2016 ASHRAE Annual Conference

Simulations were conducted in order to make assessments regarding life cycle costs of hybrid ground source heat pump systems with different pumping configurations on an elementary school design.  The long term simulations evaluated the systems’ performance over a 20 year period. The basecase in this study is an all-ground heat exchanger (all-GHX) configuration.  A closed circuit cooling tower (CCCT) and dry fluid cooler (DFC) were separately modeled to provide heat rejection in the hybrid systems.  Life cycle costs of the all hybrid systems presented herein for an example building are estimated between 35-40% less than an all-GHX configuration.  The basecase life cycle cost is estimated at $894,000 while the hybrid options ranged from $565,000 to $578,000. These likely do not represent the lowest life cycle cost designs which would balance the sizing of the system components including the ground source heat exchanger and supplement heat rejection device with the associated energy costs’ present value.  In many instances, there are space constraints on sizing individual components, most notably the ground heat exchanger.  As such, the alternate hybrid designs arbitrarily utilize 140 bores which is one-half the size of the base design at 280 bores, with the addition of a supplement heat rejection device which is sized to provide acceptable borefield temperatures. The control scheme utilized across all hybrid systems considered common in industry (Thornton 2014),  inherently allows loop temperatures to elevate and forms a solid basis upon which to make valid conclusions regarding life cycle comparisons between an all-GHX design and Hybrid systems.  The heat pump entering water loop temperature (EWT) target was designated as 95⁰F, but iterations to reduce the sizing of the supplemental heat rejection device and allow higher temperatures of no more than 100⁰F were acceptable if resultant life cycle costs were favorable.  DFC physical size and acoustical concerns were peripheral considerations. Hybrid systems are designated as Case One thru Case Four. Additional simulations with further reduced borefields are briefly explored which indicated possible life cycle costs savings of over 40% with total life cycle costs of around $500,000, and presented as Case 1A and 4A.  Pumping configurations included the use of dual individual circulator pumps, single circulator pumps w/central variable speed pump, and a single central variable speed pump.  The basecase which utilized dual circulator pumps used the most pump energy, while a central variable speed pump achieved 60% pump energy savings.

Ground Source Heat Pumps (GSHP) provide with the opportunity to be coupled with refrigeration units. In principle, the heat rejected by refrigerators can be harnessed to raise the efficiency of the heat pumps.
This paper analyses the operational and economic performance of this innovative system deployed in Sainsbury’s supermarkets. First, the efficiency of the GSHP is evaluated, throughout the stores and over the period under consideration. Then, an economic analysis comparing the efficiency of investing in GSHP rather than in gas boiler systems is conducted. Recommendations on cost reductions are finally developed. Results show the Coefficient of Performance (COP) of GSHP systems to be highly dependent on the period of the year. During the summer, efficiency is roughly 40% less than during the winter. Overall, the efficiency of all the GSHP systems appear to be above the eligibility threshold for the Renewable Heat Incentive (RHI), with the average Seasonal COP (SCOP) of the stores being 3.0 in 2014. From an economic perspective, this average performance leads to roughly £120,000 of operational savings per year compared to gas boiler systems, with significant contribution stemming from the improvement in the refrigeration systems. Calculations show an investment payback time (PBT) of less than 8 years, a figure projected to rise slightly in the upcoming years as electricity becomes more expensive than gas.
Finally, this research project highlights cost reductions, achievable through two different approaches. First, by turning off heat pumps only when most economically convenient, up to 5.5% of the electricity costs can be saved among the stores and nearly 15% in stores boasting high thermal efficiency. Second, the profitability of the system deprived of the boreholes is evaluated. Despite the ineligibility for the RHI, the small CAPEX of this configuration could lower the PBT to 6 years.

Steady-state heat transfer inside boreholes is usually assumed when sizing geothermal boreholes and a constant borehole thermal resistance is used to calculate the temperature difference from the fluid to the borehole wall. Thus, heat rejected into the fluid is assumed to be transferred immediately at the borehole wall. In reality, steady-state borehole heat transfer is rarely present. Rejected heat will heat the fluid and the grout before reaching the borehole wall and be transferred to the ground. These transient effects, caused by the fluid and grout thermal capacities, are beneficial as they reduce the peak ground loads and, consequently, the required borehole length. This paper proposes improvements to the ASHRAE vertical borehole sizing equation to account for borehole thermal capacity. In the first part of this study, annual TRNSYS energy simulations are performed on a residential ground-source heat pump system. Borehole models that account for thermal capacity are used to quantify the borehole transient effects for a range of operating conditions. In the second part of the paper, modifications to the current ASHRAE sizing equation are proposed to consider borehole thermal capacity. Results show that neglecting borehole transient effects leads to oversized boreholes and overestimated heat pump energy consumption. By considering the fluid and grout thermal capacity, it appears that borehole length can be reduced by about 10% and heat pump energy consumption by 5%. The largest reductions occur when heat pumps operate intermittently.

Geothermal heat pumps are a greener alternative to the traditional heating and cooling systems for buildings. Instead of using as much fuel or electricity to heat and cool a building as a conventional system does the geothermal heat pump saves energy by using the ground or nearby water source as a heat sink to displace the thermal energy. They tend to be rather large and have an expensive initial cost but in the long run they save money and fossil fuels. Most heat pump’s pipe configurations are buried under the ground where they will not be easily accessed at a later date. Because of this, the design of the configuration must be right the first time. This can be very difficult because the thermal properties of the ground varies from location to location. The ground in one place might be mostly sand and a mile away the ground might be mostly clay, so it becomes very difficult to design configurations and they can be easily oversized or undersized for the building depending on the thermal properties of the ground. This paper will explain a test process that can be done to test for the thermal properties of the ground before designing the configuration for a geothermal heat pump. This experiment is performed at the site of which the heat pump would be installed to gain the thermal properties of that particular location. A low cost and simple to use system consisting of a pump, tank, thermocouples, flow meter and data collector is used. By using this information a proper pipe configuration can be designed to best fit the needs of the building and configured to fit the available land on the property.