As already mentioned, the large value of the radiation flux incident on the filter-PV array means that a significant absorptance loss will occur even for small values of the filter spectral absorptance. Consider the interference-plasma filter presented in Chapter 4 (Figure 4.22). That filter has very low absorptance (« 0.01) at all wavelengths except in the band-pass region around X = 1500nm where it reaches a maximum of 0.15. Using the optical properties of this filter and the emitter and PV array properties used for Figure 8.2, the system performance has been calculated using the Mathematica program. These results are shown in Figure 8.3.
As a result of the filter absorptance, 70W of power is lost from the system. The large reflectance (> 0.96) for X > 2000nm results in bettrer cavity efficiency, pc = 0.63, compared to pc = 0.57 for the BSR case of Figure 8.2. However, a lower PV efficiency, pPV = 0.27, compared to pPV = 0.31 for Figure 8.2, results in a lower TPV efficiency, pTPV = 0.17, compared to pTPV = 0.18 for Figure 8.2. The PV efficiency is lower even though the temperature is considerably higher, TE = Tb = 1505K, compared to TE = Tb = 1231K. The higher temperature and cavity efficiency means there is more convertible radiation incident on the PV arrays. However, for the case in Figure 8.3 most of the 70W absorbed by the filter occurs in the range 1200 < X < 2000nm where the PV spectral response is highest. As a result, the short circuit current is lower than for the case in Figure 8.2 and as a result the PV efficiency is lower.
TE = Tb = 1505 K, TcI = Tc2 = Tc3 = 300 К Cavity efficiency, ric = ,63 PV efficiency, t|pv = .27 TPV efficiency, Т|тру = t|cT|pv = .17 Electrical power out, PEL = 42 W Power absorbed by filter = 70 W
Figure 8.3.—Performance of planar, square geometry TPV system with the same properties as used in Figure 8.2 except the back surface reflector (BSR) has been replaced by the interference-plasma filter with the optical properties shown in Figure 4.22. Note that the incident PV array radiation fluxes shown are the useful fluxes, not the total fluxes.
If a front surface filter is used for spectral control in a TPV system, it must have negligible absorptance, especially in the wavelength range where the PV arrays have the largest spectral response. If a filter can be fabricated using only dielectric (nonconducting) materials, then a non-absorbing filter is possible.