Energy sustainability and climate change are two major challenges in the 21st century. One proposed renewable energy strategy to address both concerns involves the use of a thermochemical cycle to convert CO2 and H2O to CO and H2 (syngas) using concentrated solar energy to drive the endothermic reduction portion of the cycle. The production of fuels from the thermochemical cycling of metal oxides occurs in two steps: (1) thermal reduction of the metal oxide at high temperatures, which generates oxygen; and (2) re-oxidation of the metal oxide at lower temperatures by steam and/or carbon dioxide, which produces the chemical fuel (Figure 1).
Figure 1. Two-step solar thermochemical cycle. Metal oxide (MO2) is thermally reduced at high temperatures (step 1) and re-oxidized by H2O and/or CO2 to produce H2 and/or CO (step 2).
While the concept is very attractive, the major challenges are the high temperature needed in the reduction step, low thermal conductivity of the metal oxide material, and the need of thermal cycling of the working material (metal oxide).
Heat-Pipe-Based Solar Reactor
A new refractory metal, the heat-pipe-based solar reactor was developed at ACT to address some of these issues. The focus is to increase the heat transfer area to enable rapid heating of the metal oxide material subject to concentrated solar heat flux while performing the thermal cycling without moving parts. In this design, the incident solar flux is directed into a cavity receiver containing one or more high-temperature pressure-controlled heat pipe (PCHP) and Constant Conductance Heat Pipe (CCHP) combinations. Specifically, the evaporator of the PCHP heat pipe is connected to the cavity housing where the incident solar flux is then transferred to the heat pipe. The condenser end of the PCHP is connected to the evaporator end of a CCHP. The length of the CCHP condenser can be tailored to effectively spread the concentrated solar radiation over a large surface area. The porous metal oxide material would then be located around the condenser end of the CCHP (Figure 2). To perform the thermal cycling, non-condensable gas will at times fill in the condenser end of PCHP to block the heat transfer to the connected CCHP and therefore the metal oxide. This design enables a large heat transfer area for the low conductivity metal oxide as well as performing thermal cycling without moving parts.
Figure 2. Illustration for a heat pipe-based reactor. Two reactors are cyclically heated by solar energy. Pressure-controlled heat pipes (PCHP), controlled by the amount of non-condensable gas (NGC), are used to perform the thermal cycle for two constant conductance heat pipe (CCHP) reactors.
To assess the feasibility of a heat pipe-based reactor, a superalloy-based heat pipe was first considered because it is capable of operating in an air environment and is commercially available. ACT has product lines for superalloy heat pipe-related products. A single superalloy-based heat pipe reactor system was designed and tested. An overall schematic of the system is shown in Figure 3A. The packed bed reactor contains a porous metal oxide material. The packed bed was formed in an annular configuration and positioned concentrically around a high-temperature heat pipe that both transferred heat into the reactor and aided in isothermalizing the packed bed (Figure 3B). Haynes 230 was chosen as the heat pipe envelope based on its excellent creep strength, oxidation resistance, ability to machine, and ability to operate continuously at temperatures up to 1100oC. The internal working fluid in the heat pipe was sodium, which is effective for temperatures up to 1100oC (Figure 3C).
Figure 3. (A) Schematic of the experimental setup containing a silicon carbide heating element (can be heated to 1650oC). (B) The packed bed surrounds the heat pipe obtaining a high- heat transfer area. (C) Initial testing of the heat pipe at high temperature (1050oC) shows relative isothermality (uniform red color) prior to integration with the reactor assembly.
The temperature limitation of a superalloy heat pipe is about 1050 to 1100oC due to the long-term creep created by the sodium vapor pressure at high temperatures. To further optimize the thermochemical conversion efficiency, higher reduction temperatures are required. A TZM + Lithium refractory metal heat pipe with operating temperature > 1400oC was then developed. Figure 4 shows the finished refractory metal heat pipe heated by an induction heater in a bell jar. The thermal radiation from the fill tube indicates that no condensable gas was in the pipe. Due to the oxidation issue with refractory metals, the refractory metal heat pipe must operate in an oxygen free-environment or vacuum environment for example where the heat can be radiatively transferred to the target working material.
Figure 4. TZM lithium heat pipe operating in the vacuum environment.
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