For the past week I've begun researching Geoexchange or the process of how Geothermal Heat Pumps work. A geothermal heat pump works by utilizing the earth as either a source or sink depending on the time of year. In the winter the area beneath what is called the freeze line is warm in comparison to the ambient temperature. In the summer the earth is cooler. Liquids such as types of refrigerant or mixtures of water and anti-freeze make thermal connections with the ground and are either heated or cooled. Ultimately, the heat phase change of the liquid is exchanged to heat water or cool appliances. What happens in between depends on the type of system being used.
Direct exchange systems are the more simple and eficient methods of ground heat exchange. They are made up of a single loop of circulating refrigerant that is in direct thermal contact with the ground. Cooper tubing is used because of it's high thermal conductivity, thus less tubing is used and less drilling is required. In return the installation and maintenance costs are less. This system is the most popular since it is the most direct method and requires the least amount of equipment and initial investment.
Closed Loop Systems consist of two loop systems, one under the ground where the thermal exchange from ground to a water, anti-freeze liquid mixture occurs and one above the ground where the heat exchange is transferred to a more conductive refrigerant which then flows to heat water or to cool large appliances. These closed loop systems are either orientated vertically or horizontally depending on many site factors such as soil type, moister content, and ground temperature. An example of a vertical system designed to heat a building with a 10kW heating capacity would require three 260-360 foot tubes, thus the boreholes would also be this deep, about 5-6 meters apart. This system is used when space is limited but more drilling cost more money. An example of a horizontal system designed to heat the same building would require the same tubes but they would only have to be 3.3-6.6 meters deep. Shallow wells receive more heat from the sun but also lose more heat to the ambient temperature. Thus it is ideal to find a nice 'middle ground'.
The team and I started by looking at which buildings on campus are not connected to central heating systems. It is more difficult than it is worth to take a building off central heating and tie it to a geothermal pump so we decided to only include buildings off the central heating grid in our study. This limited our prospects to only a few areas. We went out and surveyed areas on Trinity Campus, areas around the Blundle House, police services, and around waterman. The factors that will determine the potential of these sites are land size, building load, and the information of the surrounding wells, soils and minerals. I have been in contact with the University of Vermont's Geology department to ask them about their test wells. Geothermal Conductivity is one of the most important pieces of information we are looking for to continue or study.
I have been developing many skills over the past weeks of working on this project. I have been writing reports and participating in peer review sessions. Working as part of a team has helped to encourage task distribution and giving and receiving feedback. The nature of the project has allowed me to be in complete control of how I schedule my work and has helped me begin to be more critical of my usage of time. I look forward to working more with the geothermal portion of the project and to continue writing solar PV reports. -RPS