Magnetocaloric effect and magnetic refrigeration

M Agnetocaloric M Aterials

Magnetocaloric effect: From materials research to refrigeration devices

Similar to the efforts to move toward electric vehicles, much research has focused on the idea of a more electric aircraft (MEA). The motivations for this research are similar to that for vehicles and include goals to reduce emissions and decrease fuel consumption. In traditional aircraft, multiple systems may use one type or a combination of types of energy, including electrical, hydraulic, mechanical, and pneumatic energy. However, all energy types have different drawbacks, including the sacrifice of total engine efficiency in the process of harvesting a particular energy, as with hydraulic and pneumatic systems. The goal for future aircraft is to replace most of the major systems currently utilizing nonelectric power, such as environmental controls and engine start, with new electrical systems to improve a variety of aircraft characteristics, such as efficiency, emissions, reliability, and maintenance costs. This paper provides an in-depth look into how the systems have—or will be—changed. Future aircraft capabilities such as electric taxi and gas–electric propulsion for aircraft are also included for discussion. Most recent commercial transport aircrafts are described as the current state-of-the-art electric aircraft system. Future goals, including those of NASA, are presented for future advances in MEA.

More Electric Aircraft: Review, Challenges, and Opportunities for Commercial Transport Aircraft

Selection of materials and expected magnetocaloric effects are discussed for magnetic cooling applications at elevated temperatures (400–800 K). Various considerations result in the selection of rare earth‐transition metal compounds such as Sm2Fe17−xCox for this task. These materials offer a wide range of suitable magnetic ordering temperatures as a function of x. They also show relatively high effective magnetic moments per volume. Molecular field models are developed for analytically predicting entropy changes at and above the ordering temperature. Concomitant adiabatic cooling ΔT is accordingly computed for these compounds near the ordering temperatures. It is found that for a family of compounds ΔT values increase somewhat with increasing ordering temperatures due to the decreasing influence of the lattice heat capacity at higher temperatures. Adiabatic cooling of ΔT=−7.5 K at 70 kOe to ΔT=−9.2 K at 70 kOe is predicted for materials Y2Fe17−xCox near their Curie points of 300 and 600 K, respectively (c...

Magnetic cooling near Curie temperatures above 300 K

Meeting NASA's N+3 goals requires a fundamental shift in approach to aircraft and engine design. Material and design improvements allow higher pressure and higher temperature core engines which improve the thermal efficiency. Propulsive efficiency, the other half of the overall efficiency equation, however, is largely determined by the fan pressure ratio (FPR). Lower FPR increases propulsive efficiency, but also dramatically reduces fan shaft speed through the combination of larger diameter fans and reduced fan tip speed limits. The result is that below an FPR of 1.5 the maximum fan shaft speed makes direct drive turbines problematic. However, it is the low pressure ratio fans that allow the improvement in propulsive efficiency which, along with improvements in thermal efficiency in the core, contributes strongly to meeting the N+3 goals for fuel burn reduction. The lower fan exhaust velocities resulting from lower FPRs are also key to meeting the aircraft noise goals. Adding a gear box to the standard turbofan engine allows acceptable turbine speeds to be maintained. However, development of a 50,000+ hp gearbox required by fans in a large twin engine transport aircraft presents an extreme technical challenge, therefore another approach is needed. This paper presents a propulsion system which transmits power from the turbine to the fan electrically rather than mechanically. Recent and anticipated advances in high temperature superconducting generators, motors, and power lines offer the possibility that such devices can be used to transmit turbine power in aircraft without an excessive weight penalty. Moving to such a power transmission system does more than provide better matching between fan and turbine shaft speeds. The relative ease with which electrical power can be distributed throughout the aircraft opens up numerous other possibilities for new aircraft and propulsion configurations and modes of operation. This paper discusses a number of these new possibilities. The Boeing N2 hybrid-wing-body (HWB) is used as a baseline aircraft for this study. The two pylon mounted conventional turbofans are replaced by two wing-tip mounted turboshaft engines, each driving a superconducting generator. Both generators feed a common electrical bus which distributes power to an array of superconducting motor-driven fans in a continuous nacelle centered along the trailing edge of the upper surface of the wing-body. A key finding was that traditional inlet performance methodology has to be modified when most of the air entering the inlet is boundary layer air. A very thorough and detailed propulsion/airframe integration (PAI) analysis is required at the very beginning of the design process since embedded engine inlet performance must be based on conditions at the inlet lip rather than freestream conditions. Examination of a range of fan pressure ratios yielded a minimum Thrust-specific-fuel-consumption (TSFC) at the aerodynamic design point of the vehicle (31,000 ft /Mach 0.8) between 1.3 and 1.35 FPR. We deduced that this was due to the higher pressure losses prior to the fan inlet as well as higher losses in the 2-D inlets and nozzles. This FPR is likely to be higher than the FPR that yields a minimum TSFC in a pylon mounted engine. 1

/pdf/turboelectric-distributed-propulsion-engine-cycle-analysis-7up1rf7ovc.pdf

Turboelectric Distributed Propulsion Engine Cycle Analysis for Hybrid-Wing-Body Aircraft

/pdf/turboelectric-distributed-propulsion-engine-cycle-analysis-1o512c5sut.pdf

The magnetocaloric effect in polycrystalline Gd was measured at temperatures from 190 to 370 K in applied fields from 1 to 7 T. The magnetocaloric temperature changes were combined with existing zero‐field specific heat data to construct a T‐S diagram for Gd near the Curie point. Experimental values of ΔT were also compared with values calculated from a simple mean field theory, which predicts rather well both the general shape of the magnetocaloric curve and the relative magnitudes of the temperature changes in various measuring fields. The maximum ΔT with applied field was about 14 K at 7 tesla, and maxima in all applied fiels occurred near 292 K. The relatively large magnetocaloric effect in Gd near room temperature is attractive for potential magnetic refrigeration applications, and the experimental T‐S diagram may now be used to refine estimates of the performance of Gd as a solid magnetic refrigerant.

T‐S diagram for gadolinium near the Curie temperature

The benefits that turboelectric propulsion may offer transport aircraft due to the flexibility of electrical distribution of power have been discussed by various authors and are briefly summarized. Estimates of the weights and efficiencies of the electric components, based on approximate sizing models for fully superconducting motors and generators and on aggressive estimates for cryocoolers and inverters, were presented at ASM2009 in a baseline turboelectric system study. An SBIR study has since predicted that the apparently aggressive cryocooler weight and performance estimates (5 lb/input-hp at 30% of Carnot efficiency) can be met. Another SBIR study has predicted that a cryogenically cooled inverter can exceed the 10 hp/lb specific power (including cooler) and the 98.8% efficiency (including cooler) that were assumed in the baseline study. The inverter with its cooler constituted half of the baseline electrical system weight. On the other hand, new estimates for the superconducting motors and generators, based upon inclusion of additional components in the sizing models and more complete ac loss models, predict somewhat heavier and slightly less efficient motors and generators. The lighter, more efficient inverter roughly offsets the adverse motor and generator changes for machines wound with a hypothetical future high temperature superconductor with low ac losses. For such a material, the weight of the entire electric system changes very little, and the already high efficiency improves slightly, from the ASM2009 estimates. For machines wound with the intermediate temperature superconductor MgB2, the system weight increases about 25%, because MgB2 must operate much colder. The modeling and performance estimates of each component are discussed and compared to the baseline estimates and to the current state-ofthe-art. The dependence of motor and generator weights and efficiencies upon some important design parameters are presented in an appendix. These results quantify the benefits of technology development of lighter cryocoolers and of superconductors with lower ac loss. A zero-th order estimate of the benefits of lower fan pressure ratio and of boundary layer ingestion, offset by the electric system weight and inefficiency penalties from the added components, gives a net fuel burn reduction of about 9%, before iterating and resizing.

Weights and Efficiencies of Electric Components of a Turboelectric Aircraft Propulsion System

The performance of the N3-X, a 300 passenger hybrid wing body (HWB) aircraft with turboelectric distributed propulsion (TeDP), has been analyzed to see if it can meet the 70% fuel burn reduction goal of the NASA Subsonic Fixed Wing project for N+3 generation aircraft. The TeDP system utilizes superconducting electric generators, motors and transmission lines to allow the power producing and thrust producing portions of the system to be widely separated. It also allows a small number of large turboshaft engines to drive any number of propulsors. On the N3-X these new degrees of freedom were used to (1) place two large turboshaft engines driving generators in freestream conditions to maximize thermal efficiency and (2) to embed a broad continuous array of 15 motor driven propulsors on the upper surface of the aircraft near the trailing edge. That location maximizes the amount of the boundary layer ingested and thus maximizes propulsive efficiency. The Boeing B777-200LR flying 7500 nm (13890 km) with a cruise speed of Mach 0.84 and an 118100 lb payload was selected as the reference aircraft and mission for this study. In order to distinguish between improvements due to technology and aircraft configuration changes from those due to the propulsion configuration changes, an intermediate configuration was included in this study. In this configuration a pylon mounted, ultra high bypass (UHB) geared turbofan engine with identical propulsion technology was integrated into the same hybrid wing body airframe. That aircraft achieved a 52% reduction in mission fuel burn relative to the reference aircraft. The N3-X was able to achieve a reduction of 70% and 72% (depending on the cooling system) relative to the reference aircraft. The additional 18% - 20% reduction in the mission fuel burn can therefore be attributed to the additional degrees of freedom in the propulsion system configuration afforded by the TeDP system that eliminates nacelle and pylon drag, maximizes boundary layer ingestion (BLI) to reduce inlet drag on the propulsion system, and reduces the wake drag of the vehicle.

https://ntrs.nasa.gov/api/citations/20120000856/downloads/20120000856.pdf

Gerald V. Brown

Papers

Turboelectric Distributed Propulsion Engine Cycle Analysis for Hybrid-Wing-Body Aircraft

Turboelectric Distributed Propulsion Engine Cycle Analysis for Hybrid-Wing-Body Aircraft

T‐S diagram for gadolinium near the Curie temperature

Weights and Efficiencies of Electric Components of a Turboelectric Aircraft Propulsion System

Turboelectric Distributed Propulsion in a Hybrid Wing Body Aircraft