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Table 2: Results from static verification with data from DSA simulations The average error is 0.61%, 1.2% and 1.1% for the 100 kW, 70 kW and 50 kW case. A few larger errors such as the pressure drop and the pressure ratio increase the average error significantly for the part load cases. Power generated: This is the most important output and should therefore be exactly the same as the reference power. This is accomplished by the control system and is not really a verification of the thermodynamic model, but rather a verification of the control system. Speed: The speed variable shows indirect efficiency of the model. If the speed of the model is lower than the speed data from DSA, the efficiency of the model is too high, because the model can have a lower speed and still produce the same electric power output. TIT (temperature at the inlet of the turbine): This variable is used in the control system and is limiting the efficiency, since the turbine blades have a maximum temperature they are designed to sustain. TOT (temperature at the outlet of the turbine): This variable is also used in the control system and limits the efficiency, but in this case it is the recuperator that cannot sustain too high temperatures without breaking. Pressure ratio over the compressor: In the verification process, there has to be at least one pressure measurement. The value of the pressure ratio is better than an absolute pressure value since the pressure ratio is dimensionless and therefore not influenced by the pressure of the ambient air at the inlet. The pressure ratio over the turbine could have been used without any differences. Added fuel power: This variable shows how much fuel has been used in the unit kW. The fuel power is preferred over the variable fuel mass flow since different fuels have different energy of combustion. The mass flow through the compressor: This is one of the most important variables. The mass flow through the turbine could also been used without any difference. Pressure drop from the compressor to the turbine: This variable gives information about how accurate the pressure drop models used in the heat exchangers and the combustion chamber. Efficiency of the recuperator: This variable shows how well the recuperator works. Another variable that could have been used was total heat transferred in the recuperator, but as for a similar reason as above a dimensionless variable is preferable. Efficiency of the gas/water heat exchanger: Same as above but for the gas/water heat exchanger instead. Efficiency of the compressor: Since this is a variable that is difficult to model, it is important to verify the results of the model. Efficiency of the turbine: Same as above. Total efficiency: This important variable tells us if the complete model has the same performance as the real microturbine. From the results of the static verification above, we see that the largest error is in the compressor mass flow and pressure ratio. In a model, where the complete microturbine system is emphasized, the errors of 2-4% can be acceptable. The errors are probably caused by errors in the ellipsoid curves that model the compressor mass flow. The pressure ratio and the mass flow for a certain speed are uniquely determined by the ellipsoid curve for that particular speed, see figure 16. With correction parameters, the equilibrium point can be adjusted and moved along the curve. In this model, both mass flow and pressure ratio are too low, which means that the true equilibrium point lies above the model curve. Therefore the errors of the mass flow and pressure ratio have a close relation. The error of the turbine mass flow model at lower part load might also increase the error of the compressor mass flow. In Gustafsson (1998), it was also the compressor mass flow model 46PDF Image | Modelling of Microturbine Systems
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