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Development of a novel integrated thermally driven adsorption−absorption (AD−AB) refrigeration system

Nikbakhti, R ORCID: 0000-0001-5222-0003 2020 , 'Development of a novel integrated thermally driven adsorption−absorption (AD−AB) refrigeration system', PhD thesis, University of Tasmania.

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Abstract

Over the last decades, demand in the field of cooling devices has been considerably increased to satisfy the requirements for human comfort. The increased demand for energy required to power the conventional compression refrigeration systems has led to both energy and environmental concerns. To overcome these concerns, the researchers have focused on the use of sorption cooling technologies driven by low-grade thermal energy sources such as solar, geothermal, and waste thermal energy.
Current research of thermally driven sorption cooling technology is first reviewed. Absorption and adsorption chillers have been proven to be effective means to convert the thermal energy into useful cooling and have been wide applied in the waste heat recovery. However, these two systems cannot work efficiently when the heat source temperature drops to below 75 °C. Such temperature energy sources are abundant existing in solar, geothermal, and waste thermal energy. Development of sorption cooling system driven by low grade thermal temperature is essential to enhance the energy utilization efficiency and reduce global warming. The challenges to develop such sorption cooling system is then identified. It is found that the evaporating and condensing pressures limit the sorption capability for both absorption and adsorption cooling systems at low-grade thermal heat source temperatures.
By analysing the operating characteristics of both absorption and adsorption cooling systems, a completely novel integrated adsorption-absorption (AD−AB) refrigeration system is proposed and studied. The innovation of the proposed system is that the generator of absorption cycle operates as the evaporator of adsorption cycle; hence, the generation and evaporating pressure is no longer determined by the cooling and chilled water temperatures as it is associated with the heat source temperature. This results in an effective operation at lowgrade thermal sources by adjusting the intermediate pressure in the system. A fully dynamic lumped-parameter mathematical model is then developed to investigate the operating characteristics of the proposed integrated AD-AB system. The model is first validated using experimental data for absorption and adsorption cooling systems, respectively. Then the model is applied to evaluate the operating performance of the integrated AD-AB system. The results show that the CC and COP of the proposed system are as high as 13.7 kW and 0.4, respectively at a thermal source temperature of 60 °C. The COP of the integrated system is significantly higher compared to the conventional adsorption system at heat source temperatures below 65 °C. The cooling capacity of the integrated system is increased by up to 100% as compared with a conventional adsorption system under the same operating conditions. It is also found that the COP of the proposed system does not change significantly at heat source temperatures between 50 and 85 °C. This indicates the integrated AD–AB system could work efficiently across a wide range of low-temperature heat sources. Furthermore, the results also reveal that the COP of the integrated system is largely affected by the cooling water inlet temperature if the heat source temperature was lower than 55 °C.
In order to further evaluate the efficacy of the AD-AB system, optimization of effective parameters on the system’s performance in terms of maximizing the energy performance of the proposed system is also conducted. The key parameters such as intermediate pressure and solution concentration are optimized to achieve the best system performance. Furthermore, different configurations of the integrated AD-AB system and effect of the flow arrangement of hot and cold fluid are also studied. The results show that as the heat source temperature increases from 50 to 85 °C, the optimal intermediate pressure vary gently from 2.36 to 2.16 kPa while the optimal solution concentration increases significantly from 52.4 to 65%. An COP of 0.4 can be achieved for the optimized intermediate pressure and solution concentration at the heat source temperature as low as 50 °C. When compared the two different configurations, the results show that the cooling capacity and COP of the system configuration with AB as bottom cycle are about 5 to 10% higher than that of the system configuration with AD as bottom cycle. The investigation on the effect of flow arrangement of the heating and cooling fluids shows that the parallel flow arrangement for the cooling and heating can provide the best system performance. The maximum specific cooling power (SCP) is around 194 W kg-1 when the hot water flow arrangement between generator and desorber and cooling water flow arrangement between absorber, adsorber and condenser are both parallel. In contrast, the lowest SCP value is 157.9 W kg-1 obtained in the series scheme for both hot and cooling water flow arrangements.
Australia has very rich solar radiation energy in the world. The application of solar energy in the proposed integrated cooling system is then examined for seven major Australian cities including Adelaide, Brisbane, Canberra, Darwin, Melbourne, Perth, and Sydney. Three different solar system configurations are considered; (i) cooling system is directly connected to the solar thermal collector, (ii) hot water storage tank is installed between the cooling unit and solar collector, and (iii) auxiliary heater with a set point of 60℃ is placed at the exit of storage tank.
Results indicate that the average daily cooling capacity produced through the second system configuration with storage tank is greater than that produced with the first configuration. It is also found that the coefficient of performance is considerably more stable during the operating hours in the second solar configuration. Results reveal that all the examined cities particularly Darwin city have high potentiality for installing the proposed solar cooling system with high cooling production. In both system configurations, the greatest average cooling energy is achieved in summer months specially in January in all locations due to the high available solar radiation. The system performance was directly influenced by the collecting area in both configurations. It is observed that the average daily cooling capacity experiences an increase by expanding the collector field while the change in the average daily COP curve is too minimal, especially in the system with the storage tank.
An energetic and economic optimization is then conducted in order to find the optimum system design in each Australian city. Considering the optimal design parameters including the storage tank volume and solar collector obtained for each city, it is revealed that Adelaide city with the internal rate of return of 25.2% is the most appropriate location while Melbourne with the internal rate of return of 19.4% is less suitable city for installing the proposed solar cooling system. Lastly, an economic comparison is also investigated, and the results indicates that considerably amount of energy of around 65% on average can be saved in the Australian cities by the solar integrated cooling system as compared to the conventional compression cooling systems.

Item Type: Thesis - PhD
Authors/Creators:Nikbakhti, R
Keywords: Integrated AD-AB cooling system; Adsorption; Absorption; Thermal effeciency; Coefficient of performance
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Copyright 2020 the author

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