Enhancement of Heat Exchange Capacity of Ground Heat Exchangers by Injecting Water into Wells

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1 PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 24-26, 2014 SGP-TR-202 Enhancement of Heat Exchange Capacity of Ground Heat Exchangers by Injecting Water into Wells Yohei Inoue, Ryuichi Itoi, Naokatsu Chou, Hiroaki Okubo and Hikari Fujii Address: 744, Motooka, Nishi-ku, Fukuoka, , Japan Keywords: Ground source heat pump system, Water injection, Numerical simulation ABSTRACT Field experiments of cooling operation were conducted using a ground source heat pump (GSHP) system installed at greenhouse in Itoshima city, Fukuoka, Japan. There are water loss zones in the lower part of heat exchange wells which were completed without grouting. Groundwater was injected into the wells by varying the flow rate for evaluating the effects of its flow rate on enhancement of heat exchange capacity of ground heat exchangers. The results showed that significant enhancement of heat exchange capacity was achieved by injected water at the flow rate about 9 L/min in the field. Then, a numerical model was developed to simulate heat transport with injected water in the wells. Sensitivity studies were carried out with the model changing the flow rate and temperature of injected water. The results showed that injecting water at 1 L/min/well of 18 o C could improve heat exchange rate by 1.7 times compared to that without injection case. 1. INTRODUCTION In GSHP systems groundwater flow could enhance heat exchange capacity of ground heat exchangers (GHEs). Improvement of heat exchange performance would result in the reduction of the total length and number of GHEs. Then, high initial installation cost mostly occupied by drilling vertical wells could be reduced. Some researchers have already studied on the effects of groundwater flow on heat exchange performance of GHEs. Diao et al. (2004) developed equations on conduction-advection to estimate the impact of groundwater flow on performance of GHEs. Wanget et al. (2009) estimated that the thermal performance of a GHE was enhanced by 9.8% in heat injection by groundwater flow when the thickness of aquifer occupies 10.6% of the borehole length. Gustaffson et al. (2010) indicated that natural convection in ungrounted GHE decreases borehole thermal resistances through laboratory experiment and computational fluid dynamics (CFD) modeling. Fujii et al. (2012) showed that vertical groundwater flow caused by head differences between aquifers significantly improve heat exchange capacity of GHE by disposing heat to the formation. Choi et al. (2013) examined the effects of both direction and rate of groundwater flow on the performance of various types of GHE arrays using a two-dimensional model. Komaniwa et al. (2013) showed that heat exchange performance of GHE could be increased by injecting water into the GHE through thermal response tests (TRTs) under different flow rate of injected water. In this research, in order to evaluate the thermal advection effects of water injection in GHEs on heat exchange performance, cooling operation for greenhouse was conducted using GHEs with water injection linked to GSHP system. Numerical simulation was also carried out to investigate the relationship between flow rate and temperature of injected water and heat exchange rate. 2. TEST FIELD AND GSHP SYSTEM 2.1 Test Field Cooling operation for greenhouse was conducted in the Maebaru field of Kyushu Electric Power Company, located in Itoshima City, Fukuoka, Japan. In this field, four heat exchange wells (GHE-1, GHE-2, GHE-3 and GHE-4) with the depth of about 60 m were already completed before this research. Then, two heat exchange wells (GHE-5 and GHE-6) were newly drilled to the depth of about 40 m for this research. Figure 1 shows the location of all GHEs, well specification and the geological column of GHE-2 and GHE-5. As the rock type was identified not with core sample but with cuttings during drilling of the boreholes, it is difficult to accurately identify the geology. Therefore, the geological column in GHE-2 and GHE-5 are not identical. Casing was set to the depth of 38 m in GHE-2 because of unstable formation conditions above 38 m. Thus, weathered granite depth would be between the depth of 8 m and 38 m, and if so, granite appears in GHE-2 and GHE-5 at the same depth. The elevation of GHE-5 and GHE-6 is higher compared to that of GHE-1 and GHE-2 by about 3 m. While silica sand was filled in GHE-2, filling material was not inserted in GHE-5 to easily occur forced convection by injected water. Thus, GHE-5 is filled with groundwater from the well bottom of 40.8 m to the groundwater level of 6.7 m from the surface. However, the sand around GHE-5 flowed into the well through opening of slotted liner below depth at 20 m. Consequently, GHE-5 might have been filled with sand between the depth of 20m and 38m, and groundwater filled GHE-5 to the water level of 6.7 m depth from the ground surface. Therefore, injected water would be considered to flow down in the well above 20 m depth, and then flowed out to the formation through upper part of slotted liner. 1

2 Inoue et al. Figure 1: Location of GHEs, well specification and geological column of GHE-2 and GHE GSHP System The diagram of GSHP system installed in Maebaru field is shown in Figure 2. Two GSHPs (HP1 and HP2) were used for cooling greenhouse. HP1 is connected in parallel to GHE-1 and GHE-2, and HP2 is installed in parallel to GHE-5 and GHE-6. Fan Coil Unit (FCU) 1 and FCU2, FCU3 and FCU4 are connected to HP1 and HP2, respectively. Specification of FCU is given in Table 1. Double U-tube as a heat exchanger is inserted in each GHE. The U-tubes in GHE-1 and GHE-2 are installed to the bottom of each GHE. On the other hand, the inserted depth of the U-tubes in GHE-5 and GHE-6 are 38.5 m and 38.7 m, respectively. Therefore, the total length of GHEs in HP1 (114 m) is about 1.5 times longer than that in HP2 (77.2 m). The outer diameter and inner diameter of the U-tubes are 32 mm and 26 mm, respectively. As for heat medium circulating in GHE and FCU, water is used because the heat medium temperature is always to be higher than 0 o C during this operation. The objective of this research is to investigate the effects of water injection on heat exchange capacity. Water was injected into GHE-5 and GHE-6 connected to HP2, then variation in heat medium temperature and Coefficient of Performance (COP) were compared between HP1 and HP2. Although groundwater was used for water injection, the groundwater was first pumped up in a storage tank. Thereby, the temperature of water in the tank increased due to high ambient temperature before injecting into GHEs. The temperature was in the range from 18 o C to 21 o C during cooling operation. Figure 2: Diagram of GSHP system Table 1: Specification of FCU FCU1, FCU4 FCU2, FCU3 Air volume (m 3 /min) Power consumption (W) Cooling capacity (kw) Heating capacity (kw)

3 Inoue et al. 3. COOLING OPERATION For evaluating the thermal advection effects of injected water in the GHE, cooling operations for greenhouse using GSHP systems were carried out. The conditions of each cooling operation are shown in Table 2. Temperature of injected water decreased according to the fall in ambient temperature. Flow rate of injected water was changed in the range from 5.8 L/min to 9.7 L/min. The flow rate represents the total value injected into GHE-5 and GHE-6, and flow rate was adjusted to be equal in each GHE using control valves. The measured flow rates in Operation 2 and Operation 3 are 6.0 L/min using flowmeter as shown in Table 2. However, there is a possibility that water was injected only at low flow rate because hose for water injection was bent at the well head. The temperature entering FCU is set to 10 o C. The following data were regularly measured every 30 seconds or 1 minute. Heat medium temperatures at inlet and outlet of GHE and FCU Temperature of injected water, greenhouse and ambient temperatures Flow rate of heat medium to GHE and FCU, and flow rate of injected water Electrical power of compressor and circulating pump of heat pumps (HPs) Table 2: Conditions of cooling operation Name Operation period Temperature of injected water ( o C) Flow rate of injected water (L/min) Preset temperature of greenhouse ( o C) Operation /9/15 ~ 2013/9/ Operation /9/25 ~ 2013/9/ Operation /9/30 ~ 2013/10/ Operation /10/4 ~ 2013/10/ Operation /10/17 ~ 2013/10/ Operation /10/22 ~ 2013/10/ Operation /10/29 ~ 2013/10/ Variation in Heat Medium Temperatures at Inlet and Outlet of GHE Figures 3 and 4 show the variation in heat medium temperatures at the inlet and outlet of GHE from 15 th Sep. to 4 th Oct., and from 4 th Oct. to 31 st Oct., respectively. In these graphs, the heat medium temperatures were plotted when HPs were in operation. During these operations water was injected into GHE-5 and GHE-6 connected to HP2, not injected to GHE-1 and GHE-2 connected to HP1. The heat medium temperature continues to increase while HP is continuously operating. During the continuous operation in long time, the temperature at the inlet of GHE reached higher than 35 o C. On the other hand, when HP repeats start-stop at relatively low ambient temperature such as 29 th Sep. to 30 th Sep. and 8 th Oct. to 9 th Oct., the heat medium temperature gradually increases or decreases. For flow rate of injected water larger than 9 L/min, the heat medium temperatures at the inlet of GHE in HP2 is equal to or lower than that in HP1. In addition, at 5.9 L/min injection between 7 th Oct. and 31 st Oct., the heat medium temperatures in HP1 and HP2 are same as shown in Figure 4. Despite the shorter length of GHEs in HP2 (77.2 m) compared to GHEs in HP1 (114 m), the heat medium temperature in HP2 was equal to or lower than HP1 because of water injection. These results indicate that the increase of the heat medium temperature is suppressed by forced convection and by disposing heat in the GHEs to the formation through slotted liner due to injected water. Furthermore, these results show that the cooling effects are larger with an increase of injecting water flow rate. It can be concluded that the larger the temperature difference between heat medium and injected water is, the more effective the effects of injected water becomes. Since there is a possibility that water was injected significantly less than 6.0 L/min because hose was bent at the well head between 25 th Sep. and 4 th Oct., the heat medium temperature in HP2 is higher than that in HP1 as shown in Figure 3. The hose for water injection was replaced with a new rigid hose at 17 th Oct. before restarting Operation 5, and water was properly injected to GHEs from the results that heat medium temperatures in HP1 and HP2 are about the same. 3

4 Figure 3: Changes in heat medium temperature at inlet and outlet of GHE from 9/15 to 10/4 Figure 4: Changes in heat medium temperature at inlet and outlet of GHE from 10/4 to 10/ COP and SCOP Coefficient of Performance (COP) is used for evaluating the efficiency of GSHP system. COP represents the ratio between power consumption of compressor of HP and cooling capacity. The cooling capacity Qc and COP are calculated with Equations (1) and (2), respectively. (1) (2) where, Qc : Cooling capacity (kw) ρ : Density of heat medium (kg/m3) Cpw : Specific heat of heat medium (kj/kg/k) To : Heat medium temperature at outlet of FCU (K) Tin : Heat medium temperature at inlet of FCU (K) qw : Flow rate of heat medium (m3/s) COP : Coefficient of performance (-) Wcom : Power input of a HP compressor (kw) Figure 5 shows variations with time in daily-averaged COP of HP1 and HP2, ambient temperature and operation time per day. COP of HP2 with water injection is higher than COP of HP1 except for the period from 25th Sep. to 4th Oct. when the water injection rate could have been little. This is because COP in cooling operation is higher with the decrease of heat medium temperature at the outlet of GHE. In fact, the average heat medium temperatures after heat was discharged to the ground of HP1 and HP2 were 28.1oC and 26.4oC between 5th Oct. and 9th Oct., respectively. Therefore, COP in the same period of HP2 is higher than COP of HP1 by about 0.5. COP of HP2 with water injection remains more than 5 throughout the cooling operation. These results confirm that water injection could enhance efficiency of GSHP system in cooling operation by decreasing the temperature rise in heat medium temperature. 4

5 Figure 5: Changes in daily-averaged COP, ambient temperature and operation time COP of the overall system (SCOP) is calculated by cooling capacity divided by the power input to the overall GSHP system, or power input of compressor and circulating pumps of HP, FCU, and groundwater pump for water injection. Since the lifting pump is used for water injection into GHE-5 and GHE-6 connected to HP2, the power consumption of the pump is included only in HP2, not in HP1. SCOP and power consumption of the pump are calculated by following Equations (3) and (4), respectively. (3) where, SCOP : System coefficient of performance (-) (4) W cp : Power input to circulating pumps of HP (kw) W FCU : Power input to FCU (kw) W gp : Power input to groundwater pump for water injection (kw) ρ : Density of groundwater (kg/m 3 ) g : Gravity acceleration (m/s 2 ) q H : Pumping rate of groundwater (m 3 /s) : Pumped up height (m) η p : Pump efficiency (-) η m : Motor efficiency (-) W cp is estimated to be kw (0.15 kw in primary pump plus kw in secondary pump) using performance curve of GSHP. In calculating W gp, H, η p, η m are assumed 10 m, 0.2, 0.8, respectively. Figure 6 shows changes with time in daily-averaged SCOP of HP1 and HP2, ambient temperature and operation time per day. Unlike COP, SCOP in HP1 and HP2 shows about same value because power input to groundwater pump is included only in SCOP calculation of HP2. In other words, the difference between SCOP and COP values is larger in HP2 compared to that in HP1. Nevertheless, the difference of SCOP between HP1 and HP2 is not large. These results imply that the efficiency of GSHP system with shorter length of GHEs would be improved by injecting water into the GHEs. Thus, the required length and number of GHEs could decrease by injecting water with lower temperature than heat medium temperature. Consequently, the high initial cost for drilling GHEs would be reduced by installing the system to inject water into GHEs. COP and SCOP are not plotted in Figures 5 and 6 from 12 th Oct. to 15 th Oct., because the heat medium temperature at the outlet of FCU could not be properly measured because of the fluid level drop in pipeline caused by leakage of the heat medium. 5

6 Figure 6: Changes in daily-averaged SCOP, ambient temperature, operation time 4. NUMERICAL SIMULATION A numerical model to simulate heat transport and water injection was developed using the FEFLOW simulator, simulation software using the finite element method. The model was verified by the history matching of heat medium temperature at the outlet of GHE between observed and simulated data. 4.1 Grid System Figure 7 illustrates a three-dimensional and a two-dimensional horizontal views of the model. The model has 40 m in horizontal directions. The size of z-direction is 50 m. The developed model has 22 slices and 21 layers, and the total number of element is The fine elements represent two GHEs including casing and slotted liner. The thickness of the casing and slotted liner in the model is larger than that in real ones in order to reproduce the outflow of injected water to the formation through slotted liner. Figure 7: 3D and 2D views of numerical model 6

7 4.2History Matching History matching of heat medium temperature at the outlet of GHE connected to HP2 was carried out to confirm the validity of the model Simulation Conditions The simulation period was 5 days in history matching according to the change in flow rate of injected water. Initial ground temperature of 18 o C was assigned in each slice based on the measured data in GHE-3 (15 m, 30 m, 45 m, and 60 m depth) only in the first simulation from 15 th Sep. to 20 th Sep. After the history matching of 25 th Sep., simulated ground temperature was used as initial conditions for the subsequent simulation. As for initial conditions of flow, water level of -6.7 m from the ground surface was set in all slices. As for thermal boundary conditions, the constant temperature was given in the top and bottom slice and side of the model. The temperature of injected water per 12 hours was given to one of the nodes inside of the casing. As for GHE boundary, diameter of borehole, inner and outer diameter of U-tube, and thermal properties of grout material and heat medium were set at the center of each GHE. In consequence, thermal resistances in and around GHEs were automatically calculated. As for boundary conditions of flow, water level of -6.7 m from the ground surface was assigned in the top and bottom slice and side of the model. The averaged flow rate of injected water during simulation period was given to the same node where the temperature of injected water was specified. While the hydraulic conductivity in the grid blocks inside of the casing above 20 m is 1 m/s, that below 20 m is 0.01 m/s because of the lower section of the borehole being filled with sand. Other physical conditions in the formation were given as shown in Table 3 depending on geological information. Table 3: Physical properties of the formation Geology Thermal Conductivity (W/m/K) Hydraulic Conductivity (m/s) Porosity (-) Results of History Matching Soil x Sand / Sand gravel x Granite x Measured data of HP2 during cooling operation was given as input data: flow rate of heat medium, heat medium temperature at inlet of GHE, flow rate and temperature of injected water. Heat medium temperature at the outlet of GHE was calculated as output. The outlet heat medium temperature was compared between measured and simulated values during history matching simulation. The parameter which significantly influences on simulation is thermal conductivity of grout. Thermal resistances in and around GHE were automatically calculated by giving parameters as mentioned in The thermal resistances become smaller by giving large thermal conductivity to grout. As thermal resistances decrease by forced convection due to water injection in the GHE, the thermal conductivity of 0.9 W/m/K was set, larger value compared to thermal conductivity of water (about 0.6 W/m/K). This value was decided by trial and error. Figures 8 and 9 show the results of history matching of outlet heat medium temperature from 15 th Sep. to 30 th Sep. and from 17 th Oct. to 27 th Oct., respectively. While operation of HP repeats start and stop with short intervals such as from the night of 15 th Sep. to morning of 16 th Sep. and from 23 rd Oct. to 25 th Oct., the calculated heat medium temperatures are higher than measured values. It seems to be difficult to simulate heat transport properly with the present model when input data of flow rate is sharply changed. However, good agreements between measured and calculated values were obtained during continuous operation when heat medium temperature significantly increases. Therefore, this model could reproduce heat transport and water injection of cooling operation in the test field, and the validity of the model was confirmed. Figure 8: History matching results from 9/15 to 9/30 7

8 Figure 9: History matching results from 10/17 to 10/27 4.3Sensitivity Studies The sensitivities of flow rate and temperature of injected water were evaluated using the numerical model above. The flow rate of injected water was changed in the range from 0.0 L/min/well to 10.0 L/min/well and the temperature was also changed in the range from 15 o C to 24 o C Simulation Conditions A part of the simulation conditions was changed from history matching simulation for simplification of the calculation. The simulation conditions in the sensitivity studies are summarized below. Initial ground temperature : 18 o C Water level : -7 m Flow rate of injected water : 0.0, 0.5, 1.0, 2.5, 5.0, 10.0 L/min/well Temperature of injected water : 15, 18, 21, 24 o C Heat medium temperature at inlet of GHE : 30 o C (Constant) Flow rate of heat medium : 30 L/min (Constant) Length of GHE : 80 m (40 m x 2wells in series) Grout material : Groundwater (Ungrouted) Type of heat exchanger : Double U-tube Operation time of HP : 12 hours/day (From 8:00 a.m. to 20:00 p.m.) Simulation period : 30 days Results of Sensitivity Studies Figure 10 shows relationship between the heat exchange rate and flow rate and temperature of injected water. The heat exchange rate without water injection of 33 W/m was used as a reference for the vertical axis. Heat exchange rate becomes larger with an increase of flow rate and with a decrease of temperature of injected water. However, increase rate in the heat exchange rate becomes smaller according to the flow rate increase of injected water. In addition, the more the flow rate of injection increases, the more significant the effects of injection temperature on heat exchange rate become. As shown in Figure 10, the water injection at 1.0 L/min of 18 o C same as initial ground temperature could improve heat exchange rate by 1.7 times compared to that without water injection. 8

9 Figure 10: Results of sensitivity studies on water injection 5. CONCLUTIONS Improvement of heat exchange rates by water injection was evaluated by cooling greenhouse using GSHP system in Itoshima City, Fukuoka, Japan. During the cooling operation, groundwater was injected into ungrouted GHEs completed with a slotted liner. After the operation, heat exchange performance of GHEs with and without water injection, and coefficient of performance (COP) and overall efficiency (SCOP) of GSHP system were compared. A numerical model was also developed to simulate heat transport and water injection. The results are summarized as follows: 1. Injecting lower temperature water than heat medium effectively cools the heat medium by forced convection and disposes the discharged heat in the GHEs into the formation through slotted liner. An increase in heat medium temperature with water injection at more than 6 L/min was suppressed due to this effect. 2. COP with water injection was higher than COP without water injection and maintained more than 5 throughout cooling operation. SCOP was almost same considering power input of groundwater pump for water injection. 3. A numerical model was developed and validated by history matching of heat medium temperature at the outlet of GHE with water injection between measured and calculated ones. 4. Sensitivity studies indicated that heat exchange capacity of the GHEs with water injection of 1.0 L/min and 18 o C could be improved by 1.7 times compared to that without it. ACNOLEDGEMENT We would like to thank the staff of Kyushu Electric Power Co., Inc. and YBM Co., Ltd. for assisting us in our experiments. REFERENCES Choi, J.C., Park, J., and Lee, S.R.: Numerical evaluation of the effects of groundwater flow on borehole heat exchanger arrays, Renewable Energy, 52, (2013), Diao, N., Li, Q., and Fang, Z.: Heat transfer in ground heat exchangers with groundwater advection, International Journal of Thermal Sciences, 43, (2004), Fujii, H., Komaniwa, Y., Ioka, S., Watabe, A., and Inoue, Y.: Evaluation on the Effect of Crossflow of Groundwater on Heat Exchange Rates in Vertical Ground Heat Exchangers, Journal of the Geothermal Research Society of Japan,34 (4), (2012), Gustafsson, A.-M., Westerlund, L., and Hellström, G.: CFD-modelling of natural convection in a groundwater-filled borehole heat exchanger, Applied Thermal Engineering, 30, (2010), Komaniwa, Y., Fujii, H., Maehara, T., and Chou, N.: Evaluation of Enhanced Heat Exchange Performance of Vertical Ground Heat Exchangers with Water Injection, Journal of the Geothermal Research Society of Japan, 35 (4), (2013), Wang, H., Wi, C., Du, H., and Gu, J.: Thermal performance of borehole heat exchanger under groundwater flow: A case study from Baoding, Energy and Buildings, 41, (2009),