Reference State Temperature Research Articles

A comprehensive assessment of energetic and exergetic performance for the dehumidification system of a processed pistachio production unit

AbstractThe present study aims to conduct a comprehensive evaluation of the energetic and exergetic performance of a dehumidification system utilized in the processing of raw pistachios. The assessment involved the application of the first and second laws of thermodynamics to calculate the exergy aspects of each component of the system, including input and output exergy rates, output/input exergy efficiency, product/fuel exergy efficiency, exergy destruction rate, exergy loss rate, exergy improvement potential rate, and specific exergy consumption. Furthermore, the effect of variations in reference state temperature on the exergy parameters was also investigated. The results indicated that the pre‐dryer chamber had the highest input exergy rate among all the components of the dehumidification system. The product/fuel exergy efficiency is specified to be 35.10%, 9.47%, and 60.43%, for the electro‐fan, heater, and pre‐dryer chamber, respectively, while their output/input exergy efficiency are 87.87%, 22.10%, and 56.28%, respectively. The values of the exergy destruction rate of these components are 0.83, 147.14, and 1.12 kW whereas the exergy loss rate values are found to be 0.03, 4.12, and 9.24 kW, respectively. The improvement potential rate values of these components are obtained to be 0.10, 117.83, and 4.53 kW, while the amount of specific exergy consumption for the dehumidification system is determined as 481.85 kJ/kg. The study also reveals that the exergy parameters vary with changes in reference state temperature and that the exergy efficiencies decrease linearly as reference state temperature rises. Therefore, the findings of this investigation demonstrate the potential for using exergy analysis as an effective tool to improve the performance of dehumidification systems in industrial settings, specifically in the production of pistachios.Practical applicationsPistachio nut known as green gold because of its high economical value, is one of the popular nuts over the world. Dehumidification system based on drying technology could be used in food industry with many distinct advantages for processing of particulate crops like pistachios. For a comprehensive assessment of this system, analysis of the first and second laws of thermodynamics is applied. Exergy aspect based on the thermodynamic analysis the is an important stage for designing, modeling, optimizing, and performance assessment of the dehumidification system to produce processed pistachio. The results of this study show that the performance of dehumidification system could be improved by incorporating a steam compressor, a secondary heat exchanger, self‐heat recuperation technology, and a pinch method. It is believed that such a study would contribute to the industrial exploitation of pistachio which can provide insight into decreasing energy consumption and reducing capital costs for engineers and factory owners.

Exergy analysis of a pistachio roasting system

Roasting is one the most widely used unit operations for the processing of pistachio nuts. The operation is aimed at improving their appeal to consumers. The process is energy-intensive, requiring considerable amounts of electrical and thermal energy. Accordingly, this study is aimed at determining the key points in an industrial-scale pistachio roasting plant for energy improvements using the concept of exergy. The system is comprised of two principal subunits: i.e. roasting and drying units. Exergetic indicators for all components of each of the subunits are individually formulated and then computed based on actual operating data. The amounts of exergy utilized and destroyed during processing a given amount of pistachio nuts are determined using the mass allocation approach. A sensitivity analysis is used to show how the exergetic indicators of the process change if the reference state temperature is modified. The heater of the drying unit (114 kW) and the furnace of roasting unit (78.3 kW) are responsible for the majority of the exergy destruction rate (due to irreversibilities) of the overall plant. The exergy utilization and destruction per kilogram of unshelled pistachio are, respectively, found to be 5030 and 4120 kJ. The contributions of the roasting and drying units to the exergy utilization for 1 kg unshelled pistachio are found to be 20.3 and 79.7%, while their contributions to the exergy destruction for 1 kg unshelled pistachio are computed as 19.6 and 80.4%, respectively. The product/fuel exergetic efficiencies of the overall roasting and drying units are found to be 1.58 and 0.44%, while their output/input exergetic efficiencies are determined to be 9.96 and 8.65%, respectively. The investigated exergetic parameters vary as the reference state temperature changes. More specifically, the product/fuel exergy efficiencies of the overall roasting and drying units are almost constant with increasing reference state temperature, while their output/input exergy efficiencies decrease linearly as reference state temperature increases. The output/input exergy efficiencies of the overall roasting and drying units decrease, respectively, up to 8.4 and 12.2% compared with the base reference state temperature (15 °C).

Assessing the exergy performance of heat pump systems without using refrigerant thermodynamic properties

This paper tackles the assessment of the exergy performance of heat pumps. This work is based on a modeling that does not require the evaluation of refrigerant thermodynamic properties and is applied to two commercial (water–water and air–air types) units for various operating conditions (i.e., temperatures at the inlet side of the evaporator and the condenser). The total exergy destruction and the irreversibility distribution into the components are estimated and cover a broad range of values. As well, the nature of the sources of exergy destruction, divided into non-renewable (electrical) and renewable (thermal) energy flows, is also investigated. This distribution, which allows the use of electricity that is really paid by the user to be clearly identified, provides different optimal operating conditions, as compared with the total exergy destruction. Finally, the influence of the exergy reference state temperature (either constant or variable) is examined; in particular, with the air–air heat pump that permits the space heating demand of a building to be achieved. Although the effect on the exergy destruction is relatively low, varying the exergy reference state temperature strongly affects the exergy efficiency, which then appears to appropriately evaluate the use of energy in heat pumps.

Effect of Reference State Characteristics on the Thermal Exergy Analysis of a Building

The exergy analysis is a way to assess the impact of an energy system on the natural resources. In a previous paper the Authors developed a building exergy model, called ⬓Thermal Exergy analysis of a building⬽. The exergy allows to assess a building from a Thermodynamics Second Law point of view. However the assessment result is function of the reference state chosen. In this paper the Authors explore the effect of the reference state thermal properties on the assessment result. To this purpose, an existing building has been modeled through the transient simulation software Trnsys. The model shows that the result changes when describing the reference state through hourly temperatures, monthly average or yearly average. The difference is low if the temperature of the energy flows is far from the reference state temperature and high if their temperature is close to that of the reference state.

An analysis of the performance of an ejector refrigeration cycle working with R134a

In the context of recent developments in the field of energy, the aspect related to energy consumption is of great importance for specialists. Many industries rely on refrigeration technologies, a great challenge being expressed by attempts in energy savings in this sector. In this respect, efforts oriented towards efficient industrial refrigeration systems have revealed the necessity of a proper design. The most commonly used method of cooling is based on vapor compression cycles. Compared to vapor compression refrigeration systems, an ejector refrigeration system shows an inferior performance, indicated by the Coefficient of Performance of the cycle, but it is more attractive from energy saving point of view. In this respect, the present study deals with a theoretically analysis of an Ejector Refrigeration System, started with the presentation of the typical ejector design. It is stated that ejector refrigeration is a thermally driven system which requires low grade thermal energy for its working. After a short description of the analyzed system, are given equations for thermal loads and Coefficient of Performance calculation, on First Law basis. The working fluid considered in this research is Freon R134a. The developed study is focused on the effect of generating temperature variation on the Coefficient of Performance (COP) and on the work input to the pump when the cooling effect, the condensation temperature, the evaporation temperature and the reference state temperature are kept constant. Are obtained results in the following conditions: the condensation temperature is tc = 33°C, the evaporation temperature is te = 3°C, the reference state temperature is to = 23°C. The generating temperature varies in the range 82 ÷ 92°C and the cooling effect is 1 kW. Also, are known the isentropic efficiencies of the ejector, which are 0.90, and the isentropic efficiency of the pump, which is 0.75. Calculation will reveal that the Coefficient of Performance is increasing together with the increase of the generating temperature values, the best COP value being 0.178, in the considered range for the mentioned temperature. In the same time, the generating temperature increase leads to the increase of the work input to the pump.

Performance analysis of a grate stoker coal-fired power plant based on the second law of thermodynamics

In this paper, the performance of a coal-fired plant is assessed using exergy analysis. The plant components were analysed, the exergy destruction of each component was calculated, and the plant overall exergetic efficiency was determined. The boiler and steam turbine were found to have the highest exergy destruction. The percent exergy destruction in the boiler and turbine were 77% and 14.5% of the total exergy destruction; respectively. In addition, the exergy destruction of each component was simulated for different reference state temperatures and for different turbine inlet temperature. It was found that increasing the turbine inlet temperature will increase the efficiency of the power plant cycle up to 42% while the reference state temperature has no significant effect on the main conclusions of the analysis. Thus, implementation of higher temperature and pressure steam boiler will reduce the coal consumption, CO2 emission, and ash generation. The policy implications are also discussed.

Experimental exergetic performance evaluation of a novel solar assisted LiCl–H2O absorption cooling system

This study proposes a novel solar assisted absorption cooling (SAAC) system and performs its exergetic analysis and assessment. The system was designed, constructed and tested in Izmir, Turkey. Water was used as refrigerant in the system. The performance of the whole system along with its main components was assessed through exergy analysis method. The results obtained gave the exergy destruction rate, the exergy efficiency in various forms, the relative irreversibility and sustainability index in both charging and discharging processes. A parametric study was also undertaken to investigate the effect of the various dead (reference) state temperatures ranging from 30°C to 42°C with an interval of 4°C on the entire system efficiency. In this context, the exergy efficiency of the whole system varied between 13.1 and 43.2% as the dead state temperatures increase from 30°C to 42°C. The highest relative irreversibility values of 66.1% and 34.6% belonged to the solar collectors and the absorption chiller in charging and discharging processes at a dead state temperature of 34°C, respectively.

Low exergy modelling and performance analysis of greenhouses coupled to closed earth-to-air heat exchangers (EAHEs)

The present study deals with modelling, analyzing and assessing the performance of greenhouse heating systems with earth–pipe–air heat exchangers (EAHEs) in closed loop mode. In this regard, an EAHE system is considered as an illustrative example. This system starts with the power plant, through the production of heat (EAHE), via a distribution system, to the heating system and from there, via the greenhouse air, across the greenhouse envelope to the outside environment. Exergy analysis method (the so-called low exergy or LowEx approach) has been and still being successfully used to design and evaluate sustainable buildings. It is applied to all components of this EAHE system for the first time to the best of the author's knowledge in this study. The overall energy efficiency value for the EAHE system studied is determined to be 72.10% while the overall exergy efficiency value is calculated to be 19.18% at a reference state temperature of 0°C. The exergy efficiency of the whole EAHE system decreases from 19.18% to 0.77% with the increase in the reference environment temperature from 0 to 18°C. The sustainability index values for the whole EAHE system decrease from 1.24 to 1.01 as the reference state temperature increases.

Methodology for the physical and chemical exergetic analysis of steam boilers

This paper presents a framework of thermodynamic, energy and exergy, analyses of industrial steam boilers. Mass, energy, and exergy analysis were used to develop a methodology for evaluating thermodynamic properties, energy and exergy input and output resources in industrial steam boilers. Determined methods make available an analytic procedure for the physical and chemical exergetic analysis of steam boilers for appropriate applications. The energy and exergy efficiencies obtained for the entire boiler was 69.56% and 38.57% at standard reference state temperature of 25°C for an evaporation ratio of 12. Chemical exergy of the material streams was considered to offer a more comprehensive detail on energy and exergy resource allocation and losses of the processes in a steam boiler.

Investigation of varying dead state temperatures on energy and exergy efficiencies in thermal power plant

In this study, the energy and exergy analysis of a 160 MW Cayorhan thermal power plant, which is operational in Ankara, Turkey, is performed. The effects of varying dead state temperatures on energy and exergy in the plant are investigated. Exergy destruction (irreversibility), entropy generation, per cent ratio to exergy destruction and heat loss, energy and exergy efficiencies are determined for the dead state temperatures ranging from 5 to 30uC. The varying dead state temperatures do not have an effect on the energy efficiency (first law efficiency), whereas they affect the exergy efficiency (second law efficiency). The energy efficiency is computed as 42?76% for the whole plant. The exergy efficiency values for the whole plant are obtained as 35?62 and 32?97% for varying dead state temperature values between 5 and 30uC respectively. It is observed that dead state temperatures directly affect the exergy efficiency of the thermal power plant. In addition, the comparison between heat losses and exergy destruction of the individual component of the plant shows that the maximum heat losses occur in the condenser, whereas the maximum exergy destruction occurs in the boiler for all the reference state temperatures. Furthermore, the analysis shows that the boiler is the major source of irreversibility in the plant.

Exergoeconomic (Thermoeconomic) Analysis and Performance Assessment of a Gas Engine–Driven Heat Pump Drying System Based on Experimental Data

Exergoeconomic analysis has been used as a powerful tool to study and optimize various types of energy-related systems. In this study, we use the specific exergy cost (SPECO) method to calculate exergy-related parameters and display cost flows for all streams and components in a gas engine–driven heat pump drying system based on the experimental data. We analyze and evaluate the performance of the drying system components and the drying process for three different medicinal and aromatic plants from an exergoeconomic point of view. We also investigate the effect of varying dead (reference) state temperatures on exergoeconomic performance parameters for the drying system components and drying process. Although the condenser and drying chamber of the gas engine–driven heat pump dryer were significantly affected by the ambient temperature, the gas engine was slightly influenced by the ambient temperature. At low ambient temperatures, the exergy rates increased and the most effective performance obtained from this dryer was at 0°C. The performance of the drying process also increased at low ambient temperatures. This study demonstrated that exergoeconomic analysis can provide more information than exergy analysis, and the results obtained from the exergoeconomic analysis provided cost-based information, suggesting potential locations for drying system improvement.

Performance investigation of a transportation PEM fuel cell system

In this study, a comprehensive performance analysis of a transportation system powered by a PEM fuel cell engine system is conducted thermodynamically both through energy and exergy approaches. This system includes system components such as a compressor, humidifiers, pressure regulator, cooling system and the fuel cell stack. The polarization curves are studied in the modeling and compared with the actual data taken from the literature works before proceeding to the performance modeling. The system performance is investigated through parametric studies on energy, exergy and work output values by changing operating temperature, operating pressure, membrane thickness, anode stoichiometry, cathode stoichiometry, humidity, reference temperature and reference pressure. The results show that the exergy efficiency increases with increase of temperature from 323 to 353 K by about 8%, pressure from 2.5 to 4 atm by about 5%, humidity from 97% to 80% by about 10%, and reference state temperature from 253 to 323 K by about 3%, respectively. In addition, the exergy efficiency increases with decrease of membrane thickness from 0.02 to 0.005 mm by about 9%, anode stoichiometry from 3 to 1.1 by about 1%, and cathode stoichiometry from 3 to 1.1 by about 35% respectively.

Comparing the Energetic and Exergetic Performances of a Building Heated by Solar Energy and Ground-Source (Geothermal) Heat Pump

In this study, we considered a building, which had a volume of 336 m3 and a floor area of 120 m2, with indoor and outdoor air temperatures of 20 oC and 0 oC, respectively. For heating this building, we selected two options, namely (i) a ground-source (geothermal) heat pump system (Case 1), and (ii) a solar collector heating system (Case 2). We employed both energy and exergy analysis methods to assess their performances and compare them through energy and exergy efficiencies and sustainability index. We also investigated energy and exergy flows for this building and illustrated from the primary energy transformation through the heat production system and a distribution system to a heating system, and from there, via the indoor air, across the building envelope to the surrounding air. We calculated that the total exergy efficiencies for Cases 1 and 2 were 4.7%, and 26.1% while sustainability index values for both cases were 1.049 and 1.353 at a reference (dead) state temperature of 0 oC, respectively.

A comparative investigation of various greenhouse heating options using exergy analysis method

This study deals with modeling and analyzing the performance of greenhouses from the power plant through the heating system to the greenhouse envelope using exergy analysis method, the so-called low exergy or LowEx approach, which has been and still being successfully used in sustainable buildings design, for the first time to the best of the author’s knowledge. For the heating applications, three options are studied with (i) a solar assisted vertical ground-source heat pump greenhouse heating system, (ii) a wood biomass boiler, and (iii) a natural gas boiler, which are driven by renewable and non-renewable energy sources. In this regard, two various greenhouses, the so-called small greenhouse and large greenhouse, considered have heat load rates of 4.15 kW and 7.5 MW with net floor areas of 11.5 m 2 and 7.5 ha, respectively. The overall exergy efficiency values for Cases 1–3 (solar assisted vertical ground-source heat pump, natural gas boiler and wood biomass boiler) of the small greenhouse system decrease from 3.33% to 0.83%, 11.5% to 2.90% and 3.15% to 0.79% at varying reference state temperatures of 0 to 15 °C while those for Cases 1 and 2 (wood biomass and natural gas boilers) of the large greenhouse system decrease from 2.74% to 0.11% and 4.75% to 0.18% at varying reference state temperatures of −10% to 15 °C. The energetic renewability ratio values for Cases 1 and 3 of the small greenhouse as well as Case 1 of the large greenhouse are obtained to be 0.28, 0.69 and 0.39, while the corresponding exergetic renewability ratio values are found to be 0.02, 0.64 and 0.29, respectively.

Exergetic modeling and experimental performance assessment of a novel desiccant cooling system

New approaches to space conditioning of buildings are required to resolve economic, environmental, and regulatory issues. One of the alternative systems that is brought to agenda is the desiccant cooling systems, which may provide important advantages in solving air conditioning problems. This study deals with the performance analysis and evaluation of a novel desiccant cooling system using exergy analysis method. The system was designed, constructed and tested in Cukurova University, Adana, Turkey and has been successfully operated since 2008. This system consists of a desiccant wheel, heat exchangers, fans, evaporative cooler, electric heater unit and refrigeration unit. The exergy transports between the components and the destructions in each of the components of the desiccant cooling system are determined for the average measured parameters obtained from the experimental results. Exergy efficiencies of the system components are determined in an attempt to assess their individual performances and the potential for improvements is also presented. The exergetic efficiency values for the whole system on the exergetic product/fuel basis are calculated to range from round 32% to 10% at the varying dead (reference) state temperatures of 0–30°C.

Exergoeconomic analyses of a gas engine driven heat pump drier and food drying process

Exergoeconomic analysis of a pilot scale gas engine driven heat pump (GEHP) drying system is performed based on the experimental values using Exergy, Cost, Energy and Mass (EXCEM) analysis method in this study. The performance of the drying system components is discussed, while the important system components are determined to improve the system efficiency. The performance of the drying process is also analyzed for three different medicinal and aromatic plants from the exergoeconomic point of view. A comprehensive parametric study is conducted to investigate the effect of varying dead (reference) state temperatures on exergoeconomic performance parameters for both drying system components and drying process. The correlations between the performance parameters and dead state temperatures are developed. The results have indicated that the dead state temperature affects the performance parameters, particularly the drying process parameters. Rising the dead state temperature leads to an increase in the exergy efficiencies of the drying process and a decrease in the ratio of the thermodynamic loss rate to the capital cost (R˙ex) values in a polynomial form. R˙ex values of the drying process are obtained to be very higher compared to those of the drying system components.

Energy and exergy analyses of ice rink buildings at varying reference temperatures

This study deals with energy and exergy analyses of ice rink buildings. An ice rink building with a net area of 648 m 2, which is considered to be closed type and located in Turkey, is assessed. Based on the capacity of the ice rink area, the refrigeration system consists of two circuits with the same basic system components, where two types of refrigerants R-134A and R-744 (CO 2) are used. Exergy analysis is based on Lowex approach, while the effect of varying reference (dead) state temperatures on the system exergy efficiency is investigated. It includes exergy transmission load, air infiltration load, exergy load room, exergy demand distribution, exergy load generation, total exergy system efficiency and exergy flexibility factor. Total exergy input rate is calculated to be 253.66 kW, while exergy destruction rate is 227.45 kW. The minimum and maximum exergy efficiency values are found to be 1.72% and 19.05% for reference state temperatures of 10 °C and −10 °C, respectively.

Earth's core convection: Boussinesq approximation or incompressible approach?

Most geodynamo models are based on the Boussinesq approximation that simplifies the mathematics of the problem by neglecting the compressibility of the liquid in the Earth's core. However, the Boussinesq approximation not only disregards the compressibility in the momentum and the continuity equations, but also neglects adiabatic cooling in the heat transport equation which is connected with the reference state temperature distribution . Whereas the former simplification introduces an acceptable inaccuracy, the latter changes the scene in a substantial way and makes the Boussinesq approximation nonadequate for the problem, especially at the onset of the convection. We prove that adiabatic cooling does not vanish in the incompressible limit used by the Boussinesq approximation and propose a new incompressible approach (IA), which neglects the direct influence of compressibility, but takes into consideration its indirect cooling effects. There is not only adiabatic cooling, but also Archimedean cooling (heating) connected with Archimedean work. This last effect allows Ohmic dissipation to be included consistently in the heat transport equation. Using the adiabatic cooling as a natural inhomogeneity in the heat equation allows the typical amplitudes of the superadiabatic temperature, flow velocity, and magnetic field in the solution to be estimated. These amplitudes are in good agreement with those given by the simulation of Glatzmaier and Roberts (An anelastic evolutionary geodynamo simulation driven by compositional and thermal convection. Physica D, 1996, 97, 81–94). Finally these amplitudes are used to obtain dimensionless equations of the IA. The coefficient with the Archimedean force, usually called the Rayleigh number, takes a moderate value of the order of R m instead of the enormous quantities which the Boussinesq approximation usually attributes to it.

Performance assessment of an internal combustion engine at varying dead (reference) state temperatures

This study deals with investigating the effect of varying dead state temperatures on exergy efficiency of a high-oleic methyl ester (HOME) fueled internal combustion engine (ICE). This engine is a 4.5L, four-stroke, four-cylinder, turbocharged, 66.5 kW maximum power capacity John Deere 4045T diesel engine run with HOME, which is genetically modified with a high-oleic soybean oil methyl ester. The test speed is 1400 min −1 at a full load. In the analysis, actual operational data obtained from a study conducted by one of the co-authors at Iowa State University, USA are used. Exergy efficiency values at various dead state temperatures are calculated for comparison purposes since these types of engines may be operated under different outdoor air conditions. The results obtained are discussed from the exergetic point of view. It was found that exergetic efficiency increased as dead state temperature decreased. As a result, exergy efficiency values ranged from 29.78% to 34.93% based on dead state temperatures between −5 °C and 30 °C.

An experimental study of a direct expansion ground-coupled heat pump system in heating mode

In this paper, an experimental performance evaluation of a direct expansion ground-coupled heat pump (DX-GCHP) system in heating mode is presented. The DX-GCHP uses R134a as the refrigerant, and consists of three single U-tube copper ground heat exchangers (GHEs) placed in three 30 m vertical boreholes. During the on–off operations from December 25, 2007, to February 6, 2008, the heat pump supplied hot water to fan-coil at around 50.4°C, and its heating capacity was about 6.43 kW. The energy-based heating coefficient of performance (COP) values of the heat pump and the whole system were found to be on average 3.55 and 3.28 at an evaporating temperature of 3.14°C and a condensing temperature of 53.4°C, respectively. The second law efficiency on the DX-GCHP unit basis was around 0.36. The exergetic COP values of the heat pump and the whole system were obtained to be 0.599 and 0.553 (the reference state temperature was set equal to the average outdoor temperature of −1.66°C during the tests), respectively. The authors also discussed some practical points such as the heat extraction rate from the ground, refrigerant charge and two possible new configurations to simultaneously deal with maldistribution and instability of parallel GHE evaporators. This paper may reveal insights that will aid more efficient design and improvement for potential investigators, designers and operators of such DX-GCHP systems. Copyright © 2009 John Wiley & Sons, Ltd.