Use of cross g-functions to calculate interference between ground heat exchangers used in ground-source heat pump systems

Ground-source heat pump systems are increasingly popular for providing single-family home heating in Nordic countries. As the density of installations increase, questions sometimes arise as to the influence of new systems on existing systems. These questions cannot be readily answered, as design and simulation techniques developed over the last 35 years have focused on analysis of individual systems without regard to the influences of other systems.Response factor models of ground heat exchangers utilize pre-computed response functions known as g-functions. These g-functions give the response of the ground heat exchanger (non-dimensionalized temperature) to the past and current heat rejection or extraction of the ground heat exchanger. We might call this a “self-g-function.”In this paper, we define a “cross g-function” that gives the response of one ground heat exchanger to heat rejection or extraction of another ground heat exchanger. With this formulation, it is possible to determine the impact of a neighboring ground heat exchanger with a different load profile and history. This has many possible applications, but we demonstrate its use to study the sensitivity of nearby residential ground heat exchangers upon one another.Peer reviewedMechanical and Aerospace Engineerin

Influence on thermal response test by groundwater flow in vertical fractures in hard rock

In this paper different approaches to groundwater flow and its effect in the vicinity of a borehole ground heat exchanger are discussed. The common assumption that groundwater flow in hard rock may be modelled as a homogeneous flow in a medium with an effective porosity is confronted and models for heat transfer due to groundwater flow in fractures and fracture zones are presented especially from a thermal response test point of view. The results indicate that groundwater flow in fractures even at relatively low specific flow rates may cause significantly enhanced heat transfer, although a continuum approach with the same basic assumptions would suggest otherwise. (C) 2003 Elsevier Science Ltd. All rights reserved

Data from: Performance of a mixed-use ground source heat pump system in Stockholm

This data set is associated with the paper "Performance of a mixed-use ground source heat pump system in Stockholm." The data consists of three XLSX files, four XSLM files and one README file associated with the methodology and documentation to allow for replication and verification of findings. The XSLM files contain VBA source code used to post-process the data.The 6300 m2 two-story Studenthuset building at Stockholm University in Stockholm, completed in 2013, was thoroughly instrumented. Space heating and hot water are provided by a ground source heat pump (GSHP) system consisting of five 40 kW off-the-shelf water-to-water heat pumps connected to 20 boreholes of 200 m depth in hard rock. Space cooling is provided by direct cooling from the boreholes. This system has now been monitored for five years. This paper presents the results in the form of a range of performance indicators that describe the short-term and long-term system performance. Performance factors are computed for several boundaries defined by the IEA HPT Annex 52 boundary schema. Seasonal, monthly, daily, and binned performance factors for both heating and cooling operation are presented and discussed. Contrary to expectations based on thermodynamic theory, the performance is better correlated to the quantity of heating or cooling provided than it is to the exiting fluid temperatures from the ground heat exchanger. Despite being in Stockholm, the building rejects about 30% more than it extracts, leading to a minimal temperature increase over the five measured years. The analysis indicates that if operated as is, the GHE will not exceed its temperature constraints for many decades. The five-year seasonal performance factor (SPF) for combined heating and cooling is 5.2±0.2 considering only the heat pump and source-side circulating pump. However, the load-side distribution system and Legionella protection systems result in a significant decrease in the 5-year combined heating and cooling SPF at the outer boundary to 1.8±0.3.Mechanical and Aerospace Engineerin

The influence of the thermosiphon effect on the thermal response test

The issue of natural and forced groundwater movements, and its effect on the performance of ground heat exchangers is of great importance for the design and sizing of borehole thermal energy systems (BTESs). In Scandinavia groundwater filled boreholes in hard rock are commonly used. In such boreholes one or more intersecting fractures provide a path for groundwater flow between the borehole and the surrounding rock. An enhanced heat transport then occurs due to the induced convective water flow, driven by the volumetric expansion of heated water. Warm groundwater leaves through fractures in the upper part of the borehole while groundwater of ambient temperature enters the borehole through fractures at larger depths. This temperature driven flow is referred to as thermosiphon, and may cause considerable increase in the heat transport from groundwater filled boreholes. The thermosiphon effect is connected to thermal response tests, where the effective ground thermal conductivity is enhanced by this convective transport. Strong thermosiphon effects have frequently been observed in field measurements. The character of this effect is similar to that of artesian flow through boreholes. (C) 2003 Elsevier Science Ltd. All rights reserved

Ground source heat pumps: observations from UK ground thermal response tests

The United Kingdom is experiencing a period of rapid growth in the use of ground source heat pump systems. Most installations in the United Kingdom use vertical ‘borehole’ heat-exchanger arrays, the design of which depends on four parameters: formation thermal conductivity, formation heat capacity, heat-exchanger resistance and heat-exchanger grout material heat capacity. Conventionally, two of these parameters (conductivity and resistance) are obtained from a thermal response test carried out on a trial heat exchanger at the site of interest by fitting thermal response data to classical line-source heat conduction theory. This test method gives no information on the heat capacities of the formation and grout material and requires an assumption about the former to enable the heat-exchanger resistance parameter to be extracted. In this work, a new method is developed for extracting all four parameters using a trust-region search algorithm in conjunction with a detailed numerical model of the test heat exchanger. Results give excellent agreement between the fitted-model predictions of heat-exchanger outlet water temperature and measured outlet water temperature for 13 test cases. A further advantage of the method developed here is that it can be used with data sets that contain disturbances and discontinuities. Practical applications : Most of the ground source heat pump installations in the United Kingdom use vertical ‘borehole’ heat-exchanger arrays. The design of these arrays requires information about the rock formation thermal conductivity and volume specific heat capacity and the borehole heat-exchanger thermal resistance and grout material volume specific heat capacity. These design parameters are usually obtained from a thermal response test carried out on a trial heat exchanger at the site of interest. In this work, thermal response test results from 13 UK sites are presented and a new method for obtaining the four design parameters is developed and proposed