A mean solar field temperature of 250°C is required to operate the engine due to the temperature drop in the steam generator, if pressurized water is used as heat transfer medium in the solar field. Alternatively the steam can be directly generated in the collectors, resulting in lower temperatures and less thermal losses (Hennecke et al, Zarza et al). At 250°C an efficiency of 65% at 800 W/m2 irradiation and an annual yield of about 450 kWh/m2*a can be expected from the collector including losses caused by heat capacities and in piping. A solar field of 1850 m2 aperture area would be necessary for the nominal load of the engine. Due to its size the field can only be erected with horizontally mounted collectors.

According to TRNSYS-based calculations the annual electrical output from the engine related to the solar aperture area is 47 kWh/m2*a. Heat delivery from the condenser of up to 100°C accumulates to 390 kWh/m2 per year. At times of low irradiation, when less than 30 % of the engine’s design heat input is delivered by the solar field, electricity production will be stopped.

Apart from the above-described thermodynamics in stationary conditions, effects as heat capacity and controllability will play an important role. These can be best investigated in existing installations though.

Costs and Revenues

As there are no experiences about high-efficient, medium size parabolic trough collector field costs, assumptions are based on the EuroTrough technology and the PTC 1800 collector of the company Solitem. The EuroTrough collector cost is estimated to about 200 €/m2 (Geyer et al) in its first power plant project, whilst the PTC 1800 costs about 500 — 600 €/m2 (including peripheral equipment as pumps, control and installation) for a 400 m2 field size in its first installations. Basing on these numbers collector field costs of 400 €/m2 installed are assumed to be achievable. The solar field costs would amount to 740.000 €. The Spilling engine costs are 160.000 € including installation and control facilities. The investments amount to about 900.000 € in total.

While the electrical power can be fed to the grid, a typical application for the condensation heat could be domestic hot water distributed in district heating for about 500 housing units. For such an application the equivalent value of the thermal energy is about 0.05 €/kWh, amounting in income of 35.000 € per year for the solar heat. Under German regulations solar electricity from such an installation can be sold to the grid for 0.457 €/kWh,
amounting to 37.000 € income per year. This means that the electricity from the engine adds 50% of the revenues at only 20% of the investment costs.

A relation of 72,000 € savings and income per year and 900,000 € of investment cost can be regarded as acceptable for the first installations of such a system, but not in the long run though. So what are the potential future improvements in efficiency and cost?