Problem 4.13 - The BSU Geothermal Heat Pump System (Summertime)

Introduction and Description

With the global quest for energy efficiency, there is renewed interest in Geothermal Heat Pumps which were have been in limited use for more than 60 years. 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 (around 55°F (13°C)), warmer than the air above it during winter, and cooler during summer. 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: Geothermal Heat Pumps. Another description of geothermal heat pumps has been provided by David White Services of Southeastern Ohio and includes a youtube video by WaterFurnace: GEOTHERMAL How does it work.

Problem 4.13 - On Friday Sept 24, 2010 professor John Vann from Ball State University in Indiana came to Ohio University to speak about BSU's switch from coal powered heat to geothermal heat. This impressive project is one of the nation's largest closed geothermal energy systems. The talk did not include many technical details, however he did describe the overall BSU system, which includes 4100 boreholes to extract or reject heat to the earth, and then transfer that heat through two energy stations to a network of hot water and chilled water loops flowing through the entire campus to provide hot water (at around 150°F) and space heating or cooling as required. (Update: The BSU geothermal system was officially dedicated in March 2012)

We were intrigued by the concept and would like to evaluate the thermodynamic feasibility and performance of a geothermal heat pump system. The following system diagram represents a possible system for summertime usage, in order to provide hot water at 65°C, and space cooling using chilled water at around 13°C. Note that this system was devised by us for purposes of this concept feasibility check only, and no data about the system was obtained from BSU. We have used the Refrigerant R134a, since this is the only refrigerant for which we have tables available. In fact we had to add new data to our tables, since with a limit of 1.6 MPa we could not reach the required temperature of 65°C. Note that the mass flow and actual power required is not specified, thus this model will represent a system suitable for any size. All energy results will be in units of kJ/kg.

 

Using the conditions shown on the diagram do the following