Molten salt pump
The high-temperature molten salt pumps of different temperature types (such as 300℃, 400℃, 450℃, 600℃, 750℃, and 850℃) produced by SDP Pump Industry have been widely used in multiple solar thermal power generation projects, promoting the technological development of extreme high-temperature molten salt pumps and realizing the localization of similar products.
Specific projects include:
● Dunhuang 10MW Molten Salt Tower CSP Project
● Delingha 10MW Molten Salt Tower Renovation Project
● Shouhang Hi-Tech 100MW Molten Salt Tower CSP Project
● Delingha 50MW Molten Salt Tower CSP Project
● Jinfan Energy Aksai 50MW Molten Salt Tower CSP Project
● Yumen Xinneng 50MW Molten Salt Secondary Reflection CSP Project
● China Power Hami 50MW Molten Salt Tower CSP Project
● Haixi Prefecture Multi-energy Complementary Integrated Optimization Demonstration Project 50MW Molten Salt Tower CSP Project
● Lanzhou Dacheng Dunhuang 50M Molten Salt Linear Fresnel CSP Project
● 50MW CSP Key Technology Research Project
● Delingha 50MW Molten Salt Tower CSP Project
● Jiangsu Guoxin Jingjiang Power Plant 2*660M Unit Project
● Jinta “100,000 kW CSP + 600,000 kW PV” Project
● CGN Jixi Lugu 490 MW DC Integrated Energy Project
● CNNC Yumen “CSP Energy Storage + PV + Wind Power” Demonstration Project (100,000 kW CSP Energy Storage EPC Project)
● Liaohe Oilfield Molten Salt Heat Storage and Steam Injection Test Station Project
By 2025, my country’s photovoltaic industry has achieved significant progress in new installed capacity. On February 27, the National Energy Administration issued the “Guiding Opinions on Energy Work in 2025,” which set a target of over 200 GW of new renewable energy power generation capacity among its key goals for 2025. In 2023 and 2024, my country’s total new wind power and photovoltaic power generation capacity will reach 296 GW and 358 GW, respectively.
According to the latest data, by the end of 2024, China’s cumulative installed photovoltaic power generation capacity will have reached approximately 885.68 GW, with over 55% of this capacity installed in the past two years. However, with the adjustment of policy objectives and changes in market mechanisms, my country’s new photovoltaic installed capacity is expected to be between 215GW and 255GW in 2025, a decline from 277.57GW in 2024.
Globally, the photovoltaic industry also maintains its growth momentum. According to TrendForce, global photovoltaic capacity will reach approximately 596 GW in 2025, a 10% year-on-year increase. Despite a slowdown in growth, this trend continues to demonstrate the industry’s strong potential and resilience.
Facing the challenges of “double highs” (high volatility and high intermittency), the rise of grid-connected energy storage systems offers a new approach to addressing the randomness and weak support issues of power systems. Furthermore, emerging markets such as the Middle East and Africa, with their abundant sunlight resources, are becoming new hotspots for photovoltaic industry development. Improved grid infrastructure and the establishment of market-based mechanisms in these regions will further accelerate the growth of photovoltaic installations.
Overall, while my country’s new photovoltaic capacity may decline slightly in the short term due to policy adjustments, the long-term prospects for the global photovoltaic industry remain promising. In recent years, continuous technological innovation and declining production costs have made photovoltaic power generation the most cost-effective energy solution in regions with abundant sunlight, further driving the growth of photovoltaic installed capacity.
Molten Salt Thermal Storage System
Molten salt thermal storage (energy storage) technology is an advanced thermal energy storage system within photovoltaic technology. Its core principle is to store thermal energy in a high-temperature molten salt solution for subsequent conversion into electricity or heat. It boasts low theoretical cost, high operating temperature, and environmental friendliness.
The key components of a molten salt thermal storage project include cold/hot salt storage tanks, electric heaters, heat exchangers, and supporting steam turbines, as shown in the figure below.
During the charging phase, an electric heater uses external electricity to generate heat, which is used to heat the salt solution, raising its temperature to a high temperature, typically reaching 600°C. The energy storage medium in a molten salt thermal storage system is typically a mixture of potassium and sodium nitrate, which has excellent thermal properties and a long lifespan. It not only stores large amounts of heat at high temperatures, but is also environmentally friendly, preventing leakage or environmental damage.
During the discharge phase, the hot molten salt transfers heat energy to a fluid medium, typically high-temperature oil, through a heat exchanger. This fluid is then used to generate high-temperature, high-pressure steam to drive a steam turbine to produce electricity. Simultaneously, the cooled molten salt is pumped back into the cold salt storage tank, ready for the next energy storage cycle.
In theory, molten salt thermal storage is an efficient and environmentally friendly energy storage system. Its ability to provide heat sets it apart from other energy storage technologies. It can be used in the power sector to directly supply high-temperature steam for power generation, or it can directly provide heating and high-pressure steam to industrial parks, helping to reduce industry’s reliance on fossil fuels and lower carbon emissions. This technology has broad application prospects and is expected to provide strong support for the sustainable development of the energy industry.
Due to its unique heat storage capacity, molten salt energy storage has slightly different application scenarios from currently mainstream electrochemical energy storage. In recent years, molten salt thermal storage technology has begun to emerge in a variety of fields, such as concentrated solar power plants, thermal power plant flexibility conversion, industrial waste heat recovery, thermal storage air conditioning, and urban heating. Solar thermal power generation and thermal power plant flexibility conversion have been the most widely promoted and applied, with some projects already achieving commercial operation.
Solar thermal power generation process
Currently, the commonly used solar thermal power station technologies are trough and tower types, while linear Fresnel power stations are not widely used.
Trough power station
The design principle of a trough power plant is to use parabolic mirrors to reflect sunlight onto a central pipe, heating it. The medium flowing within the pipe absorbs and carries the heat away. The medium used to absorb and transfer heat is typically thermal oil.
During the day, some of the heated thermal oil is pumped to the boiler, heating it, which generates steam, which drives the steam turbine to generate electricity. Another portion of the thermal oil heats the molten salt in the molten salt system, storing energy. (Cold molten salt is pumped from the cold tank by a cold pump and transported to a heat exchanger, where it is heated by the thermal oil and then transferred to a hot tank for storage.)
At night, the stored hot molten salt becomes a heat source, releasing energy. (Hot molten salt is pumped from the hot tank by a heat pump and transported to a heat exchanger, where it heats the thermal oil and is then transferred to the cold tank. The heated thermal oil is then transported to the boiler, heating it, which generates steam, which drives the steam turbine to generate electricity.)
Schematic diagram of a typical trough power station flow chart
Tower power station
The operating principle of a tower power plant is that a mirror array reflects sunlight onto a solar collector at the top of the tower. Molten salt is then transported to the collector, where heat exchange occurs.
During the day, cold molten salt is pumped from a cold tank by a cold pump and transported to the collector at the top of the tower for heating. It is then transported to a hot tank for energy storage. In tower power plants, hot molten salt is used to heat the boiler. The boiler water is continuously heated by the hot molten salt pumped from the hot tank, which then drives the steam turbine for power generation. After the heat exchange, the hot molten salt cools down, becoming cold molten salt, which is then transported to the cold tank. It is then pumped out again by the cold pump and transported to the collector, where it participates in the heat exchange.
During the day, the liquid level in the cold tank gradually decreases, while the liquid level in the hot tank gradually increases, allowing for energy storage. At night, the reverse occurs.
Schematic flow chart of a typical tower power plant
Typical molten salt operating characteristics:
- Typical two-component mixture: 60% NaNO₃ + 40% KNO₃
- Density: 1900 kg/m³ (300°C); 1800 kg/m³ (400°C); 1600 kg/m³ (600°C);
- Vaporization pressure: <0.01 bar;
- Viscosity: 3.6 cp;
- Operating temperature: 300°C to 400°C for trough power plants; 300°C to 600°C for tower power plants;
- Condensation temperature: 240°C.
Main equipment of molten salt heat storage
Molten salt
Molten salts are the core of molten salt heat storage technology. They are salts that melt under high temperatures. Typically, they refer to the melt of inorganic salts, including a range of common salts such as NaNO₃, KNO₃, Ca(NO₃)₂, Na₂CO₃, K₂CO₃, NaCl, and KCl. These are common raw materials for agricultural fertilizers and are chemically stable at room temperature. Broadly speaking, they also include oxides and organic salts.
Currently, the industry mostly uses mixed salts as molten salts. Compared to single-component molten salts with high melting points and excessive thermal stability, mixed salts have lower melting points and higher decomposition temperatures, meeting the high-temperature heat transfer and temperature requirements of various applications. While adapting to different operating temperatures, they maintain the advantages of molten salts, such as thermal stability and low saturated vapor pressure.
Adjusting the composition and ratio of the mixed salts can produce a variety of mixed salts with different melting points, decomposition temperatures, and applicable operating temperature ranges. Therefore, finding mixed molten salts with low melting points and high decomposition temperatures has become a key research focus in molten salt heat storage technology.
Molten salts offer many advantages over other heat storage media. First, molten salts are ions with excellent electrical conductivity, far exceeding that of conventional electrolyte solutions. This makes the electric heating process very convenient. Second, molten salts have a wide operating temperature range, typically between 280°C and 1000°C. New low-melting-point salts even have a minimum operating temperature of 80°C, meeting the temperature requirements of most industrial applications.
Furthermore, molten salts have a low saturated vapor pressure, reducing equipment costs and improving safety. Furthermore, their low viscosity in liquid form provides excellent chemical and thermal stability. Furthermore, the raw materials are readily available and relatively inexpensive.
Salt storage tanks
Salt storage tanks are key equipment for storing molten salt, typically constructed of stainless steel or carbon steel. The exterior and foundation of the tanks are typically covered with thick insulation, which helps minimize heat loss and thus reduces energy loss.
The tank roof typically features a connection port for the molten salt pump and a safety valve to maintain the pressure within the tank within the atmospheric pressure range. An immersion heater is typically installed within the tank to maintain the temperature of the molten salt when the system is idle or when needed, ensuring that the stored heat can be effectively released when needed.
In addition, to prevent overheating of the concrete layer, a ventilation layer is added. Strategically arranged steel pipes within the ventilation layer ensure temperature control. This meticulously designed system ensures the safety and efficiency of molten salt thermal storage technology.
Other components of the molten salt thermal storage system include heat exchangers, molten salt pumps, molten salt valves, and salt traps, each playing a vital role in the overall system.
Molten Salt Pump
Based on the molten salt application environment in CSP, the operating characteristics of molten salt pumps are as follows:
High medium temperature, 300°C to 600°C;
Extremely long length, with a total length of over 20 meters;
Variable frequency operation, wide operating speed range;
Frequent starts and stops;
High maintenance costs, requiring high reliability;
Low NPSH requirements to minimize molten salt discharge from the tank bottom.
In a CSP plant, a high-temperature molten salt pump is one of the most critical conveying equipment, used to circulate and transport 565°C molten dibasic nitrate (60% NaNO₃-40% KNO₃) molten salt. The pump unit is approximately 20 meters long and must operate safely and stably in harsh environments for long periods of time, placing a significant strain on its overall performance. Failure and downtime of the high-temperature molten salt pump during operation can cause significant economic losses to the overall operation of the CSP plant. Therefore, the pump’s operational reliability under high-temperature conditions and its safety and stability during frequent starts and stops are crucial.