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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]