Photovoltaic: Technology of Next Millennium
Posted May 23, 2008on:
Photovoltaic, or PV for short, is a solar power technology that uses solar cells or solar photovoltaic arrays to convert light directly into electricity with no emission of dangerous gases and with least amount of industrials waste. The first few years of 21st century have witnessed a large development in the Photovoltaic energy generation and utilization. Due to variety of reasons including the concerns of deteriorating earth atmosphere and global warming, the PV technology has seen large increase in solar panel manufacturing and deployment world over, particularly in Japan and Germany. About 30-40% growth in the sector in last few years is a great incentive for investment. Entrepreneurs, venture capitalists and big industrial houses in the country are coming forward to establish industries in this area. The technology is expected to make a big splash in the Indian industrial world and solve the power crisis being faced in many states.
The main issue with the technology is its affordability. Intense R&D efforts are being made world over to find new materials, processes, and device structures to increase power conversion efficiency of basic unit so as to bring cost of solar generated power equivalent to that of power obtained from conventional sources.
The photovoltaic effect was first observed in 1839 by Alexandre-Edmond Becquerel. The first modern solar cell was patented by Russel Ohl in 1946. In 1954 Bell Laboratories found that silicon doped with certain impurities was very sensitive to light. This signaled the start of the modern age of solar power technology. The first practical application of photovoltaic was to power orbiting satellites and other spacecraft and pocket calculators, but today, the majority of photovoltaic modules are used for terrestrial grid connected power generations. There is a smaller market for off grid power for remote dwellings, roadside lamp posts, emergency telephones, remote sensing, and cathodic protection of pipelines. Based on different technologies and materials, the solar cells can be grouped into 4 different generations:
First generation photovoltaic cell: The cell consists of a large-area, single-crystal, single layer p-n junction diode, capable of generating usable electrical energy from light sources with the wavelengths of sunlight. The cells are typically made using a diffusion process with silicon wafers. These silicon wafer-based solar cells are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the terrestrial solar cell market.
Second generation photovoltaic cell: These cells are based on the use of thin epitaxial deposits of semiconductors on lattice-matched wafers. There are two classes of epitaxial photovoltaic – space and terrestrial. Space cells typically have higher AM0 efficiencies (28-30%) in production, but have a higher cost per watt. Their “thin-film” cousins have been developed using lower-cost processes, but have lower AM0 efficiencies (7-9%) in production. There are currently a number of technologies/semiconductor materials under investigation or in mass production. Examples include amorphous silicon, polycrystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. An advantage of thin-film technology theoretically results in reduced mass so it allows fitting panels on light or flexible materials, even on textiles. Second generation solar cells now comprise a small segment of the terrestrial photovoltaic market, and approximately 90% of the space market.
Third-generation photovoltaic cell: They are proposed to be very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photo-generated charge carriers. For space applications quantum well devices (quantum dots, quantum ropes, etc.) and devices incorporating carbon nanotubes are being studied – with a potential up to 45% AM0 production efficiency. For terrestrial applications, these new devices include photoelectrochemical cells, polymer solar cells, nano-crystal solar cells, dye-sensitized solar cells and are still in the research phase.
Fourth Generation Photovoltaic cell: This hypothetical generation of solar cells may consist of composite photovoltaic technology, in which polymers with nano-particles can be mixed together to make a single multi-spectrum layer. The multi-spectrum layers can be stacked to make multi-spectrum solar cells more efficient and cheaper.
Out of the four generations listed above, first two have been commercialized. Bulk of the photovoltaic modules deployed so far consist of crystalline silicon. The efficiency of crystalline silicon modules varies from 17-22%, though theoretical limit is around 29%. Using these modules, large solar farms connected to grid, stand alone power stations to electrify villages and small localities have been established. The silicon modules have also been integrated to house hold and commercial buildings to provide the main or alternate electrical energy.
Efficiency of a solar cell depends on its ability to absorb solar radiation. Larger the fraction of solar radiation it absorbs, larger will be its efficiency and larger power it will generate. Taking this into account, multi-junction solar cells have been fabricated. The efficiency of triple junction, state of the art solar cell of 40.7% has been recorded by Spectrolab, USA. The R&D work to improve efficiency further is going on by using 4, 5 or 6 junction solar cells. Using these high efficiency solar cells and focusing solar light to 500X, high efficiency solar concentrator has been devised to give electrical power of few KWp, enough to light up a household of small family. The use of light reflector have reduced the actual device size of the device, thus reducing the usage and price of much costlier semiconductor materials.
The efficiency of basic solar cell unit has further been increased to 42.8% using an altogether novel concept. The credit goes to a group of scientists under the leadership of University of Delaware, USA. The cells achieve this by splitting the incoming light into high, low and medium energy chunks. The light is then directed to different state of the art devices optimized to respective chunks of radiation leading to higher efficiency. The concept is yet to be commercialized.
The last two generations of solar cells are still at research and development stage. It will take some more years to understand the underlying science and technology to bring them to commercial level.