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Fig. 35. Experimental values of the heat transfer coefficient, HTC, vs. G, for all ex- perimental superficial gas velocities. The studies confirmed that upflowing particles can be used in the novel design of a solar receiver, both in UBFB and CFB mode. UBFB heat transfer coefficients up to 1100 W/m2 K were obtained for operation at low superficial gas velocities, thus limiting sensi- ble heat losses by the exhaust air. Although CFBs could be interesting for large scale operations, due to the higher solid flux achieved, the comparison of Table 28 tentatively illustrates the main advan- tages of using the UBFB principle, even for high receiver capacities. This selection accounts for sizes, complexity and operating costs. The high heat transfer coefficient obtained within a given range of superficial gas velocities, U and solid flux, G, and the possible op- eration of the powder upflow bed at high temperatures (>750 °C) moreover offers new opportunities for high efficiency thermody- namic cycles, with a conversion increase from the current 40% to over 46% (leading to a reduction of the receiver and heliostat requirements). Table 28 At equivalent values of the specific heat of molten salts and powders, and with a lower powder temperature after heat recov- ery (~150 °C against ~300 °C for molten salts), the higher receiver temperatures of the powders reduce the storage requirement on a mass basis by ~50%, however only about 30% on a volumetric basis, due to the lower bulk density of the powder bed. Parasitic power consumption is also significantly reduced due to the reduced pumping energy, and lack of heat tracing of the circuits. The com- bination of these technological advantages will considerably reduce the LCOE of the SPT concept. The dense particle suspension is a promising option, justifying additional testing (e.g. using multiple receiver tubes), and allowing a more correct analysis of economic advantages in both operating and investment costs, as tentatively proposed in Table 28. A multi-tube receiver (16 parallel tubes) has been studied within the CSP2 project [27] at the CNRS 1MW solar furnace, and results are presented in a separate paper. 7. Conclusionandrecommendationforpriorityresearch This work reviewed some recent developments and design rec- ommendations for the high temperature heat storage, where sensible/latent/reaction heat each offers a contribution with re- versible reactions being the most effective (heat storage >>1000 kJ/ kg), followed by latent heat storage (500–1000 kJ/kg) and sensible heat storage materials (<500 kJ/kg). This is dealt with in Section 1 of the paper. Section 2 of the paper specifically dealt with sensible heat storage. Sensible heat storage materials are well-documented, have the lowest cost materials but at the same time the lowest storage capacity com- pared to PCM and TCS. Most commonly used minerals’ costs range from 0.05 €/kg to 0.1 €/kg, hence cheaper than latent heat storage costs sensible heat materials have excellent thermal conductivi- ties: 1.0 W/mK–7.0 W/mK for sand-rock minerals, concrete and firebricks, 37.0 W/mK–40.0 W/mK for ferroalloys. Their disadvan- tage is their heat capacity, ranging from 0.56 kJ/kg K to 1.3 kJ/kg K, making storage unit sizes to be bigger than when using latent heat. The energy stored by a SHS material towards variations of 1 °C in its temperature can be 100 times lower compared to the energy storage capacity when it has a phase change, as can be noticed in H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 35 Comparison of UBFB and CFB solar receiver (10 MWth) with SiC powders (50 μm, 3100 kg/m3 and specific heat at 500 °C of 1.2 kJ/kg K) [203]. Parameter Voidage, ε Pressure drop, ΔP/L (mbar/m) Heat transfer coefficient, HTC (W/m2 K) Riser diameter, D (m) Solids circulation flux, G (kg/m2 s) Solids flow in riser tube, Qs (kg/s) Temperature increase in riser passage (K) Average temperature in riser, Tr (K) Temperature of riser wall, Tw (K) Driving ΔT (K) Required heat exchanging surface, S (m2) Height of riser, H (m) Number of parallel riser tubes (–) Pressure drop, ΔP (mbar) Air flow to riser, Qa (N m3/h) Heat loss by air flow (90% recycle), ΔHloss (kWth) Air pumping power, P (kWel) Equation or reference ρp(1 − ε) (πD2/4) × G Inlet ~250 °C Outlet ~750 °C (Tin + Tout)/2 ΔT≈ (Tw−Tb) (10 MWth × 106)/(HTC × ΔT) – S/(πHD) ΔP/(LH) +disperser + separator UBFB: 0.05 Nm/s (i.e. 0.14 m/s at 750 °C) CFB: 2 Nm/s (i.e. 5.5 m/s at 750 °C) [QaCpa(Tout − 20)] ×0.1 UBFB: blower CFB: fan UBFB 0.6–0.7 [21,28] 124 950 0.04 [21,28] 50 [21,28] 0.063 500 500 800–850 300 30 e.g.3×2meach 3×40 (2×124)+50≈300 ~30 14564 kJ/h 0.8 kWth <1 kWel CFB 0.95–0.99 [235,236] 6.2 600 for G > 400 kg/m2 s [231] 0.08 [231] >400 [231,235] 2.0 500 500 800–850 300 55.5 Commonly ≥ 6 m 36 (6×6.2)+50≈90 1300 123,370 kJ/h 35 kWth <4 kWel The UBFB application results in lower heat losses, but a slightly higher pressure drop. Towards the number of parallel receiver tubes, the CFB is more advantageous. The conveying of the powders from the cold storage to the feeding system of the receiver is supposed to be operated by mechanical conveying (as a series of bucket elevators). Further scale-up work is required to confirm the tentative economic and design assessment.PDF Image | Thermal energy storage: Recent developments
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