Startup and control methods for an ORC bottoming plant
(19) AUSTRALIAN PATENT OFFICE(54) (51)G Title Startup and control methods for an ORC bottoming plant International Patent Classification(s) F01K 25/08 (2006.01) 7BMEP F01K F01K 13/02 (2006.01)13/02 F01K 25/08 20060101ALI2006061 20060101AFI2006061 7BMEP PCT/US2005/016019 (21)(87)(30) (31)(43) (71) (72) (74) Application No: 2005241109 (22) Application Date: 2005.05.06 WIPONo: WO05/108750 Priority Data (33) Country US Number (32) Date 2004.05.OG 10/840,775 Publication Date : 2005.11.17 Applicant(s) United Technologies Corporation Inventor(s) Kang, Pengju; Cogswell, Frederick James Agent/Attorney Watermark Patent & Trademark Attorneys, The Glasshouse 290 Burwood Road, Hawthorn, VIC, 3122 (57) Abstract: The invention is a system and method for smoothly starting and controlling an ORC power plant. The system comprises a cascaded closed loop control that accounts for the lack of relationship between pump speed and pressure at start-up so as to control pump speed and pressure, and that smoothly transitions into a steady state regime as a stable operating condition of the system is attained. The cascaded loop receives signals corresponding to a superheat setpoint, a pressure at an evaporator exit, and a temperature at an evaporator exit, and controls the pump speed and pressure upon startup to provide smooth operation. The system and method can further comprise a teed torward control loop to deal with conditions at start up and when external disturbances are applied to the ORC power plant. 1. A closed loop control system for an ORC, said ORC including a pump, said control system including:
a comparator that compares a superheat setpoint input and a calculated superheat value input, and provides a superheat error signal; a superheat controller responsive to said superheat error signal, said superheat controller providing a superheat control signal; an adder that adds said superheat control signal and a pressure signal, and provides a summed signal; a range limiter that accepts as input said summed signal, and produces a range limited signal within a limit range; a subtractor that subtracts from said range limited signal a duplicate of said pressure signal, said subtractor providing as output a subtracted signal; and a pressure controller that accepts said subtracted signal and produces in response thereto a pressure control signal; whereby said closed loop control system controls a superheat of said ORC when said range limited signal is below a maximum value of said limit range, and said closed loop control system controls a pressure of said ORC when said range limited signal is at a maximum value of said range limit. 2. The closed loop control system for an ORC of claim 1, wherein a mathematical model of a pump is employed to determine whether said pump is operating in a pressure-limited regime. 3. The closed loop control system for an ORC of claim 2, wherein, in response to a determination that said pump is operating in a flow-limited regime, said control system prevents said pump from increasing a rotation speed until said pressure attains said pressure limit. 4. A method of starting an ORC, said method including the steps of:
providing a dosed loop control system for an ORC, said ORC including a pump and an evaporator having a heat input, said control system including:
a comparator that compares a superheat setpoint input and a calculated superheat value input, and provides a superheat error signal; a superheat controller responsive to said superheat error signal, said superheat controller providing a superheat control signal; an adder that adds said superheat control signal and a pressure signal, and provides a summed signal; a range limiter that accepts as input said summed signal, and produces a range limited signal within a limit range; a subtractor that subtracts from said range limited signal a duplicate of said pressure signal, said subtractor providing as output a subtracted signal; and a pressure controller that accepts said subtracted signal and produces in response thereto a pressure control signal; applying heat to said evaporator, said heat being applied at a fraction of the enthalpy flux desired at steady state operation; operating said pump at reduced speed; setting a high pressure limit to a value of pressure that can be achieved at steady-state at said reduced pump speed; waiting until the operating condition of said ORC attain a pressure plateau of an operating curve of said pump curve; increasing the pressure limit to a nominal operating value; operating the pump at a faster speed consistent with the increased pressure limit; permitting the operating mode of said system to switch from pressure control to superheat control at a pressure at or below said nominal operating value of said pressure limit; and increasing and controlling the heat flux to bring the system to full load. 5. A closed loop control system for an ORC substantially as herein described with reference to the accompanying drawings. 6. A method of starting an ORC substantially as herein described with reference to accompanying drawings, UNITED TECHNOLOGIES CORPORATION WATERMARK PATENT AND TRADE MARKS ATTORNEYS