Bileşik ısı güç sistemlerinin ekonomik yönden olurluklarının araştırılması

Abstract

Yaklaşık 50 yıllık bir süre içerisinde hidrokarbona dayalı enerji kaynaklarının tükeneceği tahmin edilmektedir. Dolayısıyla bu durum enerji dönüştürme çevrimlerinin daha etkin kullanımım zorunlu kılmaktadır. Bileşik ısı - güç sistemleri de bu zorunluluktan dolayı geliştirilmiştir. Bu çalışmada farklı bileşik ısı - güç sistemlerinin değişen elektrik ve ısı yükleri karşısındaki performansları incelenmiştir. Bunun için önce sistemleri simüle eden ve daha sonra bu sistemler için ekonomik analiz yapan bir bilgisayar programı geliştirilmiştir. Programla beş farklı sistem ve üç değişik yatırım alternatifi incelenebilmektedir. İncelemelerin sonucunda uygun finans yöntemi ve kurulacak sistemin ekonomik yönden fizibil olup olmadığı tespit edilmektedir. Yapılan analizler sonucunda banka kredisi ve 'leasing'in finansal yönden iyi sonuçlar verdiği gözlemlenmiştir. Ancak bu iki finansal alternatifden birinin diğerine üstünlüğü konusunda somut bir sonuca varılamamıştır. Çok sayıda parametre oluşu ve koşulların çeşitliliği bu yönde bir sonuca varılmasını imkansız hale getirmektedir.

The fact of hydrocarbon based fuels will be exhausted in the near future is making scientists to evolve new methods in energy conversion. Therefore combined heat and power systems are products of such studies. In industrial countries lots of energy policies have been produced in order to cope with energy gap. Increasing coal production, featuring natural gas and oil search, using renewable energy sources (wind energy, geothermal sources, tide energy etc.), developing nuclear energy and a general treatment of energy usage are among these policies. The last one has lots of methods like effective isolation for heat pump usage, increasing efficiency of heat resources usage, usage of waste materials as fuel and combined heat and power production. In conventional power systems only one third of fuel energy can be converted to electricity. Two thirds of fuel energy is wasted to ambient. With the usage of combined heat and power systems energy utilization factor increases. Combined heat and power systems have been being used throughout the world for years. That's why their superiorities to conventional power systems have been proved for many times. In this work performance of combined heat and power systems under fluctuating heat and power demand have been examined. Economic conditions in Turkey were especially emphesized. That's why in order to find out specific results a software have been evolved. This software makes simulations according to the heat and electricity demands then by the help of some values from simulations analyze the system in economical way. So selection of proper system can be carried out. Reason of including economic analysis in this work can be easily seen from EK- F. Because electricity and fuel prices increased rapidly it was an obligation to include economic analysis in this work. xn Above all this software takes values of heat and electricity demands and makes simulation by these values for the system intended to be installed. For five different combined heat and power systems simulation can be carried out. These are extraction steam turbine system, back - pressure steam turbine system, gas engine system, gas turbine system and diesel engine system respectively. After simulation have been carried out values such as fuel consumption, electricity consumption, amount of electricity sold obtained from simulation are used in economic analysis. In this part of software three financial alternatives are used. These alternatives are bank credit, leasing and payment in advance respectively. After evaluations made in this part proper combined heat and power system can be determined. The thermodynamics of conventional power systems have been interesting for engineers for a long time. Goal was always to reach maximum efficiency. Designer try to reduce amount of fuel inlet to system in order to get maximum work because this process ensures advantage at managing facility. Goal for the designer of a combined heat and power system is different. Because in this situation there is both heat and power production. Now facility can earn money by both heat and electricity. Therefore the designer concerns himself (or herself) with not only reaching maximum efficiency but producing maximum heat amount as well. Hence combined heat and power systems must be considered in both thermodynamic and economic ways. For combined heat and power systems there are lots of performance criteria. The most common is energy utilization factor (EUF). This criterion is defined as the ratio of total energy taken from system ( work + useful heat) to energy given to system. Although energy utilization factor is 1 according to the first law of thermodynamics, in practice all of the wasted heat can not be used, therefore this value occurs between 0.7 and 0.9. Another criterion is heat to power ratio which is used in combined heat and power system selection. It is defined as the ratio of produced heat to produced electricity. Although it is a parameter which can be used in combined heat and power system selection, for the facilities which have fluctuating heat and power demand values usage of computer programs are more suitable. Gaining heat by combined heat and power systems is achieved by heat recovery systems. These systems use engine jacket water heat and exhaust gases heat. With these heat sources hot water is produced. In addition to these systems there are systems producing steam from engine directly. But they cost much. As I mentioned before there are five types of combined heat and power systems. Extraction steam turbine type can be used for a range of different heat to power ratio values. So this type of systems can be used in facilities in which heat demand varies in time. Back -pressure steam turbines are used when heat demands are greater than electricity demands. In gas turbine systems heat energy of exhaust gases is recovered by a waste heat recuperator or a waste heat boiler with an auxiliary firing. Auxiliary firing is carried out by dispatching additional fuel to the exhaust gases in which there are still enough oxygen. The amount of oxygen in exhaust gases limits the xm amount of fuel dispatched for auxiliary firing. In diesel engine systems the heat recovery systems are also used. Finally gas engine systems use same heat recovery systems too. In gas engine systems natural gas and propane can be used. Both diesel engine systems and gas engine systems can be used for facilities which have low heat and power demands. Before thermodynamic simulation heat and power values demanded by facility is given to software. Program can take these values in five different types. These are : 1- Annual values 2- Monthly values 3- Weekly values 4- Daily values 5- Hourly values Entering demand values operator can choose one of the combined heat and power plant types. For each type of plant values of some parameters must be given. These are : - Electrical power - Electrical efficiency - Thermal efficiency - Efficiency of additional boiler - Purchase price of electricity - Selling price of electricity - Annual working period of facility in hours - Fuel price - Selling price of steam (this value is calculated automatically by program) After entering parameters mentioned above, program executes a cycle in which simulation is done. In simulation process different conditions are considered. These conditions occur according to comparison of heat and power demand values of the plant for which the system is intended to be installed. By this comparison economical benefits and losses appear. In economic analysis section three financial alternatives are considered. The values (TFcg = total fuel amount used in combined heat and power plant, TFbek = total fuel amount used in additional boiler, TEAlış= amount of purchased electricity, TWcg = amount of electricity produced, TESatış = amount of electricity sold, TBSatış = amount of steam sold ) obtained from thermodynamical simulation are used in this section. All economical evaluations are in U.S. dollar currency. In economic analysis there are two main groups : annual expenditures, annual revenues. Parameters considered in annual expenditures are as follows : 1- Oil consumption xiv 2- Internal power consumption 3- Fuel consumption 4- Operation costs Parameters considered in annual revenues are as follows : 1- Heat production income 2- Electricity production income After annual revenues and expenditures has been found out, search for proper financial alternative begins. In this part of financial analysis net present value method (NPV) is used. This method shows the total value of investment. Investment which has the maximum net present value is the most suitable one. In order to determine the amount of investment, cost of plants per unit power produced has been used. According to this, costs of plants have been taken as follows : - Extraction steam turbine system 1200 $/kW - Back - pressure steam turbine system 800 $/kW - Gas engine system 700 $/kW - Gas turbine system 700 $/kW - Diesel engine system 600 $/kW Linear method has been used in calculation of amortization for all financial alternatives. In the bank credit section some parameters must be entered to the program. These parameters are : Interest rate Amount of investment MARR (minimum rate of return) Equity fund amount Incentive amount Tax rate - Period of amortization - Life of investment - Salvage value of the investment - Expenses - Revenues - Credit period In the leasing section different parameters compared with the parameters of bank credit section are as follows : - Leasing interest - Period of leasing - Purchasing price after leasing In payment in advance section a kind of incentive called 'Investment tax credit' has been considered. This incentive is a decrement from the taxable income at the end of the first year of investment. Therefore corporation pays less tax at the end of the first year. In section four, results and analysis can be found. In this section software has been used for three different facilities which have fluctuating heat and power demand xv values. Effect of fuel and electricity prices has been examined and the results have been graphed. Usage of combined heat and power systems necessitates different choices for different applications. Values of heat and electricity demands affect choice. Diesel engine and gas engine systems are more proper for facilities which need little amount of electricity. Back - pressure steam turbine systems and extraction steam turbine systems must be preferred for facilities in which heat demands are greater than electricity demands. Gas turbine systems are flexible because useful heat production can be done easily by auxiliary firing. Because combined heat and power systems have high energy utilization factors interest on these systems is increasing day by day. Furthermore their lower emissions in comparison with emissions of conventional power systems make them more preferable. It can be said that critical factors affecting feasibilities of combined heat and power systems are not only thermodynamical but economical as well. Interest rate, plant life, fluctuation in fuel and electricity prices are main factors. In conclusion it can be said that combined heat and power systems can be financed by bank credit and leasing. But it is impossible to say bank credit usage is advantageous or vice versa, because of lots of parameters and conditions. That's why proper financial alternative must be chosen according to the highest NPV.