GEORGIA INSTITUTE OF TECHNOLOGYThermodynamicsGEORGIA INSTITUTE OF TECHNOLOGY George W. Woodruff School of Mechanical Engineering ME 3322 Summer 2005 Thermodynamics Geothermal Heating and Cooling System: Thermodynamic Analysis Submitted by: Justin Clark Submitted to: Nader Sadegh Submitted on: July 29, 2005Introduction/Summary With energy costs mounting, many researchers are beginning to find alternatives to the old energy-guzzling ways. One successful attempt to save energy is an alternative air conditioning system known as Geothermal. Geothermal uses the idea that heat can be transferred from cooler ground or water temperatures rather than producing heat as a traditional air conditioner would. One type of Geothermal system uses an open loop system with a water pump that pumps water from a nearby water source—a well, lake, or other source—to a pressure tank that stores the water. An indoor heat pump then uses the stored water to regulate indoor temperatures. Heat Pump The Geothermal heating and cooling system uses a vapor-compression refrigeration and heat pump system. The heat pump is a reversible system controlled by the thermostat in the building. The heat pump has four components: a condenser, evaporator, compressor, and expansion valve. The condenser and evaporator switch roles depending on the climate. During the winter, the open loop system acts as the evaporator and refrigerant absorbs the heat from the water pumped from the nearby lake. During the summer, the condenser is the open loop system, because the water absorbs heat from the refrigerant. 2CondenserEvaporatorCompressorExpansion Valve Figure 1. Winter Geothermal cycle. For example, during the summer, to cool a house, the thermostat would be set to a certain temperature, like twenty degrees Celsius, and set on cool, rather than heat or fan. Once the air in the house exceeds twenty degrees, the heat pump begins a cycle. Warm air passes through the evaporator, where the refrigerant pumping through the system is a cool liquid and absorbs heat from the air. The air blows back into the room, now at a cool temperature. The refrigerant then passes through the compressor, which raises the temperature and pressure of the refrigerant, compressing the liquid into a superheated vapor. The change in pressure causes the vapor to continue moving through the system, where it contacts the condenser, or the open loop system. The cooler water absorbs the heat from the refrigerant, which is now a saturated vapor, and continues through the system to the expansion valve, where the pressure drops and the refrigerant changes back into the liquid state and goes back to the evaporator. The cycle continues as long as the temperature in the room is above twenty degrees Celsius or the thermostat undergoes some sort of change. During the winter, when the house needs heating instead of cooling, the condenser and evaporator essentially switch places. 3Open Loop System The Geothermal system also uses an open loop system which pumps water from a nearby water source. The water pump uses polyethylene piping at least 10 feet underwater in a sufficiently large lake or pond. One acre of water in warm climate can produce enough water for twenty tons of the heat pump capacity for the system, which is sufficient for a 3000 square foot area. Since water temperatures in large ponds and lakes stay more constant than air temperatures, water makes an ideal alternative to cooling or heating with air. Water temperatures in Georgia are about fifteen degrees Celsius in winter and about twenty-seven degrees Celsius during the summer. Another important part of the open loop system is piping that discharges the used water back to the water source. Because the water never actually accumulates more chemicals from the heat pump, the water can safely be returned to the lake or pond, where it can replenish the displaced water. The water pump only needs to pump more water to the pressure tank when the tank drops below a fixed pressure. With a sufficiently large body of water, the temperature of the water has no net change, even though heat is transferred into the refrigerant during the heat pump and refrigeration cycles. System Analysis There are four states of heat transfer in the Geothermal system. These states are represented by Figure 2 using temperatures for the summer cycle. For the summer, State 1 occurs when Refrigerant 22 leaves the evaporator. State 2 occurs after Refrigerant 22 leaves the compressor. States 3 and 4 occur before and after the expansion valve, respectively. The winter cycle is similar to the winter cycle, just in the opposite direction. The states, however, occur in 4the same place because the condenser and evaporator switch places. Table 1 shows the thermodynamic analysis at each state during the summer and Table 2 shows the thermodynamic analysis during the winter using Refrigerant 22, which is recommended for Geothermal systems. Figure 2. States of the Geothermal system for summer. Table 1. Summer thermodynamic analysis where m is the mass of the refrigerant State Temperature (°C) Pressure (bar) Specific Volume (m3Internal Energy (kJ) Enthalpy (kJ) Entropy (kJ x K) 1 20 9.1030 30259m 232.76m 256.37m .8996m 2 50 11 .002583m 250.85m 277.57m .9556m 3, 4 28 11.315 .000848m 78.09m 79.05m .2936m 5Table 2. Winter thermodynamic analysis where m is the mass of the refrigerant. State Temperature (°C) Pressure (bar) Specific Volume (m3) Internal Energy (kJ) Enthalpy (kJ) Entropy (kJ · K) 1 16 8.1268 .0291m 231.59m 255.21m .9048m 2 20 9 .03082m 252.95m 280.68m .9795m 3, 4 20 9.103 .0008263m 68.33m 69.09m .2607m In an ideal Geothermal system, the only heat transfer occurs during the heat pump or refrigeration pump cycle. Because the system uses vapor compression refrigeration, the coefficient performance of the system is represented by Equation 1 for the summer as a refrigeration cycle and Equation 2 for the winter as a heat pump cycle. Using Refrigerant 22 and Equation 1, the coefficient of performance is 9.86 in the summer and 8.64 in the winter. 1241hhhh−−=β Equation 1. Coefficient of performance for vapor compression refrigeration. 1232hhhh−−=γ Equation 2. Coefficient of performance for vapor compression heat pump. According to the first law of thermodynamics, the work cycle is equal to h2-h1, which is 18.09m kJ in the summer and 21.36m kJ in the winter.