Subterranean thermal energy storage using thermosiphon arrays

Increasing evidence of ecosystem damage caused by burning fossil fuels has sparked an interest in finding new methods of storing and utilizing energy that does not contribute to release of carbon dioxide. There is untapped potential for storing cold energy within the ground in climates with extreme winters and summers. This stored energy can be employed as a method of cooling buildings if effective heat transfer to and from the storage media and the building is obtained. A thermosiphon array is considered a means of enhancing that heat transfer. A test site has been studied at the University of Utah Sill Center consisting of two concentric thermosiphon arrays. It was designed to operate such that its performance is indistinguishable from traditional cooling systems. The research objective of this paper is to document the initial stages of the installation and operation, as it has spanned many years. The data collected from the thermosiphons is presented as a baseline for performance of the system. Ground temperature data is analyzed by finding the thermal diffusivity of the soil at the site. The scope of this work includes implementing the data acquisition system, ensuring the thermosiphon array was operational and leak proof and presenting preliminary performance data as a background for future work. Results include the preliminary performance data for two charged thermosiphons as well as ground temperature data. The thermal diffusivity of the ground was calculated using iterative methods to match the actual temperature distribution found in the ground from February to April 2015 to a mathematical model. The mild temperatures occurring late in the winter of 2015 stifled the freezing in the ground near the thermosiphons but still allowed detailed measurements of the thermal response to various atmospheric temperatures. It was also discovered that the thermosiphons were not receiving sufficient refrigerant, and that the addition of a flow meter and a pump would be beneficial. However, this project is to be continued for seasons to come in an effort develop a cost-effective method to reduce carbon dioxide emissions from cooling of commercial and residential buildings.