Performance analyses of basic cogeneration basic cogeneration plants in agriculture and food production

. Energy in agriculture and in food production have vital importance that affect directly the cost and the quality of the products. Micro cogeneration plants are widely used in the World to produce heat and electricity at the same time in a same plant. The aim of this study, to analyze the performance of the production of power and heat for the food production process and the agricultural needs. For that reason, a basic cogeneration plant is taken to analyze the performance by using 1 st and 2 nd laws of thermodynamics and exergy analysis method. The electric-heat exergy rates, the energy and exergy efficiencies, the combustion chamber and the gas turbine outlet temperatures, the total electric and heat energies and exergies, and the specific works were calculated for various ambient temperatures and for various excess air rates of the basic cogeneration plants fueled with methane. The effects of the various ambient temperatures and the various excess air rates on the performance of the basic cogeneration plants were obtained, discussed and analyzed. For the best performance and for the best working conditions of the basic cogeneration plants in agriculture and food production process and food industry recommendations were done. It was found that, lower ambient temperatures give higher electric efficiency, but lower heat efficiency. However, an optimum excess air rates for the best performance was found at the value 2.3 of the excess air rates of the basic cogeneration plants fueled with methane.


Introduction
Energy in agriculture and in food production have vital importance that affect directly the cost and the quality of the products.Micro cogeneration plants are widely used in the World to produce heat and electricity at the same time in a same plant.The aim of this study, to analyse the performance of the production of power and heat for the food production process and the agricultural needs.For that reason, a basic cogeneration plant is taken to analyse the performance by using 1st and 2nd laws of thermodynamics and exergy analysis method.The electric-heat exergy rates, the energy and exergy efficiencies, the total electric and heat energies and exergies, and the specific works were calculated for various ambient temperatures and for various excess air rates of the basic cogeneration plants fuelled with methane.
Karaali and Ozturk, [1,2] in their articles have studied the performance, exergetic, parametric analyses and optimization of a simple cogeneration plants.They showed that the excess air coefficients are more effective than the compression ratio on the simple cogeneration plants' performances, exergetic, and electrical efficiencies.In their parametric studies, they showed the best working conditions of this plants and systems.* Corresponding author: rabikar@gmail.comKaraali and Keven [3,4], in their researches they have calculated parametric performance analyses of the three various cogeneration systems those were named basic, air heating, absorption cooling, and air-fuel heating cogeneration systems.
In that study, the basic cogeneration plant taken from literature (reference [5,6]) by taking out the recuperator from the given system.Combustion chamber and gasturbine exit temperatures, energy and exergy efficiencies, electric-heat exergy ratios, specific work, and also, electric and heat energy and exergy were calculated by taking various environmental temperatures and various excess air coefficient of this new system.The effects of the environmental temperatures and the excess air coefficient on the performance of the basic cogeneration plants were investigated, discussed, and obtained.

Materials and methods
Basic cogeneration cycles obtain some devices in it.These devices are gas-turbine (main and most important), combustion compressor, chamber, heat recovery steam generator.The assumptions made in the analysis of the cycle is given in reference [6].Natural gas is accepted as methane.The heat losses accepted as 2% of the fuel's UHV of the combustion process.As can be seen in Figure 1, air is taken from the inlet the compressor for compressing.The compressed air and the fuel are combusted in the combustion chamber.Some of the exhaust gases energy is converted to mechanical energy in the gas-turbine, after the combustion chamber.The exhaust gases are at low pressure and hot at the exit of the gas-turbine.These hot gases are used to produce steam at the heat recovery steam generator (HRSG).In Table 1, the basic cogeneration plant's energy, mass, and entropy equations are shown.In Table 2, the basic cogeneration plant's exergy, and exergy efficiencies equations are given.In the calculations, in our analyses and in our FORTRAN coding program, equations in Table 1 and Table 2 are taken to be used.The reactions of the combustion between air and fuel in the combustion chamber were as;

Results and discussion
In Fig. 2, combustion chamber outlet temperatures as a function of excess air coefficient for basic cogeneration plants in various ambient temperatures are shown.For that cycles the environment temperatures are effective on the combustion chamber's exit temperatures.At the high environment temperatures combustion chamber outlet temperatures are high.The excess air coefficients are very effective on the outlet temperatures since the air fuel ratios are changed.Increasing excess air coefficient from 1.3 to 3.5 decreases the outlet temperatures of the combustion chamber about % 37.In Fig. 4, the power as a function of excess air coefficient for the basic cogeneration plant in various ambient temperatures are obtained.Higher environmental temperatures mean higher total electrical and heat energies.However, increasing in excess air coefficients from 1.3 to 3.5 decrease the power about % 13.5.In Fig. 5, electric and heat exergy as a function of excess air coefficient for the basic cogeneration plants in various environmental temperatures are obtained.Higher environmental temperatures show lesser total electric and heat exergy.The reason is higher environmental temperatures increase the compression work.The cycle has a maximum exergy point at 2.3 of the excess air coefficients which is very important.At 2.3 of the excess air coefficient the system's working conditions for maximum exergy is optimum.In Fig. 6, specific work as a function of excess air coefficient for the basic cogeneration plants in various environment temperatures is obtained.In higher environmental temperatures lesser specific works were gained.However, increasing in excess air coefficients from 1.3 to 1.5 increase the specific work, and at 1.5 of the excess air coefficients the specific works are optimum.Higher 1.5 of the excess air coefficients decrease specific works.In Fig. 7, variation of energy efficiency as a function of excess air coefficient for different ambient temperatures for the basic cogeneration plants is obtained.It can be said that, higher environmental temperatures mean higher energy efficiency.However, increasing in excess air coefficients decrease the energy efficiency about % 13.7.In Fig. 8, electric heat exergy rates as a function of excess air coefficient for the basic cogeneration plant in various ambient temperatures is obtained.It can be said that, cold weather means higher electric heat exergy rates.Increasing in excess air coefficients increase the electric heat exergy rates about % 141.In Fig. 9, effects of excess air coefficient on fuel consumption per exergy for the basic cogeneration plants in various ambient temperatures is obtained.As it is seen that, the minimum fuel consumption per exergy is obtained at 2.3 excess air coefficient.In Fig. 10, effects of excess air coefficient on Z factors (ratio of lost exergy to useful exergy) for the basic cogeneration plants in various ambient temperatures is obtained.As it is seen that, the minimum Z factor is obtained at 2.3 excess air coefficient.

Conclusions
In this study, a basic cogeneration plant is analysed to obtain the efficiencies and the best parameters of the performance by using exergy analysis method and 1st and 2nd laws of thermodynamics.Combustion chamber and gas-turbine outlet temperatures, energy efficiencies, total electric and heat energies and exergies, electricheat exergy rates, specific works, Z factor (ratio of lost exergy to useful exergy) are calculated by considering various environmental temperatures and various excess air coefficients.The effects of the excess air coefficient and the environmental temperatures on the working parameters of the basic cogeneration systems were discussed, and obtained.For the best exergetic performance and parametric working conditions, some conclusions were done such as, the basic cogeneration systems can obtain and give the best parameters at 2.3 excess air coefficient which is the optimum point.Also, lower environmental temperatures give better exergy and electric efficiencies, but lower heat exergy.

Table 1 .
The basic cogeneration plant's energy, mass and entropy equations[2,5,6].In the investigation, the normal conditions were P0 =101.30kPa, and T0 = 25.0°C.The air mass flow of the compressor was mair,in = 91.30kg/s, and the flow of the inlet fuel mass is mfuel,in = 1.640 kg/s.And the isentropic efficiency of the gas-turbine and the compressor are ηiz,C = ηiz,T = 0.860, steam-hot water obtained temperature is THRSG,steam = 485.60K, and the HRSG outlet temperatures is Texh,out= 426.0 K[6].

Fig. 2 .
Fig. 2. Effects of excess air coefficient on combustion chamber exit temperatures for the basic cogeneration plants in various ambient temperatures.In Fig.3, the effects of excess air coefficient as a function of the gas-turbine outlet temperatures for the basic cogeneration plants in various environment temperatures are shown.Increasing the environment temperatures are effective on the outlet temperatures of the gas-turbine.In higher environment temperatures the outlet temperatures of the gas-turbine are higher about %4.The excess air coefficients are effective on the

Fig. 3 .
Fig. 3.The effects of excess air coefficient as a function of the gas-turbine outlet temperatures for the basic cogeneration plants in various environment temperatures.

Fig. 4 .
Fig. 4. The power as a function of excess air coefficient for the basic cogeneration plants in various ambient temperatures.

Fig. 5 .
Fig. 5. Electric and heat exergy as a function of excess air coefficient for the basic cogeneration plants in various environmental temperatures.

Fig. 6 .Fig. 7 .
Fig. 6. Specific work as a function excess air coefficient for the basic cogeneration plants in various environment temperatures.

Fig. 8 .
Fig. 8. Electric heat exergy rates as a function of excess air coefficient for the basic cogeneration plant in various ambient temperatures.

Fig. 9 .
Fig. 9. Effects of excess air coefficient on fuel consumption per exergy for the basic cogeneration plants in various ambient temperatures.

Fig. 10 .
Fig. 10.Effects of excess air coefficients on Z factors (ratio of lost exergy to useful exergy) for the basic cogeneration plants in various ambient temperatures.