Design and Construction of a 700kW th High-Tempera-ture Sodium Receiver

. The Australian Solar Thermal Research Institute (ASTRI) has been developing technologies designed to collect and store solar energy at high-temperature to drive a new high-efficiency power block based on the supercritical CO 2 Brayton cycle. ASTRI is pursuing two alternative pathways: one based on the use of liquid sodium as a heat transfer fluid, and the other based on the use of solid particles. The current work describes ASTRI’s progress towards design and construction of a 700kW th prototype sodium receiver suited to this type of system, which will be installed and tested on Solar Field 2 at the CSIRO Energy Centre in Newcastle, Australia.


Introduction
The Australian Solar Thermal Research Institute (ASTRI) has been developing technologies designed to collect and store solar energy at high-temperature to drive a new high-efficiency power block based on the supercritical CO2 Brayton cycle.ASTRI is pursuing two alternative pathways: one based on the use of liquid sodium as a heat transfer fluid, and the other based on the use of solid particles.The current work describes ASTRI's progress towards design and construction of a 700 kWth prototype sodium receiver suited to this type of system, which will be installed and tested on Solar Field 2 at the CSIRO Energy Centre in Newcastle, Australia.

Design criteria
The sodium receiver prototype was developed as part of ASTRI's objective to further the development and demonstration of next generation, higher temperature solar thermal technologies to increase concentrating solar thermal system performance and market competitiveness through lower cost and improved efficiency.In particular, ASTRI focussed on technologies compatible with operation of a supercritical CO2 power cycle, and set the operating temperature range to 520-740°C in alignment with the US Generation 3 CSP program, in particular the Liquid Pathway project [1].As this will be a first-of-a-kind demonstration of a sodium receiver at this high temperature range, the highest priority in design was given to safety and ensuring successful demonstration of operation.However, this priority was not set completely at the expense of performance.Sodium is often discussed as beneficial for receiver performance in comparison to other fluids [2], in particular because high thermal conductivity reduces the front-to-back receiver tube temperature and associated thermal stresses, and therefore allows higher solar fluxes and more compact receiver designs.This provides the potential to reduce both thermal losses and capital cost.Therefore the design criteria established included both these factors, along with several others as listed below.
• Demonstrate the successful operation and integration of a 700kWth sodium cavity receiver • Design and test a receiver to match expected operating conditions for a commercial plant featuring a supercritical CO2 power cycle • Demonstrate a design that is simple, scalable, and durable, supporting the objective of lowering the cost of CSP • Validate receiver performance models using experimental data • Increase understanding of receiver design interfaces with system balance of plant • Demonstrate a receiver efficiency greater than 86%

Concept design development
Two cavity-type receiver concepts were initially evaluated by ASTRI.Inspired by the SG4 dish receiver [3], the first of these was based on a deep cylindrical/conical type design, with surrounding spillage skirt and a "ribcage"-like flow path to allow drainage [4].The second was based on a concave-tube-bank-in-cavity concept, with similar overall geometry to the CO2 receiver developed by Abengoa, and tested previously at CSIRO [5].While the first of these two designs was evaluated to have potential performance advantages, the simplicity and scale-up feasibility of the second concept were decisive factors in its down-selection for further development.
Potter et al. [6] describe the concept performance modelling process for this design, in which ray tracing and heat transfer modelling were implemented using CSIRO's Heliosim software.The initial design was a tilted cavity, with both the circular aperture and banks of tubes angled down towards the heliostat field.However, in a further iteration, the tube banks were re-oriented vertically to simplify their mounting (on spring hangers), and to reduce the likelihood of sodium egress from the aperture in the event of a sodium leak.Heliostats aim at the centre of the aperture, with different combinations of heliostats from CSIRO Solar Field 2 selected depending on the sun position to maintain the 700 kW design output (where possible), while also trying to maximise receiver efficiency and respect flux limits (discussed further below).
The final receiver geometry from the conceptual design phase is shown in Figure 1, also showing the predicted absorbed flux at design point (equinox solar noon).There are 10 tube banks, each consisting of 7 seam welded tubes, 25.4 mm × 1.65 mm minimum wall thickness, from Alloy 625 (UNS NO6625) Grade 2 material split into left and right sections.Two separate fluid supply lines (with separate pumps) introduce sodium to the two banks at the centre of the receiver, and then fluid flows in a serpentine manner through the tube banks to the outside.In this way the highest flux region corresponds the coldest fluid region.Table 1 details the tube and flow path design.Figure 2 has key dimensions.Performance parameters simulated at design point are listed in Table 2

Structural integrity analysis
Initial design screening used a reduced 2D generalised plane strain analytical model implemented in Python nashTubeStress [7] to evaluate the risk of excessive deformation (plastic), thermal ratcheting (cyclic incremental plastic deformation), and adherence to the linear elastic material strength limits dictated by creep-rupture data.Flux limit tables derived from thermoelastic-stress were generated for the range of metal temperatures expected on the prototype receiver, specific to its flow conditions and tube dimensions.
As the design developed, a more detailed 3D Finite Element Analysis (FEA) evaluation was developed to include tube banks, their connection via headers to interconnect piping, and the overall structural fixture of panels to framework.Stresses considered were primary stresses due to internal pressure (200 kPa gauge) and dead-weight (vessel and contents), and secondary stresses resulting from thermal expansion under live loads (temperature and flux).Two nickelbased superalloy tubing materials were considered for fabrication of the prototype receiver: 25:4 mm OD x 1:65 mm WT Alloy 625 (UNS N06625) and 33:4 mm OD x 1:32 mm WT Alloy230 (UNS N06230).Due to sourcing restrictions, solution annealed (Grade 2) Alloy 625 was chosen despite Alloy 230 being preferable (due to lower thermal stresses and lower susceptibility to over-ageing).Figure 3a shows design tensile strength f-values derived from manufacturer datasheets for Alloy 625 alongside values given for allowable stress intensity S m and yield strength Sy from ASME BPVC II, Part D, Table 1B-2010, based on: • stress to cause rupture in time t at design temperature SRt divided by 1.5; • yield strength at design temperature ReT divided by 1.5; and • ultimate tensile strength at design temperature RmT divided by 3.5.
The lowest curve of this combination of limits was used to construct the time-dependent design tensile strength or f-value curves in time t.Given the range of bulk sodium temperature being considered for this technology it can already be observed that the design tensile strength fvalues will be governed by creep.Applying the design tensile strength at temperature to the design point conditions of a particular tubing product results in a specific peak allowable net flux. Figure 3b shows this for time independent design tensile strength (the lower curve) and time-dependent f-values derived from manufacturer's data-sheets.Given the short duration of the planned test program, the flux limits for the prototype receiver were set based on creeprupture lifetime of 1000 h (the orange curve in Figure 3b).More aggressive flux limits and longer lifetimes, including up to the typical 30 year design life of a CSP plant, are feasible at this temperature range, as was demonstrated in the analysis carried out as part of the Gen3 CSP Liquids Pathway project [1].However sourcing suitable materials (e.g.Alloy 740H) and carrying out the necessary inelastic thermo-mechanical analysis was not feasible within the budget and time constraints of this project.The bulk sodium temperature at this point is 555°C and the maximum surface temperature is 668°C.Assuming constant material properties for UNS N06625 at 650°C in nashTubeStress with a net absorbed flux of 0.76 MWm -2 results in an outer fibre equivalent (von Mises) stress of 262 MPa.A 2D generalised plane strain FEA model of the same point with temperature dependent mechanical properties results in reduced stress of 255 MPa.However, the generalised plane strain 3D panel model and analysis also incorporates the headers and tube bends, which act like springs to allow bending, so that despite the assumption of generalised plane strain at the junction to the interconnect piping, the resulting peak outer fibre equivalent stress of 195 MPa is significantly lower than that calculated for 2D generalised plane strain tubes (Figure 4c).

Detailed design
With the concept design established, the detailed design phase commenced.Apart from the tube-banks themselves, the receiver consists of a receiver frame and cladding, a panel mounting system (on spring hangers), instrumentation (thermcouples), a mechanical door, a spill tray, interconnecting piping including fill and drain lines and valves, heat tracing, and insulation.The cavity was enlarged slightly from the conceptual design to accomodate increased inter-tube-bank spacing and the spill tray.In addition, as a result of recommendations from a Hazard and Operability (HAZOP) study, thermal cameras were added to monitor frontside tube temperature.

Design standards
The receiver was designed in accordance with Australian Standard AS1210 2010 Class 1.
Structural consultants FE Consultants were engaged to assist in the detailed design phase, to help with code interpretation, review the design by analysis techniques and compliance with standards for all the relevant load cases and assessment criteria, and to complete the detailed design drawings.A commercial, finite element code (Abaqus 2020) was used, with stress analysis now incorporating the interconnecting pipework and panel support sytem.The consultants concluded that the sodium receiver and piping design compied with AS1210 and sound engineering practice.Some of the design details are listed below: • Operating pressure is 200 kPa (gauge) and design pressure 300 kPa (gauge).
• Creep design life is 1000 h and number of design thermal cycles is 500, although it is noted that the potential operating life is expected to be between 10,000 and 100,000 hours.• The design low cycle temperature is 300°C, noting the intention is to maintain this temperature during the night and non-operational periods during testing campaigns, to minimise the ingress of impurities during the draining and filling process.• The pressure related hazard level level according to AS4343 is C (low pressure) • Total sodium volume in the receiver is approximately 85 L.
• Post weld heat treatment is not required.However, a post bending anneal was specified for interconnect piping bends with a strain rate >15% as per the material manufacturer's guidelines • Top headers were supported via a dummy leg, supported by spring hangers Bends at the ends of the receiver tubes allow for deflections, essential for stress relief given flux and tube temperature variation across individual panels.The tubes are bent in an alternating fashion (refer to Figure 5) to allow enough spacing for the butt welds of the tubes to the headers.Interconnecting piping is also designed with sufficient length and bends to cope with thermal expansion mismatch between tube banks, in accordance with AS1210 and ASME B31.3.The receiver is designed with drain and fill lines that allow sodium to fill from the bottom in normal operation, pushing the cover gas (argon) up and out through the vent lines.The valves on these lines are closed in operation, but fail open so that in the event of incident, the receiver would completely drain, with sodium returning to the drain tank in the main sodium loop.
Figure 6a shows the final design, which includes a door that can be closed when the receiver is either not in use or is in warm standby, or automatically closes (assisted by a counterweight) in the event of tube failure during operation to minimise smoke egress.Seals around the door, combined with water shedding features on the receiver enclosure, prevent water from entering the enclosure.Figure 6b shows a section view with the door removed to show the aperture position relative to the tube bank.A drip tray at the base of the cavity space drains to a sealed plenum area, capable of containg the entire inventory of sodium from the receiver in case of tube rupture.The receiver enclosure is connected via a vent line (with an actuated valve) to the main sodium loop enclosure, which would allow smoke to be drawn into the scrubber system in the event of a sodium leak.The receiver tubes are painted with the absorber coating Pyromark 2500, generally following the procedure outlined by Ho et al. [8], although the final curing step is at 700°C to avoid unnecessarily rapid aging/degradation of the coating that occurs at higher temperatures [9].Instrumentation includes 32 Type-K stainless steel sheathed thermocouples clamped to the backs of the tubes.Most of these are located along the horizontal centreline of the receiver, at the left-and right-most tubes of each panel (i.e.20 in total).Two of the panels (one at the centre, one at the edge) are more densely instrumented, with additional thermocouples located along the centreline, and at locations on the tube bends at the top and bottom.A further 35 thermocouples are located on interconnecting pipework, drain and vent lines, and both inside and outside the insulation at various locations around the enclosure.Tube temperature on the front side will be monitored by two thermal cameras mounted on supports cantilevered in front and below the receiver.
The insulation is SiO2-CaO-MgO (Superwool Plus) blanket, pinned in layers to the inside of the housing.The thickness and blanket density (either 96 or 128 kg/m 3 ) varies depending on location, for example, 525 mm at the top, 225 mm at the front around the aperture, 350 mm on the sides, 50 mm on the door, and 450 mm behind the tubes at the back.At the rear, the insulation and housing are arranged in removable "plugs" to improve access to this region.Total mass of insulation on the receiver is 666 kg.All pipework except the receiver tubes themselves are installed with heat tracing (HTS/Amptek Heavy Insulated Duo-Tape), to ensure sodium can fill the loop without blockage.Heating the receiver tubes will be done with solar flux from the heliostat field.

Construction
The receiver tubes were bent and welded to the headers to form the tube banks by MCM Manufacturing in Newcastle.They were then painted with the Pyromark 2500 absorber coating, and transferred to Performance Engineering Group.There the receiver enclosure and door were fabricated, and the tube banks mounted within along with interconnecting pipework and valves.Acceptance testing included standard procedures such as hydrostatic testing, non-destructive testing of welds (100% RT of pipe butt welds, and 100% DPI of non-butt welds), as well as a water ingress test to confirm the enclosure and door were adequately water tight.The key challenge during construction related to welding of the Alloy 625 material, which is not a simple material to weld.Weld rectification work was required for many of the welds in order to pass the NDT requirements.Another challenge was sourcing of Alloy 625 grade 2 in the small quantities required for the prototype.For the 25.4mm diameter tubing, seam-welded material was able to be sourced.It is noted that the tube bank manufacturer measured the thickness of the tubing to be approximately 2.4mm, which is outside the tolerance for minimum wall thickness tubing (+28 / -0%).However, due to the sourcing challenges, this material was used for fabrication of the tube banks.Figure 7 shows some images of the receiver at various stages of fabrication.

Conclusion
Fabrication of the receiver was completed in July 2022.The receiver is designed to interface with a new sodium loop at CSIRO Newcastle, as described by Gardner et al. [10] in a companion paper.The receiver and loop will be skid-mounted so that they can be tested first on the ground (off-sun), then lifted onto the tower in Field 2 for the on-sun testing campaign.Design and procurement of major equipment items for the loop has been completed by CSIRO and Vast Solar, with the sodium loop assembly soon to commence.On-sun testing is expected to commence in mid-2023.

Figure 1 .
Figure 1.Heliosim 3D surface model of the conceptual receiver design, with a section mid receiver showing the predicted absorbed solar flux at design point.

Figure 2 .
Figure 2. Overall dimensions of the conceptual receiver design, showing cross section from west (left) and above (right).

Figure 3 .Figure 4 .
Figure 3. (a) Design tensile stress f for Alloy 625 showing time-dependent values derived from manufacturer's data compared to time-independent (Sm) allowable stress values from ASME BPVC II, Part D. (b) Peak allowable net flux (that which is absorbed by the tube, not including loss to ambient via radiation and convection) for UNS N06625 using creep-rupture data from manufacturer's data-sheets and ASME allowables for a mass flow of 0.18 kg s -1 .

Figure 4a shows the
Figure4ashows the bulk fluid temperature entering east and west flow paths in the middle from the top and continuing in panels of seven tubes towards the left and right outer edge in a serpentine fashion.Fluid temperature is raised from the inlet at 520°C to a maximum of 747°C at the outlet by the net absorbed flux, as shown in Figure4b.The highest flux absorbed is 0.76 MWm -2 on the 5th tube of the first panel of the east flow-path (i.e.near the centre of the receiver).The bulk sodium temperature at this point is 555°C and the maximum surface temperature is 668°C.Assuming constant material properties for UNS N06625 at 650°C in nashTubeStress with a net absorbed flux of 0.76 MWm -2 results in an outer fibre equivalent (von Mises) stress of 262 MPa.A 2D generalised plane strain FEA model of the same point with temperature dependent mechanical properties results in reduced stress of 255 MPa.However, the generalised plane strain 3D panel model and analysis also incorporates the headers and tube bends, which act like springs to allow bending, so that despite the assumption of generalised

Figure 5 .Figure 6 .
Figure 5. 3D model of the sodium receiver showing the tube bank arrangement, including the drain and fill lines.

`Figure 7 .
Figure 7.The sodium receiver in various stages of construction showing (a,b) the completed tube banks, (c) the tube banks welded to the interconnecting piping, (d) the competed receiver enclosure with door and (e) a view from inside the receiver showing the insulation.

Table 1 .
. Details of the tube, tube bank spacing, layout and flow path for the conceptual receiver design.

Table 2 .
Design point conditions and performance for the ASTRI 700 kWth sodium receiver concept design.