ORC 2017

14:20 Session 4B: Waste Heat Recovery (2)
Chair: Sven Andersson

14:20 20 mins	Design and off-design analysis of an ORC coupled with a micro-gas turbine Alberto Benato, Anna Stoppato, Alberto Mirandola, Marco Del Medico Abstract: In the recent years, the possibility of recovering heat from gas turbine (GT) exhaust gases using Organic Rankine Cycles (ORC) have been largely explored. However, it is difficult to identify working fluids properly matching with micro-GT exhaust gases. For this reason, the Authors have developed a computer tool, implemented in Matlab environment, able to perform the fluid selection and the plant layout optimization of an ORC unit which recovers the exhaust gases of a 65 kW micro-gas turbine. During the optimization process different configurations are considered which include an intermediate oil loop between the main heat source and the ORC, regenerative or not, subcritical or transcritical. Exergo-economic analyses are also performed to estimate the exergy destruction rate and evaluate the economic feasibility of the optimized solutions. The optimum fluid is selected among 115 pure working fluids and their mixtures. The fluid thermodynamic properties have been retrieved from REFPROP and CoolProp database. The isentropic efficiency of the expander is estimated through the axial and radial flow turbines efficiency charts. In the present work, the optimization goal is the maximization of the net electric power while the optimized variables are: the heat source outlet temperature, the working fluid evaporation pressure, the turbine inlet temperature and the concentration if the fluid is a mixture, the minimum temperature difference in the evaporator, recuperator (if present) and condenser, the recuperator efficiency and the condensation temperature. The maximum net electric power is reached with cyclopentane (12.07 kW) and R141b (12.03 kW) as working media and with a recuperative configuration. In order to find out the most suitable ORC unit, an off-design analysis has also been performed. By means of Aspen Exchanger Design & Rating, the ORC heat exchangers detailed geometry has been determined. The heat exchangers geometry, the pump and the turbine maps have been uploaded in Aspen Plus and, then, the plant off-design behavior has been predicted. Adopting a management strategy that maintains the turbine inlet temperature equal to the design value, the best off-design performance has been obtained with R141b as working fluid.
14:40 20 mins	Off design study of gas pipelines recompression station Sonia Laura Gomez Alaez, Veronica Brizzi, Dario Alfani, Paolo Silva, Andrea Giostri, Marco Astolfi Abstract: An emerging field of application for ORC is the WHR from gas turbine compressor drives in gas pipeline recompression stations. The total length of gas pipelines worldwide is more than 2.7 million km with the need of recompression stations to compensate the pressure drops every 200 km. Each recompression station consists in a small gas turbine (25MW) that mechanically drives the natural gas compressor; hence a large amount of heat (20-30 MW) at relatively high temperatures (500-600°C) is available for waste heat recovery (WHR). ORC are the most suitable power system for this application as demonstrated by the fifteen ORMAT plants in North America installed since 1999 for a total power greater than 75 MW. The advantage is the high replicability of the same overall design for different recompression stations thanks to the relatively low variability of exhaust gas thermodynamic conditions and mass flow rate with clear benefits in terms of scale economies. On the other hand, the design and the operation of these power systems is not trivial because, during the year, they experience strong variations in both the heat input specifications and the ambient conditions especially if they are installed at high latitudes. The natural gas mass flow rate shows a seasonal pattern with higher values in the cold months because of the need of buildings heating systems: this determines in summer season a reduction of the heat available for recovery and a higher turbine outlet temperature with a marked effect on off-design of the ORC Primary Heat Exchanger (PHE). Moreover, the seasonal thermal excursion may be relevant (more than 40°C) with clear effects on both the gas turbine operation and the condensing pressure of the ORC. This paper focuses on the definition of the optimal design and the optimal off-design operation of an ORC coupled with a gas turbine compressor drive on a yearly basis. The ORC is designed as a superheated recuperative cycle provided by a once through PHE and a dry Air Cooled Condenser (ACC). Working fluid is R245fa. The main parameters to be determined are the design condition of the compressor drive gas turbine (namely the size) and the design ambient temperature (namely the condensation temperature and the turbine design). The recompression station gas turbine part-load map is obtained from manufacturer datasheets conveniently scaled to match different machine duty while the ORC off-design map is numerically evaluated with a dedicated tool. For each combination of heat source condition and ambient temperature the off-design operation of the ORC is optimized aiming at maximising the net power output of the system while taking into account a realistic behaviour of the components and different operational constraints. Final results are presented highlighting the impact of different designs on the annual performance and the need of flexibility because of the sliding pressure control on the turbine inlet pressure. In particular the Once-Through PHE is expected to shift from subcritical to supercritical and from supercritical to subcritical operation depending on the turbine load with a clear impact on the component design.
15:00 20 mins	Intermittent waste heat recovery: Investment profitability of ORC cogeneration for batch, gas-fired coffee roasting Antonio Pantaleo, Julia Fordham, Oyeniyi Oyewunmi, Christos Markides Abstract: The operation of batch, gas-fired coffee roasters equipped with afterburners is well known and widely described in literature. "Drum" and "air roasting" are the two most common coffee roasting methods. Both methods include an heating air process, generally by means of a modulating gas burner, followed by passing the hot air through the coffee beans, either passively, in the case of drum roasters, or under pressure, in the case of air roaster, with a batch process that lasts around 15 min. The hot air travels from the roasting chamber through an afterburner to remove the volatile organic compounds (VOCs) and CO and is finally discharged at high temperature into the atmosphere. This single-pass roasting method is often substituted to a semi-closed loop where part of the roaster gases are recycled and mixed to the hot air flow in order to roast the beans with the desired time-temperature curve. A big challenge for roasting is to rapidly heat the air before introducing it into the batch. To achieve this fast heating, the roasters use a very energy intensive and quite low efficiency process. In this paper, energy and material balances of a rotating drum coffee roasting with partial hot gas recycling and costs assessment methodologies are adopted to compare the profitability of three cogeneration systems integrated into the process. The case study of a major coffee processing firm with 500 kg/hr production capacity and the Italian subsidy mechanism for energy efficiency are assumed to carry out the thermo-economic assessment. The CHP options under investigation are: (i) regenerative topping micro gas turbine (MGT) coupled to the existing modulating gas burner to generate hot air for the roasting process; (ii) fluctuating waste heat recovery from the hot flue gas through an organic rankine cycle (ORC) with a thermal storage buffer; (iii) not regenerative topping micro gas turbine with direct recovery of turbine outlet air for the roasting process by means of afterburner to modulate heat demand of roasting. The sensitivity of investment profitability to the main techno-economic process parameters and in particular the daily roasting operating hours is provided, to assess the key factors influencing the feasibility of such intermittent heat recovery options.
15:20 20 mins	Control variables and strategies for the optimization of a WHR ORC system Andrea Baccioli, Marco Antonelli Abstract: In this paper, the dynamic behavior of a WHR (Waste Heat Recovery) ORC system with positive displacement rotary expander has been analyzed and an optimal control strategy was defined to increase the system efficiency and flexibility. Input heat flow was varied in time by varying the heat source mass flow and inlet temperature, according to two different load cycles. Three different control strategy were implemented and compared. The first strategy was sliding pressure, where expander speed was kept constant and system power output was controlled by evaporator pressure variations. The second control strategy was sliding velocity, where expander speed was varied to keep the evaporating temperature to a constant set point value. The third control strategy was a combination of sliding-pressure and sliding velocity: the set point of evaporating pressure varied according to a suitable function of easily measurable variables, with the objective of maximizing system efficiency. A function of the heat source admission temperature and of the product of the volume flow rate by the admission pressure was used to define the evaporating temperature set point. This function was evaluated in steady-state conditions from the model of the plant. Results showed that the last control strategy, maximized system efficiency and flexibility, and that the control parameter chosen were suitable to drive the set point variation.