5.16 Summary
The price of PV modules has decreased dramatically over the past few decades and thin film processes are starting to have a further effect on decreasing prices. Other components of PV systems have also to be considered and their costs reduced. The degree of financial viability of a PV system depends on subsidies and the price received for exported PV electricity.
With decreasing prices, large-scale implementation becomes more plausible – most likely as grid-connected embedded generation. At a certain level of market penetration this will present problems for electricity supply because of the variability of the resource. This will require either development of long-term storage technologies or the integration of PV with other renewables and cleaner fossil fuel technologies.
As with any technology there is an environmental impact. With PV this impact is nearly all in manufacture and the bulk of this is caused by the energy used to produce the cells. However, this energy is 'paid back' within 10% of the useful life of the PV cells. The emission of pollutants and greenhouse gases associated with the manufacture and life of PV systems is less than that for the same amount of energy derived from fossil fuels.
Renewable energy technologies offer a route to reducing carbon dioxide emission and mitigating global warming if implemented on a large scale. Any near-term systems have to work within the existing infrastructure and this means a mix of renewable and non-renewable capacity.
Photovoltaics is the most modular renewable technology that is suitable for the urban environment. Integration with other renewables enables it to contribute significantly to the reduction of greenhouse gas emissions.
The resource of solar energy is essentially inexhaustible. Sunlight comes in a continuous range of wavelengths, or photon energies, and this results in a compromise in the optimum materials employed. PV cells are made from semiconducting materials under conditions of very high purity. They are solid state devices with no moving parts and are hence silent and almost maintenance free. There are a number of different materials employed with varying efficiencies, environmental impacts and applications.
There are parallel drives to produce either high efficiency cells by bulk crystalline processing or lower cost thin film cells with lower efficiency. The cost of the thin film technologies in particular decreases as production increases, see The next generation.
The next generation
Thin film processing offers a big decrease in manufacturing costs when applied to large-scale production. Other techniques are being researched that offer even greater reductions in cost. These include the use of semiconducting polymers and titanium dioxide activated by a light absorbent dye as the chief active materials in photocells. The deposition techniques include techniques in which a paste is spread or sprayed on the substrate and then dried. These techniques are all in the next league down in terms of cost and energy, but also, unfortunately, of efficiency.
Applications range from stand-alone remote power systems, through power for satellites to embedded generation on domestic houses or offices. Implementation in a wider energy strategy would involve integration with energy storage mechanisms, in batteries, connection to the national grid or a chemical fuel. Economic viability remains a key issue.
Activity 46 (self-assessment)
From the information you have gained on different material types for PV cells and the discussion of systems and applications, what do you think are the main considerations for system designers in choosing cell types for particular applications?
Answer
The application and load to be supplied are the prime consideration, then come the area available for modules and the budget available. You could also include in your answer aspects such as ease of installation, types of system, e.g. stand-alone or grid connected. You might also mention particular high efficiency cells based on semiconductors other than silicon, or the flexible nature of some thin film cells. There are, in fact, a large number of permutations available to an engineer and a wide range of materials and systems to choose from.