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Electricity from solar cells

In Central  Europe,  photovoltaic energy conversion from solar cells has by far the greatest proven technological potential for the production of electricity from renewable energy sources. Yet, its current contribution to the electricity supply is still at levels that  are insignificant  in terms  of the energy  industry. Although photovoltaics has had  annual  global growth rates of over 30 % for the past decade or so, it will take several decades before  it can make a perceptible contribution to German electricity supplies.  In the long  run, however, photovoltaics will prove  to be one  of the most important pillars of a sustainable energy  supply system.

Continued committed market  development of photovoltaics technology will be essential  if it is to become one  of the major components of a future energy  system.  We may assume  that photovoltaic electricity, which is still very expensive in comparison with electricity  from the grid in industrialised countries, will fall to price levels which,  taking  into account external costs in the energy  system,  will make it economically  competitive. Solar electric power  is already  commercially competitive in most standalone applications where it is able to compete with battery-produced electricity  or diesel-electric energy  transformation, or with the costs of grid expansion respectively. This sector  of photovoltaics encompasses a good third of the world market.

The essential  condition for a large-scale activation of the potential of photovoltaic electricity production is a further  significant  cost reduction. This will be supported mainly by research oriented towards the long run, both into the basics of materials  and  processes and the specific conversion technologies (cells, modules, systems). This can be achieved particularly  by increasing efficiency, reducing material  usage  and  developing high-productivity manufacturing technologies.

Like all renewable energy  technologies, photo- voltaics offers major benefits  from the ecological point  of view compared to conventional technologies for electricity generation. Using current state-of-the-art system  technology, a photovoltaic installation in central  Europe will generate the amount of energy  used  for its production in about three  years. There will be further  large reductions in this energy  payback time in the near future  as new technologies are used.

Research and  development requirements

As it is not  yet possible  to finally assess the various technological approaches in respect to their long-term development prospects, it is necessary  to continue to support the wide range of different  photovoltaic technologies:

Basic research

Completely new physics approaches are necessary  to reduce costs. Some examples are:

•   The development of solar concentrator cells with efficiencies of up to 40%
•   The development of new component structures for solar cells
•   Solar cells with highly structured absorbers and  nanostructures on the surface
•   The development of photon management
•   Target-oriented semiconductor diagnostics

Silicon wafer solar cells

Up to now,  progress in solar cell technology has been achieved almost  exclusively by developing the already  sophisticated silicon wafer techno- logy which  dominates the market. This techno- logy consists  of processing monocrystalline or multicrystalline wafers that  are 200-300 µm thick. The potential for further  cost cutting is, however, far from being  exhausted. Above all, this involves developing new technologies aimed at:

•   Using thinner and  even ultra-thin silicon wafers
•   New kinds of cell structures
•   Achieving higher  efficiency
•   Simplified process  technologies
•   Lower-cost  production of solar silicon (solar- grade Si) and  thin silicon wafers

Thin-film solar cells

Thin-film technologies are considered to have a high  potential for cutting costs:

•   CIS (chalkopyrite) and  CdTe thin-film solar cells
•   Chrystalline  silicon thin-film solar cells
•   Amorphous silicon
•   Nanocrystalline silicon
•   Modified  production technologies
•   Thin-film solar cells based  on dyes and organic  semiconductors
•   Research into materials  and  processes for thin-film technologies

Organic solar cells

Organic solar cells based  on fluid semi-conduct- ing mixtures can be applied to large flexible substrates by means of screen-printing. Despite their relatively short  service lives and  relatively low efficiencies, these  cells could  dominate niches on the market  for off grid photovoltaics. The following areas are being  researched for the further  development of organic  solar cells:

•   Evaluation of new organic  semiconductor systems  with improved absorption of these solar spectrum and  optimized charge transport properties
•   Further  development of current cell concepts
•   Modified  production technologies
•   Module  wiring
•   Encapsulation, especially of flexible solar cells
•   Light management

Module  technology

Photovoltaic cells must  be encapsulated to ensure  the long-term, safe operation of these energy converters and  allow for integration in construction and  technical structures. The research and  development issues include:

•   The development of methods to greatly expand the service life of modules
•   The development of new electrical wiring methods in module technology
•   The development of module technologies optimally  modified for the aesthetics and mechanics of specific applications, such as flexible modules.

Photovoltaic power  plants  and systems

In the midterm, photovoltaic power  plants  and systems  will probably be available with an out- put  ranging from several 100 kW to several MW to cover a peak loads (such as for the operation of cooling  systems).  Greater  research and development is required for:

•   The development of appropriate solar cells, concentrating optics,  and  mechanical system  technologies

 PV system  technology

The goal is to develop inexpensive photovoltaics inverters  that  are highly reliable with long
service lives that  match those  of PV panels.  At the same  time,  the wide variety of system configurations that  require  customized inverters solutions  must  be taken  into consideration.

To this end,  cooperation with system  analysis is necessary  for the evaluation of PV systems  and components in order  to improve  the reliable operation and  design  of PV systems.

Lifecycle analysis and  recycling

As production capacities grow  for solar cells, recycling  issues, technical service lives, and energy  payback  increasingly  play an important role and  move  more  into the focus of research and  development projects:

•   Reduction of material  and  energy consumption in manufacture
•   Reusability of photovoltaic elements and materials
•   Calculations of aging  and  creation of kinetic
models  for damage to PV panels



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