Solar Thermal Power

occasional blogger john hare

I have listened to some doomsday energy crap (gas $5.00 gallon and more forever, Japanese nuclear devastation, etc) in the last few weeks that started me thinking of alternatives again. It may be possible that one of the solar thermal generating concepts could find funding for terrestrial applications in the current mood. It may even be worth something later on out there.

A turbine engine intakes air and compresses it, heats it, and runs the expanded air through a turbine to extract the energy and run some sort of machine, often a generator. One of the solar thermal schemes has a fairly low pressure gas as a working fluid to do this job. I don’t have references for the original concept. The cool gas runs through a low tech compressor (possibly fiberglass or other in situ material) and then through an enormous solar heat collector before expanding through the (low tech of in situ materials) turbine to generate power. The exhaust heats the compressed gas in a heat exchanger before the compressed gas enters the solar collector proper. The somewhat cooled exhaust then  is sub cooled in shadow or heat sink before repeating the cycle. I think the original was lunar based.

I was unable to insert a cartoon sketch of the concept this time. In raw form, the same sysem using ambient air could be used here on Earth, and could possibly generate some investment or even revenue fairly rapidly in the current energy hysteria. The heat exchanger could be clear visqueen on a roof or hillside with a dark material under it, like shingles or dark rocks. A few leaks would cost energy, but not sleep. The turbine and compressor equipment could be  built from cheap fiberglass, or even wood.  The idea is to build so extremely cheap that efficiency is very secondary in importance.

While this is a bit off topic, it could create some in situ class experience with other peoples’ money, and might even generate a bit of solid rocket fuel of the green folding type for the right company.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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22 Responses to Solar Thermal Power

  1. ken anthony says:

    How would a turbine compare with a stirling engine?

  2. Ed Minchau says:

    I worked on a concept very much like this in New Mexico a few years ago. We used cheap Fresnel lenses to focus sunlight on boilers, and we got some great results before running out of funding. (videos here and here) I have continued doing conceptual work on the project since then on my own, and have come up with a boiler and turbo-generator design that ought to make home solar-thermal generators economical. And the design scales up very nicely.

  3. David says:

    Good idea and good to have you posting again.

  4. A_M_Swallow says:

    Most turbines are designed to have the air flow through them in one direction so the system may need a one way valve.

    Homes need about 3 kW.
    Villages need n * 3 kW.

    If a turbine is inefficient then the mirror, heat collector, pipes and cooling radiator need to be bigger.

  5. johnhare john hare says:

    In terms of technically efficient operation, a Stirling should win. In terms of investment cost per unit of energy, this has the possibility of considerably better return. In terms of ability to implement for individuals, this ‘could’ be massively better.

    Thanks, interesting times in the Chinese curse sense.

    I am not sure where the one way valve would be. A compressor forces the air through the heat collector before it drives the turbine. Visually, it is a jet engine with a solar collector instead of a burn section.
    For terrestrial operations, I see this as no mirrors and few pipes. The whole system would be relatively low pressure to avoid structural cost escalation. Once a demonstrator unit proves (or disproves)the concept, it seems pretty basic that you would have a good turbine designer optimize for the application. For the home owner market I’m thinking of, a factory with molds for fiberglass turbine and compressor discs could deliver on the real cheap. This would be competative with the home windmill market.

  6. Chris (Robotbeat) says:

    What you are describing (solar thermal with air as the working fluid using ISRU materials) has been and is being explored in real life by these guys:

  7. johnhare john hare says:

    They seem to be doing some interesting stuff, though not what I described. They are going for a higher efficiency steam engine with concentrators and pistons. I think what I am talking about has the possibility of lower investment and much lower maintenance. They reject turbines for their lower efficiency, while I like them for their lower operational costs.

    The initial acceptance of turbine engines on airliners was based on increased speed. Only later did the airlines realize that turbines were running thousands of hours between overhauls which piston engines couldn’t.

  8. A_M_Swallow says:

    Even on an open system you are going to need a pipe to connect your solar collector to the turbine and heat sink.

    Heat engines work on temperature difference so if the sun light is not concentrated you will only get a small amount of energy out. Mirrors are not expensive. Curved mirrors can be made from guttering with a shiny surface stuck on it.

    A spinning compressor will act as its own one way valve. However if you have a closed loop do you really need two fans?

  9. Eric Collins says:

    Why not have your solar collector be something like a parking lot? In the summer, nothing gets hotter than nice black pavement. The system I’m thinking of would have something like water running through pipes a few inches below the surface and a few feet apart. The water could then be pumped (or naturally convected, though I’m less sure what is required to make that work) to a heat exchanger/turbine or Stirling engine. The heat sink could be a nearby lake, a water main, or possibly just a big circulating loop that goes down about 20 or 30 feet into the Earth. This might also have the added advantage of being reversible, and therefore able to defrost an iced over parking lot in the winter.

    Hmm.. Now that I think about it. You might want to use a working fluid which has a lower freezing point than you system is likely to see in winter, or perhaps a fluid that doesn’t have that nasty habit of expanding when it freezes.

    Just thinking out loud here. Feel free to modify, improve, or ignore.

  10. David P says:

    Have you run any preliminary numbers to see how much power is extracted from the turbine to spin the compressor? And then how much of the remaining energy is available for extraction?

    You would also need a way to start the cycle (and then restart the cycle if it fails/stops for whatever reason). That would “cost” you energy, which could really eat into your overall efficiency.

    It seems you’re talking about a closed loop system at “fairly low” pressure in your writeup… how low are you talking about? It’s an interesting idea what wouldn’t be hard to run some simple numbers against.

  11. Paul Roberts says:

    Having worked in gas turbine design for 15+ years, one thing about this is apparent; the efficiency will be appalling. It’s one thing to do this on earth where gross inefficiency can be acceptqable due to the ready availability of infrastructure, but it’s another thing to imagine applying it to an off-planet solution.

    I know you’re not explicitly suggesting that in your piece, but the world is full of many, many types of ISRU (after all, here on Earth, everything we do is a case of ISRU, really) power generation. The real problem here is to get the efficiency high enough and the costs low enough to be better than solutions that are reaping huge benefits due to scale.

    The system you describe is likely to have efficiencies so bad that you need truely large machines to get any usable amount of power from them, so large that $5/litre gas is cheap. Notice I said $5 a litre. Most of the world is already doing fine on $5 a gallon gas. $5 a gallon is NOT the end of the world and neither is $20 a gallon. But at $20 a gallon, a lot of other things become much more economic, on a relative basis. 🙂


  12. Chris (Robotbeat) says:

    I don’t make much money at all, but I’m pretty confident that $5/gallon gasoline wouldn’t stop me from driving. Sure, I’d drive more efficiently and consider alternatives more (like a vegiesel, for instance), but it’s not a heck of a lot different from $3.50/gal. But we’re well within the realm where shale oil and coal-to-liquids make economic sense. And I once calculated that anything north of $3/gallon makes the extra cost for a hybrid car worth it (though only if you’re buying new). We’re now just about within the realm of electric cars making a lot of sense for commuting (it’s all about the battery costs, and battery costs don’t track directly with gasoline costs, like diesel, ethanol, etc. tend to do).

    Of course, this has almost nothing to do with solar thermal power.

    If you want a cheap source of power on a relatively small scale (i.e. for a household or farm) without fuel costs, wind power is pretty good. On a larger scale, geothermal is even better since it’s baseload and still pretty cheap… provided you live near geothermal hot-spots.

  13. A_M_Swallow says:

    A home power system basically has to be self starting. Although you may be able to include a rechargeable battery.

    At the equator the mid-day sun gives off 1kW per square metre. A realistic value may be 0.7 kW/m2.

  14. Ed Minchau says:

    The system you describe is likely to have efficiencies so bad that you need truely large machines to get any usable amount of power from them, so large that $5/litre gas is cheap.

    Indeed, some working fluids are better than others. Some of the solar thermal installations around today use a 50/50 blend of water and antifreeze or else oil or molten salt as the working fluid, which for terrestrial applications is much better than a low pressure gas.

    For an orbit-based solar thermal system, it might be preferable to use Helium as the working fluid. It is also worth noting that in space at 1AU the insolation is around 1370W/m^2, and the “cold sink” temperature is 3 Kelvin. The working fluid would only need to be heated to 300 Kelvin to achieve 99% conversion efficiency.

  15. johnhare johnhare says:

    I threw this idea out there without doing serious numbers first. As I described, it is not worth pursuing. My point remains that in the current hysteria about energy prices, a market window currently exists for alternate energy. If some of you can push your ideas down the road in this emotional time, they might have a higher level of maturity when you really need off planet power from in situ materials.

    People that post here know that $5.00 a gallon (or litre) gas is not the end of the world. There are many people that don’t grasp that truth. Much purchasing is emotional, and if some people are going to react that way, the ideas in comments here would be a worthwhile place for them to spend it.

  16. Stellvia says:

    Googled: “£1.35 per litre in USD per US gallon” = $8.34 per gallon. That’s what I paid yesterday.

  17. A_M_Swallow says:

    The ISRU designs for the Moon and Earth are going to be very different.

    For the Earth ISRU is not make from natural material but what the local hardware store sells. Pipes, guttering and reflective are easy to find. A generator is needed. Either remove the diesel engine from a purchased generator or try turning a cooling fan into a turbine.

    For the Moon a parabolic reflector can be made by piling up regolith, sinterring and covering with a thing layer of aluminium or iron.

  18. Paul D. says:

    We don’t have a shortage of affordable electricity, in places where markets are allowed to operate effectively. We’re running into constraints on liquid fuels. The likely solution there will be some kind of synfuel, for example methanol from natural gas or coal (which is already being used in the gray market in China as a gasoline extender.)

  19. Axel says:

    Ed, 3K heat sink (space/moon) is cool. But radiator efficiency goes down radically with temperature. I think it was ~1/T^4. Ah, here it is:

    So cost of the radiator becomes dominant. As a guesstimate: using less than 100 K for the heat sink does not look promising. “[…] at 100 K the energy flux density is 5.67 W/m2”. So to generate 5 kW with a 100 K heat sink, you need at least 1000 square meters of radiator. In comparison to collect 5 kW of solar energy, less than 10 square meters of sun shine are needed.

  20. A_M_Swallow says:

    A temperature of 100 K is -173 C, (-280 F) very cold. Oxygen only becomes a gas at -183 C.

    A warm room temperature of 27 C is 300 K (81 F).

  21. Axel says:

    A_M: 100K may seem to be cold, but the tempting point of the 3K value as Ed said is in conversion efficiency = 1-Tc/Th near 100%. See
    Here on Earth the only feasible way is to use high Th (hot temperature) of 1000K or more. A moon/space environment allows to go for a lower Tc (cold temperature). So theoretically room temperature waste heat could be used to generate electricity, instead of wasting electricity to pump away waste heat (like our air-conditioners do). E.g. 300K to 100K would allow for efficiencies up to 1-100/300=66%. Not bad. Could be worth it, if radiator surface is cheap. However going close to 100% efficiency by using Tc=3K will almost certainly be prohibitively expensive.

  22. Peterh says:

    The system John Hare describes would be low efficiency, and need to be large to generate much power. But at the temperatures and pressures described, inexpensive lightweight materials can be used for turbine, compressor, collector tubes, and reflectors. Low cost installation and maintenance trumping efficiency. Injection molded plastic could do the job for most of it.

    No need for a heat exchanger to cool, just release the warm air after it does it’s work on the turbine.

    As for starting, I’m thinking close off both ends, let the air in the collector heat, then release through the turbine to start it.

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