1.1 Technical Description

Within this chapter the integration of a solar collector into a 20 MWei biomass Rankine cycle will be presented with respect to technical and mainly economical aspects. The following benefits will result from the combination of both technologies:

• The plant is operated exclusively with renewable energy-sources

• Biomass is a limited source of renewable energy. With solar thermal electricity production the amount of biomass substituted by solar energy can be used in other energy applications

• No net-production of CO2 during operation

• Typical live steam parameters of a biomass plant correspond to those of the Fresnel-Collector (approximately 450°C and 70 bar)

• The plant can be operated 24 hours a day without the need for heat storage.

• Compared to Solar Only plants, the efficiency of converting solar radiation into elec­trical energy may be higher in hybrid power plants since the steam cycle always runs at full load.

In order to use realistic assumptions for the power block within this study, its layout is based on existing biomass power plants of E. ON in Zolling or Landesbergen, Germany. However this kind of system would not be appropriate to be built in Germany because of the limited solar energy yields. The technology as described is applicable for example in South European regions.

Figure 1 shows the water / steam cycle of the hybrid plant. Both heat sources — solar field and biomass boiler — are connected in parallel. In order to supply a constant electricity output (20 MWel), the biomass boiler provides the amount of thermal energy that can not be supplied by

the solar field. To ensure reliable operation, the biomass boiler runs at 50% or more of its nominal thermal load. Accordingly, the solar thermal input rises up to 50% depending on the availability of solar irradiation.

The size of the solar field is varied within these simulations from 0 m2 to 190’000 m2. The solar share, i. e. the percentage of the annual amount of solar steam fed into the turbine is pictured in figure 2. The solar field size 0 m2 corresponds to the pure biomass plant without

solar field.

The amount of solar energy depends approximately linearly on the solar field size between 0 and 65’000 m[1] [2]. Solar fields greater than 65’000 m2 lead to dumping of solar energy during hours with full irradiation, because the solar field must not provide more than 50% of the nominal steam mass flow for the turbine.