3.2.1. Hybridization for Power Enhancement
Considering power boosting mode, in
Table 12, it may be observed how it has been possible to increase both the gross and the net power generation, resulting in higher values of gross and net efficiency and thus reducing the specific fuel consumption. Nevertheless, fuel mass flowrate usage is the same as for the original combined cycle, so this set of alternatives, although being useful for increasing power generation capacity, do not represent a direct positive environmental impact, as they do not reduce the associated carbon emissions of the cycle.
Figure 6 shows the most representative results of the power boosting operation mode for all the studied alternatives.
In
Figure 6a, the increases in the gross and net power of the different alternatives with respect to the base case study are presented. The results from the injection of the steam generated from the molten salts before the last high-pressure superheating circuit, corresponding with alternative A, show the highest increase in gross and net power values, with an additional power generation of 19,152 kW and 19,468 kW, respectively. This result is in line with the minimization of exergy destruction in the mixing of streams, as both have closer temperature values. Alternative B, injection before the second reheating circuit, corresponds to an increase of 15,954 kW and 15,768 kW in gross and net power. For alternative C, injection before the intermediate-pressure turbine inlet, gross power increases by 16,765 kW and net power by 16,578 kW above the reference case values. For alternative D, injection before the low-pressure turbine inlet, gross and net power values rise by only 5206 kW and 5048 kW. Finally, for alternative E, injection before the high-pressure turbine inlet, gross and net power rise by 17,034 kW and 16,773 kW.
3.2.2. Hybridization for Fuel Saving
Considering the enhancement of the cycle efficiency,
Figure 6b shows the increase in net efficiency for all the studied alternatives. With respect to the original value of 57.94 % without solar hybridization, alternative A shows the highest increase in the net efficiency, 2.92%, followed by alternative C, with 2.55%; alternative E, with 2.53%; alternative B, with 2.44 %; and alternative D, with 1.04%. Finally, if the net specific fuel consumption values are compared, as in
Figure 6c, it may be observed how consumption is reduced from the original value of 6214 kJ/kWh to 6103 for alternative D (
%) and 5962 for alternative B (
%). Alternatives E and C reach similar values, 5953 (
%) and 5952 (
%), whereas the maximum reduction in net specific fuel consumption is obtained with alternative A, 5915 (
%).
On the other hand, regarding the fuel-saving operation mode, results have been collected in
Table 13, with the most representative ones presented in a graphical way in
Figure 7.
As can be observed, gross and net power remain almost constant. It must be considered that the algorithm employed by THERMOFLOW® optimizes the thermal efficiency of implemented cycles. Therefore, although a constant value of power was expected in fuel-saving operation, small differences below 0.01% in net power were found due to the optimization algorithm. Apart from that, the most interesting effect is the reduction in the necessary fuel consumption rate for generating that power thanks to the thermal power provided by the molten salts. As a consequence, the cycle efficiency increases while generating the same net power. Finally, in opposition to the power boosting mode, the fuel saving mode results in lower carbon emissions with respect to the original cycle, with the corresponding positive environmental effect.
Figure 7a shows the increase in net efficiency, where it may be observed that alternative A is again the best performing one, with an increase of 3.01%, followed by alternative E, with 2.66%. Then, option C increases the efficiency by 2.51% and option B by 2.35%. Finally, injection at the low-pressure line, as in alternative D, only increases the efficiency by 1.32%. Note that differences in the efficiency values between the power-boosting and fuel-saving operation modes for the same integration points can be related to the partial-load operation of the steam turbine when operating the cycle in the fuel-saving mode. Considering the results of net specific fuel consumption, as shown in
Figure 7b, higher reductions with respect to the power boosting mode are obtained. The best alternative, A, reaches a reduction of 308 kJ/kWh (
%), followed by alternative E, with a reduction of 274 kJ/kWh (
%). Then, option C allows us to reduce the net specific fuel consumption by 258 kJ/kWh (
%), followed by option B, with a reduction of 243 kJ/kWh. Finally, alternative D is only able to reduce it by 139 kJ/kWh (
%).
Nevertheless, the most interesting result for the fuel saving operation mode is the reduction in net fuel consumption, passing from the original value of 15.68 kg/s to 15.43 for alternative D, 15.16 for alternative B, 15.12 for alternative C, 15.09 for alternative E, and 15.00 for alternative A, while maintaining the same net power generation capability. This reduction in fuel consumption is translated directly into a reduction in carbon dioxide emissions from the reference value of 43.11 kg /s. The highest reduction is observed for alternative A, avoiding the emission of 1.86 kg /s, followed closely by alternative E, with a reduction in emissions of 1.61 kg /s. Options C and B avoid the emission of 1.52 and 1.41 kg /s, respectively, whereas alternative D only avoids emissions of 0.67 kg /s.
Comparing results globally, it may be affirmed that the best alternative for injecting the steam generated by the molten salts is alternative A.
Figure 8 shows the T-Q diagrams for this alternative in power boosting and fuel-saving operation modes. It may be observed that steam heating curves have shifted upwards, closer to the exhaust gas lines with respect to the original cycle without hybridization. These changes may be related to a better performance in heat transfer.
In addition, alternative A maximizes power generation with the least fuel consumption. Hence, when operating in power-boosting mode, a net power of 461,622 kW is obtained, 2.06% higher than for the original cycle without solar hybridization. On the other hand, in the fuel-saving mode, alternative A is able to reduce thermal power absorption at the combustion chamber to 725,807 kW, using 54,003 kg/h of fuel and emitting 145,331 kg /h. With respect to the reference case, this represents a reduction in thermal power absorption at the gas turbine combustion chamber by 32,760 kW, a fuel saving of 2448 kg/h of natural gas, and it avoids the emission of 6696 kg /h. Thus, for the integration of steam generation with solar power using molten salts, we recommend extracting water from the first high-pressure economizer, evaporating and superheating it in the solar field, and injecting it back before the last high-pressure superheating circuit. The second best alternative would be option E, injecting the steam before the high-pressure turbine inlet. Alternative B, injection before the second reheating circuit, and alternative C, injection before the intermediate-pressure turbine inlet, yielded similar results. Both correspond to steam injection at the intermediate-pressure line. Both are good options: although improvements are not as good as for alternatives A and E, the cycle efficiency is substantially improved. Finally, alternative D, steam injection before the low-pressure turbine inlet, is the least beneficial alternative in terms of cycle improvement.
Finally, it is worth highlighting that the developed model may be useful in similar contexts, where the hybridization of an existing combined cycle with solar thermal power is under the scope, allowing one to test different configurations and find the optimal one.