228 research outputs found
The universal influence of contact resistance on the efficiency of a thermoelectric generator
The influence of electrical and thermal contact resistance on the efficiency
of a segmented thermoelectric generator is investigated. We consider 12
different segmented -legs and 12 different segmented -legs, using 8
different -type and 8 different -type thermoelectric materials. For all
systems a universal influence of both the electrical and thermal contact
resistance is observed on the leg's efficiency, when the systems are analyzed
in terms of the contribution of the contact resistance to the total resistance
of the leg. The results are compared with the analytical model of Min and Rowe
(1992). In order for the efficiency not to decrease more than 20%, the contact
electrical resistance should be less than 30% of the total leg resistance for
zero thermal contact resistance, while the thermal contact resistance should be
less than 20% for zero electrical contact resistance. The universal behavior
also allowed the maximum tolerable contact resistance for a segmented system to
be found, i.e. the resistance at which a leg of only the high temperature
thermoelectric material has the same efficiency as the segmented leg with a
contact resistance at the interface. If e.g. segmentation increases the
efficiency by 30% then an electrical contact resistance of 30% or a thermal
contact resistance of 20% can be tolerated.Comment: 8 pages, 8 figure
An analytical model for the influence of contact resistance on thermoelectric efficiency
An analytical model is presented that can account for both electrical and hot
and cold thermal contact resistances when calculating the efficiency of a
thermoelectric generator. The model is compared to a numerical model of a
thermoelectric leg, for 16 different thermoelectric materials, as well as the
analytical models of Ebling et. al. (2010) and Min \& Rowe (1992). The model
presented here is shown to accurately calculate the efficiency for all systems
and all contact resistances considered, with an average difference in
efficiency between the numerical model and the analytical model of
pp. This makes the model more accurate than previously published
models. The maximum absolute difference in efficiency between the analytical
model and the numerical model is 1.14 pp for all materials and all contact
resistances considered.Comment: 8 pages, 5 figure
The demagnetization factor for randomly packed spheroidal particles
We investigate if the demagnetization factor for a randomly packed powder of
magnetic spheroidal particles depend on the shape of the spheroidal particles
and what the internal variation in magnetization is within such a powder. A
spheroid is an ellipsoid of revolution, i.e. an ellipsoid with two semi-major
axis being equal. The demagnetization factor is calculated as function of
particle aspect ratio using two independent numerical models for several
different packings, and assuming a relative permeability of 2. The calculated
demagnetization factor is shown to depend on particle aspect ratio, not because
of direct magnetic interaction but because the particle packing depend on the
aspect ratio of the particles. The relative standard deviation of the
magnetization across the powder was 3\%-8\%, increasing as the particle shape
deviates from spherical, while the relative standard deviation within each
particle was relatively constant around 5\%.Comment: 7 pages, 9 figure
The performance of a combined solar photovoltaic (PV) and thermoelectric generator (TEG) system
The performance of a combined solar photovoltaic (PV) and thermoelectric
generator (TEG) system is examined using an analytical model for four different
types of commercial PVs and a commercial bismuth telluride TEG. The TEG is
applied directly on the back of the PV, so that the two devices have the same
temperature. The PVs considered are crystalline Si (c-Si), amorphous Si (a-Si),
copper indium gallium (di)selenide (CIGS) and cadmium telluride (CdTe) cells.
The degradation of PV performance with temperature is shown to dominate the
increase in power produced by the TEG, due to the low efficiency of the TEG.
For c-Si, CIGS and CdTe PV cells the combined system produces a lower power and
has a lower efficiency than the PV alone, whereas for an a-Si cell the total
system performance may be slightly increased by the TEG.Comment: 10 pages, 6 figure
Topology optimized permanent magnet systems
Topology optimization of permanent magnet systems consisting of permanent
magnets, high permeability iron and air is presented. An implementation of
topology optimization for magnetostatics is discussed and three examples are
considered. First, the Halbach cylinder is topology optimized with iron and an
increase of 15% in magnetic efficiency is shown, albeit with an increase of 3.8
pp. in field inhomogeneity - a value compared to the inhomogeneity in a 16
segmented Halbach cylinder. Following this a topology optimized structure to
concentrate a homogeneous field is shown to increase the magnitude of the field
by 111% for the chosen dimensions. Finally, a permanent magnet with alternating
high and low field regions is considered. Here a figure
of merit of 0.472 is reached, which is an increase of 100% compared to a
previous optimized design.Comment: 10 pages, 10 figure
Modeling the microstructural evolution during constrained sintering
A numerical model able to simulate solid-state constrained sintering is
presented. The model couples an existing kinetic Monte Carlo (kMC) model for
free sintering with a finite element model (FEM) for calculating stresses on a
microstructural level. The microstructural response to the local stress as well
as the FEM calculation of the stress field from the microstructural evolution
is discussed. The sintering behavior of a sample constrained by a rigid
substrate is simulated. The constrained sintering results in a larger number of
pores near the substrate, as well as anisotropic sintering shrinkage, with
significantly enhanced strain in the central upper part of the sample surface,
and minimal strain at the edges near the substrate. All these features have
also previously been observed experimentally.Comment: 9 pages, 7 figure
The efficiency and the demagnetization field of a general Halbach cylinder
The maximum magnetic efficiency of a general multipole Halbach cylinder of
order is found as function of . The efficiency is shown to decrease for
increasing absolute value of . The optimal ratio between the inner and outer
radius, i.e. the ratio resulting in the most efficient design, is also found as
function of and is shown to tend towards smaller and smaller magnet sizes.
Finally, the demagnetizing field in a general -Halbach cylinder is
calculated, and it is shown that demagnetization is largest either at or . For the common case of a Halbach cylinder
the maximum values of the demagnetizing field is either at at
the outer radius, where the field is always equal to the remanence, or at at the inner radius, where it is the magnitude of the field in the
bore. Thus to avoid demagnetization the coercivity of the magnets must be
larger than these values.Comment: 5 pages, 5 figure
The lifetime cost of a magnetic refrigerator
The total cost of a 25 W average load magnetic refrigerator using commercial
grade Gd is calculated using a numerical model. The price of magnetocaloric
material, magnet material and cost of operation are considered, and all
influence the total cost. The lowest combined total cost with a device lifetime
of 15 years is found to be in the range \^{+++}$ refrigeration unit. In a
rough life time cost comparison between the magnetic refrigeration device and
such a unit we find similar costs, the former being slightly cheaper, assuming
the cost of the magnet can be recuperated at end of life.Comment: 17 pages, 17 figure
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