magnetocaloric cooling; caloric regenerator; heat transfer fluid; zeotropic blend, temperature glide; efficiency and capacity
Solid-state caloric cooling cycles are considered to be a promising alternative to conventional cooling technology. Theoretically, they can compete with the conventional vapor compression cycle and are much more efficient. However, this technology does have one problem: its performance decreases rapidly if the temperature difference between the condenser and evaporator gets larger. The environment where cooling technology is used often has large temperature differences between condenser and evaporator. For instance, in a typical automotive system, the condenser operates at 45°C and the evaporator at 5°C. Thus, the temperature difference between the condenser and evaporator reaches 40°C. The cooling load of the caloric cycle decreases by 86% when the temperature difference increases from 5°C to 20°C. This weakness prevents the magnetic refrigeration cycle from becoming commercialized and competing with vapor compression cycle. To solve this issue, the authors have applied two-phase zeotropic blends as heat transfer fluid in the caloric cycles. There are two reasons. First, using two-phase flow can increase the Nusselt number. The flow is laminar when the system uses single phase water, so the Nusselt number cannot be improved in the regenerator. Second, two-phase zeotropic mixture can generate a temperature gradient in the regenerator to overcome the large temperature difference. Therefore, an ethane/isobutane mixture has been chosen due to its high temperature glide (~ 40°C). This selection is different from that of the vapor compression cycle which normally prefers small glide zeotropic mixture. This paper shows that using two-phase zeotropic mixture through the entire regenerator has much better performance than using single and two-phase flow together in the caloric cooling cycle. Optimized zeotropic refrigerant system was chosen to compare with a water system. As a result, the system using two-phase zeotropic blend can increase temperature difference between cold and hot side of the regenerator because Nusselt number increases and lower mass flow rate is required to reach the optimal displacement ratio. Therefore, the amount of heat transfer in the cold heat exchanger significantly increases and the system's efficiency ultimately gets much better. This is first time to use two-phase flow zeotropic mixture in the caloric refrigeration system and this paper expands the application of two-phase zeotropic blends to caloric cooling systems.