Off-Design behavior and control strategies of small scale cycles with organic fluids.

Energy saving is one of the priority of our society, involved in worldwide challenges such as climate change, energy resource depletion, conflicts between nations and economic crisis. This objective must be achieved both by energy end-users through an increased awareness about wasted energy and the use of high efficiency devices, and by energy producers. In the electric energy production field, the possibility of decentralizing the production allows to reduce the plant size, gives the possibility to end users to become independent energy producers, to exploit renewable energies and to increase the efficiency of the systems by recovering low-temperature heat. Because of the stochastic nature of some of those energy sources, it is expected that future energy systems will be forced to become increasingly flexible, in order to deal with this challenge. In this scenario, Organic Rankine Cycles (ORC) are one of the fastest growing technologies, due to their ability of exploiting low temperature heat sources, typical of renewable energies and of waste heat streams, to their simplicity and low costs. The aim of this doctoral thesis is to contribute to the knowledge of the modeling and optimization of small scale organic cycles systems for low temperature applications, such as low concentration solar application, waste heat recovery and micro-geothermal systems. Particularly various control strategies and control parameters have been defined to improve efficiency, flexibility and system management, both for organic Rankine cycles and for organic flash cycles (OFC), which can be considered an advanced architecture of the basic Rankine cycle. The use of a positive displacement rotary engine, which is particularly suitable for small scale cycles under variable working conditions, due to its high simplicity and flexibility, has allowed to simulate the operation of a small scale solar ORC, according to a sliding-velocity control strategy and without any thermal storage, reducing in this way the number of required solar collectors and simplifying the system layout. The dynamic analysis of the plant highlighted the effect of various transient phenomena which caused a variation in the prediction of annual plant production with respect to steady-state approach. The comparison of sliding-velocity and sliding-pressure control strategy of a small scale Waste Heat Recovery (WHR) ORC, has highlighted the ineffectiveness of both of them in the case of highly variable heat sources and has lead to the definition of an optimal combined sliding-pressure and velocity control strategy, controlled by easy measurable variables: the optimized function which allowed operation according to this control strategy could be extrapolated, in steady-state conditions, both from system data and even from expander data through a simple approximation of the heat exchangers off-design behavior. Dynamic simulations have confirmed that both methods lead to better results in terms of system flexibility, efficiency and safety. One of the major problems of ORCs is the constant temperature evaporation phase, which increases entropy production during the isothermal heat transfer and in the case of sensible WHR systems, keeps the exhaust temperature of the heat flux high. Organic flash cycles could be an alternative solution to bypass this problem: however, the architectures proposed in the literature for low temperature WHR systems have the drawback of high specific costs. A new regenerative architecture with the same thermodynamic performance of the original architecture has been defined, with the result of a decrease in systems cost, leading the specific cost of the main component of the cycle to be equal to that of basic ORC systems, in the case of very small scale applications. The off-design analysis comparison of single flash cycle with and without regeneration has highlighted the higher flexibility of the regenerative solution, and the possibility of adopting an optimal combined sliding-pressure and velocity control strategy, acting on the expander speed and on flash pressure, in a similar manner as in ORC systems.