Laboratory investigation on the use of thermally enhanced phase change material to improve the performance of borehole heat exchangers for ground source heat pumps

Ground source heat pumps have high efficiency and high capital cost primarily due to borehole drillings. This research investigates the inclusion of high‐conductivity phase change material (PCM) in the borehole heat exchanger of a ground source heat pump to reduce the borehole length required and improve its coefficient of performance (COP). In the laboratory model, the borehole heat exchanger was represented by a cylindrical electrical heater having a total power of 9.216 W, operating for 1 hour while resting for 3 hours. Surrounding the heater in the annular region, either soil, PCM, or high‐conductivity PCM was used as grouting material. The annular region was surrounded by a large amount of soil enclosed in a large bin as a representation of ground soil. The high‐conductivity graphite was impregnated with the commercial PCM “PureTemp29.” Results from the experiments revealed that the PCM is able to decrease the temperature fluctuations in the annular and soil regions, while graphite increases the thermal conductivity of the annular region and hence increases the rate of heat dissipation from the heater to the soil surrounding it. The maximum COP values of a ground source heat pump calculated assuming ideal reversed Carnot cycle for cooling mode showed an increase of approximately 81% with PCM and by 112% with graphite‐enhanced PCM.     

Experimental study on a small Brayton air refrigerator under −120 °C

A small Brayton air refrigerator with high-speed gas bearing turbo-expander and compact plate-fin heat exchanger was designed and fabricated. Performances of the two key components (turbo-expander and heat exchanger) and the refrigerator as a whole were investigated. The bearings used in the refrigerator were compliant foil journal bearings with elastic support, and stability inspection was conducted on them. The results indicate that the refrigerator had a high cooling capability, and that the gas bearings used here achieved good performance in a wide speed region up to 250,000 rpm. While the expander operated at its design speed (220,000 rpm), the refrigerator had a cooling capacity of 1500 W at ?80 °C, and the lowest no-load refrigeration temperature of ?124.5 °C was obtained. Moreover, the influence of nozzle number and velocity ratio on the turbo-expander and the effect of flow rate and cold inlet temperature on the heat exchanger are discussed in detail.     

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

The use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy-porosity model. The study includes the effect of inlet temperature and flow rate of high-temperature-fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.     

Performance simulation of a low‐swirl burner for a Stirling engine

A new type of gas burner for Stirling engine that can recover adequate heat from exhaust gas was designed based on the plate heat exchanger and low‐swirl combustion technology, which consists of three components: a cyclone, a burner, and a circular plate heat exchanger. The circular plate heat exchanger tightly wound around the combustion chamber plays a high efficiency of heat recovery role. In consideration of the radial symmetry of the burner, a three‐dimensional numerical simulation was carried out by Ansys15. The velocity distribution, temperature distribution, and pressure distribution of the combustion gas were presented respectively. Strong backflow that came from the exhaust gas around the root of the flame in the combustion chamber and a vortex below the inlet of the exhaust gas channel were found, which were beneficial for the combustion and improving the uniformity of temperature distribution. Combustion behaviors of the burner under standard operating conditions were obtained, the highest temperature was about 2200 K in burner and the exhaust gas entered the plate heat exchanger at the temperature of 1375 K and exited at 464 K, with the waste heat recovery efficiency over 65.8%. And, the air‐fuel ratio and combustion power had negligible effect on the waste heat recovery efficiency.     

Analysis of an encapsulated phase change material‐based energy storage system for high‐temperature applications

The capability of an encapsulated phase change material (EPCM)‐based thermal energy storage (TES) system to store a large fraction of latent energy at high temperatures was examined. A 3‐dimensional simulation of a prototype heat exchanger was conducted employing sodium nitrate as the phase change material (PCM). The k‐ω SST model was used to capture the turbulent flow of the HTF, while the melting front was tracked using the enthalpy‐porosity method. The results show that the use of metal deflectors yields a nearly constant heat transfer coefficient over the capsule's surface. Despite this, the presence of the void in the capsule and natural convection within the molten PCM influenced the storage characteristics of the system affecting the shape of the isotherms and melting front. Furthermore, the EPCM capsules consecutively undergo the same heat transfer starting from the capsule closest to the inlet. The EPCM capsules store 80% of the energy lost by the HTF. The 17.7 kg of sodium nitrate stores 14.5 MJ of energy where 20% of the energy stored is via latent heat. Of the energy released by the heat transfer fluid, 80% was absorbed by the EPCM capsules with the remaining energy going into the test section walls. A total of 14.5 MJ of energy was stored by the 17.7 kg of NaNO₃, of which 20% is attributed to the latent heat. The fraction of energy stored as latent heat would be larger if a smaller operating temperature range was used. Thus, an EPCM‐based latent heat TES system is capable of storing a large fraction of the supplied energy and presents efficient means of storing thermal energy for high‐temperature applications. Additionally, the strong agreement between the numerical and experimental works demonstrates that the numerical methods employed can predict the behavior of an EPCM capsule not only within a single capsule but on the system scale as well. Therefore, the applied numerical methods can be used for further design and optimization of EPCM‐based latent heat TES systems.     

Cascaded latent heat storage for parabolic trough solar power plants

The current revival of solar thermal electricity generating systems (SEGS) unveils the still existing need of economic thermal energy storages (TES) for the temperature range from 250 °C to 500 °C. The TES-benchmark for parabolic trough power plants is the direct two tank storage, as it was used at the SEGS I plant near Barstow (USA). With the introduction of expensive synthetic heat transfer oil, capable to increase the operating temperature from former 300 °C up to 400 °C, the direct storage technology became uneconomical. Cascaded latent heat storages (CLHS) are one possible TES alternative, which are marked by a minimum of necessary storage material. The use of a cascade of multiple phase change materials (PCM) shall ensure the optimal utilization of the storage material.This paper reports experimental and numerical results from the investigation of cascaded latent heat storages with alkali nitrate salts like NaNO₃, KNO₃ and others more. The experiments were conducted with vertical shell and tube type heat exchanger devices under realistic operation parameters. The experimental results were used for a numerical model to simulate different CLHS configurations. Dymola/Modelica was used to conduct the simulation. The outcome of this work shows on the one hand, that the design of CLHS for this temperature range is more complex than for the temperature range up to 100 °C. And on the other hand, the low heat conductivity of available PCM is an obstacle which must be overcome to make full use of this promising storage technology.     

Performance analysis of a novel hydrogen production system incorporated with hybrid solar collector and phase change material

In this technical article, a novel experimental setup is designed and proposed to produce a hydrogen by using solar energy. This system comprises a hybrid or photovoltaic Thermal (PVT) solar collector, Hoffman's voltameter, heat exchanger unit and Phase Change Material (PCM). The effect of PCM and mass flow rate of water on the hybrid solar collector efficiency and hydrogen yield rate is studied. This experimental results clearly showed that by adding the thermal collector with water, decreases PV module temperature by 20.5% compared with conventional PV module. Based on the measured values, at 12.00 and 0.011 kg/s mass flow rate, about 33.8% of thermal efficiency is obtained for water based hybrid solar collector. Similarly, by adding Paraffin PCM to the water based thermal collector, the maximum electrical efficiency of 9.1% is achieved. From this study, the average value of 17.12% and 18.61% hydrogen yield rate is attained for PVT/water and PVT/water with PCM systems respectively.     

Heat storage in direct-contact heat exchanger with phase change material

This paper describes the development and performance of a direct-contact heat exchanger using erythritol (melting point: 391 K) as a phase change material (PCM) and a heat transfer oil (HTO) for accelerating heat storage. A vertical cylinder with 200-mm inner diameter and 1000-mm height was used as the heat storage unit (HSU). A nozzle facing vertically downward was placed at the bottom of the HSU. We examined the effects of flowrate and inlet temperature of the HTO using three characteristic parameters of heat storage – difference between inlet and outlet HTO temperatures, temperature effectiveness, and heat storage rate. The temperature history of latent heat storage (LHS) showed three stages: sensible heat of solid PCM, latent heat of PCM, and sensible heat of liquid PCM. Further, the operating mechanism of the DCHEX was proposed to explain the results. The average heat storage rate during LHS was proportional to the increase in flowrate and inlet temperature of HTO. Thus, latent heat can be rapidly stored under large HTO flowrate and high inlet temperature in the DCHEX.     

Effect of thermo physical properties of heat exchanger material on the performance of latent heat storage system using an enthalpy method

In this paper, a simple two‐dimensional theoretical model based on enthalpy formulation of a latent heat storage system has been developed to study the effects of thermo physical properties of heat exchanger container materials on the thermal performance of the storage system. Numerical results show that thermal conductivity, specific heat and density of the heat exchanger container materials increases, the melting time of the PCM decreases. Numerical results also show that high value of thermal conductivity of the heat exchanger container materials did not make significant contribution on the melt fraction. It is also found that initial temperature of the PCM does not have very important effects on the melting time, while the boundary wall temperature play an important role during melting. Copyright © 2005 John Wiley & Sons, Ltd.     

Working fluids for low-temperature heat source

The performance of different working fluids to recover low-temperature heat source is studied. A simple Rankine cycle with subcritical configuration is considered. This work is to screen working fluids based on power production capability and component (heat exchanger and turbine) size requirements. Working fluids considered are R134a, R123, R227ea, R245fa, R290, and n-pentane. Energy balance is carried out to predict operating conditions of the process. Outputs of energy balance are used as input for exergy analysis and components (heat exchanger and turbine) design. The heat exchanger is divided into small intervals so that logarithmic mean temperature difference (LMTD) method is applicable. R227ea gives highest power for heat source temperature range of 80–160 °C and R245fa produces the highest in the range of 160–200 °C. There is optimal pressure where the heat exchanger surface area is minimum. This optimal pressure changes with heat source temperature and working fluid used. The least heat exchanger area required at constant power rating is found when the working fluid is n-pentane. At lower heat source temperature (80 °C), the maximum power output and minimum heat exchanger surface area for different working fluids is comparable.     

内可逆空气制冷机性能的生态学优化

基于[火用]分析的观点，运用有限时间热力学方法对内可逆空气制冷机进行生态学优化，导出了换热器热导最优分配时的最佳制冷功率、熵产率以及生态学（E）目标函数的解析式，进一步求得最大E目标值时的工质等熵温比（压比）界限及相应的制冷系数、制冷功率和熵产率；采用数值计算分析了热源温比、换热器总热导以及高温热源温度和环境温度之比对该制冷机生态学最优性能的影响。结果表明：生态学目标函数不仅反映了[火用]输出率和熵产率之间的最佳折衷，而且也反映了制冷功率和制冷系数之间的最佳折衷。     

Performances of a heat exchanger and pilot boiler for the development of a condensing gas boiler

In this research, design factors for a heat exchanger and boiler were investigated using a simplified model of a heat exchanger and pilot condensing boiler, respectively. Specifications of each heat exchanger component (e.g., upper heat exchanger (UHE) and lower heat exchanger (LHE); coil heat exchanger (CHE); baffles) were investigated using a model apparatus, and the comprehensive performance of the pilot gas boiler was examined experimentally. The heating efficiency of the boiler developed was about 90% when using the optimal designed heat exchangers. Compared to a conventional Bunsen-type boiler, the heating efficiency was improved about 10%. Additionally, NO_x and CO emissions were about 30 ppm and 160 ppm, respectively, based on a 0% O₂ basis at an equivalence ratio of 0.70, which is an appropriate operating condition. However, the pollutant emission of the boiler developed is satisfactory considering the emission performance of a condensing boiler, even though CO emission must be reduced.     

Thermoelectric conversion of waste heat from IC engine‐driven vehicles: A review of its application,issues, and solutions

Thermoelectric generator (TEG) is a promising thermoelectric (TE) conversion technology to effectively recover and convert waste heat from vehicle exhaust into useful energy, ie, electricity. Exhaust TEG (ETEG) is a system that is incorporated into the exhaust manifold of a vehicle. Exhaust TEG comprises of a heat exchanger, TEG modules, heat sink, and power conditioning unit. The present work reviews different vehicular ETEGs based on engine type, engine‐rated power, type and number of TEG module, efficiency of ETEG and TEG, exhaust and coolant temperature, and power output of ETEG . In addition to these, the technical issues faced in these ETEGs are addressed under 2 categories, viz., primary (TEG with low ZT TE material and inefficient heat exchanger and heat sink) and secondary issues (low operating temperature TEG modules and installation position of ETEG). In addition to it, effects of vibration and thermal cycling of exhaust system on TEG modules that may arise in ETEG are also discussed. A review of preventive solutions to the issues is also presented. Finally, the economic aspects of an ETEG are also discussed. The review highlights the need of commercialization of TE materials with ZT > 2, high‐temperature operating range, and segmented TEG modules in large volumes so that their practice can be extended in vehicular applications. Heat exchanger modeling using computational fluid dynamics and interfacing with heat transfer theory is essential to maintain temperature uniformity across the TEG modules. Installation of ETEG in the exhaust pipe should be such that it does not affect the performance of the engine. It is also realized that sturdy TEG modules should be developed for long‐term operation to prevent degradation due to mechanical vibration and thermal cycling of the vehicle. Further, ETEG is economically beneficial in vehicles such as trucks owing to availability of high thermal energy in their exhaust stream.     

Size Effects of Miniature Refrigeration and Liquefaction System

This article examines the size effects on the performance of miniature refrigerators and liquefiers operated by the Linde cycle. The system sizes are cased into a function of the compressor characteristic length and the heat exchanger length while several cycle operation parameters are held constant. Simplified models of a Hampson-type counterflow heat exchanger and a reciprocating-type compressor were considered in the present analysis.

For both the refrigerator and the liquefier, it was found that only for certain ranges of the compressor size is the system able to produce a heat exchanger effectiveness greater than the required minimum value. It was also found that there exists an optimal compressor size for obtaining maximum heat exchanger effectiveness, cooling effect, mass fraction of liquefied product, and coefficient of performance. For the refrigerator, the optimal compressor size for obtaining the maximum heat exchanger effectiveness is different from that for obtaining the maximum cooling effects because of the mass flow rate effect. For the liquefier, the optimal compressor sizes for obtaining the maximum heat exchanger effectiveness, mass fraction of liquefied gas product, and FOM are approximately the same.

When the Claude cycle is employed, it is found that it theoretically offers a more satisfactory performance than the Linde cycle in the small size range. The FOM of a mesoscale system with the Claude cycle can reach the range of 20 to 23% with the liquid product temperature ranging from 65 to 90 K.     

Heat extraction from salinity-gradient solar ponds using heat pipe heat exchangers

This paper presents the results of experimental and theoretical analysis on the heat extraction process from solar pond by using the heat pipe heat exchanger. In order to conduct research work, a small scale experimental solar pond with an area of 7.0 m² and a depth of 1.5 m was built at Khon Kaen in North-Eastern Thailand (16°27′N102°E). Heat was successfully extracted from the lower convective zone (LCZ) of the solar pond by using a heat pipe heat exchanger made from 60 copper tubes with 21 mm inside diameter and 22 mm outside diameter. The length of the evaporator and condenser section was 800 mm and 200 mm respectively. R134a was used as the heat transfer fluid in the experiment. The theoretical model was formulated for the solar pond heat extraction on the basis of the energy conservation equations and by using the solar radiation data for the above location. Numerical methods were used to solve the modeling equations. In the analysis, the performance of heat exchanger is investigated by varying the velocity of inlet air used to extract heat from the condenser end of the heat pipe heat exchanger (HPHE). Air velocity was found to have a significant influence on the effectiveness of heat pipe heat exchanger. In the present investigation, there was an increase in effectiveness by 43% as the air velocity was decreased from 5 m/s to 1 m/s. The results obtained from the theoretical model showed good agreement with the experimental data.     

Technical feasibility assessment of a PEM fuel cell refrigerator system

PEM Fuel Cells (PEMFCs), fueled by hydrogen, are electrochemical devices that convert hydrogen to useful power and two by-products: heat and water. They cover an important part of power applications namely in the transportation area, and in other practical applications that are either stationary or portable. In particular, the domestic refrigerator is one of the daily and indispensable applications but with a high-energy demand due to the high running time cycles. This work is a technical assessment of the feasibility of building a coupled “PEM Fuel Cell – Refrigerator” system. Real technical data for the refrigerator are collected, processed and evaluated. The obtained results show reasonable flows consumption rates. In fact, the refrigerator requires a flow rate of 1.607 slpm of hydrogen and 8 slpm of air at a pressure of respectively 3 atm and 1 atm. The water is produced at a rate of 1.285 10⁻³ slpm. The annual amount of hydrogen consumed by the refrigerator is estimated to 28, 47 kg. The energy provided to the refrigerator is about 130 W and the energy needed by the air compressor is 28, 24 W. A technical solution is suggested at the end of this work to reduce the start and stop cycles of the fuel cell.     

New method to evaluate the heat storage density in latent heat storage for arbitrary temperature ranges

Phase change materials (PCM) are used to store heat or cold in narrow temperature intervals with high storage density. In applications with an exact temperature range, the heat storage density can be calculated straight forward and different PCM can be compared. However, in many applications no exact temperature range is known or the temperature range is not fixed. In such cases, the evaluation of the storage density and the comparison of different PCM are quite difficult and the standard approaches do not give accurate and easy to read results. In this paper we present a new method that is simple, accurate, and allows a visual evaluation of the heat storage density for arbitrary temperature ranges. This is possible by plotting the enthalpy difference in a 2-dimensional contour plot with the upper and lower storage temperatures as the two dimensions. In a second step, the temperature differences used for heat transfer, for example at a heat exchanger, can be included. This way, the new method can be used as an aid in the design of a storage and for its technical and economical optimization.     

Experimental study on small-scale hydrogen liquefaction of 0.5 L/h

A small-scale hydrogen liquefaction device based on two-stage G-M refrigerator was designed and manufactured. Many practical operation processes on the liquefaction device were conducted in the open-air test base. The experimental results shown that 1) The direct liquefaction scheme of micro-positive pressure with normal temperature hydrogen realized by two-stage pressure reducing valve was feasible and effective; 2) Design of four-stage heat exchanger for G-M refrigerator cold head was reasonable and reliable; 3) The liquefaction rate in pure hydrogen was 0.47 L/h, and liquefaction pressure can maintain the range of about 120 kPa～160 kPa; 4) After venting hydrogen-helium mixture, the liquefaction rate of hydrogen was 0.439 L/h again. In other words, the previously filled helium in the liquid hydrogen Dewar could be discharged through multiple venting method. The residue helium had little effect on the hydrogen liquefaction rate; 5) The scheme of simultaneous liquefaction and transmission was proved to be feasible; 6) Operation process experience and safety precautions on the hydrogen liquefaction were summarized. The testing results provided a technical support for design and operation of small-scale hydrogen liquefactions.     

Characterization of melting and solidification in a real-scale PCM–air heat exchanger: Experimental results and empirical model

This paper describes the experimental studies carried out to test thermal cycling of a real-scale PCM–air heat exchanger at ambient temperatures. To achieve this goal an experimental setup previously designed and used for testing real-scale prototypes of PCM–air heat exchangers is modified. The PCM used is commercially available, organic, and paraffin based. The total energy exchanged during melting and solidification, as well as the time elapsed until total melting/solidification are determined from the power curves experimentally obtained. The influence of the inlet air temperature and air flow is studied, and results show that the continuous thermal cycling of the unit is a repetitive process: running experiments with similar conditions leads to the same thermal behavior, no degradation in the PCM properties is noticed. Pressure drop is measured for different air flows. Depending on the inlet air temperature, full solidification of the PCM could be achieved in less than 3 h for an 8 °C temperature difference between the inlet air and the average phase change of the PCM. Average thermal powers of up to 4.5 kW and 3.5 kW for 1 h are obtained for melting and solidification stages, respectively. An empirical model is developed from the experimental results, which could be a useful designing tool for applications that use such technology: green housing, curing and drying processes, plant production, HVAC, and free-cooling.     

On the coupling of PCM stores to active solar systems

This paper investigates the problems arising in the combination of active solar systems with PCM storage matrices. A new heat exchanger design is used, based on the rolling cylinder principle. The store is managed by a dedicated microcomputer which interacts with the system control computer. The store computer configures the store in real time to operate it in near optimal conditions, according to the current system operating point.