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10:00   Operational experience I
Chair: Dr. Joost Brasz
20 mins
Marco Astolfi, Roberto Bini, Ennio Macchi, Marco Paci, Claudio Pietra, Nicola Rossi, Alessio Tizzanini
Abstract: Recent international focus on the value of increasing renewable energy supply highlights the need for revaluating all alternatives, particularly those that are large and well-distributed. One such option is geothermal energy from hydrothermal low temperature resources. The most efficient and cost-effective way to exploit this type of reservoir is based on the use of binary cycles. Purpose of ENEL Research in this field is to develop and design an optimized and highly flexible binary cycle to be applied to the exploitation of water dominant geothermal resources acquired by ENEL worldwide and to study options for its integration with solar energy. Different Rankine cycles for geothermal resources at temperature levels between 120°C and 180°C have been extensively studied and optimized to maximize overall conversion efficiency and to minimize plant costs, in collaboration with Politecnico di Milano. This analysis allowed the assessment both of subcritical optimized cycles and supercritical innovative advanced cycles using different working fluids (e.g. hydrocarbons and refrigerants). Moreover the carried out studies showed that in a supercritical cycle the possibility to operate outside the fluid saturation curve during the heat adduction phase guarantees a greater power production (no pinch-point problems) and operational flexibility with respect to subcritical cycles (e.g. reduced performance decline due to external condition variability such as brine temperature and mass flow rate and environment air temperature). Based on the carried out theoretical analysis, the decision to demonstrate an advanced, high efficient binary cycle at the pilot scale was taken. A preliminary design of the main components for a 500 kWe prototypal-sized supercritical binary power plant was carried out in collaboration with Turboden and Politecnico di Milano. The plant was designed and realized by Turboden and was installed in the Enel experimental platform (Livorno, Italy) at December 2011. In the first part of 2012 the pilot plant was put in operation and characterized for different geo-fluid conditions (e.g. temperature and flow rate) and ambient conditions. The experimental tests carried out on the supercritical pilot plant validate the achieved theoretical results, taking special care to plant performance flexibility and main component design criteria. This paper will give an overview of the theoretical and experimental activities carried out in order to characterize the ORC supercritical technology.
20 mins
Marco Frassinetti, Dario Rizzi, Aldo Serafino, Lorenzo Centemeri, Claudio Spadacini
Abstract: All the existing ORC cycles worldwide that have been based on turbomachinery are employing axial or radial inflow turbines. Exergy has introduced a new alternative configuration: the radial outflow turbine. The radial outflow turbine has many unique characteristics which result in high ORC engines efficiency. The high efficiency of the machine is combined with the better overall cycle efficiency as a result of higher pressure ratio and volumetric ratio. In detail, Exergy radial outflow turbine most distinguishing features are: - The generally high inlet/outlet volumetric flow ratio is well combined with the change in cross section across the radius; - Compared to an axial turbine, the low inlet volumetric flow is compensated by higher blades at the first stage. This is due to the section change available along the radius, so that there is no need for partial admission; - The constant peripheral velocity along the blade span leads to constant velocity triangles and to prismatic blades; - Tip leakages and disk friction losses are minimized by the multi-stage configuration on a single disk; - The single disk/multi-stage configuration allows a solution with higher number of stages in overhung arrangement without affecting the rotordynamics of the turbine; - The intrinsic limits of a radial outflow expander to develop high enthalpy drop is not relevant for this cycle, presenting itself a very low enthalpy drop. Moreover the tip speed is limited by the low speed of sound and consequently this kind of expander suits well with this cycle arrangement. The predicted results about the radial outflow turbine presented at the 1st International Seminar on ORC Power Systems are reviewed and they are compared with empirical data from field. Firstly a brief description of the turbines is provided and the instruments used to perform the measurements are presented. Secondly the measured data are plotted, compared with the thermodynamic simulations and CFD analysis performed during the design phase. Finally an evolution of the radial outflow turbine is proposed and analyzed.
20 mins
Stefano Ganassin, Quirijn Eppinga, Jos van Buijtenen
Abstract: Organic Rankine Cycle systems are generally meant to convert relative low temperature heat into electricity. However, ORC systems have also proven themselves to be more versatile in cases of small sizes up to 1 MWe, irrespective of temperature. In the case of biomass combustion, the heat source temperature is high enough to motivate a water/steam cycle, unless the amount of heat to convert is small. Here ORC systems come also into play. As most ORC working fluids have a limited chemical stability at high temperatures, intermediate fluid circuits are applied for heating the evaporator. This can be either a thermal oil loop, or a system based on pressurized water. Apart from the extra investment, complexity and need for emergency cooling, such systems tend to increase losses in terms of heat transfer and pumping power for circulation. Moreover, the available temperature is not used for optimal Carnot efficiency. Based on the application of a chemically stable hydrocarbon, a system was designed for the direct heating of the working fluid by flue gasses from the combustion of biomass. Flue gasses, as they leave the biomass combustor, are cooled down to 530 °C by mixing with ambient air or recirculated flue gas. This temperature level is sufficient for the ORC working fluid to reach its desired highest cycle temperature of 325 °C. Moreover, condensable salts condense to particles and are separated before they can hit the heating surface of the evaporator, thus avoiding the formation of eutectic layers on the heating surfaces. Two units have been build using the Triogen 165 kWe ORC system: one based on a moving grid combustor and one based on a fluidized bed combustor. Specifics of plant lay-out will be given, together with first operating experience. The combined systems of biomass combustion and ORC open the possibility to use biomass for small scale electricity production. Units, so far only designed for the supply of heat for drying or district heating, can be extended to cogeneration plants for very flexible power-to heat ratios.