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Performance Enhancement of Urban Ground Source Heat Pumps through Interactions with Underground Railway Tunnels.

INTRODUCTION

A major part of the energy used in the UK and elsewhere is for heating or cooling. In particular, more than 40% of fossil fuels are burnt for low temperature heating of buildings (Nera Economic Consulting, 2009). This equates to more than 24% of UK carbon dioxide (C[O.sub.2]) emissions (DECC, 2013). In March 2007, the European Council made a commitment to reduce greenhouse gas emissions by 20% by 2020 (European Commission, 2009). The UK Government went further and established a target to reduce the nation's C[O.sub.2] emissions overall by 80% by 2050 in comparison to the 1990 reference point (The Stationery Office, 2008). In addition, The Mayor of London supports the greater use of renewable and low carbon generation technologies, and has set a target for London to generate 25% of its heat and power requirements through the use of local, decentralised energy systems or also called district heating networks (DHNs) by 2025 (GOV.UK, 2015). Renewable decentralised energy opportunities including the use of energy from secondary sources such as sewers, electricity cable tunnels or underground railways (URs) are also supported in London. It has been shown that the total heat that could be delivered from secondary sources in London is of the order of 71 TWh/year (2.4E14 Btu/year) (The Greater London Authority and Buro Happold, 2013).

The thermal environment of the LU railway

Since 2005, LU has used temperature sensors and data loggers to record the air temperatures at numerous platforms and stations. The recorded data showed that during summer peak-hour operation temperatures can reach as high as 32[degrees]C (89.6[degrees]F) (Gilbey et al. 2011). Due to the high thermal capacitance of the underground system the tunnel air temperature in winter is in excess of 20[degrees]C (68[degrees]F) (Thompson et al., 2008). The work of Gilbey et al. (2011) showed that there is an approximate linear relationship between platform and outside air temperatures and it can be expressed as [T.sub.vlat] = 0.36 X [T.sub.amb_air] + 19.5. The authors work also showed that tunnel temperatures are typically 2 to 3[degrees]C (3.6 to 5.4[degrees]F) cooler than platform air temperatures. This was explained as being a result of the heat produced by the braking mechanism concentrating at the platforms. The soil surrounding a typical deep UR tunnel also contains a large amount of heat energy due to the heat sink effect that the ground provides the tunnel. Ampofo et al. (2004) have shown that the heat absorbed by the earth surrounding an UR accounts for 30% of the total heat release, and contains approximately 4,500 GJ (4.2E9 Btu) of heat energy per km (0.62 mi) of tunnel. This energy is low-grade and ranges in temperature from approximately 20 to 30[degrees]C (68 to 86[degrees]F) (Thompson et al., 2008). This low grade energy could potentially be extracted by nearby vertical ground heat exchangers (GHEs) or ground source heat pump (GSHP) systems. This paper investigates to what extent the heat in the soil surrounding an UR tunnel could enhance the operation of urban GSHPs installations.

NUMERICAL INVESTIGATIONS

Since the operation of the URs and GSHP involves complex, transient, three-dimensional (3D) transport phenomena and extreme geometrical aspect ratios, 3D numerical models of URs and GHEs were independently developed and validated. These individual models were then built into the same modelling environment for their combined analysis. Dimensions, physics and boundary conditions of this combined model were detailed in Revesz et al. (2017). The model was then used to carry out a parametric analysis, consisting of four different studies which are discussed in this paper. The analysis aimed to investigate the potential improvements on the GSHPs heat extraction rates due to the proximity of the UR tunnel(s). For the analysis, London was chosen as a case study and the model was based on typical conditions. When UR-GSHP interactions are being investigated, it is important to consider the initial effect of the UR operation on its surroundings. Starting a simulation of an UR-GSHP model from a uniform soil temperature profile would not be realistic if a London based case study is considered, since the operation of the URs over an extended period would have impacted on the surrounding soil temperature prior to the installation of the GSHP. For this reason the numerical analysis first considered an initial 50 years long simulation period with the operation of the UR. After that many years of operation the warming effect of the UR on the surrounding soil is almost negligible. The results of this initial study provided initial conditions for the anlaysis detailed in this paper. The analysis has 2 years long simulation period with a maximum time step of 1 day. This period does not achieve full convergence of the results. However, over 90% of the change does occur in the first 2 years making this a suitable point to consider the impact of any of these changes. However, the results should not be considered as a final steady state response.

Parametric analysis

Within the parametric analysis four studies were conducted considering different geometrical configurations of the UR(s) and the nearby GHE array. These studies were as follows: 1) Single vs multiple tunnels, 2) GHEs between multiple tunnels, 3) Single vs double looped GHEs, 4) Proximity variation.

Model inputs

UR: The model inputs related to the operation of the UR system were fixed during the parametric analysis and are the same as detailed in Revesz et al. (2017). This is due to the fact that the operational characteristics of the deep level URs in London do not differ significantly. In terms of the geometrical parameters of the UR railway, the only variation was to compare a single tunnel scenario with a multiple tunnel one.

GHEs: Within all four studies, the pipe length of the GHE array was kept the same. This allowed the comparison of the impact of nearby tunnels on GHE arrays with the same size but with different layouts. The overall pipe length of the GHE array was selected as 8000 m (2.6E4 ft). This equates to 40 single looped GHEs with a depth of 100 m (328 ft) and with a spacing of 6 m (19.69 ft). This is a medium sized GSHP installation that typically could be found in London (a GSHP system with a heating capacity of ~ 300 kW (1023.64 kBtu/hr)). The inner diameters of the GHE pipes were 2.4 mm (0.84 in) with a pipe wall thickness of 2.4 mm (0.09 in). The pipe shank spacing between the down and up flow sides of the U-shaped pipes were set as 100 mm (3.93 in). The GHEs operational characteristics were kept the same within the four studies. That is, there were fixed temperature and volume flow rate boundary conditions applied at the inlets of the GHE, 5[degrees]C (41[degrees]F) and 0.1 l/s (1.5 gpm) respectively. These are typical operating conditions for a London based GSHP system which is functioning in its heating mode. In order to enhance computational efficiency of the numerical model an assumption was made that the temperature profile across the thin (150-200 mm (5.9-7.8 in)) borehole filling material which normally is a mixture of sand and bentonite was not of primary interest. Thus its physical geometry surrounding the GHE pipes could be removed from the model. This way the number of the finite element nodes required to solve the model were reduced significantly consequently the number of degrees of freedom of the model. The validity of the assumption rested on the fact that the thin borehole material has similar thermal characteristics to that of the surrounding soil.

Soil: The different ground conditions that typically exist down to 115 m (377.3 ft) below the surface within the central London area were taken into account in building the model. These typical soil layers and their thermo-physical properties are summarized in a work of Revesz et al. (2015).

Monitored model outputs

In order to investigate the impact of the UR tunnel heat load on the nearby GHEs the following outputs from the numerical model were monitored and analysed:

a) The GHEs' average fluid temperature at the pipe outlets [[degrees]C ([degrees]F)].

b) The GHEs' average heat extraction rate [kW (kBtu/hr)].

Study 1: Single vs multiple tunnels

Introduction of Study 1: The aim of this investigation was to establish whether the heat load from multiple running tunnels would have a larger impact on the nearby GHE array than a single tunnel. The aspect ratio of the GHE array was chosen as 1x40 (i.e. a single line of GHEs near the UR tunnel). The horizontal -wall to wall- distance between the systems was set as 3 m (9.8ft). This is the closest permitted proximity for any underground structures built near UR tunnels in London.

Results of Study 1: Figure 1 shows the simulated temperatures in the middle two-dimensional (2D) cross sections of the 3D model geometries at the end of the 2 years simulation period. The left side shows the single tunnel whilst the right side shows the multiple tunnels scenario.

It can be seen that the ground warms up more and the thermal plumes surrounding the tunnels are reaching further when the heat loads of multiple tunnels are simulated. Because of this, the GHEs' fluid temperature is heated more. This is shown in the left side of Figure 2 which shows the simulated averaged temperatures of the GHEs fluid at the pipe outlets. It can be seen that the fluid temperature increased by an average of 0.7[degrees]C (1.3[degrees]F) due to the tunnel heat load from a single tunnel and of 0.9[degrees]C (1.6[degrees]F) when a multiple tunnel heat load scenario was applied in the model. This higher temperature profile started from the beginning of the simulation, and lasted until the end of it. This shows that the initial ground temperature which is affected by the URs is an important characteristic to consider when UR-GSHP interactions are being investigated. The increment in percentage was measured as 8.9% for single and 11.5% for multiple tunnel geometry. On the other hand, the GHEs average heat extraction rate increased by 24.7% when the single and by 31.8% when the multiple tunnel heat load was applied within the model. This is shown in the right side of Figure 2.

Study 2: GHEs between multiple tunnels

Introduction of Study 2: Study 2 considered a geometrical scenario whereby the vertical GHEs are placed between multiple tunnels. It was expected that such an arrangement would increase the interactions; however in reality, such a geometrical scenario would apply to only a small range of places in London and would not be common unless part of an integrated holistic design of a new build structure, for example an UR station. The GHE array aspect ratio was kept the same as it was in Study 1, i.e. 1x40 GHEs.

Results of Study 2: Simulated temperatures at a 2D mid-cross section of the 3D model geometry at the end of the 2 years simulation period are shown in Figure 3. The left side of the Figure 3 shows the model simulated temperatures when only the GHEs operation was considered without the initial effects of the URs. It can be seen that the impact of the GHEs operation is causing a fairly linear temperature decline on its surroundings and the maximum temperatures in the soil surrounding the array are reaching at around 11[degrees]C (51.8[degrees]F). However within the simulation results where the heat load from the tunnels were also accounted for (see right side in Figure 3), the temperature field around the GHE array is warmer (~16 to 17[degrees]C (60.8 to 62.6[degrees]F)), especially at the depths where the tunnels are operating. Because of this, the GHEs' circulated fluid temperature is heated more. This is shown in Figure 4. The left side of Figure 4 shows the average fluid temperatures leaving the GHEs with and without the multiple tunnel heat loads. It can be seen that when the tunnel heat loads were applied in the model (both initially and during the two years simulation period), the temperature of the fluid leaving the pipes increased on average by approximately 1.2[degrees]C (2.2[degrees]F), which equates to an increment of 14.8% compared to a scenario where the tunnel heat loads were neglected.

In addition Figure 4 (right) shows the average heat extraction rates with and without the tunnel heat loads. It can be seen that the GHEs heat extraction rates of the GHEs have significantly increased, by approximately 41% due to the heat loads from the tunnels.

Study 3: Single vs double looped GHEs

Introduction of Study 3: Study 3 aimed to investigate how UR-GSHP interactions are affected, when the GHEs are constructed in double looped configuration near to a single tunnel. The array aspect ratio and the overall GHE pipe length were kept the same as they were in the previous studies (1x40 and 8000 m (2.6E4 ft)). However, double looping the GHE pipes made the depth of the GHEs shorter, to 50 m (164 ft).

Results of Study 3: Figure 5 shows the model simulated temperatures when only the double looped vertical GHEs operation was considered without the initial effects of the UR (left) and the results where the heat load from the tunnels were also accounted for (right). It can be seen that the temperature field around the entire depth of the GHE array is warmer when the UR heat load was accounted for. Because of this, the GHEs' circulated fluid temperature is heated more. This is shown in Figure 6.

The left side of Figure 6 shows the average GHE fluid temperature increment at the pipe outlets due to the UR heat load. The right side of the same figure highlights the estimated average heat extraction increment based on the temperature increment of the GHE fluid due to the tunnel heat load.

It can be seen in Figure 6, that the average fluid temperature leaving the pipes is lower than it was when the deeper single looped GHEs were simulated in the previous studies. This is because the soil temperatures are higher at greater depths which resulted by the geothermal gradient condition imposed on the soil domain. On the other hand, when the tunnel heat load was applied, the heat extraction rate of the GHEs significantly increased, by almost 43%, compared to the scenario when the tunnel heat load was neglected. This is a substantial increase and it is about 18% higher than the increment observed in Study 1 with a single UR tunnel. The reason for this is that although the length of the GHE pipes were kept the same 8000 m (2.6E4 ft) as within the previous studies, the double looped configuration applied in Study 3 reduced the overall depth of the GHE array. This in turn allowed greater interactions between the two systems by having built majority of the GHE array within the soil regions where the impact of the UR is higher.

Study 4: Proximity variations

Introduction of Study 4: Within studies 1 to 3, a fixed UR-GHE horizontal distance, 3 m (9.8 ft) was used. Therefore these previous studies have explored UR-GHE interactions by considering geometrical options with the largest interaction potential possible. Study 4 aimed to explore to what extent the interactions are affected by moving the GHE array further away from the wall of the UR tunnel. The base case scenario, to which the newly built geometrical options were compared, was set as the original 3 m (9.8 ft) horizontal separation distance. This was then gradually increased in order to investigate how the UR-GHE interactions were affected by horizontally separating the systems. The base case scenario, option a, and the other four horizontal distance options (options b to e) were set at 3, 6, 12, 24 and 35 m (9.8, 19.6, 39.3, 78.7 and 114.8 ft) between the wall of the tunnel the vertical GHEs.

Results of Study 4: Simulated temperature results at 2D mid-cross sections for the five different geometrical arrangements are illustrated in Figure 7. For each plot, the results represent the end of the two years simulation period considering both the UR and GHE array operation. It can be seen that as the horizontal wall to wall distance increases the UR impact on the GHE array becomes smaller. This lesser impact was confirmed by the results in the GHEs' fluid temperature variation in Figure 8 (left).

It can be seen in Figure 8 that the average temperature of the GHEs fluid leaving the pipes is reduced as the UR-GHE horizontal separation distance increases. However, it can be seen that in option b, the impact from the tunnel was almost as high as it was in option a. This suggests that at a 6 m radial distance from the wall of the UR tunnel the thermal impact from it is still relatively large. The results also suggest that after about 24 m (78.7 ft) from the wall of the tunnel the impact on the GHEs is small and after about 35 m (114.8 ft) it is almost negligible.

In the context of the GHEs heat extraction performance, Figure 8 (right) illustrates the average heat extraction rate for each geometrical option studied. It can be seen that the GHEs extract less heat as their horizontal distance increased from the tunnel. The results showed that constructing the GHE array at 6 m (19.6 ft) distance from the wall of the tunnel could still result in a significant (above 20%) heat extraction rate improvement compared to a scenario where there was no additional heat source available from a nearby UR tunnel. On the other hand, the results suggest that at 35 m (114.8 ft) from the tunnel wall, the GHEs heat extraction rates would only improve by less than 5%.

CONCLUSIONS

In this paper the authors aimed to enhance the understanding of the interactions between GSHPs and URs. An earlier introduced 3D numerical model was used to perform a number of studies, which investigated how certain geometrical parameters would impact on GHEs heat extraction rates built near UR tunnel(s). The key conclusions drawn from this paper are that the efficiency of GSHPs operating in the proximity of UR tunnels could be enhanced significantly (up to ~43%) if the GHEs are installed at close proximity to the UR tunnels. It can also be concluded from the results, that UR-GSHP interactions are unlikely to occur if the GHE array is built further than ~ 35 m (114.8 ft) from the wall of the UR tunnel. Therefore if the aim is to enhance the heat extraction rates of urban GSHP systems, constructing the GHEs as close as possible to the UR tunnel is essential.

ACKNOWLEDGMENTS

The authors would like to express their gratitude for the support by London Underground Ltd, London South Bank University and the interdisciplinary centre for Storage, Transformation and Upgrading of Thermal Energy (i-STUTE).

NOMENCLATURE

D1 = one-dimensional

2D = two-dimensional

3D = three-dimensional

DHN = district heat network

GHE = ground heat exchanger

GSHP = ground source heat pump

LU = London underground

UR = underground railway

Subscripts

plat = platform

amb_air = ambient air

in = inlet

out = outlet

REFERENCES

Ampofo, F., Maidment, G. and Missenden, J. 2004. Underground railway environment in the UK Part 2: Investigation of heat load, Applied Thermal Engineering, 24 (5-6), pp. 633-645. DOI:10.1016/j.applthermaleng.2003.10.018.

DECC. 2013. The Future of Heating: Meeting the challenge. Available from: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/190149/16_04-DECC-The_Future_of_Heating_Accessible-10.pdf [Accessed 6 October 2014].

European Commission. 2009. Decision No 406/2009/EC on the effort of Member States to reduce their greenhouse gas emissions to meet the Community's greenhouse gas emission reduction commitments up to 2020. Official Journal of the European Union.

Gilbey, M., Thompson, J. and Duffy, S. 2011. The Potential for Heat Recovery from London Underground Stations and Tunnels, in: CIBSE Technical Symposium. Leicester UK.

GOV.UK. 2015. Policy 5.5 Decentralised energy networks, London City Hall. Available from: https://www.london.gov.uk//what-we-do/planning/london-plan/current-london-plan/london-plan-chapter-five-londons-response/poli-0 [Accessed 25 March 2017].

Nera Economic Consulting. 2009. Renewable Heat Technologies for Carbon Abatement: Characteristics and Potential, Final Report to the Committee on Climate Change.

Revesz, A., Chaer, I., Thompson, J., Mavroulidou, M., Gunn, M. and Maidment, G. 2015. Interactions of ground source heat pumps with nearby underground railway tunnels in an urban environment, in: Yokohama, Japan: International Institute of Refrigeration.

Revesz, A., Chaer, I., Thompson, J., Mavroulidou, M., Gunn, M. and Maidment, G. 2017. The potential for integration of ground energy from underground railway tunnels, in: Las Vegas, Nevada, USA: ASHRAE Winter Conference.

The Greater London Authority and Buro Happold. 2013. London's zero carbon energy resource. Available from: http://www.london.gov.uk/sites/default/files/031250%20GLA%20Secondary%20Heat%20-%20Summary%20Report.pdf [Accessed 11 December 2015].

Thompson, J., Gilbey, M. and Maidment, G. 2008. Geothermal cooling of underground railways-the opportunity, in: UK, London: The Institute of Refrigeration, Volume 105.

Akos Revesz

Student Member

Maria Mavroulidou, PhD

Issa Chaer, PhD, CEng

Mike Gunn, PhD

Jolyon Thompson, PhD, CEng

Graeme Maidment, PhD, CEng

Akos Revesz is a postgraduate researcher at London South Bank University, London, UK. Issa Chaer is a Associate Professor and Course Director at London South Bank University, London, UK. Jolyon Thompson is a Senior Tunnel Ventilation Engineer at WSP, Surrey, UK. Maria Mavroulidou is a Reader at London Southbank University, London, UK. Mike Gunn is a Professor Emeritus in Geotechnics at London South Bank University, London, UK. Graeme Maidment is a Research Professor in Refrigeration at London South Bank University, London, UK.
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Author:Revesz, Akos; Mavroulidou, Maria; Chaer, Issa; Gunn, Mike; Thompson, Jolyon; Maidment, Graeme
Publication:ASHRAE Conference Papers
Article Type:Report
Date:Jan 1, 2018
Words:3686
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