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The integration temperature difference Δ T i considers the heat transfer routes, linking the heat transfer process with the thermodynamic behavior of heat exchangers. The first and second non-dimensional integration temperature differences are defined as Δ T i, h ∗ = Δ T i / T h, i and Δ T i, s ∗ = Δ T i / ( T h, i - T 0 ) respectively, where T h,i is the heat source temperature and T 0 is the environment temperature. This paper is the first to experimentally verify the significance of the non-dimensional integration temperature differences on organic Rankine cycle (ORC) systems. The first non-dimensional temperature difference is shown to have linear relationship with the revised entropy generation numbers ( N s ). With increases of the second non-dimensional integration temperature difference, the expander powers, system thermal and exergy efficiencies had parabola distributions. They simultaneously reached maximum at Δ T i, s ∗ = 0.282 , under which the vapor cavitation in the expander disappears and the exergy losses of heat exchangers are acceptable to elevate the expander efficiency. Beyond the optimal point, the ORC performance is worsened either due to the vapor cavitation in the expander, or due to the poor thermal matches in the evaporator and condenser. The second non-dimensional integration temperature difference comprehensively reflects the effects of heat source temperatures, heating powers and organic fluid flow rates and pressures, etc. It balances exergy destructions of various components to optimize the system. Thus, it can be an important parameter index to maximize the power or electricity output for a specific heat source. The usefulness of the integration temperature difference and the future work are discussed in the end of this paper. [ABSTRACT FROM AUTHOR]

Latent heat thermal energy storage is an attractive technique as it can provide higher energy storage density than conventional heat energy storage systems and has the capability to store heat of fusion at a constant (or a near constant) temperature corresponding to the phase transition temperature of the phase change material (PCM). This paper provides a state-of-the-art review on phase change materials (PCMs) and their applications for heating, cooling and electricity generation according to their working temperature ranges from (−20 °C to +200 °C). Four working temperature ranges are considered in this review: (1) the low temperature range from (−20 °C to +5 °C) where the PCMs are typically used for domestic and commercial refrigeration; (2) the medium low temperature range from (+5 °C to +40 °C) where the PCMs are typically applied for heating and cooling applications in buildings; (3) the medium temperature range for solar based heating, hot water and electronic applications from (+40 °C to +80 °C); and (4) the high temperature range from (+80 °C to +200 °C) for absorption cooling, waste heat recovery and electricity generation. Different types of phase change materials applied to each temperature range are reviewed and discussed, in terms of the performance, heat transfer enhancement technique, environmental impact and economic analysis. The review shows that, energy saving of up to 12% can be achieved and a reduction of cooling load of up to 80% can be obtained by PCMs in the low to medium–low temperature range. PCM storage for heating applications can improve operation efficiency from 26% to 66%, depending on specific applications. Solar thermal direct steam generation (DSG) is the most common electricity generation application coupled with PCM storage systems in the high temperature range, due to the capability of PCMs to store and deliver energy at a given constant temperature. The recommendations for future research are also presented which provide insights about where the current research is heading and highlights the challenges that remain to be resolved. [ABSTRACT FROM AUTHOR]

Ground Source Heat Pump (GSHP) systems, if improperly designed may lead to overheating or overcooling of the ground. Good designs ensure properly balanced energy storage through adequately sizing ground-loops. Hybrid systems combine conventional Heating, Ventilating, and Air Conditioning (HVAC) with GSHPs in order to significantly reduce the high installation costs of GSHPs. The hybrid systems are designed in such a way that GSHPs provide the base building energy demands while the conventional HVAC is used only during the peak hours. In general, all buildings can be divided into two main categories: cooling dominant and heating dominant. If a building is cooling dominant, ground temperature increases with time and in heating dominant cases it decreases. A severe ground temperature increase/decrease is referred to as ‘ground fouling’ because it can render the GSHP inoperable, as temperature differences are required to maintain controlled heat flow. This paper compares long-term operation of hybrid and non-hybrid GSHP systems in order to investigate the effectiveness of hybridization at alleviating ‘ground fouling’. A homespun 2D finite-volume model is proposed to study heat transfer in ground coupled heat pump systems and is verified against an analytical solution as well as experimental data. Through simulation of different building types, it is demonstrated that hybridization has potential to reduce ‘ground fouling’ but only in limited cases for which a large portion of the energy demands is being met by the conventional HVAC. [ABSTRACT FROM AUTHOR]

A novel linear Fresnel reflector which employs the evacuated tube, CPC secondary reflector, and uses molten salt as the heat transfer fluid (HTF) was designed and studied in this paper. A 3D optical model was developed to simulate the radiation transmission within the system with Monte Carlo Ray Tracing (MCRT) method. Based on the model, firstly, the optical performance of the systems using cylindrical and parabolic mirrors was compared. Then the local solar flux distribution on the absorber surface and the optical efficiency were computed. Then the effects of the slope error, time and location, etc. were investigated. Finally, the thermal performance was investigated by coupling the MCRT with the Finite Volume Method (FVM). The optical simulation results indicate that the system with optimized cylindrical mirrors can achieve nearly the same performance as the one with parabolic mirrors. The solar flux distribution on the absorber exhibits a non-uniform characteristic which can be improved by using mirrors with proper slope error. The instantaneous optical efficiency of 65.0% at normal incidence and the annual mean optical efficiency which ranges between 55.2% and 34.8% from the equator to N50° can be achieved. The numerical results indicate that the temperature profiles on the absorber follow the non-uniform solar flux. The collector efficiencies are all above 46.0% under the studied conditions. Both the thermal efficiency and the collector efficiency increase with decreasing salt temperature and with increasing radiation. These results suggest that the introduced system is a feasible choice for using molten salt as the HTF in Fresnel system. [ABSTRACT FROM AUTHOR]

The heat transfer through a vertical stack of rectangular cavities filled with phase change materials (PCMs) is experimentally analysed in terms of both melting and solidification processes. This paper provides data that are useful for benchmarking and validation of numerical models that account for natural convection in the molten PCM. Two different PCMs are investigated: the free-form PCM-Rubitherm® RT 28 HC; and the microencapsulated PCM-Micronal® DS 5001 X. In terms of practical applications, the main goal is to discuss which PCM type is better for building applications. The time required for the melting and solidification fronts to reach the mid-plane of the cavities is presented as a function of the PCM type. During charging, the control-temperature value on the hot surface of the test-sample and the period of thermal-regulation are investigated. It is shown that the free PCM is preferable for the thermal control of vertical systems as both parameters are improved due to natural convection. The use of microencapsulated PCMs allows accelerating the charging process with almost no thermal stratification. However, in this case the control-temperature effect and the thermal-regulation period are both reduced. Regarding the discharging process, subcooling plays an important role during the solidification of the free PCM and its effect cannot be neglected when modelling. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation of thermal and thermodynamic performance of a receiver for a parabolic trough solar collector with perforated plate inserts is presented. The analysis was carried out for different perforated plate geometrical parameters including dimensionless plate orientation angle, the dimensionless plate spacing, and the dimensionless plate diameter. The Reynolds number varies in the range 1.02 × 10 4 ⩽ Re ⩽ 7.38 × 10 5 depending on the heat transfer fluid temperature. The fluid temperatures used are 400 K, 500 K, 600 K and 650 K. The porosity of the plate was fixed at 0.65. The study shows that, for a given value of insert orientation, insert spacing and insert size, there is a range of Reynolds numbers for which the thermal performance of the receiver improves with the use of perforated plate inserts. In this range, the modified thermal efficiency increases between 1.2% and 8%. The thermodynamic performance of the receiver due to inclusion of perforated plate inserts is shown to improve for flow rates lower than 0.01205 m 3 /s. Receiver temperature gradients are shown to reduce with the use of inserts. Correlations for Nusselt number and friction factor were also derived and presented. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the effects of inlet temperature and rib height on the fluid flow and heat transfer performances of the ribbed channel inside the high temperature heat exchanger are presented. The inlet temperature varies from 850K to 1250K and the ratio of rib height to channel height varies from 0.083 to 0.333. The results indicate that increasing the rib height can enhance the flow disturbance and hence improve the heat transfer performance. The inlet temperature has little effect on the basic structure of fluid flow and the heat transfer is enhanced due to the increased velocity. Compared to increasing the rib height, more heat can be transferred by increasing the inlet temperature with less pressure drop. The high pressure drop is more serious as the inlet temperature increases. It is proposed to use the compact heat transfer structure at the low temperature region and replace it by loose heat transfer structure at the high temperature region. It also demonstrates that the Nusselt number and friction factor are unsuitable to compare the heat transfer and pressure drop performances among different temperature conditions because the physical properties of fluid change with temperature variation. [Copyright &y& Elsevier]

Abstract: This work analyzes influence of anisotropy of anisotropic soil on the processes of heat transfer by ground heat exchangers used in ground-coupled heat pump systems. First, this paper uses the heat-source theory to develop several explicit exact solutions for heat conduction in anisotropic infinite or semi-infinite media with internal line, cylindrical-surface, or spiral-line sources. These solutions will reduce to common heat-source solutions for isotropic media. Next, these general solutions are applied to borehole ground heat exchangers and energy piles (or pile ground heat exchangers). Then, temperature responses of borehole and pile ground heat exchangers are examined in depth. Especially, the cylindrical-surface source model and the spiral-line source model are compared when applied to energy piles with spiral coils. An interesting result of this comparison is that these two models yield the almost same average temperature at the pile wall, implying these two models should be equivalent for modeling responses of energy piles with spiral coils. However, compared to the cylindrical-surface model, spiral-line model is physically reasonable but is computation-intense. Moreover, the theoretical analysis shows that anisotropy of the medium has little effect on the short-term performance of ground heat exchangers but influences the long-term temperature responses obviously. [Copyright &y& Elsevier]

Abstract: Recently, oxy-fuel combustion has received increasing attention as one of the short-term solutions for capturing CO2 from power plants. Despite their quick development, there are several issues that need further research. Combustion characteristics, heat transfer, emissions levels, optimum oxygen concentration and flue gas recycle are the main concern of the literature. All these issues may introduce several changes in power plant and boiler configuration that causes uncertainty in the final design of future installations. In this respect, oxy-fuel combustion in fluidized bed seems to present advantages in terms of flexibility, operational adaptability and simplicity of design modifications respect to air combustion. These issues have been though scarcely analysed in open literature. Oxy-fuel combustion in fluidized beds would require the use of additional heat transfer surfaces in order to control the bed temperature and the pollutant formation, minimising the CO emission and taking advantage of the fuel energy in the steam cycle. Moreover, the adaptability to different operational conditions (flue gas recycle, oxygen concentration, boiler load, etc.) inherent to fluidized bed characteristic would require an accurate location of heat transfer surfaces. To achieve this aim, this paper describes and compares the simulation results of a circulating fluidized bed boiler with an external heat exchanger, working under air and oxy-firing conditions. Calculation of fluidization conditions and comparison with different oxy-fuel combustion characteristics are presented. Results show the suitable location of heat transfer equipment in circulating fluidized bed to obtain similar temperature profiles along the boiler. A broad range of conditions have been run for different boiler designs, and several operational limitations have been set for selected boiler geometries. [Copyright &y& Elsevier]

Abstract: Thermal energy storage (TES) has shown potential in improving the overall performance in energy systems, through shifting of thermal load demand, and through matching of uneven energy availability in time and in space. Latent heat TESs demonstrate advantages over sensible heat TESs for their high storage density and small temperature swing; however, lack of accurate knowledge in novel material properties and lack in a holistic design protocol often lead to difficulties in reaching technically viable storage design. With the aim of proposing a sound latent heat based TES design-to-validation protocol, this paper covers material property characterization through Temperature-history (T-history) method, heat exchanger design through heat transfer modeling, and model validation through experimental verification. A model for submerged cylindrically finned heat exchanger latent heat storage unit with phase change material was built. The results show that performance of gelled salt-hydrate based TES can be assessed with a pure conduction based model. This material property characterization-to-model verification approach may serve as a standard in providing accurate storage design for performance evaluation. [Copyright &y& Elsevier]

Abstract: The paper presents an analysis of thermodynamic losses in thermal reservoirs due to irreversible heat transfer and frictional effects. The focus is upon applications to large-scale electricity storage for which it is the loss in availability (or exergy) that is most relevant. Accordingly, results are presented as loss coefficients which are defined as the fractional loss of the entering availability. Only losses stemming from irreversibility are considered – heat losses to the surroundings are not included in the analysis. A number of simplifying assumptions have been adopted, but the results nonetheless clearly demonstrate the dependence of the losses on operating temperatures, reservoir geometry and mode of operation, and point the way towards methods of optimisation. Estimates for a typical installation suggest that the losses are not insignificant, particularly for one-off charge and discharge (i.e., for long-term storage), but remain acceptable for cyclic operation, so as to make the use of thermal reservoirs attractive for electricity storage schemes. [Copyright &y& Elsevier]

Abstract: In one tube pass and one shell pass counter-flow heat exchangers, when both streams change temperature by different amounts, the effectiveness is defined as the temperature change for the stream with lower capacity divided by the maximum possible change and the effectiveness depends on the number of transfer units and the thermal capacity ratio. In this paper, an attempt has been made to formulate a simple-to-use method which is easier than existing approaches, less complicated and with fewer computations for accurate and rapid estimation of effectiveness in one tube pass and one shell pass counter-flow heat exchangers as a function of number of transfer units and the thermal capacity ratio. The proposed method permits estimating the exit temperature for a one tube pass and one shell pass counter-flow heat exchanger without a trial-and-error calculation. The average absolute deviations between the reported data and the proposed correlations are found to be less than 2% demonstrating the excellent performance of proposed correlation. The tool developed in this study can be of immense practical value for engineers and scientists to have a quick check on the effectiveness in one tube pass and one shell pass counter-flow heat exchangers at various conditions without opting for any experimental measurements. In particular, practice engineers would find the predictive tool to be user-friendly with transparent calculations involving no complex expressions. [Copyright &y& Elsevier]

Abstract: In the paper, two systems for solar thermal power plants (STPPs) are devised for improving the overall performance of the plant. Each one attempts to reduce losses coming from two respective sources. The systems are simulated and compared to a reference STPP. They consists on: (a) a double thermal energy storage (DTS) with different functionalities for each storage and (b) the subdivision of the solar collector field (SSF) into specialised sectors, so that each sector is designed to meet a thermal requirement, usually through an intermediate heat exchanger. This subdivision reduces the losses in the solar field by means of a decrease of the temperature of the heat transfer fluid (HTF). Double thermal energy storage is intended for keeping the plant working at nominal level for many hours a day, including post-sunset hours. One of the storages gathers a fluid which is heated up to temperatures above the nominal one. In order to make it work, the solar field must be able to overheat the fluid at peak hours. The second storage is the classical one. The combination of both allows the manager of the plant to keep the nominal of the plant for longer periods than in the case of classical thermal energy storage. To the authors’ knowledge, it is the first time that both configurations are presented and simulated for the case of parabolic through STPP with HTF technology. The results show that, if compared to the reference STPP, both configurations may raise the annual electricity generation (up to 1.7% for the DTS case and 3.9% for the SSF case). [Copyright &y& Elsevier]

Abstract: Magnetic refrigeration is an emerging technology based on the magnetocaloric effect in solid-state refrigerants. This technology offers a smaller global environmental impact than the refrigeration obtained by means of the classical vapor compression machines operating with fluids such as HFCs. The Active Magnetic Regenerative Refrigeration (AMRR) is currently the most studied ant tested magnetic cycle. It combines the regenerative properties of a high specific heat solid porous matrix with the ability of performing thermo-magnetic cycles thanks to the magnetocaloric property of the refrigerant; while a fluid pulsing through the regenerator works as a heat transfer medium. An active magnetic regenerator can provide larger temperature spans making up for the local small temperature variation of the refrigerant. In the present paper, a practical model for predicting the performance and efficiency of an AMRR cycle has been developed. The model evaluates both the refrigerant properties and the entire cycle of an AMR operating in conformity with a Brayton regenerative cycle. The magnetocaloric material of choice is gadolinium, while the heat transfer medium is liquid water. With this model can be predicted the refrigeration capacity, the power consumption and consequently the Coefficient of Performance. The results show a greater COP when compared to a classical vapor compression plant working between the same temperature levels. [Copyright &y& Elsevier]