• Energy Research

Abstract Achieving energy savings with domestic off peak air conditioning using phase change materials (PCMs) has always proved a challenge. Although the energy efficiency ratio of an air conditioner is higher during the night, this improvement often does not offset the exergy loss experienced when using thermal storage. Simulations have been conducted using the effectiveness-number of transfer units (ɛ-NTU) representation of a PCM system to determine the instantaneous heat transfer when coupled to an inverter chiller cooling system. Results show that although 85% of the energy consumption for cooling could be shifted to the off-peak period with an ice based system, the energy demand increased by 7.6%. The investigation demonstrated that by using a PCM with a melting point of 4 °C, it is possible to achieve an energy saving for cooling. A savings of around 13.5% can be achieved using a PCM with a melting point of 10 °C. Energy usage was increased with a more efficient PCM storage system. This unexpected result was due to which period the storage system was charged. A more efficient storage system charged quicker during the warmer part of the evening. Therefore energy minimisation requires optimal charging during the coldest part of the night.

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 154.4 kJ kg−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.

Abstract An experimental validation for a computational fluid dynamics (CFD) and an effectiveness-number of transfer units (e-NTU) model for tubes in a large phase change material (PCM) tank has been conducted. The inlet and outlet heat transfer fluid (HTF) temperatures as well as twelve temperature locations in the PCM tank were compared with the CFD results. The average effectiveness of the phase change process of each experimental point was also compared with results from the CFD as well as the e-NTU models. From this study, it was concluded that the CFD model and the e-NTU model developed can accurately predict the behaviour of the thermal storage system during the freezing process. There are however, discrepancies in the melting process due to the exclusion of the effect of natural convection in the models. Using the experimental results, an effective thermal conductivity has been determined to account for buoyancy for various distances of tubes. The paper gives details of the CFD model of the phase change thermal storage system, and presents results from the CFD model, experiments and e-NTU model.

Abstract The tube-in-tank is a compact configuration well suited for PCM thermal storage systems. However limited research has investigated the impact of the boundary condition applied to the PCM achieved through differing tube arrangements. In Part 1, using CFD and considering the discharging condition for CSP applications, it was determined that when the heat transfer fluid flow was in parallel, poor extraction of latent energy occurs, whereas in a counterflow arrangement maximum latent energy is extracted. In Part 2, the impact of mass flow rate and PCM thermal conductivity on the extraction of latent energy for these tube arrangements was investigated. It was discovered that the counterflow arrangement can experience poorer heat transfer which can be avoided through design. Furthermore, little investigation has considered the impact of the effectiveness of heat transfer with PCM systems with increased amounts of sensible energy, typical for CSP applications. It was determined that for latent dominant storage systems, the counterflow tube arrangement should be applied, while for sensible dominant PCM storage systems, parallel flow should be considered.

The proliferation of non-scheduled generation from renewable electrical energy sources such concentrated solar power (CSP) presents a need for enabling scheduled generation by incorporating energy storage; either via directly coupled Thermal Energy Storage (TES) or Electrical Storage Systems (ESS) distributed within the electrical network or grid. The challenges for 100% renewable energy generation are: to minimise capitalisation cost and to maximise energy dispatch capacity. The aims of this review article are twofold: to review storage technologies and to survey the most appropriate optimisation techniques to determine optimal operation and size of storage of a system to operate in the Australian National Energy Market (NEM). Storage technologies are reviewed to establish indicative characterisations of energy density, conversion efficiency, charge/discharge rates and costings. A partitioning of optimisation techniques based on methods most appropriate for various time scales is performed: from “whole of year”, seasonal, monthly, weekly and daily averaging to those best suited matching the NEM bid timing of five minute dispatch bidding, averaged on the half hour as the trading settlement spot price. Finally, a selection of the most promising research directions and methods to determine the optimal operation and sizing of storage for renewables in the grid is presented.

An experimental validation for a computational fluid dynamics (CFD) model for tubes coiled in a phase change thermal energy storage system has been conducted. Using the validated CFD model, three CFD models have been developed. The first model was developed having pins embedded on a tube with heat transfer fluid (HTF) flowing in it, with PCM surrounding the tube. Different configurations of pins on the tube have been analysed. The second model developed is similar to the first model; however, fins were embedded instead of pins. Different configurations of fins on the tube were also investigated. The last model developed was a plain copper tube surrounded by PCM with HTF flowing in it. This model was used as a benchmark for comparison for the first two models. The models were analysed for the freezing process. From this study, it was concluded that fins on the tube is better than pins on the tube. The paper gives details of the CFD models and presents the results obtained from simulations carried out using these models.

Abstract Night cooling strategies are gaining popularity with the raise in profile of Green Buildings and Sustainable rating systems. The use of night ventilation to exploit lower diurnal temperatures to pre-cool the building structure in preparation for the following day's gains is well known; however, the role which the facade has to contribute to night ventilation is not fully understood. Researchers are familiar with economiser cycles operation for central air conditioning systems and the ability of these to operate in night ventilation mode with simple modifications to the control strategy requires validation. Simulations were carried out for a typical office building in Adelaide to demonstrate that a traditional economiser cycle operating 24 h each day under thermostatic control delivers energy savings. A number of facade structures were considered and the effect of varying the location of the thermal mass within the structure was investigated. The paper gives details on the model used for the simulations and discusses the results obtained. It was found that increasing the mass on the inside of the facade is preferred over the external for the warm marine climate zone of Adelaide, South Australia.

Abstract An investigation into characterising and optimising the useful latent energy that can be stored within a tube-in-tank phase change thermal energy storage system has been conducted, with particular reference to off peak thermal storage applications for cooling buildings. This process involved determining the actual useful energy that can be stored within a phase change material (PCM) storage system coupled to a low energy night time cooling system using a cooling tower. The useful energy that can be stored within the PCM was determined using a validated effectivess-NTU model. The energy storage effectiveness of the PCM system was determined, which directly indicates how much of the PCM was useful. This storage effectiveness was optimised delivering a storage effectiveness of 68% and 75%. This parameter can be directly compared to sensible storage systems and it was found that tube-in-tank systems can store more than 18 times more useful energy than sensible storage systems per unit volume.

Abstract Thermal energy storage systems for both heat and cold are necessary for many industrial processes. High energy density and high power capacity are desirable properties of the storage. The use of latent heat increases the energy density of the storage tank with high temperature control close to the melting point. Tube in PCM tank is a very promising system that provides high packing factor. This work presents an experimental study of a PCM tank for cold storage applications. Two different configurations and different flow rates of the heat transfer fluid were studied. The effectiveness of the PCM storage system was defined as that of a heat exchanger. The results showed that the heat exchange effectiveness of the system did not vary with time, decreased with increasing flow rate and increased with increasing heat transfer area. The effectiveness was experimentally determined to only be a function of the ratio m ˙ /A. This equation was found to be adequately be used to design a PCM storage system, and a case study is presented. It was shown that the tube in tank design together with a low temperature PCM is suitable as a thermal storage facility for cold storage.