Solar Thermal hits 30% efficiency

Science Friday had Steve Cowman, the CEO of Stirling Energy Systems, as a guest speaker during the show this last Friday. He said they are getting efficiencies of 30% out of their latest configuration of production solar thermal systems. By efficiency he means that 30% of the solar energy hitting the mirror dish gets converted into electrical energy. Which is pretty good, but the total energy per installation still depends upon the site.

Many of these are going into projects located around the Imperial Valley in Southern California, a location with some of the best solar thermal energy annual incomes. Which is a nice way of saying the place is a hot, sun-drenched desert.

On the environmental impact:
Some of those on the extreme conservation end of the scale are going to find this intrusion of man-made devices, and the roads & power lines that link them, objectionable. I would like to remind those folks that their ideal of pristine wilderness is already back in the past. Our effects on the planet's climate show that we already assumed the role of gardener for all of the biosphere. And that while we have been quite neglectful gardeners up until now, we must still take all of the biosphere into account.

We need to abandon the old notions of Man vs. Nature. The extreme conservationists share the same model as the greedy industrialist; they just prefer that civilization lose territory to wilderness over the industrialists' strategy of displacing wilderness wherever convenient.

We have to embrace our role as Gardeners of Earth and civilization intertwined with nature. Specifically in the case of building solar thermal arrays in the Mojave, those that would preserve the desert should remember that the deserts are growing. I find it to be an almost poetic symmetry that the ever expanding deserts we would like to avoid as Earth's Gardeners can be the source of the clean energy that will slow their expansion.

And to forestall the conservationist's argument that the solar industries will just adopt the über-industrialists' strategy and cover the all the deserts in mirrors and solar panels, please remember that the solar energy that reaches Earth's surface is about FIVE THOUSAND TIMES the energy our civilization uses. The science fiction writer in me enjoys playing around with what we might do with an abundance of say ten times our actual energy needs. But the economist in me says never fear, conservationists; as we tried filling the deserts with clean energy collection, the price of energy would drop so low as to make each new solar collector too expensive to be economically viable.

According to John Smart, we would need only 100 square miles of these Stirling Solar Thermal Collectors, an area 10 miles by 10 miles, to provide all the US electrical needs. Now there could be some bias in that calculation, but even if that means that number is off by a factor of 10, we are still only talking about an area 32 x 32 miles. I think the deserts can spare us this space.

I also think some of the wildlife can successfully co-exist with these solar installations; the flora and fauna that prefer some shade in the desert might even prosper from such projects. As apposed to the people, who would not really be fighting for such real estate. The wildlife will only have to deal with some scattered fields of nearly silent, mostly immobile structures, even rarer maintenance crews, and the power lines. Imagine a developer trying to sell investors on the notion of putting up a mall out in the empty, sun-drenched desert.

This is a young industry that needs to be fostered. I think that around the time we hit one terrawatt of installed solar power, the majority of society will have been convinced that this is cheap, clean, abundant energy with almost no downside. And the consensus might be those who were making the most noise about solar energy's downsides were either receiving some kind of personal benefit from the fossil fuel industries, or were generally opposed to change in any form. History does not look kindly on those who resist the next paradigm.

On Improving Efficiency Further:

According to thermodynamics, particularly the Carnot heat engine, the maximum mechanical energy, and therefor electrical energy after conversion, is related to the heat difference between the heat source and the heat dump. The redesigned mirror dish system and Stirling engine design that Stirling Energy Systems will be using in upcoming projects probably improves mostly on the heat source and the heat-to-mechanical-to-electrical energy efficiency. They might be overlooking the heat dump side of the system, and thus sacrificing some potential efficiencies.

I have made a diagram suggesting further efficiency that could be gained through placing a fan, radiator fins and possibly an air flow cowling that could result in more effective heat flux though convective means. I'm not sure if driving the fan would cost more than the additional electricity created, which would be the deciding factor for adding it to the system. But a further thought occurs that might make this decision much easier.



The dishes will be sited in the hot, arid desert. The ambient air will not be the ideal for removing the heat from the system, so improving the ambient air might be beneficial enough for adding to the design. My suggestion is burying geothermal cooling tubes below the dishes. The ground a couple feet below the surface will be much cooler, so running the ambient air through the tubes prior to use in cooling will mean the heat flux will be higher, increasing the power generated. This effectively increases the scope of the system to to include some of the reservoir of relative coolness in the ground underneath the dishes.

The same notion could be applied to the long trough-style solar thermal collectors. It would probably be more effective as well. With longer underground tubes spread out over a larger area, the external intake air would heat up the ground near the tubes more slowly. And the air would be closer to the underground temperature as it was brought to the steam turbines. (The cooling phase should take place after the steam has flowed through the turbine; cooling the expanded steam will cause it to decrease in volume, creating a relative vacuum and sucking more steam in from the turbine exhaust, resulting in a stronger flow.)

The designs that use molten salt as heat storage medium could also benefit from the geothermal tubes. During the day cycle, the time from the sun being too low to generate any more useful heat to sometime well into the night will probably be the time of highest demand upon the heat storage system, and the geothermal cooling would offset the day's heating of the ambient air. Later in the night when power demand has fallen, the coolness of the ambient air might even be effectively stored in the ground near the tubes.

Of course, water would be a much better energy transfer medium than the air. Perhaps the most efficient scheme would be to 1) run the steam exhaust from the turbines through the geothermal cooling tubes, condensing and cooling it, then 2) using the cooled water at the cooling stage of the turbine process, creating that relative vacuum from cooling the steam while preheating the water to just below boiling prior to its conversion to steam by the molten salt phase. An added benefit should be much less water demand since the water is in a mostly closed circuit system.

The grand scale notion then is to concentrate the solar thermal heat and the geothermal cool into the same system where they can deliver more power to the turbines. This should be a more efficient use of a given patch of land out in the desert.

I will try and post a diagram of this process as well. I will likely be describing these notions in a fictional story I'm working on, so if there are any critiques on the scientific soundness of my notions, I would welcome them. I'd hate to use some bad science in my fiction.

EcoGeekDan