One-Dimensional Fluid-Dynamic Study of a Molten Salts Thermal Energy Storage System

Eurotecnica is an international engineering and contracting company active in the fields of melamine, chemicals, refinery and solar. A staff of more than 100 highly skilled employees is the core of the company. To date Eurotecnica has successfully carried out more than 130 projects, implemented all over the world. Eurotecnica is the world leader in melamine plants and technologies and is now actively investing in solar power plants.

The world is bustling with new projects for solar power stations. Solar power station can get energy from the sun during daytime only, but the energy requirements from the grid have different timing and the turbines in the power island cannot be operated on a continuous stop and go basis. The solution to that is to store the thermal energy from the sun in the form of a mixture of molten nitrates, to be held in huge tanks, and then use it during night. While the idea seems simple, putting it into practice is not that easy because the scale of the system is far bigger than what has been experienced up to now: storage capacity is expressed in terms of tens of thousands metric tons of molten salts and the size of all the equipment and machineries is huge. On the other hand, a faulty thermal energy storage systems may jeopardize an entire solar power project. For these reasons absolute reliability is paramount.

Figure 1 - Flowmaster network modelling molten salts thermal energy storage system. From right to left it is possible to note Tank 1, the immersed pump, Valve A, the six heat exchangers (green rectangles), Valve B controlled by a controller (yellow component), the distribution torus and Tank2.

In the present work the detailed study of different operating conditions of a molten salts thermal energy storage system is presented. In particular, the emergency closure of a valve is studied in two different conditions, namely the beginning and the end of the cycle. Target of the simulations is to find the minimum valve closing time that guarantees the safety of the system, i.e. the minimum time for which the peak pressure is below the maximum allowable pressure for the system. The system is simulated by means of Flowmaster, the thermo-fluid system simulation software.
The System
The system to be studied is composed by two tanks of about 15 m height and 40 m diameter. In each tank there is an immersed pump and a distribution torus. The two tanks are connected by a pipeline in which there are mainly two valves and six heat exchangers in between. Each valve is in the proximity of a tank. During the day hot molten salt, warmed up indirectly by parabolic troughs via the six heat exchangers, is pumped from one tank to the other one. During the night molten salt is pumped the other way round and, being still warm enough, it releases the heat accumulated during the day through the six heat exchangers. In the present work the tank from which molten salt is pumped will be called Tank 1 while the tank into which molten salt is pumped will be called Tank 2; similarly, the valve near Tank 1 will be called Valve A while the valve near Tank 2 will be called Valve B. In the present work the flow of molten salt from Tank 1 to Tank 2 is considered (the reverse flow being symmetrical) and the emergency closure of Valve B in two different operating conditions is studied. The system is studied at the beginning of the cycle when Tank 1 is full and Tank 2 empty and at the end of the cycle when Tank 1 is empty and Tank 2 full. In these simulations molten salt is at a temperature of 286°C and has a density of 1907 kg/m3; under these conditions the speed of propagation of sound wave is about 1850 m/s. The high density and the high sound speed of molten salt are likely to produce a severe pressure surge when Valve B closes. For this reason an accurate fluid-dynamic study is mandatory in order to prevent serious safety problems. The focus of this study is in the pressure surge phenomena and not in the heat transfer phenomena that occur in the system.

Figure 2 - Results of the parametric analyses: maximum pressure as a function of the valve closing time for the start of run and the end of run.

In Figure 1 the Flowmaster network used for modelling the molten salts thermal energy storage system is presented. Each component of the network is characterised by geometrical and performance data provided by the manufacturer. Moreover, since in the simulations heat transfer phenomena are neglected, each heat exchanger is modelled by means of a discrete loss (green rectangles in Figure 1) as well as the distribution torus. Finally, the closure of Valve B is controlled by an appropriate controller component (yellow component in Figure 1). All the fittings (bends, junctions, diffusers) connecting these components are modelled in the network. The system presents also an important vertical deployment; the maximum height of the system being about 20 m. This is an important factor to be accounted for in the simulation of pressure surge phenomena. The system is designed to work between vacuum condition and a maximum relative pressure of 25 bar. Since ambient pressure is 0.888 bar, the maximum allowable absolute pressure is 25.888 bar.

The Simulations
In order to evaluate the valve closure time that meets safety standards, two sets of parametric analyses were performed for the start of run and the end of run conditions. In Figure 2 the results of the two parametric analyses are presented. It can be noted that the maximum absolute pressure decreases significantly as valve closure time increases until about 20 seconds; after that, maximum absolute pressure decreases very slowly. The valve closing time to be used in the case of an emergency manoeuvre needs to be unique for the entire cycle and needs to guarantee a reasonable safety margin. A valve closure time of 20 seconds guarantees good safety margins for both start and end of run conditions.

In Figure 3 and in Figure 4 the detailed results of the simulations performed with a valve closure time of 20 seconds at the start and at the end of the cycle are presented. In particular the maximum pressure in the system, the pressure at the pump outlet and the mass flow rate at the pump outlet are presented together with the valve closure time. In both cases a strong pressure surge is established, nevertheless the maximum pressure in the system never exceeds the maximum allowable pressure for the system. Moreover, it can be noted that the peak pressure is larger at the end of run. Finally, in both cases a reverse flow at the pump occurs.

Figure 3 - Detailed results for the simulation of the start of run: valve closure (brown), maximum pressure in the system (blue), pressure at the pump outlet (red) and volumetric flow rate at the pump outlet (green).	Figure 4 - Detailed results for the simulation of the end of run: valve closure (brown), maximum pressure in the system (blue), pressure at the pump outlet (red) and volumetric flow rate at the pump outlet (green).

Conclusions
The one-dimensional fluid-dynamic simulations performed with Flowmaster allowed to study the detailed behaviour of the system early in the design phase considering different operating conditions. Specifically, the present work allowed for the precise definition of emergency manoeuvres that guarantee the safety of the system during the entire operating cycle. The precise definition of the valve closure time also allows for the identification of the appropriate motor to be used for manoeuvring the control valve. This work demonstrates the importance of numerical simulation early in the design phase of a large plant in which absolute reliability is paramount.

Alberto Deponti - EnginSoft
Francesco Castelletta - Eurotecnica

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info@enginsoft.it

Article published in the Magazine: EnginSoft Newsletter Year 7 n. 2