Alternatif süperkritik karbondioksit çevrimlerinin termodinamik analizi
Thermodynamic analysis of alternative supercritical carbon dioxide cycles
Yazar: HACI OSMAN BOYAN
Danışman: PROF. DR. ÜNER ÇOLAK
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Enerji Bilim ve Teknoloji Ana Bilim Dalı / Enerji Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Enerji = Energy ; Makine Mühendisliği = Mechanical Engineering ; Mühendislik Bilimleri = Engineering Sciences
Dizin: Süperkritik karbon dioksit = Supercritical carbon dioxide ; Termodinamik analiz = Thermodynamic analysis

Özet:
Fosil kaynak temelli insan yaşamının yol açtığı etkiler, tabiatta ve günlük hayatta artık görünür hale gelmiş ve tehlikeli boyutlara ulaşmıştır. Enerji üretiminde, hava, kara ve deniz taşımacılığında, ulaşımda neredeyse tamamen fosil kaynaklara bağlılığımız atık ve iklim sorunlarına yol açmıştır. Geri dönülemez noktaya hızla ilerlerken fosil kaynaklara bağlı olduğumuz sektörlerdeki alışkanlıkların da değişmesi gerekmektedir.
Dünyada, enerji üretiminde, yaklaşık %60 oranında fosil yakıtlı santraller, %10 civarı ise nükleer santraller kullanılmaktadır (IEA, 2020). Karbondioksit ve diğer sera gazı salımlarına sebep olan fosil yakıt kullanımının azaltılması için yenilenebilir kaynakların kullanımının yaygınlaşması gerekmektedir. Güneş enerjisinin ısıl uygulamaları için büyük bir potansiyel vadeden süperkritik karbondioksit (sCO2) çevrimleri dikkat çeken bir konudur. Güneş enerjili santrallerin yanında, yeni nesil nükleer santraller, jeotermal santraller, fosil yakıtlı santraller, ağır sanayide atık ısı geri kazanımı uygulamaları gibi çeşitli ısı kaynaklarına uygulanabilme esnekliği, bu çevrimler için geniş bir araştırma alanı oluşturmaktadır. sCO2 çevrimlerinin tasarım noktası ve tasarım dışı durumlardaki davranışlarını değerlendiren modellere ihtiyaç vardır. sCO2'nin bir güç çevriminde iş akışkanı olarak kullanılması fikri 1950 yılında Sulzer tarafından ortaya atılmış ve sCO2 Brayton çevriminin patentini almıştır (Sulzer, G., 1950). Daha sonra yapılan araştırmalarla yüksek potansiyel vadeden bir konu olduğu görülmüş ve son 10 yılda yapılan çalışmalar da büyük bir ivme kazanmıştır.
Bu çalışmada ısı kaynağından bağımsız, sabit bir ısı girişinde üç farklı sCO2 çevriminin performansının modellenmesi amaçlanmıştır. Tez kapsamında, üç çevrim için (Basit Çevrim, Yeniden Sıkıştırma Çevrimi, Birleşik Çevrim) tasarım noktaları belirlenmiş ve bir termodinamik model oluşturulmuştur. EES (Engineering Equation Solver) programında oluşturulan modeller için belirlenen tasarım noktası esnektir ve modelin belirlenen farklı tasarım koşullarında çalışmasına olanak verir. 
Modellenen 3 çevrim için en uygun tasarım noktalarının hesaplanması hedeflenmiştir. Tasarım noktası için, kompresör giriş sıcaklığı 32 °C, türbin giriş sıcaklığı 600 °C ve türbin giriş basıncı 25 MPa seçilmiştir. Tasarım noktasında en yüksek verime birleşik çevrimde (~%50), en düşük verime de basit çevrimde (~%35) ulaşılmıştır. Çevrimlerin basınç oranı, yeniden sıkıştırma ayrılma oranı, kompresör giriş sıcaklığı, türbin giriş sıcaklığı gibi parametrelerinin değişimlerine çevrim veriminin tepkisi incelenmiş ve en uygun noktalar belirlenmiştir. Çevrimler de sadece o çevrime ait olan parametre değişimi açısından değerlendirilmiştir (Örneğin, Birleşik Çevrimdeki sıkıştırma ve genişleme oranları). Bu değişkenlerin ısıl verim üzerindeki etkisi ayrı ayrı incelendikten sonra, ikili, üçlü ve dörtlü olarak aynı andaki değişimlerine karşılık en yüksek ısıl verim değerleri araştırılmıştır.
Bu çalışma sonucunda ve yapılan diğer araştırmalar incelendiğinde, belirli bir ısı kaynağıyla birlikte tüm diğer değişkenlerin hesaba katıldığı ayrıntılı bir sCO2 çevrimi modellemesi bu konudaki araştırmaların ve deneysel çalışmaların, ticari bir uygulamaya dönüşmesini hızlandıracaktır.

Abstract:
The effects of fossil fuel consumption have currently become noticeable in both nature and daily life and have reached hazardous levels. The world's nearly exclusive reliance on fossil resources in energy production, air, land and sea transportation has led to waste disposal problems and climate change. As we move rapidly to the point of no return, the habits in the sectors where we depend on fossil resources must change. 
About 60% of the world's energy supply comes from fossil fuel power plants and 10% comes from nuclear power plants (IEA). In order to reduce the use of fossil fuels which cause carbon dioxide and other greenhouse gas emissions, the use of renewable resources should globally become widespread. Supercritical carbon dioxide (sCO2) cycles, which promise great potential for concentrating solar power (CSP), are a topic of interest. In addition to solar power plants, their flexibility to be applied to various heat sources such as new generation nuclear power plants, geothermal power plants, fossil fuel power plants and waste heat recovery and recycling applications in heavy industry creates a wide research area for these cycles. The physical properties of carbon dioxide such as being chemically inert, non-flammable and non-toxic, easily accessible and cheap, and its critical temperature (~32 °C) close to the ambient temperature have led to the idea that it could be used in power cycles. Moreover, the possibility of reaching high efficiency at high temperatures, a wide operating temperature range, adaptability to a wide variety of energy sources, smaller cycle components and lower investment costs make the sCO2 cycle clearly noticeable from other thermodynamic cycles.
The advantages of the sCO2 cycle in thermal applications of solar energy are as follows: It is more efficient than the currently used superheated steam cycle, its power range (~10 – 150 MWe) allows for the design of large CSP systems, its compactness enables design flexibility when placing the power cycle inside the CSP system (one tenth of any size compared to a Steam turbine), its small size and simplicity represents a considerable potential capital cost saving over existing steam cycles, and lastly, the use of heat transfer fluid (sCO2) which is compatible with existing thermal energy storage methods and which also minimizes the pinch points of the heat exchangers. 
Among the waste heat recovery technologies, heat-power conversion systems are remarkable in terms of their flexibility in reuse of waste heat and economically recoverable costs. The Organic Rankine Cycle, which is successful in medium temperature applications, is less preferred due to the low chemical inertness of the working fluid at high temperatures. Instead, sCO2 cycles can be a promising alternative for high temperature waste heat recovery applications. The high turbine outlet temperatures in the sCO2 cycle create great opportunities for heat recovery and thus the cycle efficiency can be increased. Among the available options, compact microchannel heat exchangers known as Printed Circuit Heat Exchangers seem to be suitable for sCO2 power cycling. 
The idea of using sCO2 as a working fluid in a power cycle was put forward by Sulzer in 1950 and the sCO2 Brayton cycle was patented (Sulzer, G. (1950). In later studies, this idea has been regarded as a subject with high potential and the studies carried out in the last 10 years have gained a great momentum.
This study attempts to model the performance of three different sCO2 cycles at a constant heat input independent of the heat source. The first of these three cycles is the simple recuperator cycle, in which a recuperator is used for heat recovery. The second is the recompression cycle, in which the recuperator and the sCO2 stream both split into two. After these separated sCO2 streams are compressed in two different compressors, they merge before the high temperature recuperator. The third is the combined cycle. In this cycle, two turbines are used with two different main compressors along with a recompression compressor. As for the two main compressor streams, intercooling method is employed while reheating is used between the two turbine streams. These cycles were analyzed according to the 1st law of thermodynamics and the parameters that made the cycle efficiency the highest were investigated. Within the scope of the thesis, design points for three cycles (Simple Cycle, Recompression Cycle, Combined Cycle) were determined and a thermodynamic model was created. The design point determined for the models created in the EES (Engineering Equation Solver) program is flexible and allows the model to work under different design conditions. 
The study aims to calculate the most suitable design points for the 3 modeled cycles. For the design point, the compressor inlet temperature is 32 °C, the turbine inlet temperature is 600 °C, and the turbine inlet pressure is 25 MPa. At the design point, the highest efficiency was achieved in the combined cycle (~50%), and the lowest efficiency was achieved in the simple cycle (~35%). The response of the cycle efficiency to the changes in the parameters of the cycles such as pressure ratio, recompression friction, compressor inlet temperature, turbine inlet temperature was examined and the most suitable points were determined. The cycles were also evaluated in terms of the parameter change within themselves only belonging to that cycle (For example, compression and expansion ratios in the Combined Cycle).
Increasing the turbine inlet temperature improves efficiency in all cycles, but the material limitations presents a challenge to be overcome. Since sCO2 cycles are highly efficient at high operating temperatures, their research has accelerated. The chemical destabilization of organic fluids used in organic cycles when it is exceeded 450°C and the corrosive effect of water vapor at high temperatures have brought sCO2 to the forefront as a stable fluid. sCO2 can work at high temperatures, is inert, does not have as corrosive effect as other fluids. However, the problem is that the components that will operate at high temperatures is not yet sufficient in terms of materials. Long-term studies are needed for nickel alloys or other high tempeature materials that are considered suitable for high temperatures.
Choosing the compressor inlet temperature close to the critical temperature will increase the efficiency. The compression work of sCO2 around the critical point is low and therefore the efficiency is higher. However, as you move away from the critical point, the efficiency decreases rapidly and then more slowly. It is thought that the compressor inlet temperatures will increase up to 50 °C, related to the region where the system operating with the sCO2 cycle is installed. For example, in the focused solar power (CSP) plant designed for an arid region, where dry cooling is planned, 45 °C is forecasted as the compressor inlet temperature. Since it will be difficult to stay very close to the critical point in operating conditions, it will be more achievable to stay a little above the critical temperature.
When the response of the efficiency curve to the pressure ratio change was investigated, it was seen that the efficiency reached its highest value at one point, and the optimum pressure ratio of the combined cycle was higher. The combined cycle achieved a thermal efficiency of 51% at a pressure ratio of 3.6, a recompression cycle of 45% at a pressure ratio of 3.3, and a thermal efficiency of 35% in a simple cycle at a pressure ratio of 3.3.
As the compression ratio increases, up to a point the total compressor work will decrease and the net work will increase. As the net work increases, the thermal efficiency will increase. However, after the peak efficiency point, the total compressor work increases and the net work starts to decrease. This also reduces efficiency. It exceeded 51% efficiency at the point where the combined cycle compression ratio was about 40%.
In the cost analysis studies, it has been determined that as the complexity of the system increases, its efficiency increases, but its cost also increases at the same rate or more. For a particular heat source, a cycle-specific cost analysis study is needed, with all variables defined. The cost of the higher efficiency combined cycle is much higher than the slightly lower efficiency recompression cycle. There are more turbomachines in the cycle and require larger recuperator area. Here, the cost gain that can balance the efficiency difference should be investigated and the most appropriate cycle should be determined. For this, more specific, more detailed analyses are needed.
In order to reduce the water consumption in the cycle, there is a need to determine the effects of dry cooling in detail. It is absolutely essential to reduce water use in energy production systems against the danger of drought that awaits the world.
When the results of this study and the previous studies are examined, it can be clearly seen that a detailed sCO2 cycle modeling, which takes into account all variables including a specific heat source, will certainly hasten the transformation of research and experimental studies on this subject matter into a commercial application.