[3] Aksamija, A., 2016. Regenerative design and adaptive reuse of existing commercial buildings for net-zero energy use. Sustainable Cities and Society, 27, pp.
Thermal performance investigation of borehole heat exchangers for a commercial building in Istanbul Ahmet GULTEKİN Ahmet GULTEKİN, Research Assistant, Istanbul Technical University Energy Institute Abstract In commercial buildings thermal energy consumption covers one of the biggest portions of the energy cost. Consequently, energy storage technology is becoming more and more popular for the efficient utilization of energy. Ground source heat pump (GSHP) systems are one of the foremost technologies due to their high efficiencies and low running costs for heating and cooling applications. In this study, a commercial building in Istanbul is investigated on the basis of heating and cooling loads. Loads are obtained by modelling the state of the art of the building via using dynamic simulation tools. Periodic heat loads with period of 1 year are examined, with compensation of winter heating with summer cooling. Based on the consumption results, the required length of boreholes is calculated. There is a necessity to drill more than one borehole heat exchangers (BHE) in larger GSHP applications. In this situation, thermal interaction between BHE has a negative impact on total performance of BHE field. Thus, placement of multiple BHE in application field and borehole spacing between them become important troubles. Therefore, with a variety of configurations and borehole spacing are analyzed by finite element simulations for building. The results can be used during the engineering design of a BHE field to maximize the thermal performance of a given application field. 1. Introduction Because of their functional and operational features, commercial buildings relatively consume more energy than other types of buildings [1]. In these buildings, significant amount of energy consumed for the heating, ventilating and air-conditioning system (HVAC). Energy efficient design and operation of HVAC systems in commercial buildings can offer great potential for large scale energy savings [2]. Therefore, thermal energy efficiency in building is a popular research topic in the recent years [3-5]. To reduce energy costs, energy storage technology is important for the efficient utilization of energy. Nowadays, using ground as a thermal energy storage is becoming more and more popular for heat pump systems owing to high efficiency, environmentally friendly and low running cost for heating and cooling applications [6-9]. To provide heat exchange between ground and working fluid through the pipes, ground heat exchangers are embedded to the ground and they can be either horizontally installed in trenches or as U-tubes in vertical boreholes. The vertical ground heat exchangers, commonly named borehole heat exchangers (BHE), are generally constructed of inserting single or multi polyethylene U-tubes to the ground. Time dependent heat transfer analysis of BHE is an important work to calculate the required total length of BHE [10]. There is a necessity to drill more than one BHE in larger GSHP applications. In this case, thermal interaction between BHE has a negative effect on total performance of BHE field [11, 12]. These adverse effects will become more important over the operation duration. Consequently, placement of multiple BHE in application field and borehole spacing between them become important matters [13,14]. There are numerous studies on placement of multiple BHE and their thermal performance analysis can be found in the literature [15-22]. In the present paper, a commercial building in Istanbul is investigated on the basis of heating and cooling loads. Loads are obtained by modelling the state of the art of the building via using dynamic simulation tools. Based on the consumption results, the required length of boreholes is calculated. Then, BHE fields with different aspect ratio of geometry allocation and borehole spacing are investigated. Long-term thermal performance of BHEs for the building is investigated with reference to possible configurations and working conditions. 2. Modelling building and BHE field 2.1 Modelling building The case study is a 900 m2 commercial building with two floors, as shown in Figure 1, located in Istanbul where both winter heating and summer cooling are necessary. Annual thermal loads, as shown in Table 1, are obtained by modelling the building using EnergyPlus™ which is a building energy simulation program to model energy
consumption. In this case, the heating demand in the winter months is more than the cooling demand in the summer. Therefore, ground temperature will decrease in the long-term operation due to imbalance seasonal loads.
Figure 1. Simulation of a commercial building. Month
Total base thermal energy Exchanged energy and unit HTR value (MWh) (MWh) (W/m) January 19.16 14.38 37.45 February 18.08 13.56 35.32 March 10.96 8.22 21.41 April 2.04 1.54 4.01 May -2.72 -3.62 -9.43 June -5.40 -7.18 -18.70 July -8.18 -10.88 -28.34 August -8.88 -11.82 -30.78 September -5.34 -7.10 -18.49 October 3.04 2.28 5.94 November 6.28 4.70 12.24 December 15.1 11.32 29.48 Table 1. Monthly thermal energy variation in terms of energy exchanged with the ground and the corresponding unit HTR value, where positive (negative) values indicate heating (cooling). In the design of BHE in GSHP systems, the determination of the required length of boreholes in a borehole field is an important stage. Undersized length of BHE may cause to system failure owing to return working fluid temperatures that may be outside the operating range of the heat pumps. Oversized length of BHE have high installation costs that may increase the return of investment of GSHP systems. Number of BHE, BHE depth, placement of multiple BHE and borehole spacing are important parameters which affect the thermal performance of borehole field. In other words, these parameters effect directly the required length of boreholes. Applying better configurations and borehole spacing can decrease the overall length of boreholes. To meet the thermal energy needs, a borehole field which has 16 BHEs with 50-m depth (800 m borehole length) is planned for different BHE placement and borehole spacing. The thermal load is assumed to be equally distributed each BHEs. The biggest part of the required energy is exchanged with the ground, while the remaining energy is supplied by additional heating or cooling systems. A periodical heat transfer rate per unit borehole length (unit HTR value) with a regular sine shape and period of 1 year is assumed for a BHE to investigate how the annual thermal load pattern influences the thermal performance of BHE with long-term operating time. 𝑞𝐿 = 𝐶1 + 𝐶2 ∗ Sin [
𝑡 8760
∗ 2𝜋] + 𝐶3 ∗ Sin [
𝑡 24
∗ 2𝜋]
(1)
where t is in hours, C1 controls the annual load imbalance; C2 is the half-amplitude of the annual load variation; C3 control the half-amplitude of daily fluctuations. The annual load scenario was considered with C1 = 10, C2 = 30 and C3 = 5, and Figure 2 shows the annual unit HTR value variation of a single BHE. The thermal load is assumed as positive if heat is collected by the BHE, negative in the opposite case.
40
m qL
W
20
0
20
0
2000
4000
6000
8000
t hour
Figure 2. Annual unit HTR value (qL) variation of a single BHE. The coefficient of performance (COP) value of a heat pump is related to the average temperature of the working fluid in winter/summer and the temperature of the circulating water in the heating/cooling system that must keep the required building temperature. In general, the lower temperature difference between the working fluid in the BHE and the circulating water of the heating/cooling system in building means the higher COP. The inlet temperature to the heat pump doesn’t go outside the operating range of the heat pumps. Operating range in heat pump is between around -5 oC and 45 oC. The heating demand in the winter period is more than the cooling demand in the summer. Because of imbalance seasonal loads, too low ground temperatures will occur in the long-term operation. Therefore, -3 oC has been considered as the minimum acceptable working fluid (water with 25% glycol) temperature for design purposes. This limitation has been taken into account for long-term investigations. 2.2 Modelling BHE field A schematic diagram of a usual borehole with single U-tube is illustrated in Figure 3 which comprises of three domains; polyethylene pipes, grout and ground. Grout and ground properties are isotropic and homogeneous. Initial and undisturbed uniform ground temperature is 17 oC. The heat transfer in the ground is assumed to be carried out with insignificant groundwater movement. Moreover, temperature variation along the vertical axis is negligible since the difference between input and output temperatures is very small. This condition allows to reduce 3D heat conduction problem into 2D one. The values of geometrical and material parameters used in the model are taken from the Ref. [22].
out Borehole Ground PE Pipe
Grout
Figure 3. Sketch of a single U-tube borehole cross section
Time dependent 2D heat conduction problem in the domains are solved for each BHE concurrently in COMSOL Multiphysics environment. COMSOL Multiphysics is a finite element analysis software commonly used to solve various physics and engineering equations. Arrays with same number of BHE (4x4 and 2x8) are considered with different borehole spacing to investigate the effects of borehole configuration and borehole spacing on the long-term thermal performance of BHE fields. To reduce the computational study and solution time, symmetric nature of the problem is considered and symmetry boundary conditions are applied as shown in Figure 4, a quarter of BHE field is chosen to model. Symmetry boundary conditions are applied on both bottom and left sides. Similarly, undisturbed temperature conditions are applied as boundary condition for the other sides of ground domain. The domain size of ground is selected large enough for each case to ensure that temperature distribution around BHE is not effected by the domain size.
Figure 4. Sketches of 4x4 and 2x8 configurations of 16 BHE and boundary conditions. 3. Results and discussion Figure 5 and Figure 6 show the minimum acceptable temperature of working fluid for a time interval of 20 years. Working fluid temperature decreases under the minimum acceptable temperature after 5 and 15 years for 4x4 configuration when borehole spacing is 3 m and 6 m respectively. For 2x8 configuration this duration is around 12 years when the distance is 3 m. However, 6 m borehole spacing is quite enough for 2x8 configuration. The results indicate that using 2x8 configuration is more appropriate in terms of design for thermal performance and borehole life since the performance loss of BHEs reach to their minimum values due to the less thermal interactions.
Figure 5. Minimum annual value of working fluid for 4x4 configuration when distance 3 and 6 m.
Figure 6. Minimum annual value of working fluid for 2x8 configuration when distance 3 and 6 m. 4. Conclusions The effects of borehole configuration and borehole spacing on the long-term thermal performance of BHE fields, for a commercial building in Istanbul, are numerically investigated on the basis of heating and cooling loads. The results show that comparing with a 4x4 configuration using a 2x8 configuration is more proper in terms of design for thermal performance because there is a larger perimeter in the geometry, which makes available more area for heat to dissipate to surrounding soil. 3 m distance isn’t enough for both configurations for long-term operation duration. However, 6 m distance is quite enough for 2x8 configuration whereas this distance isn’t enough for 4x4 configuration at long-term operation. The results can be used for more efficient and high performance engineering design of GSHP systems. 5. Acknowledgement The author is deeply grateful to Ergin KUKRER for his valuable suggestions on the modelling building. References [1] Fasiuddin, M., Budaiwi, I., 2011. HVAC system strategies for energy conservation in commercial buildings in Saudi Arabia. Energy and Buildings, 43, pp. 3457–3466. [2] Valdiserri, P., Biserni, C., 2016. Energy performance of an existing office building in the northern part of Italy: Retrofitting actions and economic assessment. Sustainable Cities and Society, 27, pp. 65–72. [3] Aksamija, A., 2016. Regenerative design and adaptive reuse of existing commercial buildings for net-zero energy use. Sustainable Cities and Society, 27, pp. 185–195. [4] Khosrowpour, A., Gulbinas, R., Taylor, J. E., 2016. Occupant workstation level energy-use prediction in commercial buildings: Developing and assessing a new method to enable targeted energy efficiency programs. Energy and Buildings, 127, pp. 1133–1145. [5] Ruparathna, R., Hewage, K., Sadiq R., 2016. Improving the energy efficiency of the existing building stock: A critical review of commercial and institutional buildings. Renewable and Sustainable Energy Reviews, 53 pp. 1032–1045. [6] Bayer, P., Saner, D., Bolay, S., Rybach, L., Blum, P., 2012. Greenhouse gas emission savings of ground source heat pump systems in Europe: a review. Renew Sust Energy Rev, 16 pp. 1256 -67. [7] De Moel, M., Bach, P.M., Bouazza, A., Singh, R. M., Sun, J.L.O., 2010. Technological advances and applications of geothermal energy pile foundations and their feasibility in Australia. Renew Sust Energy Rev, 14 pp. 2683-96. [8] Lee, J.Y., 2009. Current status of ground source heat pumps in Korea. Renew Sust Energy Rev, 13 pp. 1560-8. [9] Yuan, Y.P., Cao, X.L., Sun, L.L., Lei B., Yu N.Y., 2012. Ground source heat pump system: a review of simulation in China. Renew Sust Energy Rev, 16 pp. 6814-22. [10] Zeng, H., Diao, N., Fang, Z., 2003. Heat transfer analysis of boreholes in vertical ground heat exchangers. International Journal of Heat and Mass Transfer, 46 pp. 4467–4481 [11] Qian, H., Wang, Y., 2014. Modeling the interactions between the performance of ground source heat pumps and soil temperature variations. Energy for Sustainable Development, 23 pp. 115–121.
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