| There has been growing interest in thermal powered air conditioning and refrigeration in the recent years due to their energy saving potential and environmental benefits. Sorption refrigeration machines can effectively utilize thermal energy sources such as solar energy or waste heat as the primary driving energy. Therefore they can lower electric power loads on grid electricity. Additionally, they can use environmentally friendly refrigerants such as ammonia, methanol and water, instead of CFC's; hence, can reduce ozone layer depletion and global warming.However, one important drawback of thermal energy and in particular, solar energy is the variation of solar insolation with time, location and season of the year. This makes it difficult to utilize the solar energy resource effectively during the low and high insolation periods, especially with the conventional solar sorption refrigeration systems. Moreover, shut downs during seasons of low solar radiation lengthens the payback periods of solar powered refrigeration systems; thus, making them unattractive. This therefore, raises the need to develop refrigerators which can be regulated according to available thermal energy. This study addresses this problem by proposing and investigating a novel multimodal refrigeration machine whose operation can be regulated based on available heat source temperature.The investigated system consists of three sorption refrigeration thermodynamic cycles that require different levels of heat source temperature, and can be alternately operated based on the available solar energy insolation namely: double way, double effect and double stage cycles. For double-way sorption cycle, both adsorption refrigeration and resorption refrigeration were combined to increase the cooling capacity. For double-effect sorption cycle, one internal heat recovery process was employed to enhance the energy utilization efficiency. For two-stage cycle with internal heat recovery process, a secondary reactive salt was used to lower the driving regeneration temperature.The manufactured multimodal machine consisted of two sorption beds, one containing MnCl2-expanded graphite composite adsorbent, and the other containing NaBr-expanded graphite sorbent. Ammonia was used as the working refrigerant. A theoretical study was first done to analyze the performance of the multimodal system, followed by experimental investigation. The performances of the single reactors under the conventional cycle were also evaluated before doing a full machine tests under the different working modes. Finally, second law analysis was done to indentify the entropy generated by thermal coupling.Theoretical results showed that the maximum COP based on the ratio between the evaporation enthalpy and reaction enthalpy for the double way cycle, the double effect cycle and the double stage cycle are limited to 1.19, 0.92, and 0.47, respectively. When the metal and adsorbent mass ratio of 5 was considered, the COP for the three cycles mentioned above reduced to 0.76, 0.60 and 0.31, respectively. The solar COP for the three cycles reached 0.41, 0.33, and 0.17, respectively, when similar metal/adsorbent mass ratio and the application of a parabolic trough collector (PTC) were assumed. The feasibility of the internal heat recovery processes between the HTS and the LTS reactors for the double effect and double way cycle showed that the internal heat recovered from HTS was sufficient for desorption of the LTS.Tests done with the NaBr bed showed interesting results for air conditioning application. The cooling power obtained varied from 1.32-3.36 kW, whereas the COP ranged from 0.28-0.54, for the different conditions tested. The inlet evaporator fluid temperature was varied from 7 oC to 13 oC, whereas the reactor inlet fluid temperature during regeneration period ranged from 59.3 oC to 70.2 oC. The half cycle time ranged from 16 to 34 minutes.Test results of the HTB with MnCl2 showed poorer performance than was expected. The COP obtained ranged from 0.05 to 0.08 when the evaporation temperature was varied from -15 oC to 15 oC. The highest desorption temperature for all the conditions was 155 oC. The poor performance was attributed to design problems which lead to large heat loss and low heat transfer in the bed. Because the HTB played a key role in the operation of the multimodal system, the design problems of this bed therefore had serious negative effect on the performance during different working modes.The performance of the double way cycle was evaluated at different cooling temperatures. The COP ranged from 0.07 to 0.15 when the cooling temperature varied from 10 oC to 15 oC, whereas the maximum desorption temperature of the HTB was constant at 155 oC. Besides the problem of the HTB, the large sensible heat of the LTB contributed to a large loss of useful cooling during the resorption process. Moreover, improper matching of the salts also affected cooling during the resorption process.For the double effect cycle, the total cooling capacity varied from 3629 kJ to 4684 kJ whereas the COP was between 0.16 and 0.20, when the refrigeration temperature in the low temperature evaporator varied from -18 oC to -5 oC, and the high temperature evaporator was kept at 10.7 oC. The maximum bed temperature of the HTB during desorption was 155 oC.Results for the double stage cycle showed a variation in the cooling capacity and COP ranging from 523 kJ to 2713 kJ and 0.07 to 0.15, respectively, when the refrigeration temperature varied from -25 oC to -5 oC. The maximum bed temperature of the HTB during desorption was 135 oC.A second law analysis indentified the entropy generation in different components and processes of the cycles. The predominant entropy generated due to thermal coupling occurred during the desorption process of the HTB. This was followed by that generated during the adsorption process of the same bed. Moreover, it was established that the double effect cycle had the highest Carnot COP among the three cycles comprising the multimodal refrigeration system.
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