Konsantre güneş enerjisi sistemleri için termoklin termal enerji depolama sistem analizi

Abstract

Konsantre güneş enerjisi (KGE) sistemleri yenilenebilir enerji kaynakları arasında gelecek vaadeden çözümler arasında gösterilen ve yenilikçi teknolojileri bünyesinde barındıran sistemlerdir. Günümüzde güneşten yararlanmada fotovoltaik sistemler kadar yaygın ve uygun maliyetli olmasalar da dünyanın farklı coğrafyalarında bir çok farklı uygulama mevcuttur. Dünyada güncel olarak yapılan akademik projeler incelendiğinde, desteklenenen Ar-Ge konularına bakıldığında, yeni geliştirilen ve yapım aşamasındaki konsantre güneş enerjisi projeleri de göz önüne alındığında, bu teknolojilerin önümüzdeki yıllarda daha da yaygınlaşacağı ve potansiyelini yansıtmaya başlayacağı aşikardır. Bu çalışma kapsamında ele alınan konsantre güneş enerjisi teknolojilerinden kule tipi konsantre güneş enerjisi teknolojisi bir çok farklı avantajıyla diğer KGE teknolojilerine göre bir adım daha öne çıkmaktadır. Enerjinin depolanabilmesi yine günümüzün güncel problemlerinden bir tanesidir. Tüm dünyanın yoğun olarak üzerinde çalıştığı ve mevcut teknolojileri daha da geliştirmeye çalıştığı enerji depolama teknolojileri iki ana başlığa ayrılmaktadır. Bu çalışma da yaygın olarak bilinen akü teknolojileri değil, diğer depolama yöntemi olan ve büyük kapasitelerde depolamaya imkan tanıyan termal enerji depolama sistemleri üzerinde durulmuştur. Termal enerji depolama (TED) teknolojileri hakkında gerekli bilgiler çalışmanın ilgili kısmında verilmiştir. Termal enerji depolama sistemleri ile büyük kapasitelerde enerji depolanabilmesi hem güneş enerjisi sistemlerinin sadece güneşin olduğu zamanlara bağlı kalmadan gün boyu üretim yapılabilmesine olanak sağlamakta, hem de gün içerisinde bulutlanma gibi üretimi aksatabilecek kısa süreli operasyonel aksamaları ortadan kaldırmaktadır. Ayrıca bu iyileştirmeler sayesinde sistem ömrü uzamakta ve üretilen elektriğin kalitesi de artmaktadır.Gelecek vaadeden bu iki yeni ve güncel teknolojinin tanıtılarak ele alındığı ve aynı sistem içerisinde birleştirildiği bu çalışmada, mevcut bir kule tipi konsantre güneş enerjisi santraline kurulabilecek tek tanklı, iki fazlı (akışkan malzeme ve dolgu malzemesi) bir termal depolama sistemi göz önüne alınmıştır. Çalışmanın ana kısmı olan 4. Kısımda bir boyutlu (1D) termal enerji depolama sistemi, Schumann denklemlerinin boyutsuz sayılar yardımıyla ayrıklaştırılması yapılmış ve geliştirilen modelin MATLAB programı yardımıyla nümerik sonuçları elde edilmiştir. Depolama sistemi çeşitli parametre değişiklikleri yapılarak incelenmiş ve sonuçlar değerlendirmeler ve sonuçlar başlığı altında yorumlanmıştır.

Concentrated solar energy systems are shown as one of the promising technologies among the renewable energy systems. Even though concentrated systems are not common and as cost efficient as photovoltaic systems; there are many different applications of this system all over the world. When the current academic projects and supported R&D topics around the world are reviewed, and also considering the newly developed concentrated solar energy systems, it is in evidence that the applications of that kind of systems will become more widespread. In this paper few CSP Technologies introducing shortly in the second part; Parabolic trough, parabolic dish, fresnel and tower. Tower-type concentrated solar system steps forward among the all types with its several advantages. Within the scope of this thesis the description of tower-type solar systems are given and then the main topic, which is thermal energy storage method is explained in detail. Energy storage, which is one of today's biggest issues and attempted to develop by whole world, is divided into two. The first one is battery technologies that is commonly known.The other one, also the main subject of this thesis is thermal energy storage. Thermal energy storage system enables to store the energy in large capacities. In this way, it would be possible to produce electrical energy not only in daylight but all day long. And it also eliminates the short-term barriers of producing energy such as clouding so the life-span of system would be increased and the produced energy would be more quality. In the scope of this thesis, these two promising current and new technologies are introduced individually and they are combined in the same system. A thermal storage system with one tank and two phase that could be installed in an existing tower-type concentrated solar power plant is considered and the Schumann equations are discretized by method of dimensionless numbers to analyse temperature distribution unidimentional in the tank. Last of all, the equations are solved, and the graphs are obtained by MATLAB to compare results. Then the results are interpreted. With this thesis, it is aimed to develop a model that provides quick and sufficient accuracy results with a simplified approach that can be utilized in the design phase of single-tank thermocline energy storage systems. Literature studies in this direction have shown that one-dimensional (1D) models can be solved more easily than two (2D) and three-dimensional (3D) models. Fast and high accuracy solutions are obtained in previous studies. The options for fluid and solid storage materials to be used in the storage system have been investigated in detail and the materials used in existing similar systems and the new materials which are that have not yet been used in the systems but are promising for the future applications are also examined within the scope of the study. As a result of the evaluations made, it was decided that the system should be sensible heat energy storage system. It is planned to use solid storage material as well as heat transfer fluid in order to reduce system cost. In this context, molten salt (HITEC) was selected as a fluid and Cofalit was selected as a storage filler.When determining the model, the main purpose is to observe the temperature distribution in the tank and determine the time required for charge-discharge of the tank. After these times can be determined, the plant will be operate more optimally. Another study in this thesis is calculation of tank storage capacity. Fluid phase and solid phase materials calculated separately than collected together for determine to the designed tank store capacity. Whereafter the calculation of capacity of the storage system, it was determined in related part of this study. The mathematical equations studied on the solution within the thesis are the equations developed by Schumann, in which both the solid phase and the fluid phase can be calculated. The simultaneous solution of these two equations is quite difficult. Due to this difficulty, the equations have been tried to be reduced to a single differential equation, which is dependent on the dimensionless time expression. The unique dimensionless numbers determined in this study. While solving of the equations were reduced by the differential equations, the dimensionless transfer unit number (NTU) method used by John A. Duffie which is already explained in his book `Solar Engineering of Thermal Processes` was used.Once the equations are discretized, the MATLAB program is used to solve the charge and discharge cycles. The equations obtained in the 4.4 Charge Cycle solution and 4.5 Decharge Cycle solution titles are added to the main code when variables are defined after the program has been defined. The initial conditions and the boundary conditions are also included in the same program. By running the program, the temperature time distribution graphs for the charge cycle for the tank (Figure 4.9) and temperature time distribution graphs for the discharge cycle (Figure 4.10) have been successfully obtained. According to these graphs, the single tank thermocline energy storage system designed for the study is charging in approximately 8 hours; this energy stored in the same way can be used for approximately 8 hours as all losses are neglected. Another part of this study is observing and interpreting the effects of parameter changes on the storage system. In this context, different heat transfer fluids have been compared and determine which fluid will be more advantageous for the system. It has been observed that liquid sodium is the most advantageous fluid among the comparative fluids. Another parameter for comparison is storage materials, which is to be observed for the storage system. Within this scope, different storage materials with potential for use have been identified and compared.Third, the effect of the solid-liquid ratio in the tank on the performance of the storage system was tried to be understood. This system is needed in order to determine optimum working conditions while system design is being done. As a result of comparison, it was observed that the thermocline region becomes thinner as the molten salt ratio increases in the storage system consisting of molten salt - cofalite. This result will be different when different thermal properties are used.This thesis was prepared with the aim of becoming a basis for the future studies. The designed model is a model with a practical approach that yields fast and has a limited error margin. In future works, the number of dimensions of the model can be increased, neglected heat exchanges (inside the tank and its surroundings) and other losses can be taken into consideration. In this study, the variables determined in the acceptance can be included in the model by making detailed calculations.