Bir alüminyum profil fabrikasındaki soğurmalı soğutma sisteminin karşılaştırmalı termodinamik analizi
Comparative thermodynamic analysis for an absorption refrigeration system on an aluminum profile factory
Yazar: ERDEM TURHAN
Danışman: PROF. DR. ÜNER ÇOLAK
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Enerji Bilim ve Teknoloji Ana Bilim Dalı / Enerji Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Enerji = Energy
Dizin: Enerji = Energy ; Enerji analizi = Energy analysis ; Kojenerasyon santrali = Cogeneration plants ; Soğurmalı soğutma sistemleri = Absorption cooling systems ; Soğutma sistemleri = Cooling systems

Özet:
Bu tez çalışmasında alüminyum profil fabrikasına ait bir bileşik soğutma-ısı-güç üretim sisteminin soğutma çevrimi ele alınmıştır. Tek etkili soğurmalı soğutma sisteminin termodinamik modeli oluşturulmuş ve akış şeması ortaya konulmuştur. LiBr – H2O ikilisi kullanılan soğutma çevriminde soğutucu olarak su, soğurucu madde olarak LiBr kullanılmıştır.
Buharlaştırıcıdan soğutmanın sağlandığı bu sistemde, alüminyum profil fabrikası içerisinde üretilen profillerin eloksal kaplamasının yapıldığı havuzları ve fabrika ortamını soğutmak amaçlanmıştır.
Gaz motorunun atık egzoz gazlarından yararlanarak, kızgın buhar üreticide su ve lityum brömür birbirinden ayrılır ve soğutucu akışkan olan su kızgın buhar üreticiden kızgın buhar olarak çıkar. Yoğuşturucuda yoğuşan soğutucu akışkan olan su daha sonra kısılma vanasından geçerek basıncı düşürülür. Sonrasında buharlaştırıcıdan geçerek eloksal kaplamanın yapılacağı havuz için gerekli olan soğutma enerjisini aktararak soğurucuya girer. Soğurucuda lityum bromür tarafından soğurulan su ile fakir eriyik oluşur. Oluşan fakir eriyik, eriyik pompası yardımıyla kızgın buhar üreticisine doğru gönderilir. Varsa eriyik eşanjöründen geçen fakir eriyik kızgın buhar üreticisine girerek su ve lityum brömür birbirinden ayrılır. Kızgın buhar üreticisinden çıkan sudan arındırılmış LiBr ise zengin eriyik olarak adlandırılır ve kısılma vanasından geçip basıncı düşürülerek, soğurucuya girer ve burada suyla karışarak fakir eriyiği oluşturur. Bu şekilde çevrimin devamlılığı sağlanır.
Tezin devamında LiBr – H2O ikilisinin kullanıldığı tek etkili soğutma çevriminin soğutma performans katsayısı hem eşanjörlü durumda hem de eşanjörün olmadığı durumda, EES (Engineering Equation Solver) programı ile hesaplanmıştır. Sonrasında eşanjörsüz sistem için her bir bileşenin tasarım parametreleri yani çalışma sıcaklıkları değiştirilerek, soğutma performans katsayısı tekrar hesaplanmıştır. Her bir eleman için sıcaklık ve soğutma performans katsayısı arasındaki ilişki grafikler ile gösterilmiştir. Bu yöntem ile optimum çalışma sıcaklıkları belirlenmeye çalışılmış ve verim hangi durumlarda daha fazla arttırılabilir bulunmuştur.
Son olarak soğutucu akışkan olarak su yerine amonyak kullanılarak çevrimin soğutma verimi bir daha hesaplanmıştır. Çıkan sonuçlar karşılaştırılarak hangi ikilinin belirtilen sıcaklık değerlerinde daha verimli olacağı tesbit edilmeye çalışılmıştır. Bu karşılaştırma da yapıldıktan sonra en uygun çalışma şartları belirlenerek, kızgın buhar üreticisinde ihtiyaç duyulan ısı gücüne göre kataloglardan uygun gaz motoru seçilmiş ve çalışma sonlandırılmıştır.

Abstract:
Energy sector has an important role to provide social and economic improvements for countries. Fossil fuels are used widely in the world and also in Turkey. Because of using fossil resources, negative environmental impacts continously increase. As a result of  fossil fuel consumption, CO2 emissions are going up and climate change is observed around the world.  
Despite ever-growing energy demand depends on growing population and industral activities in Turkey, natural resources are limited. Accordingly, Turkey imports its energy raw materials. It means that Turkey is a country dependent on foreign supplies. Currently, our country is dependent to foreign oil with 98% and natural gas with 91%. Considering these dependencies, the importance of long term and effective planning increase significantly in the energy sector.
If our country is considered to have limited energy sources, the project of minimizing the losses and increasing the efficiency during energy production and utilization has become more significant. Cogeneration, in other words combined heat and power generation systems, means that  both heat and electricity production are made by a single energy source. Producing energy by the combined heat and power system is more efficient than conventional energy systems producing heat and electricity separately. Besides, establishing the cogeneration systems in the facility where it is used is important in terms of availability and economicy.     
Waste energy utilization is also another area to consider. In this direction, even the cogeneration systems with higher efficiency compared to conventional systems, some exhaust gas is released. Especially in summer times, absorbtion refrigeration systems can be used to benefit from waste heat for cooling. In the same way, if electricity, heat and cooling are produced from a single energy source this kind of systems are called trigeneration. There are two types of absorption cooling systems that are widely used in practice. These are systems using ammonia-water and lithium bromide-water solutions. 
The purpose of this thesis is evaluating the cooling load of the factory under consideration. This load is a design parameter for absorption refrigeration system. Specifying the working temperatures of cooling system components  and carrying on thermal analysis of absorption cooling system by using stated data on EES (Engineering Equation Solver) software are also aimed. Once the coefficient of performance of the cooling cycle is calculated on EES programme, some design parameters are changed and calculated again on EES to find the most efficient system.
In this study, a cooling cycle of the trigeneration system belongs to an aluminum extrusion rail factory is examined. Thermodynamic model of the single-effect absorption cooling system is created and thermodynamic flow scheme is shown. LiBr-H2O binary solution is used for absorption refrigerant system and water is used as refrigerant while LiBr used as absorbant.  
In this system, cooling process is made from evaporator. The energy of cooling is used in the pool which is made of anodic coating of aluminum extrusion rails in the factory in order to provide cooling and is used for air cooling in the factory.
Cooling cycle of absorption refrigeration system runs through these stages; water and lithium bromide separate from each other in the generator. Water, which is refrigerant exits from generator as superheated vapor. After that, superheated water vapor comes to condenser and condenses there. Condensing water passes through the throttling valve and its pressure is reduced to minimum pressure of the cycle. After water enters to evaporator and transfer its cooling energy to the pool which is used to make of anodic coating, the refrigerant comes into the absorber. In the absorber, refrigerant water is absorbed by the lithium bromide and poor solution is constituted. This poor solution is pumped to the generator. If there is a heat exchanger in the system, the poor solution passes through the heat exchanger and water and lithium bromide are separated from each other in the generator. Dewatered LiBr in the generator is called rich solution and passes through throttling valve by reducing the pressure after exiting the generator. Then, it enters the absorber and there it becomes a mixture of refrigerant water again. Thus, the continuity of the cycle is ensured.
The coefficient of cooling performance of the single-effect cooling cycle in which LiBr and water binary solution is used is calculated by EES for each system; with heat exchanger and without it. After that the working temperatures of each component for the system without heat exchanger are changed and coefficient of performance of the refrigeration system is analysed again. The relationship between temperature and coefficient of performance for each device is shown graphically and optimum working temperatures are specified with this method. Also, in which cases the efficiency can be increased more is investigated.
An other important point for absorption cooling cycle is crystallization. Crystallization is a phenomenon in which a solid material dissolved in a solvent is precipitated and separated from the liquid under certain conditions. This phenomenon usually occurs at the exit of the generator and inlet of  the absorber of cooling system because of the high temperature and low pressure values respectively. At high temperature rich solution exiting from generator may start to crystallize when it started cooling. Accordingly, it is very difficult to pump the mixture via the pump and the cycle can not be completed. As a result of this event, cooling process stops.
Finally, ammonia is used as refrigerant in the cycle instead of water and accordingly the cooling performance of the system is estimated once again. Due to the low freezing point of ammonia, it is more suitable to work at temperatures below 0°C. This wide operating temperature and pressure range of ammonia makes the ammonia  water fluid couple one of the most used pairs for absorption cooling systems. Also, due to its extreme volatility, it is much easier to separate the water from the generator. In this case, the design parameters are the same as before cases but due to the high volatility characteristic of the ammonia, the degree of dryness in the steps after the generator is entered as "1" to EES software because of the presence of vapour phase of ammonia. In the light of this information output of the analysis is compared and it is decided which binary solution is more efficient at the specified temperature values. After all these comparisons, by determining the most suitable working conditions, the appropriate gas engine is selected from the catalogs according to the heat power required by the generator and the study is terminated.