“Renewable energy” gets said so often that the words have all but lost their meaning. Like the word “sustainable” or “globalization, “renewable energy” tends to go in one ear and right out the other. But concern is rising over the environmental impact of fossil fuels, their supply, and their unpredictable prices.
The geopolitics of energy are far beyond the scope of this post. This post is just an overview of the renewable energy resources–the ones that are already deployed and the ones in development.
Of all the renewable resources available to us, solar is one of the most promising. After all, we’ll always have the sun. And, under ideal conditions, a single square meter of Earth’s surface receives 1,000 Watts of sunlight on average. That’s enough to run a computer or two, or a kitchen appliance. And that’s falling on every square meter.
The trouble is that harvesting solar energy requires harvesting light, and light is hard to hold onto. But there are lots of interesting technologies in the works.
For me, solar power conjures up images of fields of blue-black solar panels. These are photovoltaic panels: sheets of specially-prepared silicon that produce usable current when light shines on them. This is the best-developed solar technology, growing in step with the semiconductor industry.
But photovoltaic panels are only one type of solar generator. Solar-thermal plants use mirrors to focus sunlight on heat exchangers, boiling water and driving steam turbines.
There are downsides. (Aren’t there always?) Solar cells are fragile. Their efficiency falls if they get dirty. As for solar-thermal plants, their production drops off dramatically if it’s cloudy. And neither type can produce power at night.
This last issue is bigger than it sounds. These days, the power grid never switches off, so in order to compete, solar power requires an efficient way to store and release large quantities of energy. We’ll cover that a little later.
Hydroelectric power might be humanity’s oldest means for generating mechanical power. Water wheels go back over two thousand years ago, to a time when they were used to power water pumps, ore crushers, and grain mills. Electric generators driven by hydroelectric turbines date back to the 1880s and 1890s–the dawn of modern electricity.
Hydropower has a lot of attractive features. Nature pumps the water, in the form of rivers. All humans need to do is build a dam and a turbine. It helps that, for historical reasons, the largest and most power-hungry cities tend to be located near rivers.
But the need to build a dam and create a reservoir is hydropower’s main disadvantage. Dams cause flooding upstream, and dams and reservoirs take up a lot of space, and disrupt ecosystems.
Windmills have been found dating back at least a thousand years. Like water wheels, they were first used to grind grain and pump water. And, like water wheels, their use as power generators dates back to the dawn of electricity.
Wind power is attractive for a lot of the same reasons as hydro and solar: all that’s needed is a decent collector. But wind, like sun, isn’t constant, so energy storage is still a necessity for wind farms to compete. And modern windmills are big and need a lot of space.
I started the last two sections talking about how old wind and water power are. Biofuel, in the form of fire, is much older. Evidence of fire’s use by humans goes back no less than forty thousand years, and we might have been using it as far back as a million. The use of fire for industry dates back five thousand years, when it was used to smelt ores and fire pottery.
Of course, when people hear “biofuel,” they don’t usually think of kilns. They think of plant-derived fuels like ethanol, methanol, and biodiesel. But whether it’s one of these, wood, wood-gas, or something else, biofuels have the same set of advantages and disadvantages.
One of biofuel’s most attractive advantages is that it’s carbon-neutral. All the energy-storing carbon compounds in plants are built using carbon dioxide pulled from the air. When biofuel is burned, it can, at worst, return that same amount of CO2.
But biofuel isn’t quite as straightforward to produce as traditional fuels like diesel and gasoline. The latter can be distilled straight from crude oil. The latter have to be fermented from feedstock. And while fermentation is hardly a new technology, scaling it up to meet the world’s fuel demands has proved a challenge. On top of that, since it’s plant-based, biofuel feedstock either has to be diverted from the existing supply of crops like corn, soy, and canola, or grown on fertile land, which is a precious resource.
The earth’s interior is warm. Some of this heat is left over from the Earth’s formation. The rest is produced by the decay of radioactive elements in the mantle. In the upper crust, the temperature of the rock rises about 72 degrees Fahrenheit for every mile of depth, on average. By extracting naturally-heated groundwater, this heat can be tapped and either used to heat houses, or to boil water and drive turbine generators.
We can go a step further than that. There are proposals to produce artificial geothermal wells by injecting high-pressure water into bedrock, creating mazes of fractures that turn the rock into a natural heat exchanger. Of course, creating geothermal wells this way has the same issues as hydraulic fracturing (fracking) for petroleum: the fracturing fluid can contaminate groundwater, and disrupting bedrock can trigger seismic activity.
A major confounding issue is that many renewable energy sources are intermittent. The wind isn’t always blowing. The sun isn’t always shining. And even with steadier energy sources like hydroelectric or geothermal, it’s not always easy to step up production to keep up with surges in demand.
A big requirement for making renewable energy competitive is developing a decent way to store it in large quantities so that it can be released on demand, even when the elements aren’t cooperating.
Batteries are the most obvious option. Lead-acid batteries are mechanically sturdy and electrically robust, tolerating a fair bit of punishment (which is why they’re used in cars). Their cousins, nickel-iron batteries, are even tougher, and can tolerate outright abuse without losing capacity. But both types require corrosive electrolytes, and don’t store very much energy for their weight, compared to other battery types.
Lithium-ion batteries have a much higher energy density, but they also have to be treated a lot more gently. Traditional lithium batteries can fail quite dramatically if they’re mechanically damaged or electrically abused (sometimes dramatically enough to cause a fire or explosion). On top of that, lithium is a relatively scarce and expensive metal. Even so, the energy density of lithium-ion cells is so high that some companies are commercializing them for large-scale energy storage anyway. Plus, far more durable technologies like lithium-ceramic batteries might solve the problem altogether. But there are other interesting options in the works.
The energy input required to move a mass against gravity is substantial. Raising a 2,000-pound concrete block 500 feet (the height of a medium skyscraper) consumes 1.356 million Joules (0.375 kilowatt-hours). That energy isn’t lost–it’s stored as gravitational potential energy. A system like this is already in development. It uses a sloping railroad track, along which railroad cars carry enormous concrete weights. The railcars are driven up the slope by electric motors, storing energy in the elevation of the weights. When that energy is needed, the railcars are allowed to roll downhill with the weights, using their motors as generators. A similar proposal uses computer-operated cranes to stack concrete weights.
A related concept is already in widespread use for storing surplus energy. Rather than using weighted railcars, this system works a lot like a traditional hydroelectric dam. But in this case, the reservoir is filled not by a river, but by a turbine which pumps water into it. That stores potential energy, and when energy is needed, the water is allowed to flow backwards through the turbine, retrieving the energy. (Well, most of it.)
Yet another technology in the same vein describes a floating platform (essentially a large, square zeppelin) covered with solar panels, which proposes using cables to haul smaller weights to an altitude of almost 12.5 miles.
Compressed air offers another possible means of storing energy. It’s already used in some data centers, where maintaining customer data requires instant power in an outage, until a traditional generator can start. Larger-scale plans propose pumping air into abandoned mines, limestone caverns, or air-bags submerged on the seafloor, and storing the energy that way.
Another technology uses large, fast-rotating flywheels, housed inside vacuum tanks and suspended by magnetic bearings to reduce friction. A spinning flywheel can store a surprising amount of energy, can hold it for a long time, and can deliver it almost instantly when it’s needed.
There are other options, of course, but in the interest of brevity, I won’t cover them here.
The human population is growing scarily fast. Our hunger for energy is growing even faster. The Earth’s deposits of coal, oil, natural gas, and nuclear fuel are finite. Environmental concerns are going to continue to drive interest toward renewable alternatives. There are certainly plenty available. They all have their own sets of issues to overcome, but even so, renewable power stands poised to take over a substantial portion of the energy market. With energy-storage technology finally catching up, there may soon come a time when we can provide secure, reliable, locally-produced power to everybody, with minimal environmental impact.