Direct Solar Energy
Wp), but also on lifecycle gains, that is, actual energy yield (kWh/Wp or kJ/Wp over the economic or technical lifetime).
High-productivity manufacturing, including in-process moni- toring and control. Throughput and yield are important parameters in low-cost manufacturing and essential to achieve the cost tar- gets. In-process monitoring and control are crucial tools to increase product quality and yield. Focused effort is needed to bring PV manu- facturing to maturity.
Environmental sustainability. The energy and materials require- ments in manufacturing, as well as the possibilities for recycling, are important parameters in the overall environmental quality of the product. Further shortening of the energy payback time, design for recycling and, ideally, avoiding the use of materials that are not abundant on Earth are the most important issues to be addressed.
Applicability. As discussed in more detail in the paragraphs on BOS and systems, standardization and harmonization are important to bring down the investment costs of PV. Some related aspects are addressed on a module level. In addition, improved ease of installa- tion is partially related to module features. Finally, aesthetic quality of modules (and systems) is an important aspect for large-scale use in the built environment.
Solar-assisted air-conditioning technology is still in an early stage of development (Henning, 2007). However, increased efforts in techno- logical development will help to increase the competitiveness of this technology in the future. The major trends are as follows:
Research in providing thermally driven cooling equipment in the low cooling power range (less than 20 kW);
Developing single-effect cycles with increased COP values at low driving temperatures;
Studying new approaches to enhance heat transfer in compart- ments containing sorption material to improve the power density and thermal performance of adsorption chillers;
Developing new schemes and new working fluids for steam jet cycles and promising candidates for closed cycles to produce chilled water; and
Research activities on cooled open sorption cycles for solid and liq-
Photovoltaic electricity generation
This subsection discusses photovoltaic technology improvements and innovation within the areas of solar PV cells and the entire PV system. Photovoltaic modules are the basic building blocks of flat-plate PV systems. Further technological efforts will likely lead to reduced costs, enhanced performance and improved environmental profiles. It is useful to distinguish between technology categories that require specific R&D approaches.
Funding of PV R&D over the past four decades has supported innovation and gains in PV cell quality, efficiencies and price. In 2008, public budgets for R&D programs in the IEA Photovoltaic Power Systems Programme countries collectively reached about USD2005 390 million (assumed 2008 base), a 30% increase compared to 2007, but stagnated in 2009 (IEA, 2009c, 2010e).
For wafer-based crystalline silicon, existing thin-film technologies, and emerging and novel technologies (including ‘boosters’ to the first two categories), the following paragraphs list R&D topics that have highest priority. Further details can be found in the various PV roadmaps, for example, the Strategic Research Agenda for Photovoltaic Solar Energy Technology (US Photovoltaic Industry Roadmap Steering Committee, 2001; European Commission, 2007; NEDO, 2009).
Advanced technologies include those that have passed some proof- of-concept phase or can be considered as 10- to 20-year development options for the PV approaches discussed in Section 3.3.3 (Green, 2001, 2003; Nelson, 2003). These emerging PV concepts are medium to high risk and are based on extremely low-cost materials and processes with high performance. Examples are four- to six-junction concentra- tors (Marti and Luque, 2004; Dimroth et al., 2005), multiple-junction polycrystalline thin films (Coutts et al., 2003), crystalline silicon in the sub-100-m-thick regime (Brendel, 2003), multiple-junction organic PV (Yakimov and Forrest, 2002; Sun and Sariciftci, 2005) and hybrid solar cells (Günes and Sariciftci, 2008).
Even further out on the timeline are concepts that offer exceptional per- formance and/or very low cost but are yet to be demonstrated beyond some preliminary stages.These technologies are truly high risk, but have extraordinary technical potential involving new materials, new device architectures and even new conversion concepts (Green, 2001, 2003; Nelson, 2003). They go beyond the normal Shockley-Queisser limits (Shockley and Queisser, 1961) and may include biomimetic devices (Bar- Cohen, 2006), quantum dots (Conibeer et al., 2010), multiple-exciton generation (Schaller and Klimov, 2004; Ellingson et al., 2005) and plas- monic solar cells (Catchpole and Polman, 2008).
Efficiency, energy yield, stability and lifetime. Research often aims at optimizing rather than maximizing these parameters, which means that additional costs and gains are critically compared. Because research is primarily aimed at reducing the cost of electric- ity generation, it is important not to focus only on initial costs (USD/
PV concentrator systems are considered a separate category, because the R&D issues are fundamentally different compared to flat-plate technologies.As mentioned in Section 3.3.3, CPV offers a variety of tech- nical solutions that are provided at the system level. Research issues can be divided into the following activities: 1) concentrator solar cell