It’s rare these days that one reads about new geothermal technology (there have been only three geothermal posts in all of GPB history), so I like to highlight new developments when I see them.
Recent Department of Energy awards for small clean tech businesses included one to Cool Energy, a Boulder, Colorado, company that has developed a new Stirling engine that converts low temperature heat energy into electricity. The grant will support a demo program to build the company’s first 20kW prototype engine and use it to generate electricity from untapped heat from distributed geothermal sources.
Cool Energy owns at least eight U.S. patents and patent applications, including U.S. Patent No. 7,617,680, entitled “Power generation using low-temperature liquids” (’680 Patent). The ’680 Patent is directed to systems and methods of generating power through the use of thermodynamic engines and liquid cryogens (materials having a low-temperature boiling point).
The core innovation of the patented invention is recited in independent claim 1:
1. A method of generating power, the method comprising:
providing a liquid cryogen in thermal communication with a thermodynamic engine to maintain a temperature differential across the thermodynamic engine with a heat source;
running the thermodynamic engine to convert heat provided in the form of the temperature differential to a nonheat form of energy;
collecting cryogen-vapor produced by vaporization of the liquid cryogen; and
combusting the cryogen vapor to generate additional energy.
This basic cycle is described in detail in the ’680 Patent, and the different positions of the Stirling engine at the major steps of the cycle are illustrated.
FIG. 3 of the ’680 Patent shows an embodiment of the invention in which the thermodynamic engine (304) is in an ambient environment (300). The temperature differential is established by providing a cryogen (308), on one side of the engine (304). Exemplary cryogens have a boiling point less than -150ºC and include liquid nitrogen, helium, liquid hydrogen, carbon monoxide, and argon.
The temperature differential across the thermodynamic engine (304) may be used to extract energy (312). The conditions of the ambient environment (300) may be manipulated to improve efficiency, such as increasing the temperature differential with an external heat source (328).
The temperature differential may also be boosted by combusting vaporized cryogen with a combustion unit (310), which receives vapor from the cryogen source (308) by a direction mechanism (316). An oxidation source (320) may be provided to promote burning. Waste heat generated by combustion of cryogen vapor may be used to locally increase temperature on the hot side of the engine through mechanism (324).
According to the ’680 Patent, the efficiency of energy extraction depends on the size of the temperature difference across the engine, and “the overall energy output of the combination is increased by energy E2 314 to provide total energy generation E1+E2.”
Cool Energy’s press release says the demo project will use the recoverable heat from co-produced liquids (presumably, the cryogens discussed in the ’680 Patent) at oil and gas wells.