10:30
Session 1B: Dynamic Simulation
Chair: Francesco Casella
10:30
20 mins
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Model predictive control of an Organic Rankine Cycle system
Xiaobing Liu, Adamu Yebi, Paul Anschel, John Shutty, Bin Xu, Mark Hoffman, Simona Onori
Abstract: Organic Rankine Cycle (ORC) waste heat recovery systems offer promising engine fuel economy improvements for heavy-duty on-highway trucks. An ORC test rig with parallel evaporators to recover both tailpipe and EGR waste heat from a 13L heavy duty diesel engine was developed and used in this work to demonstrate a novel control strategy based on Model-Predictive Control (MPC). The main control objectives for the ORC system are: (i) regulation of working fluid temperature, (ii) safe turbine operation - away from 2-phase region, and (iii) maximization of waste heat recovery. The MPC uses a built-in moving boundary evaporator model to predict future system response and generate optimal actuator reference commands. Two variants of MPC were considered in this work: an adaptive linear MPC (LMPC) and a nonlinear MPC (NPMC). Compared with the traditionally used PID controller, MPC demonstrates more accurate temperature control and improved disturbance rejection in simulation. Finally, the LMPC and NMPC controllers were implemented on the ORC test rig and showing promising initial test results.
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10:50
20 mins
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Dynamic modeling of an Organic Rankine Cycle to recover waste heat for transportation vehicles
José Galindo, Vicente Dolz, Lucía Royo, Aldrik Brizard
Abstract: According to the International Council on Clean Transportation, the target for CO2 emissions in passenger cars was reduced until 2007 an average of 1 % per year, however from 2008 more restrictive targets were imposed with an average of 4 % per year. In 2020 the target of CO2 emissions for passenger cars and light-commercial vehicles will be reduced from 2012 up to 28% and 18%, achieving values of 95 g/km and 147 g/km respectively. Therefore, waste heat recovery technologies seem to assume an essential role in the CO2 reductions of the forthcoming decade.
This paper deals with a 0D simulation model of an Organic Rankine Cvcle coupled to a 2 l naturally aspirated gasoline engine and using ethanol as working fluid. The main purpose of this paper is to illustrate the performance and capabilities offered by the interaction between both systems by using the LMS Imagine.Lab Amesim platform. An experimental facility of an ethanol ORC using a swash-plate expander coupled to gasoline engine has been used to calibrate it.
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11:10
20 mins
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The study of dynamic process of ORC variable conditions based on control characteristics analysis
Yunli Jin, Naiping Gao, Tong Zhu
Abstract: The effect of the characteristics of the ORC components including equipment and parameters on system performance has been explored deeply in previous studies. The fluctuation of heat source and load demand in practical application, leads to the change of ORC operation environment. Operation condition variation of ORC system responding to this change should be done, which realized by certain control strategy. The way that control strategies impose influence on ORC system has not been investigated. The purpose of this paper is to study the impact of control strategy on ORC performance. The control characteristic analysis is introduced which means the dynamic characteristic of ORC system under certain control mode. Four different control strategies based on different regulated variables, control variables, control algorithm and operation mode are proposed and compared. The dynamic process model is built and simulated. The results show that the selection of regulated variables, control variables and control algorithm directly influences the dynamic behavior of condition changing, not of final steady state. The operation mode selection of fixed or slide pressure has the effect on ORC in both dynamic process and final steady state.
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11:30
20 mins
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Dynamic analysis of off-grid ORC plants with various solution for the thermal storage
Paolo - Iora, Gioele Di Marcoberardino, Costante M. Invernizzi, Giampaolo Manzolini, Paolo Belotti, Roberto Bini
Abstract: Real time matching of electric power generation is a crucial aspect in off-grid systems as well as in case of use of renewable intermittent sources. In this paper, with reference to 1 MWel Turboden biomass ORC plants operating in off-grid systems, we study the possibility to store thermal energy in the form of sensible heat within a storage composed by a bunch of steel or cast iron pipes with variable thickness and different coating materials. Thermal power is taken in and out of the storage by a flow of thermal oil, heated by a biomass furnace, and eventually supplied and converted into electricity by the ORC plant running in an off-grid area. Starting from steady state conditions at 30% of the nominal power of the furnace, we assume an instantaneous increase to 100% of the electric power demand and we study the transient of the system in correspondence of the furnace power ramp. To this purpose, we developed a dynamic finite difference model , for the system composed by the furnace, the ORC plant, the furnace inlet and outlet piping and the thermal storage. It comes out that with a storage system properly designed as function of the furnace power ramp, it is possible to run the ORC at 100% during the analyzed transient, thus allowing a real time matching of the power demand.
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11:50
20 mins
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Dynamic modeling and optimization of an ORC unit equipped with plate heat exchangers and turbomachines
Matteo Marchionni, Giuseppe Bianchi, Apostolos Karvountzis-Kontakiotis, Apostolos Pesiridis, Savvas A. Tassou
Abstract: Nowadays environmental concerns call for a transition towards an economy based on fossil fuels to a low carbon one. In order to achieve this goal, efficiency optimization of existing energy systems through waste heat to power conversion units based on bottoming Organic Rankine Cycles (ORC) is one of the actions that appears to be suitable and effective both from cost and environmental perspectives. Indeed, these units are able to increase the overall efficiency of production processes, existing facilities and renewable power plants with a limited payback time. However, despite the increasing number of ORC installations at megawatt scale, the waste heat rejected by industrial processes has rather a widespread nature. Hence, ORC units with a power output in the range of kilowatts should be developed to address this opportunity for heat recovery and for business.
In the current research activity, a dynamic model of an ORC system was developed in a commercial 1D Computer Aided Engineering software platform. Sub-models of the two plate heat exchangers and of the multi-stage centrifugal pump were developed and calibrated using performance data of industrial components at design and off-design conditions. On the other hand, the R245fa radial turbine design was accomplished using a design procedure that provided geometrical and performance data for the mapping of the device by means of a 1D tool. A steady-state off-design analysis at different operating conditions at the evaporator was further carried out optimizing pump and turbine speeds to maximize the net power output. Furthermore, the thermal inertial effects at the evaporator were assessed with reference to a sample heat load profile of the water hot source and at different time scales.
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12:10
20 mins
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Design methodology for waste heat recovery systems in vehicles considering optimal control
Philipp Petr, Wilhelm Tegethoff, Jürgen Köhler
Abstract: In a motor vehicle, the internal combustion engine satisfies the prior demands such as mechanic energy for propulsion and auxiliaries as well as thermal energy for compartment heating. An applied waste heat recovery system based on the Organic Rankine Cycle (ORC) is a subordinate process. Thus, it underlies varying exhaust gas temperatures and mass flow rates as well as limitations given by superior systems like the cooling system or the after-treatment of exhaust gases. Inadequate system design and controls challenge the cost effectiveness of these small-scale recovery systems.
In order to minimize the fuel consumption of a vehicle over the total drive, the design of the ORC system and particularly the control strategy have to be optimized with respect to dynamically changing boundary values. Consequently, the optimum expander inlet state is not static but depends on the actual exhaust gas inlet temperature and mass flow rate, the actual losses and time constants of the ORC components. The resulting (dynamic) volume flow rates and pressure ratios have to be considered when designing the ORC components.
In this article, a methodology and framework for the design of waste heat recovery systems especially for transient operation is presented. In a first step, in total 32 working fluids are evaluated under various component parametrizations for a broad range of exhaust temperatures. Beside optimum expander inlet states, impacts on heat exchanger and expander design were assessed in quasi-static cycle simulations.
For the design process, (dynamic) Modelica models of the ORC system implying working fluid models, controls and other involved systems are enhanced regarding simulation speed and robustness. The system models are exported as FMU for further investigations and optimization in the developed framework. In order to optimize component parameters (i.e. heat exchanger geometry), controller parameters are computed for the tested parametrization over a broad range of operating conditions to ensure accurate control and comparable conditions. In the next step, the cycle performance for the parametrization is evaluated. Therefore, an optimizer calculates optimal control variables in small time steps. By means of the developed framework, the design optimization process of the evaporator geometry for a waste heat recovery system in an omnibus is presented. Finally, the potential reduction of the fuel consumption of the presented methodology is evaluated in virtual test drives.
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