Scholarly article on topic 'Design Features and Control Concepts of ALSTOM Molten Salt Receiver'

Design Features and Control Concepts of ALSTOM Molten Salt Receiver Academic research paper on "Materials engineering"

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{"Molten Salt Central Receiver" / "Concentrated Solar Power"}

Abstract of research paper on Materials engineering, author of scientific article — A.K. Das, P. Iñigo, R.J. Terdalkar, A. Joshi, C. Wang, et al.

Abstract The tower type molten salt solar thermal power plant consists of a large heliostat field consisting of thousands of mirrors with capabilities to track the sun, positions itself in order to reflect the solar energy onto the molten salt central receiver (MSCR) atop a tower optimally located in the heliostat field. The receiver consists of tube banks arranged in panels with molten nitrate salt flowing within. The concentrated energy from the heliostat field heats the molten salt from 290oC to 565oC while it passes through the receiver. The hot molten salt is stored in a tank and utilized to produce steam for power generation as per the demand. The molten salt central receiver serves as the interface between the solar field and the molten salt heat transfer medium. The various components comprising the MSCR have been designed to maximize the received solar energy, the transfer of solar energy to salt, and the capacity of hot salt production. The state of the art for molten salt central receiver technology offer new opportunities to optimize the design and control of the same. This paper discusses the advanced design features and control concepts of the ALSTOM molten salt central receiver. It includes an optimized receiver surface to minimize pressure drop while maximizing the utilization of the heliostat field, modular design which is easy to construct and maintain, multiple system features for controlling salt temperatures and a health monitoring system for accurate, on-line life assessment, as prominent features.

Academic research paper on topic "Design Features and Control Concepts of ALSTOM Molten Salt Receiver"

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Energy Procedia 69 (2015) 350 - 359

International Conference on Concentrating Solar Power and Chemical Energy Systems,

SolarPACES 2014

Design features and control concepts of ALSTOM molten salt receiver

A. K. Dasa*, P. Inigoa, R. J. Terdalkara, A. Joshia, C. Wanga M. M. Clarka, D. McGranea, L. Denga

aALSTOMPower Inc., 200 Great Pond Drive, PO Box 500, Windsor, CT 06095, USA

Abstract

The tower type molten salt solar thermal power plant consists of a large heliostat field consisting of thousands of mirrors with capabilities to track the sun, positions itself in order to reflect the solar energy onto the molten salt central receiver (MSCR) atop a tower optimally located in the heliostat field. The receiver consists of tube banks arranged in panels with molten nitrate salt flowing within. The concentrated energy from the heliostat field heats the molten salt from 290 oC to 565 oC while it passes through the receiver. The hot molten salt is stored in a tank and utilized to produce steam for power generation as per the demand. The molten salt central receiver serves as the interface between the solar field and the molten salt heat transfer medium. The various components comprising the MSCR have been designed to maximize the received solar energy, the transfer of solar energy to salt, and the capacity of hot salt production.

The state of the art for molten salt central receiver technology offer new opportunities to optimize the design and control of the same. This paper discusses the advanced design features and control concepts of the ALSTOM molten salt central receiver. It includes an optimized receiver surface to minimize pressure drop while maximizing the utilization of the heliostat field, modular design which is easy to construct and maintain, multiple system features for controlling salt temperatures and a health monitoring system for accurate, on-line life assessment, as prominent features.

© 2015Published byElsevier Ltd.Thisisanopen access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG Keywords: Molten Salt Central Receiver; Concentrated Solar Power

* Corresponding author. Tel.: +001-860-285-4607; fax: +001-860-285-3436. E-mail address: apurba.kumar.das@power.alstom.com

1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi: 10.1016/j .egypro .2015.03.040

1. Introduction

The molten salt solar plant decouples the availability of solar energy from the electric grid demand by storing the solar thermal energy in the form of hot molten salt. The molten salt plant as presented in Fig 1 illustrates the process with major plant components. The molten salt solar plant is divided in five major sections including the solar field, molten salt central receiver (MSCR), thermal energy storage (TES), steam generator (SG) and power block. The plant operation involves transferring the salt from the cold salt storage tank to the hot salt storage tank in the TES through the MSCR. The MSCR is a heat exchanger that utilizes solar energy to heat the salt from 290 oC to 565 oC. The molten salt stored in the hot storage tank at 565 oC is utilized to generate steam in the SG. The molten salt exits the SG system at 290 oC and is returned to the TES cold storage tank. The steam produced is throttled through a turbine to extract work and generate power for the grid.

The use of molten salt directly as the heat transfer fluid (HTF) in the solar receiver, as opposed to steam or another HTF, eliminates the inefficiency of the indirect heating of molten salt for thermal storage. The direct steam solar plant works using steam as the HTF to collect and transport the energy collected from the solar irradiation to the steam turbine for power generation. Excess energy collected from the solar field in the direct steam central

I Receiver I

Fig 1: Process schematic of the molten salt central receiver power plant

I Receiver ■

Fig 2: Direct steam solar plant with molten salt storage

receiver (DSCR) can be stored in the TES using a steam to salt heat exchanger to heat up the salt to 565 oC. Once the diurnal cycle is completed the energy stored in the TES is used to continue the power generation cycle. The energy is extracted from the molten salt using a SG similar to the process illustrated in Fig 1 for the molten salt solar plant. The additional heat exchanger in the direct steam solar plant for transferring energy from steam to salt is not present in the molten salt solar plant thus making it a more efficient power cycle. Direct steam with molten salt storage system representation is shown in Fig 2.

The MSCR system design includes advanced features for optimized start-up and shut-down operation, robust response for cloud events, and design for optimal parasitic load and efficiency. It leverages decades of design experience, construction and operation knowledge of ALSTOM's high temperature, high pressure boiler and HRSG products, including the use of advanced design tools for transient simulation, cyclic fatigue and creep analysis along with knowledge of high grade material fabrication.

2. Molten salt central receiver

The molten salt central receiver serves as the interface between the solar field and the molten salt heat transfer medium. The various components comprising the ALSTOM MSCR have been designed to maximize the received solar energy, the transfer of solar energy to salt, and the capacity of hot salt production.

Compressed

Fig 3: Simplified Process flow schematic of MSCR

The MSCR is positioned atop a structural tower for optimal solar field interface. A process flow diagram schematic of the MSCR including the interface to the TES is presented in Fig 3, for illustration of the process. The salt in the TES cold salt tank is pumped up the tower to the MSCR via the cold salt supply pipe to the MSCR Inlet Vessel. The inlet vessel is designed such that a minimum volume of salt is always available at sufficient pressure to support proper shutdown during an unplanned event. A pressurized air pillow is maintained over the salt working volume with the necessary pressure available in the event of a process trip to allow uninterrupted flow while the solar field defocuses the solar irradiation from the receiver panels. The inlet vessel also serves as a buffer between the TES cold salt storage tank and the flow control within the MSCR. The same air pillow that supports salt flow during a process trip also provides stable inlet conditions for the MSCR flow control valves CV1 and CV2.

From the inlet vessel, the salt flows through a series of tube panels arranged in passes, which comprise the heat transfer surface. Each panel is fabricated of alloy tubes spanning from an inlet header to an outlet header. Each tube in a panel is independent of the neighboring tubes, except for the connections to the common supply and discharge headers. The panels are fabricated such that there is effectively no gap between tubes when in operation. The panel arrangement is such that spilled solar energy is minimized while allowing for differential thermal expansion of tubes within a single panel. Multiple panels comprise a single pass and the MSCR has several passes, much like the Superheat System of a steam boiler. General arrangement views of the MSCR are presented in Fig 4 and Fig 5 with identification of the major receiver subsystems and equipment.

Fig 4: MSCR Isometric View with Major Equipment and Subsystem Identification

Fig 5: MSCR Isometric View Cut with Major Equipment and Subsystem Identification

The passes are arranged into two separate parallel flow paths called branches as illustrated in Fig 3. The two branches each occupy one half of the perimeter of the MSCR. Each flow path has its own flow control (CV1 and CV2), such that the flow may be tailored for the available energy from the solar field. Flow from pass to pass within a branch is of serpentine arrangement. Flow changes direction in the vertical orientation as salt is discharged from one pass to the next. The serpentine flow pattern is illustrated in Fig 3 with blue color passes representing downward flow and red color passes representing upward flow.

The two branches discharge into a common outlet vessel. The outlet vessel acts as a buffer to the master flow control unit (CV4 and CV5) at the inlet to the hot tank of the TES. The outlet vessel is also sized to accept the cooling flow from the inlet vessel during an emergency trip, assuming that the master flow control unit (CV4 and CV5) may have closed, resulting in a unit trip. The hot salt return pipe provides the high temperature salt from the outlet vessel to the Hot Salt Storage Tank of the TES.

3. Process control

The MSCR is controlled such that within normal unit flow range a continuous supply of 565 oC is provided to the Hot Salt Storage Tank regardless of the solar field conditions. The MSCR process control schematic for one branch is presented in Fig 6. Salt flow is varied with respect to the available load and operational limits for heat flux application are defined as a function of salt flow.

The pressure in the inlet vessel is controlled by the pressurized air system. The TES cold tank pumping system controls the salt level in the MSCR inlet vessel, which allows for a reliably constant pressure at the inlet of the flow control valves (FCV1 for branch 1, Fig 6) thereby providing a more accurate and reliable molten salt flow rate through the MSCR. Stable flow control allows for stable receiver outlet temperature control, especially during load transitions.

3.1. Molten salt temperature control

MSCR outlet MS temperature in each path (of two) is controlled by a combination of one or two stages of attemperation MS control valves (primary and final if there are two) and the amount of receiver main MS flow [1]. In a situation when the MS temperature deviates from the setpoint signal by more than an allowable deviation, the

solar field will be asked to adjust heat flux by an appropriate amount based on relevant temperature and flow measurements in the MSCR. Attemperation MS control valves are positioned by a cascade control scheme consisting of a primary (master) and secondary (slave) controller to provide fast and effective control action. The molten salt flow rate in each branch is trimmed as necessary to support the control of molten salt temperature at the outlet of the corresponding branch such that the total attemperation molten salt flow in that branch stays at a nominal flow rate in the long term. A schematic of the MSCR outlet MS temperature control concept is depicted in Fig 6.

Fig 6: MSCR Process Control Diagram

3.2. Cloud event transients

During a severe cloud event the pressurized air system is engaged and the MSCR is placed in Standby / Bypass Mode. In this mode, the heat transfer surface is forced empty while the rest of the receiver remains full, awaiting a restart. The inclusion of an individual path bypass will allow each path to be individually placed in standby while the rest of the system continues operation. Bypass mode for the MSCR is illustrated in Fig 7. Parts of the MSCR containing molten salt during bypass mode are colored red and those containing air are colored blue.

The I&C design for the receiver provides appropriate levels of process instrumentation to allow the operational mode decisions to take advantage of the full MSCR design flexibility. Instrumentation for performance monitoring is applied to individual panels to evaluate the heat pickup per pass and pass-to-pass pressure drops. Thermocouples applied to the backside of the tubes of the heat transfer panels allow for monitoring of metal temperature to determine when to fill the panels and help verify filling and draining progress within the receiver.

In spite of the rather harsh process conditions, the MSCR instrumentation package is designed, in coordination with instrument suppliers, to the accuracy and reliability standards expected by the utility power generation sector.

4. Compressed air receiver management system

An independent dry compressed air system is utilized to manage different modes of operation of the MSCR system. The compressed air system should have built in redundancy and backup power, in order to have maximum availability of the MSCR system. As discussed before, during normal operation the inlet tanks are pressurized with the compressed air system. The other advantages of using the compressed air system in the ALSTOM MSCR design are discussed below.

4.1. During cloud events:

This will provide the opportunity to drain both the branches separately - enabling operation of one branch while no or very low solar flux is available for the other branch. The receiver panels can be isolated from the rest of the receiver system such that the tanks can be maintained at operating levels and the pump can be operated in bypass mode in order to have a quick restart.

4.2. After shutdown:

The compressed air management system is utilized to keep the MSCR system pressurized to a slight positive pressure using the dry compressed air, such that ingress of atmospheric moisture can be avoided. The various aspects of corrosion of materials in molten salt service exposed to moisture have been discussed in literature [2-4]. The compressed air system used to keep the system (partially/completely) above atmospheric pressure will greatly reduce the chance of such corrosion process in the various components of the MSCR system as the ingress of moisture from the environment is eliminated. The approach has been utilized successfully for other molten salt receivers to preclude the occurrence of corrosion. This was the case for Solar Two after initial failure of a 304 stainless steel [2-4].

Fig 7: Final MSCR State after Execution of Bypass Mode

4.3. During shutdown:

The compressed air system will provide a more reliable means of draining only the receiver panels. With some purging, it can get rid of all the molten salt from above the drain valves and substantially reduce the possibility of a salt freeze. The drain event will be faster and hence operation of the molten salt receiver can use more of the available solar energy.

5. Transient performance

Solar receivers are subject to daily cyclic operation both due to the daily solar cycle and to the variations in weather conditions. The design of the MSCR must withstand the inherent cyclic nature of solar thermal power plant operation, operation at high temperatures & thermal stresses, and the corrosive nature of the molten salt. The various considerations for design and operation of a solar receiver have been discussed by Terdalkar et al. [5]. In order to properly design the receiver the operating modes of the receiver must be well understood. The cycling operation is an important element in the fundamental design of the receiver. As such a simulation and analysis of various operating modes is made, including the typical start-up & shut-down cycles and less frequently occurring events, such as unplanned process runbacks and trips. A list of events considered in the transient analysis follows:

Table 1: List of events considered for transient analysis

List of events

Daily Normal Operations

- Daily Start-up

- Daily Maximum Operation

- Normal Load Transients

- Daily Shutdown

- Night Preservation Intermittent Normal Operation

- Cold Start-up

- Hot Start-up

- Extended Shutdown

Cloud and Special Operation

- Cloud Induced Transient, System Standby and Panel Bypass

- Restart from Standby/Bypass

- Single Branch Operation Emergency Operation

- Runback

- Plant Blackout

- Emergency Shutdown

Using a dynamic modelling tool, a representative model is constructed and tuned to match the defined operational steady state performance. The events described above are simulated with input and constraints from the solar field supplier and plant integrator. Detailed functions such as filling the system with salt at startup and draining the salt at shutdown are analyzed. Results from the simulations are used for various purposes including unit life assessment, control logic development, instrumentation sizing, risk mitigation, defining the preventative maintenance cycle, and real time plant optimization development. Through coordinated iteration with the solar field supplier and Plant Integrator, the MSCR unit design is revised to achieve an optimal plant configuration.

The optimized MSCR design includes operational procedures for cloud events to minimize downtime and maximize available thermal load utilization for optimal Plant production. Results from life assessment and mechanical integrity studies ensure the highest transient ramp rates while considering typical Plant operational lifetime. Nonoperational events, such as overnight preservation, are evaluated to be cost effective and efficient.

6. Optimized receiver design

As discussed before, the heat transfer surface of the receiver comprises of multiple passes guiding the salt in a serpentine (up and down flow directions) path while picking up solar energy. The optimized receiver heat transfer surface design is achieved by tailoring the material and mass flux of various passes [6]. This is discussed using the following simple example without loss of generality to a more complex realistic situation.

The heat flux applied by the heliostat field needs to be within a predefined Maximum Allowable Heat Flux (MAHF). The MAHF is defined as the maximum heat flux that can be applied on the tubes under the conditions of pressure, temperature and mass flux for the given material of the tube. A design space thus created by plotting

MAHF vs the process temperature is termed as heat flux design space (HFDS) for the purpose of discussion. For illustration an arbitrary HFDS is shown by a solid line in Fig 8 (a) corresponding to a mass flux i1 through the tubes. The Actual Heat Flux (AHF) applied on the surface needs to be bounded by the HFDS created by the MAHF curve. The practical considerations in designing a solar field might limit AHF to be much below the MAHF. In this simple example demonstrated in Fig 8, same AHF is applied on all the passes marked by the horizontal broken line. The difference between design space available due to MAHF and design space utilized due to the AHF is marked by the area hatched by red lines in Fig 8 is termed as lost heat flux design space (LHFDS). An optimized design will minimize the LHFDS for the MSCR surface. The simple example in Fig 8 (a) demonstrates the case where the mass flux through each pass remains constant for the MSCR whereas Fig 8 (b) demonstrates the case where the mass flux through each pass is tailored to minimize the LHFDS. The result is demonstrated by the difference in area hatched by red lines.

This optimization procedure also ensures minimum pressure drop across the MSCR heat transfer passes. The pressure drop across the MSCR system is an important consideration due to availability of pumps and parasitic power consumption. A final optimized receiver configuration might have several other considerations like, supply chain, manufacturing, site requirements, customer requirements, etc.

Heat Transfer Medium Temperature Heat Tmnsfer Medlum Temperature

Fig 8: (a) Arbitrary HFDS corresponding to a given tube material with mass flux ij; (b) Arbitrary HFDS corresponding to a given tube material

with various mass fluxes ii to i4.

7. Mechanical design

The inlet & outlet vessels, panels and link piping have been designed to the rigorous requirements of ASME code. MSCR pressure parts must meet the specific code requirements of the local jurisdiction; however, the MSCR design has a firm basis having been developed in accordance with ASME Boiler & Pressure Vessel Code Section VIII - Rules for Construction of Pressure Vessels, ASME B31.1 Power Piping, and ASME B16.34 Valves among others.

Additionally ALSTOM's strict internal design and material standard is adhered to for design of the one sided heated tubes for the heat transfer surface which undergoes considerable cyclic stresses. The direct, flow-through design of the receiver inherently reduces inter-connecting piping compared to conventional, direct steam generators. The molten salt flows in a serpentine fashion as it progresses through the passes of the heat transfer surface. The relatively low internal pressure of the molten salt receiver allows for the use of lighter schedule pipe as compared to direct steam receivers. The resultant design is elegantly simple, benefiting equipment cost and field erection.

Proper functioning of the MSCR Receiver depends on the interconnecting piping between equipment, manifolds, headers and tanks. Stress analysis of such piping adhering to applicable codes and standards, and considering the modes of operation, is required. The design maintains proper slope of the pipe for draining, and proper flexibility to

minimize terminal forces and moments at equipment nozzles for unit life. Pipe supports are located such that piping loads are transferred to the steel structure as necessary, considering site conditions for wind and seismic loads.

The mechanical design of the panel support consists of rigid hangers supporting the upper headers. A portion of the weight of the panels contributes to the load on the upper headers. The lower headers are guided and spring supported to allow the header to move downward with thermal expansion of the tube panels during operation. Horizontal buckstays, which provide structural bracing for wind and seismic loads, and also retain the heat transfer panel tubes in plane, are spaced appropriately. Each horizontal buckstay is connected to a panel frame. The panel frame, when connected to the main structure, provides permanent support for the panel; it also functions as a shipping and lifting frame, and potentially can be utilized as a fabrication rigging table. The horizontal buckstays are connected to the tube panel wall to provide horizontal support to the tubes while allowing vertical movement.

8. Structural design

The structure of the MSCR is designed appropriately for site environmental conditions - wind and seismic. The MSCR is merged with the supporting tower for dynamic structural analysis so that a realistic dynamic response is captured. All anticipated probable load cases and load combinations are taken into account, such as operating weights, empty weights, hydro test weights, live loads, and thermal loads. The heat transfer panel modules arrive to site fully fabricated and the panels are designed such that each panel may be installed or removed independently of the rest of the heat transfer surface. In this way the heat transfer surface is easily attached to the internal MSCR structure. The entire receiver structure is protected from solar flux by a system of heat shields. The optimized MSCR structure can be made of steel or concrete based on site economics, supply chain, schedule etc.

Fig 10: Schematic of concrete MSCR structure

8.1. Steel structure

The MSCR structural design considers standard structural shapes, general construction practices, and is arranged for optimal on-site erection. The steel is shipped to site in the most economical arrangement. The structure is arranged into self-supporting modules, such that on site, parallel construction of the modules may occur allowing for

the fastest and safest erection possible. The structural steel modules supporting the parallel construction concept of the MSCR are presented in Fig 9 with a module highlighted in red. The structural connections between modules are designed to accommodate field fit up and typical erection tolerances.

8.2. Concrete structure

The MSCR structure can be also made of concrete based on the site condition and economics as well as project requirements. This will allow usage of locally sourced materials and workforce to build the MSCR structure. The concrete structure can also be integrated with the tower on which the receiver is placed. This allows integrated construction of the tower with MSCR structure. A simple schematic of the concrete structure is shown in Fig 10.

9. Summary

ALSTOM leverages years of expertise and experience in boiler and pressure vessel development, design, material and manufacturing to design the MSCR for utility-scale, solar tower technology, renewable power generation. The critical part of the development is the MSCR located on top of the tower, which acts as the interface and accepts high heat fluxes from the heliostat field to heat up the molten salt. The various design features of the MSCR are discussed in this work. The components comprising the MSCR have been designed to maximize the received solar energy, the transfer of solar energy to salt, and the capacity of hot salt production. Foreseeable opportunities for design optimization for the full scale MSCR design have been discussed in conjunction with ALSTOM's proposed path forward. The knowhow of this new technology is developed through researching the information in literature as well as continual improvement of the understanding of various aspects of the receiver components through analysis and testing.

References

[1] Joshi A, McCombe J, Yang S. System and Method for Controlling Molten Salt Temperature, USPTO 2013; Application no.: 14/074,782

[2] Kelly B. Lessons Learned, Project History, and Operating Experience of the Solar Two Project, Sandia National Laboratories 2000; Report No.: SAND2000-2598

[3] Pacheco JE. Final test and evaluation results from the Solar Two project, Sandia National Laboratories 2002; Report No.: SAND2002-0120

[4] Litwin RZ. Receiver system: lessons learned from solar two, Sandia National Laboratories 2002; Report No.: SAND2002-0084

[5] Terdalkar, R., Qian, H., Ye, G. Unique Challenges in the Design and Operation Philosophy of Solar Thermal Power Plants. Energy Procedia 2014; 49: 2521-2531

[6] Das A, McGrane D, Simiano M, Boschek E, Gan XP, Improved Solar Receiver Design, USPTO 2013; Application no.: 14/139,904