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32 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 appropriate de-dusting equipment and heat recovery on the exhaust gas. The application of CFB seems hence limited to very large storage and re-use capacities. The rotary drum and screw conveyer can offer advantages for heat storage, but can be excluded as heat capture option due to the low heat transfer rate achieved. Among the dif- ferent concepts, the UBFB (low to moderate CSP capacities) and CFB (large capacities) are recommended. The BFB, CFB and moving bed concepts can however be applied in the TES and thermal block of the plant, with some tentative il- lustrations are given in Fig. 33 and Fig. 34 as example applications. The excellent heat transfer characteristics of powder beds more- over facilitate the integration of E-PCM, as temporary heat storage medium. The loop of Fig. 33 is generally composed of a hot storage silo fed from the discharge of the heat or energy receiver, and feeding a fluid bed heat exchanger, where the particles transmit their energy to submerged tubes inside whose a working fluid (for example steam) is generated and further expanded in a turbine: the fluid bed heat exchanger is a common device in the electrical power indus- try (mostly implemented for coal and biomass combustion in fluidized beds). The cooled particles exit the boiler (continuous circulation) and are returned towards the cold storage tank by mechanical convey- ing. Finally, a conveying system raises the particles from the cold silo to the heat receiver inlet. Particles are used as heat transfer fluid and heat storage medium, possibly complemented by using an E-PCM. The loop of Fig. 34 includes the same unit components. Heat is supplied to the hot-side of the Stirling engine in a BFB of the par- ticle heat carrier. 6.2.1. Denseup-flowofparticles Literature concerning dense up-flow systems, with forced up- flow and bubble-induced mixing is scarce. Initial research mostly covered moving packed beds. Fluidized bed up-flow reactors were described by several researchers. The dense up-flow column is fed at its bottom by a circulation flow of solids at an appropriate pres- sure to overcome the pressure drop of the upwards-moving bed of solids. The column itself is either aerated from the windbox [3,7] or by a combined windbox aeration and additional aeration at the bottom of the riser column itself [21,28]. 6.2.2. ParticleloopsinSPTandCSPapplications Since the particle solar receiver is the key component, previ- ous investigations in the field of solar receivers using particles as HTF are dealt with in Table 27. Tubular absorbers are generally pro- posed for use in current solar thermal receivers. The first studies on direct absorption solar receivers started in the early 1980s with three concepts, the fluidized bed receiver [208], the free falling par- ticle receiver [209] and the rotary kiln receiver [210,211]. In the free falling particles concept, the solids are dropped di- rectly into the concentrated solar beam from the top of the receiver and are heated during their pass through the concentrated solar ra- diation. The principle was validated by on-sun experiments at pilot scale [212]. A receiver prototype was tested at the National Solar Thermal Test Facility (NSTTF) in Albuquerque NM, USA. Short batch runs were performed for a total particle inventory of about 1800 kg. The receiver efficiency generally increased with the particle flow rate and varied from about 35% to 52%, thus in good agreement with simulated data. A review of the falling particle receiver was pro- posed by Tan and Chen [213]. CNRS (FR) developed a “sand heater loop” using sand particles as HTF [211]: it combined a solar rotary kiln that delivered hot sand to a heat storage/heat recovery sub- system consisting of a hot and a cold heat storage bin and a multistage fluidized heat exchanger. The air jet flow concept of Röger et al. [214] used falling par- ticles that line the inner wall of a cylinder closed at its top, whilst the bottom part was facing the concentrated solar beam. Heat re- covery from the hot storage is then possible using fluidized bed heat exchangers as described by Warerkar et al. [215], or by moving bed particle-air heat exchangers tested by Al-Ansary et al. [216]. In the former study, storage bins are integrated at the top of the tower. Direct and indirect absorption particulate receivers were dis- cussed in detail by Zhang et al. [28]. Indirect receivers operate at lower flux density in the range 200–400 kW/m2, but offer a better control of particle circulation within the receiver and a possible man- agement of operating pressure and heat losses. The particle mass flow rate inside the solar receiver is one of the main issues for a high power solar concentrating system using par- ticles as HTF. In industry, BFB and CFB are widely used at large scale in oil refineries and in combustors [217,218]: the dense phase flu- idized bed can be used in standpipes to provide an important and steady downward flow of solid as shown by Bodin et al. [219], Smol- ders et al. [220], and Chan et al. [221]. In this regime, the suspension is uniform, it has a low voidage, and it circulates slowly (a few cm/ s), thus limiting both the energy consumption and its use as HTF. Novel concepts use a dense suspension of small size solid par- ticles, patented by Flamant and Hemati [222], and are currently developed in the framework of both a French National and a Eu- ropean project [30]: they involve a dense particle upflow suspension receiver (solid fraction in the range 30–40%) in vertical absorbing tubes submitted to concentrated solar energy. 6.2.3. Recent experimental results Recent experimental investigations of using a particulate HTF were presented by Flamant et al. [21] and Zhang et al. [28], where the concept of using UBFB and BFB particle suspension carriers was tested at the solar receiver of Font Romeu (F). The selected par- ticles were 64 μm silicon carbide (3120 kg/m3), conveyed in a single tube at air velocities between 0.03 and 0.22 m/s, achieving powder circulation fluxes between 8 and 50 kg/m2s. The tube wall temper- ature reached up to ~800 °C. The experimental results of the heat transfer coefficient, HTC, against solid flux and for all values of operating gas velocity are il- lustrated in Fig. 35. The heat transfer coefficient is a clear function of G. Values of HTC range from >430 W/m2 K to 1120 W/m2 K. These measured values include both the contribution of particle convec- tion and radiation at higher temperatures. The radiation contribution can be separately assessed from the Stefan–Boltzmann equation and ranges from ~30 W/m2 K at low wall and bed temperature, to 200 W/ m2 K for high temperatures [28]. CFB experiments were also carried out and were partly re- ported by Zhang et al. [231]. Solids circulation was achieved via a downcomer and L-valve. Heat supply was by hot water or thermal fluid (Santotherm 350). The downcomer was water-cooled through a 0.2 m long concentric cooler. Temperatures were measured and recorded (wall, different lo- cations in the CFB, feeding and overflow lines of the fluid). The HTC was calculated from the known exposed surface area, Aex, the mea- sured temperature difference, ΔT, and heat input, Q. The bed material used was rounded sand of 75 μm, 2260 kg/m3, and Cps = 1.05 kJ/ kg K. Various combined (U,G) values were tested to scan the different riser hydrodynamic regimes (dilute flow, core/annulus flow, dense flow): G was varied from 8 to ~500kg/m2 s and U – UTR from 1 to 18 m/s (with U, the superficial gas velocity, and UTR, the required onset superficial gas velocity for CFB riser flow). The HTC varied from ~20 W/m2 K (G = 0) to ~400 W/m2 K (G = 500 kg/m2 s). If a higher temperature would have been used (wall temperature >600 °C), the radiation contribution should have in- creased the measured HTC. The required superficial gas velocity in a CFB (>>3 m/s) significantly exceeds the low value in the UBFB (~0.1 m/s).PDF Image | Thermal energy storage: Recent developments
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