Heat transfer and exergy analysis of cascaded latent heat storage with gravity-assisted heat pipes for concentrating solar power applications

A thermal network model is developed to predict the performance of latent heat thermal energy storage (LHTES) systems including cascaded phase change materials (PCMs) and embedded heat pipes/thermosyphons. Because the design of LHTES systems involves a compromise between the amount of energy stored, the heat transfer rate, and the quality of the released thermal energy, an exergy analysis is also carried out to identify the preferred LHTES design. It is found that the LHTES with the lowest melting temperature PCM yields the highest exergy efficiency. However, a cascaded LHTES recovers the largest amount of exergy during a 24 h charging–discharging cycle. Quantitatively, the cascaded LHTES recovers about 10% more exergy during a 24 h charging–discharging cycle compared to the best non-cascaded LHTES considered in this work.     

Lauric and palmitic acids eutectic mixture as latent heat storage material for low temperature heating applications

Palmitic acid (PA, 59.8 °C) and lauric acid (LA, 42.6 °C) are phase change materials (PCM) having quite high melting temperatures which can limit their use in low temperature solar applications such as solar space heating and greenhouse heating. However, their melting temperatures can be tailored to appropriate value by preparing a eutectic mixture of the lauric and the palmitic acids. In the present study, the thermal analysis based on differential scanning calorimetry (DSC) technique shows that the mixture of 69.0 wt% LA and 31 wt% PA forms a eutectic mixture having melting temperature of 35.2 °C and the latent heat of fusion of 166.3 J g⁻¹. This study also considers the experimental determination of the thermal characteristics of the eutectic mixture during the heat charging and discharging processes. Radial and axial temperature distribution, heat transfer coefficient between the heat transfer fluid (HTF) pipe and the PCM, heat recovery rate and heat charging and discharging fractions were experimentally established employing a vertical concentric pipe-in-pipe energy storage system. The changes of these characteristics were evaluated with respect to the effect of inlet HTF temperature and mass flow rate. The DSC thermal analysis and the experimental results indicate that the LA–PA eutectic mixture can be a potential material for low temperature thermal energy storage applications in terms of its thermo-physical and thermal characteristics.     

Experimental and computational evolution of a shell and tube heat exchanger as a PCM thermal storage system

A combined experimental and numerical study has been designed to study thermal behavior and heat transfer characteristics of Paraffin RT50 as a phase change material (PCM) during constrained melting and solidification processes inside a shell and tube heat exchanger. A series of experiments are conducted to investigate the effects of increasing the inlet temperature of the heat transfer fluid (HTF) on the charging and discharging processes of the PCM. The computations are based on an iterative, finite-volume numerical procedure that incorporates a single-domain enthalpy formulation for simulation of the phase change phenomenon. The molten front at various times of process has been studied through a numerical simulation. The experimental results show that by increasing the inlet HTF temperature from T_H = 70 °C to 75 and 80 °C, theoretical efficiency in charging and discharging processes rises from 81.1% to 88.4% and from 79.7% to 81.4% respectively.     

Comparison of energy and exergy analysis of fossil plant,ground and air source heat pump building heating system

The energy and exergy flow for a space heating systems of a typical residential building of natural ventilation system with different heat generation plants have been modeled and compared. The aim of this comparison is to demonstrate which system leads to an efficient conversion and supply of energy/exergy within a building system.The analysis of a fossil plant heating system has been done with a typical building simulation software IDA–ICE. A zone model of a building with natural ventilation is considered and heat is being supplied by condensing boiler. The same zone model is applied for other cases of building heating systems where power generation plants are considered as ground and air source heat pumps at different operating conditions. Since there is no inbuilt simulation model for heat pumps in IDA–ICE, different COP curves of the earlier studies of heat pumps are taken into account for the evaluation of the heat pump input and output energy.The outcome of the energy and exergy flow analysis revealed that the ground source heat pump heating system is better than air source heat pump or conventional heating system. The realistic and efficient system in this study “ground source heat pump with condenser inlet temperature 30 °C and varying evaporator inlet temperature” has roughly 25% less demand of absolute primary energy and exergy whereas about 50% high overall primary coefficient of performance and overall primary exergy efficiency than base case (conventional system). The consequence of low absolute energy and exergy demands and high efficiencies lead to a sustainable building heating system.     

Maximising the energy output of a PVT air system

Simultaneously generating both electricity and low grade heat, photovoltaic thermal (PVT) systems maximise the solar energy extracted per unit of collector area and have the added benefit of increasing the photovoltaic (PV) electrical output by reducing the PV operating temperature. A graphical representation of the temperature rise and rate of heat output as a function of the number of transfer units NTUs illustrates the influence of fundamental parameter values on the thermal performance of the PVT collector. With the aim of maximising the electrical and thermal energy outputs, a whole of system approach was used to design an experimental, unglazed, single pass, open loop PVT air system in Sydney. The PVT collector is oriented towards the north with a tilt angle of 34°, and used six 110 Wp frameless PV modules. A unique result was achieved whereby the additional electrical PV output was in excess of the fan energy requirement for air mass flow rates in the range of 0.03–0.05 kg/s m². This was made possible through energy efficient hydraulic design using large ducts to minimise the pressure loss and selection of a fan that produces high air mass flow rates (0.02–0.1 kg/s m²) at a low input power (4–85 W). The experimental PVT air system demonstrated increasing thermal and electrical PV efficiencies with increasing air mass flow rate, with thermal efficiencies in the range of 28–55% and electrical PV efficiencies between 10.6% and 12.2% at midday.     

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.     

Performance investigation of thermal energy storage system with Phase Change Material (PCM) for solar water heating application

In order to harvest solar energy, thermal energy storage (TES) system with Phase Change Material (PCM) has been receiving greater attention because of its large energy storage capacity and isothermal behavior during charging and discharging processes. In the present experimental study, shell and tube TES system using paraffin wax was used in a water heating system to analyze its performance for solar water heating application. Energy and exergy including their cost analyses for the TES system were performed. Accordingly, total life cycle cost was calculated for different flow rates of the Heat Transfer Fluid (HTF). With 0.033 kg/min and 0.167 kg/min flow rates of water as HTF, energy efficiencies experienced were 63.88% and 77.41%, respectively, but in exergy analysis, efficiencies were observed to be about 9.58% and 6.02%, respectively. Besides, the total life cycle cost was predicted to be $ 654.61 for 0.033 kg/min flow rate, which could be reduced to $ 609.22 by increasing the flow rate to 0.167 kg/min. Therefore it can be summarized that total life cycle cost decreases with the increase of flow rate.     

Modeling and optimizing parabolic trough solar collector systems using the least squares support vector machine method

Investigating the complicated thermal physics mechanisms of the parabolic trough solar collector systems plays a vital role in efficiently utilizing the solar energy. In this paper, the least squares support vector machine (LSSVM) method is developed to model and optimize the parabolic trough solar collector system. Numerical simulations are implemented to evaluate the feasibility and efficiency of the LSSVM method, where the sample data derived from the experiment and the simulation results of two solar collector systems with 30 m² and 600 m² solar fields, and the complicated relationship between the solar collector efficiency and the solar flux, the flow rate and the inlet temperature of the heat transfer fluid (HTF) is extracted. Some basic rules, such as the solar collector efficiency increases with the increase of the solar flux and the flow rate of the heat transfer fluid, and decreases with the increase of the inlet temperature of the HTF, are obtained, which indicates the LSSVM method is competent to optimize the solar collector systems. As a result, the new approach will provide meaningful data for developing the parabolic trough solar thermal power plant in China.     

Energy and exergy analyses of OMW solar drying process

The energy and exergy analyses of the drying process of olive mill wastewater (OMW) using an indirect type natural convection solar dryer are presented. Olive mill wastewater gets sufficiently dried at temperatures between 34 °C and 52 °C. During the experimental process, air relative humidity did not exceed 58%, and solar radiation ranged from 227 W/m² to 825 W/m². Drying air mass flow was maintained within the interval 0.036–0.042 kg/s. Under these experimental conditions, 2 days were needed to reduce the moisture content to approximately one-third of the original value, in particular from 3.153 g_water/g_{dry matter} down to 1.000 g_water/g_{dry matter}.Using the first law of thermodynamics, energy analysis was carried out to estimate the amounts of energy gained from solar air heater and the ratio of energy utilization of the drying chamber. Also, applying the second law, exergy analysis was developed to determine the type and magnitude of exergy losses during the solar drying process. It was found that exergy losses took place mainly during the second day, when the available energy was less used. The exergy losses varied from 0 kJ/kg to 0.125 kJ/kg for the first day, and between 0 kJ/kg and 0.168 kJ/kg for the second. The exergetic efficiencies of the drying chamber decreased as inlet temperature was increased, provided that exergy losses became more significant. In particular, they ranged from 53.24% to 100% during the first day, and from 34.40% to 100% during the second.     

Exergy analysis of cogeneration power plants in sugar industries

In this paper, exergy analysis of a heat-matched bagasse-based cogeneration plant of a typical 2500 tcd sugar factory, using backpressure and extraction condensing steam turbine is presented. In the analysis, exergy methods in addition to the more conventional energy analyses, are employed to evaluate overall and component efficiencies and to identify and assess the thermodynamic losses. The analysis is carried out for a wide range of steam inlet conditions selected around the sugar industry’s export cogeneration plant. The results show that, at optimal steam inlet conditions of 61 bar and 475 °C, the backpressure steam turbine cogeneration plant perform with energy and exergy efficiency of 0.863 and 0.307 and condensing steam turbine plant perform with energy and exergy efficiency of 0.682 and 0.260, respectively. Boiler is the least efficient component and turbine is the most efficient component of the plant.     

Numerical simulation of modified Trombe-Michel Walls with exergy and energy analysis

Numerical simulations of three configurations of a modified Trombe-Michel Wall (TMW) are carried out and the numerical results are employed to carry on the energy and exergy analyses. The three configurations of the modified Trombe-Michel Wall (TMW) investigated are with sharp edges, with rounded edges and with the guided flow. For each configuration and glaze spacing, spanning from 0.1 m up to 0.5 m, the velocity and the mean air temperature in the channel, the heat flux, the exergy gain and the conductive heat transfer through the massive wall are evaluated numerically. The comparison among the three configurations shows that the guided flow presents the highest energy and exergy efficiency.     

Overall thermal energy and exergy analysis of hybrid photovoltaic thermal array

In this paper, overall thermal energy and exergy analysis has been carried out for different configurations of hybrid photovoltaic thermal (PVT) array. The hybrid PVT array (10.08 m × 2.16 m) is a series and parallel combinations of 36 numbers of PV modules. A one-dimensional transient model for hybrid PVT array has been developed using basic heat transfer equations. On the basis of this transient model, an attempt has been made to select an appropriate hybrid PVT array for different climatic conditions (Bangalore, Jodhpur, New Delhi, and Srinagar) of India. On the basis of high grade energy (i.e. overall exergy gain), case-III has been selected as the most appropriate configuration because overall exergy for case-III is 12.9% higher than case-II. The overall thermal energy and exergy gain for Bangalore is 4.54 × 10⁴ kW h and 2.07 × 10⁴ kW h respectively which is highest in comparison to the other cities.     

Performance and thermo-economic assessments of geothermal district heating system: A case study in Afyon,Turkey

In this study energy, exergy and exergoeconomic analysis of the Afyon geothermal district heating system (AGDHS) in Afyon, Turkey is performed through thermodynamic performances and thermo-economic assessments. In the analysis, actual system data are used to assess the district heating system performance, energy and exergy efficiencies, exergy losses and loss cost rates. Energy and exergy losses throughout the AGDHS are quantified and illustrated in the flow diagram. The energy and exergy efficiencies of the overall system are found to be 37.59% and 47.54%, respectively. The largest exergy loss occurs in the heat exchangers with 14.59% and then in the reinjection wells with 14.09%. Besides, thermo-economic evaluations of the AGDHS are given in table. Energy and exergy loss rates for the AGDHS are estimated to be 5.36 kW/$ and 0.2  kW/$, respectively.     

Hot water cooled electronics: Exergy analysis and waste heat reuse feasibility

We report an experimental study on exergetically efficient electronics cooling using hot water as coolant. It is shown that water temperatures as high as 60 °C are sufficient to cool microprocessors with over 90% first law (energy based) efficiency. The chip used in our experiment is kept at temperatures of 80 °C or below so as not to exceed any allowable industrial specifications for maximum microprocessor chip temperature. The use of hot water as coolant will eliminate the requirement for chillers typically used in air-cooled data centers and, therefore, significantly reduce the power consumption. An exergy analysis shows that a six fold rise in second law (exergy based) efficiency is achieved by switching the water inlet temperature from 30 °C to 60 °C. The resulting high exergy at the heat sink outlet is a measure of the potential usefulness of the waste heat of data centers, thereby helping to design data centers with minimal carbon footprint. A new metric for the economic value of the recovered heat, based on costs for electricity and fossil fuels, heat recovery efficiency and an application specific utility function, is introduced to underscore the benefits of hot water cooling. This new concept shows that the economic value of the heat recovered from data centers can be much higher than its thermodynamic value.     

Optimum temperatures in a shell and tube condenser with respect to exergy

This paper focuses on evaluation of the optimum cooling water temperature during condensation of saturated water vapor within a shell and tube condenser, through minimization of exergy destruction. First, the relevant exergy destruction is mathematically derived and expressed as a function of operating temperatures and mass flow rates of both vapor and coolant. The optimization problem is defined subject to condensation of the entire vapor mass flow and it is solved based on the sequential quadratic programming (SQP) method. The optimization results are obtained at two different condensation temperatures of 46 °C and 54 °C for an industrial condenser. As the upstream steam mass flow rates increase, the optimal inlet cooling water temperature and exergy efficiency decrease, whereas exergy destruction increases. However, the results are higher for optimum values at a condensation temperature of 54 °C, compared to those when the condensation temperature is 46 °C. For example, when the steam mass flow rate is 1 kg/s and the condensation temperature increases from 46 °C to 54 °C, the optimal upstream coolant temperature increases from 16.78 °C to 25.17 °C. Also, assuming an ambient temperature of 15 °C, the exergy destruction decreases from 172.5 kW to 164.6 kW. A linear dependence of exergy efficiency on dimensionless temperature is described in terms of the ratio of the temperature difference between the inlet cooling water and the environment, to the temperature difference between condensation and environment.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.     

Effects of meteorological variables on exergetic efficiency of wind turbine power plants

This present paper deals with exergy efficiency results of the Wind Turbine Power Plants (WTPPs). Effects of meteorological variables such as air density, pressure difference between state points, humidity, and ambient temperature on exergy efficiency are discussed in a satisfactory way. Some key parameters are given monthly for the three turbines. Exergy efficiency differs from 0.23 to 0.27 while temperature is changing from 268.15 K to 308.15 K with air density 1.368–1.146 (kg/m³). While pressure difference (ΔP) between inlet and outlet of the turbine differs from 100 to 1100 (Pa), exergy efficiency decreases fairly for different wind speeds. While specific humidity is changing from 0.001 to 0.015 (kg_water/kg_{dry air}), exergy efficiency decreases gently. Generally these meteorological variables are neglected while planning WTPPs, but this neglect can cause important errors in calculations and energy plans. Obtained results indicate that while planning WTPPs meteorological variables must be taken into account.     

Experimental study of saturated flow boiling heat transfer in an array of staggered micro-pin-fins

This study concerns water saturated flow boiling heat transfer in an array of staggered square micro-pin-fins having a 200 × 200 μm² cross-section by a 670 μm height. Three inlet temperatures of 90, 60, and 30 °C, six mass velocities for each inlet temperature, ranging from 183 to 420 kg/m² s, and outlet pressures between 1.03 and 1.08 bar were tested. Heat fluxes ranged from 23.7 to 248.5 W/cm². Heat transfer coefficient was fairly constant at high quality, insensitive to both quality and mass velocity. Heat transfer was enhanced by inlet subcooling at low quality. Possible heat transfer mechanism was discussed.     

A novel polygeneration system integrating the acetylene production process and fuel cell

This paper presents a novel polygeneration system that integrates the acetylene process and the use of fuel cells. The system produces acetylene and power by a process of the partial oxidation/combustion (POC) of natural gas process, a water–gas shift reactor, a fuel cell and a waste heat boiler auxiliary system to recover the exhaust heat and gas from the fuel cell. Based on 584.3 kg/h of natural gas feedstock, a POC reactor temperature of 1773 K, an absorber pressure of 1.013 MPa and a degasser pressure of 0.103 MPa, the simulation results show that the new system achieved acetylene production of 1.9 MW, net electricity production of 1.7 MW, power generation efficiency of 26.8% and exergy efficiency of 43.4%, which was 20.2% higher than the traditional acetylene production process. The new system's exergy analysis and the flow rate of the products were investigated, and the results revealed that the energy conversion and systematic integration mechanism demonstrated the improvement of natural gas energy conversion efficiency.     

The heat transfer characteristics of liquid cooling heat sink with micro pin fins

This paper numerically and experimentally investigated the liquid cooling efficiency of heat sinks containing micro pin fins. Aluminum prototypes of heat sink with micro pin fin were fabricated to explore the flow and thermal performance. The main geometry parameters included the diameter of micro pin fin and porosity of fin array. The effects of the geometrical parameters and pressure drop on the heat transfer performance of the heat sink were studied. In the experiments, the heat flux from base of heat sink was set as 300 kW/m². The pressure drop between the inlet and the outlet of heat sink was set < 3000 Pa. Numerical simulations with similar flow and thermal conditions were conducted to estimate the flow patterns, the effective thermal resistance. It was found that the effective thermal resistance would reach an optimum value for various pressure drops. It was also noted that the effective thermal resistance was not sensitive to porosity for sparsely packed pin fins.