out very slowly. The rate of spin of the turbine is tuned to meet the rate of alternating current on the grid.3 Total power out depends on how much water flows and from what height (called the “head”). The size of the reservoir – really an artificial lake – indicates how much power sits in storage.
Like Niagara Falls, many stations can pump water uphill into the reservoir when electrical demand is low. “Hydro storage” is the only grid-sized storage method in commercial use today. Since grids are large and interconnected, power from anywhere on the grid can be used to pump the water. There’s no reason the electrons from a wind farm hundreds of miles away can’t do the work!
Niagara Falls and a few other spots like it are ideal for hydropower. What’s needed is a lot of two things: flowing water and height. A natural waterfall clearly has both. When there is no natural head, or height from which the water can fall, one can be created by building a dam.
The biggest hydroelectric plant in the world is the enormous Three Gorges Dam, on China’s Yangtze River. Though it’s already generating some electricity, when it’s fully complete it will be able to deliver an astounding 22.5 gigawatts. The dam is more than 600 feet (185 metres) high and will serve 26 generators. To put that in perspective, it has 22 times the power of a medium-sized coal or nuclear plant. Most of the big hydro projects under construction are in China.
How does it work? As a kid, I used to build a dam on the creek that ran through our farm. When I was done, the water would rise on the upstream side of the dam. The same idea applies to large hydro: Dam up a river and let the water rise on one side. The result is a giant artificial lake or reservoir, which stores energy. The height from which the water can now fall is measured from the top of that lake. The rest of the story is the same: Spin a turbine4 and generate electricity.
In the case of the Hoover Dam, a technical marvel of its time, the reservoir is the lovely Lake Mead, which stores energy and is enjoyed for recreation at the same time. Hoover Dam takes advantage of the surrounding natural geography, and the flooded area is a high, narrow canyon. This means that as the water rose, the area of land that was flooded was relatively small.
But many of these dams have a downside. The reservoirs are not always a center of watery recreation in the middle of a desert. In the case of the Three Gorges Dam, more than 400 square miles (1,000 square kilometres5) were flooded in order to generate a good-sized “head” and build a large reservoir of stored power. Slowly and inexorably, as the waters rose, whole villages, forests and even archaeological sites were flooded. Three Gorges has displaced about 1.25 million people. That’s a lot of new homes to be built – not to mention a lot of human misery. And that’s not the only downside – all those drowned trees will eventually rot, releasing methane, a potent greenhouse gas.
A few summers ago, while cycling up my favorite hill in Newfoundland – it climbs away from the Atlantic Ocean and through the town of Petty Harbour – I saw a curious sight: an enormous, old wooden pipe that followed the road down to the harbor. The pipe, which was taller than me, sprayed water from small leaks that had formed between the slats comprising the exterior shell. It looked like a long, leaky barrel. Climbing the same hill the next summer, I saw that it had been replaced by a modern steel version. It turns out that pipe carries water from a river at the top of the road and delivers it, under pressure, to a small generating station at the bottom. I didn’t realize it then, but that leaky wooden pipe was part of an old “run-of-river” power station.
Run-of-river hydro roughly means relying on a river’s natural flow to generate power, without disrupting it with large dams and reservoirs.5 The central point is to minimize the impact on the local ecosystem and to avoid the creation of a large, artificial lake.
How does it work? There are two ways to do it. The first is to dam the river, forcing all of the water to flow through turbines – but at low speeds and without building up the level of the river. In this case, the turbines are usually more propeller-shaped,6 to be efficient at the lower flow speeds. The second way is to build a pipe that diverts some of the river’s water downstream beside the river to a turbine. Because the water is trapped in the pipe – like a giant straw blocked at the bottom – the same pressure is built up at the turbine just as if it were a vertical drop. The pipe creates the head in the same way a vertical dam does – minus the dam.
As you might expect, the system isn’t perfect. Run-of-river projects are generally thought to have minimal impact on the environment, but it’s not zero. Roads and power lines sometimes need to be built if the river is in a wilderness area, and the amount of diverted water can be large enough to change the river’s natural ecosystem. These are valid concerns. British Columbia recently opened up huge wilderness areas for private run-of-river development, and debate about ecological impact is fierce. Also, since run-of-river projects don’t build any significant storage capacity in a reservoir, they’re completely reliant on natural precipitation patterns. Droughts mean blackouts!
Finer points aside, there’s no doubt that run-of-river, developed with care, has a role in our renewable energy future. There are thousands of potential sites, ranging from the small off-the-grid building beside a little creek, to commercial scale projects churning out renewable energy along the world’s major rivers.
All the hydropower we’ve seen so far is based on the potential energy water gains when it’s evaporated by the sun and carried into the sky. Water leaves the salty oceans and flows back to the sea as freshwater rivers. Now there’s another way to use the stored-up solar energy in all that evaporated water – this time by exploiting the chemical difference between freshwater and saltwater. There are two competing visions of how to do it.
The Norwegian power company Statkraft is building a small saltwater generator on a fjord near Oslo. Freshwater flows into one chamber and saltwater into another; the end result is enough electric current to power several homes. This pilot project generates only four kilowatts, but the hope is that it’s the first step toward building a 25-megawatt power plant enough for 2,500 homes – by 2015. The only exhaust is a briny mix of fresh and saltwater, which can be dumped into the sea.
How does it work? Osmosis is the principle that freshwater will spontaneously flow into saltwater if the two are separated by a special membrane. Osmosis in reverse is how we squeeze drinking water from seawater in desalination plants. Osmosis raises the pressure of the saltwater section as the freshwater flows in, and that increased pressure can be used to spin a turbine.
A laboratory-sized saltwater battery that employs a technique dubbed “Blue Energy” has sparked a competing technology that could give Statkraft a run for its money. Creator Westus has teamed up with a company called Reds tack to take that coffee-mug-sized contraption and build a pilot project in the Netherlands on the same scale as Statkraft’s. Salty wastewater from a salt mine is pumped into one pipe, freshwater from a local river is pumped into another, and the result is electrical current. This time, no turbine is needed, since the current is created directly – like a battery.
How does it work? Saltwater contains lots of charged particles; freshwater does not. When salt, or sodium chloride, is dissolved in water, it splits into positively charged sodium ions and negatively charged chloride ions. Freshwater and saltwater flows into a series of chambers, which are separated by membranes that allow only one type of ion to pass through. The membranes allow the different ions to separate, generating a current. Positive ions flow one way and negative ones flow the other, resulting in a saltwater battery.
That’s fine for pilot projects, but how well will this idea scale to commercial-sized operations? Technical problems remain – like how to keep the membranes from getting clogged with silt. Good engineers can figure that out. A deeper question is: How much energy can be generated by our river systems without wrecking the rivers? The Blue Energy folks estimate that half the river flows of the world could generate 7% of world energy needs. That’s pretty significant. Statkraft’s technology isn’t quite as efficient – a similar amount of diverted water would meet only 1%, of the planet’s energy needs.
That seems like a lot of diverted water, but it’s diverted at the mouth of the river, then dumped into the sea. That’s where it was headed anyway, so running it through some big batteries on the way doesn’t seem so bad.
There’s no question that both large and small-scale hydro are viable (we’ll forget about the “river meets the sea” stuff for now). Hydro is the single largest and most reliable source of renewable energy, rivaling coal and nuclear for sheer scale of production. Hydro not only provides vast quantities of power, but that power is also responsive – which means production can be ramped up (or down) very quickly to react to grid demands. Hydro proved its worth long before carbon was an issue. How big hydro grows from here is a matter of debate. There’s a tradeoff between protection of land and mitigation of carbon.
Concerns about projects like the Three Gorges Dam and its effects on both people and the landscape are valid. The criticism that rotting, flooded forests produce methane can be addressed with smart forestry practices prior to flooding, and even deepwater forestry afterward. Other concerns aren’t so easy to address. Flooding from the Three Gorges has displaced entire villages, and a lot of beautiful land, valuable archaeology and ecosystems have disappeared.
But if we’re going to ask China to stop building coal plants – and we plan to do the same in the west – we must find other ways of producing power. The Three Gorges Dam will prevent more than 100 million tons of carbon dioxide from entering the atmosphere7 – that’s almost twice the emissions of all8 the cars on Canada’s roads. These projects are so massive, and displace so much fossil fuel, that it’s hard to argue, given intelligent management, that there’s no net benefit.
Run-of-river projects are not ecologically neutral, although some come pretty darn close. They can affect fish populations, sometimes require transmission lines and roads to be built in what are often pristine wilderness areas, and they do alter the natural flow of the river, at least to some extent. What we need is balance. Not all run-of-river projects bring severe ecological interference. That pipe flowing along my favorite cycling hill in Newfoundland hardly seems problematic.
Hydro is a present-day, viable and large-scale renewable alternative to coal. Given what’s at stake if carbon levels get too high, all else being equal, large-scale hydro should proceed as fast as we can put up the dams.
There is, however, another problem that could be very hard to deal with. If precipitation patterns change as predicted, there may not be much in the way of flowing water to keep those plants going. But our eggs are all in one basket anyway – we need to act to stop that scenario from happening, regardless-so that hardly seems a reason not to proceed apace.
The potential? The world has already harnessed one-fifth of what’s technically feasible, and it’s reasonable to think we could double that figure, using both large-scale hydro and run-of-river. If electrical consumption were to remain constant, that would raise hydro’s share to one-third. But since consumption will double-at least-by 2050, hydro’s share would remain the same, at about one-sixth.