Large scale Geothermal developement

[BobMonger]

I've been looking for some designs for hot air engines that can be used with geothermal heat sources that might power a small community. I live in Carlin, Nevada USA on the upper reaches of the Humboldt River in the northeast corner of the state. Although we are quite isolated here our area is practicly awash in hot springs, geysers, and the like but this resource is being underutilized by the larger power companies and co-ops. Although there are experimental "flash-back" power generation projects in the area, the heat required for these is not optimal. The Stirling design holds promise but to have community support for developement there needs to be something on an industrial scale that would justify the land and water usage; something a VERY conservative City Council might consider to offset rising electricity costs. We are looking at something that would power a generation system of around 5 megawatts or enough for a town of about 2500 people. I would be glad to hear of any ideas along these lines. Thanks
BobMonger
Carlin, Nevada USA

[dolanbaker]

I would imagine that such engines would be extremly large and expensive to build.
Presumably you would require multiple engines, one by each hot spring.
What sort of water temperatures are you getting, you would probably need to use a low temperature differential design.

A small scale system could work quite well, but a system generating 5MW may not be practical.

[gary350]

I have a picture of a 40 HP factory engine. It is 4 cylinder, each cylinder is about 3 ft diameter. It is used to run a line shaft in a factory. The information in the book says the engine had to be completely rebuilt every 3 weeks and it took 2 days to do the work. Lots of maintenance to keep such a large engine running. Not very practical if you ask me.

I also have a picture of a tiny engine someone built it with a bellows for the power piston. No friction on the power piston so maintenance should be very low. You should think about an engine design that is low maintenance something that has to run 24 hours a day 365 days a year to supply power to a city will need to be low maintenance.

[Deferr]

This sort of engine would definitely need to be low temperature difference. This means a large displacer diameter to stroke ratio is required for efficient operation. This also means a shorter power piston stroke would be necessary.

It should be noted that the efficiency of a heat engine cannot exceed the Carnot efficiency. This is stated as follows: N=1-Tc/Th, where N=Carnot Efficiency, Tc=absolute temperature (K or R) of cold reservoir, and Th=absolute temperature (K or R) of the hot resovoir. It can be seen by this equation that the greater the temperature difference the greater the theoretical efficiency of the heat engine. This type of rough calculation can sometimes help get a feel for the feasibility of such a project.

[kaess]

A thermal engine will never reach carnot efficiency. Carnot efficiency suppose infinite stable thermodynamics transformations.
The best you can get is Curzon Ahlsborn efficiency.
For geothermal use, efficiency is not the main target. What you need is the max power. Then we can talk about thermo economical efficiency.

A new engine is under design for this purpose. Delta T is about 80° more or less. Power around 250 kW !!! Efficiency can be ajusted between Curzon and Carnot ( To high... Engine stop ).

It is not a Stirling engine. For low delta the only engine you can built is a Carnot engine. It is what I'm doing now...

SK

[Deferr]

A thermal engine will never reach carnot efficiency. Carnot efficiency suppose infinite stable thermodynamics transformations.

This is important. I should have mentioned it in my post. The Carnot cycle is the ideal efficiency of any thermodynamic cycle operating between two temperature resovoirs. There are other limits for particular cycles. I only bring it up because it is often the measure that specific designs are compared against.

For geothermal use, efficiency is not the main target. What you need is the max power. Then we can talk about thermo economical efficiency.

Again, well said. If you can obtain heat for free (geothermal, solar, etc.) then it is not necessary for the cycle to be as efficient as if the energy was a direct cost (gas, electric, etc). It would be great to get more talk on the forum about thermo-economical efficiency.

One more comment. If you are able to express the efficiency of the engine as a function of the temperatures then you can find the amount of heat input needed to drive whatever size engine you need. This can then size heat exchangers, etc, in order to calculate the capital investment involved. This can also be useful in the early parts of the design phase to make general decisions about which technologies to use. These decisions should be made on the basis of cost, logistics, sustainability, low maintenance, etc.