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Concentrating collectors use a reflective parabolic-shaped surface to reflect and concentrate the sun’s energy to a focal point, where the absorber is located. Concentrating collectors use only direct sunlight; most other types of collectors can also use indirect sunlight. Concentrating collectors come in two types: imaging reflectors and non-imaging reflectors. A non-imaging reflector collector looks much like a flat plate collector, as the mirrors and receivers are mounted in a glazed frame of similar dimensions to flat plate collectors. Non-imaging reflectors do not track the sun but remain stationary while still retaining the ability to focus the sun’s energy on the receiver as the sun moves across the sky. These collectors make up a very small percentage of the total collector market. To work effectively, imaging reflectors must track the sun. These collectors can achieve very high temperatures because the diffuse solar resource is concentrated on a small area. In fact, the hottest temperatures ever measured on the earth’s surface have been at the focal point of a massive concentrating solar collector. This is sort of like starting a fire with a magnifying glass on a sunny day.

Imaging collectors have been used to make steam that spins an electric generator in a solar power station. This is commonly referred to as Concentrating Solar Power (CSP). Large electricity generating plants using imaging concentrating collectors have been constructed in dry climates around the world with great success. A lot of research is being done in this field, and we can expect this technology to grow significantly in the near future.

Figure 3.6: Concentrating collector

Although there have been attempts to use imaging collectors in domestic water heating systems, no successful or durable products have been developed. Problems were encountered with the tracking mechanisms, the precision needed in the mechanisms and the durability of the reflectors and linkages. Recently a couple of firms have been releasing imaging collectors for small commercial and industrial use. Given the poor track record of imaging collectors, we are still unsure whether this second attempt will be a success. They are particularly vulnerable to snow and ice buildup in cold climates.