http://www.diva-portal.org
Postprint
This is the accepted version of a paper presented at ICNTSE 2018.
Citation for the original published paper:
Fasci, M L. (2018)
Shallow Geothermal Heat Pumps: a study of the resource potential at a neighbourhood scale.
In:
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-246395
Shallow Geothermal Heat Pumps: a study of the resource potential at a neighbourhood scale.
Maria Letizia Fascì, Alberto Lazzarotto, José Acuña, Joachim Claesson KTH Royal Institute of Technology, Brinnelvägen 68
Stockholm, Sweden
mlfasci@kth.se, alberto.lazzarotto@byv.kth.se, jose.acuna@energy.kth.se , claesson@energy.kth.se
Abstract—The residential sector accounts for a relevant share of global energy use; therefore it is important to use as much renewable energy as possible to satisfy its demand. Geothermal energy, among others, is nowadays used for this scope: more and more buildings in several countries are exploiting the underground to satisfy domestic heating, cooling and hot water demand by means of ground-source heat pumps. On the long run heat extraction/injection can lead to depletion of the ground as heat source/sink. Current tools only allow a designer to take into account the depletion of the ground caused by the system she or he is designing. However, the actual total heat depletion is also influenced by the surrounding systems. With the growing diffusion of ground-source heat pumps the ability of estimating the total underground heat depletion is of paramount importance. The aim of the article is to give an insight of the problem: the goal is to show what will happen in the underground if residential ground source heat pump systems are designed without taking into account the presence of neighbouring installations. The study is performed for different types of soil and borehole heat exchangers designs.
Index Terms— Ground source heat pumps, neighbouring boreholes, thermal influence, geothermal sustainability.
I. INTRODUCTION
The concept of sustainable development was discussed for the first time in the Brundtland Report, in 1987 [1]; since then environmental issues have been relevant part of the political agendas and efforts have been made by the international community to set and accomplish targets within the sustainable development framework [2, 3, 4]. Within this framework, diminishing energy consumption, especially from non-renewable sources, is of paramount importance.
The residential sector accounts for a relevant share of global energy use. In 2016, the residential sector in the European Union accounted for 25.4% of final energy consumption, with space heating, domestic hot water and space cooling accounting respectively for 64.7%, 14.5% and 0.3% of the total households’ consumption. Only 16% of the total consumption was satisfied by renewable energies [5].
Efforts are directed towards both decreasing the energy need of this sector and increasing the share of energy demand satisfied through renewable energy sources [6]; one of the sources that can provide a significant contribution is shallow geothermal energy (SGE). SGE can in fact be used to satisfy
the heating, cooling and hot water demand in residential buildings.
SGE is usually exploited by means of heat pumps (HPs):
heat can be extracted from the ground and used as heat source in the evaporator of a HP operating in heating mode, while if a HP is operating in cooling mode the heat released in the condenser can be rejected into the ground. Heat can be extracted from or rejected to the underground by means of, among others, borehole heat exchangers (BHEs). A BHE consists of a 10-20 cm diameter hole dug in the ground, whose length can vary from tens to hundreds of meters; a pipe for the circulation of a secondary fluid; and grouting material or underground water to fill the gap between the borehole wall and the pipe.
Despite the renewable nature of SGE, as pointed out for example by Vienken et al. [7] and Hähnlein et al. [8], care must be taken not to locally overexploit this source, i.e., not to produce high temperature variations in the underground surrounding the systems, referred to as local thermal anomalies. In fact, local thermal anomalies in the underground should be kept within certain limits due to technical and environmental reasons; in particular, focusing on the technical reasons, temperature changes on the BHE wall are responsible for decrease in the systems’ performance and possibly ground freezing.
In this study, extending the definition of sustainable development given in the Brundtland report ("Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs") [1], we define a GSHP system as sustainable if its use does not lead to the depletion of the ground as heat source/sink, i.e., it can be used for indefinite time to extract/reject heat at the initial design rate without exceeding the imposed temperature limits. Therefore, it is necessary to understand how the temperature on the BHE wall evolves in time to assess its sustainability. If the temperature never exceeds the limits the GSHP can be considered sustainable.
Local thermal anomalies become stronger as the number of neighbouring BHEs increases. This is well known and many tools are available to correctly design borehole fields: fields were many BHEs belonging to the same installation are present [9] [10] [11] [12] [13] [14]. However, still little has been done regarding neighbouring BHEs belonging to
different installations, for example BHEs in densely populated areas [15]. As the number of installations is increasing [16], addressing this problem becomes more and more urgent to avoid wrong forecasts of systems performance and not sustainable systems designs.
In this paper we want to show the potentiality of the shallow underground as sustainable energy source, but also the limitations posed by the presence of neighbouring systems and the importance to take their presence into account to avoid an overexploitation and unsustainable use of this source.
II. METHODOLOGY
In order to evaluate the technical sustainability of a GSHP system it is necessary to evaluate the evolution of the BHE wall temperature variation. In this study we perform this calculation for several scenarios. Several geometrical configuration were considered: the first configuration considers an isolated BHE, i.e., the absence of neighbouring systems; the other configurations consider 2, 3, 4, 9 and 16 neighbouring BHEs in symmetric, regular configurations (fig.
1).
For the configurations with neighbouring BHEs, for each configuration, 3 different grid distances (GDs) were considered: 5 m, 10 m and 20 m. The GD represents the minimum distance between two consecutive BHEs. For each scenario the temperature variation on each BHE wall, due to the operation of all the BHEs was calculated. All the scenarios were finally compared with the first one (isolated BHE) to quantify how much energy can be potentially extracted by an isolated BHE and how the presence of neighbouring systems reduces this potential.
A. Configuration 1: isolated BHE
The temperature variation on the wall of an isolated BHE depends on ground properties, BHE load and BHE geometry.
Regarding the ground properties, 4 different types of ground
were considered: limestone, clay, and two types of granite (table I).
TABLE I. GROUND PROPERTIES
Property Ground type
Limestone Clay Granite LK Granite HK Conductivity k
[Wm-1K-1] 1.3 1.28 1.70 4.00 Density ρ
[kgm-3] 2300 1450 2750 2750 Specific heat cp
[Jkg-1K-1] 900 880 890 890 Diffusivity α
[ms-2×10-7] 6,28 10 6,95 16,34
For what concerns the BHE load, it was assumed to have the same profile as the heating and hot water demand of a typical Nord-European villa [17][18]; the intensity of the load was calculated so to give an annual-averaged linear BHE load of 10W/m; the BHE is extracting only. For what concerns the BHE geometry, the buried depth and BHE radius were set to 6 m and 0.1 m respectively; 2 different BHE lengths were considered: 50 m and 400 m. The BHE was considered as a continuous line heat source with uniform and constant heat flux in a semi-infinite, uniform, isotropic media with constant properties. The finite line source (FLS) model and the superposition method in time were then used to evaluate the temperature variation on the BHE wall [19]:
(1) where ΔTb,is is the temperature variation on the wall of an isolated BHE with respect to the undisturbed ground temperature, tn is the nth time step, k is the ground conductivity, qi is the linear BHE load at time i; g is a function of tn-ti, BHE length Hb,is, BHE buried depth Db,is, BHE radius rb,is, and ground diffusivity α. The formulation given by Claesson and Javed [20] was used for the calculation of the ground thermal response g.
B. Other configurations: non-isolated BHEs
The temperature variation on the wall of a non-isolated BHE depends on ground properties, BHE load and BHE geometry, like for the isolated BHE, but also on the distances from the surrounding BHEs and their loads and geometries. It was calculated using the FLS model and superposition in space [18]:
(2) ΔTb,n_is is the thermal variation on the non-isolated BHE, ΔTb,is
is the thermal variation of the same BHE if isolated, N is the number of neighbouring BHEs, j is the jth neighbouring BHE, qj is the annual average linear BHE load of the jth BHE, Hj and Dj are respectively the length and buried depth of the jth BHE, dj-b,n_is is the distance between the non-isolated BHE and the jth
Conf.2 Conf.3 Conf.4
Conf.5 Conf.6
Fig. 1. BHEs configurations. BHEs of the same type (same colour and letter) and belonging to the same configuration have the same BHE wall temperature variation.
BHE, the other symbols are the same as for the isolated BHE.
All the BHE have the same annual average BHE load and geometry. It can be noticed from Eq. 2 that the load profile of neighbouring BHEs was ignored and their load was assumed constant and equal to the annual average. This approximation was shown to lead to negligible errors by a previous study by the authors [17].
III. RESULTS AND DISCUSSION
A. Scenario 1: isolated BHE
The temperature variation on the wall of an isolated BHE with the respect to the temperature of the undisturbed ground is shown in figure 2. What is shown is the absolute value of the temperature variation, therefore it should be kept in mind that the temperature variation on the BHE wall is negative as heat is extracted (the ground cools down).
Figure 2 shows that the temperature variation is strongly dependent on the ground properties: materials with relatively high conductivity (granites) have a lower thermal response compared with low conductivity materials (clay and limestone). The figure shows that a steady-state condition is
reached: there is no variation between the thermal response during the 49th and 50th year of operation. Moreover, the steady-state condition is reached relatively fast: the temperature response after 1 year of operation is around 90%
of the thermal response after 50 years of operation for the 50 m BHE; the steady state is reached more slowly for the 400 m BHE. The steady-state condition is reached when the heat extracted from the BHE wall is equal to the heat supplied to the BHE wall from the surrounding ground. The heat capacity of the ground can be considered infinite compared to the heat extracted in GSHP applications, this is what makes possible the achievement of a steady-state condition. Therefore, if a GSHP is designed so that, during the steady state, the limits imposed on the temperature variation on the BHE wall are not exceeded, then heat can potentially be extracted/reinjected for an infinite time at the designed rate and the source can be considered renewable and sustainable.
For example, if the maximum temperature variation allowed on the borehole wall is 6 ⁰C, 4.38 MWh/y (corresponding to an average load of 10 W/m on a 50 m BHE) can be sustainably extracted by a 50 m BHE in granite HK, but not in the other materials considered. If the maximum temperature variation allowed is 5 ⁰C the same BHE would operate without exceeding the temperature limits during the first years of operation, but would exceed the limits during the steady-state, therefore, according to the definition used in this paper it could not be considered sustainable; in order to sustainably extract the same amount of heat it would be necessary to use a deeper BHE.
B. Scenarios 2, 3 and 4
A comparison between the temperature response of the wall of a BHE in configuration 2 (green line), 3 (yellow line), 4 (red line) and the isolated case (blue line) is shown in figure 3 for 50 m BHEs. Only the behaviours in clay and granite HK are shown; they represent the extreme cases.
Figure 3 shows that the BHEs influence each other already during the first year if GD = 5 m and already during the second year if GD = 10 m. This is true both for the BHEs in clay and granite HK. If GD = 20 m it takes longer for BHEs to influence each other, though the influence is visible after 50 years, and consequently during the steady state. It is important to consider that, differently from the isolated BHE for which the steady state is reached in few years, when the number of BHEs increases the time to reach the steady state increases, besides the further the BHEs the longer it takes to reach the steady state. Therefore, especially for the case GD = 20 m, the steady- state temperature variation might be higher than the temperature variation after 50 years.
To have a more quantitative visualization of the phenomenon, the temperature responses for the isolated BHEs and the other configurations are shown in table 2. For the isolated BHEs the minimum and maximum responses during the year are shown. For the other configurations only the additional responses due to the neighbouring BHEs are shown (therefore the isolated BHE response has to be summed up to have the total response). The additional response for
Fig. 2. Temperature variation (in absolute value) on the BHE wall of a 50 m- (top) and 400 m-length (bottom) isolated BHE in clay (black line), limestone (yellow line), granite LK (purple line), and granite HK (red line).
configuration 3 is not shown, it is simply two times the additional response for configuration 2.
From the data underlined in table 2 some deductions can be done: the additional thermal response due to the neighbouring
Fig. 3. Temperature response of BHEs in configuration 2 (green line), 3 (yellow line), 4 (red line) compared with the isolated case (blue line) for clay (on the top) and granite HK (on the bottom). r stays for GD.
BHEs can, in certain situations, be even higher than the thermal response due to the BHEs of interest; the relative influence of the neighbouring boreholes increases as the time increases; and finally, in absolute terms, low conductivity materials have higher thermal responses, as for the isolated
case, but during the timespan analysed the relative influence of neighbouring BHEs in granite HK is higher than the relative influence of neighbouring BHEs in clay. Therefore, if a system is designed using relative safety coefficients, there are more risks in granite than in clay to exceed the acceptable temperature variation during the first years of operation.
Indeed, when the steady-state condition for the system is reached there is no difference between the relative influence of neighbouring BHEs in different materials; this can be explained looking at the physics of the problem, but is beyond the scope of this paper.
C. Scenarios 5 and 6
For scenarios 5 and 6 table 3 is analogous to table 2 shown for scenarios 2 and 4. In scenarios 5 and 6 the BHEs belonging to the same configuration are divided into 3 types (A, red; B, yellow; and C, green); BHEs of the same type and same configuration have the same thermal response.
In configurations 4 and 5 the BHEs types are affected differently by their neighbours, with BHEs As being more affected than BHEs Bs and BHEs Bs being more affected than BHEs Cs. From the data underlined in yellow it can be observed that the relative differences between different BHEs of the same configuration are higher for higher GDs and decrease with time. The data underlined in green show that the BHEs in configuration 4 have the same behaviour as the BHEs in configuration 5 after 5 years of operation, but the thermal response of the BHEs in configuration 5 is higher than the thermal response of the BHEs in configuration 4 after 50 years, showing that, being equal the GD, in the short term small and big neighbourhoods have the same thermal behaviour, but in the long term bigger neighbourhoods are subject to more interaction between the BHEs.
In general the data show that as the size of the neighbourhood increases the thermal influence between the
BHEs increases; this can lead to temperature variation on the BHEs wall significantly higher than the
variations expected from calculations based on the assumption that the BHEs are isolated.
The data obtained for 400 m BHEs are not shown, they show that longer BHEs influence more the surrounding ones, with this effect being more pronounced as time increases.
Clay Limestone Granite LK Granite HK
5 y 20 y 50 y 5 y 20 y 50 y 5 y 20 y 50 y 5 y 20 y 50 y
Is.BHE 0.1 m 2,5/14,9 3,0/15,4 3,1/15,5 2,5/13,6 3/14,2 3,3/14,4 1,9/10,6 2,3/11,0 2,5/11,2 0,8/5,1 0.9/5.2 0.9/5,3
Conf 2
5m 1,4 1,8 2,0 1,2 1,7 1,9 0,9 1,3 1,4 0,5 0,6 0,6
10 m 0,7 1,1 1,2 0,5 1,0 1,2 0,4 0,8 0,9 0,3 0,4 0,4
20 m 0,2 0,5 0,6 0,1 0,4 0,6 0,1 0,3 0,4 0,1 0,2 0,2
Conf 4
5m 3,9 5,1 5,5 3,2 4,7 5,3 2,6 3,7 4,1 1,4 1,7 1,8
10 m 1,8 2,9 2,1 1,3 2,6 3,1 1,1 2,0 2,4 0,7 1,0 1,1
20 m 0,5 1,3 1,6 0,2 1,0 1,5 0,2 0,8 1,1 0,2 0,5 0,5
TABLE II. TEMPERATURE VARIATIONS IN CONFIGURATIONS 2 AND 4 FOR 50 m BHEs
IV. CONCLUSIONS
Shallow geothermal energy is potentially a sustainable energy source for heating and cooling of residential building, and its use can help decreasing the CO2 emissions associated to the residential sector. However, care must be given to the way this source is exploited in order to use it in a really sustainable way. In particular, this study has shown the need to know with low uncertainty the ground properties of the installations and the need to take into account the presence of neighbouring systems during the design phase of a GSHP; in fact a design that is sustainable for an isolated BHE is not necessarily sustainable for a BHE in dense neighbourhoods.
Designing sustainable BHEs neighbourhoods is still possible as the temperature variations they generate in the underground reach a steady-state condition; in order to do so new dedicated modelling tools are necessary, the authors themselves are working on their development.
ACKNOWLEDGEMENT
This project is supported by the Swedish energy agency.
REFERENCES
[1] World Commission on Environment and Development, “Our common future”, Oxford: Oxford University Press, 1987.
[2] United Nations Conference on Environment and Development,
“Rio Declaration on Environment and Development”, 1992.
[3] United Nations Framework Convention on Climate Change,
“The Kyoto Protocol”, 1998.
[4] United Nations Framework Convention on Climate Change,
“The Paris Agreement”, 2015.
[5] Eurostat, “Energy consumption in households”.
http://ec.europa.eu/eurostat/statistics-
explained/index.php/Energy_consumption_in_households (accessed July 6, 2018)
[6] European Commission, “Horizon 2020 - The Framework Programme for Research and Innovation”, 2011.
[7] T. Vienken, S. Schelenz, K. Rink, P. Dietrich, “Sustainable Intensive Thermal Use of the Shallow Subsurface—A Critical View on the Status Quo”, Groundwater 53, pp. 356–361, 2015.
[8] S. Hähnlein, P. Bayer, G. Ferguson, P. Blum, “Sustainability and policy for the thermal use of shallow geothermal energy”, Energy Policy 59, pp. 914–925, 2013.
Clay Limestone Granite LK Granite HK
5 y 20 y 50 y 5 y 20 y 50 y 5 y 20 y 50 y 5 y 20 y 50 y Is.BHE 0.1 m 2,5/14,9 3,0/15,4 3,1/15,5 2,5/13,6 3/14,2 3,3/14,4 1,9/10,6 2,3/11,0 2,5/11,2 0,8/5,1 0.9/5.2 0.9/5,3
Conf.5 BHE.C
5m 6,9 10,1 11,1 5,5 9,1 10,5 4,4 7,1 8,1 2,6 3,4 3,6
10 m 2,6 5,1 6,0 1,7 4,3 5,6 1,4 3,4 4,3 1,1 1,8 2,0
20 m 0,5 1,8 2,5 0,2 1,3 2,1 0,2 1,0 1,7 0,3 0,7 0,9
BHE.B
5m 8,2 11,5 12,5 6,7 10,5 11,9 5,4 8,2 9,2 3,1 3,9 4,1
10 m 3,4 6,2 7,2 2,4 5,3 6,7 2,0 4,2 5,2 1,5 2,2 2,4
20 m 0,7 2,4 3,2 0,4 1,8 2,8 0,3 1,5 2,2 0,4 0,9 1,1
BHE.A
5m 9,8 13,2 14,2 8,2 12,1 13,6 6,5 9,4 10,4 3,6 4,4 4,6
10 m 4,5 7,5 8,5 3,2 6,6 8,0 2,7 5,2 6,2 1,8 2,6 2,8
20 m 1,1 3,1 4,0 0,5 2,4 3,6 0,5 2,0 2,8 0,6 1,2 1,4
Conf.6 BHE.C
5m 9,0 14,5 16,3 6,7 12,8 15,3 5,5 10,0 11,9 3,6 5,0 5,4
10 m 2,8 6,4 8,0 1,7 5,2 7,2 1,5 4,2 5,7 1,3 2,4 2,7
20 m 0,5 1,9 2,9 0,2 1,3 2,4 0,2 1,1 1,9 0,3 0,8 1,0
BHE.B
5m 11,2 17,0 18,9 8,7 15,2 17,8 7,1 11,9 13,8 4,4 5,8 6,2
10 m 3,9 8,2 9,8 2,6 6,8 9,0 2,2 5,4 7,0 1,8 2,9 3,3
20 m 0,8 2,7 3,9 0,4 1,9 3,3 0,3 1,6 2,6 0,4 1,1 1,4
BHE.A
5m 14,0 20,1 21,9 11,2 18,2 20,9 9,0 14,2 16,1 5,3 6,8 7,2
10 m 5,4 10,3 12,1 3,6 8,7 11,2 3,0 6,9 8,7 2,3 3,7 4,0
20 m 1,1 3,8 5,2 0,5 2,8 4,5 0,5 2,3 3,5 0,6 1,5 1,8
TABLE III. TEMPERATURE VARIATIONS IN CONFIGURATIONS 5 AND 6
[9] P. Eskilson, “Superposition Borehole Model: Manual for Computer Code” University of Lund, Lund, Sweden, 1986.
[10] C. Yavuzturk, J.D. Spitler, S.J. Rees, “A transient two- dimensional finite volume model for the simulation of vertical U-tube ground heat exchangers”, ASHRAE Transactions 105 (2), pp. 465-474, 1999.
[11] M. Cimmino, M. Bernier, “A semi-analytical method to generate g-functions for geothermal bore fields” International Journal of Heat and Mass Transfer 70, pp. 641-650, 2014.
[12] L. Lamarche, B. Beauchamp, “A new contribution to the finite line-source model for geothermal boreholes.” Energy and Buildings 39 (2), pp. 188-198, 2007.
[13] A. Lazzarotto, “Developments in Ground Heat Storage Modeling” (doctoral thesis), KTH, 2015.
[14] P. Monzó, P. Mogensen, J. Acuña, F. Ruiz-Calvo, C. Montagud,
“A novel numerical approach for imposing a temperature boundary condition at the borehole wall in borehole Fields”, Geothermics 56, pp. 35-44, 2015.
[15] S. Hähnlein, P. Bayer, P. Blum, “International legal status of the use of shallow geothermal energy. Renewable and Sustainable Energy Reviews” 14, pp. 2611–2625, 2010.
[16] B. Sanner, “Shallow geothermal energy–history, development, current status, and future prospects”, European Geothermal Congress. Strasbourg, France: pp. 19–24, 2016.
[17] M.L. Fascì, A. Lazzarotto, J. Acuña, J. Claesson, “Thermal influence of neighbouring GSHP installations: relevance of heat load temporal resolution”, unpublished
[18] R. Ulseth, K.B. Lindberg, L. Georges, M.J. Alonso, Å. Utne,
“Measured load profiles and heat use for “low energy buildings”
with heat supply from district heating.” 15th International Symposium on District Heating and Cooling. Seoul, South Korea, 2016: pp. 180–190.
[19] J.D Spitler, M. Bernier, “Vertical borehole ground heat exchanger design methods”, In: S.J. Rees Advances in Ground- Source Heat Pump Systems. London: Woodhead Publishing, 2106.
[20] J. Claesson, S. Javed, “An analytical method to calculate borehole fluid temperatures for time-scales from minutes to decades”. ASHRAE Transactions. 117, pp. 279–288, 2011.
