This Small Business Innovation Research (SBIR) Phase I project will develop an earth heat exchanger that has a lower installation cost, lower operating cost, and smaller footprint than the currently available ground source heat pumps. High ground loop installation costs are the primary barrier to broader acceptance of ground source heat pump technology despite its efficiency advantage. By replacing the circulating liquid pipes with a "heat-pipe" arrangement which consists of a closed pipe containing a saturated working fluid which is cyclically evaporated and condensed at different locations within the heat-pipe, heat transfer efficiency between the earth and the heat pump refrigerant is increased. A heat-pipe heat exchanger would require no electricity in heating mode and only a very small amount of electricity in cooling mode. A novel working fluid which has a low vapor pressure, is non-toxic, inexpensive, and is not a potential ground water contaminant has been identified. Successful completion of this project will result in a heat-pipe exchanger that is more economical to install and operate.
The broader/commercial impacts of this research are increased adoption of ground source heat pumps, the most efficient means of heating and cooling. The heat-pipe will have a lower internal thermal resistance and will use a larger pipe diameter than CLPS which will increase the contact surface area per unit length. This will allow less boring/trenching, will simplify installation, and will lower installation costs. A significant reduction in the cost and footprint of installations could increase adoption in both new building and retrofit applications. World demand for HVAC equipment will rise more than five percent per year through 2012, exceeding $70 billion. The US market is growing at 3.2% per year and exceeded $14B in 2009. The new technology will be licensed to heat pump manufacturers.
The purpose of this project is to develop an improved earth heat exchanger for use in conjunction with a ground source heat pump (GSHP). This heat exchanger will have a lower installed cost than conventional GSHP earth heat exchangers. A reduction in the installed cost of GSHPs will result in wider adoption of this technology which will result in lower per capita energy consumption. The new heat exchanger utilizes a heat pipe principle and includes a method for pumping working fluid so that the heat pipe can be operated both in heating and cooling mode. A small scale engineering model of the GSHP heat pipe system was designed, constructed and tested. A suitable working fluid has been selected and an appropriate pump and drive means have also been developed and tested. The engineering model was tested above ground for convenient access and observation and a water jacket was fitted to the lower "earth" section of the heat pipe in order simulate the thermal mass of the soil. A one ton (12,000 BTU/hr) "mini-split" heat pump was adapted for this testing. The experimental heat pump / heat pipe system was fitted with appropriate pressure and temperature sensors as well as a means of determining refrigerant flow rate. The heat pipe / heat pump system was successfully operated in both heating and cooling mode and operating data was collected and are being used to estimate the size, cost and performance of an in-ground heat pipe suitable for a three ton GSHP application.
