Solar-Electric Company Runs on Steam

Nov. 24, 2013
"On a sunny day, the stored steam contains enough energy that it can be taken down through all four stages of the compound steam engine."

TerraJoule, in Redwood City, Calif., is developing 100-kW-scale electrical power-generation systems that use the sun to superheat water into steam, which then runs reciprocating steam engines in order to spin conventional ac generators.

Full disclosure: I’ve known about these folks for about a year because my wife, who’s an EE, works there. But I couldn’t write about them until now because they were in stealth mode. Their website went live last week, so the cat’s out of the bag.

The original target market was small communities that could use reliable electricity, but that are too remote to ever be connected to a national grid. (Many of the founders are Australians; some have family in India. Yet, as it turns out, there is a much more diverse market than that, some of which is described on the Website

Here’s the basic idea:

1.                    Start with a long solar trough; at the focus of the trough there’s a black pipe through which the working fluid circulates. The trough rotates about its long axis to follow the sun's elevation.

2.                    The working fluid is heated by the sun until it gets really hot. Then it goes through a heat exchanger that boils a supply of distilled water until it’s superheated.

3.                    The steam runs a multi-stage compound steam engine.

4.                    The steam engine turns the generator.

Here are the benefits:

1.     No fragile photovoltaics

2.     Single solar collector (no heliostats)

3.     The trough doesn’t move (unlike heliostats)

4.     No heavy batteries.

5.     No fancy inverters, just a generator.

6.     The reciprocating-piston steam reached its acme of sophistication around 1930; it can achieve as close to 100% of Carnot efficiency as you’re going to see and mechanically, it’s not nearly as fussy as a turbine.

Here’s a little more detail. It’s been a long time since I took a thermodynamics course, but this much stayed with me.  The steam is superheated and the tank it is stored in is heavily insulated. On a sunny day, the stored steam contains enough energy that it can be taken down through all four stages of the compound steam engine. Through the night, you back off on the number of stages. Each stage, of course represents a Δt for the steam going in and the steam going out.

(What power output does that represent per stage?  That’s what my wife is developing the instrumentation to work out. The CTO knows what he expects, but optimizing the actual full-size steam engine in the lab will the acid test. After that, the testing moves outdoors to the next stage of testing, where the apparatus will be directly pumping water for farming.  Then it will be on to generating electric power.)

That’s the short version of the story; there’s more detail than that on the terrajoulecorp Website.

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