Monday, March 06, 2006

Stirling Engines. The Diff between HTD & LTD

Dean Kaman of Segway fame has a ‘new’ project.

There is a post about it on treehugger


...In 1993 a product of the year was his water purifier; his latest idea for the third world is his stirling engine powered electricity generator that can be powered by anything, including the omnipresent cow patties in India. ...


... He has developed a microfinance model to support it and may soon be building them in a factory in Bangladesh. ...


Last time I looked there were 4 comment also posted.

The 1st comment was a negative politial speal.

The 3rd was a interesting non techo comment with a link.

From comment 4:

I will buy one as soon as available. Biomass heating is far cheaper than electricity. I heat with $2.00 a bushel corn.


Note: a bushel is a volume (not a wieght), there are 36 bushel to ton of wheat (old language).

The last (4th) comment is a feel-good statement. It can’t be acted on until you know the characteristics & costs of the sterling engine and the system that it’s a part of.

The second comment is the one I wish to deal with in a little more detail.


At least this time around he's not pretending it's all his inventions. And if the whole project fails, that means that there will be a mess of stirling engines lying around for other people to use. I've been thinking of using a stirling engine to generate electricity from a solar water heater, but have been discouraged by the utter lack of any affordable engines to run the generator. If Deano's great leap forward™ craters, the bankruptcy sale might yield a few cheap stirling engines.


I’d like to make two comments here.

Firstly, a failed venture may provide a few engines, but is no basis for building a co-generation industry.

Secondly, the comment ‘using a stirling engine to generate electricity from a solar water heater’ shows a lack of understanding. It show that the writer does not understand the difference between High Temperature Differential (HTD) and Low Temperature Differential (LTD) engines.

Low Temperature Differential situations are generally where the difference between the hot side and cold side is less than 100 degrees C. I.e. a good strong cup of tea could drive it.

High Temperature Differential situations are generally where the difference between the hot side and cold side is greater than 100 degrees C. I.e. a naked flame or 'stronger’. Non-combustion examples of HTD energy sources are solar furnaces and super heated geothermal springs/vents.

The engines in the article are HTD engines. The engines that the authour would like to get hold of are LTD engines. Solar hot water would only just get these HTD engine to turn over (if at all). This would not be cost effective. Because HTD engines have an intense heat source, they can be compact and small. LTD are large bulky things, they need to be, to get any meaningful amount of energy transfer.

This is the reason for the 'utter lack of any affordable' LTD engines.

LTD engines are the dream engine. With this type of engine you could build the massively distributed power networks that would be particularly terrorist resistant, in the same way that the Internet is. Not single points of failure.

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