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For design and optimization purposes, understanding the performance characteristics of different configurations of PCM systems during charging and discharging processes is crucial, particularly for thermal storage in CSP. A numerical study using FLUENT has been performed to explore and compare the characteristics and behavior of PCM in different shell and tube configurations; namely counter flow and parallel flow in vertical and horizontal orientations. These designs have been compared with that of an alternative design using plate fins. The main practical advantage of the alternative design is decoupling the PCM location from the path of flow of the heat transfer fluid that reduces the constraints on the materials. Confined heating/cooling pipes in a channel at the bottom of the PCM enclosure makes it more flexible to match the required working pressures, and simplifies the design and material (PCM/plate) selection. This is a significant issue as higher temperatures and pressures are being required for improving the overall solar thermal system performance. Based on the same heat transfer area, amount of PCM and input energy, a performance comparison was determined based on exergy maximization. Using sodium nitrate as the PCM, the results show a higher heat transfer rate for the vertical arrangement of plate fins compared to the counter flow shell and tube configurations, as well as a more uniform heat transfer rate compared to all shell and tube arrangements for both the charging and discharging processes. The proposed design leads to less redundant PCM, as well as a smaller and more cost effective PCM system as a heat storage unit. Overall, the vertical plate fin configuration maximizes the useful heat that can be extracted for a given amount of heat and PCM, which has direct applicability to thermal storage solutions for CSP and other applications. Moreover, its modular design facilitates the selection and optimisation for different scales and applications e.g. as a heat storage for electricity production or as a protection system for a receiver from high thermal stresses in a CSP plant. [ABSTRACT FROM AUTHOR]

A numerical study has been conducted on a shell and tube latent heat storage system whereby the inlet heat transfer fluid direction is periodically reversed during charging and discharging. The impact of varying the boundary condition at the interface of the tubes carrying the heat transfer fluid and phase change material (PCM) on the evolution of the phase change front, heat transfer area and heat transfer rate have been evaluated during the charging and discharging processes. Results for the charging processes show a higher heat transfer area develops during the early stages and amplification of natural convection after 40% melt fraction, leading to a higher heat transfer rate. In comparison to the fixed flow condition, periodic flow reversal for the discharge cases results in an increased heat transfer area for a longer period of time, leading to a higher heat transfer rate particularly after 75% solidification. This effect is more important for discharging cases in the absence of convection heat transfer. Periodically reversing the direction of heat transfer fluid, which produced a periodic boundary condition at the tube-PCM interface, also resulted in a lower temperature gradient in space and time and consequently higher exergy recovery, and about a 6% increase in the time-average heat transfer rate in the charging and discharging cases. The novel reversal flow method provides a means to implement a periodic boundary condition without changing the heat source/sink, enhancing the thermal performance and cost effectiveness of latent heat storage systems. Phase change storage systems incorporating periodic flow reversal provide higher energy delivery rates, greater power density and more exergy recovery. This method can support fast heat release to respond to a peak load in a CSP plant or fast heat storage to protect a tubular receiver from high thermal stresses. [ABSTRACT FROM AUTHOR]

This paper presents a new TRNSYS model of a refrigeration system incorporating phase change material (PCM) for mobile transport. The PCTSU is charged by an off-vehicle refrigeration unit and the PCM provides cooling when discharging and the cooling released is utilized to cool down the refrigerated space. The advantage of this refrigeration system compared to a conventional system is that it consumes less energy and produces significantly lower greenhouse gas emissions. A refrigeration system for a typical refrigerated van is modelled and simulations are performed with climatic data from four different locations. The main components of the TRNSYS model are Type 88 (cooling load estimation) and Type 300 (new PCTSU component), accompanied by other additional components. The results show that in order to maintain the temperature of the products at −18 °C for 10 h, a total of 250 kg and 390 kg of PCM are required for no door opening and 20 door openings during the transportation, respectively. In addition, a parametric study is carried out to evaluate the effects of location, size of the refrigerated space, number of door openings and melting temperature of the PCM on the thermal performance. [ABSTRACT FROM AUTHOR]

Abstract: An innovative refrigeration system incorporating phase change material (PCM) is proposed to maintain refrigerated trucks at the desired thermal conditions. The advantage of using PCM to maintain low temperatures is that a conventional refrigeration system does not have to be located on-board the vehicle. In addition, the system consumes less energy and produces much lower local greenhouse gas (GHG) emissions. The phase change thermal storage unit (PCTSU) is charged by a refrigeration unit located off the vehicle when stationary. The PCM is discharged and provides cooling when in service. A new PCM with a lower cost than currently available PCMs was developed, suitable for maintaining the refrigerated truck at a temperature of −18°C. The PCM has a melting temperature of −26.7°C and a latent heat of 154.4kJkg−1. A prototype system was constructed and test results proved that the proposed refrigeration system is feasible for mobile transport. An analysis shows that delivery of refrigerated products can be made with a PCM system having a weight comparable to that of an on board conventional refrigeration system with less than half of the energy cost. [Copyright &y& Elsevier]

Highlights • Radiant wall used as TES for a heat pump coupled to a PV array. • Deterministic control concepts were optimized for radiant wall pre-cooling. • Investment in PV panels was fully exploited only under solar control strategies. • A minimum of PV nominal power was required for optimizing solar controls. • Best solar control had lower operation cost than best peak load shifting control. Abstract Thermally activated building systems (TABS) can work as thermal energy storage (TES) systems, which are useful in shifting the energy use of space cooling and heating in buildings. The present study analyses and optimizes simple deterministic control concepts for radiant wall supplied by a heat pump for cooling purposes. First, the "solar" concept was studied, which was focused on exploiting the output of a photovoltaic (PV) array. Secondly, a "peak load shifting" concept exploiting the low electricity cost and high heat pump energy efficiency during night periods was evaluated. The results showed that the "solar" concept saved between 57% and 95% in comparison to a conventional control in different PV installed capacities. Moreover, the optimized "peak load shifting" concept had lower operation cost than the conventional control with most of the PV configurations proposed. Therefore, the study showed that the investment in the PV array was fully harnessed only with specific controls. Furthermore, the "solar" control concepts were found to help achieving the goals of net-zero energy buildings by maximising self-consumption of renewable energies in the building, as well as reducing the total imported/exported energy. [ABSTRACT FROM AUTHOR]

In the present paper, an experimental study is carried out to evaluate the effect of the dynamic melting concept in a cylindrical shell-and-tube heat exchanger using water as the phase change material (PCM) and a potassium formate/water solution as the heat transfer fluid (HTF). The dynamic melting concept is a new heat transfer enhancement technique which consists of recirculating the liquid PCM during the melting process with a pump and thus increasing the overall heat transfer coefficient as a result of the dominance of the forced convection. The HTF flow rate was kept constant at 1 l/min and four different PCM flow rates of 0, 0.5, 1 and 2 l/min were tested. Results from the experimental analysis showed enhancements up to 65.3% on the melting period, up to 56.4% on the effectiveness, and 66% on the heat transfer rates when the PCM flow rate was twice the HTF flow rate. From these experiments, it can be concluded that dynamic melting is an effective technique for enhancing heat transfer during melting of PCM. [ABSTRACT FROM AUTHOR]

The intermittency of renewable energy systems remains one of the major hurdles preventing a large scale uptake of these technologies and concentrated solar power (CSP) systems are no different. However, CSP has the benefit of being able to store excess heat using thermal energy storage (TES). For the uptake of CSP with TES it must be demonstrated that the technology is both economically as well as environmentally feasible. This paper aims to investigate the economic and environmental impact of several TES options that are available for CSP systems. The investigated systems include an encapsulated phase change material (PCM) system, a coil-in-tank PCM system and a liquid sodium TES system. The economic impact in the current study refers to the capital cost (CAPEX) of each system including the tank, storage material, encapsulation cost (if applicable) and allowances for construction and engineering. The environmental impact of each system is accounted by calculating the embodied energy of each of the system components. Each storage system will be required to store a comparable amount of energy so that reliable conclusions can be drawn. The results from this analysis conclude that the encapsulated PCM (EPCM) and coil-in-tank system represent an embodied energy of roughly one third of the corresponding state-of-the-art two-tank molten salt system. Furthermore, the EPCM and coil-in-tank systems result in CAPEX reductions of 50% and 25% over the current state-of-the-art two-tank molten salt system. The liquid sodium system was found to result in higher embodied energy and CAPEX than any previously studied TES system. Finally, the advantages and disadvantages of each system was discussed and compared to previous literature. [ABSTRACT FROM AUTHOR]