Without water, there seem to be eight pieces of PV cells because light is reflected by mirrors in the case.
Without water, there seem to be eight pieces of PV cells because light is reflected by mirrors in the case.
[Click to enlarge image]
With water, the case seems to be filled with PV cells.
With water, the case seems to be filled with PV cells.
[Click to enlarge image]

Nippon Telegraph and Telephone Corp (NTT) prototyped a "cogeneration-type amorphous photovoltaic power generation module" and demonstrated it at NTT R&D Forum 2011, which runs from Feb 22 to 23, 2011, in Tokyo, Japan.

NTT integrated photovoltaic (PV) cells and a solar-based water heater into one module to improve the conversion efficiency of the PV cells and utilize solar-heated water.

The module was made by (1) installing four pieces of Sanyo Semiconductor Co Ltd's amorphous silicon (Si) PV cells in a sloping stair-like case, (2) filling it with water and (3) putting a transparent cover on it. When there is no water in the case, those PV cells generate electricity simply by using light. When the case is filled with water, however, the appearance of the module and its output drastically change.

With water, the PV cells can be seen from various directions due to the refraction and reflection of light. As a result, the usage rate of solar light improves, increasing the amount of power generated by the cells. In the demonstration, the current value improved by about 50% by using water.

Amorphous Si PV cells have a high usage rate of visible light, but their usage rate of infrared light is low. Because the unused infrared light can be used to warm water, the amorphous Si PV cells and the water are both playing important roles, NTT said. The module has a water inlet and outlet so that heated water can be actually used.

The problem of the module is that its manufacturing cost is still high because of its relatively complicated structure.