تجزیه و تحلیل عملکرد تجربی سیستم گرمایش گلخانه ای خورشیدی پمپ حرارتی به کمک منبع زمین

Ground-source heat pumps (GSHPs), also known as geothermal heat pumps (GHPs), are recognized to be outstanding heating, cooling and water heating systems, and have been used since 1998 in the Turkish market. Greenhouses also have important economical potential in Turkey’s agricultural sector. In addition to solar energy gain, greenhouses should be heated during nights and cold days. In order to establish optimum growth conditions in greenhouses, renewable energy sources should be utilized as much as possible. It is expected that effective use of heat pumps with a suitable technology in the modern greenhouses will play a leading role in Turkey in the foreseeable future. The main objective of the present study is to investigate to the performance characteristics of a solar assisted ground-source heat pump greenhouse heating system (SAGSHPGHS) with a 50 m vertical 1 × 1/4 in. nominal diameter U-bend ground heat exchanger using exergy analysis method. This system was designed and constructed in Solar Energy Institute of Ege University, Izmir, Turkey. The exergy transports between the components and the destructions in each of the components of the SAGSHPGHS are determined for the average measured parameters obtained from the experimental results. Exergetic efficiencies of the system components are determined in an attempt to assess their individual performances and the potential for improvements is also presented. The heating coefficient of performances of the ground-source heat pump unit and the overall system are obtained to be 2.64 and 2.38, respectively, while the exergetic efficiency of the overall system is found to be 67.7%.

The climatic conditions of the region are the prime factor, which effect the development of the plant and the economics of the greenhouse production. It becomes necessary to take measures to heat the greenhouses when the temperature goes below 12 °C in order to obtain high quality and high yield crop, that is especially important for export purpose [1]. Greenhouses also have important economical potential in Turkey’s agricultural sector. In addition to solar energy gain, greenhouses should be heated during nights and cold days. In order to establish optimum growth conditions in greenhouses, renewable energy sources should be utilized as much as possible [2]. Heating applications in the greenhouses have an important effect on yield as well as on quality and the cultivation time of products [3], [4], [5], [6] and [7]. Effective use of heat pumps with a suitable technology in the modern greenhouses plays a leading role in Turkey in the foreseeable future. Although in greenhouses not only the possibility of heating, but also the ability of cooling and dehumidification has been recognized, only a restricted number of practical applications have been realized [2]. The use of solar energy is considerable interest for two reasons. First, it leads to diminution of fossil fuels consumption. Second, solar energy is a non-polluting source of energy [8]. In this regard, investigations conducted on solar assisted heat pumps are getting more and more importance [9], [10], [11], [12], [13] and [14]. An exergy analysis has proven to be a powerful tool in the thermodynamic analyses of energy systems [15], [16], [17], [18], [19] and [20]. In order to calculate exergy, the environment must be specified. Because of the lack of thermodynamic equilibrium in the surrounding nature, only its common components can be used for the above-mentioned purpose. The ability of an energy carrier to do work expresses the general ability to be converted into other kinds of energy, and therefore exergy can be used not only to analyze the process of power plants and of other mechanical machines, but also to investigate technological process. An engineer designing a system is expected to aim for the highest possible technical efficiency at a minimum cost under the prevailing technical, economic and legal conditions, but also with regard to ethical, ecological and social consequences. Exergy is a concept that makes this work a great deal easier. The impact of energy resource utilization on the environment and achievement of increased resource utilization efficiency are best addressed by considering exergy. The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change. The latter point suggests that exergy may be, or provide the basis for, an effective measure of the potential of a substance or energy form to impact the environment [21]. Exergy analysis can also indicate the possibilities of thermodynamic improvement of the process under consideration, but only an economic analysis can decide the expediency of a possible improvement [18]. Various studies have been undertaken by many investigators on exergy analysis of solar assisted heat pumps [8], [22], [23], [24] and [25]. However, to the best of authors’ knowledge, no studies on the performance testing of a SAGSHPGHS with a 50 m vertical 1 × 1/4 in. nominal diameter U-bend ground heat exchanger using exergy analysis method have appeared in the open literature. The study reported here includes the performance analysis of a SAGSHPGHS with R-22 as the refrigerant in the heating mode by using exergy analysis. A flat-type solar collector is directly installed into the ground-coupled loop. An experimental set-up, described in the next section, is constructed and tested for the first time on the basis of a university study performed in the country. This study also describes an easy-to-follow procedure for exergy analysis of SAGSHPGHSs and how to apply this procedure to assess the heating system performance by calculating exergy destruction, and thus showing the direction for improvements.

The performance of a SAGSHPGHS, along with its each effects, was evaluated by using exergy analysis, which aims at better identifying process efficiencies and losses. The data used were obtained from the measurements made in a SAGSHPGHS, which was designed and installed in the Solar Energy Institute of Ege University, Izmir, Turkey. In this regard, expressions for an energy and exergy analysis of a SAGSHPGHS were derived using mass, energy and exergy balance equations. The exergy efficiency values of each of the components were also given, while the potential for improvements were discussed. The main conclusions that may be drawn from the present study are listed below. (a) The exergy efficiency values for the GSHP unit and the whole system on a product/fuel basis are obtained to be 71.8 and 67.7%, respectively. (b) The highest irreversibility on a system basis occurs in the greenhouse fan-coil unit, followed by the compressor, condenser, expansion valve and evaporator, sub-regions I and V for the GSHP unit and the whole system, respectively. Besides this, the remaining system components have a relatively low influence on the overall efficiency of the whole system. (c) Experimental results also show that monovalent central heating operation (independent of any other heating system) cannot be met overall heat loss of greenhouse if ambient temperature is very low. The bivalent operation (combined with other heating system) can be suggested as best solution in Mediterranean and Aegean region in Turkey, if peak load heating can be easily controlled. (d) The analysis should provide a designer with a better, quantitative grasp of the inefficiencies and their relative magnitudes. Furthermore, the results can draw an engineers attention towards the components where the most availability is being destroyed and quantify the extent to which modification of one component affects, favorably or unfavorably, the performance of other components of the system.