Converting Low-Grade Heat into Electrical Power







Low-Grade Heat Sources

        Solar Thermal
        Industrial Waste Heat

 Low-Grade Heat Sources--


    The Earth's temperature increases with the depth from the ground. It was reported that the geothermal gradient is 25-30C per km of depth in most of the world, not including the tectonic plate boundaries adjacent area. Geothermal reservoirs can reach temperature up to 370C, and they are powerful sources of energy.

    A typical geothermal extraction process would be injecting a cold fluid deep into the ground, and pumping it back when it is heated by the underground heat. Geothermal is cost effective, sustainable, and reliable. Although geothermal wells release greenhouse gases trapped deep within the earth, these emissions are much lower per energy unit than burning fossil fuels. Therefore, geothermal is environmental friendly.

Fig. 1 Extracting geothermal from the Earth


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    In stead of digging wells, there are places where geothermal can be obtained directly from hot springs, hot geysers, or steam vents. For example, in Iceland, the Nesjavellir Geothermal Power Station uses the steam rise spontaneously  from underground.  Beside Iceland, there are reliable geothermal source in many places like Hungary, Kenya, the Philippines, and the United States.

Fig.2 Steam rising from the Nesjavellir Geothermal Power Station in Iceland

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    Below shows the geothermal potential in the United States. It is remarkable that the entire United States are suitable for geothermal heat pumps. Currently, the geothermal reservoirs occur primarily in the western states.

Fig. 3 The geothermal potential in the United States





Thermodynamic Cycles for the Conversion

        Kalina Cycle
        Goswami Cycle
        Trilateral Flash Cycle
        Organic Rankine Cycle
        Supercritical Rankine Cycle