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.     

Thermal modeling of a packed bed thermal energy storage system during charging

The process of charging of an encapsulated ice thermal energy storage device (ITES) is thermally modeled here through heat transfer and thermodynamic analyses. In heat transfer analysis, two different temperature profile cases, with negligible radial and/or stream-wise conduction are investigated for comparison, and the temperature profiles for each case are analyzed in an illustrative example. After obtaining temperature profiles through heat transfer analysis, a comprehensive thermodynamic study of the system is conducted. In this regard, energy, thermal exergy and flow exergy efficiencies, internal and external irreversibilities corresponding to flow exergy, as well as charging times are investigated. The energy efficiencies are found to be more than 99%, whereas the thermal exergy efficiencies are found to vary between 40% and 93% for viable charging times. The flow exergy efficiency varies between 48% and 88% for the flows and inlet temperatures selected. For a flow rate of 0.00164 m³/s, the maximum flow exergy efficiency occurs with an inlet temperature of 269.7 K, corresponding to an efficiency of 84.3%. For the case where the flow rate is 0.0033 m³/s, the maximum flow exergy efficiency becomes 87.9% at an inlet temperature of 270.7 K. The results confirm the fact that energy analyses, and even thermal exergy analyses, may lead to some unrealistic efficiency values. This could prove troublesome for designers wishing to optimize performance. For this reason, the flow exergy model provides the most useful information for those wishing to improve performance and reduce losses in such ITES systems.     

Energy metrics of photovoltaic/thermal and earth air heat exchanger integrated greenhouse for different climatic conditions of India

In this paper, a study is carried out to evaluate the annual thermal and exergy performance of a photovoltaic/thermal (PV/T) and earth air heat exchanger (EAHE) system, integrated with a greenhouse, located at IIT Delhi, India, for different climatic conditions of Srinagar, Mumbai, Jodhpur, New Delhi and Bangalore. A comparison is made of various energy metrics, such as energy payback time (EPBT), electricity production factor (EPF) and life cycle conversion efficiency (LCCE) of the system by considering four weather conditions (a–d type) for five climatic zones. The embodied energy and annual energy outputs have been used for evaluation of the energy metrics. The annual overall thermal energy, annual electrical energy savings and annual exergy was found to be best for the climatic condition of Jodhpur at 29,156.8 kWh, 1185 kWh and 1366.4 kWh, respectively when compared with other weather stations covered in the study, due to higher solar intensity I and sunshine hours, and is lowest for Srinagar station. The results also showed that energy payback time for Jodhpur station is lowest at 16.7 years and highest for Srinagar station at 21.6 years. Electricity production factor (EPF) is highest for Jodhpur, i.e. 2.04 and Life cycle conversion efficiency (LCCE) is highest for Srinagar station. It is also observed that LCCE increases with increase in life cycle.     

Experimental study on the discharge characteristics of two eutectic solder packed bed latent heat storage systems

Metallic solder based PCMs possess higher thermal conductivities, larger storage masses and exhibit lower subcooling effects compared to their organic or inorganic counterparts. It is thus justified to investigate their potential usage for medium temperature applications. These solders are relatively expensive and can be combined with cheaper PCMs in cascaded storage systems which are more thermodynamically efficient compared to single PCM systems as reported recently. The aim of the research is thus to compare two packed bed storage systems during discharging cycles using eutectic solder (Sn63/Pb37), that is widely available worldwide. The single PCM system (40 capsules) consists of encapsulated spheres of eutectic solder, whereas the second cascaded system consists of encapsulated spheres of eutectic solder and erythritol in an equal storage ratio in the tank. For the cascaded system, the eutectic solder capsules are placed at the top and erythritol at the bottom of the storage tank (20 capsules at the top and 20 at the bottom). The effect of the discharging flow-rates of 4 mL/s, 6 mL/s and 8 mL/s is investigated in relation to the temperature profiles, energy rates and exergy rates. Increasing the flow-rate, increases heat transfer rate thus shortening the discharging time as well as increasing thermal profile reversals during discharging. The peak energy and exergy rates increase with the increase in the flow-rate for the two storage systems. The single PCM system shows slightly higher average energy and exergy rates compared to the cascaded system possibly due to its higher thermal conductivity. The cascaded PCM system shows higher average stratification numbers at all the flow rates considered. The non-cascaded system exhibited slightly higher exergy recovery efficiencies compared to the cascaded PCM system possibly due to its higher thermal conductivity at all flow-rates considered. The effect of the initial discharging temperature is also investigated with a discharging flow-rate of 6 mL/s after charging with set heater temperatures of 260°C, 280°C and 300°C, respectively. Comparable thermal profiles are seen for both systems for the three set temperatures; however, the single PCM system shows slightly higher storage temperatures. The single PCM shows slightly higher but comparable peak and average discharging energy rates compared to the cascaded system. The exergy rates for the two systems are also comparable. However, the cascaded system shows slightly higher exergy rate values for the lowest set temperature whereas the single PCM system shows slightly higher exergy rate values for the other two set temperatures. Energy and exergy rates are almost independent of the initial storage tank temperatures induced by different set charging temperatures. The average stratification number shows no correlation with set temperature for both storage systems. The cascaded system shows slightly higher average stratification numbers at different set temperatures. Exergy recovery efficiencies for different set heater temperatures are comparable for the two storage systems and vary only marginally with the increase in the set temperature. Overall, the effect of the flow-rate is more pronounced than the effect of the set heater temperature.     

Investigation of heat transfer and pressure drop in plate heat exchangers having different surface profiles

It would be misleading to consider only cost aspect of the design of a heat exchanger. High maintenance costs increase total cost during the services life of heat exchanger. Therefore exergy analysis and energy saving are very important parameters in the heat exchanger design. In this study, the effects of surface geometries of three different type heat exchangers called as PHE_flat (Flat plate heat exchanger), PHE_corrugated (Corrugated plate heat exchanger) and PHE_asteriks (Asterisk plate heat exchanger) on heat transfer, friction factor and exergy loss were investigated experimentally. The experiments were carried out for a heat exchanger with single pass under condition of parallel and counter flow. In this study, experiments were conducted for laminar flow conditions. Reynolds number and Prandtl number were in the range of 50 ? Re ? 1000 and 3 ? Pr ? 7, respectively. Heat transfer, friction factor and exergy loss correlations were obtained according to the experimental results.     

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.     

Energy analysis of heat surplus storage systems in plastic tunnels

This paper presents the findings of a theoretical analysis and experimental verification on the storing of heat excess in soil and liquid accumulators located in a foil tunnel. There was positive verification of the formulated macroscopic heat exchange model in both accumulators (maximum error 81%) and the quantity of heat stored in them was defined. During the experiments, under existing weather conditions, the amount of stored heat stood between 6 MJ and 45 MJ in the liquid accumulator and between 9 MJ and 130 MJ in the soil accumulator. The quantity of heat supplied from the accumulator to the interior of the tunnel during discharging, which stood between 0.6 MJ and 46 MJ, was also described. The COP was determined for the tested system both for the accumulator charging process and the discharging of the soil accumulator. Furthermore, the quantity of heat used for heating up heat originating from the discharging of the accumulator whilst heating the tunnel for favourable and unfavourable surrounding climate conditions was determined.     

α–h Diagram and principle of exergy coupling of GAX cycle

Ammonia absorption chiller systems of a single-stage cycle and a Generator Absorber heat exchanger cycle (GAX) were simulated and studied. At heat source temperatures of T_H = 120 °C, T_M = 25 °C and T_L = 5 °C, the coefficient of performances of the two cycles are 0.589 and 0.776, the GAX cycle is higher 31.8% than the single-stage cycle. And the exergy efficiencies of the two cycles are 15.4% and 27.4%, the GAX cycle is higher up to 77.9%. This paper proposes a new method that adopts the energy quality factor α as a evaluation criterion and also uses the α–h diagram as a thermodynamic analysis tool graphically, and a concept that divides absorption cycle to a heat pump subcycle and a heat engine subcycle. By means of the α–h diagram, the thermodynamic frameworks of the two cycles were illustrated. The comparison analysis indicates that the improvement of cycle performance depends on its thermodynamic perfectibility. In fact, the exergy demand of heat pump subcycle in the GAX cycle is as the same as that of the single-stage cycle, however, the energy cascading use and the exergy coupling framework of the heat engine subcycle in GAX cycle is retrofitted, so that the exergy consumption is reduced and the increased benefit is obtained from the overall cycle.     

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.     

Energy and exergy analyses of the oxidation and gasification of carbon

Exergy losses in gasification and combustion of solid carbon are compared by conceptually dividing the processes into several subprocesses: instantaneous chemical reaction, heat transfer from reaction products to reactants (internal thermal energy exchange) and product mixing. Gasification is more efficient than combustion because exergy losses due to internal thermal energy exchange are reduced from 14–16 to 5–7% of expended exergy, while the chemical reactions are relatively efficient for both processes. The losses due to internal thermal energy exchange may be reduced by replacing air with oxygen, although this introduces additional process losses for separation of oxygen from air, or alternatively, preheating of air by heat exchange with product gas. For oxygen-blown gasification of fuels with high calorific value, such as solid carbon, it is advisable to moderate the temperature by introduction of steam. At optimum gasification temperatures in the ranges of 1100–1200 K (for atmospheric pressure) and 1200–1300 K (for 10 bar pressure), up to 75% of the chemical exergy contained in solid carbon can be preserved in the chemical exergy of carbon monoxide and hydrogen.     

Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems

Selection of an appropriate HTF is important for minimising the cost of the solar receiver, thermal storage and heat exchangers, and for achieving high receiver and cycle efficiencies. Current molten salt HTFs have high melting points (142–240 °C) and degrade above 600 °C. Sodium’s low melting point (97.7 °C) and high boiling point (873 °C) allow for a much larger range of operational temperatures. Most importantly, the high temperatures of sodium allow the use of advanced cycles (e.g. combined Brayton/Rankine cycles). In this study, a comparison between the thermophysical properties of two heat transfer fluids (HTFs), Hitec (a ternary molten salt 53% KNO₃ + 40% NaNO₂ + 7% NaNO₃) and liquid sodium (Na), has been carried out to determine their suitability for use in high-temperature concentrated solar thermal central-receiver systems for power generation. To do this, a simple receiver model was developed to determine the influences of the fluids’ characteristics on receiver design and efficiency. While liquid sodium shows potential for solar thermal power systems due to its wide range of operation temperatures, it also has two other important differences – a high heat transfer coefficient (～an order of magnitude greater than Hitec) and a low heat capacity (30–50% lower than Hitec salt). These issues are studied in depth in this model. Overall, we found that liquid sodium is potentially a very attractive alternative to molten salts in next generation solar thermal power generation if its limitations can be overcome.     

Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material

This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain form-stable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity.     

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.     

Energy and exergy analysis of timber dryer assisted heat pump

In this research, poplar and pine timbers have been dried from the moisture contents of 1.28 kg water/kg dry matter and 0.60 kg water/kg dry matter to 0.15 kg water/kg dry matter in heat pump dryer functioning on the basis of 24 h operation. The change in weight in all of the timbers was followed in the drying chamber and drying stopped when the desired weight was achieved. At 40 °C dry bulb temperature, 0.8 m/s air velocity, and initial moisture content of the poplar timbers 1.28 kg water/kg dry matter, the moisture content was reduced to 0.15 kg water/kg dry matter moisture content in 70 h, and the moisture content of the pine timbers which was 0.60 kg water/kg dry matter was reduced to the same amount in 50 h. All data collected while drying were saved on computer and analysed afterwards. For this system, energy analysis was made to determine the energy utilization. Exergy analysis was accomplished to determine of exergy losses during the drying process.     

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.     

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.     

Experimental study of a compact PCM solar collector

The performance of a compact phase change material (PCM) solar collector based on latent heat storage was investigated. In this collector, the absorber plate–container unit performs the function of both absorbing the solar energy and storing PCM. The solar energy was stored in paraffin wax, which was used as a PCM, and was discharged to cold water flowing in pipes located inside the wax. The collector's effective area was assumed to be 1 m² and its total volume was divided into 5 sectors. The experimental apparatus was designed to simulate one of the collector's sectors, with an apparatus-absorber effective area of 0.2 m². Outdoor experiments were carried out to demonstrate the applicability of using a compact solar collector for water heating. The time-wise temperatures of the PCM were recorded during the processes of charging and discharging. The solar intensity was recorded during the charging process. Experiments were conducted for different water flow rates of 8.3–21.7 kg/h. The effect of the water flow rate on the useful heat gain (Qu) was studied. The heat transfer coefficients were calculated for the charging process. The propagation of the melting and freezing front was also studied during the charging and discharging processes. The experimental results showed that in the charging process, the average heat transfer coefficient increases sharply with increasing the molten layer thickness, as the natural convection grows strong. In the discharge process, the useful heat gain was found to increase as the water mass flow rate increases.     

Economic evaluation of latent heat thermal energy storage using embedded thermosyphons for concentrating solar power applications

An economic evaluation of a latent heat thermal energy storage (LHTES) system for large scale concentrating solar power (CSP) applications is conducted. The concept of embedding gravity-assisted wickless heat pipes (thermosyphons) within a commercial-scale LHTES system is explored through use of a thermal network model. A new design is proposed for charging and discharging a large-scale LHTES system. The size and cost of the LHTES system is estimated and compared with a two-tank sensible heat energy storage (SHTES) system. The results suggest that LHTES with embedded thermosyphons is economically competitive with current SHTES technology, with the potential to reduce capital costs by at least 15%. Further investigation of different phase change materials (PCMs), thermosyphon working fluids, and system configurations has the potential to lead to designs that can further reduce capital costs beyond those reported in this study.     

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.     

Performance of a dual-purpose solar continuous adsorption system

A conceptual design and performance of a dual-purpose solar continuous adsorption system for domestic refrigeration and water heating is described. Malaysian activated carbon and methanol are used as the adsorbent–adsorbate pair. The heat rejected by the adsorber beds and condensers during the cooling process of the refrigeration part is recovered and used to heat water for the purpose of domestic consumption. In a continuous 24-h cycle, 16.9 MJ/day of heat can be recovered for heating of water in the storage tanks. In the single-purpose intermittent solar adsorption system, this heat is wasted. The total energy input to the dual-purpose system during a 24-h operation is 61.2 MJ/day and the total energy output is 50 MJ/day. The latter is made up of 44.7 MJ/day for water heating and 5.3 MJ/day for ice making. The amount of ice that can be produced is 12 kg/day. Using typical value for the efficiency of evacuated tube collector of water heating system of 65%, the following coefficient of performances (COP's) are obtained: 44% for adsorption refrigeration cycle, 73% for dual-purpose solar water heater, 9.1% for dual-purpose solar adsorption refrigeration and 82.1% for dual-purpose of both solar water heater and refrigerator.