Performance of a Covered Closed Loop Shallow

Performance of a Covered Closed Loop Shallow Geothermal Greenhouse Heating System V. Firfiris, P.G. Kougias, Ch. Nikita-Martzopoulou, G.G. Martzopoulos Faculty of Agriculture, Aristotle University of Thessaloniki, 54124 (229), Thessaloniki, Greece Keywords: Geothermal energy, heating system, ground source heat pumps Abstract A great variety of heating applications are being used nowadays in order to cover the greenhouse thermal needs. During the last decade a significant increase in installation of Shallow Geothermal Heat (SGH) systems assisted by closed loop Geothermal Heat Pumps (GHP) has been observed. The SGH systems are highly efficient, renewable energy systems for greenhouse heating and cooling. The main advantage of these systems consists on the simplicity of the application, the low cost of structure and the fact that no drilling is required. The SGH systems exploit the underground stored heat, even in shallow depths. The external air temperature variations interact directly only with the surface soil temperature, due to the soil heat capacity. As a result, the effect of the external air temperature variations is being reduced at deeper ground layers. The underground temperature value remains almost constant at a depth greater than 5.0m, while in 2.0m depth the underground temperature value has been proved that do not change significantly if the ground is under cover. Therefore, a fluid of a more or less constant temperature can be supplied to the primary GHP circuit by installing a ground heat exchanger. GHPs, operating with such a system, consume less energy than air-to-air heat pumps which are widely used. The present study presents a technical analysis concerning the installation and performance of a covered closed loop GHP used in a SGH system for greenhouse, without an auxiliary conventional heating system. The innovation of the examined system relies on the fact that the ground surface, above which the piping installation is under a polyethylene film cover or it is located inside a greenhouse. This system was proved experimentally to improve the heating performance of the GHPs. INTRODUCTION The main objective of a greenhouse is the intensive out of season green crop production, which can be achieved by maintaining the optimum temperature at every stage of the crop growth (Sethi and Sharma, 2008). In order to fulfill the condition above, more energy is required. Usually, this energy is derived from the consumption of fossil fuels, which are characterised as pollutants, since they emit carbon dioxide when burned, causing so far, the greenhouse effect. To reduce the emission of greenhouse gases and atmospheric pollutants, and to cope with recent rise and rapid fluctuations in oil prices, an energy-efficient alternative heating system for greenhouses is required (Lee, 2009). Thus, the necessity of energy saving and the increasing interest in the reduction of greenhouse energy cost, led to extensive efforts of exploitation of renewable energy sources, such as geothermal energy, biomass and solar energy.

Proc. IS on GreenSys2011 Eds.: Kittas et al. Acta Hort. 952, ISHS 2012

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Among the several renewable energy systems, the Shallow Geothermal Heating (SGH) systems are considered as highly efficient systems for greenhouse heating and cooling. In general, this system is combined with one of the following three types of GHPs: (a) Closed loop systems: Closed-loop systems use a continuous loop of buried plastic pipes as the ground heat exchanger (GHE). The pipes are connected to the heat pump to form a sealed, underground loop through which a heat transfer fluid (water or antifreeze solution) is circulating in a closed circuit and transfers heat to or from the ground (Healy and Ugursal, 1999). (b) Open loop systems: Open loop systems use local groundwater (i.e wells) or surface water (i.e., lakes) as a direct heat-transfer medium. Open-loop systems consist primarily of extraction wells, extraction and reinjection wells, or surface water systems (Mustafa Omer, 2008). (c) Hybrid systems: Hybrid systems consist of closed or open loop systems in conjunction with technologies that take advantage of other renewable energy sources (i.e. solar energy), or in combination with conventional heating and cooling systems. In recent years, a lot of experimental work has been conducted in order to exploit geothermal energy for greenhouse heating and cooling (Nikita- Martzopoulou, 1990; Martzopoulos, 1991; Bakos et al., 1999; Tong et al., 2010). Lund et al., (2010) reported that the worldwide use of geothermal energy used for greenhouse heating has increased by 10% in installed capacity and 13% in annual energy use. Many authors have investigated the possibility to satisfy the greenhouse thermal needs by using GHPs with either horizontal or vertical closed loop and the conclusions of their research seems to be very encouraging, since on the one hand the greenhouse is heated properly and on the other economic and energy saving are achieved (Ozgener and Hepbasli, 2005; Benli and Durmus, 2009; Bakirci, 2010). The present study investigates the possibility of greenhouse heating using a covered closed loop GHP system, without an auxiliary conventional heating system. The innovation of the examined system relies on the fact that the ground surface, beneath which the piping installation is located, is covered by polyethylene film (greenhouse simulation) or it is located inside a greenhouse. MATERIALS AND METHODS A north-south orientated greenhouse of modified tunnel type located in the Aristotle University Farm (40o32’ N latitude and 22o59’ longitude), Thessaloniki, Greece, was used for the study. This greenhouse belongs to the Center of Agricultural Structures Control, has a total floor area of 120.0m2 and the ridge and side heights are 4.2 m and 2.6 m respectively. The greenhouse was covered by FRP (fiberglass reinforced plastic) at the sidewalls and with UV stabilized polyethylene (PE) on the roof. Method for calculating the greenhouse heating requirements In order to calculate the greenhouse heating requirements, the minimum desired internal air temperature for any cultivation must be taken into account. The external temperature is considered to be the lowest value of the mean minimum temperatures recorded for the months that the experiment lasted. Considering the above, the thermal requirements (Q) of a greenhouse can be determined by calculating the greenhouse losses (Qo)due to convection, conduction, radiation and due to the ventilation process (Qvent), using the following equations: Q = Qo + Qvent

(1)

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Qo = U A (ti – to),

(2)

where Q is the rate of heat flow (W), U is the overall heat consumption coefficient (W m-2 oC), A is the surface area of the greenhouse covering (m2), ti is desired internal air temperature (oC) and to is external air temperature (oC)

1

Qvent = p N V cp (ti – to),

(3)

where Qvent is the heat losses due to ventilation (W), p is the density of the inside air, Kg m-3, N is the infiltration rate (s-1), V is the volume of the greenhouse (m3), cp is the specific heat of the inside air, (J Kg-1 oC-1), ti is desired inside air temperature (oC) and to is external air temperature, (oC) Description of the heating system The heating system used to cover the greenhouse thermal needs was based on a closed loop GHP. The heating distribution system inside the greenhouse was consisted of a network of black polypropylene (PP-R) tubes. The heat was retrieved from the soil with the use of a ground heat exchanger (GHE), consisted of a plastic pipe system (High Density Polyethylene, HDPE, tubes) and a circulator. The pipe system was buried at 2m depth and the soil surface area was covered with a PE film. GHE sizing is concerned mainly with the determination of the length of the buried pipes circuit. The required GHE length (Lh), based on heating requirements qh, can be calculated as follows (RETScreen International, 2005):  COPh − 1   COP ( R p + Rs Fh )  h  Lh = qh   Tg ,min − Tewt ,min     

where COPh is the design heating coefficient of performance of the heat pump system, Rp is the pipe thermal resistance, Rs is the soil/field thermal resistance, Fh is the GHE part load factor for heating, Tg,min is the minimum undisturbed ground temperature and Tewt,min is the minimum design entering water temperature at the heat pump. RESULTS AND DISCUSSION According to regional meteorological data, the monthly mean minimum temperature for the total experimental period was 4.5oC. The desired temperature inside the greenhouse was considered to be 13oC. Thus, the amount of the greenhouse heating requirements was calculated to be 13.75 kW. This amount of energy also indicated the power output of the needed GHP. The GHE was buried at 2.0m depth and the area was under a PE film cover. The underground temperature value remains almost constant at a depth greater than 5.0m, while in 2.0m depth the underground temperature values have been found not to be changed significantly where the ground surface area was under cover. Previous experimental results (Kougias et al., 2011), proved that the PE cover presented a great impact on the increase of COPh. A similar solution to exploit this influence of PE on COPh is to install the GHE beneath the greenhouse structure. Thus, no further area is needed for the installation of the ground heat

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exchanger. It was calculated that for the present geothermal system, the COPh value reached up to 4.25. Table 1 presents the average ground temperature values at 2.0m depth concerning the period from November to April. The surface area, beneath which the temperature sensors were installed, was under a PE cover. These data were derived from a preliminary experiment. As it can be seen, the minimum undisturbed ground temperature for the total experimental period was recorded at 13.13 oC (during January). According the above mentioned results and using the equation given in previous paragraph, the length of the GHE was calculated to be 598 m. Fig. 1 depicts the installation of the piping system at 2.0m depth. Table 2 summarizes all the results referring to the selection and installation of the GHP. The economical sustainability of a closed loop GHP system for greenhouse heating and cooling compared with other conventional systems (i.e. systems that use fossil fuel or natural gas) can be achieved considering the installation, operation and maintenance costs of each technology. The above comparison is possible to lead to accurate conclusions concerning the depreciation period of each heating and cooling system. Conclusions have been reported by many authors, that have studied analytically the economic feasibility of GHPs in comparison with several alternative heating and cooling systems (Healy and Ugursal, 1997; Esen et al., 2006; Pulat et al., 2009). CONCLUSIONS The present study presented a technical analysis concerning the installation of a covered closed loop GHP system for a greenhouse, without an auxiliary conventional heating system. The innovation of the examined system relied on the fact that the ground surface, beneath which the piping installation is located, is covered by polyethylene film (greenhouse simulation) or it is located inside a greenhouse. LITERATURE CITED Bakirci, K. 2010. Evaluation of the performance of a ground-source heat-pump system with series GHE (ground heat exchanger) in the cold climate region. Energy 35:3088-3096. Bakos, G.C., Fidanidis, D. and Tsagas, N.F. 1999. Greenhouse heating using geothermal energy. Geothermics 28:759-765. Benli, H. and Durmus, A. 2009. Evaluation of ground-source heat pump combined latent heat storage system performance in greenhouse heating. Energy and Buildings 41:220-228. Esen, H., Inalli, M. and Esen, M. 2006. Technoeconomic appraisal of a ground source heat pump system for a heating season in eastern Turkey. Energy Conversion and Management 47:1281-1297. Healy, P.F. and Ugursal, V.I. 1997. Performance and economic feasibility of ground source heat pumps in cold climate. International Journal of Energy Research 21:857-870. Lee, J.-Y. 2009. Current status of ground source heat pumps in Korea. Renewable and Sustainable Energy Reviews 13:1560-1568. Kougias, P.G., Firfiris, V., and Martzopoulos, G.G., 2011. Improvement of a Heat Pump Coefficient of Performance Used in Greenhouse Heating and Cooling Systems. In: Greensys 2011, Halkidiki, Greece, 5-10 June 2011. Lund, J.W., Freeston, D.H., and Boyd, T.L., 2010. Direct utilization of geothermal energy 2010 worldwide review. In: World geothermal congress, Bali, Indonesia., 25–29 April 2010. Martzopoulos, G.G. 1991. The influence of three geothermal heating systems on the growth and the yield of tomato crop. Acta Hort. (ISHS) 287:77-88.

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Mustafa Omer, A. 2008. Ground-source heat pumps systems and applications. Renewable and Sustainable Energy Reviews 12:344-371. Nikita-Martzopoulou, C. 1990. Greenhouse heating systems with geothermal energy of low enthalpy in Greece. Acta Hort. (ISHS) 263:183-190. Ozgener, O. and Hepbasli, A. 2005. Experimental performance analysis of a solar assisted ground-source heat pump greenhouse heating system. Energy and Buildings 37:101-110. Pulat, E., Coskun, S.Unlu, K. and Yamankaradeniz, N. 2009. Experimental study of horizontal ground source heat pump performance for mild climate in Turkey. Energy 34:1284-1295. RETScreen International. 2005. Clean Energy Project Analysis: RETScreen Engineering & Cases. Minister of Natural Resources Canada. Sethi, V.P. and Sharma, S.K. 2008. Survey and evaluation of heating technologies for worldwide agricultural greenhouse applications. Solar Energy 82:832-859. Tong, Y., Kozai, T.Nishioka, N., and Ohyama, K. 2010. Greenhouse heating using heat pumps with a high coefficient of performance (COP). Biosystems Engineering 106:405411. Tables Table 1. Average undisturbed ground temperature at 2.0m depth under PE cover. Month Temperature, oC November 18.23 December 14.45 January 13.13 February 13.65 March 16.05 April 19.85

Table 2. Calculated parameters and measured data used for the selection of the GHP. Parameter Unit Value o Monthly mean minimum temperature C 4.5 o Desired temperature inside greenhouse C 13 Greenhouse heating requirements KW 13.75 GHP coefficient of performance 4.25 o Minimum undisturbed ground temperature C 13.13 Pipe length of GHE m 598

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Figures

Fig.1. The installation of the piping system at 2.0m depth.

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