Nicholas E. Powell

An optical comparison of silicone and EVA encapsulants for conventional silicon PV modules: A ray-tracing study

Ray-trace simulation is used to quantify the optical losses of photovoltaic modules containing silicon cells. The simulations show that when the modules encapsulant is silicone rather than ethylene vinyl acetate (EVA), the modules short-circuit current density under the AM1-5g spectrum is 0.7–1.1% higher for screen-printed multi-cSi cells, 0.5–1.2% higher for screen-printed mono-cSi cells, and 1.0–1.6% higher for high-efficiency rear-contact cells, depending on the type of silicone. This increase is primarily due to the transmission of short-wavelength light (≪420 nm) and is therefore greatest when used with low UV-absorbing glass and cells of a high IQE at short wavelength. We also quantify absorption in the glass, EVA and silicone at longer wavelengths and describe the influence of an encapsulants refractive index on escape losses.

An optical comparison of silicone and EVA encapsulants under various spectra

Under the AM1–5g spectrum, the efficiency of a c-Si module can be increased by 0.5–1.5% (relative) by using a silicone encapsulant rather than EVA. This increase is primarily due to photons of wavelengths less than ∼400 nm being transmitted by silicone but blocked by EVA. The highest increase in efficiency arises for cells with a good ‘blue response’ and for silicones of a higher refractive index. In this work we show that when calculated for the AM1–5g spectrum, the optical advantage of silicone over EVA tends to be an underestimate of what can be expected in the field. This is because incident spectra are frequently ‘bluer’ than the AM1–5g spectrum, particularly in summer and on cloudy days. In such cases, a larger fraction of photons have a wavelength less than 400 nm. With ray tracing, we find that the relative advantage of silicone over EVA is increased by an additional 0.3% on a sunny summer day in Phoenix AZ, and by 0.7% on a cloudy summer day in Brownsville TX. Still greater increases are expected for maritime climates and for installations with high albedo.

Examination of an optical transmittance test for photovoltaic encapsulation materials

The optical transmittance of encapsulation materials is a key characteristic for their use in photovoltaic (PV) modules. Changes in transmittance with time in the field affect module performance, which may impact product warranties. Transmittance is important in product development, module manufacturing, and field power production (both immediate and long-term). Therefore, an international standard (IEC 62788-1-4) has recently been proposed by the Encapsulation Task-Group within the Working Group 2 (WG2) of the International Electrotechnical Commission (IEC) Technical Committee 82 (TC82) for the quantification of the optical performance of PV encapsulation materials. Existing standards, such as ASTM E903, are general and more appropriately applied to concentrated solar power than to PV. Starting from the optical transmittance measurement, the solar-weighted transmittance of photon irradiance, yellowness index (which may be used in aging studies to assess durability), and ultraviolet (UV) cut-off wavelength may all be determined using the proposed standard. The details of the proposed test are described. The results of a round-robin experiment (for five materials) conducted at seven laboratories to validate the test procedure using representative materials are also presented. For example, the Encapsulation Group actively explored the measurement requirements (wavelength range and resolution), the requirements for the spectrophotometer (including the integrating sphere and instrument accessories, such as a depolarizer), specimen requirements (choice of glass-superstrate and -substrate), and data analysis (relative to the light that may be used in the PV application). The round-robin experiment identified both intra- and inter-laboratory instrument precision and bias for five encapsulation materials (encompassing a range of transmittance and haze-formation characteristics).

Improved spectral response of silicone encapsulanted photovoltaic modules

In this work the benefit of using optically superior silicone encapsulant materials over the incumbent ethylene vinyl acetate is demonstrated. Optical characterization of the two materials demonstrates improved transmission in the UV region of the solar spectrum. Single cell mini-modules were prepared using two different manufacturers of screen printed textured monocrystalline Si PV cells. Spectral response measurements demonstrate a 0.74–1.03% relative increase in short circuit current density when silicones are used rather than ethylene vinyl acetate, clearly in the region of improved UV transparency. IV measurements demonstrate a 0.31–1.45% improvement in current for silicone.