PV is a method of generating electrical power by converting solar radiations into current using semiconductors that exhibit the photovoltaic effect. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy acting as a charge carrier for an electric current. This effect was first observed by Edmond Becquerel in 1839, when he was experimenting with illuminated metal electrodes in an electrolyte.[8] However, the photovoltaic effect was explained in 1905 with the pioneering theoretical work of Albert Einstein on the photoelectric effect for which he received the Noble prize.[9]
1.3.1 The Solar Spectrum
Solar irradiation can be well estimated by a black body at a temperature of 5800 K emitting according to Plank’s distribution.[10] The solar radiation is attenuated by absorption, reflection, and scattering. Sunlight is absorbed in the visible and UV region by molecular nitrogen, oxygen, ozone, nitrous oxide, and methane in the mid-infrared region by water vapor and in the infrared region by carbon dioxide. The spectrum is strongly confined between the far infrared and near ultraviolet by the time it reaches the earth’s surface. The air mass coefficient is used to characterize the solar spectrum after the solar radiation travels through the atmosphere and hence it is commonly used to characterize the performance of solar cells under standardized conditions.
The Air Mass (AM) is the ratio of the path length (y) of the sun light passing through the atmosphere when the sun is at a given angle θ to the zenith, to the path length (x) when sun is at its zenith (Figure 1-6). This relation is approximated as below,
AM =
xycos θ1 (eq. 3)
Figure 1-6. Cartoon drawing illustrating AM.
The solar irradiance outside the earth’s surface (AM 0), at sea level (AM 1) and the standard reference spectrum are compared in (Figure 1-7). The spectrum outside the atmosphere, the 5,800 K black body, is denoted as AM 0, meaning ‘zero atmospheres’. Solar cells for space power applications are generally characterized using AM 0. The spectrum after travelling through the atmosphere to sea level with the sun directly overhead is referred to as AM 1, meaning ‘one atmosphere’. AM 1 (θ = 0o) to AM1.1 (θ = 25o) range is used to estimate performance of solar cells in equatorial and tropical regions. The standard reference spectrum in PV is denoted by AM 1.5 G, which corresponds to the total global (hemispherical) irradiance under specified atmospheric conditions at an incident angle of 48o.
500 1000 1500 2000 2500 3000 0.0
0.5 1.0 1.5 2.0 2.5
Spectral irradiance / Wm-2 nm-1
Wavelength / nm
AM 0 AM 1.5 Direct AM 1.5 Global
Figure 1-7. Solar irradiation spectra above atmosphere and at surface.
1.3.2 Photovoltaic Market overview
Renewable energy continued to grow strongly as investments increase, prices falls, and policies spread. In the span of last five years, total global installed capacity of many renewable energy technologies grew at very fast rates. Solar PV capacity in operation at the end of 2011 (70 GW) was about ten times the global total (7 GW) just five years earlier (Figure 1-8). It grew the fastest of all with operating capacity increasing at an average of 58%
annually. It was followed by concentrating solar thermal power (CSP), which increased almost 37% and wind power increased 26%. In spite of this admirable progress, renewable energy only shares 17% shares of global energy consumption. Biomass, solar and geothermal collectively share a tiny amount of 3.3% among the all renewable resources. By the end of 2011, 30 GW of operating capacity of solar PV was added, increasing the total global capacity by 74% to almost 70 GW, sufficient to generate 85 TW/year.[11]
Figure 1-8. Solar PV Total World Capacity.[2]
Solar PV is now the third most important renewable energy source following hydro and wind power, in terms of globally installed capacity. The number of countries having more than 1 GW capacity to their grids increases from three to six. So far the best efficiency solar cell is a multi-junction concentrator solar cell with the overall efficiency 44% (Figure 1-9). The highest efficiency of 35.8% was obtained by sharp corporation using a triple-junction technology in 2009[12] and Boeing Spectrolab have achieved 40.7% using a triple layer design. Crystalline silicon based modules are facing great competition by thin-film solar cells, CdTe, amorphous Si, and microcrystalline Si, which are expected to account for 31% of the global installed power capacity by 2013. San Jose based company Sunpower produces cells with energy conversion ratio of 19.5%, which is well above the market average of 12-18%.[13] Solar cell efficiency varies from 6% for amorphous silicon-based solar cells to 44%
with multiple-junction concentrated photovoltaics. But for the commercially available photovoltaic modules, the efficiencies are around 14-22%. The cost of PV has already reached well below nuclear power in 2011 and is set to fall further. The average solar cell prices as monitored by Solarbuzz group fell from $3.50/watt to $2.43/watt over the course of 2011 and the prices below $2.00/watt are looking inevitable. For large-scale installations, prices reached below $1.00/watt. The declining prices of PV are directly proportional to the installation capacities.
Emerging technologies, such as DSSCs and organic solar cells are expected to grow rapidly in next few years. Although they have lower module efficiencies, their cost per watt is estimated to be three to four times lower than the conventional c-Si based systems. Currently these emerging technologies are being developed industrially in pilot plants and are very close to commercialization.