The Influence of Geometry on the Performance of a Helical Steel Pile as a Geo-Exchange System

Abstract Foundation piles have the potential to improve the economic and technical feasibility of ground source heat pump (GSHP) systems. They simultaneously provide structural support and energy for space heating and cooling. In this study, a thoroughly verified and validated numerical model of a novel in-ground heat exchanger for GSHP systems is developed and used to simulate and optimize performance. Commercially available helical steel casings with nominal sizes according to the American Petroleum Institute (API) of API 13.5, API 23, API 29, API 53, API 60, and API 68 were considered. The flow rates considered were 1 L/min, 2 L/min and 4 L/min for laminar flow and lower pressure drops. Results show the performance to increase with increasing pile size owing to improved convection and longer residence times. Optimizing performance with an API 68 steel casing, with a 2” nominal plastic pipe) give a capacity increase of 0.01 ton/pile (or 8.3%), which would reduce the approximate pile array required for a 3 ton cooling system from 13 to 12. Doubling the size of this pile and keeping other parameters constant gives an 18.5% capacity improvement with an output capacity of 0.28 ton/pile, and an 11, 20 m pile array requirement to meet a 3 ton cooling load. With a maximum heat exchange rate of 58.6 W per meter depth, this shallow in-ground heat exchanger has the potential to minimize energy and costs for small-scale implementation. In addition, the possible low flow rates help reduce pumping power requirements. This study provides a foundation for sizing and design of helical steel piles. Moreover, the study also gives insights into pile performance in multi-layered soils where the thermal conductivity varies with depth.