• Energy Research

Abstract There is growing interest in using direct contact heat transfer in thermal storage with phase change materials (PCM). Previous research has predominantly focused on the heat transfer improvement mostly using liquid as the heat transfer fluid, with limited consideration for volume change and pumping losses both of which reduce the useful energy storage density of the system. An experimental investigation was undertaken using air as the heat transfer fluid and water as the PCM subject to freezing only. Unity heat exchange effectiveness was identified over the entire phase change process demonstrating the excellent heat transfer characteristics of this concept. A volume increase of 30% was measured with potential for significant reduction. Pumping losses were found to be significantly higher than expected, and should represent the primary focus of future research. If pumping losses can be reduced, gas based direct contact PCM storage can potentially achieve a higher useful storage density than conventional PCM systems which rely on a large heat exchange area.

Thermal storage systems with phase change materials are predominantly designed, analysed and optimised through numerical modelling. An alternative simplified method is being proposed for the characterisation of these phase change thermal storage systems. The method is based on the effectiveness-number of transfer units (e-NTUs) technique. A simplified mathematical representation has been analytically developed using the e-NTU technique for a cylindrical tank filled with phase change material (PCM), with heat transfer fluid flowing through tubes inside the tank. Experiments have been carried out on a cylindrical tank filled with PCM and with one, two and four coils of tubes to validate the technique. Experimental results for the systems with a high heat transfer area compare well with those calculated from the model. The results show that this technique can readily be used as a design tool for sizing and optimising a thermal storage unit with phase change materials. From this study, it may be concluded that the model based on the e-NTU technique can accurately predict the average heat exchange effectiveness of the thermal storage system with a high heat transfer surface area during charging and discharging.

Abstract Phase change materials (PCMs) can store large amounts of heat or cooling in a small amount of material, they potentially have less weight and volume compared with other thermal energy storage materials. Thermal energy storage applications such as solar hot water systems and off peak refrigeration systems are able to use PCMs to store heat or cooling. However, research has shown that the effectiveness of these systems heavily depends on the arrangement of the PCM system, which affects both the storage density and the thermal resistance to heat transfer. However, specifying as well as determining an effective PCM system has been difficult in the past because it involves using numerical modelling which is time consuming. This paper presents the results of an experimental investigation carried out on a tube-in-tank design filled with PCM for cold storage applications. The PCMs used are salt hydrate with phase change temperature of −27 °C and water. From the experimental measurements, the average heat exchange effectiveness of the storage tank was determined and a characteristic design curve has been developed as a function of the measured average NTU.

Dew point cooling (DPC) is a novel indirect evaporative cooling concept capable of delivering air temperatures approaching the dew point. Coupling this technology with CO2 refrigeration is well suited to minimising transcritical operation when the coefficient of performance (COP) is dramatically reduced in hot climates. A substantial experimental program was conducted to characterise this combination by testing a 20 kW CO2 refrigeration system subject to ambient temperatures above 40 °C. It was demonstrated that DPC operation not only avoided transcritical operation during such weather conditions, but also increased the COP by up to 140% compared to the conventional system. The combination of these technologies was successfully mathematically modelled, from which the optimum condenser inlet air temperature was identified for each condenser temperature. Using this optimum condition, it was possible to maximise the COP for a range of conditions applicable to the psychometric chart. An annual case study for Adelaide, Australia was conducted which demonstrated that optimally coupling DPC with CO2 refrigeration can reduce the annual energy consumption and peak demand by 16% and 47%, respectively, compared to a conventional CO2 booster system. Furthermore, the number of hours of transcritical operation was reduced from 3278 to 27.

Abstract Numerical modelling is commonly used to design, analyse and optimise tube-in-tank phase change thermal energy storage systems with fins. A new simplified two dimensional mathematical model, based on the effectiveness-number of transfer units technique, has been developed to characterise tube-in-tank phase change material systems, with radial round fins. The model applies an empirically derived P factor which defines the proportion of the heat flow which is parallel and isothermal. This P factor was determined using a validated computational fluid dynamics model. This method can be used to design and optimise tube-in-tank salt based phase change thermal storage units with finned tubes.

Abstract An experimental investigation was undertaken in which the thermal resistance for the heat flow through a typical timber framed pitched roofing system was measured under outdoor conditions for heat flow up. The measured thermal resistance of low resistance systems such as an uninsulated attic space and a reflective attic space compared well with published data. However, with higher thermal resistance systems containing bulk insulation within the timber frame, the measured result for a typical installation was as low as 50% of the thermal resistance determined considering two dimensional thermal bridging using the parallel path method. This result was attributed to three dimensional heat flow and insulation installation defects, resulting from the design and construction method used. Translating these results to a typical house with a 200 m 2 floor area, the overall thermal resistance of the roof was at least 23% lower than the overall calculated thermal resistance including two dimensional thermal bridging. When a continuous layer of bulk insulation was applied to the roofing system, the measured values were in agreement with calculated resistances representing a more reliable solution.

The applicability of the effectiveness–NTU method for characterising a PCM thermal energy storage system was experimentally investigated. The system consisted of PCM encapsulated in spheres with a liquid heat transfer fluid. Freezing and melting tests have been carried out for a variety of conditions on a tank filled with 60 spheres. The investigation demonstrated that a correlation existed between the effectiveness of heat transfer and the mass flow rate in accordance with the effectiveness–NTU relationship for condensers and boilers. It has been proven experimentally that the effectiveness–NTU method is applicable for PCM encapsulated in spheres in a tank.

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. The authors acknowledge the South Australian Department of State Development who have funded this research through the Premier’s Research Industry Fund – International Research Grant Program (IRGP 33). The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER), ENE2015-64117-C5-3-R (MINECO/FEDER), and ULLE10-4E-1305). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. The project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia. The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537) and the city hall of Puigverd de Lleida.

Abstract Thermal energy storage in concentrated solar thermal power plants improves the dispatchability and eliminates the miss-match between the energy supply and demand. Recently, considerable attention has been made to latent heat thermal energy storage due to its high energy density per unit mass and volume at nearly constant temperature. This paper presents a computational fluid dynamics (CFD) model using ANSYS FLUENT 15.0 for phase change material (PCM) in a vertical triplex tube thermal energy storage system and its validation through experimental results. To enhance the heat transfer inside the PCM, eight fins have been incorporated between the internal and external tubes. Experiments were conducted for both freezing and melting processes. The CFD model endeavoured to simulate both the freezing and melting processes of the PCM. The inlet and outlet temperatures of the heat transfer fluid (HTF) as well as six temperature locations in the PCM were compared with the CFD results. The average effectiveness as well as the duration of the phase change process at each experimental point were compared with results from the CFD model and found to be in good agreement. The variation between the experimental and CFD for the phase change duration are within an average of 5.8% for freezing and 1.6% for melting.