A different kind of solar power

[Zachary]

Well this is fairly simple so I guess there's a flaw in my 'plan' somewhere :p, but hey here goes. I'm putting this in babbling and not general science because it's just a bit of banter; if this is a credible method of generating power I'll eat my hat!

Ok, current solar cells are only about 25% efficient, however instead of using sunlight to directly generate electrical current why not use it as a heat source?

Of course sunlight is normally low grade heat, but why can't giant lens systems be used to focus the sunlight on a smaller area? Lenses are just ground class and they could be pretty crude, and the only energy losses would be from sunlight reflecting off the surface of whatever they're heating. If you heated a fairly volatile liquid with a low reflectivity (um...water with black dye? :p ok maybe something more refined than that) then you would only need to heat up the water to just a bit over boiling point to have a theoretical maximum efficiency of >25% (if your cold reservoir was the sea or something cold like that), and considering how tightly you could focus the sunlight the efficiency could be bumped to well over that of a conventional solar cell, and all that using ground glass and a simple heat engine.

So where's the flaw? why hasn't it been done? would the yeilds be too low to bother with the whole thing? Is making giant lenses harder than I think?

Cheers,
Zac.

[LurchGS]

I think a variant is being done already - quite a few homes around here use 'solar heat' - that is, what you think are solarelectric panels are not - they're panels for trapping solar heat. Water is pumped up from a storage tank and allowed to trickle over a black surface - heat is exchanged.

Water from the storage tank is used to heat the building. It's also used for bathing and other hot-water needs around the house.

I don't think it comes close to boiling, though - but for residential/small business needs, you don't need it.

[Lord Jubjub]

It's been producing power right now in eastern California.

One MAJOR drawback is that it takes a rather large amount of land to produce a decent size powerplant. Thus, only in the desert southwest is it really feasible to do it.

BTW, they use oil, not water for the liquid at the foci. The hot oil is then pumped into a water bath to produce steam and drive turbines.

[Trebuchet]

And mirrors rather than lenses to concentrate the light. They're easier to make in large sizes. The oil doesn't have to be black, either, just the tube it runs through.

[Swift]

A webpage (http://www.solardev.com/SEIA-makingelec.php) about solar thermal.

[tlbs101]

There is this large solar mirror / boiler experiment located on Kirtland AFB. It was built by the US department of energy about 15 years ago. A large field of mirrors tracked the sun and focused sunlight onto a target (which obviously got very hot).

I'm not clear why they quit experiments, but the thing is still there. You can see the collector tower in the distance (on the base) if you drive south on Wyoming Blvd.

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We also have had friends who heated their swimming pool with passive solar collector panels (pipes running under black heat-absorbing material). They work quite well.

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[zebo-the-fat]

How about this idea, http://www.wentworth.nsw.gov.au/solartower/

Solar energy heats the air in a tower, the hot air rises and turns the turbine to produce power.

[LurchGS]

sounds like a variant of a Flettner sail

[mugaliens]

So where's the flaw? why hasn't it been done? would the yeilds be too low to bother with the whole thing? Is making giant lenses harder than I think?

Cheers,
Zac.

No flaw, and it has been done (http://en.wikipedia.org/wiki/Solar_One).

Only microscale production uses fresnel lenses. Anything larger uses reflective mirrors.

By the way, Solar One is at Yermo, the Marine Corps supply depot. It's about half a mile south of I-15, not on I-40 like the wiki article mentions (although it might be visible from I-40). It powers the entire base and feeds the rest to the grid.

[mugaliens]

How about this idea, http://www.wentworth.nsw.gov.au/solartower/

Solar energy heats the air in a tower, the hot air rises and turns the turbine to produce power.

Sounds like a lot of hot air, to me...

Seriously, I don't think it would nearly as land efficient as a boiler design. Might be cheaper to build, though.

I once saw a schematic where the light was gathered and sent underground, via light pipes, to the boiler, rather than being reflected by mirrors. It was more expensive, though.

[Ronald Brak]

Seriously, I don't think it would nearly as land efficient as a boiler design. Might be cheaper to build, though.

It's only about 0.5% efficent. On the other hand it can provide baseload power by storing heat in saline ponds. Some say a full scale solar updraft tower could produce electricity as cheaply as coal, but there is dispute about those figures.

Personally I think there is a very good chance PV panels win out in the end. This is because I am guessing we will eventually work out cheap ways to make highly efficient flat panels and they will beat solar thermal on price as far as electricity is concerned.

But is is possible that solar PV and solar thermal could be combined. Solid state devices can generate temperature from a temperature difference. Some PV concentrators get quite hot. Maybe someone could put the two together?

[mr obvious]

I'd be interested in learning more, from a technical stance, about the Solar Tower idea. Several questions leap out (and the links do not carry much in terms of technical information).

How do they expect the device to work 24 hours a day? They mention re-radiation from the ground during the night. Why do they expect that to be enough to drive the turbines? How tall would the tower need to be to take advantage of that?

Are they going to use a capacitative mechanism to store the air until it's hot enough? If they use an open vent in the tower, then relatively small differences in temperature will cause the air to start flowing, and I'm guessing steady-state won't generate much electricity (200,000 homes worth is quite a bit). I could be wrong, but air doesn't have the density of fluids and thus you'd need a lot more flow to generate the same amount of momentum transfer.

Finally, how does this compare to using oil (via solar heating, not burning the oil), per unit land area?

[Ronald Brak]

How do they expect the device to work 24 hours a day? They mention re-radiation from the ground during the night. Why do they expect that to be enough to drive the turbines? How tall would the tower need to be to take advantage of that?

They they have salty ponds that get hot during the day and release the heat at night. As long as the 38 square kilometer covered area is warmer than the outside air, wind will blow. The tower is supposed to be a kilometer high.

Currently there is talk about building a smaller version that will supply 100,000 homes instead of 200,000.


Finally, how does this compare to using oil (via solar heating, not burning the oil), per unit land area?

Solar troughs can have efficiencies of around 20% so you would need 1/40th the land of a solar updraft tower, but no power at night. The Solarcube, which is made in my town has an efficiency of 37% and so would need 1/76 the amount of land. Here is popular press article on it:

http://pesn.com/2006/02/01/9600228_Sun_Ball_Released/

Note that the article discusses marginal costs, not what you would really be paying. It's good, but it's not fantastically good. Basically if you have money sitting in the bank it may be a good investment here in Australia.

[mugaliens]

But is is possible that solar PV and solar thermal could be combined. Solid state devices can generate temperature from a temperature difference. Some PV concentrators get quite hot. Maybe someone could put the two together?

Good points, Ronald. How about putting a traditional solar furnace in the tower, with traditional mirrors heating the boiler in the middle. Only have the heat is used to create steam, the remainder collected in hot oil storage tanks in the ground (used for night). The excess heat rises, powering the turbine, and towards the top, we use the solid state devices to extract as much as we can. We could also use the solid state devices to line the hot oil storage tanks, and the back of the mirrors...

There's got to be a price point cutoff there somewhere. Probably cheaper just to insulate the storage tanks with rock wool.

[Ronald Brak]

You'd also need to consider how much it might cost to simply generate electricity and store it in a modern flow battery which might possibly be cheaper than hot oil storage, but I don't know.

Perhaps if you had a combination of wind and solar, you would generally be able to meet demand during the day and at late at night, but you would use flow batteries and gas turbines for the evening peak.

[mr obvious]

Thanks for the info. With 40 times the area needed for the Solarcubes, or more, it appears that the heated-air towers aren't that great of a deal. Using storage via batteries might be useful for going through the night, but are there efficient batteries that don't use toxic or caustic compounds?

Nightfall might be a problem, but I recall there was some speculation a few years back about sending some solar panels into space and transmitting the energy via microwaves to receivers on Earth. By using enough of these panels, there'd be pretty much nonstop power generation. Any idea if that went anywhere?

[Ronald Brak]

Thanks for the info. With 40 times the area needed for the Solarcubes, or more, it appears that the heated-air towers aren't that great of a deal. Using storage via batteries might be useful for going through the night, but are there efficient batteries that don't use toxic or caustic compounds?

A solar updraft tower might be worth it if you had cheap land, such as in Australia. Also, you could presumably farm the covered land, so it wouldn't be going to waste.

You wouldn't want to fall into a flow battery, but they are pretty safe. Or at least probably safe compared to the alternatives.


Nightfall might be a problem, but I recall there was some speculation a few years back about sending some solar panels into space and transmitting the energy via microwaves to receivers on Earth. By using enough of these panels, there'd be pretty much nonstop power generation. Any idea if that went anywhere?

It's far too expensive at the moment. Currently it's much cheaper to put solar cells on your roof and store power for use at night. Although space based power might be cheaper in the future, the cost of earth based power is also likely to come down as technology improves, so it's possible it may never be economical.

[mugaliens]

Nightfall might be a problem, but I recall there was some speculation a few years back about sending some solar panels into space and transmitting the energy via microwaves to receivers on Earth. By using enough of these panels, there'd be pretty much nonstop power generation. Any idea if that went anywhere?

You're talking about solor power satellites. It's an interesting idea, but the construction requirements make the ISS look like it was built as a small, desktop model.

And that's for just one.

For enough to meet our requirements, we'd need a fairly large fleet of them.

Rather daunting, I'm afraid, and over a 20-year lifespan, it would cost about $38 Billion for a 5 GW design, yet produce only $43 Billion over the twenty years.

Yeah! It makes more than it costs, right? Actually, no, because the cost is paid up front and the payback trickles in over the next twenty years. Given current inflation estimates, that comes to a net loss of revenue of around $11 Billion. Compared to the power it produces over it's lifetime, that comes to around 2 cents per kW-hr, which is actually less than half the cheapest rate here on Earth.

But the initial cost for enough to supply the world's needs would be absolutely staggering! Imagine multiplying $38 billion times the tens of thousands or hundreds of thousands of what we'd need?

It'd be well into the quadrillions of dollars, that's a given.

[Zachary]

No flaw, and it has been done (http://en.wikipedia.org/wiki/Solar_One).

Only microscale production uses fresnel lenses. Anything larger uses reflective mirrors.

By the way, Solar One is at Yermo, the Marine Corps supply depot. It's about half a mile south of I-15, not on I-40 like the wiki article mentions (although it might be visible from I-40). It powers the entire base and feeds the rest to the grid.

Why are fresnel lenses only used for small scale applications? They can be made of plastic so surely they'd be cheaper than mirrors?

Also maybe I'm wrong here (I'm no engineer) but surely the tracking mechanism would be easier. All you have to do is tilt the lens from west to east (or whatnot) and then move the heat engine to the focal point of the lens (which would be pretty easy considering it's on the ground and not in mid air).

Anyway, nice to know I had a point. You say the solar cube is 37% efficient, which is much more efficient than normal photovoltaic cells (especially considering there's a 4-10% power loss in coverting DC to AC), so why not the widespread use?

And why have photovoltaic cells grabbed the public's imagination as a great source of renewable energy when solar thermal goes basically unnoticed?

[Zachary]

oh yes, now this might sound a bit farfetched but isn't the troposphere about 100 degrees colder than the atmosphere at ground level? Would putting power plants in the atmosphere via large balloons be too ridiculous to take advantage of the much improved cold reservoir the troposphere would give?

[Ronald Brak]

Anyway, nice to know I had a point. You say the solar cube is 37% efficient, which is much more efficient than normal photovoltaic cells (especially considering there's a 4-10% power loss in coverting DC to AC), so why not the widespread use?

They use very expensive, high efficiency solar cells but save on costs by concentrating light onto them. I'm pretty sure the efficiency rating doesn't include inverting from DC to AC.

The company is definitely planning on large scale use. They say they plan to produce 50,000 meter square Solarcubes per month by the end of the year and will build additional assembly lines depending on market demand. They are also licencing overseas manufactures to produce them. Provided it works as claimed, I imagine it will be popular, unless a competiter manages to produce something even cheaper. They should be extremely popular in remote parts of Australia where power from diesel generators costs about $1.60 U.S. a kilowatt-hour.

[mr obvious]

Why are fresnel lenses only used for small scale applications? They can be made of plastic so surely they'd be cheaper than mirrors?

Also maybe I'm wrong here (I'm no engineer) but surely the tracking mechanism would be easier. All you have to do is tilt the lens from west to east (or whatnot) and then move the heat engine to the focal point of the lens (which would be pretty easy considering it's on the ground and not in mid air).

Anyway, nice to know I had a point. You say the solar cube is 37% efficient, which is much more efficient than normal photovoltaic cells (especially considering there's a 4-10% power loss in coverting DC to AC), so why not the widespread use?

And why have photovoltaic cells grabbed the public's imagination as a great source of renewable energy when solar thermal goes basically unnoticed?

I'm not an expert, but my understanding is that you want to maximize the collection of sun. That is, you want the largest area concentrated onto the small region where the material (e.g., oil, salt) is being heated. One can pretty much use a 500 foot or larger radius circle with mirrors to accomplish this. If you wanted a fresnel lens, you'd need to make a single 500 foot piece (recall that if you use many smaller pieces, you have the same problems making individual adjustments as the mirrors). Such a large piece of optics is likely to deform under its weight, be susceptible to wind (vibrations) and damage (from birds or other inquisitive animals), and has to be mounted above the surface being heated.

Then add to that the fact that the focal region is probably not going to be too small. Each mirror's reflection is of some size. More mirrors might smear that focal spot because adjustments aren't too precise. However, a single standard fresnel lens probably can't focus all the light from a 500 foot-radius circle into a spot that's comparably small. (As an anecdotal piece of evidence, I have yet to see one of those fresnel-like page magnifiers that can do more than 2 or 3x.) You might be able to engineer the optics so that it can work, but then this becomes quite expensive, when mirrors are considerably cheaper. I think.

[mugaliens]

Folks, large-scale utilization of the sun, by default, barring no scientific arguement, involves solar mirrors, a collection basin, and steam turbines. This is precisely why the best authorities on this subject decided on such a pilot project umpteen years ago. Solar 1 proved itself viablem, which is why Solar 2 is in the process of being built.

Duh....

[mugaliens]

I'm not an expert, but my understanding is that you want to maximize the collection of sun. That is, you want the largest area concentrated onto the small region where the material (e.g., oil, salt) is being heated. One can pretty much use a 500 foot or larger radius circle with mirrors to accomplish this. If you wanted a fresnel lens, you'd need to make a single 500 foot piece (recall that if you use many smaller pieces, you have the same problems making individual adjustments as the mirrors). Such a large piece of optics is likely to deform under its weight, be susceptible to wind (vibrations) and damage (from birds or other inquisitive animals), and has to be mounted above the surface being heated.

Then add to that the fact that the focal region is probably not going to be too small. Each mirror's reflection is of some size. More mirrors might smear that focal spot because adjustments aren't too precise. However, a single standard fresnel lens probably can't focus all the light from a 500 foot-radius circle into a spot that's comparably small. (As an anecdotal piece of evidence, I have yet to see one of those fresnel-like page magnifiers that can do more than 2 or 3x.) You might be able to engineer the optics so that it can work, but then this becomes quite expensive, when mirrors are considerably cheaper. I think.

Uh, yeah. Beyond a few meters, mirrors are far more efficient, all things considered, which is precisely why solar power stations around the world have adopted this approach.

[Ronald Brak]

Folks, large-scale utilization of the sun, by default, barring no scientific arguement, involves solar mirrors, a collection basin, and steam turbines. This is precisely why the best authorities on this subject decided on such a pilot project umpteen years ago. Solar 1 proved itself viablem, which is why Solar 2 is in the process of being built.

Duh....

I'll make an argument, and it's one based on economics. I think which ever is the cheapest method will be used.

The first commercial thermal solar plant has come online in Spain. It cost $47 million and at 12 cents a kilowatt-hour would produce about $2.9 million dollars worth of electricity per year. This comes to a return of about 6.2%. A concentrating PV (Photovoltaic) system costing $1,000 in a similar location can produce about $84 worth of electricity per year for a return of about 8.4%. Now technological improvements and larger scale construction will probably reduce the costs of solar thermal, but the same applies to PV and concentrating PV. If I had to bet on which would end up being the cheapest in a decade I would put my money on PV and concentrating PV. PV also has the advantage that its simplicity makes it more suitable for small-scale point of use applications.