Solar, Simpler

One of the nice things about energy is that there's many, many different forms of it. There's many, many different ways to convert between those forms.

Yesterday we discussed the problem of turning the energy in sunlight into electricity via the circuitous route of sunlight to electricity (in a space solar panel) to microwave beamed down to earth, to electricity in a field of antennas. The advantages are continuous operation and high collection efficiency, and of course the disadvantage is the truly head-spinning cost.

Solar panels on the earth's surface are also expensive, not always very environmentally friendly to make, and rather inefficient. They also have the problem of not operating when light is unavailable in sufficient quantity, as at night or during poor weather. There's no way around those - either the solar energy is there or it isn't. But there are at least a few ways around the first set of problems. The solution is to turn sunlight into electricity by a different route. One such route is so-called solar thermal energy. Instead of taking the direct light -> electricity route, it does what most current power plants do and turns the initial source of energy into heat, which is then turned into mechanical energy, which is then turned into electricity. It's a more complicated route, but it's also a cheaper and easier route because you can skip the expensive and fragile panels entirely. You simply set up mirrors:


The mirrors focus light at the engine, and the engine converts the thermal energy into mechanical work, which turns a generator and produces electricity. Now it's not actually possible to take something hot and get energy out of it without having somewhere cooler to move the heat. It's actually differences in temperature that can be made to produce work. The maximum theoretical efficiency of this process is call the Carnot limit, and it's given by


Where the hot and cold temperatures are given in Kelvin. To increase the efficiency, you want to get that engine as hot as possible. The nice thing about this scheme is that its simplicity and lack of expense. Mirrors and engines are much less of a hassle than solar panels. The disadvantage is that even worse than solar panels the efficiency collapses when the sunlight (and thus the temperature) drops. But in particularly sunny areas this could turn out to be a nice way to get energy at economically competitive costs. Indeed in many places it already is.


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Actually, the efficiency doesn't collapse; you just get less total energy. A benefit of solar-thermal is that superheated water in insulated tanks is actually a reasonably efficient and cost-effective form of medium duration energy storage (assuming the energy came in as heat. If the energy came in as electricity, it's terrible), and thus solar-thermal can be used to effectively deliver power at night.

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By Nathan Myers (not verified) on 01 Oct 2009 #permalink

I've heard that heating your house water is actually much more economical than generating electricity. Water heaters are generally inefficient and if you insulate a tank you can generate enough hot water over the course of a day to fuel your showers and dishwashers and washing machines over night.

As for the whole "You can't get solar electricity at night" problem, there's a fairly efficient work around for this involving hydroelectric pump storage.

Taking into account evaporation losses from the exposed water surface and conversion losses, approximately 70% to 85% of the electrical energy used to pump the water into the elevated reservoir can be regained.[1] The technique is currently the most cost-effective means of storing large amounts of electrical energy on an operating basis, but capital costs and the presence of appropriate geography are critical decision factors.

So you build excess solar generators, store the power in the form of pumped water, and release the energy at night as it becomes needed.

If you want to be really clever, you can pump the water where it needs to go anyway, using wind turbines to restock depleted lakes or rivers with underwater reserves, or channeling rain water from flood plain collection points up into water towers. So now you're solving water needs and electricity needs side-by-side.

Even if you don't want to use the water for anything useful, doesn't something like 60%+ of the world population live on the coasts? Just pumping sea water up and down a pipe would work fine (assuming you can deal with salt water corrosion and such).

It's still not shown that solar thermal is better.

With solar panels you basically need only panels which require minimal maintenance.

With solar thermal you need mirrors, servos to rotate them and a common thermal powerplant. Way too much stuff.

By Alex Besogonov (not verified) on 01 Oct 2009 #permalink

Dearest Mr. Springer,

I've been BEGGING Sci-Port: Louisiana's Science Center to let me build a mini solar power station for the last year an a half. I mean, old giant satellite dishes are a dime a dozen here, as you well know. I mean, one of those, a little mylar, and something to point it at, and BOOM! Great exhibit, right?
You should email them as a dignified science man of the internet convincing them so.

The trade off between the initial investment pollution/energy cost for the solar panels vs heat transfer technology would be a nice thing to get a grip on. But somehow it seems that the low maintenance one would assume from solar cells over their 30? year life would provide quite a benefit to offset the other costs.

@3 - hydroelectric pump storage

Maybe instead of using solar electricity to operate a pump, it might be possible to have the heat directly pump the water into the elevated reservoir?

Conversion to steam and letting the steam rise into the reservoir would be a horribly bad idea unless you can somehow recover the heat of condensation. Perhaps some sort of device with no (or very few) moving parts that uses a hot spot? Say you have a column of water. By boiling just a tiny bit of it at the base of the column, the rest would get pushed up. Sure, the bubble of steam then rises to the top and the energy that boiled it is lost, but it might be reasonably efficient regardless. I wonder if the hydraulic ram idea could be adapted.

To increase the efficiency, you want to get that engine as hot as possible.

[Engage pedant mode] No, you want the difference between the hot and cold reservoirs to be as large as possible, and the cold reservoir as cold as possible. It doesn't matter how hot you get the hot side if your delta T is low, but you can get high efficiencies with a low delta T as long as the cold side is very, very cold.

On a completely different note, have you heard about the idea of using osmotic pressure to run a power plant? Norway is building one.

Solar farms, voltaic or thermal, are stooopid on the scale of (unsubsidized) electrical utilities. Two-axis steering issues are minor. North American deserts have wind-blown silicate dust, and larger. Daily washing acres of optical surfaces is tip of the iceberg. An hour sandstorm in San Berdoo will frost your winshield and strip paint off your car. Soutwestern US deserts enjoy frequent annual Santa Ana winds and such - sand riding sustained 50 mph wind with gusts to 80+ mph. BTW, solar farms are sails, too.

"On a completely different note, have you heard about the idea of using osmotic pressure to run a power plant? Norway is building one."

Uh, what? It generates power by separating fresh water from salt water? There are a few details that need explaining here.

ppnl: Obviously extracting fresh water from salt requires power. By inference, then, one should be able to extract power by combining fresh water with salt.

By Nathan Myers (not verified) on 02 Oct 2009 #permalink

This is nice but irrelevant. The environmental lobby will not allow these things to be built. Recall Ted Kennedy and the MA wind farm. Also Sen Boxer has a bill in Congress that would prohibit any solar projects in the Mojave.

The fact of the matter is, the environmental lobby wants to reduce the human population to a few million (whole planet) paleolithic hunter-gatherers. Read John Holdren or Cass Sustein. If they get their way, there will be NO industry, NO agriculture, NO science, NO art or literature. Just skin-clad savages dying in ignorance and poverty by age 50.

By Bob Sykes (not verified) on 02 Oct 2009 #permalink

When you're obsrerved blatantly lying about your opponents, it naturally raises the question of whether there's anything at all you won't lie about. The null hypothesis is "no".

By Nathan Myers (not verified) on 02 Oct 2009 #permalink


"Obviously extracting fresh water from salt requires power. By inference, then, one should be able to extract power by combining fresh water with salt."

Ok but so far all you have is a battery not an energy source. As far as I can tell a low energy density and very inefficient one at that.

I need a mechanism and some details in order to judge this.

If Iceland can get their geothermal from volcanos
we ought to tap the Yellowstone caldera - it might
be able to provide power for the whole continent.

Just skin-clad savages dying in ignorance and poverty by age 50.

You mean they want to turn us into tea party protesters?

I need a mechanism and some details in order to judge this.

When you have salt water and fresh water separated by a semi-permeable membrane, water is drawn into the salty side by osmosis. If that occurs in a sealed vessel, this process can produce a significant amount of pressure. That pressure can be used to drive a generator. This is known as pressure-retarded osmosis and is the approach taken by the new Norwegian plant, but there are several other approaches possible.

For more detail, see, for example, Recent Developments in Salinity Gradient Power.

"The osmotic pressure difference between fresh water
and seawater is equivalent to 240 m of hydraulic head. In
theory, a stream flowing at 1 m3/s could produce 1 MW of
electricity. The worldwide fresh to seawater salinity
resource is estimated at 2.6 TW"