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10:00   Parallel Session: System Design, Optimization and Applications I
Chair: Vincent Lemort
20 mins
Emiliano Casati, Piero Colonna
Abstract: Thermodynamic conversion of solar radiation is one of the high-potential solutions to the global energy problem [1]. One of the key issues is the high capital cost and relatively low conversion efficiency of current technology [2]. We are exploring the concept of a small capacity modular (30-50 kWe) system powered by linear solar collectors. Economy of scale can be achieved by large series production. The small footprint makes it suitable for easier delivery and installation and the system is applicable even if only limited surface is available for the solar field. The adoption of the supercritical cycle configuration, which does not pose acute problems in terms of operating pressure and temperature with the adoption of complex molecule working fluids, arguably entails benefits from the efficiency and cost point of view, as well with respect to thermal storage and controllability. Challenges are identified with respect to, among others, the high compression ratio pump and the turbo-expander. A siloxane has been selected as the working fluid. A model of the solar absorber is presented, together with its coupling to the model of the power unit, which is capable of off-design simulations. A preliminary design of the equipment has been performed in order to investigate feasibility. Results of system simulations encompassing a solar year are presented. They lead to the preliminary thermodynamic optimization of the system, which is analyzed and discussed. Future work is briefly outlined.
20 mins
Matthew Orosz, Alexander Fanderl, Christian Muller, Harold Hemond
Abstract: The lack of suitable expanders hinders the use of ORCs (organic Rankine cycle engines) for solar power or waste heat recovery at small (kilowatt) scales. Purpose-built turbomachinery is cost prohibitive at these scales. Positive displacement machines derived from HVAC scroll compressors are generally inexpensive alternatives and are available in various displacements. A drawback of HVAC scrolls as expanders, however, is their inbuilt volume ratio, usually ~3, which is only useful with typical ORC fluids at thermal resource temperatures of ~100°C, i.e. much lower than the potential of concentrating solar and some waste heat sources. Kane addressed this problem by using modified HVAC scrolls and operating two ORCs in series, driven by solar thermal and diesel exhaust gas at 150°C [1]. This solution has the drawback of requiring the duplication of equipment needed to match the fluid and machine volume ratios. We report here on three alternative approaches using a single ORC operated from a 150°C thermal source and various configurations of positive displacement expanders: a pair of HVAC scrolls in series, an HVAC machine incorporating a purpose-built high volume ratio scroll, and a piston expander. The compounding of HVAC scroll expanders is greatly facilitated by removing the limitation of asynchronous generation at grid frequencies. Without this constraint, testing at MIT on a 3kWe ORC test bench confirms that HVAC scrolls of suitable displacements can be paired and loaded separately for effective two-stage expansion of the working fluid R245fa to a volume ratio of 9, albeit with the added complexity of recombination of electrical outputs. Starting from the geometric framework of HVAC scrolls [2], we created a new scroll design derived from an examination of the tradeoffs between volume ratio, scroll size, number of turns, and spiral equations for constant and varying wall thicknesses. We also examined the potential of applying the ubiquitous IC engine, reconfigured for two-stroke operation as an expander using redesigned cam profiles to alter the valve gear timing; this approach has the advantage of being low-cost with standard volume ratios of 8-10. For these several approaches machine characteristics were related to a thermodynamic model to derive machine speeds (RPM) at mass flow rates corresponding to a few kilowatts of power output. Sizing, specification, and performance models are developed in Matlab and EES, prototype expanders are designed, and performance characteristics are compared on the MIT 3kWe test system using R245fa at 150°C.
20 mins
Markus Preißinger, Theresa Weith, Dieter Brüggemann
Abstract: Organic Rankine Cycle (ORC) is a state of the art technology in low temperature geothermal applications. Besides, ORC systems are used in biomass fired heat and power plants as well as for waste heat recovery at high temperatures. A promising way to further increase the efficiency of ORC systems is the supercritical mode of operation. In the present work subcritical and supercritical ORC for waste heat recovery at high temperatures are analyzed. Nine potential working fluids out of three chemical classes (alkanes, alkylbenzenes, siloxanes) are investigated with waste heat temperatures in the range of 633.15 K to 823.15 K and ORC working pressures up to 1.3∙pcrit. Simulations are carried out using Peng-Robinson Equation of State (EOS). In addition, the influence on thermodynamic and plant-specific properties using other EOS are discussed within this work. Simulations for supercritical octamethyltrisiloxane (OMTS) show an increase in electric net power of more than 5.5 % compared to subcritical process. The gain in electric power of the generator is even higher (8.5 %). However, it is pointed out that the enhancement of electric power is quite sensitive to waste heat temperature. This is also valid for the optimal working pressure in subcritical process in which net power is maximized at a working pressure of 0.58 MPa (0.80 MPa, 1.38 MPa) for heat source temperatures of 633.15 K (663.15 K, 693.15 K). In addition to thermodynamic analysis further aspects like overall size of the turbine and heat transfer characteristics of the heat exchange equipment have to be considered. Therefore, volume flow rates at turbine inlet and outlet as well as heat transfer coefficients are calculated to allow for a holistic evaluation of supercritical ORC.