5.4 Annual yield of PVT collectors
5.4.3 The effect of optical coatings applied to the collector
Having determined the annual yield of PVT systems with PVT collectors of varying surface area, it is investigated next to which extent the yield can be increased by applying optical coatings to the PVT collector. Both low-emissivity coatings and anti-reflective coatings will be considered.
Low-emissivity coatings
A normal PV laminate is not spectrally selective. Its emissivity is determined by the emissivity of the glass encapsulating the cells, being approximately 85%. This
implies that in a PVT collector, besides convection, radiation is an important means of heat transport from the PV laminate to the cover glass. Part of the heat transported is subsequently lost to the ambient.
Low-emissivity coatings are frequently applied in double glazed windows to re-duce heat loss by radiation. In double glazed windows an as low as possible par-asitic absorption in the visible part of the spectrum is desirable. As described by Granqvist [95], both metals and doped semiconductor coatings can be used as low-emissivity coating in double glazed windows. If low-low-emissivity coatings are applied on the PV laminate in a PVT collector, an as low as possible parasitic absorption in the near infrared part of the spectrum is required as well, to maintain a good electri-cal efficiency. Because of the much lower absorption in the near infrared, the doped semiconductor coatings are more suitable than the metal coatings. The most impor-tant examples of doped semiconductor coatings are tin oxide, indium tin oxide and zinc oxide. Fluor doped tin oxide (SnO:F) is widely used for low-emissivity coatings in double glazed windows. Indium tin oxide (ITO) is mainly applied as transparent conductive coating in flat panel displays. Aluminium doped zinc oxide (ZnO:Al) has been developed for electronic applications such as transparent conductive oxide in thin-film solar cells [73] and has not yet been developed and applied as a low-emissivity coating. The optical properties of these doped semiconductor coatings are similar. They have a refractive index n of about 2 and an absorption coefficient α that in the visible part of the spectrum is relatively low (in the order of 5· 102cm−1, depending on the doping level) but α increases rapidly with increasing wavelength.
In general a thicker and heavier doped coating will have a lower emissivity, but will also have a higher parasitic absorption. Because of the relatively high refractive in-dex of doped semiconductor coatings they cause some extra reflection. So, when applied to the PV laminate, this extra reflection will reduce both the electrical effi-ciency and the absorption factor somewhat.
Because SnO:F is most widely used as low-emissivity coating it will be consid-ered here. Haitjema has experimentally and numerically studied the optical prop-erties of SnO:F in detail [96]. Applying a 300 nm thick SnO:F coating with a fluor doping concentration of 3· 1020cm−3will reduce the emissivity of the PV laminate from 85% to about 20%. Best SnO:F coatings prepared by Haitjema had an emissivity of 15% at a thickness of 620 nm.
Anti-reflective coatings
Both the front and back surface of the cover glass and the front surface of the PV laminate reflect about 4% of the incident irradiance each. This reflective loss can be reduced by means of anti-reflective coatings. Anti-reflective coatings have already been discussed in section 2.2.2. The anti-reflective coatings considered here are sin-gle layer porous SiO2 coatings, deposited by a dip coating technique [60]. Because these anti-reflective coatings have a thickness of only 120 nm, they do not affect the
PV laminate Cover glass
= low emissivity coating
= anti-reflective coating
A B C D
Figure 5.11:Four PVT collector configurations (drawing not to scale). A: Without coatings. B: low-emissivity coating on the laminate. C: AR coatings on both sides of the cover glass and on the PV laminate. D: AR coatings on both sides of the cover glass and a sandwich of low-emissivity coating and AR coating on the PV laminate.
emissivity of the substrate onto which they are deposited [60]. Anti-reflective coat-ings can be applied to the cover glass and on the PV laminate in the collector.
Configurations
In this section PVT systems with collectors having c-Si PUM cells are considered again. However, this time the effect of the additional 300 nm SnO:F low-emissivity and/or 120 nm porous SiO2anti-reflective coatings described above, is investigated.
The four configurations A to D that are considered are schematically shown in fig-ure 5.11. Configuration A is the standard configuration, considered in section 5.4.2, without additional coatings. Configuration B has a low-emissivity coating on the PV laminate. Configuration C has an anti-reflective coating on both sides of the cover glass and on the PV laminate. In configuration D, the cover glass has anti-reflective coatings on both sides as well. In addition the PV laminate contains a sandwich of low-emissivity coating and anti-reflective coating.
The optical model described in chapter 2 is used to determine the electrical STC efficiency and the AM1.5 absorption factor of these configurations at collector level.
Besides the effect of the cover glass and the packing density of the cells, the optical effects of the coatings have been taken into account. The results are given in the top half of table 5.6. The emissivity of the laminate ε is given in the final column. The effect of the addition of a low-emissivity coating can be seen by comparing configu-ration A and B. Because of increased reflection and because of parasitic absorption in the low-emissivity coating, the electrical STC efficiency has reduced from 12.97% to 11.84% and the absorption factor of the PV laminate has been reduced from 81.0% to 76.9%. This reflection and absorption will be discussed in more detail in section 5.5.
Comparing configurations A and C, it can be seen that the addition of the anti-reflective coatings has significantly increased the electrical STC efficiency of the
lami-Table 5.6:The electrical STC efficiency, the absorption factor and emissivity of a PV laminate with c-Si PUM cells, in a PVT collector with coating configuration A to D and in a PV module with coating configuration E and F. For a solar thermal collector with coating configuration G and H the absorption factor and emissivity of the absorber are given as well. Configurations E to H will be introduced in section 5.5.
conf. system ηcol,STCe (%) A(%) ε (%)
A PVT 12.97 81.0 85
B PVT 11.84 76.9 20
C PVT 14.19 87.9 85
D PVT 13.67 86.7 20
E PV 14.06 88.6 85
F PV 14.71 91.2 85
G thermal - 86.6 12
H thermal - 91.4 12
nate from 12.97% to 14.19% and the absorption factor from 81.0% to 87.9%. In config-uration D, both types of coating are combined. The low-emissivity coating reduces the emissivity of the PV laminate to 20% and the anti-reflective coatings increase the electrical STC efficiency and the absorption factor of the laminate, though not to the levels found for configuration C.
The effect on the annual yield
The laminate properties summarised for configurations A to D in table 5.6, are used as input parameters for the annual yield model. All PVT systems that have been con-sidered in section 5.4.2, are concon-sidered here again, but with low-emissivity and/or AR coatings. These are the PVT systems for domestic hot water with collector areas of 3, 6 and 12 m2and systems for combined domestic hot water and room heating with collector areas of 6, 12 and 24 m2. In figure 5.12 the results are shown. In the left panel the results for the domestic hot water system are shown and in the right panel the results for the combined domestic hot water and room heating system are given. The annual thermal efficiency is plotted versus the annual electrical efficiency.
The efficiencies of the ‘A’ configurations (without any additional coatings) were al-ready given in table 5.5 and the fact that increasing the collector area results in higher annual yields, but lower thermal and electrical efficiencies has been discussed in sec-tion 5.4.2.
It can be seen in each case that the addition of a low-emissivity coating to the PV laminate (compare configurations A and B), increases the annual thermal efficiency somewhat, but reduces the annual electrical efficiency by more than 1% absolute.
However, adding anti-reflective coatings (compare configurations A and C) is
benefi-8 9 10 11 12
Figure 5.12:The annual electrical system efficiency versus the the annual thermal system efficiency for PVT systems with c-Si PUM cells and coating configurations A, B, C and D. Left: For PVT systems for domestic hot water with a collector area of 3, 6 and 12 m2. Right: For PVT systems for combined domestic hot water and room heating with a collector area of 6, 12 and 24 m2.
cial for both the annual electrical efficiency, increasing by almost 1% absolute, and the thermal efficiency. Having both a low emissivity coating and anti-reflective coatings (configuration D) results in the highest thermal efficiency. The electrical efficiency however, is not as high as the case with only anti-reflective coatings (configuration C) and may in some cases even be lower than the case without any coatings (config-uration A).
Note that in systems with a high thermal efficiency, the extra heat collected be-cause of the application of coatings is used most efficiently. The effect of these additional coatings on the thermal efficiency is therefore largest in these systems.
For example consider the 3 m2collector for domestic hot water. Adding both low-emissivity and anti-reflective coatings increases the thermal efficiency from 34.5%
to 41.4%. If the same coatings are applied to a 12 m2collector, the increase is only from 14.6% to 16.3%. Both in absolute and in relative terms the effect of the addi-tional coatings is less in the system with the larger collector. This indicates that a complete system analysis, as performed here, is required to analyse the effect of the application of optical coatings.
Thickness of the low-emissivity coating
Up till now for coating configurations B and D a low-emissivity coating with a thick-ness of 300 nm has been applied to the PV laminate, which is a standard thickthick-ness for low-emissivity coatings applied in double glazed windows [60]. Next the effect of the thickness of this coating on the annual system efficiencies is investigated.
0 100 200 300 400 500
Figure 5.13: The electrical and thermal system efficiencies versus a the thickness of the SnO:F low emissivity coating applied to the PV laminate. A system for only domestic hot water with a 6 m2collector is considered.
Haitjema presents the emissivity of SnO:F coated glass as a function of coating thickness [96]. The standard reference coating considered here has an emissivity of about 0.20 if the thicknesses is 300 nm or more. If the coating thickness is reduced, the emissivity increase to 0.24 at 200 nm and to 0.40 at 100 nm. A bare glass sub-strate, without low-emissivity coating has an emissivity of 0.85. This indicates that somewhat thinner coatings still reduce the emissivity significantly. At the same time thinner coatings give rise to less parasitic absorption. The resulting effect on the electrical STC efficiency of the PV laminate was investigated using the optical model presented in chapter 2.
The reference PVT system for domestic hot water with a 6 m2 PVT collector is considered and coating configuration D is used as starting point. The PV lami-nate’s emissivity, electrical efficiency and absorption factor were determined for low-emissivity coating thickness ranging from 0 to 500 nm. These values were used as input for the annual yield model. The results are shown in figure 5.13. It can be seen that by decreasing the coating thickness below the standard value of 300 nm, the an-nual electrical system efficiency increases as a result of lower cell temperatures and the reduced parasitic absorption. The optical loss caused by additional reflection in-duced by the coating is not rein-duced. Therefore, the potential increase in electrical efficiency turns out to be limited. At the same time the annual thermal system effi-ciency is decreased. The optimal thickness of this coating for PVT applications will be discussed in section 5.5.5. The efficiencies of the same system, but with coating configuration C are also indicated. Note that because of the additional reflection inherently present in configuration D, the efficiencies for a low-emissivity coating thickness approaching 0, do not correspond to the efficiencies for configuration C.