Collectors for medium size solar fields of several hundred square meters can be fabricated cost-efficient in production halls, in contrast to power plant collectors, which have to be assembled on site in special jigs due to their size. On the other hand less units will be produced for small installations and thus installation costs are increased. In principal only small amounts of material are necessary for concentrating collectors, as a thin (and lightweight) mirror gathers the radiation onto a small absorber. Improvements in production towards light-weight collectors, a sufficient production capacity and fast installation procedures may bring costs down below 200 €/m2 installed in the next years.

Engine costs may not decrease notably, but electrical efficiency could be improved significantly, especially when engines with higher steam temperatures were available. A 50 kW prototype steam engine of the company IAV, Germany reached 24% mechanical efficiency (Buschmann et al). The company Otag, Germany aims at 16% efficiency for a 3 kW engine. 20% engine efficiency does seem to be accomplishable, but higher losses in the collector field have to be accounted for, because of higher collector temperature. Assuming achievements as 20 % electrical efficiency in the engine, 200 €/m2 collector costs (installed) and a 20 years life-time (8.7% annuity), solar heat costs of 0.05 €/kWh and electricity costs of 0.11 €/kWh can be accomplished in future installations.

Small installation for electric power range of 1 to 5 kWel

Combined solar heat and power can also serve as a technology for single-family homes. This demands small collectors, which can be installed on tilted roofs. Also small efficient heat engines are required. Several companies are currently developing small Stirling or steam engines of several kW electrical power for combined heat and power production in residential homes.

The market for small CHP systems delivering several kW heat and electricity seems to have a high potential, which could also be matched with appropriate solar systems.

Solar electricity may even be produced during night by using thermal storage, which is especially interesting for solar village systems, increasing the potential for these systems.

Conclusions

The economics of solar district heating systems can be improved by introducing an engine producing electricity under today’s German tariffs for solar electricity. Such kinds of installations currently still lack high-performance medium size parabolic trough collectors. Availability of heat engines over a wider power range would open more opportunities. However the proposed system based on the existing Spilling steam engine with 120 kW of electric power could be an option under German climatic conditions and current regulations, once a high-efficient parabolic trough collector is available for the proposed solar field size of 1850 m2 for 400 €/m2 or below.

Current developments in the small CHP market are interesting for solar and hybrid applications in residential or industrial applications. Small engines and the envisaged 20­24% of conversion efficiency are very interesting in order to reduce the cost of combined solar heat and power systems.