Garry,
Glad you brought up Peltier devices. They can also be used as generators, thermoelectric generator (TEG). They aren’t usually cost effective, but in some cases they can be. Imagine a heating device with electronic controls that MUST not fail even in the event of a power failure. A TEG can sometimes be a more reliable and cost effective solution than a battery UPS system. Amazon.com : thermo electric generator
Rindert
As I understand it, the thermocouples aren’t very efficient (possibly an area ripe for a breakthrough with nanotechnology), but for certain purposes they work very well. The products you listed show very credible energy density, 7w for 4" sq is pretty attractive. I wonder what temps they can handle, and life expectancy.
The Voyager space probes used a hot radioactive material surrounded by a thermocouple array, the one had enough power a few years ago to answer back when it crossed into galactic space. With more radiation it would function even longer.
If the price and efficiency were better, some thermoelectric pads on a wood stove would be very hard to beat.
I have mentioned this elsewhere before, but years ago Sunpower had partnered with Woodmizer, and briefly offered a sealed linear Stirling, said to generate 1kw. They sold it as part of a sawdust burning woodstove. Coleman got involved also in producing a linear Stirling refrigeration system for a cooler.
The linear Stirlings promised long life and high reliability, as parts ride on gas bearings. They had been proven mission reliable for NASA.
But now I see nothing from that company, and nothing remotely like the consumer products they had been making. Very unfortunate, as 1 kw from the wood stove would address my energy needs, and would allow many people to diversify the electrical grid.
From everything I could find last winter the technology just isnt practical yet for those thermal generators. As I recall the life span was also pretty short. More battery storage looked more cost effective here with solar.
Nonetheless, it is true that, with over 15,000 TEGs inthe field and a commercial operating history dating from 1975, failures are exceedingly rare. TEG Reliabilityreport | PDF | Reliability Engineering | Analysis
While I was rehabilitating and testing a “vintage” Acer Chromebook today, I came across a YouTube video with a Stirling engine twist which was so unique I had to say “cool.” Perhaps you will find it interesting as well.
I spent a lot of time looking at Stirling engines for off grid power generation in a CHP configuration. Two key advantages: (1) External combustion means you can use any fuel, including tarry wood gas and (2) they have very few moving parts leading theoretically to long life. But in my mind it’s a dead end:
(1) For starters… you can buy plenty of toy stirlings that spin an unloaded wheel but you can’t buy a 5-20kw Stirling for any reasonable amount of money if at all. (2) And if you try to build one the engineering constraints are a total bear. Which leads right back to why (1) isn’t going to change.
To expound:
The “hot side” requires a material that can take high gas pressure while super hot.
It wants to be spherically shaped for mechanical strength and low gas friction, cylindrical so the piston can get in and out for reduced dead space and multi thin tube arrayed for heat conduction.+
The heat exchanger wants to have high surface area and turbulent flow for rapid heat transfer with low gas volume to reduce “dead space” losses; but also high volume, low surface area and laminar flow for reduced gas velocity and piping losses. The trade offs here are super tough.
Also it wants to cycle fast for high power but slow for efficiency. High cycle rate and high gas pressure are basically required for an engine of practical size, generating practical power. Skimp on one and you have a truck sized engine putting out 15hp.
Oh and you want to use hydrogen as the working gas for best thermodynamic properties but hydrogen also embrittles the steel housing (boom) and leaks out over time. Helium is second best on physics, doesn’t embrittle but also leaks, can’t self source it and is expensive.
A cousin to the Stirling is much more promising, the Ericsson heat engine. You can’t buy one of those either, and it requires a hot valve where a stirling does not, but at least the engineering isn’t working against itself at every turn. Also uses external combustion.
In researching Ericsson engines I came across these folks:
http://www.proepowersystems.com/PROEHOME.HTM
I read their materials just nodding my head - it’s a good explanation of why stirling is mostly a dead-end. Sadly they don’t seem to have made a go of it which is too bad.
Great post AnthonyB.
“Pretty promising Rose Gardens have many thorns”, me.
S.U.
I have found something of interest to those who like Stirling Engines.
Did you know there is RIGHT NOW in the market refrigerators Freezers that use stirling engines to run them and they do not have a compressor, nor do they use freon???
These cost $9,000 and up.
I have seen a portable unit that is going for just over 1,000$
I would believe because they have so little to fail this would be a very very reliable freezer. And for being off grid would be worth it.
That is interesting but I suspect you could buy a normal refrigerator and the solar PV system to run it for far less than $9000
Right you don’t buy a diesel train engine to run your house but you can buy a diesel engine to run your tractor. They do make smaller units, and less expensive.
As always the right tool for the job, these go down to - 80 deg.
Refrigeration and power generation are very different even if a Stirling “engine” is used in each.
Power generation is efficient when the hot and cold side are as different as possible. In practice that means a very hot, hot side and an ambient temp cold side with temperature differences of many 100s of degrees.
For refrigeration, things are efficient when the hot and cold side are as similar as possible. In practice this means an ambient temp hot side and a freezing or modestly below freezing cold side… maybe not even 100 degrees C or <200 degree F separating the two halves. The materials and configuration are totally different for refrigeration and power generation because of the temps involved.
If you want refrigeration off grid… just get an efficient regular electric appliance and power it as you see fit. Maybe DIY some extra insulation and maybe add some supplementary cooling on the evaporator…
EcoRenovators has a great thread on DIY heat pumps that might serve for inspiration but we are still talking about repurposed electric refrigeration compressors.
Yeah, the main point I wanted to say is that here is a freezer that uses a stirling engine. The fact that it is a commercial product blew my mind, and the fact that it is used for medical and lab grade freezers also.
What you said about keeping the temp closer on both sides of the stirling was exactly what was said in this video.
I like working with 3d printers, and I have a friend who has built a stirling engine. I am looking at the question is it worth building this myself, or… can I just buy the stirling engine off the shelf, or buy a used fridge, what is the best use of time…
My standard fridge is doing a great job, I am always looking to do better… I do tend to end up doing the conventinal way, but I do explore new paths…
Thanks for your input.
They really did some fine engineering on this…
Improvements to stirling engine…
Id like to see this integrated into the rotary motor. Might improve the energy density for the sterling design. Liquid Piston has a design that may work with some mods.
Watching this video again now I understand why he pressurized the engine for more power.
Some Stirling engine rabbit holes to explore:
Ohio University has been active in Stirling Engines with a number of papers and patents-
This guy designed that refrigerator mentioned above, also from ohio-
This is the design “bible” more or less as produced for NASA/DoE. Very extensive…
This is a pretty advanced DIY effort on a small stirling engine-
http://diystirlingengine.com/sv-2-stirling-engine-generator/
And you’ll find the Philips MP1002C coming up over and over. That is worth some research, research that is pretty easy to find.
There is a “simple” spreadsheet floating around that will estimate engine performance based on various design parameters such as gas pressure, stroke and bore dimensions, cycle frequency, etc. I don’t have it handy but it’s worth grabbing a copy if you come across it.
I’ve been down all these rabbit holes and more. My conclusion was that I can’t buy or make a suitable stirling engine but maybe someone else will figure it out.