Papers by Keyword: Exergy

Abstract: This study compares the performance of a vapor compression refrigeration system (VCRS) with an integrated mechanical subcooled vapor compression refrigeration system (IMS-VCRS) in water-cooled centrifugal chillers. Specifically, this study uses four different refrigerants R134a, R450a, R513a, and R515a. A mathematical model of IMS-VCRS is developed to determine the performance parameters at a fixed cooling capacity of 1750 kW. This study investigates that the energy consumption of an IMS-VCRS is reduced by 11.36% for R134a, 11.98% for R450a, 13.36% for R513a, and 11.89% for R515a. The mechanical subcooled system's COP is increased by 12.84% for R134a, 13.59% for R450a, 15.40% for R513a, and 13.48% for R515a due to the low power requirement. In addition, exergetic analysis is also used to determine the system's second law efficiencies, which in this study improved by 11.25%, 11.84%, 13.41%, and 11.77% for R134a, R450a, R513a, and R515a respectively. Furthermore, parametric analyses were performed to investigate the effects of various system parameters such as evaporator and condenser temperatures.

Abstract: Various researchers have developed a model of conventional H₂O -LiBr absorption machines, with the aim of predicting their performance. In this paper, the methodology of the adaptation of the characteristic equations is applied; this model is able to represent the cooling capacity of the double effect absorption chiller by means of simple algebraic linear equation. As a result, it is concluded that a good agreement between the theoretical simulation using the thermodynamic model and the results obtained on the literature. The results achieved through the adapted characteristic equations model have a deviation less than 10% for the cooling capacity and the coefficient of performance COP, and less than 15% for the heat capacity comparing to the thermodynamic model results. The developed characteristic equations and their assessment are useful for the simulation and the control purpose.

Abstract: In recent years, solar water heating technology has got the major importance in water heating applications. For the efficient and effective working of solar water heaters, storage of the hot water is the main issue in this water heating technologies. This paper mainly presents the exergy and efficiency comparison between the solar flat plate water heater and evacuated tube water heater. A five level stratified thermal storage tank is designed and developed for this analysis. At each level of the tank, temperature of the water is collected for each one hour interval from 9:00 to 16:00 hours in both cases and the overall exergy and efficiency of the two water heaters were calculated. The average efficiency of all the levels for flat plate and evacuated tube collectors are 7.91% and 8.20% respectively. The overall system efficiency is obtained as 39.54% for flat plate and 41.00% for evacuated tube solar collector. At each level exergy is calculate for both systems and the average exergy of all the levels for flat plate collector is 4.243 kW and for evacuated tube solar collector is 4.371 kW.

Abstract: Highly increased fuel prices and the need for greenhouse emissions reduction from diesel engines used in marine engines in compliance with International Maritime Organization (IMO) on the strict regulations and guidelines for the Energy Efficiency Design Index (EEDI) make diesel engine exhaust gas heat recovery technologies attractive. The recovery and utilization of waste heat not only conserves fuel, but also reduces the amount of waste heat and greenhouse gases dumped to the environment .The present paper deals with the use of exergy as an efficient tool to measure the quantity and quality of energy extracted from waste heat exhaust gases in a marine diesel engine. This analysis is utilized to identify the sources of losses in useful energy within the components of the system for three different configurations of waste heat recovery system considered. The second law efficiency and the exergy destroyed of the components are investigated to show the performance of the system in order to select the most efficient waste heat recovery system. The effects of ambient temperature are also investigated in order to see how the system performance changes with the change of ambient temperature. The results of the analysis show that in all of the three different cases the boiler is the main source of exergy destruction and the site of dominant irreversibility in the whole system it accounts alone for (31-52%) of losses in the system followed by steam turbine and gas turbine each accounting for 13.5-27.5% and 5.5-15% respectively. Case 1 waste heat recovery system has the highest exergetic efficiency and case 3 has the least exergetic efficiency.

Abstract: Nowadays, developing solar cooling technologies, especially ejector refrigeration system, has become preferable to scientific researchers. Exergy analysis is a technique in which the basis of evaluation of thermodynamic losses follows the second law rather than the first law of thermodynamics. An experimental exergy analysis of a solar-driven dual parallel-connected ejector (DPE) refrigeration system was conducted using water as working fluid. Saturated steam with 2 bar and 120oC was provided by heat–pipe evacuated tube solar collector with an assistant of an electric heater. The saturated stream was used as a motive flow for the ejectors. The exergy destruction and exergetic efficiency of the main components of the DPE refrigeration system were determined and compared with those when using a single ejector (SE) under same operating conditions. It was found that the most irreversibilities of both systems occurred at the solar collector, electric boiler and ejectors, respectively. Also, the total irreversibility (Exergy destruction) of the system when using DPE was lower than using a SE. In additions, the exergetic efficiency of the ejector, evaporator, and overall system when using DPE were increased by 21%, 10%, and 27%, respectively. The system thermal ratio (STR) and coefficient of performance (COP) of the system using DPE compared with SE were increased by 20% and 23%, respectively.

Abstract: The research was performed on thermal energy storage prototype in Thailand. Concrete was used as the solid media sensible heat material in order to fulfill local material utilization which is easy to handle and low cost. Saturated steam was used for heat transfer fluid. The thermal energy storage prototype was composed of pipes embedded in a concrete storage block. The embedded pipes were used for transporting and distributing the heat transfer medium while sustaining the pressure. The heat exchanger was composed of 16 pipes with an inner diameter of 12 mm and wall thickness of 7 mm. They were distributed in a square arrangement of 4 by 4 pipes with a separation of 80 mm. The storage prototype had the dimensions of 0.5 x 0.5 x 4 m. The charging temperature was maintained at 180°C with the flow rates of 0.009, 0.0012 and 0.014 kg/s whereas the inlet temperature of the discharge was maintained at 110°C. The performance evaluation of a thermal energy storage prototype was investigated in the part of charging/discharging. The experiment found that the increase or decrease in storage temperature depends on the heat transfer fluid temperature, flow rates, and initial temperature. The energy efficiency of the thermal energy storage prototype at the flow rate of 0.012 kg/s was the best because it dramatically increased and gave 41% of energy efficiency in the first 45 minutes after which it continued to rise yet only gradually. Over 180 minutes of operation time, the energy efficiency at this flow rate was 53% and the exergy efficiency was 38%.

Abstract: The resource intensity of primary nickel production in China was analyzed by the indicator of exergy, which can provide a unified picture about the overall processes involved in the life cycle of nickel. The results show that primary nickel’s CExD value is 235GJ_ex/t, and the largest contributor to the CExD is electricity (46%), followed by fuels (31%), mineral (16%), and land resource (7%); the considerable proportion natural mineral and land resource account for in the result is due to the high exergy value of sulphide minerals and the low grade of natural nickel ore. The results also show that the calculation of this study is sensitive to the choice of allocation basis (mass and market value); however, market value is not recommended in this study since that nickel’s price varies greatly over time.

Abstract: This study presents four analysis at unit 4 Kamojang geothermal power plant are exergy analysis at current condition, exergy efficiency optimization, economic optimization, and exergoeconomic optimization with wellhead valve pressure as a variable. Calculations are conducted by using the MATLAB. Thermodynamics characteristic of geothermal fluid assumed as water characteristic which get from REFPROP. Wellhead pressure operational condition 10 bar has exergy efficiency 31.91%. Exergy efficiency optimization has wellhead valve pressure 5.06 bar, exergy efficiency 47.3%, and system cost US$ 3,957,100. Economic optimization has well pressure 11 bar, exergy efficiency 22.13%, and system cost US$ 2,242,200. Exergoeconomic optimization has 15 optimum condition. Exergoeconomic optimization aims to analyze the optimum wellhead valve pressure for maximum exergy efficiency and minimum cost of power plant system.

Abstract: Energy conservation is a major topic of concern since our energy sources are exhausting exponentially. This paper focuses on waste heat recovery using which scrap preheating is done in metal castings using sand molds. During solidification of molten metal, most of the heat is lost to the sand. The proposal is to prepare the sand mould with aluminium shots surrounding the mold cavity. These shots absorb some of the heat from the solidifying metal in the mold cavity. The heated shots are separated from the mold and they are allowed to transfer their heat energy to the metal scrap by conduction. The experiments indicate that at least 6.4% of heat recovery is achievable. This will be instrumental in reducing the enormous amount of energy spent to melt the metal considering the fact that casting is the most widely used manufacturing process globally.

Abstract: The entropy generation of any thermodynamic system provides a useful measure of extent of irreversibility. The irreversibility causes the loss of useful work (exergy) in the system and it has to be minimized. Thermal radiation, coming from the sun is rich in exergy. Entropy generation is one of the parameter that quantifies the loss of exergy. It is a unique parameter to measure the strength of irreversibility of thermodynamic process. The criterion for the optimal thermodynamic operation of a collector is used in terms of Entropy Generation Number (N_s) and Mass Flow Number (M). In this paper, the performance of a 10 W_p photovoltaic thermal (PVT) system is analyzed. The range of mass flow rates to be used for testing is obtained by entropy minimization method. The maximum electrical, thermal and exergy efficiency of 10.9, 23.5, 14.8 % is obtained at the mass flow rate of 0.008 kg/s.