Keeping up with the "double carbon" goal and seeking a photothermal future




By the end of 2022, the installed capacity of renewable energy power generation in China has historically exceeded that of coal power generation, and the construction of new energy system and new power system has brought new opportunities for the development of solar thermal industry. The "Carbon Peak Action Plan by 2030" issued by the State Council clearly pointed out: actively develop solar thermal power generation, and promote the establishment of solar thermal power generation, photovoltaic power generation, and wind power complementary regulation of wind and solar thermal integrated renewable energy power generation base.


With the rapid development of renewable energy power generation base, the solar thermal power generation industry is ushering in a new upsurge of development, but also brings new challenges.


Q: How to give full play to the comprehensive advantages of long-term, large-capacity, high-safety, and low-cost heat storage systems equipped with photothermal power plants to better meet the needs of building new power systems?


Q: How to tap the greater potential value of solar thermal power generation technology in the context of electricity market trading?



Huang Wenbo, chairman of Shouhang High-tech, as the executive vice-chairman of the National Solar Thermal Industry Technology Innovation Strategic Alliance, attended the meeting and delivered a speech. In the conference, on the theme of "Analysis of Key Technical Problems of Tower-type Photothermal Power Station", he first proposed that the systems of the Photothermal Power Station and the natural environment are closely related to each other from the macro level, the coordination between the various links affects the operation efficiency and investment economy of the whole life cycle of the power station, and then the analysis is carried out from the micro level, combined with the operation experience of the first high-tech 100MW molten salt tower-type photothermal power station, from the technical, construction, operation and maintenance and other aspects of the point depth analysis. Part of the report reads as follows:


First, the analysis of the problem of heliostat selection.



heliostatIt is the main investment part of the tower solar thermal power station and the core equipment of the concentrating system. Its main function is to reflect the sunlight projected to the surface of the heliostat to the surface of the heat absorber located at the top of the tower.

As we all know, for tower power stations, heliostat selection is very important. Its basic requirements include: high reflectivity, small surface error, high tracking accuracy, strong wind resistance, mass production, easy installation and maintenance, and the ultimate goal of assessment is the economy of the heliostat's full life cycle.



1. The relationship between the size of the heliostat and the size of the light spot


Q: the size of the heliostat spot must be big?


On the issue of heliostat selection, there has always been a preconceived idea that "a large size heliostat spot will be larger", but in fact this is a cognitive misunderstanding. The large-size heliostat is made of many face mirrors, and the large area of the heliostat does not mean that the spot size must be large. As shown in the figure, through the sub-mirror splicing angle adjustment (canting) technology, the light spots of each sub-mirror can be gathered together, so under the working condition that most of the sun is incident paraxial, the size of the heliostat light spot is mainly determined by the size and optical path of the single-sided sub-mirror.





2. The relationship between size and wind resistance


Q: Is it necessarily not suitable for large heliostats in windy places?



It is correct that a large size heliostat has a large overall wind load and a small size heliostat has a small overall wind load.


However, the size of the wind load should not be confused with the strength of the wind resistance, which is determined by the foundation and structural strength of the heliostat. As shown in the following figure, the size is large, the wind load is large, and the wind resistance is strong; the size is small, the wind load is small, and the wind resistance is weak. The wind resistance is not directly related to the size, and the wind resistance of the heliostat can only be evaluated by the design working wind speed and the survival wind speed of the heliostat.




3. Relationship between size and maintenance cost


The area of the heliostat is appropriately small and large, which reduces the number of heliostats and reduces the maintenance cost.




4. Analysis of glass thickness and reflectivity



The thickness of the glass mirror is thinned from thickness to improve reflectivity


From the perspective of reflectivity, the thinner the heliostat glass, the better. At present, the reflectivity of 4mm glass is only 93.5, that of 3mm is 94%, and that of 2mm can reach 95%, but the thinner it is, the back plate needs to be added to increase the wind resistance. However, at present, some molten salt tank designs in the world use a 0.95mm backplane mirror, with a reflectivity of 97%. The weight of 4mm glass is 25% higher than that of 3mm glass, and the heliostat steel structure will also increase accordingly.




5. Relationship between back plate and surface type accuracy and wind resistance


(1) The thickness of the glass reflector is thinned from thick, and the surface shape is integrally formed by a rigid back plate. Compared with the control of mechanical support points, the surface shape has higher precision and improved reflectivity.


(2) Improve the rigidity of the mirror surface and enhance the wind resistance


The two figures are to analyze the deformation of the glass under the wind speed of 16 m/s and the peak wind pressure with or without the back plate. The results show that the sub-mirror with the back plate improves the rigidity of the mirror, the deformation is smaller, and the wind resistance is stronger.




6. The relationship between the transmission mechanism and the tracking accuracy.


The high-precision transmission mechanism is the core component of the mirror field control system. There are many factors that need to be considered for the heliostat drive transmission mechanism, including transmission ratio, transmission accuracy, transmission efficiency, maintainability, and life span. Power station with large wind load, large mirror field and harsh environment, long-term economic consideration, high-precision RV reducer, high transmission efficiency and precision, strong load impact resistance, and smaller tracking error.



7. The exploration of heliostat technology route in the first flight


Since 2010, Shouhang High-Tech has successively developed heliostat products of various specifications, explored various technical routes, and conducted long-term test experiments at Tianjin Photothermal Power Generation Test Base and Dunhuang Power Station. As shown in the table, these explorations and experiments include: different heliostat areas, different heliostat shapes, with or without a back plate, different glass thicknesses, different transmission modes, different driving modes, different encoders, etc. Based on many years of heliostat design, manufacturing experience and various types of heliostat measured data, starting from the economic evaluation model of the whole life cycle, the optimal heliostat scheme is selected.



Based on many years of heliostat design, manufacturing experience and various types of heliostat measured data, starting from the economic evaluation model of the whole life cycle: small mirror, appropriate large-size heliostat, thin glass, with back plate, the use of high-precision transmission is the better scheme of large-scale photothermal power station heliostat.



The above picture is a part of the photos of the heliostats of the first flight of high tech in Tianjin and Dunhuang. These heliostats have undergone experimental tests ranging from 5-10 years. It is very clear that the exploration of the technical route of heliostats, the first flight of high tech, has been on the road......


Design selection and deformation analysis of 2. heat absorber


1. Thermal hydraulic design of heat absorber


A very important part of the design of the heat absorber is the calculation of thermal hydraulics, and the CFD method can be used to calculate the flow characteristics of the fluid inside the heat absorber, which is the basis for pump selection. The temperature distribution of the heat absorber and the molten salt can be obtained on this basis.



2. Selection and optimization of heat sink pipe diameter and pipe wall thickness


The diameter of the heat absorber and the thickness of the pipe wall are all a comprehensive optimization process. The temperature of the inner and outer walls of the pipe, thermal stress, mechanical stress, heat absorption efficiency, molten salt pump power consumption, molten salt corrosion and other factors should be considered at the same time to optimize the optimal pipe diameter and wall thickness.



Molten salt tank of 3. storage and heat exchange system: how to avoid leakage of molten salt tank


1. Main issues to be considered in design


Thermal expansion, foundation insulation, antifreeze, leak detection, etc. In the design can be used finite element technology and three-dimensional simulation of a combination of means.



2. Problems to be considered for tank foundation


It is necessary to consider the selection of ceramsite diameter, and stainless steel wire mesh can be added to ceramsite to increase reliability, so as to prevent the separation of ceramsite with different particle sizes in 30 years. At the same time, the feasibility of the next generation of domestic high-temperature large plates to solve the problems existing in ceramsite construction can be demonstrated in engineering.



3. Manufacturing and testing process


In order to ensure the quality of the storage tank, it is necessary to have a complete manufacturing process and a detailed description of the whole process and control procedures. The welding quality inspection is one of the most important factors to be considered, and 100% of the tank welds need to be NDT.



4. Foundation design of high groundwater level molten salt storage tank


At present, some projects in China have high groundwater level. For this situation, we put forward a preliminary scheme for the foundation design of molten salt storage tank with high groundwater level, as shown in the figure, which can effectively solve the technical problem of waterproof design of storage tank foundation.



IV. Operation Curves of Tower Power Station under Different Working Conditions


1. Operation curve of tower power station under cloudy condition



In cloudy weather, DNI fluctuates frequently and greatly, which requires close cooperation between the mirror field and the heat absorber and depth adjustment. At the same time, the turbine load is adjusted at any time according to the salt level, and the unit is operated at high load at night to adjust the peak of the power grid.


2. Operation curve of tower power station under strong wind condition



In windy weather, the mirror field can still operate with maximum efficiency. Under the high load operation of the unit all day, the hot salt tank reaches the maximum liquid level at 15: 00 p.m. It can be seen that the output of the mirror field still has a large margin under the strong wind background. 50% of the heliostats are evacuated when the molten salt tank is full, which shows that the design structure of large-area heliostats is reasonable, the wind resistance is strong, the operation is stable in windy environment, and the condensing capacity is good.


3. Operation curve of cloudy wind and dust condition



In cloudy, windy and dusty weather, DNI fluctuates violently. The influence of cloud changes on DNI is predicted by the sky camera. It not only automatically adjusts the flow rate of the cold salt pump, but also controls the number of mirror field inputs according to the infrared display temperature to prevent the temperature of the heat absorption screen from changing too fast, thus ensuring stable outlet temperature, collecting solar energy to the maximum extent under the premise of safety, and maintaining continuous operation of low load.


4. Operation and maintenance of Dunhuang Shouhang Photothermal Power Station after getting off the tower and turning cloudy after getting off the tower




To deal with complex and changeable weather conditions, adjust the operation strategy in time, and accurately predict the weather of the next day. The steam turbine unit will be connected to the grid to generate electricity before the weather improves, and the sun island will collect heat and store heat after DNI meets the requirements.


Five, heliostat cleaning strategy


1. Cleaning of heliostat



Under severe weather conditions, the cleanliness of the heliostat is the most critical part to ensure the power generation efficiency of the solar thermal power plant. The use of appropriate area of the larger heliostat, the unit time cleaning heliostat area will be larger, because of this adjacent heliostat spacing, can use large mechanized, automated cleaning system, high efficiency. Through radar ranging, pressure controller and high-precision sensor, it can adapt to the mirror field road without artificial hardening, automatically adjust the brush head of the brush holder, and carry out high-quality and rapid cleaning.


The figure is the first self-developed intelligent cleaning vehicle.



2. Solve the problem of frost in winter


At present, the average time for a car to clean a heliostat of 115 square meters is about 50 seconds, and six cleaning vehicles can clean the heliostat of the whole mirror field in 2.5 to 4.5 days, thus ensuring the cleanliness of the reflector to the maximum extent. At the same time, the central control room and the mirror washing car are used to control the heliostat in linkage: in which area the cleaning car is cleaned, the control system automatically changes the heliostat in this area from operation to cleaning position, thus liberating the strength of the mirror field operators in the main control room and ensuring the maximum effective utilization of the heliostat in the whole mirror field.



Six, molten salt tower power station operation and maintenance problems


Because the operation of the solar thermal power station is greatly restricted by the weather, due to different seasons, the weather has its differences and commonalities, after year-by-year operation experience accumulation, has now summed up a set of operation strategies to reduce the number of unit start and stop and ensure that the unit runs in the high efficiency load range.


1. Winter


In winter, there will be a phenomenon of mirror frost, by adjusting the winter cleaning strategy, shorten the mirror frost time, improve the efficiency of the mirror field in winter.


2. Spring


In spring, due to the influence of sand and dust weather, the mirror field efficiency is low. First, the mirror field cleaning efficiency is improved. Second, the sun island operation is adopted. The unit is started the next day after the salt level reaches the requirements, so as to ensure a longer single operation period of the unit, reduce the number of unit start and stop, and ensure the design life of the unit.


3. In the summer and autumn seasons, the sunshine time is longer, the air quality is better, and the unit adopts the strategy of long-term continuous operation. According to the forecast of different weather conditions, flexibly adjust the unit output.


The weather was bad the next day.


In the case of bad weather on the next day, the hot salt level can still ensure the operation of the unit before the heat absorption system is put into operation.


The next day the weather was fine


When the weather is good on the next day, ensure that the hot salt level reaches the minimum level before the heat absorption system is put into operation, so as to reduce the amount of light abandonment under the condition of full load operation of the unit on that day.


4. The operation strategy at different times of the day, according to the peak power demand of the power grid, give full play to the advantages of the solar thermal power station peak.




During the operation of the concentrator heat collection system from 9:00 to 18:00, the DNI-based climate prediction, when necessary, the group will operate at low load during this period, and finally the hot salt level will reach the highest value after the heat absorption system is out of operation.




During the peak period of 18:00-23:30 in the evening peak, the energy storage characteristics are used to carry out peak regulation of the power grid with the maximum load.




In the evening peak after the end of the 23:30-9:00 unit to take advantage of the low ambient temperature at night, the back pressure of the unit, the advantages of high power generation efficiency, to maximize the operating efficiency of the unit.



This is one of the more classic days in June 2022. Except for the failure of the steam turbine power generation system, the maximum power generation load is only 90%, and other systems are normal. The power generation on that day reaches 2.05 million degrees, and the average load is above 85MW 24 hours a day. The hot salt tank is full for 5 hours after 15 o'clock on that day, indicating that the photothermal efficiency of the concentrating and heat collection system exceeds the design value, although the 30000-ton heat storage tank is already the largest heat storage tank in the world, it is still too small, and the power generation system cannot be underestimated. If there is a short board, it will affect the overall efficiency and avoid losing the big because of the small.



This is June 2022, the best weather month last year, but there are still 7 cloudy days and 10 cloudy days. In this case, except for the failure of the turbine power generation system, the maximum power generation load can only be 90%, and other systems are normal. The maximum power generation in a single day is 2.07207 million degrees, the maximum power generation time is 10.93 days, and the power generation in that month is 33.764701 million degrees. If the power generation system is repaired, the power generation in the month will be higher; if you choose to build a power station in a place with a better climate environment, if you choose the same configuration as Dunhuang 100MW, there is still a lot of room for development.



It can be seen that the climate of the plant site, the condensing system, the heat absorption system, the heat storage system, the heat exchange system and the power generation system of the photothermal power station are interrelated and influence each other, and they are an organic whole with endless growth and unity of opposites. Their influence on the photoelectric efficiency varies from one to another, is closely related to the investment income, and affects the whole life cycle of the power station, from investment decision-making to project design, from product procurement to equipment production, from project construction to commissioning and operation, and even to project scrapping, it is worth the experts, scholars and industry colleagues to discuss and study together!