Andy asked me about Peltier junction TEGs (thermoelectric generators) in the Yahoo group Joulethief. The only thermoelectric generators I’ve experimented with are the thermocouples used in natural gas appliances to shut them off if the pilot light goes out. Nowadays these may be getting rarer because the energy efficiency is seriously reduced by a pilot light. I have never experimented with a Peltier junction device, even though they have been around for a long time. I think it’s obvious why few people have done any real experimentation with these. I must also state that I am not a thermoelectric engineer or expert in this field so I can only summarize what others have stated about this subject.
HOT! The first and probably main reason is that Peltiers depend on a large difference in temperature, which means that the most likely way to make them work is to make them hot on one side. So we get into heating them with something that burns, and that can pose a fire hazard. Along with that, there is the problem of the high temperatures causing corrosion and failure of the devices. High temperatures and electronic devices do not get along.
Where? I’ve looked through many electronics websites and I seldom see Peltiers. They kind of remain a scientific curiosity, mainly used for training purposes – kits that people can assemble and make a motor turn with a candle, for example. I’m not saying they aren’t used commercially. I have read that some car makers are (or will) put TEGs on the exhaust system to recover wasted energy and generate electricity.
I have also seen them used in coolers to keep them cold. But the way Peltiers work is to use power to move the heat away from the cooler. The power needed to do this is added to the heat that is moved, so the Peltiers get hot on the outside, and that heat has to be dissipated.
Back to generation. To make electricity we must have a steady and reliable source of heat. We have a source of heat in the vehicle’s exhaust system, but it varies a lot and the vehicle is not running much of the time.
The flue of a heating system, stove, boiler or similar is another source. But will this system be running year round? To save on fuel bills, it’s likely the heat will be off a lot during warmer times (well, maybe not in Antarctica). We could use a solar concentrator to get heat from the sun, but it’s not operating during the night time. And I’ve also read that the cost of solar photovoltaic panels has dropped a lot in the last few years, so it would most likely be more expensive in dollars per watt to use TEGs in place of solar PV panels.
Old Fashioned? Most people have forgotten that man’s first method of generating commercial electricity was with heat. The heat was used to boil water, to make steam, to drive a steam engine, to turn the electric generator. This is still how it’s done today, but the steam engine has been replaced with a turbine. Are we old fashioned and should we convert to Peltier TEGs? The old fashioned way is probably more efficient, and undoubtedly less expensive than Peltier TEGs.
Efficiency I have read that Peltiers are not very efficient. That’s not a problem if the TEG is being used where the heat is waste heat, in other words, the heat is a byproduct of some other process and is normally not used. A good example is the heat from vehicle exhaust. But if the TEG is going to be used in an electric generator where the fuel is expensive then the difference in efficiency could make the difference in whether or not the generator is capable of staying in business. In that case, older technology such as the steam engine would be the choice.
As I said, I’m not an expert in this field. But I have seen up close electric power plants that use heat to generate electric power, in megawatt quantities. I see a typical emergency power plant that consists of a diesel engine of well over a thousand horsepower coupled to a generator that can put out a thousand kilowatts or a megawatt. Along with the fuel tank the plant takes up room the size of half a small house. These plants are for emergency backup purposes and efficiency is not a major concern because the plant may run for much less than a thousand hours over the period of its lifetime — a few decades.
Needs Radical Change If it were to be used for a primary electricity source, then efficiency would be a major concern. In my opinion, I believe there could be improvements in the efficiency of this type of generating plant if it were radically redesigned to take advantage of modern engineering knowledge.
Since this is a fixed installation there are no concerns about weight and size. I believe that the engine and generator could be more fully integrated so that less losses and hence more efficiency could be obtained. I assume that is what the power plant designers did when they switched from engines to gas turbine generators. This is a mature technology, but it seems to have never been applied to power plants less than a megawatt in size. Why? Is it that the industry has assumed that there would be no demand for smaller power plants?
It looks like I’ve strayed far away from the topic of Peltier TEGs, doesn’t it? But could there be room for competition from another technology, called fuel cells? These have few or no moving parts, like the Peltier TEGs. There have been installations of fuel cells used for running industrial plants, but I have never heard of them being used commercially in homes or large scale power plants. I guess one reason may be that they use precious metals such as platinum, so the expense may be prohibitive.
More later…
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