Development and Validation of Numerical Tools to Implement Climate Sensitive Design and Control for Ground Source Heat Pumps with Horizontal Collectors

Abstract

At the outset of the 21st century, growth in the Sustainable Energy Technologies (SETs) market is driven by the need to secure energy supply, limit the environmental impact of fossil fuels and to generate economic growth. By 2020, the Irish government targets SETs to deliver 12% (0.65Mtoe) of annual thermal energy consumption. The Ground Source Heat Pump utilising a horizontal collector (GSHPHC) is one of the most popular thermal SETs in Ireland, accounting for 61% of all heat pump installations as of 2010. GSHPHC thermal output is relatively constant and controllable compared to wind and solar technologies. However, the Coefficient of Performance (COP) which dictates the cost effectiveness of GSHPHC, is sensitive to a broad transient system which includes the building, heat distribution system, collector design, ground type, climate and operational control. The combined influence of the latter four elements has received limited attention to date and therefore provided the focus for this HP-IRL/H study. The HP-IRL/H study was motivated by the Irish heat pump industry’s needs and identified knowledge deficits in the literature. Using a multi-disciplinary thermo-environmental analysis methodology, this study aimed to demonstrate the potential GSHPHC performance gain from a novel and holistic Climate Sensitive Design and Control (CSDC) approach in a Cool Marine climate, through the following experimental and numerical objectives: (i) conduct a literature review to identify all GSHPHC design, control and environmental parameters; (ii) construct a fully functional experimental facility; (iii) characterise ground temperature response to seasonal, diurnal and weather fluctuations as well as quantifying ground heat transfer processes and properties; (iv) characterise the sensitivity of GSHPHC’s COP to collector design, climate and operational control; (v) develop a transient GSHPHC numerical simulation method incorporating the aforementioned characteristics; and (vi) demonstrate and quantify the potential performance gains from CSDC using the numerical simulation method. The thermo-environmental analysis methodology necessitated a literature review across five distinct disciplines of climatology, soil physics, heat transfer, fluids and thermodynamics. Uniquely, the key literature from these five disciplines is presented across the first 7 chapters of this thesis for the first time, with a uniform nomenclature throughout. In the most comprehensive study to date, a full-scale testing facility comprising a 15kW heat pump and 430m2 horizontal collector, serving a 1,125m2 commercial building, delivered over 50 million experimental data points from 130 climate, ground, collector, heat pump and building sensors between 2007 and 2010. This data has allowed accurate measurement and analytical characterisation of the ground’s thermal energy resource and properties, in addition to the GSHPHC’s thermodynamic, thermal and hydraulic performance. Findings indicate an annual average ground temperature of 11.72ºC, with seasonal and diurnal mean-to-peak amplitudes of 6.7 and 1.92K respectively, while average ground thermal diffusivity and conductivity were shown to be 1.05 x 10-6 m2/s and 2.6W/mK respectively. The GSHPHC’s COP dropped by 1.67% per 1K reduction in source temperature, thermal drawdown in the source was shown to be proportional to heat extraction rate, while heat pump COP was reduced by 8 to 13% when all circulating and standby power was considered. A suite of 11 analytical equations and 5 numerical models have been compiled. The simple, yet effective numerical approaches maintained high accuracy while uniquely catering for all the HP-IRL/H collector dynamics including: closely positioned parallel, in-line pipes with thermal interference; hourly weather influence at the surface; multiple surface covers and ground layers; thermally coupled collector and heat pump performance transience; as well as new CSDC split-level collector designs and novel control strategies. The ground response to heat extraction model (NL-4) had an average error of ±0.25K over 2 months of continuous heat extraction, while the coupled collector and heat pump model (NL-5) simulated source return temperature with an average error of ±0.13K over 6 hours for both cyclic and continuous operation. A preliminary numerical test demonstrated the potential of the CSDC approach by simulating the thermal performance of alternative collector designs and control strategies. The deployment of split-level collectors at -0.5 and -1.75m utilising collector temperature feedback control, produced a 4.6% COP advantage over 2 months compared to the high performing HP-IRL/H collector, by taking advantage of reduced volumetric heat extraction and using intelligent feedback control to capture the positive elements of both the diurnal and seasonal ground thermal energy resources. Additional modifications including a southerly incline with a split-level collector can result in average COP increases of 6.5 to 7.9% over 3 months. Further developments, particularly on using the next generation of improved models to simulate intelligent control of inclined, split-level collectors coupled with thermal storage, could boost SPF by up to 10% in Cool Marine regions and further justify the CSDC approach.