Solar Energy Part 3
You probably haven't heard much about heat to electric devices because they are not primarily in the field of solar energy. There is so much waste heat available in conventional generation processes that their use in solar applications is almost forgotten. It is true to say that this field of study is generating plenty of excitement even among governmental departments. It is estimated that an additional 30 per cent of energy could be extracted from our present power plants if the waste heat could be utilized.
When one studies the waste heat generated in our cars, the results are particularly surprising. A modestly sized family car actually uses around 300 watts of electrical power, whereas it could generate 10 Kilowatts without placing an additional load on the engine - in fact the load would be less since the alternator could be removed from the engine. 10 kilowatts is enough power for at least two houses!
Much of the excitement is due to the fact that this is an area of proven science, there also being numerous ways to extract this heat, many of which do not require sophisticated equipment. However, for such proven science, there still remains the problem that we can't go to our local store and purchase such things yet!
Seebeck and Peltier
First, we should look at something that is actually available - the Peltier Heatpump Module. Also known as peltier coolers. These are quite commonly available as cooling devices or miniature refrigerators. I'll leave it to you the reader to look up how these work. I would just point out one aspect - Mr. Seebeck actually was working on single materials, and not pairs of materials as was Mr. Peltier. Nontheless, it can be said that the Seebeck effect is essentially the reverse of the Peltier effect, and vice versa.
The manufacturers of these heatpump modules say that they work both ways - producing heat or cold, or generating electrical power. That isn't quite correct. They are much better as heat producers than they are as generators. Even using a module that is specifically designed for generation purposes, an efficiency of barely 4 per cent is achieved. Now, efficiency figures in the field of solar energy are usually taken with a pinch of salt, or at least with a crocked smile on the face! But this efficiency figure is REAL! Simply put, if you want to achieve 4 watts electrical output, then 100 watts of heat energy must pass through the device! Any engineer will tell you that heating something up is easy, cooling something down is damn hard. With these devices you must do both, and constantly! It can therefore be seen that the device isn't efficient enough and the heat flow problems make it non practical for common usage. But this only applies to the commonly available devices. The principles work and we CAN make much more efficient devices. We can expect to see much better devices becoming available soon. The new devices will withstand much higher temperatures and will be constructed from much cheaper and safer materials (the present modules are highly toxic!).
Thermionic Emission
Another exciting area is that of Thermionic Emission. Old proven electronic stuff, as in vacuum tubes (valves)! Think how a simple vacuum tube diode worked. A filament heated a cathode which then emitted electrons that were attracted to a positive anode, thus forming a flow of current. Simple enough, but what if we heated the cathode by some other means (like solar energy). Yes it works! The new thermionic devices will be much smaller than the old tubes, the gap between cathode and anode being only a fraction of a millimeter. Designs for solid state (no vacuum needed) devices are available and, perhaps the ultimate, a design that needs no heating (room temperature operation) has been proposed. Unlike solar cells that produce both voltage and current, these thermionic designs require an external voltage to be applied. Now that is in fact a distinct advantage since we can choose the voltage and stabilize it. By introducing a grid element into the device, we can also produce AC type waveforms superimposed on the DC voltage, meaning we can transform the voltages up or down as required. The devices have the potential ability to produce thousands of Amperes of current. All in all, these modern vacuum tubes could provide the answer to many of the problems normally associated with solar energy devices.
Natural Chemistry>
It may seem odd to many that nature, that is so clearly Carbon based, hasn't given us a simple method of energy conversion. Afterall, photosynthesis in plants is a highly complicated process that we do not fully understand yet, and are years away from replicating. But perhaps nature has given us just what we need, if we look in the right places! Carbon, in its Graphite form, is the best known material for converting light into heat. Furthermore, its layer type structure is unique. Almost any other element can be inserted between the layers of Graphite. It is becoming obvious that this material can be manipulated by man into whatever chemical form he desires - it is the ultimate semi-conductor building block. We can expect great things to emerge shortly made of Carbon. Already graphite layer devices are proposed for waste heat recovery.
Silicon is the best known semiconductor but it isn't natural. Carbon, its next door neighbour, has just been discussed. If we mixed the two together, something good might come out! Well, nature has done that for us. Silicon Carbide, commonly known as carborundum, used in abrasives and grindstones - one of the hardest materials known. Silicon carbide can be made by heating carbon and sand together, but is obviously much cheaper to dig out of the ground. The crystals have a strange characteristic. If you connect an electric current across the crystal at just the right spot, it emits a brilliant white light. Just like the cat's whisker diode of early radio days, it is hard to find the right spot but it can be done. The problem is that we don't dig it up out of the ground, but we blast it out! It is difficult to find a perfect, undamaged crystal. None the less, this material can be expected to have many secrets within. I feel that more research into this natural semiconductor will produce worthwhile results.
There are so many more designs for thermal devices that I cannot list here. Suffice to say that this is one area that should produce practical products into the marketplace shortly. It is to be hoped that these will also be suitable for solar energy conversion. I personally see a much brighter future in this heat approach than in the light type of solar cells. Next we will look at an extremely different electronic approach.