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Wednesday, September 12, 2007

Stirling Solar Power

Photovoltaic solar cells have been improving in terms of energy output per dollar at the rate of about 5% per year. That may not sound like much but they've been improving at approximately that rate (with significant fluctuation) since the first one was built in 1883. If the average yearly improvement continues at the current rate (which I think is a very good bet), then by mid century, the use of photovoltaic solar cells will be significantly cheaper than fossils fuels as a method to generate electricity.

I've been so focused on using photovoltaic cells to generate electricity that I missed a very important development right here in my own backyard using an alternative method to convert solar energy to electricity:

Two Southern California utility companies are planning to develop a pair of sun-powered power plants that they claim will dwarf existing solar facilities and could rival fossil-fuel-driven power plants.

Southern California Edison and San Diego Gas & Electric are working with Stirling Energy Systems, a Phoenix startup that has paired a large and efficient solar dish with a 200-year-old Stirling engine design.

Poor Bob Stirling. Even though the engine he invented nearly two hundred years ago is by far the most efficient type of heat based engine and is way more efficient than the Internal Combustion Engine (that's what's in cars), and can be powered from any heat source (or cold sink), it has never really found very many useful applications. That's because while it's very efficient transforming heat into mechanical energy, it's much larger than alternatives such as the Internal Combustion Engine for a given amount of power and therefore generally more expensive to build. As long as fuels suitable for other engines remain inexpensive, the capital costs required for Stirling engines overwhelm the fuel savings.

So Bob would be thrilled to know that his design, at long last, has the potential to be widely used to transform heat from the sun into mechanical energy which will then be used to generate electricity:
Stirling Energy Systems is planning to build two separate solar farms, one with the capacity to generate 500 megawatts of electricity in the Mojave Desert near Victorville, California, for SoCal Edison, and a 300-megawatt plant in the Imperial Valley, near Calexico, California, for SDG&E. The utilities have signed 20-year deals to buy all the juice the farms can turn out, and have options to expand the plants if they are successful.
Stirling Engines are the best alternative for this application since Internal Combustion Engines can't be powered by an external heat source (the sun in this case) and other engines like the steam engine are too inefficient (and therefore cost too much) for this application:
Osborn said the Stirling dishes are 30 percent efficient -- 30 percent of the sun's energy is converted into electricity -- which is two to three times as efficient as conventional photovoltaic cells.
The efficiency is not necessarily the most important factor, however. It's the cost of electricity after all costs, including capital costs, are taken into account. I suspect this is the biggest contributor to where photovoltaic cells currently fall short of the Stirling dishes.

However, assuming that efficiency is the most important factor and assuming that the 5% per year average improvement of photovoltaic cells continues, it'll take about 30 years for photovoltaic cells to catch up with Stirling dishes. Of course, improvements in the manufacture of Stirling dishes may also occur which would make this approach competitive for even longer.

Given the expected prices of fossil fuels over the next decade or so (from oil futures prices), the Stirling dish design is competitive now and will probably continue to be over at least that period unless another technology is developed that is even more inexpensive.

On the downside, the 2005 article quoted above says that the first megawatt of generating capacity was supposed to be online in the spring of 2007. It's not, so the project has already fallen behind. However, as a founder of a startup that's behind the initial projected schedule on some of its projects, it's not necessarily the end of the world.

Hopefully, it will be successful, since we have a lot of desert here in Southern California and a huge need for electricity.


erp said...

Never doubted American ingenuity for a moment.

Susan's Husband said...

Interesting. All of the serious proposals I have seen for orbital solar power use steam turbines or Stirling engines, not photovoltaics.

Bret said...

I wonder why? I'd think the weight of the Stirling engine and generator would tip it in favor of photovoltaics. Maybe reflective surfaces in bulk are that much lighter than photovoltaics?

Susan's Husband said...

Oh yes, in space reflective surfaces are extremely light weight. You need just a thin sheet of mylar and a little bit of bracing. We're talking multiple square kilometers of reflective surface. The weight of the generator is a small fraction of the overall station size (remember, you have to put in cooling and power transmission equipment as well).