Interactions between the physical soil environment and a horizontal ...

Renewable Energy 44 (2012) 141e153

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Interactions between the physical soil environment and a horizontal ground coupled heat pump, for a domestic site in the UK Raquel Garcia Gonzalez a, b, *, Anne Verhoef a, d, Pier Luigi Vidale b, d, Bruce Main a, Guogui Gan c, Yupeng Wu c a

Department of Geography and Environmental Science, School of Human and Environmental Sciences, Whiteknights campus, The University of Reading, Reading RG6 6DW, UK NCAS-Climate, University of Reading, Reading, UK Institute of Building Technology, University of Nottingham, Nottingham, UK d Walker Institute, University of Reading, Reading RG6 6BB, UK b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 July 2011 Accepted 10 January 2012 Available online 9 February 2012

There is currently an increased interest of Government and Industry in the UK, as well as at the European Community level and International Agencies (i.e. Department of Energy, American International Energy Agency), to improve the performance and uptake of Ground Coupled Heat Pumps (GCHP), in order to meet the 2020 renewable energy target. A sound knowledge base is required to help inform the Government Agencies and advisory bodies; detailed site studies providing reliable data for model verification have an important role to play in this. In this study we summarise the effect of heat extraction by a horizontal ground heat exchanger (installed at 1 m depth) on the soil physical environment (between 0 and 1 m depth) for a site in the south of the UK. Our results show that the slinky influences the surrounding soil by significantly decreasing soil temperatures. Furthermore, soil moisture contents were lower for the GCHP soil profile, most likely due to temperature-gradient related soil moisture migration effects and a decreased hydraulic conductivity, the latter as a result of increased viscosity (caused by the lower temperatures for the GCHP soil profile). The effects also caused considerable differences in soil thermal properties. This is the first detailed mechanistic study conducted in the UK with the aim to understand the interactions between the soil, horizontal heat exchangers 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. The results of this study will help to calibrate and verify a simulation model that will provide UK-wide recommendations to improve future GCHP uptake and performance, while safeguarding the soil physical resources. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Renewable energy Horizontal ground source heat pumps Soil thermal properties Heat extraction Coefficient of Performance

1. Introduction Currently renewable energy sources provide only 1% of the UK’s total heat demand. To reach the 2020 renewable energy target, around 12% of the UK’s heat needs to be obtained from renewable sources which will help increase our annual energy savings, but also reduce CO2 emissions [1]. Ground Coupled Heat Pumps (GCHPs) are an example of a reliable renewable energy source. GCHP technology, among other renewable energy sources, needs to be developed further in order to meet that target. The UK Renewable Heat Incentive (RHI) together with new policies implemented * Corresponding author. Department of Geography and Environmental Science, School of Human and Environmental Sciences, Whiteknights campus, The University of Reading, Reading RG6 6DW, UK. Tel.: þ44 1183787902; fax: þ44 1183786660. E-mail address: [email protected] (R. Garcia Gonzalez). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2012.01.080

recently (Energy Act 2008) will help this development, revise the requirements for energy supply and will improve the availability and uptake of this technology in the UK. The seasonal temperature differences encountered in soil are harnessed by GCHPs to provide heating in the winter and cooling in the summer. The performance of a GCHP system will depend on technical factors (ground heat exchanger (GHE) type, length, depth, and spacing of pipes). Furthermore, especially for horizontally installed GHEs (the type of system considered in this paper, which represents approximately 44% of the total UK housing stock in 2004 [2]), it will be determined to a large extent by interactions between the below-ground parts of the system and the environment (atmospheric conditions, vegetation and soil characteristics) (see e.g. [3,4]). Diurnal and seasonal variations in soil temperature are a function of incident solar radiation, fluctuations in air temperature, type

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R. Garcia Gonzalez et al. / Renewable Energy 44 (2012) 141e153

and density of vegetation cover, soil textural composition, and soil depth. The soil texture (proportions of sand, silt and clay in the soil matrix) will determine to a large extent the soil’s thermal properties (heat capacity, thermal conductivity and heat diffusivity) [5e7] and hence its temperature profile. Soil moisture content has a strong effect on soil thermal properties and therefore indirectly on the soil temperature. Soil moisture content is determined by rainfall, evapotranspiration and soil hydraulic properties (e.g. those that affect infiltration, drainage and runoff). Like soil thermal properties, soil hydraulic properties also strongly depend on a soil’s relative saturation. Due to the much higher heat capacity of soil relative to air and the thermal insulation provided by vegetation, as well as the dampening effect of the soil layers (due to their finite thermal diffusivity), seasonal changes in soil temperature (as defined by their amplitudes) are much less than, and lag significantly behind, seasonal changes in overlying air temperature. Thus in summer, deep soil temperatures (roughly at 1e2 m) are cooler than the nearsurface atmospheric air while in winter they are warmer than the overlying air. Depending on the balance between extraction and rejection of heat from and to the ground, the soil temperature in the neighbourhood of the GHE may fall or rise compared to its ‘natural’ temperature; this will in turn affect water and vapour transfer, due to the coupled nature of these processes [8]. These temperatureinduced changes in water (liquid and vapour) fluxes will cause localized changes in soil moisture content and therefore influence the thermal properties of the soil surrounding the GHE, and hence ultimately the performance of the GCHP [9]. The Coefficient of Performance (COP) of a horizontal GCHP system will indirectly be affected by the hydraulic and soil thermal properties. Soil type and moisture content affect ground heat pump performance. Higher COP values are generally found for sandy soils (they have the largest thermal conductivities and diffusivities at a particular soil water content) and then for loam and clay soil textural types [9]. Therefore, a layer of sand is usually incorporated during the GCHP installation to provide a good contact for the heat exchanger pipes in order to enhance heat transfer. Under saturated conditions, no significant differences will appear for the COP for different soil texture types; however, under half-saturated conditions the heat extraction will experience a much more rapid decrease due to a decreased heat capacity [10]. Due to their installation depth (1e2 m), in the UK the soil surrounding GHEs of horizontal GCHPs is not expected to freeze, but extremely cold winters could modify normal GCHP behaviour. Furthermore, under those conditions a layer of snow may be present that would insulate the soil. Thus, the occurrence of snowfall and fluctuations in temperature from above to below freezing point and vice versa will have also important implications for the performance of GCHPs. A recent report summarised the COP of a number of GCHP installation at various locations in the UK [11]. The main recommendation was that further research should be carried out to explain the variation in measured COP for apparently similar installations in the trial. Singh et al. (2010) also note that there is a lack in UK specific data and research into this topic [2]. The first year UK field trial by the EST showed that about 80% of the installations included in this study cannot be considered a renewable source of energy under the EU Renewable Energy Directive (this requires a COP equal to or higher than 2.875). In this paper we summarize one year of observations from an on-going measurement campaign undertaken for a horizontal GCHP installation in Drayton St Leonard (Oxfordshire, UK). We present the soil physical state variables, together with the soil thermal properties and soil heat fluxes, for two soil profiles

(reference soil and soil near the GHE, in this case a slinky configuration) monitored at the field site, as well as the inlet and outlet temperatures of the GCHP system. We will summarise the effect of environment dynamics on GCHP performance of this system. This is the first detailed mechanistic study conducted in the UK with the aim to understand the interactions between the soil, horizontal heat exchangers and the aboveground environment.

2. Materials and methods 2.1. Field site description and experimental set-up Atmospheric and soil measurements near a horizontal ground source heat exchanger (slinky configuration with loop diameter of 1 m and pipe diameter of 0.04 m) installed in a field adjacent to a cottage situated in Drayton St Leonard, Oxfordshire (51 390 5100 N, 1 80 2800 W) started early November 2009 [12]. The field was predominantly bare during winter, but throughout the rest of the year a relatively sparse herb layer was present, with a maximum height of approximately 0.5 m. The experiment monitors the meteorological driving data with instruments installed on a mast, as well as soil parameters and variables, near the heat exchangers and for soil that was deemed not to be influenced by the GHEs. Further details of measurement heights/depths and sensors used can be found in Table 1. The GCHP system provides heating to the main house, outhouse and swimming pool. Four ground loop trenches of about 80 m long run in parallel; the loops are evenly spaced over 20 m with roughly 5 m between each trench. The installation depth of the loops is approximately 1.1 m. A layer of about 5 cm of sand was used during the GCHP installation directly below and above the slinky to enhance the heat transfer between the buried heat exchanger and its surrounding soil. Textural composition of this layer of sand (clay: 2.7%; Silt: 9.6%; Sand: 87.8%) and the surrounding soil (in the reference profile, see Table 2) was obtained using the laser

Table 1 Measurement depths and types of sensors used at the GROMIT Drayton St Leonard experimental site. Height or depth (m) Meteorological variables Radiation 1.5

Sensor Kipp and Zonen radiometer (type CNR1) Delta-T Devices, type AT2 Delta-T Devices, type RHT2 Delta-T Devices, type AN1 ARG rain gauge

Air temperature 1.5 Humidity 1.5 Wind speed 1.5 Precipitation Surface GCHP profile soil variables Thermistors Pt-100e Soil temperature 0.02, 0.05, 0.10, 0.25, 0.5, 0.75, 1.0a, 1.10b, 1.14c Soil moisture 0.07, 0.17, 0.37, 0.75, 1.12d Delta-T Devices, Thetaprobe type ML2xf Reference profile soil variables Soil temperature 0.02, 0.05, 0.10, 0.25, Thermistorse 0.5, 0.75, 1.0, 1.05 Soil moisture 0.07, 0.17, 0.37, 0.75, 1.0 Delta-T Devices, Thetaprobe, type ML2xf GHE inlet/outlet 10 thermocouples Shielded type T (coppermeasurements constantan)e Thermal properties 0.07, 0.17, 0.37, 0.75 Three-needle probesg Reference profile a b c d e f g

Above slinky. 2e4 cm above slinky. Below slinky. 2 cm above (slinky/18 cm away). Nominal precision (NP): 0.01 C. NP: 0.01 m3 m3: 0e40 C. NP 5% (according to Bristow et al., 2001).

R. Garcia Gonzalez et al. / Renewable Energy 44 (2012) 141e153 Table 2 Soil textural composition at different depths in the reference profile obtained using the laser granulometry technique. Composition, % Depth, cm 0 20 40 60 80 100

Clay

Silt

Sand

2.22 1.75 2.17 2.63 2.41 2.47

32.10 25.75 31.10 33.20 34.24 34.60

65.65 72.45 66.70 64.10 63.35 62.95

granulometry technique. The soil is a loamy sand (average composition; clay: 2.4%; Silt: 33.2%; Sand: 64.4%). Furthermore, dry bulk density was obtained at different depths from soil samples taken at the site (see Table 3). The trench furthest away from the garden and cottage was selected to install the first vertical array of instruments (the GCHP profile), in order to quantify the influence of the slinky on the soil’s temperature and moisture regime. The reference instrument profile was installed about 2 m away in the horizontal direction from the middle portion of the slinky. Hydraulic and thermal soil properties, soil moisture content, q (m3 m3), and soil temperature were obtained near the GHE and in the reference profile (see details in Table 1 and Fig. 1). The inlet and outlet fluid temperature of the ground heat exchanger was also measured using thermocouples on the inlet and outlet pipes that were accessible from a nearby manhole, related to the GCHP installation. Snow and freezing conditions affected the GCHP and the surrounding soil from 18 December 2009 until the middle of January 2010, when rather cold conditions prevailed all over the UK. Due to battery and storage module problems, as well as temporary flooding of the manhole, some data were lost during 2010. In these cases, meteorological data were gap-filled using information from a nearby meteorological station, Howberry Park (Crowmarsh Gifford, UK). 2.2. Methods to calculate thermal properties and soil heat flux Soil heat transfer occurs mainly by conduction, which is driven by temperature gradients and depends on thermal conductivity, which is a function of soil moisture content [5]. Heat conduction is not the only heat-transferring process that takes place in the soil. Heat transfer due to phase changes, such as condensation/evaporation and freezing/melting, can increase heat transfer substantially [5]. When measuring thermal conductivity using three-needle probes (see description later in this section) both processes are taken into account, thus we are considering an apparent thermal conductivity. Under phase changes, heat transfer near a heat exchanger will be enhanced or reduced, dependent on the soil moisture content and soil texture. Soil heat flux (G, W m2) can be estimated using Fourier’s law, assuming finite temperature differences (DT, C) for a finite layer thickness (Dz, m) and considering a homogeneous medium with uni-dimensional flow in the vertical direction:

G ¼ l

DT Dz

Depth, cm

br d

0e20 20e40 40e60 60e80 80e100

1.32 1.30 1.45 1.43 1.34

(1)

where l denotes thermal conductivity (W m1 K1). Eq. (1) can be used within soil horizons where soil thermal properties are assumed to be isotropic. The combination of Eq. (1) with the principle of heat conservation describes the heat transfer in a one-dimensional medium, given by:

vT v2 T ¼ Dh 2 vt v z

(2)

where Dh is the thermal diffusivity (in m2 s1), which is defined as the ratio of thermal conductivity to the volumetric heat capacity, Ch. These three thermal properties will determine soil heat flow and the temperature distribution, hence knowledge of their temporal evolution and how they differ between the GCHP and reference profile is important. The following will briefly describe their calculation. Thermal conductivity can be calculated using different approaches [13]. Thermal conductivity was estimated using the approach detailed in [7]:

l ¼ lsat ldry Ke þ ldry

(3)

where ldry is the dry thermal conductivity, lsat is the l at saturated soil moisture content, qs and Ke is the Kersten number. Lu et al. (2007) proposed a linear equation for the ldry calculation, which is dependent on soil porosity (n):

ldry ¼ 0:51 0:56n

(4)

Saturated thermal conductivity, as calculated from Eq. (5), was calibrated using the three-needle probes when available (only for the reference profile at 0.17 m and 0.37 m) providing a maximum (1.7 W m1 K1) and a minimum (1.2 Wm1 K1) threshold, as derived from the Johansen approach [14]:

lsat ¼ 1:2 þ 12:4 ldry 0:25 1:7

with the constraint 1:2 lsat ð5Þ

The Kersten number, following the approach by Lu et al. [7] is given by:

i h Ke ¼ exp a 1 Sra1:33

(6)

where Sr is the relative saturation (q/qs). Ke changes the shape of the leSr curve using a soil texture dependent parameter (a ¼ 0.96), and a shape parameter with the value of 1.33. Volumetric heat capacity (Ch, MJ m3 K1) was calculated from the volumetric heat capacities and volume fractions of the solid and liquid soil components. Organic matter and air were not taken into account, as their contribution can be considered negligible. Only the volumetric heat capacities for the mineral soil components (Ch,s, 2.0 MJ m3 K1) and water (Ch.l, 4.2 MJ m3 K1) were considered. Ch can then be obtained as follows:

Ch ¼ Ch;s Table 3 Dry bulk density ðb rd ; g=cm3 Þ obtained from samples collected in the reference profile.

143

dr b

rs

þ Ch;l q

(7)

where rs is the density of the solid phase (2650 kg m3), drb is the dry bulk density (kg m3) and q is the soil water content which lies between zero and the pore volume fraction (¼1 drb/rs). Note that phase changes are not considered in the calculation of Ch. Once l and Ch are known, the thermal diffusivity, Dh, can be estimated by: Dh ¼ l/Ch. Thermal diffusivity can be calculated independently from measured diurnal soil temperatures, obtained at two depths, by

144


Fig. 1. Vertical section of the sensor location for the GCHP and reference profile.

employing, for example, the logarithmic, arctangent or amplitude method [6]. However, these methods are less reliable under environmental conditions leading to small soil temperature amplitudes (i.e. low (wintertime) air temperatures, cloudy conditions, or soils strongly insulated by vegetation). Infiltrating rainfall will also reduce reliability of these methods. It was therefore decided to calculate thermal diffusivity from l/Ch, in order to achieve acceptable results throughout the whole year. Thermal properties were also measured directly in the reference profile at different depths (Table 1 for more details) using threeneedle probes, designed and installed using the approach detailed in Refs. [15,16]. Multi-needle heat pulse probes have been proved to provide reliable soil thermal properties measurements. A pulse of heat (15 s in duration) is generated in the central needle and radiates outwards. Fine-wire thermocouples in the two outer needles measure the corresponding rise and fall of temperature at approximately 1 s intervals. The maximum temperature increase (DeltaTmax) and the time taken to reach this maximum from the initiation of the pulse (Dt) are obtained. Additional processing of these temperature curves is required (non-linear curve fitting) to obtain accurate values for DeltaTmax and Dt. This method makes

use of DeltaTmax and Dt, the geometry of the three-needle probes and the magnitude of the heat pulse, to estimate the thermal properties. The installation of these sensors allowed us to compare the theoretically derived thermal diffusivity, using the Lu et al. [7] approach for calculation of l, and volumetric heat capacity, directly with the measured thermal properties. In the results section we will analyse the differences found in soil moisture content and soil temperature over the GCHP and reference profile, and the implication for the heat extraction near the GCHP. Furthermore, we will discuss the calculated and directly measured thermal properties as well as the soil heat flux derived using Fourier’s law for heat conduction. Finally, we will discuss the Coefficient of Performance, COP, results calculated using the GHE inlet and outlet fluid temperature data. 3. Results and discussion 3.1. Meteorological variables Fig. 2 shows daily averages of net radiation, air temperature, relative humidity, and wind speed at the experimental site from 1st


145

Fig. 2. (a) Daily mean net radiation and air temperature; (b) Wind speed and relative humidity at the GCHP experimental site at Drayton St Leonard. Data were gap-filled using data from the nearby Howberry Park meteorological site.

November 2009. Air temperature and net radiation were highest during the summer months, when the seasonal recharge of heat into the ground takes place. Note the low air temperatures during late December/early January 2010, this relates to a period of frost and snow in the area. The plot of wind speed reflects large shortterm fluctuations. Relative humidity shows a minimum around the middle of July; throughout the year it varies in relation to the air temperature and rainfall. 3.2. Soil physical variables 3.2.1. Soil moisture content Half-hourly soil moisture content was measured continuously from 1st November 2009 at five soil depths in the GCHP and reference profile (see Fig. 1). Unfortunately, the soil moisture probes above the slinky, at 0.07 m and 0.37 m depth, failed, so direct comparisons with the reference profile were not possible at those soil depths. It was decided not to replace the sensors in order to minimize disturbance of the ground near the slinky. Fig. 3 shows daily average soil moisture contents at 0.17 m, 0.37 m, 0.75 m and 1.0 m soil depth for the GCHP and reference profile. Overall, soil moisture content in the soil affected by the slinky was always lower, at 0.75 m and 1 m depth. This pattern was also visible at 0.17 m depth, but not between the end of April until August 2010, which coincides with a period when more vegetation (grasses and sparse herbs) grew on top of the reference profile, explaining the depletion of soil moisture content by the shallow root

layer. This may also have caused an increment in soil temperature in the upper layers of the GCHP profile (around the middle of May), compared to the reference profile temperature (see Fig. 5). This was caused by the fact that the GCHP profile surface soil received more net radiation as a result of its lower vegetation density. Soil moisture content near the GHE is influenced by the soil structure, moisture gradients and gravity effects, and temperature gradients. Moisture migration (vapour and liquid) is generally from warm to cold parts and so heat extraction by GCHPs should lead to higher moisture contents in the GHE vicinity [9,17]. A thin relatively moist region could develop near the slinky, but its thickness will depend on the ability of the surrounding soil in retaining the moisture in presence of an artificial temperature gradient and on the induced temperature gradient itself [18]. However, the soil moisture contents in the GCHP profile presented in Fig. 3 do not reflect this theoretical pattern, not even at 1.0 m depth where the slinky was installed. This could be due to the location where the Thetaprobe soil moisture sensor was positioned during the sensor installation. The Thetaprobe installed at 1 m depth was located about 18 cm away from the slinky, in order not to disturb the water and heat flow in the immediate vicinity of the slinky, which may explain why this Thetaprobe did not capture the expected increment of soil moisture content that may have occurred within a few centimetres of the slinky. If fact the sensor at w20 cm distance may have recorded the zone that dried out because of its moisture migrating to the slinky. It is possible that the sensors at shallower depths could reflect this downward migration.

146


Fig. 3. Daily average unfrozen soil moisture content (m3 m3) at (a) 0.17 m, (b) 0.37 m, (c) 0.75 m and (d) 1.0 m soil depth for the GCHP and reference profile. Fig. (d) also denotes the daily rainfall amounts recorded at the site.

Note also that only one sensor was used to monitor soil moisture content at each depth. Due to the spatial variability of the soil moisture content as a result of the existence of stones and cracks (causing preferential flow) in the soil profile, soil moisture content measurements may not be entirely representative of the site, but they can give us at least an indication of the overall differences in the amount of water between both profiles. The hydraulic conductivity decreases as a result of the decrease of soil temperatures, mainly as a result of increased viscosity [19,20]. The hydraulic conductivity for the slinky profile at 0.75 m would be about 30% lower than for the reference profile due to temperature effects on water viscosity. It can also be observed that the slope of q with time during the summer months for the reference profile at 0.75 m (Fig. 3c) is steeper compared with the slinky profile. This also indicates that the water flux for the reference profile was more pronounced due to its larger hydraulic conductivity value. The effect of soil temperature on water viscosity and thus on the hydraulic conductivity was slightly lower at 1.0 m due to different soil textural properties with lower values of soil moisture content; this can be observed in the measurements (Fig. 3d) with q-values for the reference and slinky profile being closer; in contrast, the hydraulic conductivity for the slinky during the melting periods of January and March was significantly larger than for the reference profile, when the Thetaprobe registered soil moisture content values that were twice as large as the values measured in the reference profile at that soil depth. A reduction in downward soil moisture transfer for the slinky profile may have left more water available in near-surface layers for evaporation and transpiration, meaning that less water was available to replenish deeper layers, compared to the reference profile. Finally, the differences in soil structure between both profiles will have contributed to a certain extent to differences in measured soil moisture contents. The reference profile may have been relatively undisturbed whereas the GCHP profile was dug out and then back-filled. A salient feature in Fig. 3 is two distinct peaks for soil moisture content at 1 m depth for the GCHP profile, throughout January and the second half of February. The second peak is also observed, but to

a lesser extent, for the reference profile. These peaks may have been caused by a combination of effects: preferential flow after rainfall, a relatively sudden thawing out of frozen water (note that Thetaprobes do not register frozen water), which particularly affected the GCHP profile near the GHE were below zero temperatures were more common, and a rise in groundwater table. The manhole was flooded between 6th February 2010 and 15th March 2010, which corroborates the probability of groundwater rise. Temporary below zero temperatures, followed by above-zero temperatures (i.e. evidence of thawing) were recorded on a half-hourly basis at a depth of 1.14 m between 8th January and 1st March. 3.2.2. Soil temperature Modelling exercises for the GCHP site under investigation showed that the GHE-influenced the soil temperatures to within a distance of around 0.6 m from the central long axis of the heat exchanger for a single straight heat exchanger system; this could be up to 0.8 m for a slinky system [12]. The reference profile was installed about 2 m away in the horizontal direction from the middle portion of the slinky, and far enough away as well from the GHE on the other side, so we could assume that the influence of the GHE on the reference temperature profile was negligible. Soil temperatures at 1 m depth, obtained from a thermistor installed just above the slinky, will reflect the GCHP system’s heat extraction. Fig. 4 shows half-hourly soil temperatures at 1 m depth for the slinky and the reference profile. Firstly, soil temperatures near the slinky are considerably lower than those for the reference profile. Secondly, a considerable diurnal variability, particularly in the spring, can be observed for the soil temperature at 1 m depth that is affected by the GHE, due to the interplay between the GCHP, the external air temperature and the indoor air temperature. The diurnal pattern of soil heat extraction and related soil temperatures depends partly on customer behaviour, as regulated via indoor thermostats. Fig. 5 compares daily-averaged soil temperatures between the GCHP and reference profile at 0.02 m, 0.25 m, 0.50 m and 1.0 m. Differences between the two profiles were negligible at a depth of 0.02 m, but they became more pronounced from approximately


147

Fig. 4. Half-hourly soil temperatures at 1 m depth ( C) for the slinky and the reference profile. Air temperature ( C) and precipitation (mm/hour) are also shown.

0.25 m depth onwards. Soil temperatures are generally much more spatially homogeneous (in particular for the deeper soil layers) than soil moisture content because phenomena such as preferential flow do not play a role in heat transfer, at least not directly. Hence, the observed effect of the GCHP system on the soil temperatures below approximately 0.3 m is real and significant. Overall soil temperatures in the slinky profile are considerably lower than in the reference profile; the amount by which they are lower can give us an idea of the thermal energy absorbed by the GHE. Cold conditions during the winter of 2009 (from 18th December) induced an abrupt drop of soil temperatures at the surface, down to below freezing point. The insulating nature of the surface soil layers (as well as the snow for part of the period) caused deeper soil temperatures to be >0 C for the reference profile. However, at 1.0 m depth for the GCHP profile daily average temperatures got very close to freezing (0.21 C).

We can illustrate more clearly the heat uptake by the GHE from the soil temperature differences, DTsoil, between the slinky and the reference profile. Fig. 6 shows these differences at 0.02 m, 0.25 m, 0.50 m and 1.0 m. Negative DTsoil values denote that the soil affected by the slinky was cooler than the reference soil. Soil temperature differences were approximately zero on average at 0.02 m until May 2010, when DTsoil became positive (up to 2 C), due to the growth of the vegetation causing local changes in soil shading and soil moisture content (mentioned in the previous section, see Fig. 3). These positive DTsoil values can be observed down to 0.25 m (Fig. 6a and b). Temperature differences at 0.25 m were approximately 1 C until May 2010 and their absolute values decreased during the summer months when the system was used only for domestic water heating and for heating the swimming pool. Absolute temperature differences increased for deeper soil layers, with DTsoil

Fig. 5. Daily average soil temperature ( C) at (a) 0.02 m, (b) 0.25 m, (c) 0.5 m and (d) 1.0 m soil depth for the slinky and reference profile.

148


Fig. 6. Daily average soil temperatures differences ( C) between the slinky and the reference profile, DTsoil, at (a) 0.02 m, (b) 0.25 m, (c) 0.50 m and (d) 1.0 m. The horizontal line was included to clearly distinguish between positive and negative temperature differences.

reaching values of down to 3 C at 1.0 m depth by the middle of April 2010. It was around this time that the swimming pool was switched on and an increase in the heat demand is observed, increasing even further the jDTsoil j values (up to 5 C). Approximately two months later, by the end of July, temperature differences, and thus the GCHP heat demand, were almost negligible; to come back again to previously observed negative levels when the owner switched on the heating by the middle of September. A new owner arrived on site in September 2010, however it is unlikely that this could have influenced the observed DTsoil values, because the settings for the room heating and hot water were not modified from those used by the previous owner. It is important to note that the absolute temperature differences at 1.0 m by September 2010 have increased by 3 C compared to the ones found in November 2009 (approximately from 3 to 6 C). Soil temperature very close to the slinky will react relatively instantly to any increase in demand. The heat demand increased substantially during September (see Fig. 14 for the Coefficient of Performance), which can explain the abrupt decrease observed in the soil temperatures near the slinky. High heat consumption found in September 2010 is suggesting a possible future unbalance due to a possible change in heat consumption habits (this we do not know yet as there were no data for September 2009) [21]. A second year of measurements, currently underway, will help to clarify and understand the reason of this evolution in reference and GHEinfluenced soil temperatures. The installation depth is an important factor to achieve optimal COP for horizontal GCHPs. If the system is undersized for the heat demand of the house (i.e. not enough total pipe length), it could lead to excessive heat extraction and temperature drop near the slinky. Under UK environmental conditions it is very likely that if soil temperatures near the slinky drop below freezing point, it could compromise their future performance because the ground will not be able to fully recharge for the following season during the summer. Tarnawski [22] mentioned that long-term operation of horizontal GCHP could potentially cause significant degradation of

ground thermal and moisture storage capacity. In his work, the horizontal GCHP in a site of northern Japan was working in cooling and heating mode, which would have helped to compensate the heat loss in the ground. In contrast, the system at Drayton St Leonard operates in heating mode only. Fig. 7 shows soil temperatures at 1 m depth (just above the slinky) and at 1.14 m (just below the slinky) for the GCHP profile, and at 1 m depth for the reference profile at Drayton St Leonard. Soil temperatures at 1 m depth for the GCHP profile remained above freezing point for the entire year, but soil temperature at 1.14 m depth for the GCHP profile dropped below freezing point at the beginning of January 2010 until March 2010. The sudden rise above 0 C will cause thawing of the frozen water, which may also explain the sudden peaks in soil moisture content presented in Fig. 3. Choosing the right installation depth and separation of the trenches as well as the total length during the design stage will secure a good performance of the GCHP during its lifetime.

Fig. 7. Soil temperatures ( C) for the GCHP and the reference profile at 1.0 m and 1.14 m depth.


The system will draw more heat and hence bring deep soil temperatures down more when air temperatures are low due to the increase in heat demand. Air temperature plotted against DTsoil at 1 m depth for the GCHP profile (data not shown) resulted in a r2 value of 0.56 for daily averages (0.39 if half-hourly values are used instead) for the period from November 2009 to March 2010 when the system was used for heating of the dwellings on site (as well as for provision of hot water, but this is standard throughout the entire year). 3.2.3. Comparison of the thermal properties for each profile Soil thermal properties were calculated for the GHE-profile and for the reference profiles at 0.17 m, 0.75 m and 1 m soil depths. Thermal properties at 0.37 m were only calculated for the reference profile (soil moisture content data were not available for the GCHP profile, see Fig. 3). Figs. 8 and 9 show the volumetric heat capacity (Eq. (7) and the thermal conductivity (from Eqs. (4)e(6)), respectively. Thermal properties vary with soil moisture content, decreasing when the soil moisture content declines during the summer months. Overall, the slinky profile has lower volumetric heat capacity and thermal conductivity compared to the reference profile, due to the lower soil moisture content values (see Fig. 3), apart from the two anomalies during the winter 2009 at 1 m soil depth, most likely as the result of melting of frozen soil water and/ or preferential flow. Thermal conductivity for the GCHP profile decreased more from May 2010 compared to the reference profile at 0.75 m and 1 m soil depth. The volumetric heat capacity was calculated assuming the same measured soil texture in both profiles, but it is likely that they are somewhat different, because the soil textural profile above the slinky was disturbed during the installation of the system. Backfilling of excavated soil material afterwards will also have affected the dry bulk density. This could be a source of uncertainty in our calculations. The three-needle probes measurements will allow us to compare the measured soil thermal properties with the estimated thermal properties. Thus we can implicitly assess the error

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that soil texture uncertainties may have introduced in our calculations. Fig. 10 shows the thermal diffusivity calculations for both profiles (calculated from l/Ch). Note that Dh for the GCHP profile is often higher than that for the reference profile at 0.17 m soil depth, although lower at 0.75 m and at 1 m depth. This is partly caused by the fact that Dh evolves with soil moisture content in a different way compared to Ch (a linear increase with soil moisture content) and l (an asymptotic increase with soil moisture content). Dh will increase up to certain soil moisture content, then reach a maximum and decrease slightly thereafter with a further increase in soil moisture content (see Fig. 11). Dh in the reference profile at 0.75 m, and at 1.0 m is almost constant during 2010; it drops slightly at the end of June 2010. For the GCHP profile its behaviour is different: Dh decreases considerably from May 2010 compared to the reference profile, at 0.75 m and 1.0 m. This evolution is not observed at 0.17 m, where Dh runs almost in parallel for both profiles. The thermal diffusivity decline for the GCHP profile at 1.0 m depth continued well into autumn, as caused by the fact that q in the vicinity of the GHE was lower than that for the reference profile (see Fig. 3d, i.e. q w 0.08 rather than w0.15 m3 m3 during summer). Although this difference is relatively small, Fig. 11 shows that Dh is very strongly dependent on q in this soil moisture region. The lower Dh values for the GCHP profile will have affected the heat transfer near the GHEs and hence the COP of the system. The three-needle probe sensor at 0.75 m failed, so only the three-needle probes at 0.17 m and 0.37 m were compared directly with the derived thermal conductivity calculated as explained in Section 2.2. Fig. 12 shows good agreement between the calculated thermal diffusivity and the three-needle probes at 0.17 m for the period of the year when measurements were available, although the theoretically derived thermal diffusivity at 0.37 m was not able to capture the decrease observed from wDOY (Day of Year) 150 (end of May) possibly due to local differences in the soil moisture content.

Fig. 8. Volumetric heat capacity, Ch, for (a) 0.17 m, (b) 0.37 m, (c) 0.75 m and (d) 1.0 m soil depths calculated from Eq. (7) for the GCHP and reference profile.

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Fig. 9. Thermal conductivity, l, for (a) 0.17 m, (b) 0.37 m, (c) 0.75 m and (d) 1.0 m depth calculated for the GCHP and reference profile, using Eqs. (4)e(6).

3.2.4. Manhole data and Coefficient of Performance The Coefficient of Performance (COP) is used to assess the efficiency of GCHPs and provides an instantaneous measure of performance [22,23], calculated as the ratio of the heat extracted by the pump and the electricity consumed by the pumps to circulate the refrigerant and deliver heat to the building:

COP ¼

Qhe þ Wcompressor Wpump þ Wcompressor

(8)

where Wpump is the electricity consumed by the pumps, Wcompressor is the electricity consumed by the compressor and Qhe is the heat extracted by the pump.

Fig. 10. Thermal diffusivity, Dh, for (a) 0.17 m, (b) 0.37 m, (c) 0.75 m and (d) 1.0 m depth as calculated from the ratio of the thermal conductivity and volumetric heat capacity, for the GCHP and reference profile. Note that thermal diffusivity is given in mm2/s.


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Fig. 11. Thermal diffusivity, Dh, thermal conductivity, l and volumetric heat capacity, Ch, for 1.0 m depth for the Drayton St Leonard soil as a function of soil moisture content (q). Note that thermal diffusivity is given in mm2s1.

Measuring the COP allows us to compare different installations and thus calculate the associated carbon savings. The COP was not directly measured, but it can be estimated from GHE inlet and outlet temperature observations (see Table 1) and pump technical characteristics. The heat extracted by the pump can be calculated from the inlet and outlet temperatures of the heat exchanger:

Qhe ¼ mf cf ðTout Tin Þ

(9)

Fig. 12. Comparison of the derived thermal diffusivity, Dh, calculated using the Lu et al. approach for calculation of l and volumetric heat capacity, with the three-needle probe sensor for (a) 0.17 m, (b) 0.37 m, for the reference profile.

where Tin and Tout are the inlet and outlet temperature, respectively, mf is the mass flow rate (0.57 kg s1) and cf is the specific heat extraction of the fluid (4700 J kg1 K1) (parameters obtained from technical pump specifications; see also [12]). Seeing mf and cf are constant, Tout Tin is a direct measure of heat extraction. The pump system at Drayton St Leonard works continuously to provide heating to the house during the winter, hot water during the entire year, and heating to the swimming pool during most part of the summer. A constant temperature of 20 C (quoted by owner) is set on the heat pump. The pump then maintains that temperature and adjusts the amount of heat required, according to the outside temperature. This means that the heat pump will switch on and off depending on the air temperature. Fig. 13 illustrates the temperature differences between the inlet and outlet of the heat exchanger, as a measure of the heat extracted by the GHE, as well as the air temperatures. There was little variability in Tout Tin from November 2009 until approximately May 2010, which reflects a rather constant GHE performance. A somewhat increased heat extraction can be noticed during January 2010 as a result of the cold weather conditions experienced that winter. During the summer months (from May to September) the pump provided heating to the swimming pool and was still used for domestic water heating. During that period, temperature differences between the inlet and outlet pipes decreased considerably when the system was less in use compared to the winter months, and due to the increase of air temperatures during the summer which compensated the heat extraction. The Coefficient of Performance, as calculated from Eq. (8) is shown in Fig. 14. The COP of the system decreased during December to increase again during the cold period by January 2010 due to the increase in customer heat demand. The pump had typical COP values between 2.2 and 2.57, until May 2010. From then on the pump remained inactive many days depending on the air temperature, used sporadically to heat the swimming pool and provide hot water. Summer corresponds to the period when the ground heat is recharged and when the heat demand was lower. The pump system came back to normal functioning from September 2010 onwards, with a COP that was slightly higher compared to November 2009. The average COP for this site was 2.35, which falls within the maximum average values obtained in the report released by the Energy Saving Trust (EST report, 2010), which describes the average performance of a group of selected GCHPs in the UK.

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Fig. 13. Temperature differences between inlet and outlet temperatures of heat exchanger. Air temperature is also shown. Time step is daily.

During the winter of 2009, unfrozen soil moisture content decreased significantly reflecting freezing conditions; during this period, the COP did not change and remained similar to previous days. Although the soil near the slinky just remained above freezing point, the soil below the slinky was frozen approximately until March 2010. Freezing will induce heat transfer from the liquid (molecularly less stable) to the solid phase as the water freezes. A rather constant temperature was observed from January until the middle of February at 1.14 m at the GCHP profile (see Fig. 7), which could be a sign of the phase changes (freezing/melting processes) happening at that depth. Melting periods will be relevant because they may have affected the thermal properties, through a change in q, thereby possibly decreasing the heat extraction of the GHE, at the expense of heat consumption to melt the ice into water. The depletion of soil moisture content during the summer months, and also of the thermal diffusivity, will have also affected the COP. 3.2.5. Estimation of soil heat flux Due to the temperature gradient induced near the slinky, heat transfer will be altered in the soil vertical profile. Ultimately this could modify the performance of a horizontal ground source heat pump. The slinky at Drayton St Leonard is installed approximately at 1.1 m depth. When air temperatures decrease, soil temperatures

Fig. 14. Calculated Coefficient of Performance (daily and monthly average) for the GCHP system. From November 2009 until the middle of April 2010 the GCHP system provided heating and hot water; from the middle of April, during the summer period (until September 2010) the GCHP provided hot water and heated the swimming pool.

near the slinky will drop further because of increased heat extraction. Similarly, when soil moisture content decreases, thermal conductivity will decrease as a result. Hence, soil temperatures will drop even further and the resulting temperature gradient may induce a heat flow in upper soil layers. Soil heat flux near the slinky was estimated using Fourier’s law (Eq. (1)), assuming a homogeneous medium where the calculated thermal conductivity can be assumed to be constant over the layer under consideration. The slinky is situated about 2 cm below the thermistors at 1.1 m. The heat flux near the slinky was calculated using the soil temperature gradient between 1.0 m and 1.1 m depth. In a similar way the heat flux was estimated for the reference profile at approximately the same depths using the soil temperatures at 1.0 m and 1.05 m depth. The soil heat flux density is positive when directed downwards. Fig. 15 shows daily average G-values throughout the year, i.e. net soil heat flux, for the soil where the slinky is installed and for the reference profile, at approximately 1 m depth. During winter net G is expected to be negative, while being positive during summer recharge conditions. Maximum differences between both G-fluxes are observed during the winter months, with less negative heat flux near the slinky reflecting the heat extraction that occurs downwards (i.e. leading to larger positive G-values) towards the slinky. During the summer heat fluxes at 1 m depth largely converge for

Fig. 15. Daily average heat flux (W/m2) estimated for the slinky and reference profile at 1 m soil depth.


both profiles to diverge again by early September 2010. Note that during summer months GGCHP < Gref because l is lower for the GCHP profile. 4. Conclusions This paper focussed on the interactions between the soil, horizontal heat exchangers and the aboveground environment. Furthermore, it analysed the factors that influence the efficiency of a horizontal GCHP. Soil temperatures and soil moisture content measurements showed that the GHE modified heat and water transport in the soil to such a degree that it could affect the GCHP performance. The slinky-type ground heat exchanger influenced soil temperatures up to 0.9 m from the slinky installation depth during the winter. The consistent differences in soil moisture content measurements between the reference and GCHP profile can be explained by temperature gradient induced moisture gradients and a decrease in hydraulic conductivity due to decreased temperatures (causing an increased viscosity), although soil heterogeneity will also have played a role. Acknowledgements This work was supported by the UK Natural Environment Research Council (NERC) (GROMIT: GROund coupled heat pumps MITigation potential, NE/F020368/1). We thank Geoff Wicks of CEH-Wallingford for supplying the Howberry park meteorological data. We are also grateful to the owners of the GCHP system for letting us use their land for our experimental set-up. We also thank Dr. Robin Curtis for his expert guidance since the start of the project. References [1] Blum P, Campillo G, Münch W, Kölbel T. CO2 savings of ground source heat pump systems e a regional analysis. Renewable Energy 2010;35(1):122e7. [2] Singh H, Muetze A, Eames PC. Factors influencing the uptake of heat pump technology by the UK domestic sector. Renewable Energy 2010;35(4):873e8. [3] Esen H, Inalli M, Esen M. Numerical and experimental analysis of a horizontal ground-coupled heat pump system. Building and Environment Journal 2007; 42:1126e34.

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