At the heart of a magnetocaloric device is the magnetocaloric material, which experiences a temperature change depending on the change of the magnetic field. Thus, the key to higher performance lies in achieving an optimisation of the magnetocaloric materials with the simultaneous optimisation and increase of the change of the magnetic field and the magnetised volume.
WP 1.1: Materials for magnetocaloric regenerator - PhD student at DTU
Experimental and theoretical studies of first order magnetocaloric materials will be done for the practical use of these in the magnetocaloric heat pump. Firstly, a number of different first order materials, including perovskite ceramics and La(Fe,Si)13Hx, will be studied and characterised with regards to their magnetocaloric properties. Strategies for the temperature control of the first order transition in the ceramics will be sought. Also, increasing efficiency by layering up to 10 different 1st order materials will be studied both by modelling and experimentally.
Results
Two first order material systems have been studied La0.67Ca0.33MnO3 made in-house and La(Fe,Mn,Si)13Hy provided by the partner Vacuumschmelze GmbH.
Studies on the possibly thermal hysteretic behaviour of La0.67Ca0.33MnO3, related to the order of the transition, were performed with a number of different techniques at DTU Energy and at ICL. An interesting correlation between magnetostriction and entropy change, which may shed light on the first order phase transition, has been identified. Also in collaboration with ICL, the magnetocaloric performance of a La manganite was evaluated as a function of particle size and silver impregnation, finding better performance for smaller particles, but not much improvement through impregnation.
A number of La(Fe,Mn,Si)13Hy alloys spanning the relevant temperature range have been characterised in order to supply input to the numerical model work in the project. Due to the powder form of this hydrogenated material the measurements have been demanding, but a method has been developed. Initial tests on the regenerative performance of this material and the layering effect have been done. The results have shown an improvement of a two-layer regenerator in comparison to a monolayer. Furthermore, the effect of mixing the layers was also investigated, finding a performance in between the single-layered and the two-layered regenerator, in agreement with model results.
WP 1.2: Magnet assembly modelling and design – PhD student at DTU
Numerical modelling and small scale experiments will be employed to design a magnet assembly for the magnetocaloric heat pump. The PhD student will study, understand and develop strategies for modelling and optimising magnet design, including hybrid elements such as electromagnets, with a view to tight integration with the electromotor. Upon construction of the magnet system, it will be carefully characterised and validated against the numerical model.
The deliverables from this work package are the design concepts and algorithms used in the design of the magnet system. These are intended to be of a generic nature and could be useful to the industrial partners in other areas also.
Results
A method has been developed to simulate the effect of the finite coercivity of the magnetic material on the performance of a magnetic system and predict the occurrence of demagnetization. The same framework also allows the prediction of how to decompose a given geometry into different magnetic materials without altering the performance and possibly saving on the total cost of the materials.
The performances of hybrid devices, which combine permanent magnets and electromagnets have been studied in a number of different conceived situations. For each case the resulting COP and cooling power can finally be expressed as a function of the power spent to run electromagnets. This result has been compared with the performance of non-hybrid devices only employing permanent magnet flux sources with or without an external electric heater running at the same power consumed by the electromagnets of the hybrid device. No significant advantage of electromagnets has been identified.
Analytical optimization techniques have been developed which can be employed to determine the optimal orientation of the magnetization in each point as well as the optimal shape of a magnetic system respect to a given linear objective functional. These methods will be applied to aid the design of the prototype magnet.