Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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H2SO4 at 1,130K, and the University <strong>of</strong> Tokyo Cycle #3 (UT-3) four-step cycle based on the<br />
hydrolysis <strong>of</strong> CaBr2 and FeBr2 at 1,020 and 870K.<br />
In recent years, significant progress has been made in the development <strong>of</strong> optical systems <strong>for</strong><br />
large-scale solar concentration; such systems are capable <strong>of</strong> achieving mean solar concentration<br />
ratios exceeding 2,000 suns (1 sun = 1 kW/m 2 ). Present ef<strong>for</strong>ts are aimed at reaching<br />
concentrations <strong>of</strong> 5,000 suns (Steinfeld and Palumbo 2001). Such high radiation fluxes allow the<br />
conversion <strong>of</strong> solar energy to thermal reservoirs at 2,000K and above, which are needed <strong>for</strong><br />
efficient water-splitting thermochemical cycles using metal oxide redox reactions (Steinfeld<br />
2005). This two-step thermochemical cycle (Figure 48) consists <strong>of</strong> a first-step solar endothermic<br />
dissociation <strong>of</strong> a metal oxide and a second-step nonsolar exothermic hydrolysis <strong>of</strong> the metal. The<br />
net reaction is H2O = H2 + 0.5O2, but since H2 and O2 are <strong>for</strong>med in different steps, the need <strong>for</strong><br />
high-temperature gas separation is thereby eliminated.<br />
H 2 O<br />
M x O y<br />
recycle<br />
Concentrated<br />
<strong>Solar</strong> <strong>Energy</strong><br />
SOLAR REACTOR<br />
M MxO xO y = xM + y/2 O 2<br />
150<br />
M<br />
HYDROLYSER<br />
xM + yH yH2O 2O = M MxO xO y + yH 2<br />
M x O y<br />
Figure 48 <strong>Solar</strong> hydrogen production by water-splitting<br />
thermochemical cycle via metal oxide redox reactions<br />
This cycle was examined <strong>for</strong> the redox pairs Fe3O4/FeO, Mn3O4/MnO, Co3O4/CoO, and mixed<br />
oxides (Steinfeld 2005 and citations therein). One <strong>of</strong> the most favorable candidate metal oxide<br />
redox pairs is ZnO/Zn. Several chemical aspects <strong>of</strong> the thermal dissociation <strong>of</strong> ZnO have been<br />
investigated (Palumbo et al. 1998). The theoretical upper limit in the energy efficiency, with<br />
complete heat recovery during quenching and hydrolysis, is 58% (Steinfeld 2002). In particular,<br />
the quench efficiency is sensitive to the dilution ratio <strong>of</strong> Zn(g). Alternatively, electrothermal<br />
methods <strong>for</strong> in situ separation <strong>of</strong> Zn(g) and O2 at high temperatures have been demonstrated<br />
(Fletcher 1999); these enable recovery <strong>of</strong> the sensible and latent heat <strong>of</strong> the products. Figure 49<br />
shows a schematic <strong>of</strong> a solar chemical reactor concept that features a windowed rotating cavityreceiver<br />
lined with ZnO particles that are held by centrifugal <strong>for</strong>ce. With this arrangement, ZnO<br />
is directly exposed to high-flux solar irradiation and simultaneously serves the functions <strong>of</strong><br />
radiant absorber, thermal insulator, and chemical reactant. <strong>Solar</strong> tests carried out with a 10-kW<br />
prototype subjected to a peak solar concentration <strong>of</strong> 4,000 suns proved the low thermal inertia <strong>of</strong><br />
½ O 2<br />
H 2