Chi Yan Tso

A field investigation of a solar-powered adsorption cooling system under Guangzhou's climate with various numbers of heat exchangers in the adsorbers

In the current study, a solar-powered double-bed adsorption cooling system has been built and tested in the Guangzhou climate. The effect of the pre-heating process on the cooling performance of the adsorption cooling system powered by solar energy has been experimentally investigated. A specific cooling power of 52.2 W/kg and a coefficient of performance of 0.20 were achieved under a 2-h pre-heating process with operating conditions of 26°C cooling water inlet temperature, 16°C chilled water inlet temperature, 8 L/min hot water and cooling water flow rate, 2 L/min chilled water flow rate and 600 s adsorption/desorption phase time. The average specific cooling power and coefficient of performance were improved by 26.1% and 33.3%, respectively, as compared to the case without conducting the pre-heating process. In addition, the influence of dead volume on the specific cooling power and coefficient of performance of the adsorption cooling system has also been investigated. Using various numbers of heat exchangers in the adsorber achieved different values of adsorber dead volume. The results show that a higher specific cooling power value is obtained with a smaller dead volume. Finally, the cooling performance of the adsorption cooling system was also studied under various operating conditions and a maximum specific cooling power and coefficient of performance are 180.4 W/kg and 0.29, respectively.

Evaporation of Al2O3-water nanofluids in an externally micro-grooved evaporator

Nanofluid is a mixture of liquid and solid phase nano-sized particles that shows a high thermal conductivity compared to its base fluid. With micro-grooves being a good method to increase the surface area for heat transfer, there is potential for nanofluid together with microgrooves to improve the performance of cooling systems. This article aims to study the effect of an externally micro-grooved surface on evaporation of aqueous nanofluids and discuss its potential application. In order to prepare a stable nanofluid, the duration time required for ultrasonication, which is a common technique in the preparation process of nanofluid, was first investigated. Next, experiments were conducted to investigate the effect of the micro-grooved surface on the evaporation rate of the nanofluids under different environmental conditions. Results show that the advantage of the enhanced thermal properties of nanofluids can only be manifested by the presence of micro-grooves. The use of a heat exchanger with nanofluids together with micro-grooves shows a better heat transfer performance than a heat exchanger without micro-grooves using water as the coolant. The enhancement is dependent on the vapor pressure and the largest improvement can be up to 44.6%.

Performance investigation of nano-structured composite surfaces for use in adsorption cooling systems with a mass recovery cycle

With an increase of the heat transfer coefficient and condensation rate in a condenser, a lower pressure can be achieved in a desorber, which leads to a dryer adsorber for the next adsorption phase and a better cooling performance in an adsorption cooling system. This study aims to experimentally investigate the condensation rate of different nanostructured surfaces and improve the cooling performance of an adsorption cooling system by coating a superhydrophobic–zeolite 13X adsorbent composite surface in the condenser. An experiment was designed and built to investigate the condensation rate of various nanostructured surfaces on a copper plate. The results show that a water collection rate (condensation rate) of the superhydrophobic–zeolite 13X adsorbent composite surface of 49.3 g/m2 min is achieved, which shows an enhancement of about 50% compared to that of the copper surface. A mathematic model is developed to estimate the cooling performance of the adsorption cooling system utilizing the composite surface and a mass recovery cycle. The simulation results show that a specific cooling power (SCP) of 231.4 W/kg and a coefficient of performance (COP) of 0.317 are determined, which shows an improvement of 25.0% and 7.8%, respectively, compared to that of the system without coating the nanostructured composite surface.

Development of a phase change material (PCM)-based thermal switch

ABSTRACT A thermal switch is an ON/OFF heat transfer control device which can be utilised in different modern fields such as cryogenics, solar energy systems, micro/nano-electronic cooling, the aerospace industry and building energy efficiency. In this study, paraffin wax is used to design and build a thermal switch based on phase change material (PCM). When the paraffin wax is heated and melts, one contact plate is pushed to make contact with another plate to turn the switch ON due to the pushing force generated by the volume expansion of the paraffin wax. Conversely, when the paraffin wax does not get enough heat to melt, the condition of the thermal switch is in the OFF state due to an air gap between the two plates. The ON/OFF thermal conductance ratio of the thermal switch is the major figure of merit and is investigated experimentally. The results show that the effective thermal conductivity of the PCM-based thermal switch in the ON state is recorded at 188.7 W/mK, while 6.2 W/mK is obtained in the OFF state. Therefore, the ON/OFF thermal conductance ratio is estimated at about 30.

Simulation Study of the Heat and Mass Recovery on the Performance of Adsorption Cooling Systems

In this study, simulation was conducted to investigate the effect of mass recovery, heat recovery, pre-heating and pre-cooling time on the system performance of a double-bed adsorption cooling system. Pressures of different system components were considered in the simulation. The adsorbent-adsorbate pair used was silica-gel and water. The heating and cooling temperatures were selected to be 85°C and 27°C respectively. Both the adsorption and desorption phase times were set at 15 minutes. The coefficient of performance (COP) and specific cooling power (SCP) were used to quantify the performance of the system. From the simulation, the basic cycle provided COP and SCP of 0.20 and 40.9W/kg respectively. By conducting heat recovery for 120 seconds, the system COP was largely increased by 99% to 0.40 compared to the basic cycle. The SCP was also increased to 42.3W/kg. Mass recovery, however, did not have too much effect on the system performance. The COP and SCP only increased by 4.5% and 3.9% respectively when conducting mass recovery for 4.7 seconds. For conducting heat and mass recovery, the COP and SCP were increased to 0.36 and 44.68W/kg, respectively. Pre-heating and pre-cooling can also be beneficial in improving both COP and SCP. The COP and SCP were increased by 14.5% and 10.1% respectively, to 0.23 and 45.0W/kg by conducting pre-heating and pre-cooling for 50.3 seconds. The combinations of these processes were also studied. It is suggested heat and mass recovery then pre-heating and pre-cooling should be conducted to improve COP and SCP. The improvements showed 31.2% for COP, increasing to 0.27, and 11.9% for SCP, increasing to 45.7W/kg.Copyright

Modeling a Novel Composite Adsorbent Based Adsorption Chiller Driven by Solar Energy

This paper aims to study the performance of a solar-powered adsorption chiller with a novel composite adsorbent material (silica activated carbon/CaCl2) operating during some typical months in Hong Kong. Modeling is established to investigate the cooling performance of this adsorption chiller driven by flat-type solar collectors with three different configurations of glaze: 1) single glazed cover; 2) double glazed cover and 3) transparent insulation material (TIM) cover. The simulation results show that the higher the solar collector temperature is, the better the coefficient of performance (COP) and the specific cooling power (SCP) of the adsorption chiller are. It is suggested to select a double glazed collector with a small value of the lumped capacitance for this adsorption chiller. Seasonal effects are discussed in which the solar COP achieves its highest value during autumn. However, the cooling capacities in spring, summer and autumn are similar. All in all, this newly developed composite material as adsorbent used in the adsorption chiller could achieve a mean solar COP of 0.36 and SCP of 94W/kg on a typical summer day of operation.Copyright