By definition, geothermal energy is the extraction of energy stored in Earth. However, there are two distinct types of geothermal energy depending on its origin: shallow and deep geothermal energy. Shallow geothermal energy is the solar energy stored in Earth, the origin of which will be described in Section 5.4. The temperature is typically some 10°C off that of the surface. The major application of shallow geothermal energy is to enhance the efficiency of the electrical heater and cooler (air conditioner) by using a vapor compression heat pump or refrigerator. Deep geothermal energy is the heat stored in the core and mantel of Earth. The temperature could be hundreds of degrees Celsius. It can be used for generating electricity and large-scale space heating. In this section, we will concentrate on shallow geothermal energy. Deep geothermal energy is presented in the following section.
The general behavior of the underground temperature distribution is shown in Fig. 1.18. At a great depth, for example, 20-30 m underground, the temperature is the annual average temperature of the surface, for example, T = 10° C. At the surface, the temperature varies with the seasons. In January, the temperature is the lowest, for example, T — AT = 0°C. In July, the temperature is the highest, for example, T + AT = 20°C. There are diurnal variations, but the penetration depth is very small. Because of the finite speed of heat conduction, at certain depth, typically —5 to —10 meters below the surface, the temperature profile is inverted. In other words, in the summer, the temperature several meters underground is lower than the annual average; and in the winter, the temperature several meters underground is higher than the annual average.
The solar energy stored in Earth is universal and of very large quantity. In much of the temperate zone, it can be used directly for space cooling. By placing heat exchange structures underground and guiding the cool air through ducts to the living space, a virtually free air-conditioning system can be built. In areas with average temperature
Figure 1.18 Shallow geothermal energy. Seasonal variation of underground temperature. On the surface, the summer temperature is much higher than the winter temperature. Deeply underground, e. g., minus 20 m, the temperature is the annual average temperature of the surface. In the Summer, the temperature several meters underground is lower than the annual average; in the winter, the temperature several meters underground is higher than the annual average. The energy stored in Earth can be used for space heating and cooling, to make substantial energy savings.
close to or slightly below 0°C, underground caves can be used as refrigerators, also virtually free of energy cost.
The major application of the shallow geothermal energy is the space heating and cooling systems using vapor-compression heat pump or refrigerator, taking the underground mass as a heat reservoir. Details will be presented in Chapter 6.