Bu çalışmanın amacı atık gazların soğutma elde etmek için kullanıldığı gaz türbinli bileşik ısı-güç (kojenerasyon) sistemlerinin termoekonomik çözümlemesidir. Termoekonomik çözümleme, termodinamik çözümlemenin yanında bu tür sistemlerin ekonomik olurluluğunu ve ürünlerin maliyetlerini irdeler. Bu çalışmanın, sıcak iklim kuşağında yer alan ülkelerde elektrik üretiminin bir yan ürünü olarak soğutma elde edilmesini özendirmesi umulmaktadır. Böylece birincil enerji tasarruf edilebilecektir. Bileşik ısı-güç üretimi elektrik ve ısının aynı santralden elde edilmesi anlamına gelmektedir. Bileşik ısı-güç üretimi temelde, elektrik üretiminde kullanılan gaz türbini, buhar türbini ve gaz motorları gibi ısı makinalarının atık ısısından yararlanmayı amaçlar. Böylece yakıt enerjisi daha etkin kullanılmış olur. Bunun iki önemli sonucu vardır. İlk olarak giderek tükenen fosil yakıtlardan tasarruf etmek, ikinci olarak küresel ısınma kaygısını, atmosfere daha az karbon dioksit atarak azaltmak. Bu çalışmada gaz türbini, atık ısı kazanı, buhar türbini ve absorbsiyonlu soğutucudan oluşan bir bileşik ısı-güç sisteminin sayısal modeli oluşturulmuştur. Modelin hesaplamalarını yapmak için Fortran dilinde iki program yazılmıştır. Birinci program sistemin birinci yasa çözümlemesini yapmakta, yakıt ve hava debilerini hesaplamakta, sistemin her noktasında sıcaklık, basınç ve ekserji debilerini bulmaktadır. İkinci program sistemin her noktası için maliyet akılarını ve birim ekserji maliyetlerini hesaplamaktadır. Önerilen sistem konvansiyonel sistemlerle karşılaştırıldığında, soğutma ve elektrik üretimi için hesaplanan maliyetlerin daha düşük olduğu görülmüştür. Ayrıca konvansiyonel yöntemlerle elektrik ve soğutma eldesinde enerjiden yararlanma oranı % 50 dolaylarında kalırken, incelenen sistemde enerjiden yararlanma oranı % 70’leri bulmaktadır. İncelenen sistem için geri ödeme süresi 7 ile 9 yıl arasında değişmektedir. Anahtar Kelimeler: Trijenerasyon, kojenerasyon, absorpsiyonlu soğutma.The objective of this study is the thermoeconomic analysis of the gas turbine cogeneration systems where the exhaust gases are used for refrigeration purposes. The thermoeconomic analysis involves thermodynamic considerations as well as the calculation of economic feasibility of such systems and cost rates of the products. Cogeneration is defined as simultaneous production of power and heat. In essence it aims to utilize the exhaust heat of prime movers such as gas turbines, steam turbines and gas motors for producing electricity. Thus a more effective utilization of fuel is achieved. This has two important consequences. First of all use of lesser amounts of fuel in the context of decreasing fossil fuel supplies and secondly reduced carbon dioxide emissions in view of the global warming concerns. The fact that the exhaust heat may be used in absorption chillers introduces a new direction for cogeneration. Thus besides electricity and process heat, cooling effect may be produced by cogeneration. This application is sometimes called trigeneration in the literature. Cogeneration was used in Europe and especially in former eastern block countries mainly in conjunction with district heating. But it has also gained wide usage in industry around the world in the last 20 years. There are many applications of cogeneration in industrial plants where electricity and process heat are produced simultaneously. There are two types of absorption refrigeration cycles that are widely used in practice. These are the aqua?ammonia cycle and the lithium bromide?water cycle. The former can be used for refrigeration at temperatures below 0°C. The latter is generally used in air conditioning systems and the minimum temperature is limited to approximately 4°C. The thermodynamic calculations related to these cycles are explained with the help of two numerical examples. The coefficient of performance (COP) of the aqua-ammonia system considered in the first example was calculated as 0.5. The COP of the Lithium-bromide-water system was 0.78. The COP depends on the evaporator, condenser pressures and the generator temperature. The average COP of the absorption refrigeration systems in this study was taken as 0.6. A numerical model of a cogeneration system consisting of a gas turbine system, heat recovery steam generator, a steam turbine, a pump and an absorption refrigeration unit was formed in this study. The steam turbine and the absorption refrigeration unit are coupled to the gas turbine system through the heat recovery steam generator. The gas and steam cycles were considered as steady flow systems, air and the combustion products were assumed to be ideal gas mixtures. Natural gas (methane) was used as fuel. Two programs were written to realize the computations of the model. The first program is for the first law analysis of the system, it calculates the mass flow rates of fuel and air, temperatures, pressures and exergy rates at all state points of the system. The second program calculates the cost rates and cost per unit exergy at all state points of the system. The numerical model was simulated with different values of the decision variables. These are the pressure ratio of the compressor, cost of the natural gas, the investment cost of the gas turbine and the investment cost of the steam turbine. Exergy flow rates, cost rates and unit exergy costs were calculated for each state point of the system. Furthermore the exergy destruction, relative cost difference and exergoeconomic factor were calculated for each component of the system. Finally the payback period of the system for different parameters were calculated. When the system is compared with the conventional systems it is seen that the costs for electricity and refrigeration are lower. The fuel utilization effectiveness has been found as 70 %, as compared to 50% for the separate production of products. The payback period was between 7 and 9 years. Keywords: Trigeneration, cogeneration, absorption refrigeration
