Wasim Saman

Analysis and modelling of a phase change storage system for air conditioning applications

Abstract A phase-change energy storage system consisting of sections of different materials with different melting temperatures is proposed for air conditioning applications. The Phase Change Materials (PCMs) are placed in thin flat containers and air is passed through gaps between them. A semi-analytical model has been developed and calculations carried out using the finite elements method. Results obtained from three models with different assumptions are compared. The effect of the design parameters such as, PCM slab thickness and fluid passage gap on the storage performance is also investigated. The system is being developed for energy storage in solar heating and energy efficient space heating and cooling applications.

An experimental study of a cross-flow type plate heat exchanger for dehumidification/cooling

The thermal and dehumidification behaviour of a standard cross-flow type plate heat exchanger, intended for use as a dehumidifier/cooler, has been investigated both experimentally and numerically. Three sets of experiments have been carried out where air is blown into the primary and secondary sides of the exchanger, while water and liquid desiccant were being sprayed in a counter flow arrangement. The first set represents the indirect evaporative cooling of the primary stream by the secondary air stream. The second set is with liquid desiccant only and no indirect evaporative cooling. In the third set of experiments the primary air stream is indirectly evaporatively cooled by the secondary air stream and dehumidified by the liquid desiccant sprayed into the primary side of the exchanger. The above experiments indicate that the heat exchanger performs well when used with liquid desiccant. Furthermore, for an exchanger angle of 45°, there is an optimum value of air mass flow rate at which the effectiveness and dehumidification efficiency of the plate heat exchanger are maxima. To investigate the effect of the ambient air conditions on the PHE performance, further experiments were carried out using a heater element and a humidifier. The results show that under laboratory conditions the exchanger effectiveness and dehumidification efficiency increase with increasing primary air inlet temperature and humidity ratio. The experimental results were used to validate a computer model developed for the cross-flow type plate heat exchanger/dehumidifier. Comparison indicates that the numerical results are in good agreement with the experiments.

An experimental study of a forced flow solar collector/regenerator using liquid desiccant

One of the main components of a liquid desiccant cooling system is the regenerator. In a liquid desiccant air conditioner, outside air is dehumidified by liquid desiccant and cooled within the absorber. The diluted desiccant solution thus obtained has to be concentrated for reuse, by passing through the regenerator and the cycle is, consequently, repeated. The regenerator used in this application is a forced parallel flow type solar collector/regenerator. The regenerator has been designed and optimized and the prototype of the solar collector/regenerator has been built and tested. Calcium chloride has been used as the absorbent solution. The results of the tests conducted as a parametric analysis indicate that the air and solution mass flow-rates and the climatic conditions affect the regenerator performance. Furthermore, a comparison between the experimental data obtained and a previously developed model for a forced parallel flow solar collector/regenerator reveals that the experiments are in good agreement with the model predictions. Finally, it was concluded that the proposed solar collector/regenerator performs satisfactorily under the summer conditions of Adelaide, Australia.

Modelling and performance analysis of a cross-flow type plate heat exchanger for dehumidification/cooling

Abstract This paper describes the performance analysis of a cross-flow type plate heat exchanger for use as a liquid desiccant absorber (dehumidifier) and indirect evaporative cooler. The proposed absorber can be described as a direct contact, cross-flow, heat and mass exchanger, with the flow passages separated from each other by thin plastic plates. One air stream (primary air) is sprayed by liquid desiccant solution, while the other stream (secondary air) is evaporatively cooled by a water spray. Each thin plate, besides separating the water/air passage from the solution/air passage, also provides the contact area for heat and mass transfer between the fluids flowing in each passage. A parametric study for the primary air stream at 33°C, 0.0171 kg/kg humidity ratio and secondary air stream at 27°C and 0.010 kg/kg humidity ratio using calcium chloride solution was performed in this study. The results showed a strong dependence on the heat and mass transfer area, solution concentration and ratio of secondary to primary air mass flow rates. However, negligible differences were found between the performance of a counter flow and a parallel flow arrangement. The results demonstrate that the proposed absorber will not offset both the latent and sensible load of the primary air and, therefore, an auxiliary cooler or more dehumidification/indirect evaporative cooling stages will generally be required to meet the sensible and latent load in a typical comfort application.

Modeling and performance of a forced flow solar collector/regenerator using liquid desiccant

A forced flow solar collector/regenerator is one of the effective ways of regenerating the weak liquid solution in an open cycle liquid desiccant air conditioner using solar energy. In this system, the weak solution flows over the absorber plate of a tilted collector/regenerator as a thin liquid film. The forced air stream, which flows parallel or counter to the solution film, removes the moisture which is evaporated from the liquid solution due to absorption of solar energy. The absorber plate of the collector/regenerator is blackened and glazed to enhance the solar energy absorption and protect it from the environment. To evaluate the thermal performance of the solar collector/regenerator, a computer model has been developed using calcium chloride as the desiccant. A parametric analysis of the system has been performed to calculate the rate of evaporation of water from the solution as a function of the system variables and the climatic conditions.

Geopolymer encapsulation of a chloride salt phase change material for high temperature thermal energy storage

In an effort to reduce the cost and increase the material compatibility of encapsulated phase change materials (EPCMs) a new encapsulated system has been proposed. In the current study a molten salt eutectic of barium chloride (53% wt.), potassium chloride (28% wt.) and sodium chloride (19% wt.) has been identified as a promising candidate for low cost EPCM storage systems. The latent heat, melting point and thermal stability of the phase change material (PCM) was determined by DSC and was found to be in good agreement with results published in the literature. To cope with the corrosive nature of the PCM, it was decided that a fly-ash based geopolymer met the thermal and economic constraints for encapsulation. The thermal stability of the geopolymer shell was also tested with several formulations proving to form a stable shell for the chosen PCM at 200°C and/or 600°C. Lastly several capsules of the geopolymer shell with a chloride PCM were fabricated using a variety of methods with several samples remaining stable after exposure to 600°C testing.

Constant-volume adiabatic combustion of stoichiometric hydrogen-oxygen mixtures

The paper evaluates and discusses the combustion characteristics of electrolytically produced stoichiometric hydrogen-oxygen mixtures under constant volume adiabatic conditions. The results will be used in the preliminary design of a combustion chamber of a hydrogen-oxygen fuelled engine. Maximum combustion pressures are investigated and compared to theoretical predictions based on the first law of thermodynamics, ideal gas laws and adiabatic flame temperature. High temperature dissociation is not taken into account. Experimental results are within 10% of theoretical predictions. Ignition delay time and rate of pressure rise are also investigated and discussed. The ignition delay time is a function of the initial mixture pressure and decreases exponentially from a maximum value of 6.6 ms towards zero as the initial pressure of the mixture is increased.

Numerical modeling of inward and outward melting of high temperature PCM in a vertical cylinder

Numerical study of inward and outward melting of a high temperature PCM in cylindrical enclosures were performed, using FLUENT 15. For validation purposes, numerical modeling of inward melting of a low temperature PCM was initially conducted and the predicted results were compared with the experimental data from the literature. The validated model for the low temperature PCM was used for two high temperature cases; inward melting of a high temperature PCM in a cylindrical enclosure and outward melting in a cylindrical case with higher aspect ratio. The results of this study show that the numerical model developed is capable of capturing the details of melting process with buoyancy driven convection for Ra<108, i.e. laminar flow, for a high temperature PCM and can be used for the design and optimization of a latent heat thermal storage unit.

Integrating Solar Heating and PV Cooling into the Building Envelope

Photovoltaic/thermal (PVT) systems generate electrical and thermal energy. In summer, the usage of the collected heat is limited to domestic hot water heating. By contrast in winter, more useful heat collection is favorable, however, the PVT collectors require less cooling; therefore, the improved electrical output is limited. In this paper a new one-dimensional steady-state building integrated solar collector model is presented and examined, incorporating PVT and thermal (PVTT) collectors connected in series. In summer, the PVT collector is air-cooled, and the collected heat is discarded to the surroundings while the thermal collector heats the water for domestic use. In winter, both the PVT and thermal collectors are water-cooled generating domestic hot water. The efficiencies of the new collector are compared to that of a PVT collector, with both collectors having the same total area and characteristics. Both collectors are able to meet the summer thermal load and to provide useful thermal energy in winter. The PVTT collector reduces the collector thermal stresses and provides slight additional electrical power output.

Thermal Performance Of A Pcm Thermal Storage Unit

The thermal performance of a PCM thermal storage unit (TSU) is studied numerically and experimentally. The TSU under analysis consists of several flat slabs of phase change material (PCM) with melting temperature of-26.7 °C. Liquid heat transfer fluid (HTF) passes between the slabs to charge and discharge the storage unit. A one dimensional mathematical model was employed to analyze the transient thermal behavior of the storage unit during the melting and freezing processes. The model takes into consideration the temperature variations in the wall along the flow direction of the HTF. The paper compares the experimental and numerical simulation results in terms of HTF outlet temperatures during the melting period.