Analysis of vertical ground loop heat exchangers applied to buildings in the UK

Abstract

The work presented here deals with the design and performance of ground-source heat pumps and ground-sink cooling systems using vertical borehole arrays for commercial applications in the UK. Heating and cooling energy demands for a range of building and HVAC plant options are obtained by thermal modelling applied to four HVAC plant options: space heating only; heating with chilled ceilings; fan coil units and constant volume all-air plant. Ground loop designs are conducted for each system option using an impulse-response method and the parameters extracted from this are used in 10-year simulations of plant response which have been carried out using HVACSIM+. The 10-year time horizon was used to assess any degradation in earth temperature over time. The results show that a substantial reduction in energy (and, hence, carbon) can be expected of up to and exceeding 50% when using ground source heat pumps for winter heating with direct cooling in summer in association with moderate temperature cooling systems such as chilled ceilings. A degradation of earth temperature was evident with systems utilising limited cooling or no cooling but this did not appear to influence heat pump performance greatly. Practical Applications: Design and performance data for use in vertical ground loop (borehole) heat exchanger arrays providing source heat for heat pumps as well as direct cooling for buildings are generated and reported in this paper. The data should be of help to design practitioners for the sizing of borehole arrays for both heating and cooling. Design and performance matching to a wide variety of HVAC combinations, building energy demand levels and two contrasting sets of earth thermal property data are included so that practitioners will be able to select results that suit a range of modern applications. Also included are results of 10-year energy simulations that demonstrate the required design and operating conditions needed to ensure that initial undisturbed earth conditions will not drift with time to an unacceptable extent. Comparisons are made with conventional heating and cooling methods so that estimates of carbon savings due to the use of ground-coupled heat pumps with (and without) direct cooling can be made.