STUDY OF GEOTHERMAL SEASONAL COOLING STORAGE SYSTEM WITH ENERGY PILES M.M. He, H.N. Lam Mechanical Engineering Department University of Hong Kong Pokfulam Road, Hong Kong Tel: 852-28592642 candyhe05@hkusua.hku.hk 1. ABSTRACT This paper describes a study of geothermal system with energy piles that has recently been built in China. This system stores the cooling energy in the cold winter, which can then be used to provide sensible cooling in the hot summer. With a proper system design, the need for an electric water chiller is completely eliminated. In order to assess the potential of application for this system in the sub-tropical region, computer simulations are carried out to obtain the predicted system performance. Both the results of the simulation and the actual operating data of the system indicate that the geothermal seasonal storage system is not capable of meeting the sensible cooling load of the building without relying on an auxiliary chiller. Further simulations are conducted to evaluate the performance of the geothermal heat pump system, which combines energy piles together with a heat pump. The analysis shows that this system can cater for both the sensible and the latent cooling loads. A comparison between the conventional borehole geothermal heat pump system and the energy piles geothermal heat pump system is made with respect to storage capacity, initial cost and energy consumption. 2. INTRODUCTION Geothermal (ground-source) heat pumps (GHP) are one of the fastest growing applications of renewable energy in the world, with annual increases of 10% in about 30 countries over the past 10 years (Curtis et al., 2005). Most of the installation of the GHP system is in central and northern Europe and the USA, where space heating is the greatest demand. A lot of study and research have already been carried out in these countries, and a lot of projects have already been built up. However, in the tropical and sub-tropical regions, where the cooling is the greatest demand, both research work and practical projects are limited. This paper describes a geothermal seasonal cooling storage system with energy piles built in south China. The system serves a plant and its office by storing the coldness in winter and providing sensible cooling in summer. Waste heat from a nearby steam line of the plant is used to provide heating in winter. Simulations are carried out to assess the potential of application for this system in the sub-tropical region. 3. SYSTEM DESCRIPTION The HVAC system of this project is designed according to an energy concept, which aims at minimization of energy consumption as well as optimization of thermal comfort. A central air handling unit (AHU) in the mezzanine delivers the fresh air to the building via a displacement ventilation system. A heating and a cooling coil in the AHU provide sensible heating and cooling. Air dehumidification is achieved by using a liquid desiccant dehumidification system. Heating is provided by a steam line in the form of waste heat from a co-generation process, which is located next to the site. Low temperature radiant heating is employed, serving the manufacturing plant with a floor heating system and the office building with a ceiling heating system. Cooling is provided by a seasonal cold storage system using energy piles. The energy piles are structural piles which are equipped with plastic pipes as heat exchangers (Sanner, 2003). These energy piles and other foundation structures are particularly suited to combined heat/cold production in regions with poor geotechnical foundation conditions. With little extra cost, these structures can be easily equipped and used as heat exchangers (Vuataz et al., 2003). Free cooling with a cooling tower during winter
time is used to cool down the ground below the building by energy piles. The chilled ground is then used to provide sensible cooling for the air handling units and the floor slabs. All the operating data are recorded by the Energy Management System (EMS) automatically at 15 minutes intervals. The building has a base area of 11,000m 2, which is also the area for the installation of the heat exchangers. There are totally 410 structure piles, which are 40 meters deep, and a double U-tube of DN 25 HDPE pipe is embedded in each pile. The double U-tube is connected in series, and all these double U-tubes are connected in parallel to the main pipe. Pure water is circulated inside the pipes as the medium to transfer the heat. Following the recommendation by Kavanaugh and Rafferty (1997), the water flow rate for the DN 25 HDPE pipe is chosen to be 0.315 l/s, so that the total water flow rate of the system is 130 l/s. The design cooling capacity of piles system is 640 kw, delivering around 340 kw cooling to the AHU s and 300 kw cooling for the floor slabs using chilled water at a temperature of 16ºC. 4. COMPUTER SIMULATIONS Simulations are carried out using TRNSYS which is a transient simulation program with a modular structure (Klein et al., 2002). The new version of DST, the duct ground heat storage model, is chosen to simulate the energy piles (Pahud et al., 1996). The following features are implemented in the DST version: Several ground layers with different thermal properties can be specified within the storage region. A ground water flow is specified for each ground layer. The heat transfer caused by forced convection in the storage region is estimated for each ground layer. The heat transferred by the pipe connections on ground surface can be estimated. The soil is mainly composed of soft clay and sand, and it has the thermal conductivity of 2.16 W/m K. All the value of parameters using in simulations are presented in Table 1. TABLE 1 Parameters used for new DST model Parameters Values Thermal conductivity of the ground 2.16 W/m K Volumetric heat capacity of the ground 2200 kj/m 3 K Grout thermal conductivity 2.78 W/m K Pipe thermal conductivity 0.40 W/m K Structure pile outer diameter 50 cm Structure pile inner diameter 30 cm U-tube outer diameter 3.34 cm U-tube inner diameter 2.74 cm U-tube center-to-center half distance 12.5 cm Darcy velocity under the ground 5x10-6 m/s A comparison is made between the value of outlet water temperature of the heat exchanger predicted by the new DST model and the one recorded by the EMS during the one-and-a-half-month charging period (as show in Figure 1). The inlet water temperature of the heat exchanger recorded by the EMS is used as the input to the new DST model. As expected, the simulated results match well with the recorded data, which means that the new DST model has a high accuracy in simulating the heat exchanger performance. But during this charging period, the temperature drop of the soil is not very significant, only from 20ºC to 17ºC. Assuming that the charging process can last for 4 month, from November to February, as shown in Figure 2, the underground soil temperature will decrease from 20ºC to 13.6ºC. If the chilled ground at 13.6ºC is used to provide chilled water for the floor slabs and the AHU s, as show in Figure 3, the water temperature will be higher than the maximum supply water temperature of 16ºC during the peak load period. These results indicate that this seasonal cooling storage system cannot fully cover the sensible cooling load of the building, and the failure might be due to the relatively mild weather in the winter season, resulting in insufficient underground cooling storage and consequent inadequate ground temperature drop.
Figure 1: Graph of comparison of the new DST model with experimental data over 1080 hours. Tfin_d represents the inlet water temperature of the heat exchangers recorded by EMS; Tfout_d represents the outlet water temperature of the heat exchangers recorded by EMS; Tfout represents the outlet water temperature of the heat exchangers calculated by the new DST model; and TmDst represents the ground temperature. Figure 2: Graph of charging process in winter over 2880 hours, assuming a constant temperature of 6 ºC for the inlet water of heat exchangers. Tfout represents the outlet water temperature of the heat exchangers; and TmDst represents the ground temperature.
Figure 3: Graph of temperature and cooling load profiles using the chilled ground to provide the chilled water for slabs and AHUs over a period of six months, assuming the operating schedule to be from 8:00 am to 8:00 pm. Tfout represents the outlet water temperature of heat exchangers; TmDst represents the ground temperature; and load represents the required cooling load. 5. FURTHER ANALYSIS Further simulations are performed to evaluate the performance of the geothermal heat pump system with energy piles, which combines the energy piles together with a heat pump. The heat pump is used to cater for both the sensible and the latent cooling loads for year-round operation, providing cooling in summer and heating in winter. According to the design requirement, the total cooling load is up to 1600 kw. Since the chilled water from the heat exchanger is supplied to the condenser of the heat pump, the outlet water temperature should not be higher than 35ºC during the summer time. Otherwise, this will result in a lower COP for the heat pump (Lam, 2005). In order to demonstrate the feasibility of a sustainable long-term performance of the system, simulations are carried out over a 10-year period. Figures 4 to 7 show the loading profile, the heat exchanger outlet water temperature profile, the heat exchanger inlet water temperature profile, and the ground temperature profile over 10 years respectively. As shown in these figures, under the peak load of 1600 kw, the highest outlet water temperature of the heat exchanger is lower than 35ºC, even after 10 years operation. Meanwhile, the ground temperature increases slowly from 20ºC to 24 ºC, and finally reaches to the stable state, ensuring a sustainable long-time performance of the heat pump. Figure 4: Cooling load profile over 10 years Figure 5: Inlet water temperature profile over 10 years
Figure 6: Outlet water temperature profile over 10 years Figure 7: Ground temperature profile over 10 years In order to further evaluate the application potential of geothermal heat pump system with energy piles, a comparison between this system and the conventional borehole geothermal heat pump system has been made. The advantages of the geothermal heat pump system with energy piles are as follow: The concrete piles have better heat transfer characteristics than conventional boreholes since the concrete has higher thermal conductivity than sand or other common grouting materials. The capacity of energy piles system is thus larger than that of the conventional system. There is no need for additional land to install the heat exchangers if they are embedded in the foundation piles. Also, there is no need for drilling of holes to install the heat exchangers. Thus, the initial cost will be much lower for the energy piles, especially for areas where land availability is limited and drilling cost is high, such as Japan and Hong Kong. 6. CONCLUSIONS AND DISCUSSIONS A geothermal seasonal cold storage system with energy piles has been installed for a manufacturing plant and its office in order to minimize the energy consumption and maximize thermal comfort. It stores the cooling energy in winter and provides sensible cooling in summer. However, after four months of cold storage, from November 2004 to February 2005, this system still cannot cater for the required sensible cooling capacity in July 2005. Simulations are carried out to study the failure of this system, using the TRNSYS simulation package and the new DST model. The one-and-a-half month s inlet and outlet water temperature data of the heat exchanger recorded by the EMS of the building at 15 minutes intervals are used to verify the new DST model, and it turns out that the new DST model has a high accuracy in simulating the heat exchanger performance. All the simulation results show that the mild winter temperature in southern China cannot store enough cold energy during the winter time to provide the required sensible cooling in summer. But if this system is connected to a heat pump for year-round operation, providing heating in winter and cooling in summer, the geothermal heat pump system can cater for both the sensible cooling load and the latent cooling load in a sustainable long-term period. A comparison between the conventional borehole geothermal heat pump system and the geothermal heat pump system with energy piles is made with respect to storage capacity and initial cost. It turns out that the latter system is capable of providing a larger capacity at a lower initial cost. The detailed study of the geothermal seasonal cold storage system provides a good reference for further detailed investigations into the application of these technologies in the sub-tropical region. ACKNOWLEDGEMENT The authors would like to gratefully acknowledge the financial support provided by the University of Hong Kong to this research project under account code: 10205139/13354/14500/323/01. REFERENCES
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