Solar Events

Not all solar energy is silent - the Stirling Engine solution

Beating Photovoltaics

Infinia Corporation of Kennewick, USA, has announced that it will be launching a solar energy product in early 2008 which rather breaks the mould.

Instead of using photovoltaics to convert solar energy directly to electricity, this system heats a gas and uses it to drive a piston back and forth, which then drives an electric generator.

The energy is collected and concentrated from a big reflector, so the system looks like an oversize satellite dish sitting in your yard.

This sounds crazily inefficient, doesn’t it? But apparently not so.

The best (and presumably most expensive) of current practical photovoltaic systems convert about 22% of the available solar energy into electricity. This mechanical system claims 24%, and all using ordinary low-environmental-impact materials and manufacturing techniques. Unlike fancy semiconductor solar panels.

Made in sufficient quantity, such a system could well be cheaper than the competition, it is suggested.

So how is this efficiency achieved ?

Obviously every component has to be tuned to perfection, and NASA has been working away at tuning similar systems for years - for use in space.

Collecting and Concentrating

You need a good collector/concentrator to get the heat onto the right part of the heat engine. It seems that the concentrator is the critical element here, collecting the energy from an imperfect big dish, and funneling it the last mile home. Silvered hollow cones (the simple solution using reflection) can get horribly hot, distort, and anyway lose a lot of the energy. Just the optical equivalent of an ear trumpet.

The top concentrator solution is a sort of transparent “crystal candlestick” into which the sunlight is injected . After shaping the beam on entry by refraction, like a lens, it then reflects it down its inside like a fancy but short and stubby light-pipe, finally squirting it out, carefully shaped, exactly where it is wanted. Because this uses internal reflections, which are “total”, there is virtually no reflection loss. A really good such concentrator is a single shaped crystal of artificial sapphire, because of its high purity, transparency and refractive index. It absorbs hardly any of the energy, it doesn’t get so hot, thus it has no need for cooling.

The Infinia website doesn’t tell us whether their efficiency figure needs them to go this far, or whether a simpler, cheaper concentrator is assumed. A good one does sound pricey.

So now we have all this precious energy captured and concentrated down optimally onto the (glowing ?) hot end of our mechanical engine. We need a really efficient next step.

The Stirling Engine

The engine of choice is called a Stirling engine, a very old concept, but heavily updated.

The energy is poured onto the outside of a closed, sealed chamber containing a carefully chosen gas. This complex little engine has two main subchambers, two pistons and some springy diaphragms.

One piston is floating freely backward and forward attached to nothing but a spring. The other is connected directly to a linear electric generator, which makes electricity as it slides backwards and forwards, driven by gas pressure.

The free piston has the heat at one end, and oscillates, pushing the gas alternately from one end of its chamber to the other. From the hot end to the cool end and back. This alternating expansion and contraction of most of the gas, as it gets hotter then colder, is what pushes the other piston back and forth and generates the electricity.

The only purpose of the free piston is to passively shift the gas from hot to cold and back, and in less refined engines this may require a power-take-off from the main piston (more moving parts - ugh). But in a fancy high-efficiency engine this is avoided by adjusting the springs and chamber sizes so that the whole thing is resonating like a very slow organ pipe, and the free piston is “bounced” naturally at just the right rate.

This is what I meant when I said “tuning”. It seems some sort of “controller” is needed to keep the bounce going as the heat input or load on the motor changes. Presumably this can be done by small automatic adjustments either to springs or chamber size.

Linear Generator

The last step in the chain is the linear generator (read “linear motor backwards”), and I think we can all assume that these things have become highly refined over years of research, so that it is fairly straightforward to spec a really good one to match in.

How does it compare ?

The surprising thing is that once we chain all these parts together, despite the loss at each stage, we can actually beat ordinary pholtovoltaics on overall efficiency.

I use the word “ordinary” because although the theoretical maximum for silicon photovoltaics is apparently 25%, we have been hearing of initial research results using techniques to frequency shift energy from outside the band of silicon sensitivity, and so capture that too. For details see this article.
If such an energy “shift” technique can work cheaply, photovoltaics could leap back into the lead.

So the jury is out, isn’t it ? Shall we all be covering our roofs in sheets of silent and expensive semiconductor ?

Or will we all have big solid 3m “satellite dishes” swiveling slowly around in our yards (following the sun) and chugging quietly like baby steam engines, probably driving the dog crazy ?

Only time will tell.

I wonder if the efficiency calculations have factored in the power required to steer the dish to follow the sun ? Just a thought.

4 Comments so far

  1. john October 30th, 2007 6:08 pm

    everything else I’ve read about Stirling engines says they are virtually silent. Apart from that good article.

  2. Peter January 28th, 2008 10:59 am

    Great details.. Now if SOMEONE could come up with a SOLAR DISH CONCENTRATOR FOR DUMMIES.. I mean for us M-F workers that have little or no solar understanding..

    If SOMEONE could Please put up a post ,link or Website with EZ to understand Step-By-Step A-Z of constructing such a unit using a Sat. Dish (prefer 20′)and Mylar.. I mean Detailed (not technical math etc)steps from Raw unit to finsihed product. Showing complete build and showing all components with parts list (modern) and index. E-Z like what a childs christmas toy instructions would look like.

    I have spend HOURS until my eyes are falling out. All i find is outdated, commercial size , peewee or food cookers. That and alot of A=2

  3. gary September 25th, 2008 9:51 pm

    I have found the same thing as you Peter. I am trying to build a solar collector using a stirling engine, but can find none for sale despite this technology being around for over 200 years. It looks like I will have to contract a machine shop and do it from scratch. Then I will sell one to you Peter!!!!!!!!!

  4. Jobe Roberts March 15th, 2009 11:24 pm

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