3. DEVELOPMENT OF CONCENTRATED SOLAR POWER AND
3.4 Description of the hybrid model
3.4.2 Control engineering of the hybrid plant
The control engineering of the hybrid plant includes the control of the steam boiler, so-lar field and the joint turbines. In the coordinative control system of the hybrid plant, the main variables to be controlled are fuel power of the steam boiler, live steam pressures of steam boiler and solar field, and the power output of the joint turbines. From the solar field point of view, the solar field is operated on fixed pressure with turbine follow mode, in which the outlet pressure of solar field is kept fixed by the main steam valve of solar field, and the steam mass flow from solar field varies according to the solar irradi-ation conditions.
From the steam boiler point of view, the steam boiler is operated also on fixed pressure mode, as the steam boiler is designed to be a natural circulation boiler. Thus, the steam boiler can be either controlled by boiler follow mode or by turbine follow mode depend-ing on the control of the main steam valve before HP turbine. In boiler follow mode, the main steam valve is used to control the steam mass flow through the turbines, and the live steam pressure of steam boiler is kept constant by adjusting the fuel power of the steam boiler. Thus, as the solar steam is injected to the inlet of HP turbine, the main steam valve of steam boiler adjusts the live steam mass flow from steam boiler, which affects to the live steam pressure and to the fuel supply of the steam boiler. On the other hand, in turbine follow mode the main steam valve is used to keep the live steam pres-sure constant, and the live steam mass flow and the power output of the turbines are controlled by fuel supply. Thus, as the solar steam is injected to the inlet of the HP tur-bine, the main steam valve of steam boiler adjusts the live steam pressure of the steam boiler, and the live steam mass flow from steam boiler is adjusted by fuel power of the steam boiler accordingly.
The hybrid plant is designed to be a new power plant, in which power boost mode is preferred over the fuel saving mode as operation mode. Thus, the control engineering of the hybrid plant is designed in order to achieve the following operation strategy, which can be divided into two parts related to the load of the solar field:
1. As the steam generation starts from the solar field, the hybrid plant is operated first on the power boost mode up to 110% load of the turbines, as the load of the solar field is approximately 10% of the nominal load of the steam boiler.
2. As more steam is generated at the solar field, power boost mode is switched to fuel saving mode without the need of oversizing the joint equipment, like tur-bines, condenser and the LP FWHs.
In the first part, the hybrid operates only on power boost mode, and the power output of the turbines is increased approximately from 134.2 MW to 147.6 MW (Figure 53), as turbines is assumed to stand 10% of extra load. Thus, electricity production is increased 10% from the nominal load, as load of the solar field is 10% of the nominal load of steam boiler. In the second part, as the solar field generates more steam, the power boost
Figure 53. The applied operation strategy of the hybrid system.
However, disadvantage of the applied operation strategy is related to the increased mass flow through turbines in power boost mode. As the steam power plant is designed to be a condensing power plant, in which the back pressure is kept constant at the condenser, the increase of mass flow through turbines causes a pressure increase at the inlet of the turbines related to the Elliptic rule (Equation 8) invented by Stodola (Raiko et al. 2013 p.66):
̇
̇ = ,
1 − ( / )
1 − ( , / , ) (8) in which
̇ is the mass flow through turbine [kg/s]
pi is the inlet pressure [bar]
po is the outlet pressure [bar]
̇ is the design mass flow through turbine [kg/s]
pi,0 is the design inlet pressure [bar]
po,0 is the design outlet pressure [bar]
By keeping the outlet pressure and the design values constant, the Equation 8 can be presented as a hyperbola function of mass flow through turbine and inlet pressure (Figure 54).
Figure 54. Hyperbola of Elliptic rule, as outlet pressure is kept constant in Equa-tion 8. Asymptote of the hyperbola is also presented with dash line. Adapted
from Raiko et al. 2013, p.67.
The increase of steam mass flow increases the inlet pressure of HP turbine, which in-creases also to the live steam pressures of steam boiler and solar field if the turbines are overloaded. Furthermore, as fluctuating solar irradiation conditions are going to cause variations to the outlet steam mass flow from solar field and to the power output of the joint turbines, it is going to cause fluctuations also to the inlet pressure of the HP tur-bine. Thus, the control of the steam boiler should adjust its load as quickly as possible while preventing pressure variations, as the steam boiler is designed to be a natural cir-culation boiler with steam drum and the operation of steam drum is sensitive to pressure variations.
The selected control method for the steam boiler is the turbine follow mode, in which the live steam pressure is controlled by the turbine, and the power output of the joint turbines is controlled by fuel supply (Figure 55). The turbine follow mode is selected as control method for steam boiler, as the control method of the solar field is also turbine follow mode. The first part of operation strategy, the power boost mode, is conducted by changing the set point for power output of the turbines directly proportional to the thermal power of the solar field. In other words, the main controller of the fuel supply of steam boiler is designed in order to allow the increase of power output of the turbines without increasing the fuel supply of steam boiler, as steam generation starts from the solar field. In the second part of the operation strategy, as more steam is generated in the solar field and the turbines are operated on 110% load, the set point for power output of the turbines is kept at 147.6 MW, and the main controller of fuel supply decreases
au-Figure 55. Schematic of the control engineering of the hybrid system.
The disadvantages of the fixed pressure mode are large throttling losses at partial load operation compared to sliding pressure mode. Furthermore, in turbine follow mode slower load changes of the steam boiler are achieved than in boiler follow mode (Figure 29). However, quick load changes are needed during start-ups and shutdowns of the solar field as well as under fluctuating solar irradiation conditions if the fuel supply of the steam boiler is adjusted accordingly to the thermal power of the solar field. Thus, it is predicted that more sophisticated and coordinative control method needs to be devel-oped for the co-operation of steam boiler, solar field and the joint turbines.
Furthermore, the turbines need to be dimensioned accordingly to the chosen operation method. In power boost mode, the turbines has to be dimensioned to the highest inlet steam mass flow, which causes additional throttling of the main steam valve of the steam boiler, as the steam boiler is operated without solar field and on partial loads.
Thus, it causes also more losses and lowers the efficiency of the plant compared to the fuel saving mode. In just fuel saving mode, the steam mass flow through turbines is not increased, and the throttling losses are smaller while operating without solar field and on partial loads. For that reason, the difference between the previously described opera-tion strategy and operaopera-tion strategy with just fuel saving mode should be investigated.