Integration of parabolic collectors and power plant simulation

The concept of integrating the parabolic collectors into the cycle is illustrated in figure

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Figure 4: Integration of the parabolic trough collectors into the water steam cycle

Because the temperature level of the solar steam is high enough, two bypasses with solar preheaters are integrated into the high-pressure and low pressure prehea­ter section of the water steam cycle.

In the solar preheater 1 the superheated steam gets condensated via heat transfer to the feedwater, which is switched into the bypass by a control valve. This heat flow is permanently varying with a maximum at noon caused by the changing irradiation in the course of the day. The condensate leaving solar preheater 1 is still at a tempera­ture level to preheat a part of the feedwater in the low pressure preheating section (solar preheater 2). The main part of the feedwater is heated in the preheaters 1 to 4 by extraction steam conventionally.

Since the solar generated steam preheats a part of the boiler feedwater, the steam consumption of the conventional preheaters 1 to 7 is reduced.

The steam remaining in the turbine can be used for an additional dynamic power generation.

A special control strategy consisting of 2 control loops has been developed that al­lows the addition of the external heat into the preheating cycle without affecting the feed-water temperature at the boiler inlet (251°C) and therefore without affecting the complex boiler control. The temperature of the feedwater through the preheaters 6 and 7 is controlled by steam valves that are installed in the steam extraction lines between the turbine and the preheaters. In order to avoid any mixing of “cold” bypass feedwater with the mainstream in the morning and evening, it gets discarded by a control valve as long as it is below the set point temperatures of 251°C in the HP sec­tion and 150°C in the LP section.

Results

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Figure 5: Irradiance, temperature and mass flow characteristics in July

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In figure 5 the results based on the average irradiation in July are illustrated.

It can be seen that the water at the outlet of the parabolic trough (PT) gets warmed up in the early morning (t=5h), is then vaporized (t=6h) and finally superheated to a controlled constant temperature of 380°C. It is obvious that the PT outlet temperature follows quite closely the irradiation curve. In parallel to the irradiance the feedwater mass flow through the parabolic trough increases till noon and then decreases.

Due to mass release effects during the vaporization phase in the morning the mass flow characteristic through the absorber tubes shows a peak. Dynamic effects like these in combination with long dead times resulting from long absorber tubes and low flow velocities complicate the development of reliable automation concepts for the water mass flow through the absorber tubes in the parabolic trough collectors [9]. Therefore a pilot control concept based on measured irradiance data to pre-adjust

the water mass flow has been applied. Figure 6 illustrates the resulting mass flow characteristics in the high pressure preheating section during the course of a day in July.

As can be seen, up to 179 kg/s of feedwater can be guided into the bypass to be preheated by the solar generated steam. In consequence, the mainstream feedwater mass flow decreases from 320 kg/s to 141 kg/s. Since the reduced feedwater mass flow needs less steam the controller closes down the extraction steam valves as it is shown in figure 7. The processes appearing in the low pressure preheating section are very similar. Closing the steam turbine extraction lines leads to a significant in­crease of the steam power plant’s power output as shown in figure 8. The power in­crease is following the solar irradiance characteristic nearly without any time delay — this is an essential result of the investigations carried out.

The combination of integrating an external heat source into the cycle and closing the extraction steam lines leads to a rise of the steam power plant output from 393 MW up to 410.5 MW at noon in July. This corresponds to a power increase of approxi­mately 4.5 percent. Compared to these results, the potential power increases in the other months are lower, for example in January the maximum additional power rises about 6.7 MW.

On the basis of this simulation data an efficiency factor can be calculated, defined as a ratio of the parabolic trough thermal power output integrated into the cycle and the achievable electrical power output increase at the steam turbine’s generator. This efficiency factor is about 28 % at noon.

In addiction to the use of average irradiance data for the simulation a single day in July showing strongly varying irradiation caused by clouds was selected as a test case. Because the irradiance is temporarily too low to preheat the feedwater in the solar preheater 1 to 251°C, the feedwater is not led back to the mainstream but dis-

Figure 7: Extraction steam mass flows into PH6 and PH7

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Figure 8: Irradiation and power output characteristics

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carded. In these periods no additional power is generated in the steam turbine. Hence significant output losses are to be noticed in the example simulated.

However, the results favourably show, that even fast changes in solar irradiation can be managed with simple control strategies.

Conclusion

The integration of external solar generated steam into the water steam cycle is a promising concept to promote renewable energy and as a consequence to reduce the CO2 emissions. The simulation results, based on meteorologic data of a location in Italy, show a power increase following the solar irradiance characteristic nearly without any time delay. This can be managed even with simple control strategies. Since the peak of energy consumption is to be found at noon, the additionally pro­duced power matches very well the electricity demand of industrial countries.

The efficiency factor defined as the ratio of the parabolic trough thermal power output integrated into the cycle and the achievable electrical power increase at the steam turbine’s generator amounts 28% maximum.