Thermodynamic simulation of the system

In this case study the simulation of the system has the advantage of a deeper knowledge of the operation temperatures and pressures in all the points of the cycle. The same simulation model presented in the previous case has been used, but with less hypotheses for the thermodynamic calculation of the cycle operating points. The pressure losses and the energy losses in heat exchangers and pipes have been ignored.

The main simulation assumptions related to the efficiency of the components are listed in Table 4.2. The mechanical and electrical efficiencies of the turbine have been set through a recursive analysis of the electric gross efficiency of the ORC cycle, with respect to design conditions. The turbine isentropic efficiency doesn't need to be defined in the simulation, as the data available allow to define the real conditions of the fluid at the inlet and outlet of the turbine, without the need to consider the isentropic transformation.

Parameter Value

Turbine mechanical efficiency ηt,m 0.955 Electric generator efficiency ηt,e 0.95 Pump isentropic efficiency ηp,is 0.70 Pump electric motor efficiency ηp,e 0.98 Pump mechanical efficiency ηp,m 0.90

Table 4.4 Main simulation assumptions

The nominal operation conditions of the unit are shown in Figure 4.9. The orange lines represent the regenerator conditions.

Operation analysis of real ORC systems

Figure 4.9 ORC simulated nominal conditions

Considering energy balances equations the mass flow rates of the MM, the thermal oil and the cooling water in the condenser have been calculated for each point of operation.

4.2.4 Operation data results

The available effective operation data of the unit covers about 3,000 hours over a time range of 12,000 hours. Figure 4.10 shows the gross output power throughout the monitoring activity. The unit mainly worked near nominal power (and in some cases also at slightly higher power), but the partial load operation has been frequent. While in summer all the available heat from thermal oil has been used for the ORC unit, in winter and middle seasons the heat is also required for space heating purposes. This aspect is particularly evident in the chart around the 8,000 h value. The unit has been operated down to 10% of the nominal power for several hours of operation.

Figure 4.11 reports the variation of the power consumption of the auxiliary systems with respect to the gross output power from the turbine. The larger share of auxiliary systems consumption is related to the pump, which causes the parabolic trend that can be noticed in the plot. For nominal output power, the total consumption of the auxiliary systems is in the range from 35 to 48 kWel. These values are in accordance to the rated consumption of 46 kWel in design conditions, which is about 4% of the gross output power of the unit. The intercept with the y-axis, representing the amount of power required also at zero load, is about 4 kWel.

Operation analysis of real ORC systems

Figure 4.10 ORC gross output power during operation

Operation analysis of real ORC systems

The main result of the application of the simulation model is the possibility of calculating the electric efficiency of the cycle. Using the model described in the previous paragraph it has been possible to obtain the chart of Figure 4.12. The gross electric efficiency shows a good performance both at full load and at partial load, remaining near the nominal value down to about 30% of the power load (ratio between the output power and the nominal power).

Figure 4.12 ORC gross electric efficiency

There is a significant amount of operation points with power load lower than 10%, with a gross efficiency of about 15%, which correspond to 75% of the efficiency at full load. This aspect is of interest as is not a common operation strategy in biomass systems. This analysis confirms that ORC units can provide a good performance also with variable loads. The net efficiency shows a similar behaviour, being the gap between gross and net efficiency almost always constant.

The scattering of the points is related to other parameters than power load, which can affect the electric efficiency (e.g. the condenser temperatures or the evaporator temperatures). A significant variability can be found in the range from 90% to 100% of the power load, where the majority of operation points lays, with about 20 ± 1% of gross efficiency. The two main clouds are related to the season of operation, which affects the condensing temperatures. With cold outdoor temperatures the condensing towers can provide a better cooling in the cycle, increasing the electric efficiency. The availability of the measurement of real temperatures allowed to verify the actual effectiveness

Operation analysis of real ORC systems

reported in Figure 4.13. The dependence from power load seems very weak, being the value almost always near 71%. It appears to be slightly decreasing near 100% of power load, probably because of the saturation of the heat exchanger capacity.

Finally, the relation between the condensing pressure and the water temperature at the condenser outlet is shown in Figure 4.14. This behaviour is in line with the design conditions and the physical relations between condensing temperature and pressure.

Figure 4.13 Regenerator effectiveness vs power load

Operation analysis of real ORC systems