Effect of Water Injection Into a Ground Heat Exchanger Drilled in a Low-ë Formation

Abstract

In ground source heat pump (GSHP) systems, the heat exchange rate per unit length of a ground heat exchanger (GHE) needs to be improved to reduce the initial cost of the system. When the formation has high thermal conductivity (ë) or fast groundwater flow because of high hydraulic conductivity (k), high heat exchange rates usually are expected at the GHE, which facilitate the introduction of the GSHP systems. Alternatively, when the ë is low or the groundwater velocity is slow, the excessive requirement of the GHE length hampers the application of the system. This research hence focuses on the enhancement of heat exchange rate by water injection into the GHEs, which could cause an artificial groundwater flow in the GHEs. A series of thermal response tests (TRTs) were carried out in an ungrouted GHE of 100 m deep drilled in an alluvial deposit in Akita City, northern Japan. The GHE was drilled in formation of low-ë and low-k, mainly consisted of clay, silt and sand. In the TRTs, water injection rates were varied as 0, 1, 3 and 5 L/min, while keeping the heat load and the circulation rate as constant. The TRTs showed that the water injection could suppress the rise of heat medium temperatures effectively even when applying small amount of water injection rates, which could improve the COP (Coefficient of Performance) of heat pumps. The improvement was more evident when the heat exchange pipes are located in the upper part of the deposit, which show higher water injectivity than the deeper zone. The field tests suggested the possibility of shortening the length of GHE in low-ë formation with minimal additional operation costs.