The objective of this study was to search the effects of the inlet air compressor pressure and evaporative cooling of four different cogeneration plants that are absorption cooling (ab), basic (bsc), air heating (airh), and air fuel heating (airfh) cogeneration systems by using the first law and the second law of thermodynamics, and the exergy analysis methods. For analysis, a program is written by the author in the FORTRAN programming language. Decreasing the atmospheric pressure or increasing the installation altitude of the plants increases the Z factor (ratio of lost exergy to useful exergy) of the four cycles about 10–13%. Also, decreasing inlet air pressure decreases the specific work about 25–28%, and the fuel energy saving ratio of the four cycles decreases about 29–30%. The method of water spray cooling of the compressor inlet air especially in the summer months, the humidity increases and the evaporative cooling can be obtained. By using this method, the ratio of lost exergy to useful exergy can be decreased for the four cycles about 0.5–2%. Also, the specific work of the four cycles can be increased about 1.2–6%.
In the conventional system, heat is produced in two separate systems in order to produce power and heat. However, in cogeneration systems, power and heat are produced in one system. In that situation, while the total energy efficiency of conventional systems is around 60%, the energy efficiency of a cogeneration plant can reach up to 90%. In other words, it is seen that there is about 30% more benefit from the energy of the fuel in the cogeneration plant. Furthermore, when a cogeneration plant is installed in systems that require heat and electrical energy at the same time, the costs of the system can be significantly reduced [1,2]. The operating method, efficiency, and other properties of the gas turbine identify the characteristics, the operating style, and the efficiency of the whole plant. Cogeneration systems can be classified as steam turbine cogeneration, gas turbine cogeneration, gas–steam turbine combined cogeneration, fuel cells, and motor cogeneration [3,4]. Energy efficiency in cogeneration plants varies between 50 and 90%. In the hot exhaust gases of the turbine outlet, there are around 1.5–3 kW of usable heat energy for each kW of electricity output. It is desired that the minimum flue gas temperature is around 150°C so that the condensation of the steam in the exhaust (corrosion formation) can be prevented [5,6,7].
Compressor inlet air pressure has serious effects on the heat and electrical energy power of the cogeneration plants. It is imperative to know the effects of the height of the chosen place or the choice of location in the installation of the facilities. As the altitude increases, the pressure of the air decreases. There are also advantages and disadvantages in working conditions. The gas turbine performance and the efficiency depend on the ambient and turbine pressures and ambient temperature. For 1 K increase in the inlet temperature, the electrical power is decreased about 0.9%. An increase of 100 m on altitude decreases the electrical power about 1.2%. For every kPa pressure, a decrease of the inlet air pressure in elements, filters, ducts, and components decreases the electrical power by around 1.2–2%. Kehlhofet et al. studied the effect of altitude above sea level on obtained work. They have drawn curves showing decreases as the height increases, and this decrease is from 10 to 75%. They have reported that these results are in agreement with the literature .
The inlet air temperatures affect the heat and electrical energy power of the plants. Water particles are injected into the inlet air, and then, this air is compressed in the summer months by this method to increase the humidity, as well as evaporative cooling in this way; it is possible to easily connect the electrical power to the temperature of the surrounding air. Depending on the evaporative humidity, it is possible to increase the electrical power between 2 and 7%. In the literature, it is found that the decrease in inlet air temperatures increases the mechanical power; for instance, a decrease from 25 to 0°C means an increase in the mechanical power about 14%; however, the heat energy decreases about 10–15% [7,8,9,10].
Ashraf in his study on this subject examined the evaporative effect of cooling on power and found similar results . Al-Fahed et al. reported in their research that the influence of ambient temperature on the performance of a simple cycle varies. The decrease in the air temperature increases the density, so less energy is required for the compression of the same mass of flow. For some regions especially in the Middle East, the difference between daytime–nighttime temperatures is large, which changes the cost greatly. In the study of Al-Fahed et al. and in the literature studies, it was reported that an increasing 1°C on the compressor inlet air temperature decreases electricity production about 0.7%. In the literature, it can be seen that the work efficiency increases about 0.7–2%, if the compressor inlet air could be cooled to 5°C [10,12,13].
Basrawi et al. have obtained the results that increasing ambient temperature decreases the mechanical efficiency; however, heat energy of exhaust gas recovery increased. They also obtained that an increase in the ambient temperature increases the heat of the exhaust gases and also the heat recovery. They also obtained that this exhaust energy to power energy ratio had the same properties with the exhaust heat energy recovery to power energy ratio .
Fernandez et al. have found in their study that an increase in the ambient temperature decreases the electrical power about 22%. They also found that for each 1°C increasing above ISO conditions, the heat efficiency of the systems decreases about 0.06%. The results showed an insight into the cogeneration systems of the tropical climate . In addition to some other works done [16,17,18,19,20,21,22,23,24,25,26,27,28,29], there are still some open questions in this field. Thus, the objective of this study is to search the effects of the inlet air compressor pressure and evaporative cooling of four different cogeneration plants that are absorption cooling (ab), basic (bsc), air heating (airh), and air fuel heating (airfh) cogeneration systems by using the first law and the second law of thermodynamics, and the exergy analysis methods.
2 Materials and methods
Cogeneration plants consist of some apparatus, and in chemical compositions, on pressures, and in temperatures changes occur in those apparatus; in addition, a chemical reaction takes place in a combustion chamber. The assumptions made in the calculations of the cycles are as follows : the ideal gas mixture laws have been applied to air and exhaust, the cogeneration plant works in a continuous regime, combustion is completed, there is no NOx formation, methane was chosen as the fuel and accepted as an ideal gas, and the combustion chamber heat loss is about 2% of the upper heating value of the fuel. The heat loss is zero in other apparatus, and the kinetic–potential energies are accepted as zero.
The compressor compresses air to high pressure. The three types of compressors are piston compressors for high-pressure (P 2/P 1 = r > 30) low-flow, centrifugal compressors for medium-pressure (r = 5–25) medium-flow, axial compressors for low pressure (less than r = 20) high flow. The gas turbine cogeneration systems mostly use axial and centrifugal compressors and their efficiencies are approximately 90 and 85%, respectively . A part of the heat of the exhaust transfers by a recuperator to the gases of the outlet of the compressor. Recuperators are generally produced in sheet type. The combustion chamber’s main function is to maintain a good combustion reaction. The combustion chambers used in gas turbines have three characteristics: mixing zone, combustion zone, and dilution zone. Gas turbine combustion chambers have the feature of producing low NOx called dry low emission. Combustion chambers with catalytic combustion, where the combustion is close to the surface of the catalyst, provide near-zero NOx emissions, and its use is becoming more widespread . There are two types of gas turbines: axial flow and radial turbines. Gas turbines are classified as reaction and impulse turbines. The 95% of applications have axial flow turbines. A part of the energy of the exhaust gases is transformed into mechanical energy in a gas turbine. Heat recovery steam generator (HRSG) passes a part of the energy of the exhaust gases to steam production. The pressure of the remaining exhaust gases is higher than the atmospheric pressure [31,32].
As can be seen in Figure 1, the compressed air at the exit of the compressor is burned in a combustion chamber with natural gas. Some of the exhaust energy is obtained as mechanical energy in a gas turbine, and the remaining energy of the exhaust is used in an HRSG device to obtain steam.
In Figure 2, the diagram of an airh cogeneration cycle is given. As can be seen, the air of the outlet of the compressor is heated by using the exhaust energy with a recuperator before entering the combustion chamber.
In Figure 3, the diagram of an airfh cogeneration cycle is given. The fuel and the compressed air, before entering into the combustion chamber, are heated with recuperators.
In Figure 4, the diagram of a cogeneration cycle added ab that the air before entering into the compressor is cooled by an absorption component that uses the heat energy of the exhaust is shown. Some of the remaining heat energy of the exhaust energy is passed to steam production.
For an open system and steady state, the first law of thermodynamics is as follows:
In the steady state, the law of conservation mass is as follows:
Chemical energy is converted into thermal energy by combustion. In this study, it is accepted that the combustion reaction happens ideally and completely. It is also assumed that the natural gases are taken as methane gas for simplifying the calculations. The following chemical reaction is taken as a basis:
For completing the theoretical combustion, the required minimum amount of air is called the stoichiometric amount of air. However, for complete combustion, more air than the theoretical amount of air is always used. The excess air coefficient is the ratio of the real amount of air to the theoretical amount of air . Exergy or availability is the theoretical maximum amount of available work, which is obtained if equilibrium with the environment is achieved at the end of a reversible process. It has two components: chemical and physical. The perfect gas mixture of physical exergy can be written in molar terms for mixed substances
Chemical exergy is the maximum useful work, which is achieved when a substance in the reference state (T 0, P 0) becomes thermodynamic equilibrium in terms of chemical composition with its surroundings . The chemical exergy of gas mixtures is calculated as follows:
Thus, the total exergy of a flow or control volume is as follows:
The exergy equation for open systems (input and output mass quantities are equal to each other) is as follows:
In Table 1, energy, entropy, and mass equations of the apparatus of the ab system are given.
|Devices||Mass equation||Energy equation||Entropy equation|
In Table 2, exergy, evaluation criteria, and exergy efficiency equations of the apparatus of the ab system are given.
|Devices||Exergy equation||Exergy efficiency|
|Overall cycle||Electrical heat ratio|
|Z factor (ratio of lost exergy to useful exergy)|
3 Results and discussion
In this research, ambient pressure and temperature are taken as 101.3 kPa and 298.15 K, respectively. For the calculations, the inlet fuel mass m fuel = 1.64 kg/s, the inlet compressor air mass flow m air = 91.3 kg/s, the turbine and the compressor isentropic efficiency η izC = η izT = 0.86, the compressor compression ratio r = 10, recuperator outlet temperature T recout = 850 K, steam temperature T steam = 485.57 K, and exhaust outlet temperature T exh = 426 K are taken.
In this study, compressor inlet air or ambient pressure depends on the installation altitude of the cogeneration system, and evaporative cooling is carried out by spraying water particles in the inlet air up to the saturation point on the 1.9% molar water vapor under normal conditions (at 25°C and 1 atm). Accordingly, the new temperature of the air was found from the amount of water injected using the psychrometric diagram and used in the calculation. Increasing the specific humidity affects the density of the air, thus the variation of specific heat with temperature. When the specific humidity is increased, it also increases the specific gas and heat constant values at a specific pressure.
In Figure 5, variation of the Z factor (ratio of lost exergy to useful exergy) with inlet air (atmospheric) pressure for ab, bsc, airh, and airfh cogeneration systems is given. It can be seen that increasing atmospheric pressure (or decreasing altitudes) decreases the Z factor (ratio of lost exergy to useful exergy) of the four cycles. Upon decreasing the inlet air (atmospheric) pressure of the four cycles from 101.3 to 70 kPa, the Z factors of the cycles increase about 10% for ab and bsc cycles and about 13% for airh and airfh cycles, which is important. Increasing inlet air (atmospheric) pressure decreases the lost exergy, which is unignorable, and should be taken into account.
In Figure 6, variation of the specific work with inlet air (atmospheric) pressure for ab, bsc, airh, and airfh cogeneration systems is given. It can be shown that decreasing inlet air pressure decreases the specific work about 28% for airh and airfh cogeneration systems. Upon decreasing the inlet air pressure, for ab and bsc cycles, the decrease in specific work is about 25%.
In Figure 7, variation of the specific fuel consumption (SFC) with atmospheric pressure for bsc, ab, airh, and airfh cogeneration systems is given. In Figure 7, it can be seen that increase in altitude or decrease in atmospheric pressure of the inlet air of the compressor increases the SFC of the four cycles. Decrease in atmospheric pressure of the inlet air of the compressor increases the SFC of bsc cycle about 27%, airh and airfh cycle about 28%, and the ab cycle about 25%.
In Figure 8, variation of the fuel energy saving ratio (FESR) with altitude and atmospheric pressure for ab, bsc, airh, and airfh cogeneration systems is given. Figure 9 shows that decreasing the pressure of the inlet air decreases the FESR of the four cycles. The FESR decreases about 29% with decrease in the atmospheric pressure for airh and airfh cogeneration systems. For ab and bsc systems, FESR decreases about 30% with decrease in atmospheric pressure.
In Figure 9, variation of the incremental heat rate (IHR) with atmospheric pressure for ab, bsc, airh, and airfh cogeneration systems is given. It can be concluded that decreasing the pressure of the inlet air increases the IHR of the four cycles. The IHRs of the ab, bsc, airh, and airfh cogeneration systems are increasing about 10% with decreasing compressor inlet air pressure.
In the summer months, the method of spraying water in the inlet air increases the humidity, so the evaporative cooling can be obtained. It is possible to connect the electrical power to the temperature of the ambient in this way. Increasing the electrical power between 2 and 6% depends on the ambient humidity. Reducing the inlet air temperature by spraying water particles into the compressor inlet air also reduces the combustion chamber outlet temperature.
In Figure 10, variation of the Z factor (ratio of lost exergy to useful exergy) with spraying water into compressor inlet air (g/kg) for ab, bsc, airh, and airfh cogeneration systems is given. As can be seen in Figure 10, the Z factor (ratio of lost exergy to useful exergy) decreases with increasing (evaporative) compressor inlet air humidity ratio for ab, bsc, airh, and airfh cogeneration systems. The ratio of lost exergy to useful exergy decreases for ab and bsc cycles about 2%, and for airh and airfh plants it is about 0.5%.
In Figure 11, variation of the specific work with compressor inlet air humidity ratio for ab, bsc, airh, and airfh cogeneration systems is given. As shown in Figure 11, this increase in evaporative inlet air humidity increases the specific work of the ab, bsc, airh, and airfh cogeneration systems. Increasing spraying water in compressor inlet air (g/kg) increases the specific work of ab and bsc cycles about 6%, and for the airh and the airfh cycles it is about 1.2%.
In Figure 12, variation of the SFC with spraying water into compressor inlet air (g/kg) for ab, bsc, airh, and airfh cogeneration systems is given. It is shown in Figure 12 that increasing spraying water into inlet air (g/kg) decreases the SFC of ab, bsc, airh, and airfh cogeneration systems. Increasing evaporative air humidity decreases the SFC of the ab and the bsc plants about 6% and decreases the airh cycle about 1.9% and the airfh plants about 1.1%.
In Figure 13, variation of the artificial thermal efficiency (ATE) with spraying water into compressor inlet air (g/kg) for ab, bsc, airh, and airfh cogeneration systems is given. As can be seen in Figure 13, increasing spraying water into compressor inlet air (g/kg) increases the ATE of ab and bsc plants about 1%, and for the airh and the airfh plants it stays almost the same.
In Figure 14, variation of the FESR with spraying water into compressor inlet air (g/kg) for ab, bsc, airh, and airfh cogeneration systems is given. It is shown in Figure 14 that increasing evaporative inlet air humidity ratio increases the FESR of the ab and the bsc plants about 6%. However, the increase for the airh plant and for the airfh plant is about 0.5%.
In Figure 15, variation of the IHR with spraying water into compressor inlet air (g/kg) for ab, bsc, airh, and airfh cogeneration systems is given. As it is shown in Figure 15, increasing spraying water into inlet air (g/kg) decreases the IHR of the bsc and ab cycles about 1%. However, for the airh and the airfh cycles, it stays almost the same.
In this study, compressor inlet air pressure and evaporative cooling effects on the performances is investigated. It is found that increasing atmospheric pressure (or decreasing installation altitudes) decreases the Z factor (ratio of lost exergy to useful exergy) of the four cycles. Upon decreasing the inlet air (atmospheric) pressure of the four cycles from 101.3 to 70 kPa, the Z factors of the cycles increase about 10% for ab and bsc cycles and about 13% for airh and airfh cycles. Increasing inlet air (atmospheric) pressure decreases the lost exergy, which is unignorable, and should be taken into account. Also, decreasing inlet air pressure decreases the specific work about 28% for the airh and airfh cycles, and for ab and bsc cycles, it is about 25%. In addition, decreasing the atmospheric pressure of the inlet air increases the SFC about 25–28% and decreases the FESR of the four cycles about 29–30%.
By spraying water into the compressor inlet air especially in the summer months, the humidity increases and evaporative cooling can be obtained. By using this method, the ratio of lost exergy to useful exergy can be decreased for the four cycles about 0.5–2%. Also, by increasing evaporative inlet air humidity the specific work of the four cycles can be increased about 1.2–6%, and the SFC can be decreased about 1.2–6%. In addition, increasing spraying water into inlet air, the FESR can be increased about 0.5–6%.
There is no acknowledgement.
Funding information: There is no funding for his work.
Author contributions: The author has equal rights on the preparation of this study.
Conflict of interest: The author declares that there is no conflict of interest.
Ethical approval: Ethical approval is not required, as the study was not performed in vivo.
Data availability statement: The processed data necessary to reproduce these findings are available upon request with permission.
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