Problem 9.4 - A R744 (CO2) Home Geothermal Heat-Pump
Introduction and Description
With the global quest for energy efficiency, there is renewed interest in renewable energy resources (refer to the U.S. Department of Energy website: Energy Efficiency and Renewable Energy.) The term Geothermal Energy has normally been associated with high temperature power generating plants, as described in the USDOE websites Enhanced Geothermal Systems and Hydrothermal Power Systems, however in this section we are mainly concerned with Geothermal Heat Pumps. Essentially this technology relies on the fact that a few meters below the surface of the earth the temperature remains relatively constant throughout the year, warmer than the air above it during winter, and cooler during summer. According to the Spring 2009 newsletter from David White, in Southeast Ohio this temperature is around 55°F (13°C). This means that we can design a heat pump which can combine hot water and space heating in winter in which the earth is used as a heat source (rather than the outside air) at a considerable increase in coefficient of performance COP. Similarly, with suitable valving, we can use the same system in summer for hot water heating and air conditioning in which the earth is used as a heat sink, rather than the outside air. This is achieved by using a Ground Loop in order to enable heat transfer with the earth, as decribed in the USDOE website: Types of Geothermal Heat Pump Systems Ground Loops.
Other interesting websites describing geothermal heat pumps include those of the California Energy Commission, the Geothermal Heat Pump Consortium, and of course the ubiquitous Wikipedia. Another relevant website is that by Mortgage Calculator titled Geothermal Resources for Homeowners (Thanks to Aaron March of Jericho, VT, for making us aware of this interesting website - Nov 21, 2011.)
Problem 9.4 - We wish to do a preliminary thermodynamic analysis of the following CO2 home geothermal heat pump system designed for wintertime hot water and space heating. Notice that with suitable valving this system can be used both in winter for space heating and in summer for air conditioning, with hot water heating throughout the year.
Notice that the gas cooler section includes both the hot water and space heater. We assume that 50°C is a reasonable maximum hot water temperature for home usage.
Using the conditions shown on the diagram:
Engineering Thermodynamics by Israel Urieli is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License