SOLUTIONS
Solar Photovoltaic (PV) Power
Thin-film PV panels made with amorphous silicon, a few
micrometers thick (far thinner than conventional PV cells).
Glass panels cover the maximum surface of any façade;
an example of building-integrated photovoltaics (BIPV).
The first PV cell containing semiconducting material was created by Bell Labs in 1954. When hit by a photon of light, such material liberates an electron that is guided into a circuit by a conductor. The electron leaves a ‘hole’ to be filled from the other end of the circuit—creating an electric current. The conversion efficiency (light into electricity) of early silicon PV cells was about 6%. Revolutionary efficiency breakthroughs are now reaching the market.
Photons from the sun have diverse energy properties. A semiconducting material possesses an energy value (called the ‘band gap’). Incoming photons need to exceed this to displace electrons. The more closely the photons match the band gap, the more efficient the system is. Silicon has an excellent band gap for the spectrum of sunlight. Over the last decade, PV cells were developed with 20% conversion efficiencies. Silicon PVs may soon reach a theoretical a conversion efficiency limit of 31%, but other materials could well go on to 70%.
Using an exemplary feed-in tariff law, Germany made itself the world’s fastest-growing PV market. It has installed PV panels generating as much electricity as 5 large coal-fired power stations. Sunnier countries such as Spain have followed this lead. China is a large producer of polysilicon and produces about 30% of the world's photovoltaics.
For PV to be widely deployed, the cost of cells should be on a declining cost-curve, and this is in fact the case. There are now three generations of solar PV cells. First generation (wafer solar) cells were followed by second-generation (low-cost, low efficiency thin film) cells and then by third generation (high-efficiency thin film) cells. In thin-film cells, a mixture of semiconductor compounds, amounting to a few percent of the material in regular PV cells, is sprayed onto a flexible base. Thin-film cells, made with a cheap blend of copper, indium, gallium and selenium (CIGS) yield 20% light conversion efficiency and a much lower cost per watt of electricity generated. Economies of scale for thin-film cells are now making solar PV a ‘grid-competitive’ power source worldwide. The industry aims to double or even triple the current 20% conversion efficiency.
1. Lester Brown [2009] Plan B 4.0
2. Herman Scheer [2007] Energy Autonomy