AMP_04_May_June_2021_Digital_Edition

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 1 8 Primary heat exchangers for Gen3 CSP systems have similarly challenging operational requirements due to the combination of pressure and tempera- ture that must be accommodated to transfer energy to the sCO 2 power cycle fluid, as described above. This require- ment has driven many SETO-supported research projects to focus on compact heat-exchanger concepts, like printed circuit designs, that utilize microchan- nel features. These designs have the po- tential for low-cost, high-power/weight ratio and simplified inlet and exit noz- zle designs. Nevertheless, joining and designing headers and manifolds for these novel heat exchangers is not trivi- al and should not be underestimated as a challenge. Due to the extreme conditions for both receivers and primary heat- ers for Gen3 CSP designs, researchers are investigating material options be- yond conventional metallic alloys. The high-temperature properties of many ceramics—strength, toughness, and chemical compatibility—are appeal- ing, but the ability to reliably manufac- ture components at low cost in relevant form factors is a challenge. SETO is addressing this challenge by funding regulatory considerations through the research phase, especially considering American Society of Mechanical Engi- neers code cases; and ensuring that materials are matched to production processes, for example, by enabling the joining or bonding of dissimilar materi- als. Achieving these goals will help de- velop a full supply chain and flexible manufacturing industry for plant com- ponents made from these new alloys. THERMAL ENERGY STORAGE Innovation in the materials of con- struction and design of TES systems holds potential to lower capital costs, improve reliability, and increase flexi- bility to make progress toward SETO’s 2030 goals. TES in commercial CSP tow- er plants uses two tanks to store hot (565°C) and cold (295°C) molten nitrate salts separately. To withstand the oper- ating temperatures, the tanks are typi- cally constructed from 347H stainless steel and carbon steel, respectively. De- signing Gen3 CSP plants that operate at higher temperatures is likely to require different storage designs, regardless of the thermal storage media used, ow- ing to the high costs of constructing hot tanks with nickel-superalloy walls [11] . To minimize capital costs, high- temperature designs are likely to rely on internal insulation to reduce wall tem- peratures and allow lower-grade ma- terials of construction. DOE supports research to develop low-cost internal insulation for high-temperature tanks. This is relatively straightforward for sol- id-particle-based storage media, as ma- terial compatibility is not a significant factor. However, for liquid media, par- ticularly molten chloride salts, an inter- nal insulationmaterial must be selected based on its chemical stability and im- permeability. These concepts may be relevant to conventional nitrate salts, as well. If internal insulation can be shown to be reliable, it may allow hot tanks to use low-cost carbon steel, reducing the cost of CSP and other TES systems. RECEIVERS AND HEAT EXCHANGERS CSP plants contain two essen- tial heat-exchange components: the receiver, where concentrated solar flux is absorbed on a surface and trans- ferred to the HTM; and the primary heat exchanger, where energy from the HTM and/or thermal storage media is trans- ferred to the power cycle working fluid. Reliable performance of these compo- nents is essential to CSP systems, as they are the key interfaces that enable energy collection and delivery. Extending the design of conven- tional tubular metal receivers to high- temperature Gen3 concepts has proved to be particularly challenging, due to the stresses imposed by the thermal gradients observed in typical cases. Ar- gonne National Laboratory developed a design guide for receivers [12] , which ap- plies to components undergoing daily cycling, where creep-fatigue damage in cyclic service or stress relaxation dam- age is a major design consideration [13] . Additionally, SETO supports research to develop and validate coatings to in- crease optical absorptivity and decrease emissivity, to improve receiver effi- ciency. Considering the life-cycle limits imposed by thermal stresses and fa- tigue-creep interactions, SETO-funded projects are also investigating alternate materials of construction, like ceramics. Fig. 3 — Researchers at Purdue University load a crucible to contain zirconium-copper liquid that will react with a tungsten carbide preform to create a zirconium carbine/tungsten cermet. Courtesy of Purdue University.