A Monthly Based Bore Field Sizing Methodology with Applications to Optimum Borehole Spacing

The required length of vertical ground heat exchangers (GHX) used in ground-coupled heat pump (GCHP) systems is determined so that the outlet temperature from the GHX remains within certain limits for the worst ground load conditions. These conditions may not necessarily occur after 10 or 20 years of operation, as is usually assumed, but often occur during the first year of operation. The primary objective of this paper is to develop a general methodology for the calculation of the total required bore field length on a monthly basis during the first year of operation using the framework of the ASHRAE bore field sizing method. It is a three phaseprocess. The first phase consists of analyzing andorderinggroundloadsaccording to the firstmonthofoperation.Next,a first setof required lengths isdeterminedbyusing the analyzed ground load components and assuming a temperaturepenaltyTp=0.Then,an iterativeprocess tocalculate the temperaturepenalty at the endof eachmonth is carried out to obtain the final required length for theworst conditions. The methodology is exemplified in a particular case with a slight annual cooling thermal imbalance and with a high influence of the hourly peak in heating. For this particular case, it is shown that the required bore field length occurs during the first year and that the starting month of operation has a strong influence on the results. Finally, it is shownthat it ispossible toreduce theborehole spacing when the annual ground load is quasibalanced. In the case studied here, the minimum length occurs for a boreholeto-borehole spacing of about 3.2 m (10.50 ft). INTRODUCTION In ground-coupled heat pump (GCHP) systems, correct evaluation of the total ground heat exchanger (GHX) length and of the required borehole-to-borehole spacing is crucial to obtain good system performance. An undersized bore field may lead to operational problems because the return fluid temperature may be outside the operating temperature limits of the heat pump.Oversizing should also be avoided as it leads to unnecessary high capital costs. A well-known procedure, based on the work of Kavanaugh and Rafferty (1997) and proposed by ASHRAE (ASHRAE 2007), is used by some software tools (e.g., presented in Philippe et al. [2010]) to determine the total bore field length. This procedure has been reformulated to take the following form (Bernier 2000).