GB/T 45313-2025Technical requirements for molten salt thermal storage system of solar thermal electric plant (English PDF)

1 Scope

This document specifies the system composition, the technical requirements and the corresponding test items for the molten salt thermal storage system of a solar thermal electric plant.

This document applies to two-tank thermal storage systems of solar thermal electric plants that use molten salt as the storage medium for sensible heat storage and whose largest tank volume is not less than 500 cubic metres, including direct storage systems in which molten salt is the heat transfer medium and indirect storage systems in which thermal oil is the heat transfer medium.

Note: a two-tank thermal storage system is a thermal storage system in which there are two kinds of tank, a high-temperature molten salt tank and a low-temperature molten salt tank; the number of tanks is two or more.

2 Normative references

The contents of the following documents constitute indispensable provisions of this document through the normative references made to them in the text. For dated references, only the edition corresponding to that date applies to this document; for undated references, the latest edition (including all amendments) applies to this document.

GB/T 6075.7-2015 Mechanical vibration - Evaluation of machine vibration by measurements on non-rotating parts - Part 7: Rotodynamic pumps for industrial applications, including measurements on rotating shafts.

GB/T 16839.1-2018 Thermocouples - Part 1: EMF specifications and tolerances.

GB/T 29529-2013 Methods of measurement and evaluation of the noise of pumps.

GB/T 30121-2013 Industrial platinum resistance thermometers and platinum temperature sensors.

GB/T 40104 Solar thermal electric plants - Terminology.

GB/T 41087 Technical requirements for the heat exchange system of a solar thermal power plant.

GB/T 44800 Technical requirements for the working medium used for heat storage and heat transfer in solar thermal electric plants - Molten salt.

3 Terms and definitions

The terms and definitions given in GB/T 40104 and the following apply to this document.

3.1 effective heat storage capacity: The heat released by the thermal storage system from the fully charged state until the fully discharged state. Note 1: the fully charged state is the state in which the charged heat and the temperature of the thermal storage system have reached their design values and charging no longer continues. Note 2: the fully discharged state is the state in which the discharged heat and the temperature of the thermal storage system have reached their design values and no further discharge is possible.

3.2 thermal storage efficiency: The ratio, over one complete charge and discharge cycle in which charging and discharging follow one another, of the energy released by the thermal storage system through the heat transfer fluid to the energy it received through the heat transfer fluid.

3.3 rated charge power: The heat received by the thermal storage system through the heat transfer medium per unit of time at the rated temperature and rated flow rate.

3.4 rated discharge power: The heat released by the thermal storage system through the heat transfer medium per unit of time at the rated temperature and rated flow rate.

3.5 unavailable salt quantity ratio: The ratio of the total quantity of molten salt lying below the start-up level of the molten salt pumps in all the tanks to the total quantity of molten salt in the plant.

4 System composition

The molten salt direct thermal storage system of a solar thermal electric plant mainly comprises the storage tanks, the molten salt pumps, the molten salt piping and valves, the heat storage medium, the instrumentation and control, and the auxiliary systems.

The molten salt indirect thermal storage system of a solar thermal electric plant mainly comprises the storage tanks, the molten salt pumps, the oil-to-salt heat exchanger, the molten salt piping and valves, the heat storage medium, the instrumentation and control, and the auxiliary systems.

5.1 Normal service conditions

5.1.1 Ambient temperature: from minus 50 degrees Celsius to 60 degrees Celsius.

5.1.2 Altitude: below 5 000 m.

5.2 System functions and performance

5.2.1 The maximum charging power of the thermal storage system should be not less than 110 % of the rated charging power, and the maximum discharging power should be not less than 110 % of the rated discharging power.

5.2.2 The operating time of the effective heat storage capacity of the thermal storage system at rated discharging power shall meet the storage duration the plant requires of the thermal storage system.

5.2.3 The equipment of the thermal storage system shall be able to withstand fluctuations of temperature, pressure and load during charging and discharging, and its load-change capability shall be not less than 10 % of the rated charging or discharging power per minute.

5.2.4 The requirements for the unavailable salt quantity ratio are given in Table 1. Where the maximum tank volume is 500 cubic metres or more but less than 2 000 cubic metres the ratio shall be not more than 20 %; where it is 2 000 cubic metres or more but less than 5 000 cubic metres, not more than 18 %; where it is 5 000 cubic metres or more but less than 10 000 cubic metres, not more than 16 %; and where it is 10 000 cubic metres or more, not more than 13 %.

5.2.5 At the annual mean temperature and mean wind speed of the site, the requirements for the thermal storage efficiency of the system are given in Table 2, which fixes the efficiency required for three bands of maximum tank volume: 500 cubic metres or more but less than 2 000 cubic metres, 2 000 cubic metres or more but less than 5 000 cubic metres, and 5 000 cubic metres or more.

5.2.6 The number of complete charge and discharge cycles over the whole life of the thermal storage system shall be not less than 12 000.

5.2.7 The design service life of the thermal storage system shall be not less than 25 years.

5.2.8 The materials of the equipment of the thermal storage system that is in contact with the molten salt shall resist molten salt corrosion, and the corrosion allowance shall meet the following requirements: a) not less than 1 mm where stainless steel is used; b) not less than 3 mm where carbon steel or alloy steel is used.

5.3 Molten salt storage tank

5.3.1 The molten salt storage tank shall be a vertical cylindrical welded steel tank. The stiffness and strength of the tank shell shall be able to carry the dead weight of the tank itself, the snow load, the dead load of the insulation and of maintenance work, the load of the fluctuating liquid level, the stresses produced by changes in the temperature of the medium, and other temporary loads. The insulation of the roof, the shell and the bottom of the tank shall meet the requirements for the temperature drop of the tank and for the thermal efficiency of the storage system.

5.3.2 At the annual mean temperature and mean wind speed of the site, the requirements for the maximum temperature drop of the tank are given in Table 3. Where the maximum tank volume is 500 cubic metres or more but less than 2 000 cubic metres, the maximum drop over 24 h shall be not more than 8 degrees Celsius for the hot salt tank and not more than 5 degrees Celsius for the cold salt tank; where it is 2 000 cubic metres or more but less than 5 000 cubic metres, not more than 5 degrees Celsius and not more than 3 degrees Celsius respectively; where it is 5 000 cubic metres or more but less than 10 000 cubic metres, not more than 3 degrees Celsius and not more than 2 degrees Celsius respectively; and where it is 10 000 cubic metres or more, not more than 2 degrees Celsius and not more than 1.5 degrees Celsius respectively.

5.3.3 A salt inlet distributor shall be fitted inside the molten salt storage tank, and the salt temperature inside the tank shall be evenly distributed in all directions. While salt is being admitted, the difference between the highest and the lowest salt temperature within the same tank should not exceed 56 degrees Celsius.

5.3.4 The molten salt storage tank should be provided with a salt drain outlet.

5.3.5 The molten salt storage tank shall be provided with facilities that prevent leaked molten salt from spreading; a sunken tank arrangement or a bund around the tank should be used.

5.3.6 Measures to distribute the load evenly shall be taken at the top of the insulating foundation of the molten salt storage tank.

5.3.7 Between the foundation and the shell of the molten salt storage tank there shall be thermal insulation measures such as an insulating layer and ventilation and heat dissipation measures such as ventilation ducts. The temperature of the upper surface of the ventilation ducts shall be not higher than the lower of the following two values: a) the local extreme high temperature with a 50-year return period plus 30 degrees Celsius; b) 90 degrees Celsius.

5.3.8 A sand bedding layer should be provided between the bottom of the tank and the insulating layer of the foundation, and the sand should be a hard medium or coarse sand.

5.4 Molten salt pump

5.4.1 The thermal storage system shall be provided with molten salt pumps such as low-temperature molten salt pumps, high-temperature molten salt pumps, temperature-regulating pumps and salt drain pumps.

5.4.2 The number of molten salt pumps shall be determined from the maximum operating condition of the system together with the reliability of the pumps. Where the number of duty pumps of the same function is three or fewer, the total flow and head of the duty pumps shall meet the delivery requirements of the maximum charging and discharging condition and one standby pump shall be provided; where the number of duty pumps of the same function is more than three, no standby pump need be provided.

5.4.3 Molten salt pumps should be vertical suspended pumps and shall be able to drain themselves under gravity.

5.4.4 The motors of the molten salt pumps shall be variable frequency motors.

5.4.5 The molten salt pump should be mounted through a mounting plate on the support platform above the molten salt storage tank, and an expansion joint should be used between the tank nozzle flange and the mounting plate.

5.4.6 The stiffness of the support platform of the molten salt pump shall meet the operating requirements under all conditions.

5.4.7 The molten salt pump should be provided with lifting lugs or a similar structure or device for hoisting.

5.4.8 A freeze protection device shall be fitted on the discharge pipe of the molten salt pump.

5.4.9 The vibration severity of the molten salt pump shall meet the requirements of group 1 in Table A.1 of GB/T 6075.7-2015.

5.4.10 The noise limit of the molten salt pump shall meet the requirements for class C given in 10.4 of GB/T 29529-2013.

5.4.11 The distance from the suction inlet of the molten salt pump to the bottom plate of the molten salt storage tank shall be greater than the sum of the thermal expansion of the molten salt pump and the normal suction height of the molten salt pump.

5.4.12 The number of starts and stops permitted over the whole life of the molten salt pump shall be not less than 15 000.

5.5 Oil-to-salt heat exchanger

5.5.1 The inlet and outlet terminal temperature differences of the oil-to-salt heat exchanger should be not higher than 10 degrees Celsius.

5.5.2 The heat transfer area of the oil-to-salt heat exchanger shall meet the working requirements under the maximum charging and discharging conditions of the thermal storage system, and an area margin of 10 % should be taken.

5.5.3 The selection and arrangement of the oil-to-salt heat exchanger shall make maintenance convenient and shall prevent the molten salt from freezing and blocking the exchanger.

5.5.4 The structure of the oil-to-salt heat exchanger shall be able to withstand thermal shock, and the number of thermal shocks it withstands over its whole life shall be not less than 15 000.

5.5.5 The materials and the performance of the oil-to-salt heat exchanger shall meet the requirements of GB/T 41087.

5.6 Molten salt piping and valves

5.6.1 The nominal size of the molten salt piping should be not smaller than DN50. The slope at which molten salt piping is laid should be not less than 0.3 %, and for molten salt piping of nominal diameter DN80 or smaller the slope should be not less than 3 %.

5.6.2 The type of construction of the valves shall suit the requirements of the molten salt system for temperature, pressure, flow, flow direction and tightness. Molten salt valves should be butt-welded and should not be connected by flanges, threads or similar means.

5.6.3 Except for molten salt piping used for emergency discharge, salt draining and venting, the normal working velocity in molten salt piping should not exceed 4.5 m/s.

5.6.4 The arrangement of the molten salt valves shall meet the requirements of the changing conditions of operation, starting and stopping, overhaul, maintenance and emergency handling of the thermal storage system.

5.6.5 A check valve should be fitted at the discharge of the molten salt pump.

5.6.6 A separate freeze protection device should be fitted on both the body and the stem of a molten salt valve.

5.7 Heat storage medium

The technical requirements for the working temperature, the flow properties, the heat transfer properties, the heat storage properties and the corrosivity of the molten salt shall conform to GB/T 44800.

5.8 Instrumentation and control

5.8.1 The instruments of the thermal storage system shall include instruments for measuring temperature, flow, pressure and vibration severity.

5.8.2 Thermocouples shall be fitted to the molten salt storage tanks and their insulating foundations. The tolerance class of the thermocouples shall be class 1 of Table 12 of GB/T 16839.1-2018 and the degree of protection shall be not lower than IP65. The thermocouples shall be arranged as follows: a) thermocouples should be installed inside thermowells; b) at least three sets of multipoint thermocouples shall be fitted inside the tank, each set evenly distributed from the tank bottom to the top of the straight shell at a spacing of not less than 2 m, with at least two measuring points at the tank bottom; c) at least three levels of thermocouples shall be fitted on the tank shell, each level evenly distributed around the circumference and preferably not fewer than four; d) at least six thermocouples shall be fitted on the tank roof, evenly distributed; e) at least twelve thermocouples shall be fitted on the upper surface of the ventilation ducts and between the concrete layers, evenly distributed at a spacing of not more than 5 m; f) thermocouples shall be evenly distributed under the lower surface of the sand bedding layer of the tank foundation at a spacing of not more than 5 m.

5.8.3 At least three level gauges shall be fitted on each molten salt storage tank, installed away from the molten salt pumps, the temperature sensing elements, the molten salt piping and anything else that would disturb the measurement. The level gauges shall meet the following requirements: a) maximum permissible error plus or minus 5 mm; b) degree of protection not lower than IP65; c) output response time not more than 200 ms.

5.8.4 A displacement monitoring device shall be fitted to the molten salt storage tank; not fewer than six shall be fitted to each tank, evenly arranged around the circumference of the tank. The accuracy of the displacement monitoring device shall be not lower than plus or minus 1 mm.

5.8.5 At least one pressure monitoring device shall be fitted at the top of a tank with a nitrogen blanket; its range should be from 0 Pa to 5 000 Pa and its degree of protection shall be not lower than IP65.

5.8.6 Platinum resistance thermometers should be fitted to the molten salt pumps. There shall be not fewer than two bearing temperature measuring points and not fewer than three motor winding temperature measuring points on each pump. The tolerance class of the platinum resistance thermometers shall be not lower than class A of Table 3 of GB/T 30121-2013.

5.8.7 At least one flowmeter and one pressure monitoring device shall be fitted at the discharge of molten salt pumps of the same function. The flowmeter shall be installed on a straight pipe of length not less than fifteen pipe diameters, with an upstream straight length of preferably not less than ten pipe diameters and a downstream straight length of preferably not less than five pipe diameters; its accuracy class shall be not lower than 1.0 and its degree of protection not lower than IP65. The class of the pressure monitoring device shall be not lower than 1.0 and its degree of protection not lower than IP65.

5.8.8 A vibration severity meter shall be fitted to the molten salt pump; there shall be not fewer than three vibration monitoring points on each pump, that is one in the vertical direction and two in the horizontal direction. The accuracy class of the vibration severity meter shall be not lower than 1.0.

5.8.9 The thermal storage system shall have the following alarm functions: a) high and low limit alarms for the liquid level in the molten salt storage tank; b) high and low limit alarms for the salt temperature in the molten salt storage tank; c) high limit alarm for the temperature of the insulating foundation of the molten salt storage tank and of its ventilation ducts; d) high limit alarms for the bearing temperature, the motor winding temperature and the vibration severity of the molten salt pump.

5.8.10 The thermal storage system shall have the following interlock protection functions: a) when the bearing or motor winding temperature or the vibration severity of a molten salt pump exceeds its limit, the molten salt pump is stopped by interlock; b) when the level in a molten salt storage tank falls below the extreme low limit or rises above the extreme high limit, the molten salt pump is stopped by interlock.

5.9 Auxiliary systems

5.9.1 Power supply. 5.9.1.1 The electrical load of the molten salt pumps, the freeze protection devices, the cooling fans of the molten salt tank foundations, the valves, the instruments and control equipment and the lighting of the thermal storage system shall be counted in the power supply capacity of the thermal storage system; the preheating and salt melting loads of the thermal storage system may be treated as temporary loads and are not counted in the power supply capacity of the thermal storage system. 5.9.1.2 The power supply category of the low-temperature molten salt pumps and the high-temperature molten salt pumps shall be class I; that of the salt drain pumps, the freeze protection devices of the molten salt storage tanks, the cooling fans of the tank foundations and the freeze protection devices of the salt drain system shall be class III; and that of the other freeze protection devices shall be class II. 5.9.1.3 A closed earthing grid shall be provided in the tank area and shall be connected with the main earthing grid of the plant to form a single whole. Each tank body shall be reliably connected to the earthing grid at not fewer than two points. 5.9.1.4 An independent uninterruptible power supply (UPS) may be provided for the thermal storage system according to the control load required, and a voltage of 220 V should be selected.

5.9.2 Salt drain system. 5.9.2.1 The salt drain system shall include the drain tank, the drain pumps and the associated valves, piping and instruments. 5.9.2.2 The salt drain system shall be able to drain the molten salt from the equipment, piping, valves and instruments of the thermal storage system other than the molten salt storage tanks and to return it to the low-temperature molten salt storage tank and the high-temperature molten salt storage tank. 5.9.2.3 Drain points shall be provided at the low points of the molten salt equipment and piping and vent points at their high points; the drain points and vent points shall be connected by piping to the drain tank or to a molten salt storage tank and shall be isolated by valves. Where venting and draining can be achieved through the high and low points of the inlet and outlet connecting pipes of a piece of equipment, no separate drain point and vent point need be provided on the equipment itself.

5.9.3 Nitrogen blanketing system. 5.9.3.1 A nitrogen blanketing system shall be provided for the low-temperature molten salt storage tank, the high-temperature molten salt storage tank, the salt drain system, the heat exchangers and other equipment and subsystems according to the need for explosion protection and oxidation resistance. 5.9.3.2 An overpressure relief device should be fitted to the nitrogen blanketing system. Where the overpressure relief device of the nitrogen blanketing system cannot entirely prevent the overpressure caused by the injection of nitrogen, an overpressure relief device shall be fitted on the molten salt storage tank. 5.9.3.3 The quality of the nitrogen in the nitrogen blanketing system shall meet the following requirements: a) nitrogen purity (volume fraction) higher than 99.9 %; b) diameter of the dust particles in the gas not more than 3 micrometres; c) dust content not more than 1 mg per cubic metre; d) oil content not more than 0.0001 %; e) dew point at the discharge pressure not higher than minus 40 degrees Celsius. 5.9.3.4 The working pressure range of the nitrogen in the nitrogen blanketing system shall be from 0 Pa gauge to 2 000 Pa gauge.

5.9.4 Freeze protection system. 5.9.4.1 Freeze protection devices shall be fitted to the equipment, piping, valves and instruments of the molten salt thermal storage system; mineral insulated (MI) electric heat tracing may be used. 5.9.4.2 The rated power of the freeze protection device should be not less than 1.5 times the heat loss from the equipment surface at the extreme low temperature with a 30-year return period. Where electric heat tracing is used as the freeze protection device, its installed power should be not less than 150 % of the rated power.

5.9.5 Preheating system. 5.9.5.1 The preheating system shall be able to heat the shell of the molten salt storage tank to a temperature more than 50 degrees Celsius above the freezing point of the molten salt. 5.9.5.2 Clean air should be selected as the heating medium of the preheating system, and the temperature of the heated air shall be adjustable. 5.9.5.3 The heating gas of the preheating system shall be evenly distributed inside the tank and shall meet the requirement that the tank heats up evenly; the maximum temperature difference across the tank shell should not exceed 56 degrees Celsius.

6 Test items

The test items for the thermal storage system are given in Table 4. The charging power test and the discharging power test verify 5.2.1; the effective heat storage capacity test verifies 5.2.2; the unavailable salt quantity ratio test verifies 5.2.4; and the thermal storage efficiency test verifies 5.2.5.