Direct Solar Energy
manufacturing; 2) optical system; 3) module assembly and fabrication method of concentrator modules and systems; and 4) system aspects, such as tracking, inverter and installation issues.
However, it should be clearly stated once more: CPV is a system approach. The whole system is optimized only if all the interconnec- tions between the components are considered. A corollary is that an optimized component is not necessarily the best choice for the optimal CPV system. Thus, strong interactions are required among the various research groups.
A photovoltaic system is composed of the PV module, as well as the balance-of-system components and system, which can include an inverter, storage, charge controller, system structure and the energy network. Users meet PV technology at the system level, and their interest is in a reli- able, cost-effective and attractive solution to their energy supply needs. This research agenda concentrates on topics that will achieve one or more of the following: 1) reduce costs at the component and/or sys- tem level; 2) increase the overall performance of the system, including increased and harmonized component lifetimes, reduced performance losses and maintenance of performance levels throughout system life; and 3) improve the functionality of and services provided by the system, thus adding value to the electricity produced (US Photovoltaic Industry Roadmap Steering Committee, 2001; Navigant Consulting Inc., 2006; EU PV European Photovoltaic Technology Platform, 2007; Kroposki et al., 2008; NEDO, 2009).
At the component level, a major objective of BOS development is to extend the lifetime of BOS components for grid-connected applications to that of the modules, typically 20 to 30 years.
For off-grid systems, component lifetime should be increased to around 10 years, and components for these systems need to be designed so that they require little or no maintenance. Storage devices are necessary for off-grid PV systems and will require innovative approaches to the short-term storage of small amounts of electricity (1 to 10 kWh, or 3,600 to 36,000 kJ), and for providing a single streamlined product (such as integrating the storage component into the module) that is easy to use in off-grid and remote applications.
For on-grid systems, high penetration of distributed PV may raise con- cerns about potential impacts on the stability and operation of the grid, and these concerns may create barriers to future expansion (see also Section 8.2.1). An often-cited disadvantage is the greater sensitivity to grid interconnection issues such as overvoltage and unintended island- ing in the low- or middle-voltage network (Kobayashi and Takasaki, 2006; Cobben et al., 2008; Ropp et al., 2008). Moreover, imbalance between demand and supply is often discussed with respect to the variation of PV system output (Braun et al., 2008; NEDO, 2009; Piwko et al., 2010). PV system designs and operation technologies can address these issues to a degree through technical solutions and through more accurate solar energy forecasting. Moreover, PV inverters can help to improve the quality of grid electricity by controlling reactive power or
filtering harmonics with communication in a new energy network that applies a mixture of inexpensive and effective communications systems and technologies, including smart meters (see Section 8.2.1).
As new module technologies emerge in the future, some ideas relating to BOS, such as micro-converters, may need to be revised. Furthermore, the quality of the system needs to be assured and adequately maintained according to defined standards, guidelines and procedures. To assure system quality, assessing performance is important, including on-line analysis (e.g., early fault detection) and off-line analysis of PV systems. The gathered knowledge can help to validate software for predicting the energy yield of future module and system technology designs.
Furthermore, very-large-scale PV systems with capacities ranging from several MW to GW are beginning to be planned for deployment (Komoto et al., 2009). In the long term, these systems may play an important role in the worldwide energy network (DESERTEC Foundation, 2007), but may demand new transmission infrastructure and new technical and institutional solutions for electricity system interconnection and opera- tional management.
Standard , quality assuranc , and safety and environmental aspects are other important issues. National and especially local authorities and utilities require that PV systems meet agreed-upon standards (such as building standards, including fire and electrical safety requirements). In a number of cases, the development of the PV market is being hin- dered by either: 1) existing standards, 2) differences in local standards (e.g., inverter requirements/settings) or 3) the lack of standards (e.g., PV modules/PV elements not being certified as a building element because of the lack of an appropriate standard). Standards and/or guidelines are required for the whole value chain. In many cases, developing new and adapted standards and guidelines implies that dedicated R&D is required.
Quality assurance is an important tool that assures the effective func- tioning of individual components in a PV system, as well as the PV system as a whole. Standards and guidelines are an important basis for quality assurance. In-line production control procedures and guide- lines must also be developed.At the system level, monitoring techniques must be developed for early fault detection.
Recycling is an important building block to ensure a sustainable PV industry. Through 2010, most attention has focused on recycling crys- talline silicon and CdTe solar modules. Methods for recycling other thin-film modules and BOS components (where no recycling procedures exist) must be addressed in the future. LCA studies are an important tool for evaluating the environmental profile of the various RE sources. Reliable LCA data are required to assure the position of PV with respect to other sources. From these data, properties such as the CO2 emission per kWh or kJ of electricity produced and the energy payback time can be calculated. In addition, the results of LCA analyses can be used in the design phase of new processes and equipment for cell and module production lines.