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Steampunk Solar Power
Jamais Cascio, 11 Aug 05

SES_dish.jpgA new agreement was just signed by Southern California Edison to guarantee 20 years' purchase of electricity from a new 4,500 acre solar farm to be built near Victorville, California. The farm will initially be designed to put out 500 megawatts, but can be expanded to 850 megawatts. This will represent the largest solar power facility in the world, and will put out more electricity than all other US solar projects combined. Funny thing, though -- it won't use a single photovoltaic cell.

Instead, these solar power generators will use a nearly 200 year old bit of technology: the Stirling Engine.

Pretty much every time we post something about solar concentrators or home cogeneration or somesuch, we get a series of comments about the neglected beauty of Stirling Engines. Admittedly, Stirling Engines -- first invented in 1816 by Scottish clergyman Robert Stirling -- are quite elegant. Here's the Wikipedia entry on how they work:

The Stirling engine works by the repeated heating and cooling of a sealed amount of working gas, usually air or other gases such as hydrogen or helium. The gas follows the behaviour described by the gas laws which describe how a gas' pressure, temperature and volume are related. When the gas is heated, because it is in a sealed chamber, the pressure rises and this then acts on the power piston to produce a power stroke. When the gas is cooled the pressure drops and this means that less work needs to be done by the piston to recompress the gas on the return stroke, giving a net gain in power available on the shaft. The working gas flows cyclically between the hot and cold heat exchangers.

The working gas is sealed within the piston cylinders, so there is no exhaust gas, (other than that incidental to heat production if combustion is used as the heat source). No valves are required, unlike other types of piston engines. [...] The ideal Stirling engine cycle has the same theoretical efficiency as a Carnot heat engine for the same input and output temperatures. The thermodynamic efficiency is higher than steam engines (or even some modern internal combustion and Diesel engines).

stirlingengine.jpgStirling Energy Systems has been working on solar power generation units for 20 years, but this is the first serious implementation of the design. The SES website has a particularly useful visualization of how the systems work (screen capture to the left), and it's one of those systems that seems almost too good to be true. If it's so simple, so straightforward, why hasn't it been done before? Parsing through the Stirling Energy Systems website, it seems the answer is cost; until recently, putting together reliable, functional systems able to produce utility-scale amounts of power remained simply too expensive.

If all goes well, the 20,000 dish system should be fully online by 2010. However, because of the modular nature of the units, the farm will start generating power as soon as the first unit is plugged into the grid. The cost of the project wasn't mentioned in the stories, but I saw a so-far unconfirmed report that power from this system is expected to run ~$.06/kWh, making it competitive with most other sources.

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Comments


Why hasn’t it been done before? It’s not only cost, but more important, reliability. Crank driven Stirling engines need some sort of lubrication to prevent the mechanism loads from wearing out the rotating and sliding parts. The lubrication has to be prevented from getting to the hot parts of the engine. That has proven to be tough to do. Witness the fact that hundreds of millions of dollars have been spent trying to do it, and still the life of crank drive (so-called kinematic) Stirlings is measured in weeks or months, not years. And when they wear their bearings, shaft seals and piston rings they have to be disassembled and rebuilt. That makes their operating costs high.

In the 80’s, Cummins tried another kind of solar Stirling- the free piston type, with better luck, but their program failed as a result of management errors. Since then the free piston Stirling has proven itself capable of reasonable life, since it uses its working gas for lubrication and does not have the bearings, shaft seals and high force sliding components that make things so tough for kinematic Stirlings.

Free piston Stirling engines are now under development by NASA for use as isotope-heated space power generators, requiring life of many years. Their heat-electricity efficiency is in the 35% range. The NASA machines are small- a few watts to several hundred watts, but the same design can be made much bigger.

Stirlings are also classic heat engines- they require heat flow across a wall, both coming in hot and coming out cooler. Heat exchangers are intrinsically expensive, and the hotter they are, the more expensive. In comparison, gasoline or diesels are dirt cheap, since they are internal combustion engines, don't require much in the way of heat exchanger and can be made of cast iron.

But then, you can't run a diesel on solar- or more accurately, it would be tough. So your oft-repeated observation still holds- if it sounds too good to believe, it is.

Anyhow, good luck, guys, we will be watching with interest. Now tell us what your mean time to failure is.




Posted by: wimbi on 11 Aug 05

I wouldnt be quite as skeptical as the previous poster. While the claims of any new technology developer are often a bit grand (i.e. unrealistic), the thing to keep in mind is that SoCal Edison is in business to MAKE MONEY. That means that they see this contract with Stirling Energy as a profitable endeavor and THAT means that they must have adequately solved the reliability issues raised by Wimbi. It's not like SoCal Edison is going to contract with a power plant that needs to cover the costs of repairing their stirling engines every few months (not unless that cost was factored into their price per kwh and it was STILL a profitable price.

I would love to hear more about the cost-competitiveness of this array. If it is really 6 cents per kwh that is amazing! That would make it very competative but I doubt the numbers are that low. If they were, we would be seeing the same kind of rush to build these stirling-solar collectors all across the southwest as we are currently seeing with wind farms. Then again, maybe this is the beginning of just that kind of rush... stay tuned I suppose.

Cheers,
WattHead (watthead.blogspot.com)


Posted by: Jesse Jenkins on 11 Aug 05

Dean Kamen is also working with Stirling engines for electricity production, with a by-product of purified water, for use in Africa and other places. These Stirlings will be using biomass as a heat source, evidently.


Posted by: gmoke on 11 Aug 05

Wow! I learned about Stirling engines a couple of years ago, and thought there must be a way to use these with the plentiful sunlight available most days at Burning Man. At that time, I found one supplier of pre-built engines with a very small unit configured for use with a parabolic mirror and a generator.

The story over at Pure Energy Systems includes a quote from someone at Southern California Edison saying that (1) they won't reveal the cost, and (2) they don't need subsidies.


Posted by: Gary on 11 Aug 05

I came across stering engines a few months ago.

I also that day, came across several concepts of bio-mass production of methane.

I see this as a very viable energy production model.

This week, I came across "fat" energy. Or grease energy.

It seems that with a few modifications to a diesel engine, [the injectors and the filters], one can literally run on the old grease from a Mickey Dees french fry cooker.

Then I ran into an institute where, through intercourse with the Federal EPA, processed grease can be used as well. This method has "Government Sanction". This liquid can be used with or without 10% diesel fuel, as a mixture.

The exhaust is half of the minmum emissions standards, under the most harshest standards of emissions control. Under either fuel regime.

Peak oil IS probably here, but I am betting on the ingenuity of rebellious Americans, who shall find a petroleum free, or a massively reduced petroleum mixture of energy providing fuel.

I expect a dragged down , knock down fight, between Big Oil, and Corporate Agriculture in the not so distant future.

It shall be interesting to see which godless corporate structure wins. Time will tell.

Submitted with respects,

Z.


Posted by: joseph zack on 12 Aug 05

One thing I've always found fascinating about Stirling Engine technology is how well it scales. Because it's NOT steam/phase-change dependent they can be designed to run off heat differential from body-heat to air, or water to air, or solar-thermal to water, etc.

...and of course they can be run in reverse; turn the crank to make one side cool and the other hot. Instant bicycle, wind or water-flow powered cooler/precipitator.


Posted by: Andrew on 12 Aug 05

I've been reading about Stirling engines for over a decade. The truth is, they were actually pretty common back around the turn of the century (as in going into the 20th century). It was not uncommon to find an apartment building or a house in an urban area with a gas-fired Stirling pumping water for the household. Electricity wasn't as widespread at the time, and most houses used gas lighting; these used the same gas supply to keep the water pressurized in the house.

SunPower in Athens, OH built the Cummins system mentioned in wimbi's post. The founder of the company invented the free-piston Stirling. If you'd like more info, find a book titled "The Next Great Thing."

I've always been fascinated with the ability to use a Stirling both as heat engine and as a heat pump. The aforementioned book also mentions how they put together a design for a replacement of most of the cooling system in a refrigerator, which would be significantly more efficient than the current systems. They also had a device capable of cooling a CPU down to nearly cryogenic temperatures; I'd LOVE to get one of those and overclock the hell out of my machine. Additionally, they played with an idea for a machine which could burn various types of biomass. Burn the biomass under the "hot" side of the engine, and evaporate some water off the "cold" side (which would effectively distill the water AND improve the efficiency of the engine) and you have the electricity-and-clean-water idea mentioned by gmoke.

Stirlings are versatile as hell. Various companies are coming up with unique ways of building them, trying to overcome their problems. The most efficient ones use hydrogen or helium as their working fluid, but you have to have VERY durable seals to contain the working fluid for a long operating life. This is true regardless of whether or not you have a crankshaft (i.e. free-piston Stirlings have this problem, too).

The most ingenious one I've seen yet is at the Quiet Revolution Motor Company. I just wish they'd show SOME signs of commercial progress. Their site hasn't changed much since I stumbled across it in the late '90's.


Posted by: Tony Chesser-Evans on 12 Aug 05

WattHead’s well reasoned comments deserve an answer. First, I must be careful to affirm that for all I know, the people in this solar Stirling work are all the very best, absolutely on the up and up, and their technology is truly going to save the world. I do hope so. But that said, I remind everybody that the world is full of very sad stories in which the people engaged in an energy development were indeed in it for the money all right, but not anything else that anyone on the outside would call worth doing. And they could get away with it because the contract payments were for something other than the ostensible target of producing power.

I give an example from my experience a long time ago. A guy came to my group proposing a big deployment of windmills, but when we examined the work proposed, we discovered that the machines were poorly designed and could not perform as specified, yet the job was to put out thousands of them. When we pointed this out, the proposer said with a wink and a wink, “Yea, sure, we know all that, but we get paid for putting these things on the ground; whether they work or not doesn’t matter”.

And there’s more. Sometimes a really good salesman can get multimegabucks for a thing that does everything just wonderfully except for one little problem that needs a little further development, but will surely be an easy fix when he gets some money to fix it. Except that it turns out that little problem never, never goes away, and the whole thing eventually collapses, leaving hardly a watt behind.

Moral of story. Whenever you see a big energy program, ask what they are being paid for. Unless the answer is watt-hours, and nothing but watt-hours, you can feel justified in being at least a little dubious.


Posted by: wimbi on 12 Aug 05

Moving pistons up and down seems so inefficient. Why not use the concentrated solar to generate steam and turn a turbine; and thus power a generator? Put a big flat black heat exchanger where the Stirling is and you have lots steam.


Posted by: Andy on 14 Aug 05

Please ignore or delete my previous post, a bit more reading answered my own question.


Posted by: Andy on 14 Aug 05

Stirling engines have been a very good idea to implement for decades now.

what bothers me is the pictures at the SES website with the little "oil tank" and "fuel injector" ...
feels like some explanations are missing.
lets not get overexited (like with fuel cells and their Hydrogen made from oil or fertilizer hungry biofuel).

i wouldnt want it to turn out to be oil fueled solar panels


Posted by: Jul on 15 Aug 05

Being skeptical is easy , however we should not forget that stirling engines have already worked perfectly in Plataforma Solar de Almería (www.psa.es) for 40.000 plus hours and I don't see any problem in future as well.For the case of using solar steam instead of solar stirling , I think for maintenance you need to close the whole plant together where as in in solar stirling it could be one dish at a time not affecting energy supply very much.


Posted by: Kashif Haseeb on 15 Aug 05

Anyone know what the practical fraction of energy is on something like this? I occasionally see news of things like 150MW wind farms, but the level that can be practically delivered on a regular basis is usually onlt 35%-40% of the rated power output. Seeing something like this rated for 500MW is good, but what is the actual output expected to be?


Posted by: Jarrod Frates on 15 Aug 05

" Worked perfectly" means different things to different folks. I once complained that my chain saw needed lots of care, while my friend said his "worked perfectly". But when I quizzed him, he admitted that he too had to replace plugs, chain sprockets, clutches, chains, clean filters etc etc etc, but while I called all that "lots of care", he discounted it as "normal" and claimed his saw "worked perfectly"!

"Aside from that, Mrs. Lincoln, how was the play?"

The meaningful number we have to look for is the mean time to failure. That is, how long, on the average, will the whole system run before somebody has to go out and fix something. On chain saws, it's a very few hours. What is it on Stirling engine solar systems??? I have not been able to find that answer.

And of course another meaningful number is the cost/kW-hr, and how much of it is people going out to fix whatever needs care.


Posted by: wimbi on 15 Aug 05

all it takes is a source of heat.

just a little provocative thought: we actually have large amount of concentrated available heat provider material, namely nuclear waste. and in some cases we are even spending energy to cool them down.

... why not?


Posted by: Michael on 15 Aug 05

Right you are, Michael. The same thought occurred to me as I was reading about a great little stirling engine for NASA that operates on plutonium and is designed to last 15 years. All we need to solve the energy problem is one of those in everybody's basement, churning out electricity and heat. No more nasty coal, no more splitting wood. And as I understand, we and the Russians are having a hard time thinking of what to do with all that under-utilized weapons-grade stuff. Here it is!


Posted by: wimbi on 16 Aug 05



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