The Development of Powder Circulation Systems for Solar Energy Capture and Storage

Abstract:Concentrated Solar Power (CSP) is an electricity generation technology that concentrates solar irradiation through heliostats onto a small area, the receiver, where a heat transfer fluid is used as heat carrier towards the heat storage and power block. It has been under the spotlight for a decade as one of the most potential or promising renewable and sustainable energy technology. The use of gas/solid suspensions as heat transfer fluid in CSP has been advocated for the first time in the 1980's and this novel application relies on its possible use throughout the full CSP plant, i.e. in heat capture, conveying, storage and heat reuse, where it offers major advantages in comparison with water/steam, thermal fluids or molten salt, the current heat transfer fluids. Since the powder 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, suspension temperatures of about 800 °C can be tolerated, with additional high efficiency thermodynamic systems being applicable. This work reviews the development of CSP and assesses both its energy efficiency and the impact of the direct normal solar irradiance on the power yield of a CSP plant. A literature review of fluidized bed systems and their use as heat transfer medium is also included. A novel bubbling fluidized bed concept, the upflow bubbling fluidized bed is selected for use in a solar power tower plant for its excellent heat transfer and potential in moderate to high receiver capacities. Circulating fluidized beds could be interesting for very large scale operations. The selection accounts for sizes, complexity and operating costs. Both upflow bubbling fluidized bed and circulating fluidized bed of fine particles feature excellent heat transfer. At the 1 MW solar furnace, PROMES CNRS France reported that a high heat transfer coefficient of 1100 W/m²K was achieved with an upflow bubbling fluidized bed receiver at superficial gas velocities below 0.2 m/s and solids circulation flux of 10 to 45 kg/m²s. A moderate heat transfer coefficient of 60 to 300 W/m²K was obtained in the circulating fluidized bed operation mode in this PhD. The values of the obtained heat transfer coefficients are dealt with by both empirical and mechanistic modeling approach, with fair to very good agreements obtained. Furthermore, the mechanistic surface renewal model provides guidelines in terms of dimensional scaling up, which is also a major contribution of the PhD. The particle circulation loop throughout the CSP installation is also examined for the remaining sections of the power generation concept, i.e. in the hot and cold storage hoppers, in the boilers of the thermal block, or as direct heat supply to the hot-side heat exchanger of a Stirling engine. Whereas in heat capture, mostly upflow bubbling or circulating fluidized beds seem appropriate, both moving beds or downcomers, and bubbling fluidized beds are recommended in the heat storage and re-use, and experimentally quantified. The novel concepts of thermal energy storage, using latent heat in phase change materials, or storing thermo-chemical heat through using reversible chemical reactions has been investigated with extensive interest. The thermal energy from a solar collection field or from transforming excess renewable electricity from wind turbines into heat can be stored for short or even long terms in such systems. However the concept applied at high temperatures is still at its early stage. Experimental phase change materials research was conducted and results illustrate the effect of different parameters. Another approach to thermal energy storage is the use of reversible chemical reactions, and they can be regenerated through the exothermic reverse reaction process. Unlike sensible or latent heat storage, mostly limited in time due to heat losses, chemical energy storage enables to bridge long duration gaps between supply and demand, hence making it particularly suitable for a stable electricity generation. Thermodynamic calculations and thermogravimetric analysis were conducted on selected reaction pairs. Results are of fundamental nature only, and it is far too early to imagine a pilot or large-scale application of thermo-chemical storage in the CSP concept. Since phase transformation or chemical reactions are involved, the materials need to be properly contained. The containment is discussed with respect to the selection concerns and criteria. The containing material or geometry should satisfy the required need of being closed and able to withstand the high temperatures envisaged, to resist possible chemical attacks by the contained materials, and to resist the pressure build up during the phase transition or reactions, in which expansion often occurs or gases and vapours are released. The PhD research is concluded by scale-up data and provides a preliminary view into the prospects and the overall economy of the system. Market prospects for both novel concentrated solar power and thermal energy storage applications are expected to be excellent. Although the PhD research provided lab-scale based design methods and equations for the key unit operations of a novel solar power tower CSP concept, there is ample scope for future development of several topics, as finally recommended.

Publication year:2016

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