## 2 Dynamics and Nonlinearity Analysis of MGT-CCP  
A MGT-CCP system, composed of an 80 kW full heat-regenerated-cycle MGT and a 75 kW lithium bromide single effect absorption chiller, is considered in this paper. The MGT-CCP system schematic diagram is shown in Figure 1. Fuel is supplied to the MGT to generate the required electric power, while the waste heat emitted from MGT is recycled to drive the refrigeration system to produce chilled water. The MGT-CCP system can generate electricity and chilled water for the power and cooling load requirements at the same time.  
The study on control strategy is based on a MGT-CCP emulator, which is built on the MATLAB R2016b environment and consists of dozens of major lumped-parameter modules and other functional components. Built on three conservation laws and thermal property equations, the emulator shows  
Figure 1: The MGT-CCP system schematic diagram.  
quite close dynamic characteristics to the primary MGT-CCP system. The detailed process of modeling is referenced in [25] and the main design data of the MGT-CCP system is given in Appendix A.  
It is worth noting that the exhaust temperature of the fully heat-regenerated MGT is 277 \({}^{\circ}\)C, which is much lower than the non-heat-regenerated MGT of about 615 \({}^{\circ}\)C. Considering the limitation of heat exchange efficiency and avoiding the corrosion of low temperature flue gas in the pipeline, the exhaust temperature of the final flue gas of MGT-CCP is designed to be 170 \({}^{\circ}\)C. Therefore, the thermal power generated by the chiller is about 75 kW. A ratio close to 1.0 between thermal power and electrical power means that power generation efficiency is more preferentially improved in such an all-heat regeneration MGT-CCP system.  
In order to help designing the controller, the MGT-CCP system can be regarded as a double-input-and-double-output dynamic system. The manipulate variables are fuel \(u_{1}(\mathrm{kg/s})\) and refrigerant valve opening \(u_{2}(\%)\). The output variables are power output \(y_{1}(\mathrm{kw})\) and chilled water temperature \(y_{2}(^{\circ}C)\).  
Table 1 is the typical operating points of the MGT-CCP system, where \(y_{1}\) is power output, and \(y_{2}\) is chilled water temperature.  
_Remark 1_: The chilled water temperature, rather than the cooling power, is chosen as the operating point for the MGT-CCP system. In the MGT-CCP system, the cooling power is proportional to the product of the temperature difference and the chilled water flow. Since the flow rate of chilled water is assumed to be constant, from a control point of view, the chilled water temperature can represent the cooling power. Although the water flow is disturbed during operation, the chilled temperature can indicate the required cooling power generation. In addition, temperature has the advantage that it can be measured directly.  
Based on the simulator, the following analysis will use two methods to expose several control issues for the MGT-CCP system. First, an open-loop step response test is performed to show the change in dynamic behavior. Second, the gap metric method is used to determine the degree of nonlinearity of the system by measuring the dynamic difference between models linearized at different operating points; thus, providing a quantitative description of the nonlinearity distribution of the MGT-CCP process.