Molten Salt Solar Power Tower Research Articles

Development, Construction, Operation and Maintenance of Shouhang Dunhuang 100 MW Molten Salt Solar Power Tower Plant

The Shouhang Dunhuang 100 MW molten salt solar power tower plant is the first 100 MW-scale commercial demonstration project in China. The plant started to break ground in October 2016, was completed and connected to the grid for power generation in December 2018, and achieved full-load operation in June 2019. This paper comprehensively introduces the development, design, and construction process of the project as well as its operation and maintenance over the past five years, and shares experiences and lessons learned from the project.

Dynamic Thermal Transport Characteristics of a Real-Time Simulation Model for a 50 MW Solar Power Tower Plant

A dynamic simulation model of a heliostat field and molten salt receiver system are developed on the STAR-90 simulation platform. In addition, a real-time simulation model coupling the above two models is built to study the photothermal conversion process of Delingha’s 50 MW solar power tower plant. The nonuniform and time-varying characteristics of the energy flux density on the receiver surface and the dynamic characteristics under different operating conditions are studied. The operational process of the receiver of a typical day is simulated. It was found that there was a strong positive correlation between the energy flux and DNI, and the maximum energy flux density on the surface of the heat absorbing tube panel moved from the first tube panel to the fourth in sequence from 12:00 to 18:00. At the same time, the energy flux density of the last four panels decreased gradually along the arrangement order of the panels. DNI, molten salt mass flow rate and inlet temperature step disturbance simulations are carried out, and the response curves of the molten salt outlet temperature and tube wall temperature are obtained. The conclusion of this paper has important guiding significance for the establishment of an operational strategy for photothermal coupling in a molten salt solar power tower plant.

Thermo-economic evaluation and optimization of solar-driven power-to-chemical systems with thermal, electricity, and chemical storage

This paper evaluates the thermo-economics of power-to-chemicals using solar energy, with the chemicals being methane, methanol, and gasoline. In addition to the optimal technology sizing and heat cascade utilization, this paper also considers the optimal molten-salt solar power tower (MSPT) design, as the MSPT significantly affects the levelized product cost. A bi-level optimization is proposed, employing mixed-integer linear programming at the lower level with heat and mass integration for optimizing sizes and operating strategies of technologies, and with heat cascade utilization and a genetic algorithm at the upper level for optimizing the MSPT design. In the upper level, the full-load storage hours, design direct normal irradiance, solar multiple, and sizes of the MSPT are optimized. The electricity sources considered are the MSPT, photovoltaic (PV) with daily electricity storage, and the electrical grid as a complementary technology to satisfy the targeted daily product demand. Cost-competitiveness of solar-driven chemical synthesis is thoroughly assessed via considering sensitivity analysis on 1) regional solar resource endowments and actual local demands; 2) electricity sources, that is, PV vs. MSPT; and 3) the scale effect represented by different chemicals’ yield. The results show that the levelized methane cost ranges from 4.5 to 8.5 €/kg, depending on the location, plant size, and annual power contribution of concentrated solar power. Due to the larger mass production, the levelized cost of methanol and gasoline is lower: 1.5–2.2 €/kg for methanol and 4–6 €/kg for gasoline. The findings highlight the significance of location choice, that is, natural endowment of solar radiation and carbon sources. Using the syngas co-electrolysis pathway and direct solar radiation 100 kWh/m2 higher, the methane production cost is decreased by 2.4 €/kg. Sensitivity analysis performed on plant scale reveals that a compact, small-scale system is far too expensive. The levelized cost of methane could be decreased by 1.2 €/kg when the plant is scaled up from 4,000 to 20,000 kg/day H2. Due to its expensive electricity storage and limited working hours, PV is typically not chosen as a power source. Overall, solar fuels are unlikely to be cost-competitive in the near future when compared to market prices for all three compounds under consideration.

Multi-objective robust optimization of a solar power tower plant under uncertainty

The optimal design of a molten salt solar power tower (SPT) plant is sensitive to the variations of uncertainties, such as solar radiation, which result in dispersion of the model output. To mitigate the impacts of uncertainties on the thermo-economic performance of SPT plant, this study develops an uncertainty-based multi-objective robust optimization design method for the case of a SPT plant in Sevilla with the expected value (i.e. the average energy cost) and the standard deviation (i.e. the dispersion of the model output) of the levelized cost of energy (LCOE) as the objectives. The Monte Carlo (MC) simulation and simulated annealing (SA) algorithm are combined to solve the robust optimization problem. The results of Pareto frontier indicate that a trade-off is needed through decision-making. The final optimal solution is determined with expectation of LCOE of 23.09 c/kWhe and standard deviation of LCOE of 1.25 c/kWhe. Compared with the deterministic optimal design, the standard deviation of LCOE of the multi-objective robust optimum is reduced by 17.22 %, which turns out to be less sensitive to the uncertainties. Moreover, the Sobol’ global sensitivity analysis results show that the direct solar radiation, heliostat field cost and receiver cost are the most sensitive to LCOE.

Control strategy of molten salt solar power tower plant function as peak load regulation in grid

Due to its inherent intermittency and fluctuation, renewable energy represented by solar energy is not friendly to the power distribution network and connect to the grid. The molten salt solar power tower station equipped with thermal energy storage can effectively compensate for the instability and periodic fluctuation of solar energy, and a reasonable operation control strategy is essential for its peak-regulating operation mode. Based on the law of conservation of energy and conservation of momentum, the dynamic model of a 50 MW solar thermal power system is established. The traditional PID control algorithm is applied to the temperature control of the receiver and the water level control of the power cycle system. Under the disturbance of different environmental factors, including random disturbance of solar direct normal irradiation, step disturbance of molten salt temperature, flow rate, and feedwater temperature disturbance experiments were carried out to obtain the dynamic response curves under different operating conditions. The thermal inertia, tangential thermal stress, and control effect of the receiver are analyzed. The meteorological data on the Spring Equinox is used for the validation of the temperature control of the receiver. The results show that the temperature control strategy of the receiver and the water level of the power cycle system has good robustness. By temperature control, the outlet temperature of molten salt in the receiver can be stabilized at the calibration value in 138 s for a 400 W/m2 DNI fluctuation in 5 s, which reduces the thermal stress caused by large changes in the receiver. By water level control, the evaporator water level can be stabilized at zero water level in 108 s for 5% working load variation, which ensures the safe operation of the steam generation system.

Simultaneous optimization of combined supercritical CO2 Brayton cycle and organic Rankine cycle integrated with concentrated solar power system

This paper addresses the simultaneous optimization of a novel combined supercritical CO2 Brayton cycle-organic Rankine cycle integrated with molten salt solar power tower plant. The supercritical CO2 Brayton cycle adopts a two-stage compression intercooling design. The organic Rankine cycle is added as a bottoming cycle to recover the waste heat of the Brayton cycle to generate extra power. Due to its complexity, the integrated system involves many degrees of freedom, which make it challenging to optimize using metaheuristic methods. To tackle this issue, we develop an equation-based model that simultaneously optimizes stream thermodynamic properties and flowsheet mass/heat equilibrium of the integrated system. The problem is formulated as a nonlinear programming model, and a tailored two-step solution strategy is developed to improve its solution efficiency. Several numerical studies are carried out to demonstrate the effectiveness of the proposed optimization method, as well as the efficiency of the integrated system. Solution time of the model is less than 224 CPU seconds. Results show that the method obtains an increase up to 3.6% in thermal efficiency compared with literature results; the optimal intercooling design increases net power output by 2.7% comparing with recompression cycle; the recompression cycle-organic Rankine cycle design obtains a 3.5% increase; and the combined intercooling cycle-organic Rankine cycle design achieves a total 4.4% increase.

Modeling and parametric study of molten salt receiver of concentrating solar power tower plant

Central receiver is a key part of concentrating solar power tower plants, as it is not only responsible for the highly effective absorption of incident energy from the heliostat field, but also for efficient energy conversion from light to heat. Its performance will directly affect the system efficiency and generating capacity of the whole plant. In this paper, a comprehensive model of molten salt receiver, which uses the mult-section lumped parameter method, is clearly developed based on a molten salt solar power tower plant. In order to improve the reliability as well as the prediction accuracy of the developed model, the dynamic characteristics of the molten salt receiver are fully investigated by a step disturbance of external parameters. Besides, in order to improve the design level of molten salt receiver, the influence of key parameters on the performance of receiver system is also extensively studied. The results show that the incident solar flux, wind speed and absorptivity of heat-absorbing tube can greatly affect the performance of molten salt receiver system. In order to verify the validity of proposed model, the simulation results are compared with the published experimental data, and the results show the model has a high accuracy. Conclusions of this paper are good references for the design, control and commissioning of molten salt receiver systems.

Working fluid selection for regenerative supercritical Brayton cycle combined with bottoming ORC driven by molten salt solar power tower using energy–exergy analysis

The objective of the present work is the investigation of a regenerative supercritical Brayton cycle (SBC) and its combination with an organic Rankine cycle (ORC). The study includes assessment, comparison, and single-objective optimization of these systems which are driven by molten salt solar power tower (SPT). First, the performances of 13 different working fluids from 18 introduced gases in the supercritical regions are studied in SBC and 6 working fluids are selected for further analysis. Then, a parametric analysis is performed to assess the effects of the compressor pressure ratio (CPR) and the Brayton turbine inlet temperature (BTIT) variations on net power output, exergy efficiency and total exergy destruction rate. After that, the optimum operating points of system performance are reported. The results show that using ORC causes all three output parameters to be improved. The highest net power output is 177321 kW and the highest exergy efficiency is 21.23% which are obtained by Helium as the working fluid. Moreover, the minimum exergy destruction is found to be 70576 kW allocating Oxygen. Finally, by comparing the examined gases in SBC, it can be concluded that using Helium leads to the best thermodynamic performance of the system.

Thermodynamic performance analysis of different supercritical Brayton cycles using CO2-based binary mixtures in the molten salt solar power tower systems

The potential to improve the performance of molten salt solar power tower system (SPT) is explored through the proposal of CO2-based binary mixture cycle in the present study. The feasibility of using xenon and butane as the additives to the S-CO2 cycle are discussed from the perspective of thermodynamic analysis. The detail parametric study is performed to reveal the effects of crucial parameters on the performance of 4 system configurations. Furthermore, the systematic comparison is conducted for 4 cycle layouts adopting CO2/xenon, CO2 and CO2/butane separately to illustrate the mechanism of performance improvement of SPT system coupled to CO2-based binary mixture cycle. The optimal performance of the SPT system is also demonstrated. Finally, the best performance system layout and suitable additives are recommended. The results indicate the following issues. Adding xenon into S-CO2 cycle can obviously improve the overall thermal efficiency and exergy efficiency. While the effects of butane as an additive are converse. The inter-cooling CO2/xenon cycle is recommended as the most suitable layout coupled to the SPT system, and the exergy efficiency is 1.18%∼1.32% higher than that of the SPT system with S-CO2 inter-cooling cycle. Detail exergy loss fraction distribution illustrates that the receiver is the highest exergy loss part and followed by the heliostat field, and butane as an additive is beneficial to reduce the receiver exergy loss for its smaller temperature difference. The study can provide a novel way to improve the SPT system performance and give a clue to the addition of CO2-based binary mixture in power cycles particularly for the application of SPT system.

Novel optimization design strategy for solar power tower plants

A novel strategy using the Sobol’-Simulated Annealing algorithm was proposed to reduce the number of optimization steps and guarantee the accuracy of a molten salt solar power tower plant design. The new method combined the Sobol’ method and the Simulated Annealing algorithm for global sensitivity analysis and global optimization, respectively. Based on the sensitivity analysis, the high-dimension global optimization problem was transformed into several low-dimension global optimization problems by parameter decoupling. In order to obtain the global minimum levelized cost of electricity of the solar power tower plant, these low-dimension models were successively optimized by utilizing the Simulated Annealing algorithm. A reference case study of the solar power tower plant with 2650 heliostats was conducted. The heliostat field, receiver, thermal storage system and power block were designed as a function of 12 parameters. It was demonstrated that the parameters related to the heliostat field and receiver were almost independent. The minimum levelized cost of electricity of 22.22 ȼ/kWhe was obtained. Furthermore, a comparison with the global algorithm and local algorithm showed that the novel method could reduce the number of optimization steps by approximately 75% compared with that of the global algorithm. A much more accurate optimal design than that of the local algorithm can be achieved herewith.

Integration between supercritical CO2 Brayton cycles and molten salt solar power towers: A review and a comprehensive comparison of different cycle layouts

In the present study, several current S-CO2 Brayton cycle layouts are reviewed, and considered to be integrated into the existing mature molten salt solar power tower (SPT) systems. The SPT systems integrated with S-CO2 Brayton cycles are completely modeled by an integrative approach. The performances of these different cycles are compared comprehensively for applications in molten salt SPT systems from the aspects of the efficiency, the specific work, and the incorporation ability with the thermal energy storage indicated by the molten salt temperature difference across the solar receiver. The results indicate: (1) The intercooling cycle can generally offer the highest efficiency, followed by the partial-cooling cycle, and the recompression cycle; The precompression cycle can yield higher efficiency than the recompression cycle when the compressor inlet temperature is high; The increase in the hot salt temperature cannot always result in the efficiency improvement of the SPT systems. (2) The partial-cooling cycle can offer the largest specific work, while the recompression cycle and the split expansion cycle yield the lowest specific work. (3) The molten salt temperature differences of SPT systems with the simple recuperation cycle, the partial-cooling cycle, and the precompression cycle are slightly larger than those of SPT systems with the recompression cycle, the split expansion cycle, and the intercooling cycle. (4) As a classical approach to improve efficiency, reheating can decrease the system efficiency in the cases with high hot molten salt temperature; SPT systems without reheating can yield larger molten salt temperature difference than those with reheating. (5) Although the current S-CO2 Brayton cycle layouts can offer high efficiency, there are still several challenges for integrating them into the SPT systems: the specific work is relatively small, and the temperature difference across the solar receiver is narrow. Further work remains to build novel S-CO2 cycle layouts with high efficiency, large specific work, and wide temperature difference.

Thermodynamic analysis and optimization of a molten salt solar power tower integrated with a recompression supercritical CO2 Brayton cycle based on integrated modeling

In the present study, a molten salt solar power tower (SPT) system integrated with a S-CO2 Brayton cycle is presented. An integrated model is developed for the integrated SPT system including the heliostat field, the molten salt solar receiver, the molten salt thermal storage, and the S-CO2 recompression Brayton cycle with reheating. Parametric analysis is conducted to investigate the effects of some key thermodynamic parameters (e.g. hot salt temperature, cycle high pressure, cycle low pressure, intermediate pressure, and the split ratio) on the integrated SPT system in terms of exergy efficiency. The parameter optimization is performed by using genetic algorithm in order to obtain the highest overall exergy efficiency. The effects of the component performance and the compressor inlet temperature on the optimum parameters are also discussed. The results indicate that the optimum hot salt temperature is 565°C which is its maximum allowable temperature when the solar salt is used as the heat transfer fluid and the thermal storage media. Besides, the optimum cycle low pressure is in the range of 7.80–10.0MPa which means that the cycle low pressure is not mandatory to be close to the critical pressure. The increase in the compressor inlet temperature leads to both the decrease in maximum exergy efficiency and the variation of the optimum thermodynamic parameters. The component performances have significant effects on the maximum exergy efficiency, but slight effects on optimum thermodynamic parameters. A novel salt with a higher maximum allowable temperature is indispensable for further improving the system efficiency. The maximum allowable temperature of 680°C is recommended for novel salts to be used in the SPT system integrated with S-CO2 recompression Brayton cycle from the viewpoint of exergy efficiency under the present conditions.

Thermal analysis of a heat pipe solar central receiver for concentrated solar power tower

A novel heat pipe solar central receiver for a molten salt solar power tower is presented. The basic element consists of a reflector, heat pipe, and receiver tube. The reflector redirects concentrated sunlight from the heliostats field onto the evaporator section of the heat pipe. After absorbing the radiative heat energy, the working fluid inside the heat pipe is vaporized at the evaporator section, and flows to the condenser section of the heat pipe where it condenses. The condenser section is inserted into the receiver tube, and is cooled by a cross flow of the heat transfer fluid inside the receiver tube. In the proposed concept, the receiver tube is free from direct irradiation by the sunlight and therefore can be kept warm by electrical heating. This will extend the daily operating time of the receiver and greatly reduce possible freezing of the molten salt. In this study, a cavity receiver with the same geometry and boundary conditions as the Molten Salt Electrical Experiment (MSEE) cavity receiver was developed. Numerical simulation for the basic element of the cavity receiver was conducted. An 88.5% efficiency was obtained, which is slightly higher than the MSEE. Moreover, the receiver efficiency under a different number of flow passes and various heat flux density on the absorbing surface was also studied. Results show that the receiver efficiency can be as high as 91.5%.

ESolar's Modular, Scalable Molten Salt Power Tower Reference Plant Design

eSolar has completed design of a molten salt solar power tower with storage based on a 50-MWt module comprised of a tower- mounted molten salt receiver surrounded by a heliostat field utilizing eSolar's small heliostat technology. This basic thermal module can be replicated, without scaling or redesign, as many times as required to create plant sizes from 50 to 200 MW with capacity factors ranging from 20 to 75%. This paper describes the details of this molten salt reference plant design as developed over the past two years, as well as a potential scenario for staged initial commercial deployment that manages both risk and cost.

CFD-simulation of a new receiver design for a molten salt solar power tower

Abstract A novel design for a molten salt solar central receiver is presented and investigated with regard to its thermal efficiency. The new receiver is for a surrounding heliostat field and consists of many hexagonal pyramid-shaped elements that are arranged alveolarly with their apexes pointing to the heliostats. Concentrated solar radiation is absorbed on the surfaces while the elements are cooled by a molten salt flow on the inside. As most of the radiative losses are re-absorbed by the neighboring pyramids, the system can almost be considered as a radiation trap. The new receiver’s efficiency has been investigated using coupled CFD-simulations for the radiation and for the heat transfer into the molten salt flow. Presuming a concentrated incident power of 1 MW/m 2 , a thermal efficiency of 91.2% was obtained. Losses by reflection could be reduced to 1.3% of the incoming radiation, while losses by emission accounted for 2.8%. The efficiency was compared to literature values of existing solar receivers, showing that the new concept is promising from the heat transfer point of view. Further simulations were performed to investigate part load behavior and the influence of geometry variations.

Development of a High-Temperature, Long-Shafted, Molten-Salt Pump for Power Tower Applications

A new hot-salt pump has been developed for molten-salt solar power tower applications that will reduce the capital cost of the plant, eliminate many of the piping, valve and sump problems associated with the handling of molten salt, and improve the reliability of a critical part of the operating system of the plant. Previous systems required that the pumps in these plants be housed in shallow sumps that were gravity fed by the storage tanks. This new pump arrangement will eliminate the sump level-control valves and the potential for overflowing the pump sump vessels. Until now, only cantilever pumps were qualified for hot molten-salt service because no suitable bearing materials had been tested. This paper describes the successful qualification of a long-shafted pump with salt-lubricated bearings tested for over 5000 hours with nitrate salt at 565°C.