9.4: Geothermal Heating

In the preceding sections of this chapter we talked mostly about using geothermal for generating electric power. But another worthwhile application is heating – either the space in people’s houses, or household water, or water in public swimming pools, or greenhouses, or the list may be long.

In geothermal heating, one can either use for this purpose all power extracted from the geothermal reservoir, or send this power first to a heat engine for generating electric power and use the waste heat from the engine for heating. As we remember, a heat engine converts only a fraction ε of the input thermal energy into mechanical work. In the best coal-fired or natural gas-fired power plants this fraction may be slightly higher than ε = 30%, the rest is the “waste heat”. But in geothermal systems the temperature of the “input heat” is lower than the steam temperature in coal or gas fired plants. Hence, the efficiency of the heat engines in geothermal installation is also lower – perhaps 20% in the dry steam system, and even less than 10% in the binary cycle systems. So, as much as 80-90% of the input heat ends up as the “waste heat” and it can be used for heating. As follows from the previously discussed example of Altheim, an electricity generation + heating combination may indeed work pretty well.

There is only one problem with heating – namely, in contrast to electric power, heat power cannot be sent over long distances. In other words, all available heat power has to be used locally. In Altheim, the geothermal installation was intentionally built next to the town, so delivering heat to homes easy. But often, in the case of huge American installations like, for instant, the famous The Geysers power plant in California, there are no human settlements nearby. So, all the waste heat has to be dispersed in the cooling towers or injected back to the underground reservoir. This is regrettable, because in the energy usage balance of an average American household, as can be seen in Fig. \(\PageIndex{1}\ more than 50% energy is used for space heating.

Figure \(\PageIndex{1}\):A “pie-style” chart showing how energy is used in an average American home – 2009 data from the U.S. Energy Information Agency Web page.

If geothermal energy could be used for this purpose instead of electricity, natural gas, or heating oil, the total CO₂ emission from American homes could be significantly reduced.

Figure \(\PageIndex{2}\): The distribution of geothermal resources in the US. Red color indicates very good conditions, orange – favorable conditions for installing binary cycle power plants, or for using geothernal water directly for heating. And the other two colors shows the areas where geothermal resources can be perhaps reached by very deep drilling, which, however, would make the costs of such ventures economically unjustified. Nevertheless, the homeowners in such areas (as well as in the other two areas) may reduce the heating costs considerably by using the so-called shallow geothermal heating (source: EIA

Well, but the country’s geography is not particularly supportive for such a noble idea! It is evidenced by Fig. \(\PageIndex{2}\), in which the distribution of the “geothermal potential” over the entire U.S. area is shown.