Exergy‐based performance analysis of packed‐bed solar air heaters

This paper presents an experimental investigation of the thermal performance of a solar air heater having its flow channel packed with Raschig rings. The packing improves the heat transfer from the plate to the air flow underneath. The dimensions of the heater are 0.9 m wide and 1.9 m long. The aluminium‐based absorber plate was coated with ordinary black paint. The characteristic diameter of the Raschig rings, made of black polyvinyl chloride (PVC) tube, is 50 mm and the depth of the packed‐bed in flow channel is 60 mm. Energy and exergy analyses were applied for evaluating the efficiency of the packed‐bed solar air heater. The rate of heat recovered from the packed‐bed solar air heater varied between 9.3 and 151.5 W m^?2, while the rate of thermal exergy recovered from the packed‐bed solar air heater varied between 0.04 and 8.77 W m^?2 during the charging period. The net energy efficiency varied from 2.05 to 33.78%, whereas the net exergy efficiency ranged from 0.01 to 2.16%. It was found that the average daily net energy and exergy efficiencies were 17.51 and 0.91%, respectively. The energy and exergy efficiencies of the packed‐bed solar air heater increased as the outlet temperature of heat transfer fluid increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Comparison of energy and exergy efficiencies of an underground solar thermal storage system

In this experimental study, solar energy was stored daily using the volcanic material with the sensible heat technique. The external heat collection unit consisted of 27 m² of south‐facing solar air collectors mounted at a 55° tilt angle. The dimensions of the packed‐bed heat storage unit were 6 × 2 × 0.6 m deep. The packed‐bed heat storage unit was built under the soil. The heat storage unit was filled with 6480 kg of volcanic material. Energy and exergy analyses were applied in order to evaluate the system efficiency. During the charging periods, the average daily rates of thermal energy and exergy stored in the heat storage unit were 1242 and 36.33 W, respectively. Since the rate of exergy depends on the temperature of the heat transfer fluid and surrounding, the rate of exergy increased as the difference between the inlet and outlet temperatures of the heat transfer fluid increased during the charging periods. It was found that the average daily net energy and exergy efficiencies in the charging periods were 39.7 and 2.03%, respectively. The average daily net energy efficiency of the heat storage system remained nearly constant during the charging periods. The maximum energy and exergy efficiencies of the heat storage system were 52.9 and 4.9%, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating

In the following work, a seasonal thermal energy storage using paraffin wax as a PCM with the latent heat storage technique was attempted to heat the greenhouse of 180 m² floor area. The system consists mainly of five units: (1) flat plate solar air collectors (as heat collection unit), (2) latent heat storage (LHS) unit, (3) experimental greenhouse, (4) heat transfer unit and (5) data acquisition unit. The external heat collection unit consisted of 27 m² of south facing solar air heaters mounted at a 55° tilt angle. The diameter and the total volume of the steel tank used as the latent heat storage unit were 1.7 m and 11.6 m³, respectively. The LHS unit was filled with 6000 kg of paraffin, equivalent to 33.33 kg of PCM per square meter of the greenhouse ground surface area. Energy and exergy analyses were applied in order to evaluate the system efficiency. The rate of heat transferred in the LHS unit ranged from 1.22 to 2.63 kW, whereas the rate of heat stored in the LHS unit was in the range of 0.65–2.1 kW. The average daily rate of thermal exergy transferred and stored in the LHS unit were 111.2 W and 79.9 W, respectively. During the experimental period, it was found that the average net energy and exergy efficiencies were 40.4% and 4.2%, respectively. The effect of the temperature difference of the heat transfer fluid at the inlet and outlet of the LHS unit on the computed values of the energy and exergy efficiency is evaluated during the charging period.     

Energy and exergy analyses of an ice-on-coil thermal energy storage system

In this study, energy and exergy analyses are carried out for the charging period of an ice-on-coil thermal energy storage system. The present model is developed using a thermal resistance network technique. First, the time-dependent variations of the predicted total stored energy, mass of ice, and outlet temperature of the heat transfer fluid from a storage tank are compared with the experimental data. Afterward, performance of an ice-on-coil type latent heat thermal energy storage system is investigated for several working and design parameters. The results of a comparative study are presented in terms of the variations of the heat transfer rate, total stored energy, dimensionless energetic/exergetic effectiveness and energy/exergy efficiency. The results indicate that working and design parameters of the ice-on-coil thermal storage tank should be determined by considering both energetic and exergetic behavior of the system. For the current parameters, storage capacity and energy efficiency of the system increases with decreasing the inlet temperature of the heat transfer fluid and increasing the length of the tube. Besides, the exergy efficiency increases with increasing the inlet temperature of the heat transfer fluid and increasing the length of the tube.     

Application of a ground source heat pump system with PCM‐embedded radiant wall heating for buildings

This paper deals with the utilization of a renewable energy‐based integrated system with the latent heat storage option for building thermal management systems. Both energy and exergy‐based assessments of the current combined system are conducted. For this purpose, phase change material (PCM)‐embedded radiant wall heating system using solar heating and ground source heat pump (GSHP) is studied thermodynamically. Heat is essentially stored within the PCMs as used in the panels to increase the effectiveness. The stored heat is released when the solar energy is not available. In the thermal energy storage analyses, four different PCMs are considered. The present results show that the overall first ‐ law (energy) and second ‐ law (exergy) efficiencies of the PCM‐free radiant heating system are much lower than the case with the PCM‐embedded radiant heating system. Therefore, it is confirmed that the energy efficiency increases from 62% to 87% while the exergy efficiency rises from 14% to 56% with the option where SP26E PCM is employed accordingly.     

An analytical model to predict the thermal performance of a novel parallel flow packed bed solar air heater

A design of a parallel flow solar air heater with packed material in its upper channel and capable of providing a higher heat flux compared to the conventional non-porous bed double flow systems is presented. An analytical model describing the various temperatures and heat transfer characteristics of such a parallel flow packed bed solar air heater (PFPBSAH) has been developed and employed to study the effects of the mass flow rate and varying porosities of the packed material on its thermal performance. The model employs an iterative solution procedure to solve the governing energy balance equations describing the complex heat and mass exchanges involved. To validate the proposed analytical model, comparisons between theoretical and experimental results showed that good agreement is achieved with reasonable accuracy. Also, PFPBSAH is found to perform more efficiently than the conventional non-porous double flow solar air heaters with 10–20% increase in its thermal efficiency. Furthermore, the effect of the fraction of mass flow rate in the upper or lower flow channel of PFPBSAH device on its performance, has also investigated theoretically. The fraction of the mass flow rate in the respective channels of the PFPBSAH is shown to be dominant parameter in determining the effective thermal efficiency of the heater.     

Experimental investigation of concentrated solar air-heater with internal multiple-fin array

This paper presents the results of experimental validation of energy conversion in a concentrated solar air-heater with internal multiple-fin array. A novel high temperature solar air heater was proposed to convert solar energy to heat for space heating in climatic conditions of Poland.The aim of the study was to verify the previously created mathematical model of heat transfer processes. The collector’s performance was analysed experimentally against the reduced temperature difference. During the experiments, an electrical air heater was used to change the air temperature at the inlet of the receiver according to the test demands. Additionally, two daily performance evaluation tests were done. During the tests the all-day measured data was collected automatically and recorded at 1-min intervals. Hourly and daily thermal efficiency was calculated. Validation of the model showed its correctness and proved that the model can be accepted.     

Thermal optimization of solar dish collector for indirect vapor generation

This work presented the performance analysis of a solar parabolic concentrator prototype. The purpose of this paper is to achieve most quantity of vapor production with different water flows. The principal component of the solar concentrator is a new absorber concept that absorbs reflected solar rays and transports it to a heat exchanger in order to generate vapor. Climatic conditions, inlet/outlet oil temperatures of the tubular solar heat exchanger, water tank temperature, and inlet/outlet water temperatures of the mixed heat exchanger were recorded experimentally during three days in November 2018. The absorbed energy, losses energy, concentrated energy, and vapor heat energy of the system were determined. Results of this work, the solar system provides thermal energy efficiency varied from 60% to 70% and a concentration factor around 350 for three water mass flow rates. In this experiment, the optimum value of vapor mass is 6 kg/h with 0.016 kg/s of water flow. Consequently, to achieve the most quantity of vapor, the water flow should be decreased.     

Exergetic performance evaluation and parametric studies of solar air heater

The present study aims to establish the optimal performance parameters for the maximum exergy delivery during the collection of solar energy in a flat-plate solar air heater. The procedure to determine optimum aspect ratio (length to width ratio of the absorber plate) and optimum duct depth (the distance between the absorber and the bottom plates) for maximum exergy delivery has been developed. It is known that heat energy gain and blower work increase monotonically with mass flow rate, while the temperature of air decreases; therefore, it is desirable to incorporate the quality of heat energy collected and the blower work. First it is proved analytically that the optimum exergy output, neglecting blower work, and the corresponding mass flow rate depend on the inlet temperature of air. The energy and exergy output rates of the solar air heater were evaluated for various values of collector aspect ratio (AR) of the collector, mass flow rate per unit area of the collector plate (G) and solar air heater duct depth (H). Results have been presented to discuss the effects of G, AR and H on the energy and exergy output rates of the solar air heater. The energy output rate increases with G and AR, and decreases with H and the inlet temperature of air. The exergy-based evaluation criterion shows that performance is not a monotonically increasing function of G and AR, and a decreasing function of H and inlet temperature of air. Based on the exergy output rate, it is found that there must be an optimum inlet temperature of air and a corresponding optimum G for any value of AR and H. For values of G lesser than optimal corresponding to inlet temperature of air equals to ambient, higher exergy output rate is achieved for the low value of duct depth and high AR in the range of parameters investigated. If G is high, for an application requiring less temperature increase, then either low AR or high H would give higher exergy output rate.     

Exergy Analysis of Electric Vehicle Heat Pump Air Conditioning System with Battery Thermal Management System

The exergy analysis of an electric vehicle heat pump air conditioning system(HPACS) with battery thermal management system was carried out by studying the exergy loss of each component. The results indicate that the compressor is the main source of system exergy loss in all operation conditions. The exergy loss distribution of HPACS is almost the same when the battery thermal management system integrated into the HPACS in cabin and battery mixed cooling mode and the system exergy loss was linearly related to the compressor speed in cooling modes. The performance of the HPACS is better than that of the positive temperature coefficient(PTC) heater in cabin heating mode. The degree of exergy efficiency improvement of the alternative mode was discussed at all operation conditions in cabin heating mode. The results indicate that the optimization effect using the electric vehicle HPACS to replace the PTC heater is obvious at lower compressor speed, surrounding temperature and internal condenser air flow rate.     

Energy and exergy analysis of a new flat-plate solar air heater having different obstacles on absorber plates

This study experimentally investigates performance analysis of a new flat-plate solar air heater (SAH) with several obstacles (Type I, Type II, Type III) and without obstacles (Type IV). Experiments were performed for two air mass flow rates of 0.0074 and 0.0052 kg/s. The first and second laws of efficiencies were determined for SAHs and comparisons were made among them. The values of first law efficiency varied between 20% and 82%. The values of second law efficiency changed from 8.32% to 44.00%. The highest efficiency were determined for the SAH with Type II absorbent plate in flow channel duct for all operating conditions, whereas the lowest values were obtained for the SAH without obstacles (Type IV). The results showed that the efficiency of the solar air collectors depends significantly on the solar radiation, surface geometry of the collectors and extension of the air flow line. The largest irreversibility was occurring at the SAH without obstacles (Type IV) collector in which collector efficiency is smallest. At the end of this study, the energy and exergy relationships are delivered for different SAHs.     

Energetic and exergetic performance evaluation of natural circulation solar water heating systems

This study deals with the energy and exergy analyses of natural circulation solar water heating (SWH) systems. The system comprises of a single glazed flat plate solar collector (FPSC) with absorber plate of 2 m², and a separate insulated well-mixed vertical water storage tank (WST) of 125 liters. The variable heat transfer coefficients, water inlet and outlet temperatures of the FPSC; and temperature of heated water stored in the WST are predicted theoretically for each interval. The daily energy and exergy efficiency of the FPSC, WST and SWH system are estimated to be about 39 and 4.36%, 67 and 38.55%, 27 and 1.01%, respectively. It is found that the water inlet temperature, optical efficiency and the solar radiation strongly influence the performance of the FPSC both energetically and exergetically. It is observed that change in the mass flow rate of water improves the exergy efficiency of the FPSC significantly. FPSC has been identified as a critical component of the system where exergy destruction of 308 W/m² takes place daily as compared to 24 W/m² in the WST against available solar exergy of about 663 W/m².     

Energy and Exergy Analysis on Drying of Banana Using Indirect Type Natural Convection Solar Dryer

Abstract

Energy and exergy analysis, in the thermodynamics, is an important tool used to predict the performance of drying system. In this work, energy and exergy analyses are made during the drying process of banana using an indirect type passive solar dryer. Solar flat plate air collector is used to heat the air. Banana gets sufficiently dried at temperatures between 28 and 82?°C. Solar radiation is measured and it is ranged from 335 to 1210?W/m². Using the first law of thermodynamics, energy analysis was carried out to estimate the amounts of energy gained from solar air heater. Also, applying the second law of thermodynamics, exergy analysis was carried out to determine exergy losses during the drying process. The exergy losses varied from 3.36 to 25.21?kJ/kg. In particular, the exergy efficiency values vary from 7.4 to 45.32%.     

Techno-economic analysis of a novel hybrid heat pump system to recover waste heat and condensate from the low-temperature boiler exhaust gas

This paper is based on the proposal of a new waste heat recovery (WHR) system, which can be utilized to heat the boiler return water, boiler supply air, and building heating air. The system is the combination of an indirect contact condensing unit (IDCCU), a mechanical compression heat pump, and two air preheaters. The system is modeled on the basis of mass and energy balance and then thermodynamically analyzed. Improved performance results were obtained in the form of an increase in the boiler's energy efficiency of about 10.47%, with 4.87% increase in exergy efficiency. The coefficient of performance (COP) of the heat pump was increased from 1.23 to 1.45 by the addition of an air heater in the conventional heat pump. The exergy destruction in each component is calculated. Sensitivity analysis was performed to check the influence of different operating parameters on the performance of the WHR system and boiler. It can be observed from the results that for a specific refrigerant temperature and a calculated amount of mass, flow rate can maximize the condensation efficiency of IDCCU by decreasing the flue gas temperature, while the use of the air heater can further reduce the flue gas temperature, and a stream of hot air can be utilized for space heating. A comparison is made with the other system on a performance basis. The results shows a clear difference in efficiencies and profit earned.     

Energy and exergy analyses of a solar driven Mg–Cl hybrid thermochemical cycle for co-production of power and hydrogen

Analysis and performance assessment of a solar driven hydrogen production plant running on an Mg–Cl cycle, are conducted through energy and exergy methods. The proposed system consists of (a) a concentrating solar power cycle with thermal energy storage, (b) a steam power plant with reheating and regeneration, and (c) a hybrid thermochemical Mg–Cl hydrogen production cycle. The results show that higher steam to magnesium molar ratios are required for full yield of reactants at the hydrolysis step. This ratio even increases at low temperatures, although lowering the highest temperatures appears to be more favorable for linking such a cycle to lower temperature energy sources. Reducing the maximum cycle temperature decreases the plant energy and exergy efficiencies and may cause some undesirable reactions and effects. The overall system energy and exergy efficiencies are found to be 18.8% and 19.9%, respectively, by considering a solar heat input. These efficiencies are improved to 26.9% and 40.7% when the heat absorbed by the molten salt is considered and used as a main energy input to the system. The highest exergy destruction rate occurs in the solar field which accounts for 79% of total exergy destruction of the integrated system.     

Energy and Exergy Analysis of a Single‐Pass Sequenced Array Baffled Solar Air Heater with Packed Bed Latent Storage Unit for Nocturnal Use

The current study presents an experimental investigation on evaluation of thermal performance of a single‐pass double‐glazed solar air heater with the use of packed bed paraffin wax as a phase change material (PCM). Moreover, the absorber plate is equipped with baffles attached over its top. Galvanized sheets with a thickness of 0.4 mm and total surface areas of 30 cm² are chosen as baffles that are placed in a sequential manner over the absorber plate. The solar energy was stored in the packed bed PCM during the diurnal period (charging process) and was released at night for nocturnal use (discharging process). The tests were performed at three different mass flow rates of 0.009 0.014 and 0.017 resulting in the creation of different Reynolds numbers along the channel. The measured parameters were inlet, outlet, and the PCM temperatures under the meteorological condition of Mashhad, Iran. Energy and exergy efficiencies of the system have been calculated according to the first and second laws of thermodynamics. The experimental results illustrate that the daily energy efficiency varied between 20.7% and 26.8%, whereas the daily exergy efficiency varied between 10.7% and 19.5%.     

Transient simulation of a solar-based hydrogen production plant with evacuated tube collector and ejector refrigeration system

This article is a careful examination of an energy poly-generation unit integrated with an evacuated solar thermal tube collector. A proton exchange membrane (PEM) electrolysis unit is used for hydrogen production, an ejector refrigeration system (ERS) is utilized for cooling demand, and a heater unit is used for heating demand. All sub-systems are validated by considering recent articles. Cooling and heating demand, as well as the net output power are calculated. The modeled poly-generation system's exergy and energy efficiency are maximized by considering the inlet temperature of the heat exchanger and primary pressure of the ejector with the parametric evaluation of the system. The proposed poly-generation set-up can produce cooling load, heating load, and hydrogen with amounts of 5.34 kW, 5.152 kW, and 63 kg/year, respectively. Based on these values, the energy ef?ciency, and exergy ef?ciency are computed to be 64.14%, and 49.62%, respectively. Higher energy and exergy ef?ciencies are obtained by reducing high pressure of the refrigeration cycle or decreasing the temperature outlet of an auxiliary heater. The heat exchanger and thermal energy storage unit have the highest cost rate among all system components with 73,463 $ and 46,357, respectively. Parametric study indicates that the main determinative elements in the total cost rate of the system are the heater, and the solar collector.     

Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector

Analysis of energy and exergy has been performed for a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. CaCl₂·6H₂O was used as PCM in thermal energy storage (TES) system. The designed collector combines in single unit solar energy collection and storage. PCMs are stored in a storage tank, which is located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM. Exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. The analyses were performed on 3 days in October. It was observed that the average net energy and exergy efficiencies are 45% and 2.2%, respectively.     

Employing exergy-optimized pin fins in the design of an absorber in a solar air heather

Heat transfer augmentation features for the improvement of heat transfer from the absorber-plate of a solar air heater to the working fluid are mostly fins however these features also increase pressure drop in the flow channel. The exergy optimization philosophy is adopted in the fin sizing for this air heating application; this optimization philosophy ensures that the maximum quantity of heat is transferred by the fins while generating the least entropy in the system thus conserving exergy. Results indicate that the higher the fin efficiency, the higher the heat conversion and delivery potential. Given an optimum fin length (efficiency) and superior coating absorptive vigour, useful energy losses in the heater can be minimized. Some important observations relevant in design are made.     

非晶硅太阳能光伏/光热空气集热器性能对比实验研究

文章设计了新型非晶硅太阳能PV/T空气集热器,该空气集热器能够解决传统太阳能PV/T热水器在高温波动情况下,晶硅电池热应力大的问题,同时避免了冬季管道发生霜冻的现象。文章通过实验对比,分析了非晶硅太阳能PV/T空气集热器、单独非晶硅光伏电池和传统太阳能空气集热器的能量效率和[火用]效率的差异。分析结果表明:非晶硅太阳能PV/T空气集热器的平均热效率为45.70%,比传统太阳能空气集热器的平均热效率降低了约25.88%;当空气质量流量增大至0.048 kg/s时,非晶硅太阳能PV/T空气集热器中的非晶硅光伏电池的平均电效率高于单独非晶硅光伏电池,它们的平均电效率分别为4.70%,4.54%;非晶硅太阳能PV/T空气集热器的总[火用]效率高于传统太阳能空气集热器的热[火用]效率和单独非晶硅光伏电池的电[火用]效率,非晶硅太阳能PV/T空气集热器总[火用]效率最大值为7.14%。文章的分析结果为非晶硅太阳能PV/T空气集热器的推广提供了参考。