One of the nice things about energy is that there’s many, many different forms of it. There’s many, many different ways to convert between those forms.
Yesterday we discussed the problem of turning the energy in sunlight into electricity via the circuitous route of sunlight to electricity (in a space solar panel) to microwave beamed down to earth, to electricity in a field of antennas. The advantages are continuous operation and high collection efficiency, and of course the disadvantage is the truly head-spinning cost.
Solar panels on the earth’s surface are also expensive, not always very environmentally friendly to make, and rather inefficient. They also have the problem of not operating when light is unavailable in sufficient quantity, as at night or during poor weather. There’s no way around those – either the solar energy is there or it isn’t. But there are at least a few ways around the first set of problems. The solution is to turn sunlight into electricity by a different route. One such route is so-called solar thermal energy. Instead of taking the direct light -> electricity route, it does what most current power plants do and turns the initial source of energy into heat, which is then turned into mechanical energy, which is then turned into electricity. It’s a more complicated route, but it’s also a cheaper and easier route because you can skip the expensive and fragile panels entirely. You simply set up mirrors:
The mirrors focus light at the engine, and the engine converts the thermal energy into mechanical work, which turns a generator and produces electricity. Now it’s not actually possible to take something hot and get energy out of it without having somewhere cooler to move the heat. It’s actually differences in temperature that can be made to produce work. The maximum theoretical efficiency of this process is call the Carnot limit, and it’s given by
Where the hot and cold temperatures are given in Kelvin. To increase the efficiency, you want to get that engine as hot as possible. The nice thing about this scheme is that its simplicity and lack of expense. Mirrors and engines are much less of a hassle than solar panels. The disadvantage is that even worse than solar panels the efficiency collapses when the sunlight (and thus the temperature) drops. But in particularly sunny areas this could turn out to be a nice way to get energy at economically competitive costs. Indeed in many places it already is.