Direct solar absorption and storage in high-purity glass melts

Terrestial solar spectrum superposed on the absorption spectrum of low-loss glass fibers.

At 1570 °C (a candidate "empty" temperature for a heat-storage glass melt,) the viscosity of soda-lime glass is only about half that of room-temperature honey; at 1800 °C (a candidate "full" temperature for a heat-storage glass melt,) its viscosity has fallen nearly to that of room-temperature motor oil. In a large storage pond of molten glass, convection will easily be turbulent; unfortunately, convection cannot be effectively driven by heating from the top.

Using the relations for the effective conductivity of soda-lime glass melts in Pilon et al. (which likely give values too low for high-purity melts because of the importance of the radiative contribution) the effective thermal conductivity of molten glass at 1570 °C is 95 W/m-°K (nearly twice that of room-temperature steel,) and it increases with temperature: at 1800 °C the effective thermal conductivity is 160 W/m-°K (comparable to room-temperature magnesium.) However, with a thermal diffusivity around 0.00004 m²/sec, a six-hour pulse of solar heating would only travel about 1 meter into the glass melt by conduction/radiation.

The only option for charging the thermal storage is a glass melt that is so transparent to solar radiation that much of the energy is absorbed in the lower half of the pond's depth. If half of the solar energy is absorbed in traveling to a 20 m depth, that is a loss rate of 3 dB/0.02 km = 150 dB/km. Data for very pure glass fibers (see diagram above) show that such transparency can be far exceeded for wavelengths shorter than 2 microns (which account for more than 90% of the AM 1.5 Direct solar spectrum, see diagram below.) It remains to be seen if such high-purity, transparent glass melts can be economical for direct solar absorption and storage.

Solar spectral distribution for AM 1.5 Direct (green line). Image quoted from http://pveducation.org .