Determination of the potential on cooling and heating of a greenhouse with aquifer thermal energy storage in Mediterranean climate

The aim of this study is to determine the heating and cooling potential of the greenhouses in the Mediterranean climatic zone, with aquifer thermal energy storage (ATED) known as one of the underground thermal energy storage application systems (UTES). In recent years greenhouse production reached to 44.000 ha in Turkey (Daşgan 2003). For high yield and quality in greenhouse crops during the winter months, inside temperature should be maintained at the critical value that can change depending on the species grown in the greenhouse. For instance, for tomatoes the critical inside temperature should be maintained not below 12-13 0C in the greenhouse. Due to this information and the last 20 year?s climate data in Mediterranean Region, a greenhouse needs approximately 150 kW heating load during 90 days in a year, 8 hours in a day (Baytorun and Abak 1995). To provide this heating load, 6 L/m2 No 6 Fuel-Oil or 9 kg/m2 coal must be consumed. The fossil fuel consumption leads to an economic burden in the operating cost, besides ashes and undesirable gas emissions from coal combustion are the biggest barriers of the greenhouses in the Mediterranean zone. Additionally, the cooling requirement of the greenhouses for summer months in the Mediterranean Climate and the advantages provided after cooling applications has been calculated.For these purposes, two separate greenhouses, each having an area of 360 m2, in the research station of the Horticulture Department - Faculty of Agriculture have been selected. One of them has been heated and cooled by ATED technique. In the second one, conventional heating and cooling systems have been used. Consequently, these two systems have been compared in terms of crops yields, economical and environmental aspect.The basic principal of the ATED systems is as follows; interior temperature can increase up to 40-70 0C during 5 months in a year and 6 hours in a day in the summer season in the greenhouse which is located in the Mediterranean climate. The waste heat in the greenhouse is transferred to ground water via interior water-air heat exchangers, hence the temperature of the water is increased from 18 0C to 34 0C. For storage purposes, this heat has been injected to the aquifer through hot well. At the end of heat storage process, during 70 days in summer months, 103.877.123 kJ energy has been stored in the aquifer. During the winter months of 2005-2006, ATED greenhouse interior temperature never decrase below critical level (11 0C).In the following winter season, when heating is required in the greenhouse, stored energy has been extracted from the aquifer and has been used for heating. In the heat recovery process, during 138 days in the winter season, extracted amount of energy from aquifer was 58.711.432 kJ. When the ambient air temperature decreased below 10 0C, this ambient cold was transferred to ground water through exterior fan-coil during the winter season, this cold energy has been stored in the aquifer via cold well. Cold storage process lasted 138 days and at the end of this research work 76.002.040 (kJ) energy has been stored. This stored cold has been used for cooling in the spring and summer months. During cold recovery process, which lasted 32 days, 15.678.065 (kJ) energy has been extracted from the ground. During the spring - summer of 2006, ATED greenhouse interior temperature never increases below critical level (30 0C).Efficiency of heat recovery process was %56 and COP of the system has been calculated as 7.64. Also efficiency of cold recovery process was found to be %20 and COP of the system has been calculated as 3.21. Consequently, a renewable heating and cooling system, which is not dependant on fossil fuels and CFC gasses respectively, has been integrated to the greenhouse.Hence, with a successful ATED application, energy cost has been reduced 70%. Economic efficiency of ATED greenhouse is calculated 56% more than conventional greenhouse. The crop yields have been increased within the range of 20-40% seasonally, in ATED greenhouse. Calculated pay back time of the system was less than 1 year.