AQUIFER THERMAL ENERGY STORAGE SYSTEM FOR COOLING AND HEATING OF ÇUKUROVA UNIVERSITY BALCALI HOSPITAL - PDF

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AQUIFER THERMAL ENERGY STORAGE SYSTEM FOR COOLING AND HEATING OF ÇUKUROVA UNIVERSITY BALCALI HOSPITAL Halime Paksoy, *Olof Andersson, Hunay Evliya, Ôaziye Abaci Çukurova University, Adana, Turkey, Tel:+90

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AQUIFER THERMAL ENERGY STORAGE SYSTEM FOR COOLING AND HEATING OF ÇUKUROVA UNIVERSITY BALCALI HOSPITAL Halime Paksoy, *Olof Andersson, Hunay Evliya, Ôaziye Abaci Çukurova University, Adana, Turkey, Tel: , Fax: *VBB Viak, Malmö, Sweden, Tel: , Fax: ABSTRACT An Underground Thermal Energy Storage (UTES) system that will conserve a considerable part of the oil and electricity being used in the heating and cooling system of the Balcali Hospital in Adana, Turkey is being planned. The system uses an aquifer for seasonal storage. Two alternatives have been studied, one without a heat pump and the other with a heat pump in the system. The results from the feasibility study show that both systems are economically and environmentally feasible. 1. INTRODUCTION UTES systems started to be developed in the 70 s with the purpose of energy conservation and increasing energy efficiency. These systems store thermal energy (natural heat and/or cold in air, soil and water, solar energy, waste heat from any mechanical process) for seasonal purposes. It is possible to make use of the summer heat for heating in winter and winter cold for cooling in summer with these systems. The time mismatch between supply and demand of energy can be closed in that sense. These systems decrease the consumption of conventional fossil fuels by enabling the usage of alternative energy sources. Replacement of conventional heating systems reduces emissions like CO 2, SO 2 and NO x from fossil fuel combustion. Mechanical cooling devices using ozone depleting substances can be eliminated. If fully exploited, UTES can play a key role in decreasing emissions that lead to global warming and ozone depletion. The methods applied for storing thermal energy storage in the underground are storage in pits, tanks and rock caverns, storage in aquifers using Aquifer Thermal Energy Storage (ATES) systems, storage in ducts using Duct Thermal Energy Storage (DTES) systems. The best medium for thermal energy storage is the underground where a large volume is both available and invisible. The ground has the capacity to store thermal energy over long periods for seasonal purposes. In addition, the technique is very simple, requiring no fancy technologies. Among the UTES applications ATES and DTES are practiced the most. Today there are several applications around the world. These systems are economically and commercially viable in a number of countries (Bakema, et.al., 1995). ATES systems compared to conventional systems can conserve energy reaching the levels of 90-95% for direct heating and cooling, 80-85% for heat pump supported heating and cooling, 60-75% for heating only with heat pump in the system, 90-95% for industrial process cooling, and 90-95% for district cooling (Andersson, 1997). Within Energy Conservation Through Energy Storage (ECES) Implementing Agreement (IA) of the International Energy Agency (IEA), a group of experts from Belgium, Canada, Germany, Sweden, The Netherlands, Turkey and USA is working in Annex 8 on the implementation of UTES systems. Turkey has joined this group in 1995 after signing the ECES-IA. UTES potential study for Turkey has been done as a subtask in Annex 8. The results from this study show that ATES is a favorable technique for Adana city in Çukurova region (Paksoy et al., 1997). There are no ATES plants in Turkey so far. A demonstration plant showing the benefits of these systems in Turkey is required. Çukurova University Balcali Hospital in Adana, Turkey has a capacity of 1400 beds that serves about people living in the region. Currently, the heating system uses roughly 4000 tons of no.6 fuel oil. In addition to this, some MWh of electricity is used annually to cool the operating rooms and administrative units. Out-going and long-term patients units are not connected to the central cooling system. The refrigerant in the cooling machines is Freon-12. In order to conserve oil and electricity of this hospital, an Underground Thermal Energy Storage (UTES) system was pre-designed and the feasibility of the system was studied. Two different concepts were proposed, with and without heat pump in the system. In both cases, heat and cold is provided to the hospital after seasonal storage in an aquifer. The study was performed as a task within IEA-ECES Annex 8. This paper gives the results from this study. 2. SYSTEM DESCRIPTION 2.1 ATES without Heat Pump The general objective with the system is to store cold from the winter and to use it as free cooling during the summer season. Then, during the winter season the system is used for preheating of ventilation air. As the main source of cold energy, ventilation air at the hospital and surface water from the nearby Seyhan Lake is being used. Seyhan River brings cold waters to Seyhan Lake from snowmelt in the Taurus mountains to the north of Adana. This fact and temperature measurements (Bozkurt, 1997) on the surface show that Seyhan Lake can be used as a source for cold storage. For more precise determination of cold source potential of the lake temperature measurements changing with depth are necessary. During winter season the groundwater is first used to preheat the ventilation air at the hospital. In this process it is chilled down to a temperature roughly 3-4 C higher than that of the air. As a second step cold bottom water is pumped from the Seyhan Lake to a heat exchanger where the groundwater is further chilled to a temperature of 8-9 C. As shown in the principle scheme (Figure 1), the surface water will then be disposed back to the lake with an increased temperature, while the cold it has provided will be stored around a number of cold wells situated in an aquifer (i.e. groundwater magazine). The groundwater circuit is closed in the sense that the water is produced from a number of warm wells and then injected through the cold wells at the same flow rate and without being exposed to air. During the summer season, when there is a demand for cold at the hospital, the cold wells are pumped providing cold water to the cooling system. In return, the groundwater will be heated by ventilation air and this heat will be stored in the aquifer through the warm wells. This heat is then stored till winter and can be used to preheat ventilation air the coming winter as earlier described. The temperature delivered from the ventilation system (air handling units) will vary depending on the outdoor temperature. However, it is believed that the mean value will be in the order of +20 C. There are two heat exchangers in the system separating the groundwater loop from the Seyhan Lake surface water (HEX 1) and the cooling loop at the hospital (HEX 2). Both heat exchangers are to be designed for a temperature drop of 2 C 2.2 ATES with Heat Pump This alternative incorporates a heat pump designed to cover 4 MW peak load for cooling at summer season and 6 MW for heating wintertime, see Figure 2. In winter it will work with groundwater from the warm wells as a source of energy. The condenser side will provide a hot water at a temperature of C. This heat will preferably be used for heating of ventilation air and to some extent also cover preparation of hot tap water. In this system, the Seyhan Lake will not be used. Instead the waste cold from the evaporators will be stored in the cold wells to be used for direct cooling ( free cooling ) of ventilation air during the summer season. At a cooling demand greater than 3-4 MW the heat pump will be used as a supporting chiller and add 2 MW cooling power. Condenser heat will then be stored in the warm wells together with waste heat from the ventilation system. 3. SIMULATIONS Pumping and reinjection of groundwater have to be simulated in order to establish the number of wells, distances between wells and the impact on the surrounding area. A thermohydraulic simulation program, CONFLOW (Probert, 1995) was used. This program simulates how the thermal front is spread around the wells as a function of time. It is also used to describe the hydraulic head in the aquifer as a function of distance. The later use will show the cone of depression and the cone of uplift under steady state conditions mainly as a function of flowrate and aquifer transmissivity. The properties of the aquifer used in the simulations are given in Table 1. Table 1. Aquifer properties used in the simulations Aquifer Properties Thickness total 14 m Top-bottom aquifer m below surface (m b s) Top-bottom aquifer m b s Top-bottom aquifer m b s Gravel size (mean) 5 mm Porosity 20 % Transmissivity (T) m 2 /s Hydraulic conductivity (K) 1, m/s Thermal conductivity 2,0 W/m K Thermal capacity, water 4,2 MJ/m 3 / K Thermal capacity, matrix 2,0 MJ/m 3 K Hydraulic gradient 3 towards south One objective with the simulations was to calculate the well configuration in order to make the storage as dense as possible. One criteria used for this purpose was to restrict the maximum cone of depression/uplift to 10 m at any point in the well field. A second criteria was not to let the thermal fronts between the cold and warm well groups interfere with one each other. However, within each well group, the thermal fronts are allowed to slightly overlap each other. The main characteristics for the two systems studied are given in Table 2. The results from simulations show that a distance between straight well groups in the order of m is thermally optimal. Between wells of the same group, the distance should be in the range of m. From a hydraulic point of view, the calculations show a maximum cone of depression/uplift in the range of 7-8 m which is well below the stipulated 10 m. The results indicate that a square well field of maximum 350 x 400 m will be enough for the storage of roughly MWh/year in the form of cold and heat at +10 C. With a heat pump in the system, the T value will be increased as well as the running hours and the mean flow rate in summer. This will extend the square well field in the order of 400x450m. Table 2. Characteristic values for the two ATES systems ATES with Heat Pump Well configuration square square Number of wells 2 x 6 2 x 6 Mean flowrate summer (m 3 /h) Mean flowrate winter (m 3 /h) Max flowrate summer/winter Storage temperature cold side ( C) Storage temperature warm side ( C) Running hours summer (h) Running hours winter (24 h/day) Stored energy (MWh) cold side (winter) warm side (summer) ATES without Heat Pump SAVINGS AND PROFITABILITY Each one of the two studied systems will save energy in the form of fuel oil and electricity. The system without heat pump has its strength in saving electricity while the system with heat pump has a favor in saving fuel oil. The savings are calculated based on the following assumptions: Mean COP chillers 2.0 Mean COP heat pump 3.0 Mean COP oil burners 0.7 Storage losses, cold side system without heat pump MWh/year system with heat pump MWh/year Storage gains, warm side system without heat pump MWh/year system with heat pump MWh/year Power for pumps system without heat pump 550 MWh e /year system with heat pump 700 MWh e /year Based on these assumptions and the stored energy shown in Table 2 the saving have been calculated to roughly 3000 MWh electricity and 7000 MWh oil each year for the system without heat pump. With a heat pump the saving of fuel oil will be considerably higher. Oil saving will be in the order of MWh/year. On the other hand the system will consume roughly 6500 MWh of electricity. This is approximately 1200 MWh more than the saving of electricity during the summer season. In Table 3, the summary of the results of the savings and their current values are shown. Table 3. Energy savings SYSTEM ANNUAL SAVING VALUE OF SAVINGS Electricity (MWh) Fuel oil (tons) Electricity (USD) Fuel oil (USD) ATES without HP ATES with HP Based on these calculations, the value of savings for the system without heat pump is only slightly lower than a system with a heat pump ( compared to USD/year). The estimated investment costs for the two systems are 1 million USD for the system without heat pump and 2.5 million USD with the heat pump. The straight pay-back time will then be approximately 2 and 5 years respectively. 5. ENVIRONMENTAL ASPECTS The implementation of the ATES system in the Balcali Hospital will result in the conservation of energy. In addition to a better energy economy, this will improve the environment by the reduction of emissions (e.g. CO 2, SO 2, NO x ) and less consumption of chloroflorocarbon (CFC) gases. The environmental benefits from this project will be reduction in energy consumption as electricity and fuel oil and replacement of chillers using ozone depleting Freon-12 gas. The savings in fuel oil (1000 m 3 /year) will approximately decrease the CO 2 emission by 1960 ton/year, SO x by 6.3 ton/year, and NO x by 7 ton/year. The replacement of 2MW of current chillers using Freon-12 will result in a saving of approximately 0.7 ton/year of Freon-12. The heat pump supported system will decrease the local emissions from the burning of oil by 5040 tons CO 2 /year, by 16.2 tons SO x /year and 18 tons NO x /year. However the replacement of chiller will decrease the usage of CFC with more than 2 tons/year. 6. CONCLUSIONS The feasibility study for the first UTES project in Turkey was performed for Çukurova University Balcali Hospital. The hospital is located in Çukurova region, where according to the UTES potential study performed, ATES is a favourable technique. The ATES method is chosen for this purpose. The basic ATES system proposed in this study is designed to meet the basal cooling and heating load of the hospital. The system at its current size and design will provide an electrical energy saving of approximately MWh/year for cooling and around 1000 m 3 of oil for heating. By increasing the differential temperature of the ATES system it is likely that the savings may considerably be higher. The total investment cost was calculated to roughly 1 million USD and the value of energy savings as approximately 0.5 million USD. This suggest a straight pay-back time of 2 years. The alternative system with a heat pump designed for 6 MW heat power and 4 MW peak load cold power save around 2500 tons of oil, but will also increase the usage of electricity to a minor extent. From an economic point of view it is also less profitable. With an investment cost of 2.5 million USD the pay-back time will be in the order of 5 years. In the further development of the project it is recommended to have a pre-design project stage to gain more accurate data for the final design of the system, with or without heat pump. ACKNOWLEDGEMENT The financial support provided by State Planning Organisation of Turkey under the project No:97K is greatly acknowledged. REFERENCES Andersson, O. (1997). ATES Utilization in Sweden- An Overview. Proc. of MEGASTOCK 97 7 TH International Conference on Thermal Energy Storage, Vol.2, pp Bakema, G., Snijders, A.L. and Nordell, B. (1995). Underground Thermal Energy Storage-State of the Art Report Arnhem, The Netherlands, 83 p. Bozkurt, A. (1997). Seyhan Baraj Gölü (Adana) Zooplanktonu. Master Thesis, Çukurova University, (in Turkish). Paksoy, H.Ö., Evliya, H. and Abaci,. (1997). The Underground Thermal Energy Storage(UTES) Potential in Turkey. Proc. of MEGASTOCK 97 7 TH International Conference on Thermal Energy Storage, Vol.2, pp Probert, T. (1995). Thermal Front Tracking Model Using Conformal Flow Technique. Lund Institute of Technology, Sweden, 56p.
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