Photovoltaics in Comparison

Photovoltaics is the most successful technique for obtaining electricity from the surroundings. It is at the same time the natural yardstick for neutrinovoltaic technology — and the comparison is worth making, because it shows where the two approaches really differ.

How a solar cell works

A photon strikes silicon and lifts an electron there across the band gap of 1.12 electronvolts. A built-in electric field at the p-n junction separates the electron from the vacancy it leaves behind, before the two find each other again. The separated charge carrier flows off as current.

The process is a surface — it takes place at the surface, in a layer a few micrometres thick.

The limits are known

What is remarkable about photovoltaics is how precisely its upper bounds have been calculated. William Shockley and Hans-Joachim Queisser derived in 1961 that a simple cell can use at most about 33 percent of sunlight.

The reason lies in the spectrum. Photons with too little energy do not clear the band gap and are lost. Photons with too much energy give off the excess as heat. Only part of the light fits.

Today's silicon cells reach just under 27 percent in the laboratory, modules in the field about 20 to 23. The technology is therefore close to its theoretical limit — a sign of maturity. One gets further only with stacked cells for different spectral ranges, silicon with perovskite for instance.

The one property that decides everything

Photovoltaics needs light. At night it delivers nothing, and that is not a technical shortcoming but the definition of the method.

In Germany this yields a capacity factor of about eleven percent over the year: an installation delivers on average one ninth of what it could at full load. The rest is night, cloud, winter.

This is precisely where all energy harvesting methods begin that do not depend on light.

The difference in approach

The master equation of neutrinovoltaic technology integrates over the volume, not over an area. That is the real difference, and it has three consequences:

The yield is meant to grow with the number of layers, not with the illuminated area. Patent WO2016142056A1 claims ten to twenty alternating layers of graphene and silicon, twelve in particular.

Orientation falls away. A solar installation has to face south and stand at an angle. A component that responds to fluxes arriving from all sides needs no orientation.

The day-night cycle falls away. The input channels named are, according to the group, present around the clock.

What can fairly be concluded

The two methods are not competitors for the same place. They aim at different situations.

Photovoltaics is established, proven in billions of modules, industrially mature and unbeatably cheap wherever the sun shines and space is available. Anyone with a roof needs no alternative.

Neutrinovoltaic technology aims at the opposite: no light, no area, no orientation. A cellar, the interior of a device, a location in the polar night. The group lists its projects expressly as being in development and describes the performance figures named as design targets.

The comparison is therefore less a competition than a division of labour: photovoltaics has shown that ambient energy can be used at scale. The question of neutrinovoltaic technology is whether that also succeeds where no light reaches.

Related

• Silicon — the shared material
• The master equation — why volume instead of area
• Energy harvesting — the wider field
• Thermoelectrics — the route via a gradient

Sources

• W. Shockley, H. J. Queisser: Detailed Balance Limit of Efficiency of p-n Junction Solar Cells, Journal of Applied Physics 32, 510 (1961).
• Fraunhofer ISE: Photovoltaics Report — efficiencies and yield figures.