The mentionned source refers to an elevator using on-board solar cells to get the power required to ascend. The solar cells are either illuminated by solar light concentrated by a set of mirrors or by a "power beamer", i.e. a high-power laser located on the ground.
The efficiency of light to electricity conversion is a critical factor, because it directly impacts on the elevator temperature, and therefore on the weight of embedded heat dissipation system.
From Wikipedia Space Elevator:
Powering climbers Both power and energy are significant issues for climbers—the climbers need to gain a large amount of potential energy
as quickly as possible to clear the cable for the next payload.
Various methods have been proposed to get that energy to the climber:
- Transfer the energy to the climber through wireless energy transfer while it is climbing.
- Transfer the energy to the climber through some material structure while it is climbing.
- Store the energy in the climber before it starts – requires an extremely high specific energy such as nuclear energy.
- Solar power – power compared to the weight of panels limits the speed of climb.
Wireless energy transfer such as laser power beaming is currently
considered the most likely method. Using megawatt powered free
electron or solid state lasers in combination with adaptive mirrors
approximately 10 m (33 ft) wide and a photovoltaic array on the
climber tuned to the laser frequency for efficiency. For climber
designs powered by power beaming, this efficiency is an important
design goal. Unused energy must be re-radiated away with
heat-dissipation systems, which add to weight.