Directed reflectivity, long life AMTEC condenser (DRC). Final report of Phase II SBIR program[Alkali Metal ThermoElectric Converter] Page: 10 of 33
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This graph gives a rule of thumb for considering the
utility of a DRC for cells of different configurations.
One such design, the radial converter, is shown
schematically in Figure 12. Given the typical
dimensions of the radial converter (inner diameter =
9 cm, outer diameter = 20 cm, cell height = 4 cm),
the view factor from the hot end to the cold end is
approximately 0.371. This view factor is close to
that for the cylindrical PX style cell with an aspect
ratio of 1.0. Assuming a comparable radiation loss
fraction of about 70 % of the total heat loss, this
gives a DRC number of 26. This projected value is
plotted as the single point in Figure 5. One can
expect that the use of DRC will reduce the energy
input required to compensate for parasitic heat losses
by about 12%. This implies that the use of a DRC
could increase the projected converter efficiency
from 30 % to 31+ % for a fully populated radial
converter, a significant improvement for critical
applications. Additional gains should be possible
through the enhanced uniformity of BASE tube
temperatures that would result from use of the DRC.
During this investigation, additional observations
were made. The data given in Table 5 shows that
even though the hot zone loses a large amount of
0.05Donut Cell PmjecOon
3 0.05 -
1 10 10C
Figure 5 Reduction in Parasitic Heat Loss As a Function of
heat via radiation, the cold end receives relatively little heat directly via radiation. Most of the heat received
by the cold end arrives via conduction because most of the radiation heat lost by the hot zone goes first to
the side walls and then is conducted to the cold end. Therefore, it was anticipated that reducing the
emissivity of the side walls would also help reduce overall heat input to the cell. In principle, a lower
emissivity (e = 0.1) can be achieved by coating the stainless steel surface with Tungsten, a metal compatible
with Sodium at high temperatures and with lower emissivity. A analytical run was made with this low value
of emissivity and it resulted in about 11.5% reduction in energy input as shown in Table 6.
Table 5: Comparison of Heat Inputs at Hot and Cold Ends for FPC
Heat Gain/Loss, Watts %
Conduction Radiation Total Radiation
Hot End 100.98 31.46 132.44 23.7
Cold End 130.36 2.14 132.50 1.6
Side Wall 29.38 29.38 0.00 N/A
Cell wall thickness: 0.165 cm, Diameter = 6.35 cm, Length = 8.9 cm, View factor = 0.1027, Steel emissivity = 0.3.
Condenser emissivity = 0.02, Hot end temperature = 873 K, Cold end temperature = 573 K.
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Hunt, Thomas K. Directed reflectivity, long life AMTEC condenser (DRC). Final report of Phase II SBIR program[Alkali Metal ThermoElectric Converter], report, September 10, 2001; United States. (https://digital.library.unt.edu/ark:/67531/metadc717643/m1/10/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.