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Energy Systems Design Accounting for Dynamic Performance temperature of the working fluid. In addition, the control system must ensure that this temperature does not exceed the safety limit anywhere in the high-temperature part of the ORC power plant during system transients. The most critical operational transient from this point of view is the trip of gas turbine B: when this happens, the network frequency drops, so the GT controller reacts by opening the fuel valve to regain the set-point frequency. Consequently, the GT exhaust flow rate and temperature increase, leading to an increase of the OTB exhaust gases temperature, which is then counteracted by the ORC controller by increasing the feed flow to the OTB and thus, eventually, also the share of the load generated by the ORC system. Preliminary simulations carried out with different designs of the system showed that, as ex- pected, the dynamic response of the ORC system is much slower than the response of the GT system, even for aggressive designs of the temperature controller. This leads to significant and po- tentially unacceptable overshoot of the pump speed during the transient. This means that the peak value of the turbine inlet temperature (which is one of the critical variables of the process) is almost insensitive to the tuning of the ORC system controller. Such peak is quickly reached due to the fast response of the GT compared to the ORC system. In particular, the response time of the ORC power system is comparatively long since the flow rate through the turbine, and thus the generated power, change very slowly with the OTB pressure. This means that the contribution of the ORC controller to the limitation of the frequency undershoot is marginal. Based on these considerations, the ORC Proportional-Integral (PI) controller was tuned in order to obtain the minimum possible settling time of the controlled variable, while avoiding the overshoot of the pump speed during the trip response transient and obtaining well-damped responses for all involved variables. The simulations showed that this is possible by setting the proportional gain to a value that is proportional to the heat exchanger volume, thus accounting for the process gain variability with the design parameters, while keeping the integral time at a suitable constant value. 5.4.3 Validation The shell and tube heat exchanger design model, described in §5.4.1, was validated using an exam- ple proposed in Ref. [16]. The differences between the simulation results and the data reported in the reference are within 1% in terms of both overall heat transfer coefficient and pressure drops. For the once-through boiler it is verified that the heat transfer coefficients and the pressure drops related to both singe- and two-phase flow are within the range of values specified in Ref. [33]. The off-design, steady-state simulation results of the gas turbine model presented in §5.4.2 were compared to the partial load characteristics given by the gas turbine manufacturer in the 10%- 100% range. Exhaust gas mass flow rate and temperature, fuel mass flow rate, and pressure in the combustion chamber were checked. The quantity showing the larger mismatch is the fuel mass flow: the relative error is about 3% for loads larger than 60%, and increases up to about 15% if the load decreases down to 10%. The dynamic model of the gas turbine was validated by comparison with simulation results of the reference model provided by the gas turbine manufacturer, which is based on proprietary experimental data. The validation scenario assumes that the three GT units initially share a total load of 24 MWE, delivering 8 MWE each. At some point in time, one unit trips, so the other two ramp up their load in order to match the total power demand, with a transient reduction of the network frequency. The result of the simulations are compared in Fig. 5.6, which shows the normalized network frequency and the load of unit B. At time t = 50 seconds, one of the gas turbines trips; subsequently, the reference model predicts a minimum normalized frequency drop of 133PDF Image | New Concepts FOR Organic Rankine Cycle Power Systems
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