On the Convenience of Using Simulation Models to Optimize the Control Strategy of Molten-Salt Heat Storage Systems in Solar Thermal Power Plants
Abstract
:1. Introduction
- Automatic control of the variables involved in the process.
- Automatic control of the operation mode.
- Automatic control of the operation strategy.
2. Materials and Methods
2.1. Control Strategies Considered
- PID control with feed-forward
- Advanced PID control with feed-forward
- Adaptive-predictive control with feed-forward
2.1.1. PID Control with Feed-Forward
- Perturbations are measurable.
- The equations relating the perturbations and the control variables of the process to be regulated are known.
2.1.2. Advanced PID Control with Feed-Forward
- Estimating the temperature reference, i.e., the set point for the hot-HTF temperature, SPTHTFH.
- Estimating the temperature of the cold salt, TSaltC_CAL, at the output of the heat exchanger.
2.1.3. Adaptive-Predictive Control with Feed-Forward
- The estimated salt flow, , is now a perturbation signal for the regulator, which allows for the dynamics of such a perturbation to be considered by the control system.
- The actual value of the control variable, , is measured and fed back into the regulator. This allows the system to include the dynamics of the other PID regulators (valves, frequency converters of the pumps).
3. Results
3.1. Simulation of the TES-System Control Strategies
3.2. Experimental Results
- Tests are to be performed on summer sunny days, when there is an excess of solar energy that cannot be used in the turbine and, therefore, must be stored.
- Each test lasts for a whole day during which the TES system is fully charged and then fully discharged.
- Since daily conditions may differ from one test to another, all the control strategies are tested several times so as to reduce the influence of such variable conditions.
- When the discharge begins, the plant counter of energy sold is consulted.
- The TES is fully discharged while the turbine is operating at full power. If at the end of the discharge the salt level in the hot tank is higher than one meter, the discharge will be considered to be incomplete and the test will be disregarded.
- At the end of the discharge, the plant counter of energy sold is read again. The difference between this reading and the one made at the beginning of the discharge will provide the net energy generated with the control strategy under test. The net energy provided during these discharges will be used to evaluate the performance of the control strategy implemented.
- For every successful discharge, the most relevant meteorological conditions during the test are written down for later use in simulations.
4. Discussion
5. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Input Variable | Description |
---|---|
SPVHi | Set Point for the speed defined by the frequency converter of the i-th hot-salt pump |
SPOC | Set Point for the opening of the main valve in the cold-salt tank |
SPOR | Set Point for the opening of the recirculation valve |
Perturbation | Description |
---|---|
THTFC | Cold HTF temperature at the input of the heat exchanger |
TSaltH | Hot salt temperature at the input of the heat exchanger |
HTF mass flow through the heat exchanger |
Output Variable | Description |
---|---|
THTFH | Hot HTF temperature at the output of the heat exchanger (variable to control) |
TSaltC | Cold salt temperature at the output of the heat exchanger |
Molten salt mass flow through the heat exchanger |
Control Strategy | eTHTFH = THTFH – SPTHTFH | |
---|---|---|
μ (°C) | σ (°C) | |
Semiautomatic | 0.40 | 1.17 |
PID | 0.20 | 0.67 |
PID with feed-forward | 0.02 | 0.30 |
Advanced PID with feed-forward | 0.01 | 0.13 |
Adaptive-predictive with feed-forward | 0.00 | 0.04 |
Control Strategy | Net Energy (MW·h) | ||
---|---|---|---|
Real (EN,Real) | Simulated (EN,Sim) | Difference (%) | |
Semiautomatic | 5453 | 5458 | −0.09 |
PID | 5522 | 5518 | 0.07 |
PID with feed-forward | 5525 | 5514 | 0.20 |
Advanced PID with feed-forward | 5643 | 5628 | 0.26 |
Adaptive-predictive with feed-forward | 5721 | 5705 | 0.28 |
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Prieto, M.J.; Martínez, J.Á.; Peón, R.; Barcia, L.Á.; Nuño, F. On the Convenience of Using Simulation Models to Optimize the Control Strategy of Molten-Salt Heat Storage Systems in Solar Thermal Power Plants. Energies 2017, 10, 990. https://doi.org/10.3390/en10070990
Prieto MJ, Martínez JÁ, Peón R, Barcia LÁ, Nuño F. On the Convenience of Using Simulation Models to Optimize the Control Strategy of Molten-Salt Heat Storage Systems in Solar Thermal Power Plants. Energies. 2017; 10(7):990. https://doi.org/10.3390/en10070990
Chicago/Turabian StylePrieto, Miguel J., Juan Á. Martínez, Rogelio Peón, Lourdes Á. Barcia, and Fernando Nuño. 2017. "On the Convenience of Using Simulation Models to Optimize the Control Strategy of Molten-Salt Heat Storage Systems in Solar Thermal Power Plants" Energies 10, no. 7: 990. https://doi.org/10.3390/en10070990