Solar Energy Part 5
Now, we will move away from the sciences of physics and chemistry, and onto the pure engineering that you can even do at home yourself on a small scale. The fascinating world of lenses and mirrors, used to capture the Sun's heat. This is really my favourite form of solar energy work!
The fact is that industry often doesn't require electricity. Many industries run quite happily with heat alone. Of course, that may be in the form of steam, hot water or as in a furnace. Oil or gas generally fuels these processes, but solar could do the job equally well in many cases. Therefore, solar energy not only provides a means toward cheaper production, but also saves our precious oil and gas reserves. The environmentalists are often satisfied as well.
In this introduction, we will merely touch the practical side - that will come later. Here, we will just look at what is possible and at some of the drawbacks hindering us in the present state of this technology. Huh! What technology? Right! To most scientists, this isn't even considered high tech stuff at all, it's get-your-hands-dirty engineering and they want no part of it! It is schoolboy physics and no self-respecting Ph.D would stoop so low! That sums up the major problem - scientists don't take it seriously.
Anyway, we over-aged schoolboys know what it is all about. Use a magnifying glass to focus the Sun's rays to a pinpoint and it's bloody hot! For the technically minded, it can be as high as 4000 degrees Celsius even from a modestly sized hand lens. Every boy-scout knows that! But some knowledge of practical science is still required. For example, how long would it take to heat a glass of water up to boiling point? The answer may surprise you - it is not possible using a hand lens! When we measure temperature, it is certainly high but the energy content is very small, much too small to compensate the heat losses that occur in the glass of water.
Going up in size, a solar oven, which is essentially a well insulated box with a glass lid, can heat water upto about 90 degrees Celsius. The absolute maximum in my experiments was 98 degrees. That's close to boiling point, isn't it? Actually no - it's very far off! We have to remember the latent heat of vaporization. Water at 100 degrees needs a huge amount of additional energy to convert it into steam at 100 degrees. But a solar oven doesn't utilize magnification of the Sun's rays. I used the word magnification since we were previously talking about magnifying glasses - of course, the correct term should be concentration. Still, 90 degrees is more than adequate for many industrial processes, not to mention our home heating and laundry requirements. There are some problems - you might have to take a bath in the evening rather than early morning, but it would be free hot water!
In order to use a magnifying glass effectively, it has to be much, much bigger. So big in fact that its sheer weight makes it impractical to use (fresnel lenses are an expensive alternative). So mirrors are used instead. These may be curved, but more commonly are faceted (lots of flat mirrors), to bend the light in much the same way that a lens does. How big? Well, in Northern Europe you might get one kilowatt of energy from a square meter of mirror. In the Middle East, 2 or perhaps 3 kilowatts per square meter are possible. Ho! I just said light - aren't we talking about heat? Yes, that is a confusing area indeed!
So, roughly speaking, one square meter of mirror can be thought of as being equivalent to an electric kettle? Probably not! It depends on how you transfer the heat into the water - the thermal conduction of the materials used, the convection currents in the water, the insulation, and so forth. What we can say is that a solar kettle would not look like an electric kettle, its design would be quite different.
Move up in size again. Can we realistically make a steam boiler? Yes, why not! The engineering isn't so complicated although it may surprise some conventional boiler engineers. Instead of insulation covered steel pipes, they may find solid aluminium covered steel pipes! Stainless steel is very good at transfering heat, whereas aluminium is good at storing heat for those times when the Sun may be hidden by clouds for example. If we make a steam boiler, then we can make a conventional steam turbine generating set for our electricity requirements. Why are we bothering with solar cells?
Fact or fiction? Yes, it is fact. A 10 Megawatt experimental solar power station was constructed and it exceeded all expectations by producing 22 Megawatts. What is more, such plants can be designed to store energy in daytime in order to produce power 24 hours a day. No longer does darkness put a stop to solar energy utilization.
So far so good. What are the drawbacks? Number one is size. More practically, how much land do you need and what would be its cost. Even the desert is owned by someone and it isn't unusual to find the price of the land shooting up when a use is found for it. Another drawback is keeping the whole thing clean, as dust and dirt would have a major effect on its working. Finally, the most obvious problem is that the Sun moves! Moving hundreds or perhaps thousands of huge mirrors to track the sun isn't easy.
Up to now, we have been very conventional. Why use water/steam? There are many other chemicals, which boil at lower temperatures, and have more convenient latent heat of vaporization figures. This is the principle behind much research into "vapour turbines". These aren't really new because, afterall, steam is a vapour too. Another oldie revived recently is the stirling engine which is growing in popularity again. In fact there are a large variety of heat engines awaiting their chance of revival.
Before we end this introduction, there is one more item to consider - efficiency. Since you will see this term quoted everywhere on solar sites, we had better understand something about it.