Terms run out of energy
ENERGY payback time, energy gain, net energy, energy harvest ratio and energy return factor. These terms and others have been used over the years to grasp the energy generated by a solar powered device.
Bryce Richards, who now lectures in renewable energy at Edinburgh’s HeriotWatt University, wants people to stop using such terminology. The most common concept, energy payback time (EPT), has perpetuated the myth that solar powered devices were unable to repay the energy invested in making them, Richards said.
EPT first arose in the 1970s after photovoltaics successful in space programs were applied on earth. In those early days it was possible that a device would never recoup its fabrication energy. Richards said EPT was appealing because of its similarity to economic payback times.
‘‘ The drawback is that EPT does not account for additional energy generated during the remainder of the economic lifetime,’’ he said. Applying EPT to a multicomponent photovoltaics system was also problematic. These systems might include a current inverter, batteries and mounting frames alongside the cells that convert light into electricity. ‘‘ If an end-user knows what the expected EPT of such a system is when it is installed, then how should that person interpret any change in EPT when the battery bank needs replacing after seven years?’’ Sarah Belfield