The Solar Turbine Group (STG) was founded for the purpose of developing and implementing a small scale solar thermal technology utilizing medium temperature collectors and an ORC to achieve economics analogous to large-scale solar thermal installations. This configuration aims at replacing or supplementing Diesel generators in off grid areas of developing countries, by generating clean power at a lower levelized cost (~$0.12/kWh compared to ~$0.30/kWh for Diesel [9, 11]). At the core of this

technology is a solar thermal power plant consisting in a field of parabolic solar concentrating collectors and a vapor expansion power block for generating electricity (figure 1). An electronic control unit is added for autonomous operation as sub-megawatt scale plants cannot justify the staffing of operating personnel. The design tradeoffs for maintaining low costs at small scales, include operating at a lower cycle temperatures (<200 DC) and using an ORC: lower temperatures enable cost savings in the materials and manufacture of the absorber units, heat exchangers, fluid manifolds and parabolic troughs.

Because no thermal power blocks are currently manufactured in the kilowatt range a small-scale ORC has been designed for this application (Figure 2). The design is based on off-the-shelf components with little modification, such as HVAC scroll compressors (for the expander), and car power steering pumps.

A novel control strategy and electronic control system is required for the components discussed above to work in concert and in a maximally efficient manner. Among the functions to be managed is the control of individual components, such as Solar ORC fluid machinery, as well as directing the energy flows between these components, battery storage and AC loads. Optimization of the control strategy, a major objective of ongoing research, will be based on theoretical and experimental characterization of all system components, and will, in practice, rely on developing a control system with feedback from diverse parameters ranging from ambient temperature to the particular load profile to be matched.

The work presented in this paper focuses on the characterization of the ORC system by developing and validating a model, which will be used to select the best components, working fluid and control strategies for the solar Rankine engine.

2. Modeling

The ORC model is built by connecting the models of its different main components. A volumetric pump and a scroll expander models are considered since they are the technologies selected for the ORC prototype presented in this paper. All models are developed under EES [5] using semi-empirical parameters that are identified with experimental data.