Comparison of Conventional and Variable Borehole Heat Exchangers for Use in a Desiccant Assisted Air Conditioning System

Abstract

The objective of this work is to analyze a gas injection borehole heat exchanger coupled with a desiccant assisted air conditioning system during cooling and heating operation. A common problem that occurs in air conditioning systems is peak loads, during which the cooling or heating power of the soil can be exceeded. To counteract this drawback, a gas injection borehole heat exchanger, which is capable of creating artificial groundwater flow along the heat exchanger by inducing a pressure difference inside the well, is used. Experimental results of the performance differences between a conventional and a gas injection borehole heat exchanger are presented. Under the same inlet conditions, a reduction in the outlet temperature of up to 2 °C is achieved compared with an equivalent conventional borehole heat exchanger in cooling mode. The maximum cooling power is increased by 26%. As a result, a fast and dynamic responding control of the heat transfer between the heat exchanger and the soil is possible. During winter operation, despite the lower drilling depth of the gas injection borehole heat exchanger system, the performance is within the range of a conventional system. The power increase is limited to around 0.2 kWth at a steady state. In conclusion, gas injection borehole heat exchangers can be promising in terms of reliable peak load handling within large geothermal fields.