March 29, 2009
Mass Produced Stirling Solar Concentrator With High Efficiency

A company in Washington State claims that by using automotive industry suppliers to mass produce parts a solar concentrator and Stirling engine can achieve high efficiency and low cost electric power generation.

Infinia's founders showed him their design for a solar-powered Stirling engine, with the heat provided by what looked like a large, mirrored satellite dish. The other end was cooled by a car-radiator system. A mechanical drive kept the dish pointed at the sun throughout the day. At night it folded up like a flower to help retain heat (with a small biofuel tank as backup).

Using the Stirling as a generator, Infinia could convert 24% of the sunlight hitting the dish into electricity - a better conversion rate than most solar panels have. Sitton needed no further convincing. He went hunting for investors just as the solar market was heating up. Since 2005 he has secured $70 million in funding from venture firms such as Vulcan Capital, owned by Microsoft co-founder Paul Allen.

Can they make this compete with other solar technologies? Anyone have insights on this?

Share |      Randall Parker, 2009 March 29 10:16 PM  Energy Solar

solar heater diy said at March 30, 2009 12:04 PM:

panels are now having an efficiency of around 29% in recent tests

dorcus said at March 30, 2009 12:22 PM:

I wonder why a combination of a solar cell + heat pipes (and this sort of set up) cant be unified for a greater than 50% efficiecy per sq foot.... Has anyone tried? At this point it sounds to be a packaging issue primarily.

bbartlog said at March 30, 2009 2:23 PM:

It's feasible, but I would bet on these guys before Infinia. It is very interesting to see two companies competing with offerings of different scale, however. If you read the spec sheets and look at the pictures, Infinia is going a smaller, presumably lower-cost route; their dish/engine setups are quite a bit smaller, maybe as much as 1/6 by surface area. Maybe this was dictated by the limitations of their off-the-shelf hardware components (like car radiators) or maybe their analysis just suggested this was a better size in terms of marketability.

In any case, the SES guys seem to have a head start in terms of orders, and their concentrators are also more efficient (31% claimed, versus 24% for Infinia). I'm guessing that Infinia would not be able to compete with SES in terms of mass deployments (i.e. power plant replacements), both for reasons of efficiency (power per unit land) and cost per KW (Infinia aims for 3.3KW peak for $20K, SES 25KW peak for $100K). However, the Infinia offering might possibly sell for high-end residential users (rural farms or anyone who wants to go off-grid). Their rig only weighs half a ton, it looks like something an ambitious do-it-yourselfer could maintain; and of course you can't buy 1/5 of an SES setup.
The main drawback of the Stirling concentrators (as I understand it) is that they lose efficiency fairly quickly if insolation is below optimal design levels. Also, the SES devices apparently still have a low MTBF (lots of breakdowns) due to the difficulties of dealing with an 800C working fluid.

All that said, I'm not sure concentrators can compete with photovoltaics on a cents-per-KWH basis. Photovoltaics nowadays are claiming $1 per watt (for 10% efficiency); the SES rig is $100K for 25KW, which is $4 per watt. Further, while it *looks* at first like the bigger nominal efficiency should mean more watts per acre (in deployment), the rotating dishes apparently have a fairly big footprint; SES only claims 8 per acre, which (if you do the math) is not much different in terms of net energy than I would get from 65% coverage with 10% efficient solar panels.

The main advantage I can see for the stirling engine dishes is that economies of scale might work in their favor. The components of their systems are all stuff that should follow conventional manufacturing rules of thumb, 10-15% reduced costs for every doubling of quantity; it wouldn't surprise me if my $100K figure (for the SES concentrators) will be quite a bit lower if they actually manufacture the 50,000 or so dishes that their two big current projects require. A lot of the cheap photovoltaics, on the other hand, are running up against supply limitations because some of their inputs are exotic ("excuse me, I need to buy the entire world supply of Indium for the next three years...").

Bottom line, I give the edge to photovoltaics, but it's too soon to count these guys out entirely. They need scale to work for them and against photovoltaics in order to succeed, but that may actually happen.

th said at March 30, 2009 3:52 PM:

If peak oil proponents weren't sucked in by so much snake oil, they might have some credibility.

K said at March 31, 2009 1:24 AM:

I agree with bbartlog. No method can be counted out. Yet PV looks better for off grid IMO.

It seems unlikely that thousands of these smaller installations, each combining a Dish/Sterling/Generator. will be cheaper than the more conventional ST arrangement of mirrors/tower.

And I also don't see how this offers an advantage over ST in storing heat for power generation at night.

That doesn't mean there will be no market or use. The grid won't reach everywhere power is needed. Which is precisely why there are a lot of solar panel installations off grid and many more coming. Can this beat the cost of PV installations located at the consumer?

And it is possible that PV will hit a resource constraint which alters the game. No unusual materials are needed for thermal.

Of course, if they could beat the costs of concentrators then utilities would welcome them. But I have already said I doubt they will.

Engineer-Poet said at March 31, 2009 6:17 PM:

No unusual materials are needed for silicon PV either; there are millions of tons of raw material (sodium fluorosilicate) lying in dumps in Florida and elsewhere, a waste product of phosphate mining.

I can see solar dishes as advantageous for providing small quanta of power after sundown.  It is relatively easy to equip a Stirling engine with molten-salt storage, which is far cheaper than batteries and lasts indefinitely.  While large ST installations can run cities from afar, dishes in the size range of 5-10 meters could be installed all over cities to provide power (and space heat, DHW and absorption A/C) while shading buildings and parking lots.  If your dish installation can run your lights and kitchen from sundown to your bedtime or closing time, you're far less dependent upon imported anything and may even be able to operate during grid outages.

Lou Bauman said at April 1, 2009 12:41 PM:

The Sterling Engine/Collector Dish depends on a machine. They breakdown and require maintenance. Solar PV is passive - no moving parts. The machine would have to beat PV by halve in cap cost to be competitive.

Darren said at April 1, 2009 1:19 PM:

What about the lifetime loss of efficiency of PV? UV degrades virtually everything, it was my understanding that PV systems decline in output steadily over their service life.

The Stirling system's output will degrade over time as mechanical parts wear, but you don't have to scrap the whole unit to restore it to original spec.

LarryD said at April 1, 2009 1:41 PM:

PV arrays do have to be kept clean, though. Dust degrades efficiency pretty significantly. And this is a problem that gets worse with scale. And the best places to site large solar power systems are also the most arid.

Steve S said at April 1, 2009 3:32 PM:

Build your PV farm 15% over planned capacity, and UV degredation is moot. No moving parts vs. lots of moving parts still wins.

sean said at April 1, 2009 4:07 PM:

I have one of these babies, a Sunpower free-piston design, cooling my beer even as we speak (they are refrigerators if you run them in reverse). Very elegant machines. I own and have built some Stirling engines, even a few that provide power, albeit at a very small scale (

Also, Infinia is working with a Free-Piston design whereas SES uses a multi-piston and crank mechanism. The advantages of free-piston designs is the use of non-contact gas-bearings that do not wear or need lubriation, hence the ability to hermetically seal them. They are inherently stout and have been in service in space for a decade. Piston/crank sytems are able to scale up better, but have to employ highly engineered lubrication/islolation scheems in order to pressurize for efficiency. Look up William Beale of Sunpower. He did most of the heavy lifting on the free-piston design.

Wayne Findley said at April 1, 2009 5:47 PM:

Of course, WhisperTech is a stirling engine (out of Canterbury Uni, Christchurch, NZ) which is commercialised for the European house heating market (gas burner), and marine applications (kero/diesel burner) at present. Hot water plus elec-a-tricity are the outputs. Anything that keeps the hot side of the engine - er - hot will do as a source...

Randall Parker said at April 1, 2009 11:04 PM:


Yes, PV degrades. The rate of degradation depends on the type of PV material. Silicon crystals last longer than at least some (all?) of the thin films.

BR said at April 2, 2009 6:05 AM:

These solar collection with a sterling engine devices have been around for about a decade and they are limited by the Carnot cycle's maximum efficiency. The equation goes like this:

Efficiency = 1 - Tc / Th

Where Tc is temperature of the cold reservoir (in an absolute sense) and Th is the temp of the hot reservoir. Take for example a day that has a freezing temperature (273K) and a 30 degree increase on the Fahrenheit scale (288K). Without any extra heat collection you get a maximum efficiency of 5.2%. These things need a fairly large array of mirrors before they can become efficient. If we assume that they are using underground storage for the cold temperature (65F or 291K) using the efficiency they have provided 24%, they are heating their engine past 388K or 238F. I don't know enough about the real efficiencies of solar powered steam, but this seems like an incremental improvement on that (efficiency wise). However none of this talk of power efficiencies deals with cost efficiency, and I have yet to see a reliable figure on cost per KW. At the rate that PV tends to be dropping I would expect that these will not be able to compete, but only time will tell.

Moresh said at August 30, 2009 4:51 PM:

I am trying to understand why Solar Dish Stirling is so expensive. I mean all the major components/modules that go into a Solar Dish Stirling are as below
Support Truss mechanism
Stirling Engine
Orientation mechanism / Heliostat
Are there any thing else ?
What drives the cost up, is it the engine ? or the cost of the mirror collector or the support mechanism. Is it that the engine is so heavy/bulky that it requires a very heavy support orientation mechanism. Thoughts ?

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