Optimization of Water Pipe Size of a Geothermal Heat Pump

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Heat pump systems are commonly used in domestic buildings to meet the heating requirements during winter season. Most heat pump systems are air to air heat pumps since they extract heats from cold ambient air and transfer it hot inside building at the expense of some external work input. Geothermal heat pumps are ground to air heat pumps because they extract heat from the ground and transfer it to hot building interior using some external work input. The focus of this project is on geothermal heat pump which is to be installed to cater the heating requirements of a domestic building located in Houston city of Texas State in USA. The pipe size of a horizontal closed loop geothermal heat pump has been calculated in this project to minimize the total piping cost of underground loop. From given heating load of 1000 kW for the building, parameters of vapour compression cycle are calculated using EES software after selection of a suitable refrigerant. The mass flow rates of refrigerant and water required to accomplish the given heating load are also determined. In the next step, total cost of the underground piping system, assuming project life of 25 years, has been estimated for various pipe sizes. The calculations of heat pump vapour compression cycle reveal that under given soil and climate conditions, the COP of heat pump system operating under ideal conditions, using ammonia as working fluid, comes out to be 6.4. As far as overall piping cost for project are concerned, it has been observed that total costs are very high for very small and very large pipe sizes. For pipe sizes of DN 50 and DN 80, total costs show minimum values. Piping costs increase when pipe size is changed on either side of the given range. Pipe size of DN 50 has been finally recommended for underground loop to minimize total piping cost of the project.