Prof. D Chandrasekharam Hyderabad, Telangana

The common two types of GHPs in use are 1) earth-couple (closed loop) system that uses sealed pipestubes-placed vertically or horizontally, through water or a mixer of water and antifreeze circulates transferring heat to and from the earth and 2) water source (open loop) system where water from the underground aquifer pumps water to the heat exchanger. Between the two, earth coupled GHPs are very popular because they are very adoptable.

Heat pumps are being in use for several decades in refrigerators and air conditioning units. It is a well insulated unit that move heat from one "space A" to " space B. When heat is removed from space 'A' and space ' A' becomes cool and 'B' becomes hot. Based on the need and space 'A' can be made cool or hot. This concept is in use since 1850 when James Harrison made the first refrigerator. The ground source heat pump works on the same principle. The ground source heat pumps (GSHP) are also known under several names: Ground Heat Pumps-GHP, Ground Coupled Heat Pump-GCHP, Groundwater Heat Pump-GWHP.The basic principle on which the GHP works is "refrigeration cycle". The refrigerant carries the heat from one "space" to another. The heat pump's process can be reversed. The earth is the main source and sink of heat. In winters it provides heat and summers it takes the heat.

The performance of a heat pump is a measure of its COP ( Coefficient of performance). Commercially available HVAC systems have COP of 3 to 4 while GHPs have greater than 6. The COP also depends on the temperature difference. When the temperature difference is small then the COP will be large. Similarly the cooling performance, measured as energy efficiency ratio (EER). The GHPs EER, depending on the seasonal temperature variation is about 80% and above.

The technology is very matured and systems can be installed adopting the local temperature variations. According to the reports published in the World Geothermal Congress 2010, up to the year 2010 (ending December 2009) the installed capacity of GSHP in the world is 50583 MWt and the energy generated was 438071 TJyear (121696 GWhy: CF of 0.27). The country that has the highest installed capacity of GSHP is USA (12611 MWt) followed by China ( 8898 MWt). In China the amount of utilizable geothermal energy (for space cooling and heating) at shallow depths, according to a news published in "Renewable Energy World.com" in 2011, is equivalent to about 350 million tons of standard coal, which is equivalent to 2.8 million GWh of electricity. If this energy source is tapped, then this will reduce emission of 500 million tons of CO2 by avoiding to mine 250 million tons of coal. The extractable geothermal energy in China's 12 major geothermal provinces is equivalent to about 853 billion tons of standard coal that could generate seven billion GWh of electric power!!