Emiliano Casati

Organic Rankine Cycle Power Systems: From the Concept to Current Technology, Applications, and an Outlook to the Future

The cumulative global capacity of organic Rankine cycle (ORC) power systems for the conversion of renewable and waste thermal energy is undergoing a rapid growth and is estimated to be approx. 2000 MWe considering only installations that went into operation after 1995. The potential for the conversion of the thermal power coming from liquid-dominated geothermal reservoirs, waste heat from primary engines or industrial processes, biomass combustion, and concentrated solar radiation into electricity is arguably enormous. ORC technology is possibly the most flexible in terms of capacity and temperature level and is currently often the only applicable technology for the conversion of external thermal energy sources. In addition, ORC power systems are suitable for the cogeneration of heating and/or cooling, another advantage in the framework of distributed power generation. Related research and development is therefore very lively. These considerations motivated the effort documented in this article, aimed at providing consistent information about the evolution, state, and future of this power conversion technology. First, basic theoretical elements on the thermodynamic cycle, working fluid, and design aspects are illustrated, together with an evaluation of the advantages and disadvantages in comparison to competing technologies. An overview of the long history of the development of ORC power systems follows, in order to place the more recent evolution into perspective. Then, a compendium of the many aspects of the state of the art is illustrated: the solutions currently adopted in commercial plants and the main-stream applications, including information about exemplary installations. A classification and terminology for ORC power plants are proposed. An outlook on the many research and development activities is provided, whereby information on new high-impact applications, such as automotive heat recovery is included. Possible directions of future developments are highlighted, ranging from efforts targeting volume-produced stationary and mobile mini-ORC systems with a power output of few kWe, up to large MWe base-load ORC plants.

Preliminary Design of a Centrifugal Turbine for Organic Rankine Cycle Applications

Organic rankine cycles (ORC) are renowned to be attractive energy conversion systems for the thermal energy sources in the small-to-medium power range. A critical component in the ORC technology is the turbo-expander; the difficulties involved in the accurate thermodynamic modeling of organic fluids and, especially, the complex gasdynamic phenomena that are commonly found in ORC turbines may result in relatively low efficiency and in performance reduction at partial loads. In this perspective, a relevant path of development can be outlined in the evaluation of nonconventional turbine architectures, such as the radial-outward or centrifugal turbine. In the present work, a critical evaluation of the feasibility of multistage transonic centrifugal turbines for ORC systems is presented. To support this study, a two-step design procedure, specifically oriented to ORC turbines, was developed. The methodology includes a 1D mean-line code coupled to an external optimizer to perform a preliminary design of the machine. The selected configurations are then verified with a CFD (computational fluid dynamics)-based throughflow solver, able to deal with any flow regime and to treat fluids described by arbitrary equations of state. The overall procedure is applied to the design of two different turbines of the same target power of about 1 MW, the former representing a transonic six-stage turbine and the latter a supersonic three-stage turbine. The two machines are characterized by very different shape and comparable performances. The results are extensively discussed in terms of both overall data and detailed flow fields.

Optimal Operation of Solar Tower Plants with Thermal Storage for System Design

Abstract Concentrated Solar Power CSP plants are increasingly being considered for construction worldwide, in order to meet the demand for renewable power generation. The most promising technology considered today employs a central receiver, illuminated by a heliostat field, using molten salts as working fluid. A distinctive feature of these plant is the possibility of thermal energy storage, providing 15 or more hours of full power operation without solar irradiation. The state-of-the-art SAM software is often use for sizing the plant and evaluating the return on investment, assuming a straightforward and short-sighted control strategy. In this paper, a model similar to that used by SAM is developed and then used to demonstrate the potential advantages of optimal control, in a context of variable tariffs with higher prices during peak hours. The modelling and optimization problems are formulated with the high-level Modelica and Optimica languages, which allows to solve the problem with minimal effort. This paper is a first step to promote the use of optimal control techniques and high-level modelling languages for the correct evaluation of the potential performance of CSP plants with thermal storage during their design phase.