You have certainly seen people on TV who have their entire house roof covered with PV (photovoltaic) panels. Per the above, if they installed 1,000 square feet of PV collectors, they likely paid around $150,000 (although most such demo homes received enormous Grants from the government or from the manufacturers of the solar equipment or from others, where they may only have spent $50,000 or $75,000 of their own money). Per the above, such a house CAN produce around 7,000 Watts of electricity ON A PERFECTLY CLEAR SUNNY DAY, AROUND NOON, as long as the tilt and direction of the house roof is correct. That sounds impressive!
Self-Sufficiency - Many Suggestions|
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There are also other matters that can affect what you might be paid, some of which are related to terrible inefficiencies in the power-grid. IF you happen to be 60 miles from their powerplant, the power grid always LOSES (by its design) 10% of the electricity they put into their end. The fact that you want to put electricity into the OTHER end still means that 10% of that electricity would never get to their powerplant. They have many employees who spend their careers in finding ways to pay you as little as possible for any electricity that they have to accept from people like you!
So you may have noticed that NO ONE ever actually SHOWS any electric bills which actually show any credit given them. They certainly TALK about the wonderful feeling of selling power back to the electric company! People tend to brag about that in the first month or two, before they actually discover how little their credit actually turns out to be. It is very rare that anyone has enough solar panels to actually see a credit of even one dollar on their electric bills from selling any solar electricity back to the utility. This situation was also true for the thousands of people in the 1980s who spent $15,000 or more to install a wind-generator, where they DID see SOME number on their electric bills to show a credit, but it was always depressingly small. (THOSE people never showed off their electric bills either!) In other words, even just the extra equipment required to convert the electricity into AC which is synchronized with the power-grid, has certainly never come close to paying for itself.
We have been considering a perfectly sunny day. But on all other days, when there is any cloudiness where less electricity is produced, that family still has to buy electricity. Even such spectacular (and spectacularly expensive) houses virtually never create sufficient extra electricity during any month to sell back to the power company to pay for the electricity they need to buy. And THAT is true even though the RESIDENTS of such houses are ALWAYS NOT common families! They NEVER leave lights on in unoccupied rooms or leave air conditioning running any more than is absolutely necessary and they have bought a lot of (expensive) appliances which are frugal in electric usage. (Actually, no one ever seems to note this, but such solar-electric houses NEVER actually have either central air conditioning or room air conditioners, because those devices consume incredible amounts of electricity, more than they can ever produce! A small three-ton air conditioner is rated at 36,000 Btus per hour. That amount of energy is equal to around 11 kiloWatts! Fortunately, modern air conditioning equipment have COP (coefficient of performance) which is good enough to require that (small) air conditioner to only actually need around 4 kiloWatts while the compressor is running. Meaning ALL of the electricity that can even be produced (briefly) at noon on that sunny day with that $150,000 of solar panels!)
Even THEN, such houses STILL do not generally produce as much electricity as the family uses, except for some hours during sunny daytimes.
The bottom line of all this is that the efficiency and the cost of current PV equipment are such that it does NOT (yet) make economical sense to buy PV equipment with the intention of producing electric power. In 50 years, or if there are impressive breakthroughs, it might, and likely WILL then make sense. The ONLY technology that I am currently aware of that seems promising in that way is an "ink" that can be applied to suitable substrate, providing the extremely thin layer that the sunlight must be able to pass through.
Peak Power Rating vs. Average Power Rating|
You may drive a car which was advertised as having a 495 horsepower engine, and that may have even affected whether you bought that specific car. That engine rating can be called a PEAK POWER RATING, being the greatest amount of power that it is capable of producing. When creating that enormous amount of power, it is realistic to expect to get around one or two MPG gas mileage. But for AVERAGE driving on an Interstate Highway, your engine only produces around 40 horsepower, during which you may get 25 miles per gallon gas mileage. This AVERAGE situation is a far more accurate description of what YOU CAN ACTUALLY EXPECT, such as regarding gas mileage. Both situations are true, but they are extremely different. One is a situation which sounds very impressive, but which you will likely NEVER actually experience, except possibly rarely for a second or two at a stoplight! The other is a situation which you may experience every day of driving! IF you were only given ONE of the numbers, which would you consider more important to know?
Whenever electricity ratings are given for alternative energy devices, they seem to always be PEAK POWER RATINGS, meaning the greatest amount of electricity or power which can be created. That is entirely different than ratings for AVERAGE USAGE CONDITIONS, which would be realistic numbers of amounts of electricity or power which might NORMALLY be expected to be provided. The discussion and calculations included here will indicate that OFTEN the realistically expectable amounts of electricity or power is only around ONE-TENTH that of the PEAK POWER RATINGS. But no one bothers to mention this important fact! So advertising makes claims of spectacular performance numbers for photovoltaic solar-electric panels, and for solar roof panels, and for electric vehicles, and for Hybrid vehicles, and for windmill-electricity-generation, and even for FUTURE giant windmills and hydrogen as a fuel. They invariably state PEAK POWER RATINGS, like that 495 horsepower engine in the car, numbers that may be technically true but are extremely misleading.
SO! If you want to buy some PV equipment AS AN EXPERIMENT, fine. You could buy just a few square feet or you could go whole hog and cover your roof, but in all cases, there is no credible way that such purchases could ever even pay for themselves and their installation, much less actually some day be producing "free electricity", at least during our lifetimes!
By the way, sadly, this situation also applies to the high-tech projects which have been created with enormous amounts of government money. Around 1978, the US government built the Solar Power Tower in New Mexico (at massive expense). Yes, it produced and produces a significant amount of electricity, often more than $1,000 worth of electricity every day. (being in the desert is a great advantage!) But their equipment is very complex, and during 1979, my visits there always witnessed dozens of technicians fixing things that had broken down! The $1,000 in free electricity each day came nowhere close to even paying the wages of all those repair technicians, much less the regular staff of that system, and much less actually ever toward paying for its enormous initial cost (or even the interest for loans to have it get built). That highly publicized Solar Power Tower has never and will never even pay for its own construction and maintenance costs.
Spain has built a similar installation recently (2007) and is bragging about its performance. But even their own videotapes happen to briefly show some meters that show the actual level of power production, which is easily seen as never being able to pay for that equipment or the maintenance or staff costs.
Germany has been granting enormous amounts of money for people who install large fields of solar-to-electric equipment, AND they also give such people a guarantee of receiving extremely high payments for the electricity they provide. Even with such subsidies, the German people who have been investing millions of dollars of their own money to install such systems, often talk about receiving $30,000 or $50,000 per year from their efforts. Even though I admire the CONCEPT, I am also aware that from a PRACTICAL viewpoint, WHY should a German landowner spend millions of dollars of his own money to only then receive LESS than if he had simply put that money in a conventional Savings Account in a bank? Worse, those farmers generally had to get LOANS to buy all the equipment, so they are likely paying MORE in interest on those loans than they receive for selling any electricity! And that is not even considering the certain fact that SOME of all that equipment will break down over future years and require (costly) repairs. And all this is the BEST available situation where the German government has committed to subsidizing 20 years of future electricity! Note also that the German owner is locked into a FIXED income over those coming 20 years, while costs of living seem certain to greatly rise. The point is that such demo projects figure to NOT inspire the needed thousands of other Germans to invest in installing similar systems, and that such an approach seems to be too financially impractical (yet) for any large scale application.
Many other countries are subsidizing similar solar electric projects. I am not aware of ANY of them which has any realistic chance of being economically feasible. By which I mean, which might amortize its own construction and maintenance and repair costs within 20 years, where then actual economic benefit might be achieved.
Specifically, IF it were not for governments providing enormous amounts of money and/or guaranteeing subsidies to the owners regarding trying to sell electricity, NONE of those highly publicized solar projects has remotely been capable of MAKING any money on its own.
Some day, when more efficient devices and technologies are developed, and when the costs of such equipment drops, solar-to-electric seems certain to be the FUTURE, but it is hard to see how that could happen for probably 50 years or so. Until then, ALL projects into making electricity from sunlight should probably be considered experiments, and if on a large scale, adventures!
Biofuels and Biogas are much the same, VERY highly promoted. But they, too, are based on first needing photosynthesis in living plants to capture the sunlight, which again, they can only do with an average efficiency of around 1%. In this case, the generally then chemically process the resulting plant materials (or let anaerobic bacteria do that for them) where methane gas is given off, which they then capture. THIS is also not a particularly efficient process. But then methane gas is bulky, so a substantial and specialized air compressor is then needed to squeeze the methane gas down to be able to store it in tanks.
The public has been (mis)lead to believe that the FUTURE of energy lies in hydrogen. Again, Reporters have never known enough science to be able to ask the right questions of the manufacturers or salespeople. For example, hydrogen gas does NOT EXIST NATURALLY, and the hydrogen has to be EXTRACTED out of some compound. The most common used compound is water, and the process is called Electrolysis. The scientific Law of Conservation of Energy makes clear that one must USE at least as much energy in extracting hydrogen as the hydrogen would later be able to release. The reality is that the Electrolysis process actually requires at least FIVE TIMES AS MUCH ENERGY (usually from electricity) to extract the hydrogen as the hydrogen could ever later release. So when a manufacturer has Reporters at a Demo where they use hydrogen to power something, there are a LOT of details that they neglect to mention, specifically the fact that the BEST overall efficiency they can hope for is around 20%, and due to a variety of practical matters, actually FAR less than that.
The public also has been led to believe that hydrogen could be BURNED in a conventional car engine. EVEN IF an engine is severely modified to do that, the overall efficiency of internal combustion engines is never much over 20%. This means that 80% of the energy in that dear and VERY expensive hydrogen is simply wasted during such demos! The ONLY way that hydrogen might actually make sense is if it is NOT burned at all, but processed in a FUEL CELL. The US government spent hundreds of millions of dollars to create fuel cells to provide a few hundred Watts of electricity for spacecraft, but even with all that, THOSE fuel cells were never close to the theoretical target of 80% overall efficiency, but around half of that at around 40%.
The point here is that NO ONE has ever yet figured out how to make a large-scale fuel cell, where the 100 kiloWatts of power needed to drive a normal-sized vehicle, and even when they will, it WILL cost many millions of dollars! The FUTURE of hydrogen as any solution to the energy crisis MIGHT come about in a few hundred years, but it is extremely unlikely in OUR lifetimes!
There have been many other ideas which have briefly claimed to be the SOLUTION to energy (electricity) production, like growing tens of thousands of acres of corn to convert it into Ethanol (the process of which requires more fossil-fuel energy than the resulting Ethanol ever has to release!), or battery-powered vehicles (which do NOT actually have any power of their own, only the ability of STORING electricity that you have to buy from somewhere else).
I realize that the public is desperately HOPING to see some actual breakthrough, but NONE of these technologies are likely to provide that, at least for fifty or a hundred years or more. Regarding PVs, there are many thousands of people trying every imaginable variant now, and again, they all get their moment of fame in the news. There are two such technologies which DO seem to have actual potential. One is a very unusual paint, which contains the silicon and similar atoms to collect sunlight and to convert it into electricity, and another which is an approach based on organic molecules. Both still have terribly low overall efficiencies and they are both still extremely expensive, but maybe in twenty or thirty years, either or both might represent a technology which MIGHT actually PAY FOR ITSELF!
THAT would be a refreshing change!
In the same way that solar heat collector panels are virtually always promoted unrealistically, the same is true regarding the creation of electricity from solar energy. We regularly hear from people who are attempting and expecting to "become independent of the power grid.". Most have already bought a few square feet of photovoltaic panels, with the various necessary special batteries, voltage regulators, circuit breakers and inverters. Some salesman had told them that they would be able to get several hundred or thousands of watts of electrical power, or some such figure.
I suppose that technically that is not quite a lie. At noon, on a perfectly clear day, it might be capable of creating the number of watts that was promoted during a sales pitch. In order to not get sued for false claims, most manufacturers of such products have done such experiments, under absolutely perfect conditions. In that specific situation, there can be as much as 340 Btu/hr/square foot of solar energy coming down. (Above the Earth's atmosphere, that 'Solar Constant' is around 429 Btu/hr/sq. ft). If there were 10 square feet of photovoltaics, they could intercept around 3412 Btu/hr, or 1,000 watts, of solar energy. But existing photovoltaic technology is not very efficient, and the quality of photovoltaics often available to the public can be around 7%. That would mean that, out of that 1,000 watts of solar energy coming down to that 10 square feet, only around 70 watts of electricity is actually produced. Could that be called a 1,000-watt system? Not really, if ethics are supposed to be involved, but salespeople tend to inflate claims of such ratings. If around $150 in purchases are involved for each square foot (photovoltaic panels, connectors, voltage regulators, inverters, storage batteries, etc), then that customer spent around $1,500 in the hopes of making electricity. If the customer was honestly informed that for that $1,500, he would, AT BEST, create around 70 watts of electricity, not even enough for a 100 watt light bulb, do you think the customer would pay out $1,500? Probably not! (An industry expert recently (mid 2008) quoted on the Charlie Rose Show that one watt of (peak) electricity from PV cells has now dropped to around $10. So enough electricity for ONE 100 watt light bulb is now possible for only about $1,000 worth of solar cells (plus installation, controls and batteries, which gets the total cost about up to the level of what we have been discussing).
(This 7% figure is for the most economical technology of solar cells, which is based on Cadmium Sulfide. That technology has a technical disadvantage called a high gap energy, which causes the disappointing efficiency. There ARE higher efficient technologies which exist, such as those based on Gallium Arsenide, but they are far more expensive and not within the price range of most people. There are even more expensive technologies that are based on silicon semiconductor technologies, which require a [metal] silicon ingot to be sliced so thin that sunlight can pass through it, which is extremely expensive to do! [Try to imagine aluminum foil so thin that you could see through it! That would be thinner than 1/100 as thick as the aluminum foil that you are familiar with!] Silicon-based technologies are also trying to grow very thin silicon crystals and making microscopic silicon pellets, which may some day result in far better prices, but not for now. So higher efficiencies exist in solar cell technology, which are reported in media stories, but they are each currently far too expensive for broad use. This all results in MOST commonly available solar cells being Cadmium Sulfide, and therefore the around 7% efficiency which we generally cite. We are being alert to practical matters, and we feel that it generally makes a lot more sense to by twice as many [cheap] 7% CdS panels than to buy 15% panels that cost ten times as much.)
NOTE: It has been quite amusing that several dozen people have e-mailed
truly vicious notes to me, claiming that I have some sort of animosity
toward these technologies where I MADE UP numbers that were not
complimentary. Some of those writers were even aware that my Degree
was in Nuclear Physics from the University of Chicago, and still they made
the most nasty of insinuations about me and my character and my intelligence.
Actually, I personally would LOVE any such technology to actually be
anywhere near as worthwhile as the promotional advertising always
implies. But as a Research Scientist, I have an overwhelming
responsibility to the truth and to accurate facts and reasoning.
This presentation is intended to neither be a commercial for or an
attack on any such technology, but rather simply an accurate
presentation of the facts as known by a Research Physicist.|
For the record, there are THOUSANDS of resources that give the numbers for the amount of solar energy which arrives. A good Public Library should have a copy of a set of the (US govt financed) ASHRAE Handbooks, which ALL heating and cooling Engineers use for their primary source of information. ABOVE the Earth's atmosphere, the amount of solar radiation coming toward the Earth is called the Solar Constant, and it is around 1,353 watts per square meter. Roughly 1/3 of the radiation from the sun immediately reflects back off to outer space (called the Albedo of Earth) which results in around 893 watts / square meter (average) actually getting to the Earth's surface. Since a square meter is around 10.76 square feet, that means that around 83 watts per square foot can be expected. It turns out that there are some circumstances of weather where that amount can be a little higher than that, perfectly clear skies, around noon, near June 21, and with an active tracking system so the panels always exactly face the sun, so I used A MORE GENEROUS figure of 100 watts per square foot!
So it is rather humorous for uneducated people who know four-letter words to be trying to insult me for using 100 watts per square foot as though it was some MINIMAL value! Instead, it is a circumstance that RARELY actually occurs, except in deserts! (Such cheap shots from uneducated people are why virtually all other Physicists choose to never communicate with the public. I can see why, and yet I still put up with such treatment just because I believe the public deserves to have some source of information which is reliably true.)
Regarding the cited 7% efficiency of solar panels: As noted above, there ARE some panels which have recently been tested in laboratories to be of much higher efficiency than I cite here. However, they are very exotic and a hundred times more expensive than the (Cadmium Sulfide) PV cells that are generally available to the public. If you have millions to spend, you could buy PV panels that are of 15% or higher efficiency! However, the CURRENT reality is that the panels still commonly available are generally only around 7% efficient. It will be great if and when technology advances to a point where reasonably priced PV panels might be 15% efficient. There ARE an assortment of possible breakthroughs which might enable that, but probably not for at least five or ten years.
CUSTOMERS have no easy way of confirming or denying information claimed by manufacturers. So if advertising would say "35% efficiency", customers would not question it! And the manufacturers KNOW that! In fact, customers tend to believe nearly everything they see on TV or read in the newspaper, based on an assumption that if they were lies, the government would somehow jump on the perpetrators! So the manufacturers and advertisers seem to feel wide freedom to be "optimistic" with their promotional claims! They know that, if in a laboratory, under absolutely perfect conditions, they could achieve a test result of XX% efficiency, they will probably never be sued over such a claim! As a Research Physicist, I tend to be in environments where more restrained information is discussed (where honesty and accuracy are paramount), and I do not recall hearing many references above 7% in such discussions, except regarding FUTURE technologies.
The calculations presented here are therefore based on an optimistic situation of 100 watts per square foot solar radiation, and even that gives such low electricity production (using the 7% figure that is commonly accepted in science), that even if that were doubled up to 15%, the total production of electricity still does not become even moderate. Actually, it would make a lot more sense to simply buy twice as many of the cheaper 7% panels than to pay more than ten or more times as much for actual 15% panels! Unless you really like to spend money or you are intent on impressing the neighbors!
The official ASHRAE charts show that for a south-facing area of solar panels, on December 21, at 10am or 2pm, the incoming solar is only around 61 watts per square foot, at 9am or 3pm, it is only around 46 watts per square foot. Multiply these numbers by 0.07 to get the amount of electricity that south-facing Cadmium-Sulfide PV panels can produce. These numbers are not even close to the glowing statements made by salesmen of solar PV panels!
So, for our ten square feet of PV panels which receive 1000 Watts of sunlight in perfectly clear weather, that total incoming sunlight, on December 21, the total incoming solar energy is liable to be around 610 Watts at 10 am or 2 pm or around 460 Watts at 9 am or 3 pm. The result of all these factors causes an annual average of solar energy to be down to around 750 watts of incoming solar energy, but that is still on a perfectly clear day. If the weather was always perfectly clear. our 10 square feet of PV panels is therefore likely to only produce 7% of that or around 50 Watts, or a daytime total of around 250 Watt-hours of electricity.
(The salesman for those products will tell you far more attractive things, of course, so that you will spend that money. After all, he knows that you really do not know that much about solar energy and photovoltaic cells, and that he has a tremendous advantage over you. He knows that if he confidently talks about Gallium Arsenide technology, you will be impressed by the big words! He knows that you really have no way of telling whether what he is telling you is accurate or exaggerated. And if it later doesn't work remarkably for you, exactly what options do you have? He technically didn't lie when he described how it performed in perfect laboratory conditions under intense desert sun!) And he and his company could nearly certainly claim that bad weather had caused your poor results, or that somehow YOU were responsible for any problems. It is tough to sue any company when they can claim such things, and they know that!
At current electric rates, that would be a savings of around two cents per average day, or about $7 per year. Now, does it make sense to pay $1,500 for any system that would save $7 per year? Or with a bigger, more expensive system, to pay $5,000 for a system that would save around $25 per year in electric bills?
If it lasted for 200 years, without needing any maintenance, and if you didn't consider the interest you would have earned in the bank with that $1,500 or $5,000, it might eventually pay for itself. After that 200 years, under those conditions, it might start producing actual savings. Do you see why that current photovoltaic electric systems are nowhere close to being cost effective?
In addition, the electricity is created as Direct Current, like a battery. And, without some sort of storage, it would just disappear and be wasted. So, you have to have substantial exotic batteries to save that potential of actual useful electricity that could be collected in a day. And, unless you intend to only use appliances that use direct current, you would need an Inverter to convert the electricity to 120 volts AC. These devices have losses, too, so final performance is reduced.
The most commonly purchased of JUCA blowers uses around 700 watts of electricity, and it is running nearly continuously during a winter day, so a total of about 16,800 watt-hours of electricity gets used up in a 24-hour day. The off-power-grid people expect to use their $5,000 photovoltaic setup to run all kinds of appliances and lights and the JUCA blower. However, we just showed that that $5,000 electricity creation system, on a nicely sunny day, could create only around 350 watt-hours of electricity, enough electricity to run ONLY the JUCA blower for around HALF AN HOUR that day/evening! So there would not even be enough electricity to run one device (the blower) through the night! If they could absolutely count on perfectly clear days, around $50,000 of photovoltaic equipment would be able to collect create around 3500 watt-hours of electricity, enough to keep the blower running for five hours of the evening. For Chicago, with its 35% clear skies, around $150,000 of photovoltaic equipment would be necessary JUST to run the blower for a few hours on the $1,400 woodstove! Seems pretty expensive, huh?
The general point being made is that salespeople of photovoltaic equipment tend to make very impressive claims for the performance of what they sell, in order to justify the rather high prices. But, since it is NOT sunny 24 hours a day, and any climate has cloudy days, and very little solar energy is collectable early or late in the day, the reality is that benefits are FAR less than the ideal scenario presented during the purchase! If you're seriously considering such equipment, ask to talk about these matters with several owners (who are NOT connected with that company!).
Next, you need to find the ACTUAL efficiency of the specific photovoltaic panel you will use. NASA has some that are better than 15% efficient, but you probably cannot afford them! And salespeople rarely admit to just what that efficiency is. They, as salespeople, will sometimes make outrageous statements, like 99% efficiency, or 70% or whatever they think you might believe. Hey, they are trying to earn a commission on whatever they might be able to sell you! The actual WRITTEN documentation on percentage efficiency is hard to get to see, but that is the number you need. As indicated above, for photovoltaic panels that are generally available to the public, they are rarely over around 7%. ASK if the PV panels are Cadmium Sulfide! They will be surprised that you know enough to ask that question, and many PV salespeople are NOT educated enough to be able to correctly answer you! (So now we're down to 1,000 * 0.07 or 70 watts of electricity, at noon, when it is perfectly clear).
In winter months, the Sun drops down pretty quickly each side of noon, so there is limited benefit more than a couple hours away from noon. If you figure a daily total of about the equivalent of three hours of noon Sun, that will probably be close. You could always actually calculate the amounts, or look them up from the ASHRAE Handbooks! In summer, the Sun stays higher for longer times, so the equivalent daily total can be five hours of noon Sun. (Now we're at daily totals of actual electricity of 210 watt-hours in winter and 350 watt-hours in summer. Remember that this is for TEN square feet of PV panels.)
At this point, you should call your local newspaper or weatherman, or again, look in the ASHRAE Handbooks, to find the cloudiness for your city for that month of the year. (Continuing our example, for Chicago with its 35% clearness [65% cloudiness], we then get from 70 watt-hours [winter] to 120 watt-hours [summer] of realistic electricity being created in an ENTIRE average day.) You probably pay around fifteen cents per kilowatt-hour of bought electricity, so these are on the order of one to two cents worth of electricity saved per day. About a maximum of around $7 maximum annual savings if you have it operating well for an entire year.
Those numbers are REALLY different from what any salesperson describes, in trying to sell photovoltaic systems to you! You might want to print out the previous four paragraphs, so you can take them with you. You could then ask him to show you how he got the impressive numbers he claimed to you! He cannot make the Sun brighter than it is, and until tremendously better photovoltaics are invented in the future, their conversion efficiencies are hard to brag about. The logic of the above paragraphs is very straightforward, and pretty simple. You are now in a position to confirm or deny any claims he might make to you about how much electricity you will generate!
Because this presentation results in such terrible electricity production, many people seem to think that I am AGAINST photovoltaic panels. Nothing could be further from the truth! But people are rarely given ACCURATE and TRUE information, especially since virtually all of the information is provided by the SALESPERSON who is wanting to make a commission on whatever he can get you to buy! He has absolutely no incentive to tell you that you REALISTICALLY would get that 100 watt-hours of electricity during each average day, because he knows that you are smart enough to not want to part with $1500 cash for such a pitiful return! OR, he may simply have never even been told these actual facts, and HE may even believe the hype that he repeats to you! In either case, your ONLY source of information is not a very good one to rely on!
There are thousands of people who are now trying to find BETTER ways to convert solar energy into electricity. One that strikes me as potentially promising involves vertical water-filled tubes that grow algae inside them. It is an interesting concept, but it is still in the early phases of finding how consistently it will work.
Another interesting possibility was actually invented around 170 years ago, but not actually practical until recently. It is a thermoelectric converter, which uses semiconductor devices to convert heat energy into electricity. The conversion efficiency is still sadly very low, but if that concept is developed, it might (eventually) represent a wonderful source for electricity.
Solar Heated House
Home Air Conditioning
C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago