DYSALT a dynamic simulator for CSP plants focused on molten salt technology

Transcription

1 DYSALT a dynamic simulator for CSP plants focused on molten salt technology Angelo Rossi 1, Giovanni A. Cossu 2 and Augusto Maccari 3 1 Senior Process Engineer at Struttura Informatica Srl, Pisa, Italy, , angelo.rossi@strutturainformatica.it 2 Business Development Manager at Struttura Informatica Srl, Firenze, Italy, Senior Engineer at Techint E&C S.p.A, Roma, Italy, Introduction In the plant design phase a number of choices have to be taken, such as architecture and components, definition of management procedures and plant control strategies. Taking into account either the quality standards required and the need of a rigorous control of the performances, could be useful and even indispensable the availability of a detailed dynamic (time depending) process simulator of the plant to be designed. Its use is moreover useful in the commissioning phase when it could help to verify: Mass and energy balance; Temperature and pressure fields; Thermal stresses generated by status and charge changes; Position of alarm and protection threshold; Control system an related tuning; Ordinary and extraordunary operational procedures such as draining and refill, start up, shut down. The plant process simulator can provide even more help by way of a powerful and effective MMI (Man Machine Interface), allowing it to be used as a proper OTS (Operator Training Simulator) and, as a PDS (Process Dynamics Simulator), its use could be very essential in caso of plant changes, repowering, etc. In order to achieve its goals, the model should correspond to high quality and operational standards. This could be obtained if its development relays on two main pillars: 1. being the simulator based on a development platform, this instrument should guarantee some features such as being deeply verified, user friendly and rich of specific functions (manageable topology, auto help and diagnostics, graphical tools and more); furthermore every components to be simulated, expecially on innovative components, should be conceived under a rigorous and scientific methodology, and should be considered approved only after having overcome the validation barrier; the platform itself should provide a specific help to achieve these goals. 2. the simulator should be generated under the most proved methodology and subsequently verifyed by way of a comparison with project data or better with experimental results; this approach implies a number of activities such as: a thorough analisys of the plant and its data, definition of functions to be obtained and their application range, topologic reduction (from POD, Process Oriented Diagram, to MOD, Model Oriented Diagram), definition and development of functional areas and their integration, verification and acceptance tests. Struttura Informatica s approach to this challenge is based on the adoption of its own platform ISAAC Dynamics the performances of which have been largely tested by leading industrial and and research institutions such as Enea, Erse, Techint. The result of this cooperation is a complete set of fully validated and cutting-edge modules allowing the Company to deliver a wide components library for CSP and other emerging fields (solar collectors based on parabolic troughs and Fresnel technology, metallic membranes and CO-Shift reactors) apart the general purpose components for the modelling of the whole power plant based on either renewable or conventional sources. 2. ISAAC Dynamics The development platform has been designed to provide an effective solution to the needs of process engineers for the dynamic modelling of conventional and renewable sources power plants and for the detailed study of specific phisycal phenomena. Due to its very general approach and open architecture, the platform may help whenever the development of models of components of every nature and technical property. ISAAC Dynamics, in the recent release 2.0 enriched of brand new features and in a renewed graphic look, allows the 1

2 development of accurate and effective dynamic simulations by way of its peculiar technical peculiarities: Advanced Model Topology now really fitting the plant flow sheet. A fast model building boosted by a powerful connection management. Image and visual components management allowing the dynamic flowsheet to get close to real plant displays. Full operation units management. Total flexibility in managing either system and user libraries. Renewed Model Editor including wizards for quick and easy development of user s mathematical models. Adoption of Restore points of dynamic simulations. Import and integration of external data sources into the dynamic simulation. Export of simulation results in several formats. Dynamic analysis with visual highlights to underline critical process events. Thermodynamic tables visualisation by way of a graphic tool. The platform technical features are at the same time sound and innovative. It is based on an object oriented architecture allowing a very effective modular construction of the software application, so ensuring on the mathematical side (the Newton-Raphson methodolgy of the solver, the double precision operations), on the process representation side (use of logical blocks) and on ease of accessing results (graphical tools, export functions), the more useful features the process engineer requires. 3. Consorzio Solare XXI pilot plant The plant on which the study has been focused will be the world s first stand alone CSP plant using molte salt as healt transfer fluid. The plant is being realized by Consorzio Solare XXI in the center of Italy close to the Archimede Solar Energy, one of the Consortium members, factory. It consists in a single loop made by six 100 meters long Solar Collector Assemblies, a double tank storage system, an auxiliary molten salt burner, a molten salt steam generator (pre heater, boiler and super heater) and a steam turbine with its generator. The overall reflecting area will be about 3000 m 2 and the nominal electrical power is 350 kw. Despite of its small size the presence of a molten salt burner and the possibility to verify on a real plant the interaction between the molten salt steam generator and the turbine, especially during the warming up and cooling down procedures, make this plant unique in the worldwide scenario. Fig. 1. Consorzio Solare XXI Pilot Plant general layout 2

3 4. Dysalt simulator As above mentioned, the carrying out of a simulator breeds a set of activities starting from the analysis of the plant and of the needs and therefore of the performances that this instrument has to be able to supply the user with. Since Techint has needed the plant dynamic simulation even in special conditions (i.e.draining and filling), Struttura Informatica had to conceive and realize new modules able to satisfy the specifications and the requirements of the client. The total performance of these procedures takes place without introducing discontinuities during the simulation and it has been a further challenge, even from the point of view of innovation, because hydraulics and thermics of the system must be considered in according to different optics laws. In particular, the modules of the Techint library (to use them is therefore required the explicit consent of Techint) are: SaltTube: receiver tube with an insulating glass casing. This module takes into account the double temperature between the side facing the mirror and that facing the sky, the possible fissure in the glass with incoming air and the thermics during the filling and draining phases; Sifo: it represents the mobile junction between two receiver tubes considering both the mass contained inside and the thermics during the filling and draining manoeuvres; Mirror: it simulates a parabolic linear mirror (trough) and takes into account the shadowing between adjacent mirror rows and the thermal losses at the ends; Sun: it supplies the user with the normal direct power (DNI, Direct Normal Irradiance) as function of the geographical coordinates, its tracking and the time (month,day, hour); Burner: it represents a CH 4 preheater including combustion and irradiation chambre and the convective one; 4.1 Simulator functional areas or tasks The plant may be considered as the collection of two functional areas: the area concerning the molten salts, including the full-blown solar field and the remaining part concerning the salt handling and the area dealing with water/steam Molten salt functional area This area is formed by: a cold tank, in which the molten salt circulation pump catches and feeds the solar field through a pipe, a manifold and a flow-rate regulating valve. The solar field is constituted by six linear trough coupled with parabolic mirrors. From this field the fluid can reach the cold tank through the interception valve (WTA001) and acting on the WA003 valve, otherwise it can flow into the hot line leading it, by means of a three ways valve, totally or partially into an auxiliary CH4 heater or/and into the hot tank. From this tank, a pump (WTB10AP001) sends the salt to the SSG (Solar Steam Generator), furthermore, through the piezometric pipe T001 and the valve WTBAA210, the hot fluid may be sent to the cold tank. The P&ID of this part of the plant is represented in the fig. 2 whilst the task implemented in the simulator is shown in fig. 3. 3

4 Fig. 2. Molten salt area P&I D Fig. 3. Molten salt task 4

5 Water steam functional area It is formed by the drum CAVITY from which starts the circulation loop helped by the pump PUMPRIS. The generated steam goes into the turbine ST1 passing through the superheater SPHT1N. The valve V1, at the turbine inlet, controls the drum pressure. The steam, discharged by the turbine, feeds the economizer and reaches the condenser; here the pump PUMP2 sends the condensed water into the preheater and then into the drum. The preheater is equipped with a recirculation loop by means of the pump PUMP1: this loop controls the temperature of the salt leaving this area. Because of modellistic reasons, in this task it has been added the molten salt circuit coming from the hot tank and, passing through the superheater, the circulation loop and the preheater, returning to the cold tank. Fig. 4. Steam cycle H&M balance Fig. 5. Water steam task 5

6 4.3 Features and operating range Considering the simulator consists of two systems and the solar field may operate in a way nearly stand-alone without the steam generator, the operating range of this instrument is the result of the combination of the different stata of the two systems, to be more clear, it is opportune to define the application borders (the stata) of each one of the two areas and then to analyze their composition Solar field production It is equivalent to the day cicle at high sun-shine; the molten salt is pumped from the cold tank to the hot one through the solar field. The receivers are in tracking (maybe defocused) Solar field recirculation It is equivalent to the day cicle at low sun-shine; the molten salt is pumped from the cold tank and comes back through the solar field. The receivers are in tracking (maybe defocused) Solar field night circulation It is equivalent to the day cicle at low sun-shine; the molten salt is pumped from the cold tank and comes back through the solar field. The receivers are in the rest state (position at 90 or 120 ) therefore inactive.. The salt flow rate is kept at a constant value (tipically at 50% of the nominal value). This status turning on occurs at the sunset and holds up till the next day SSG production This status is turned on when the molten salt level in the hot tank is high enough to hold up a steam production for a significant time SSG production and restore This status is turned on when the hot tank level is sufficient to hold up a steam production for a significant period and the cold tank temperature is less than a threshold value SSG stand-by This status is turned on when the hot tank level is less than a threshold value (typically during the night). A share of the molten salt pumped from the cold tank and sent to the solar field is turned aside through a by-pass line equipped with a control valve -before entering the solar field- and sent to the steam generator to keep the salt temperature over the salt freezing point. Steam and water inside the SSG are bottled up Stand-by with restore This status is turned on when the hot tank level is less than a threshold value (typically during the night) and the cold tank temperature is less than a limit value. This is a transitory state, because it is stopped when the hot tank level reaches the lower operative limit. Synthesizing, the simulator has an operative range varying from solar field stopped and gvs in stand-by up to solar field with full sunshine and SSG production. Besides the normal operative range, this simulator allows the user to verify the operative procedures both the common ones (charge or status changes, CH 4 auxiliary heater turning on, pipe heating, etc.) and those out of the ordinary as drainings and fillings. 4.4 Results The simulator has been verified by a campaign of tests attesting the product functionality efficiency, the application range and the result validity. In this case the following tests have been executed: Stability: drift maintaining for 30 minutes at least in corrispondence to different operative states. Draining-Filling: it has been performer a simulation of the manoeuvres leading to the full draining of the solar line, analysing the plant behaviour during this phase. This procedure has been followed by a simulation of the manoeuvres achieving the solar manifold filling and restoring the full charge functioning conditions. The 6

7 model used to simulate these two procedures schedules the turning on of the plant draining/filling phase by means of an appropriate command (DRENA=1). These manoeuvres have been executed on a model uncoupled with the water-steam cycle. Actuators workability: manoeuvres have been executed on the main actuators present in the simulated part of the plant to verify the answers of the component itself and of the whole task. All these tests have given positive indications. Hereinafter, our attention will be focused to the draining and filling phases because they are particularly significant for molten salt plants. Draining and filling logic and procedures The Consorzio Solare XXI pilot plant is located on an embarkment, so that its own peculiarity is the following one: the first half (about 300 meters) of the solar row rises towards the top of the embarkment, whereas the second half has a descending slope. The draining logic is based on the fact that the draining occurs when the siphon breaking valve, placed at the top of the row, becomes open. The molten salt flows into the cold tank both through the solar field feeding line with the pump turned off and through the draining line controlled by the appropriate valve. The filling procedure is performed turning on the solar field feeding pump and turning off the just mentioned valve. In Fig. 6, at the end of the document, it is shown the behavior, during the draining phase, of the salt masses contained in the two sections of the solar row, upstream and downstream the breaking siphon valve, and the related flow rates; in the same figure, it is also shown the trend of the salt masses, of both the sections, during the filling phase 5. Conclusions and improvements Even if not yet pointed out, the simulator has been equipped with a control system, at engineering level, to keep the main variables of the plant around the pre-fixed set-point values. This system is formed by: A drum low level control; A drum high level control; A drum pressure control; A control of the temperature of the molten salt at SSG outlet; A control of the temperature of the molten salt at solar field outlet; Under the supervision of this control system, it has been shown above that the simulator operative range may cover, even if not all, the greatest part of the plant performances: from the common ones up to the special ones. The achieved and above discussed results satisfy the engineering expectations and they have been considered valid in this first version of this simulator. The application fields of this simulator may be summarized in the following items: Definition and positioning of the alarm and protection thresholds; Settlement of control strategies and their tuning Verification of the operative procedures both normal and special (draining, filling, start up, shut down ) Its help may be extended some more further: In diagnostics (glass fissuration..) When equipped with a MMI, it may be used at OTS level; At PDS level, in case of improvement, repowering, component changes, etc. After the theoretical validation we have seen above, it is foreseen a further campaign based on the real data of the plant. 7

8 DRAINING FILLING Fig. 6. Draining and filling graphs 8