Testing of High Performance Concrete as a Thermal Energy Storage Medium at High Temperatures

In recent years, the mission to become energy independent from fossil fuels has become a higher priority. Solar energy is an ideal solution because it does not have the environmental impact of the fossil fuels. One key to the transition to solar energy is improved energy storage for when collection is prohibited by weather variation and during nighttime hours. Concrete has a low material cost and is reported to be $1/kWh_thermal [1]. Concrete as a sensible heat thermal energy storage material has been tested to temperatures of 390°C with an embedded steel heat exchanger using synthetic oil as a heat transfer fluid [2]. The energy storage efficiency of concrete can be improved by raising the maximum charging temperature. This paper reports the testing to further the peak temperature to 450–500°C by using high performance concrete developed by the University of Arkansas. A molten nitrate salt was selected along with stainless steel heat exchanger. The testing is of a laboratory scale with a single 3/4 inch pipe imbedded in a 4 inch by 4 inch concrete cross section. During testing, the differences in the materials’ thermal expansion produce stress at the pipe/concrete interaction zone. After testing with no interface material, the high tensile stress in the concrete caused large radial and longitudinal tension cracks that would hinder adequate heat transfer. Three interface materials between the concrete and stainless steel pipe were tested: Teflon© tape, Deacon 8875, and aluminum foil. For Teflon tape and Deacon 8875, the concrete stress was minimized and produced only small micro-cracks. To improve the heat transfer in the concrete, testing of a pipe with radial fins was conducted. The pipe consisted of a continuous helicoidal, auger style. During testing, large cracks occurred at each of the fin locations which demonstrated the need for a stress reducing material. Yet, only Deacon 8875 and aluminum foil were able to be applied to the unique radial fins. The results of the testing are reported and compared in the article. The Teflon^® tape had the fastest heat transfer of all the interface materials. Testing was considered a success on a single tube laboratory scale. Future multiple tube testing is a viable option.