These use a planar spring which holds the pistons away from the cylinder walls very precisely, or apparently there is another way to do it which I have not read much about which uses "gas bearings and compliance", according to Sunpower this is the way they do it, in contrast to infinia who uses springs, but I have seen videos and so forth of their engines and they are using a planar spring. Apparently the gas bearings are to maintain alignment, and the spring provides the return force for the resonant motion (it is also possible to use a gas spring but I haven't seen examples of this done). The motion of the power piston is harnessed with a linear alternator. See the sunpower library for more details.

The steam engine that is low maintenance will I think probably work out, but it still needs to be demonstrated. Now that FeF is using a dish collector it might make total sense to go for a FPSE instead for electrical generation, for solar but then the engine might as well be reused for cogen. Given that the engine currently being developed is 1 Kw it is not suitable for the lifecube anyway, where I think steam would make the most sense, so 2 engines will need to be developed.

I watched some of the videos on vimeo about the steam engine, and the main developer from SACA indicated I think 15 percent. Now I dig this might make sense for biomass if you don't mind harvesting a lot or you have a use for the excess heat. Depending on the grade it could be reused for air conditioning (though that is a project itsself).

But the sunpower engines you can see get 36% overall for an 80 watt engine at 2.7 thot/tcold, and the larger engines can get up to 40% efficiency as mentioned in the library, due partly to lower conduction losses and lower gas leakage around the piston for a given gap size between wall and piston. Now I know excessive focus on efficiency makes no sense, dollar per watt is what matter here. But if the efficiency is more than twice as high that is gong to have an impact. I don't know how much the carnot efficiency varies with temperature with these engines, but IIRC it is fairly flat, so the same engine would work well with the dish collector. We know the collector has a substantial cost onf $0.5 to $1 per watt, so cutting it to half the size would be a very real money saver.

Given their mechanical simplicity, they could be easier to replicate than a long lasting steam engine too. They do not require super accurate machining apparently. A 1 Kw might be fine with less than 25 micron machining, less only reduces efficiency due to large gap etc. The spring needs to be the right shape but not terribly accurate, and could be easily laser cut or maybe even done with a router or even etched or stamped then the edges sanded by hand. You just have to make sure all parts operate with the strain of the material below the fatigue limit and it lasts a very long time. So once the shape is open sourced it's easy to duplicate.

Conceptually they are extremely simple machines, and although apparently the gas dynamics are not trivial there is a lot of expertise on stirling engine design in the amateur community to draw on, and a decent CAE fluid dynamics program would surely be a great help. Documents coming out of sandia labs on the dish stirling engines they developed in the 70s indicates they were not having any problems. So again once the design is worked out they may be a lot easier to duplicate.

Basically it is a beta stirling (see wikipedia on stirlings) and the spring and mass of the piston+rod are a resonant unit. The main other problem is to keep them out of phase at the right angle, this is accomplished by allowing the engine as a whole to move slightly which is apparently enough. The phase of the power piston could also be adjusted by drawing the power from the linear alternators with slight modifications to slow down the piston when desired, or allow it to go faster producing a slight phase shift.



resources:
enatech natural gas cogen with fpse

sunpower library, sunpower tried to produce a natural gas cogeneration unit called the microgen but gave up, they have been working to produce a biomass powered unit but it seems unlikely they would be interested in collaboration since they are an IP company at heart there is a book "the next great thing" about the company history. William beale, company founder is the inventor of the FPSE. I called them once and talked to the head of the organization to ask if the engines were in production, no but he said though also said they last 20 years no maintenance library: http://www.sunpower.com/index.php?pg=9

hm, http://74.125.155.132/scholar?q=cache:xLq7BdTJcuIJ:scholar.google.com/+free+piston+linear+stirling+flexure+bearing&hl=en&as_sdt=0,5

machineable ceramic or ferrite cores may reduce eddy current losses inalternator and may not be expensive or that hard to work with
boch thermotechnique is working to produce natural gas cogen unit
omachron tried to make very very cheap fpse stirling engines out of stamped metal parts for like $75 for a 1 hp motor that was 22% efficient at combustion temperatures. Cant find the page again now but I say it on their own website and it was in that price range. For some reason they blamed import restriction after 9/11 for killing the project.
Infinia was a nasa spinoff called stirling tech of washington (not to be confused wtih stirling teck of washington which also makes a crappy overpriced stirling engine the st5 runs on biomass though) dish stirling commercialized

The harwell thermomechanical generator was a crappy fpse that used flexures but in a different way, inefficeint and heavy suspect this was due to the high stiffness of the flexure, to oget decent resonant frequency therefore requires large mass.

-use neon or hydrogne as working gas? n2 commonly used , methane is also suggested and may have beed used previously but might not be stable at higher temps
forum information on stirling engine groups indicates hydrogen might actually be workable, it is used in industry as a coolant often and is manageable, embrittlement occurs on hot metal though , apparently an engine designed optimally works optimally regardless of chosen gas so different gasses can be tested easily or the engine could be drained and recharged with a different gas for different uses.

- the alternator could perhaps be purchased off the shelf
flexure bearing cartridges are available but they might not be capable of the high number of cycles though given ease of manufacture I don't know why the wouldn't be.



more links , there is a lot of good information on this which should be a good help

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TWR-4JF97R6-1&_user=10&_coverDate=06/30/2006&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1745923124&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=5b9a5a10c472f4d2c55c6beca2bc1b50&searchtype=a
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=870691
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=870693

http://www.scientific.net/AMM.44-47.2065
I saw a better one once specifically on the springs for an engine rather than cooler but same idea
http://scholar.google.ca/scholar?hl=en&q=free+piston+linear+stirling+flexure+finite+element+analysis&btnG=Search&as_sdt=0,5&as_ylo=&as_vis=0
http://gltrs.grc.nasa.gov/reports/2004/TM-2004-213412.pdf

here's an example of a chinese system with solar thermoacoustic, 1kw:
http://english.cas.cn/ST/HT/ht_progress/201008/t20100819_57663.shtml

yes, if a an adequate design can be worked out they might be the way to go, although they are more experimental than fpses. I think research papers on engines that have been built may b a more useful place to start because they would actually describe the engines, like the ring shaped 30% efficient one. If I had access to these journals online etc. I would dig them up, but I don't. Marcin has access though. If the engine is 30% efficient as long as the size is reasonable you can just have more engines, so it does not have to be 1kw, although that could be nice if efficiency went up even further or it was easier to make of course.

@Dawg there are tons of papers online about them. It is actually very simple to build, and that is one of the advantages.

just start with google: thermoacoustic engine. You'll soon get info from Los Alamos and Sandia Labs. I've downloaded tons of papers and patents, but I can't remember any specific links.

I was just reading up on the possibility of getting helium from air. It's not as hard as I thought.

http://www.physlink.com/reference/AirComposition.cfm

the obvious problem is that the boiling point is extremely low, and the lower the boiling point the less efficient refrigeration processes get. However we only need a little bit.

Interestingly the stirling cycle is on eof the most efficient methods of producing low temperatures. I do not know what the energy efficiency would be to get helium by fractionation of liquid air, but a byproduct would be all kinds of gasses for use in welding and other things, and we only need a relatively little bit of helium really.

Wikipedia says it takes 220 watt hours to get 1 liter of xenon though again depends on the temp diff. Also obviously you can't use anything as the working gas directly to liquefy helium, but by using say liquid xenon and a vacuum pump the necessary low temperatures can be produced. Or you can just condense everything *except* helium.

Or they may be methods like molecular sieves or the use of gas diffusion (mulit stage) across a thin dimethylsilicone membrane, which is used in oxygen concentrators, which could be more reasonably efficient.

In any case, it is more common than xenon and some of the other gasses so maybe it would in fact be acceptable to include helium as the working gas as it is sufficiently common and abundant in this context to mesh with OSE goals.