Impact of hysteresis on caloric cooling performance

Caloric cooling relies on reversible temperature changes in solids driven by an externally applied field, such as a magnetic field, electric field, uniaxial stress or hydrostatic pressure. Materials exhibiting such a solid-state caloric effect may provide the basis for an alternative to conventional vapor compression technologies. First-order phase transition materials are promising caloric materials, as they yield large reported adiabatic temperature changes compared to second-order phase transition materials, but exhibit hysteresis behavior that leads to possible degradation in the cooling performance. This work quantifies numerically the impact of hysteresis on the performance of a cooling cycle using different modeled caloric materials and a regenerator with a fixed geometry. A previously developed 1D active regenerator model has been used with an additional hysteresis term to predict how modeled materials with a range of realistic hysteresis values affect the cooling performance. The performance is quantified in terms of cooling power, coefficient of performance (COP), and second-law efficiency for a range of operating conditions. The model shows that hysteresis reduces efficiency, with COP falling by up to 50% as the hysteresis entropy generation (qhys) increases from 0.5% to 1%. At higher working frequencies, the cooling performance decreases further due to increased internal heating of the material. Regenerator beds using materials with lower specific heat and higher isothermal entropy change are less affected by hysteresis. Low specific heat materials show positive COP and cooling power up to 2% of qhys whereas high specific heat materials cannot tolerate more than 0.04% of qhys

Harmonic analysis of temperature profiles of active caloric regenerators

The design of a thermal regenerator is initially carried out by considering the fundamental influencing variables. For novel solid-state cooling systems using active caloric regenerators, the non-linearity of the coupled phenomena of material properties, heat transfer, and hydraulic flow can complicate the interpre- tation of experimental and simulated results. Based on the boundary conditions of a sinusoidal magnetic field and fluid flow, we elucidate the operation of active regenerators by deriving easy-to-manage analyt- ical expressions for the temperature transients of the caloric materials and heat transfer fluid. An internal temperature measurement system with an estimated uncertainty of ±0.3 K for packed bed regenerators has been developed for validation. The derived expressions have acceptable accuracy relative to the exper- imental and numerical results for temperature profiles in both magnitude and sensitivity, where average and maximum errors are ∼10% and ∼15%, respectively. Useful figures of merit are post-calculated using the derived temperature profiles. We found that the average temperature profiles are linear for passive regenerators and nonlinear for active regenerators, and their transients are nonlinear functions of the configuration and operating parameters.This work was in part financed by the RES4Build project, which received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 814865. The authors wish to acknowledge Dr. Kaspar K. Nielsen for the valuable discussions of the experimental setup

The potential application of a magnetocaloric heat pump in ultra-low temperature district heating systems

Ultra-low temperature district heating (ULTDH) systems were proposed to operate at lower temperatures in combination with booster heat pumps working from 313 K to 333 K. This can increase the system efficiency and enable the exploitation of renewable heat sources, but only based on the proper design for system boundary conditions. The techno-economic analysis of ULTDH systems revealed that the seasonal coefficient of performance of booster heat pumps should be sufficiently high to ensure the competitiveness of ULTDH systems. We study the potential of a novel magnetocaloric heat pump integrated into a domestic hot water application based on ULTDH systems. The magnetocaloric heat pump is based on the active magnetic regenerator (AMR) system that uses the reversible magnetocaloric effect of a solid-state refrigerant with no global warming potential to build a heating/cooling cycle. The solid refrigerant can be layered with different magnetic phase-change temperatures to provide a good temperature match between the user water stream and the refrigerant flow. The heating capacity can be tuned in a wide range by combinatorial control of frequency, fluid flow rate, blow fraction and offset fraction. An experimentally validated model with an average error of ~9.2% was used to study the partial load operation. A COP improvement of 68% was obtained at a partial load of 47.4% by simultaneously tuning the frequency and fluid flow rate. Important factors for the magnetocaloric heat pump integration in the ULTDH system were identified

Experimental study of non-bonded packed bed active magnetic regenerators with stabilized La(Fe,Mn,Si)13Hy particles

The aim of this study is to develop more stable magnetocaloric regenerators, made from non-epoxy-bonded La(Fe,Mn,Si)13Hy particles to address the instability issues of conventional regenerators with a first-order phase transition. The stabilized magnetocaloric materials are obtained by increasing the α − Fe content at the expense of a small reduction of the adiabatic temperature change. However, the experimental results show that the non-bonded structure improves the regenerator efficiency and reduces pressure drop, potentially compensating for the reduction of the material’s magnetocaloric effect. Compared to epoxy-bonded regenerators, non-bonded regenerators exhibit a larger temperature span (10.2 K at no load) and specific cooling power (27% improvement at a span of 4 K). Due to the elimination of the epoxy, a lower friction factor and higher packing density are obtained. The long-term mechanical and chemical stabilities are verified by comparing specific heat, effectiveness, and pressure drop before and after a test period of more than one year.This work was in part financed by the RES4Build project, which received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No.814865. J. Liang is grateful for financial support of the China Scholarship Council (CSC, No. 201708440210). We wish to acknowledge Mike Wichmann for the support in fabrication of housing and flanges, and Florian Erbesdobler for maintaining the DSC device