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30 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 Table 23 Test results for AISI 321, Pdeform (pressure to initial deformation) and Prupt (pressure to rupture). calculated required wall thickness is far below the standard thick- nesses of commercial tubes. Predicted and experimental pressures to deformation and rupture are in very fair agreement. Further ex- perimental confirmation is however required. 6. PCM/TCSintegrationinTESrequiresheatcarriers 6.1. Introduction TES generally requires the use of a heat transfer fluid during the charging and discharging processes. Although water/steam, gas, syn- thetic oils and others can be used, current HTFs are commonly molten salts, when large-scale heat storage is included, however with drawbacks of temperature limitation required heat tracing of the molten salt circuits, and high pumping power requirements. The maximum temperature for the application of thermal fluids is between 350 and 400 °C, whilst molten salt applications are limited to ~560 °C. Molten salts are mostly nitrates, such as solar salt (NaNO3/ KNO3) or Hitec® (NaNO2/NaNO3/KNO3), with an upper temperature limit of ~600 °C for solar salt, and a lower temperature limit through solidification at 230 and 140 °C respectively. The molten salts have a low vapour pressure, but their moderate viscosity as a function of temperature leads to considerable pumping energy. The insta- bility of the molten salts should be carefully considered, with e.g. a slow conversion at ~538 °C (Hitec® HTS) from NO2− → NO3−, or a decomposition of NO3− salts to NO2− and O-radicals with possible formation of CO32− plugs [200], according to: NO−→NO−+12O;2NO−→O*+32O+N; 322222 and CO2 +O*→CO23− As a result of these instabilities, physical properties (μ, ρ, etc.) will change and environmental issues should be taken into con- sideration (e.g. emissions through pressure venting of the formed gases), whilst plugging problems might occur. Recently, the use of particle suspensions as HTF has been ad- vocated. Since the particle suspension has a heat capacity similar to that of molten salts, without temperature limitation except for the maximum allowable wall temperature of the receiver tube, sus- pension temperatures of up to 800 °C can be tolerated for refractory steel tubes (even higher when using ceramic or glass tubes), thus offering new opportunities for highly efficient thermodynamic cycles such as obtained when using supercritical steam or CO2, as previ- ously illustrated in Fig. 6. Despite the pre-cited advantages of using circulating particles as HTF, multiple questions need to be investi- gated concerning powder stability (attrition); equipment erosion; or the overall economy of the system when the receiver tempera- ture is increased, leading to increased power cycle efficiency, increased storage density, reduced thermal power requirements, and increased plant capacity factor. 6.2. Powdersasnovelheatcarriersinrenewableenergysystems A novel application of powders in renewable energy relies on their use as heat transfer medium for heat capture, conveying and storage. Gas, mostly air, but possibly inert gas such as N2, is used as powder carrying medium. According to the gas velocity applied, different gas–solid concepts are possible. With increasing gas flow rate, the operation moves from a fixed bed to a bubbling fluidized or ultimately circulating fluidized bed and pneumatic conveying [201]. Without, or with a limited gas flow, falling particle reactors or mechanical conveying could be used. Whereas powder reactors in chemical applications mostly use Geldart A or B powders as bed material [202], HTF applications will mostly use A-type powders in order to limit the required gas flow rate and associated sensible Number of charging/ discharging cycles KNO3 E-PCM, 39 mm O.D. 2.0 mm wall thickness Sb2O3 E-PCM, 27.6 mm O.D. 1.5 mm wall thickness Pdeform Prupt Pdeform Prupt (MPa) (MPa) (MPa) (MPa) 0 (reference) 14.8 250 14.6 500 14.5 750 14.4 1000 14.4 1250 14.1 41.6 16.8 41.5 16.2 45.0 40.9 16.0 44.6 40.6 15.7 43.9 39.8 15.9 43.5 39.4 14.8 46.2 42.2 subjected to pull and press experiments, and pressures to defor- mation and to rupture were measured and given in Table 23. Clearly, the pressure required to deform or burst the tested encapsula- tions significantly exceeds the pressure generated during the phase change (~5 bar = 0.5 MPa). Moreover, the KNO3 tube of larger di- ameter and thickness performs better, with a lower reduction in pressure after multiple cycling. These results are however in line with the theoretical predictions, as shown below. The minimum required wall thickness at the maximum oper- ating temperature is calculated according to Eq. 14. The pressure to deformation and rupture is calculated according to the Barlow formula, Eq. 15. The maximum intended operating pressure P is 3 bar; the outside diameter for cylinder 1 and 2 is respectively 39 mm and 27.6 mm. The allowable stress SE (yield strength) at 650 °C for AISI 321 is 250 MPa. The coefficient Y is 0.4 because the material is nonferrous and the factor C is taken as 0.01 mm, which is a pes- simistic estimation. The minimum required wall thickness for the two cylinders and the selected materials is summarized in Table 24. A commercial wall thickness of 1.5 or 2 mm provides a large excess wall thickness. For the pressure to deformation, S is the yield strength at 650 °C, this is the same as SE in the calculation for the minimum required wall thickness, and the safety factor φ is 1.5. To calculate the pres- sure to rupture, S is the tensile strength and the safety factor is 1. The results of the calculations for the pressure to deformation and rupture are given in Table 25. Clearly, experimental values are about 10–20% below the theoretical predictions, so the pressure equa- tions can hence be used. For high numbers of cycles, the design pressure should however be increased by 10 to conservatively 20%. Form the above data and experimental treatment, it emerges that the proper properties of the encapsulation materials as given in spe- cialized references provide a very fair basis for design calculations. Yield strength, tensile strength, and time-dependent creep strength are the most important properties that enable to assess required wall thickness and allowable pressures to deformation and rupture. Thermal cycling of E-PCM sample tubes has demonstrated that the Table 24 Minimum required wall thickness (mm) for the selected materials. Test 1 Test 2 Table 25 Pressure to deformation and rupture (MPa) for AISI 321. Pdeform (MPa) AISI 321 0.033 0.027 Prupt (MPa) 45 48 Test 1 Test 2 17 18PDF Image | Thermal energy storage: Recent developments
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