Mitigation potential of horizontal Ground Coupled Heat Pumps for current and future climatic conditions: UK environmental modelling studies

Lead Research Organisation: University of Reading
Department Name: Geography and Environmental Sciences

Abstract

An increased uptake of alternative low- or non-CO2 emitting energy sources is one of the key priorities for policymakers in order to decrease combustion of fossil carbon, thereby slowing the increase of CO2 concentration in the Earth's atmosphere. A considerable amount of UK effort has gone into investigating renewable energy sources such as wind, marine and solar power, as well as into bio-energy. However, relatively little work has been undertaken on the topic of ground coupled heat pump systems (GCHP), a relatively underused technology in the UK, in contrast to other countries such as USA, Switzerland and Sweden. To put it simply, GCHPs use temperature differences (between soil and air) to provide space heating in the winter and cooling in the summer. This is achieved by placing plastic pipes (filled with fluid containing anti-freeze) in the ground so that they can exchange heat with the soil, so called heat exchangers. This heat is 'upgraded' by a heat pump to heat homes or other buildings, thereby providing a sustainable, renewable and reliable source of energy. The performance of these GCHPs depends on the design and configuration of the heat exchangers (e.g. length of pipes, depth of installation, spacing between pipes). However, the performance of horizontally installed systems, as opposed to the more expensive vertical borehole ones, is also affected, in a rather complex way, by the environment. With this we mean soil, vegetation and atmospheric conditions, which will significantly differ over the UK and over time (diurnal, seasonal and inter-annual variation). The research described in this proposal aims to investigate how the long-term (~50 years, the average lifespan of GCHP systems) performance of these systems varies throughout the UK. Our findings would form the basis of recommendations to local governments (and users) on the location-dependent economic viability of these systems and their potential to reduce carbon emissions, while explicitly taking into account that our climate is changing at a significant rate. Also, depending on the balance between how much heat is taken away from and returned to the ground, the soil temperature in the neighbourhood of the heat exchangers may fall or rise; related to this is the movement of soil moisture away from or towards the heat exchanger. These processes will also affect the performance of the system during its life span. These intricate interactions between soil and GCHP can be mimicked by computer model simulations and various packages are available for use by GCHP designers and installers. However, these types of software have been developed to work on a site-by-site basis and moreover they simplify the effect of the environment. Also, they address short time spans only (~1-3 years). In this proposal we will use a detailed land surface model, such as the one used by the UK Meteorological Office to predict the weather. First we will improve it to ensure that all important interactions between the below-ground heat exchangers and the soil (heat and moisture flow, including groundwater) are taken into account. We will then test it and subsequently drive the model with long-term data, generated to represent the climate, soil type, and vegetation (and related properties) throughout the UK. Only then can we obtain reliable estimates about the UK-wide long-term performance of GCHP systems and their effectiveness in reducing CO2 emissions. This allows us to recommend increased uptake in specific UK areas as well as indicate how specific changes to the design and configuration of GCHP systems (e.g. type of tube and installation depth) can improve performance and hence increase its potential for reduction in CO2 emission.
 
Description Final Report GROMIT (GROund coupled heat pump MITigation potential)



By 2020, the UK will need to generate 15% of its energy from renewables to meet its contri-bution to the EU renewable energy target. Heating and cooling systems of buildings account for 30%-50% of the global energy consumption; thus, alternative low-carbon technologies such as Ground Coupled Heat Pumps (GCHPs) can contribute to the reduction of anthropogenic CO2 emissions. The seasonal temperature differences encountered in soil are harnessed by GCHPs to provide heating in the winter and cooling in the summer. The heat pump is coupled to an underground heat exchanger (HE); these HEs can be installed horizontally (generally in about 1-2 m deep back-filled trenches) or vertically.

A research project, funded by NERC (NE/F020368/1) and led by the University of Reading (UoR), ran between May 2009 and November 2012 to assess the mitigation potential of GCHPs in the UK. It brought together environmental scientists and building and energy technologists from UoR, the University of Nottingham, the British Geological Survey, and the Centre for Ecology and Hydrology (Wallingford).

The combined effect of environment dynamics, ultimately affecting the soil temperatures, and GCHP technical properties on GCHP performance were assessed using detailed simulation models; two strands of modelling were pursued. Firstly, detailed high resolution 3-D modeling using software such as FLUENT; in this case the thermal performance of various HE configurations were investigated over relatively short timescales (weeks) while the atmospheric and soil (boundary) conditions were kept constant. The model investigations were carried out for a range of slinky loop spacings, loop diameters, values of soil thermal properties, and allowing for continuous and intermittent operation. Comparisons were made for the heat transfer rate, the amount of pipe material needed, as well as excavation work required for a horizontal slinky-loop HE. The results indicated that system parameters have a significant effect on the thermal performance of the system (Chong et al., 2013a; see also Wu et al. 2010, 2011). The maximum difference in the thermal performance between vertical and horizontal slinky-loop heat exchangers with the same loop diameter and loop spacing is less than 5% (Chong et al., 2013b).

Secondly we combined a land surface model (JULES, see also Garcia et al., 2012b) with an approximation of the detailed horizontal GCHP model (JULES-HP). We analysed multi-year dynamic interactions between the environment and the horizontal GCHP HE, as well as their combined effect on potential heat extraction from the soil. Sensitivity tests were performed for three sites in the UK, studying the effects of climate, soil type, and slinky configuration. Results showed that heat pump performance does not remain constant, and will depend considerably on the installation depth and operating characteristics, as well as on the changing climatological conditions (Garcia et al. 2013, under review).

We also conducted a field experiment for verification purposes, measuring the effect of heat extraction by a horizontal HE (installed at 1 m depth to heat a domestic property in Oxfordshire) on the soil physical environment. The slinky influenced the surrounding soil by significantly decreasing nearby soil temperatures (by up to °6 C). Also, soil moisture contents were lower for the soil profile in the vicinity of the GCHP, affecting soil thermal properties (Wu et al., 2010; Garcia et al., 2012).

GROMIT is the first detailed mechanistic study conducted in the UK with the aim to understand the interactions between soil, horizontal HE and the aboveground environment. An increased understanding of these interactions will help to achieve an optimum and sustainable use of the soil heat resources in the future. Although GROMIT has now finished, we are in the process of finalising and analysing long-term gridded (1 km) JULES-HP runs (reflecting current and future environ-mental conditions) to reliably assess the mitigation potential of GCHPs over the entire domain of the UK. In this way we can identify areas that are most suitable for the installation of GCHPs (i.e. have the highest mitigation potential). Only then can informed recommendations be made to local and regional governments, and to GCHP designers and installers, avoiding under- and over-designing of future systems. Ultimately, we hope to improve future GCHP uptake and performance with these UK-wide recommendations, while safeguarding the soil physical resources for future generations.



References: Garcia-Gonzalez et al. (2013) Renewable Energy (under review); Chong et al. (2013a) Applied Energy 104: 603-610; Chong et al. (2013b) Int. J. Low-Carbon Tech. doi:10.1093/ijlct/ctt001; Garcia-Gonzalez et al. (2012) Renewable Energy, 44: 141-153; Garcia-Gonzalez et al. (2012b) Water Resources Research, 48: W05538; Wu et al. (2011) Int. J. Low-Carbon Tech. 6 (4): 261-269; Wu et al. (2010) Applied Thermal Engineering, 30(16): 2574-2583.
Exploitation Route To develop GSHP installation tools
Sectors Construction,Energy,Environment

 
Description Collaboration with members of EU Thermomap project 
Organisation British Geological Survey
Country United Kingdom 
Sector Public 
PI Contribution Drs Garcia-Gonzalez and Verhoef (University of Reading) also advised and collaborated with members of the EU-funded Thermomap project (http://www.thermomap-project.eu/), specifically Drs Jon Busby and M.Lewis (BGS). The GROMIT field location at Drayton St Leonard, Oxfordshire served as one of the geothermal test areas for the Thermomap project.
Start Year 2011