Survey of Energy Resources 2007
Geothermal - Heat Pump Applications
One of the fastest growing applications of renewable energy is the use of geothermal (ground-source) heat pumps (GHP). This direct use of geothermal energy is based on normal ground or groundwater temperatures which are relatively constant and available anywhere. There are two main types of geothermal heat pumps (Fig. 11-9 ). In ground-coupled systems a closed loop of plastic pipe is placed in the ground, either horizontally at 1-2 m depth or vertically in a borehole down to 50-250 m depth. A water-antifreeze solution is circulated through the pipe. Thus heat is collected from the ground in the winter and optionally heat is rejected to the ground in the summer. An open loop system uses groundwater or lake water directly as a heat source in a heat exchanger and then discharges it into another well, a stream or lake or even to the ground. In essence heat pumps are nothing more than refrigeration units that can be reversed. In the heating mode the efficiency is described by the coefficient of performance (COP) which is the heat output divided by the electrical energy input. Typically this value lies between three and four (Rybach, 2005).
Worldwide data relating to geothermal heat pump applications were presented at the World Geothermal Congress, Antalya, Turkey, 2005 (WGC-2005). According to those data GHPs account for 54.4% of the worldwide geothermal direct use capacity and 32% of the energy use. The installed capacity reported in 2005 was 15 384 MWt and the annual energy use 87 503 TJ, with a capacity factor of 0.18 in the heating mode. Based on the size of a typical heat pump unit of 12 kW and the total installed capacity, the total number of installations was estimated to be 1.3 million in 2005, which is over double the number of units reported in 2000.
Fig. 11-10 shows the rapid growth in the worldwide use of geothermal heat pumps, as well as those leading countries which reported at, and after, WGC-2005.
Until recently, almost all of the installations of ground-source heat pumps have been in North America and Europe, increasing from 26 countries in 2000 to 33 countries in 2005 (Lund, et al., 2005). China is, however, the most significant newcomer in the application of heat pumps for space heating. According to data from the Geothermal China Energy Society in February 2007, space heating with ground-source heat pumps expanded from 8 million m2 in 2004 to 20 million m2 in 2006. Conventional geothermal space heating in the country had grown from 13 million m2 in 2004 to 17 million m2 in 2006. This reflects the policy of the Chinese Government to replace fossil fuels where possible with clean, renewable energy. The Law of Renewable Energy of China was implemented in 2006.