Güneş Enerjili Absorpsiyonlu Soğutma Sisteminin İncelenmesi

Abstract

İnsanların büyük bir kısmı aşırı sıcak bölgelerde yaşamaktadır. Bu yüzden güneş enerjili soğutma özellikle bu bölgelerde oldukça popüler gözükmektedir. Güneş enerjili soğutma uygulamalarında kullanılan en büyük avantaj maximum talep noktasında elde edilebilir, maximum güneş enerjisi miktarıdır. Bu başvurularda iki ana uygulama vardır. Bunlardan ilki bina soğutulmasında diğeri yiyeceklerin korunması içindir. Bina soğutma başvurularında kullanılan güneşli soğutma sistemi sıcak yaz periyodunda dışarıdan ısıyı temin etmek için kullanılabilir. Bu sistemin maliyeti iki ana fonksiyon arasında pay edilebilir. İstenilen soğutma öğlen civarı maximum düzeydedir. Bu hal binanın durumuna göre değişir. Soğutma için depolanacak kapasite birkaç saat olmasına rağmen ısıtma sisteminde bu periyod oldukça fazladır. Ancak güneşli çevrimler özellikle etkili değillerdir ve güç çevrimlerinde güneşli kollektörlerin kullanımında büyük toplama alanları ve yüksek maliyet gerektirir. Güneşli enerji daha çok ısı enerjisine dönüştürülür. Mekanik ve elektrik enerjisine daha az dönüştürülür. Bir absorpsiyonlu çevrimli soğutma sistemi güneşli kollektörler için en uygun uygulama alanı olarak gözükmektedir. Absorpsiyonlu soğutma sisteminde 4 temel eleman vardır; Üreteç, yoğuşturucu, buharlaştırıcı ve yutucudur. Bu bölümler üzerinde çalışma akışkanı sirküle edilir. Çalışma akışkanı LiBr-H20 veya NH3- H2O olan soğutucu ve yutucu çiftinden oluşan akışkanlardır. Isı güneşli kollektörlerden üretece temin edilir. Bu ısı üreteç de soğutucunun bir kısmını buharlaştırmaya yol açar. Yoğuşturucuya ulaşınca soğutucu buhar yoğunlaştırılır. Yoğuşturulan buhar buharlaştırıcıya kadar gider. Hava ile ısıl kontakt ile yoğuşturucuda bu soğutucu akışkan soğutulur. Yoğuşmuş şekilde buharlaştırıcıya gelen bu soğutucu akışkan, buharlaştırıcıda dışarıdan temin edilen ısı ile yeniden buharlaştırılır. Yutucudaki çevrim kapalıdır. Yutucuda soğutucu ve yutucu akışkan yeniden birleştirilir. Birleştirilen bu çözelti üreteçe pompa ile basılır. -vıı- Kısaca bölümler hakkında bilgi vermek gerekirse öncelikle bölüm1 de esas konumuz olan absorpsiyonlu soğutucunun özet bir şekilde anlatımı incelendi. Bölüm 2 de ise soğutucu akışkanlarla NH3-H20 soğutucu akışkanlı sistem ile H20-LiBr çalışma sistemi çiftine giriş yapıldı Bölüm 3 de ise özellikle NH3-H2O Çalışma sistemi çevrim boyunca her bir kompenentde ki hal ve durumu incelendi Bölüm 4 de ise absorpsiyonlu soğutma çevriminde esas önemli olan çevrimin soğutma tesir katsayısının hangi değerlerin etkisiyle değişebileceği incelenerek sistemin çalışması için yapılan bilgisayar programı hakkında bilgi verildi.

As a very large proportion of the world population lives in overheated areas, current interest in the prospects for solar cooling is particularly strong. The great adventage of using solar energy in cooling and refrigeration applications is that the maximum amount of solar energy is avaliable at the point of maximum demand. There are two quite different major applications, the first in cooling of buildings and the second in refrigeration for food preservation. In building applications part of the solar cooling system could be used to provide heating outside the hot mid-summer period and the costs of the system could be shared between the two functions. The cooling demand is at a maximum during the early afternoon, depending on the oriantation of the building and its thermal mass, so that the storage capacity for cooling is only a few hours in contrast to the very much greater periods required for heating system. Currently two basic solar techniques are being considered: The vapour compression cycles driven by an engine or electric motor and the absorption cycles driven by an absorption aggregate. Unfertunately, existing solar engines are not particularly efficient and the power cycles using solar collectors imply large collection areas and hence high capital costs, since solar energy is more efficiently converted into heat than into electricity or mechanical energy, an absorption aggregate seems appropriate for many solar refrigeration applications at present. If and when the cost of photovoltaic cells will be reduced considerably, it would be of adventage to use vapour compression systems powered by photovoltaic generators. The absorption machine consist of four basic parts; Generator, condenser, evaperator and absorber. Through these parts a working fluid is circulated. The working fluid e.g. lithium bromide-water and ammonia-water consists of a refrigerant in an absorbent. Heat supply from the solar colletors to the generator causes some of the refrigarent to the evaporate. When reaching the condenser, the refrigarent vapour is condensed and the condansate is led to the evaporator, which is in thermal contact with the air to be cooled. In the evaperator the condansate is again vapourized upon heat supply from the ambient air. In the absorber the cycle is closed when the refrigarent recombines with the absorbent and is pomped from there to the generator. -IX- Most of the present absorption cooling aggregates are lithium bromide-water machines with water - cooled absorber and condenser. The pressure in the condenser and generator is determinated by the condenser fluid coolant temperature of the cooling fluid in the absorber. The basic solution processes of an NH3-H2O coolant are similar to those of the LiBr-H20 system, but the pressures and the pressure differences are much higher and mechanical pumps are needed to return solutions from the absorber to the generator. In many applications condenser and absorber are air-cooled, with generator temperatures in the range of 125 to 170°C, in applications where water cooling is used, generator temperatures may be in the range of 95 to 100°C. The effective performance of an absorption cycle depends on the two materials that comprise the refrigarent-absorbent pair. Desirable charecteristics for the refrigarent-absorbent pair are; l.The absence of a solid-phase absorbent 2. A refrigarent more volatile than the absorbent so that separation from the absorbent occurs easily in the generator. 3. An absorbent that has a small affinity for the refrigarent. 4. A high degree of stability for long-term operations. 5. A refrigerant that has a large latent heat so that the circulation rate can be kept at the minimum. 6. A low corrosion rate and nontoxicity for safety reasons Large absorption air contitioners are manufactured by many of the air conditioning manufacturers in the world but only one makes a residential-sized LiBr-H20 unit for solar installations. If the pump work is neglected the c.o.p of an absorption air contitioner can be calculated from the system. C.O.P=Cooling effect (qBUH.) / Heat input (qoRET.) The c.o.p values for absorption air conditioning range from 0.5 for a small single stage unit. These values are about 15 percent of the c.o.p values that can be achived by a vapor compression air conditioners. It is difficult to compare the c.o.p of an absorption air conditioner with that of a vapour - compression air conditioner directly, because the efficiency of the electric power generation or transmission is not included in the c.o.p of the vapor - compression air conditioning. -x- In order to raise the c.o.p value of an absorption chiller it is considered appropriate to take the following measures especially for solar air - conditioning. As show in figure below,it is generally effective to lower the condensing temperature both for absorber and condenser by letting cooling water flow through both compenents Cooling water 140 120 100 80 60 Cooling 40 (%) 20 0.0 cooling capacity Standart cooling 3*10 kcal/h capacity 75 80 85 90 95 100 105 110 120 Hot water temperature to generator The computer program was written in fortran 77 The main program calls the individual unit subroutines, which have been specified by user.In these program cycles, all datas was taken from the thermodynamics table in the water forms, but the LiBr values after the seperated from generator was taken from specified equation. -XI- In absorption cooling cycles mostly LiBr-Water and Water- Amonia are used as working fluids. The behaviours of working fluids and the processes in the systems are described as follows: The refrigerant fluid, Amonia for Amonia-Water systems and Water for lithium Bromide-Water system, is seperated in generator by the means of heat given to this compenent. The mass flow rate of the refrigerant depends on the temperature to which the solution is heated. After it reaches to the desired temperature, the refrigarent goes through the condenser and eveperator. Meanwhile the weak solution which has less amount of the refrigerant fluid as a result of dissolving in the generator, expands in the throttling valve and goes through the absorber. Here it absorbs the refrigerant coming from the eveperator and a strong solution which has more amount of the refrigerant, as a results of this absorption, forms. Then the strong solution pomped to the generator through the solution pump In this 4 section, in a computer program is described as we see, main desired thing that can be described by the using of heat energy is produced in the generator at the collector systems. Heat energy is entered at the begining of the program, However the other input values which were said as a absorber, condenser and eveperator temperatures. In this program eveperator temperature was taken between the 4-10 C degree, because of the using LiBr-Water systems. If the eveperature temperature is decreased under the 0 C degree. After reaching this temperature, Water liquid form becomes the solid state. This solid form effects the pipe line of the installation, i.e ice of the water is unwanted state form in this system. This state is called the cyristalization. Cyristalization state becomes above the consantration of the %68 LiBr. As a result of the main program, If the generator temperature is increased, the c.o.p. value will be sharply increased. If the condenser and absorber temperature are increased, the c.o.p. value will be decreased. We observed that the change of the eveperator temperature effects the c.o.p value i.e, If the eveperature temperature is increased between the 4-10 C degree, the c.o.p value is slowly increased. With the development of the technology and industrialization, two problems has been formedidecrease of energy sources and pollution. Especially in recent years, after the effect of freon gasses on atmosphere were found, the industrialized countries decided on decreasing the use of freon gasses problem on using vapor compression cooling systems, so absorption cooling systems which use LiBr-Water and Water-Amonia as working fluids, will gain great important. -Xll- As the conclusion, for the effective operation of absorbtion cooling system, the following criteria must be generally satisfied: The choice of working fluid and the design parameters of the compenents.For the comparable region, where temperature is above 0°C LiBr-Water system is more effective than Ammonia-Water system. However, LiBr-Water systems creates more design problems than Ammonia-Water system because the pressure values corresponding to the convenient temperatures are sub-atmospheric.