At present, low-temperature collectors are widely used. There are also many other high-quality products on the market, which follow different approaches, depending on climatic conditions and applications.

However, in view of the anticipated market development, a number of technological challenges are arising. Some of the key issues include cost reduction, higher quality, aesthetics and building integration.

With regard to cost reduction, the basic trade-off between cost and quality (performance, durability, recyclability and aesthetics) needs to be considered. The bulk of the European market has evolved towards higher-quality products and systems, but this has not been the case elsewhere (for example, in China, which accounts for more than two thirds of global sales). Looking at the European market, there is ample potential to develop cheaper products, which may be less durable and effective, but which are cheaper and can be easily replaced. This may lead to a different approach, which could be advantageous if lower performance is compensated by lower costs and thus a faster uptake of solar

thermal energy use. Building integration is relevant for both new and existing buildings. It includes issues such as the:

• inclusion of solar collectors in prefabricated roofs, awnings and facades, and

• further development of collectors conceived for vertical uses (facades), including large-area facade-integrated collectors, which can be combined with so-called active walls or with photovoltaic modules.

Higher levels of building integration require new rounds of RD&D efforts in close co-operation with architects, construction companies and manufacturers of building envelopes.

A related issue is collector size, which is traditionally relatively small (around 2m2). However, there is a recent trend towards larger collector dimensions, implying a different set of conditions for collector design, manufacturing, logistics and installation. In this area, there is significant potential for technological development. Taking into account the projected rapid increase in market size, the recycling potential of materials used in the solar collectors will be a major issue. The lifecycle assessment of the whole solar thermal system, taking into account the fuels and the materials they replace, will also be crucial. New approaches, such as the active wall that heats and cools the room behind it, are very promising and should be developed.

For all issues mentioned here, specific RD&D attention should be given to air and vacuum tube collectors. Although air collectors have great potential, particularly for applications such as space heating, ventilation and space cooling by ventilation, they are less developed than liquid-based collectors. For vacuum tube collectors, the potential for further development lies mainly in improved building integration. This includes easier tube replacement in case of failure, resistance to stagnation, possible thermal improvements and longevity.

Clearly, a major issue is the automation of manufacturing processes, particularly for collector construction, where there is still great potential for increased productivity.

Major efforts are needed in the following areas:

• More efficient ways to use conventional collector materials (metals, glass, insulation), especially with a view to developing multifunctional building components, which simultaneously act as an element of the building envelope and a solar collector.

• Evolution in the optical properties of collector components. In particular, a more systematic use of optical films to enhance heat/light transmission through glass covers and reduce this transmission during excessive exposure; and the use of colours in absorbers or covers to achieve more flexible integration concepts.

• Alternative materials for collector production: the use of polymers or plastics, the coating of absorbers optimised to resist stagnation temperatures and new materials to prevent deterioration resulting from UV exposure.

• Improvement in the recycling potential of collector components and materials in view of lifecycle cost reduction, and overall sustainability of materials.

• Special topics will include issues such as: The control of solar energy delivered by entire facades, in particular the aspects related to fault detection and the consequences of stagnation temperatures when a prolonged no-load situation coincides with peak solar radiation;

• New component testing and evaluation methods; and

• A dedicated concept for the automation of manufacturing processes and assembly techniques.