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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control and further diode developments, solar TPV systems with efficiency in the range <strong>of</strong><br />
25–35% are possible.<br />
Spectral control is <strong>of</strong> crucial importance and holds the key <strong>for</strong> TPV efficiency. The goal <strong>of</strong><br />
spectral control is to allow only photons above the band gap to reach the diode, as photons below<br />
the band gap not only represent a loss <strong>of</strong> useful energy but also reduce diode efficiency because<br />
they cause a rise in the diode temperature when being absorbed. For emission control, rare earth<br />
and transition metal-doped ceramics, refractory intermetallic coatings, thin-film and multilayer<br />
filters, plasmonic filters, and photonic crystals have been explored (Fleming et al. 2002; Licciulli<br />
et al. 2003). However, high-temperature operation <strong>of</strong> the emitters poses great challenges to the<br />
stability <strong>of</strong> the materials and structures. In comparison, filters, either stand-alone or built on the<br />
surface <strong>of</strong> the diode, suffer less from the stability issue.<br />
In 2002, <strong>for</strong> a 1.5-kW GaSb-based system used as a home furnace, the total cost was estimated to<br />
be $4,200 with $2,700 <strong>for</strong> the furnace and $1,500 <strong>for</strong> the TPV generator at ~15% efficiency<br />
(Fraas and McConnell 2002). This corresponds to $1/W. If we add in the cost <strong>of</strong> the concentrator<br />
at $1.6/W (assuming 15% efficiency, 850 W/m 2 solar insolation, and $200/m 2 concentrator cost),<br />
the cost is $2.6/W based on current technology (not counting other items that may be needed <strong>for</strong><br />
the solar TPV system). If the efficiency is doubled to 30%, reducing the cost <strong>of</strong> energy in half,<br />
then the other major opportunity <strong>for</strong> cost reduction is the concentrator cost. This cost would need<br />
to be reduced significantly to bring the total cost to a target cost <strong>of</strong> $1/W as <strong>for</strong> solar PV.<br />
Concentrated Photovoltaics. Concentrated photovoltaic systems do not involve a solar thermal<br />
process, but they share the concentrator issues <strong>of</strong> linear and central receivers. Thus, this fastdeveloping<br />
field might well be considered under the “Crosscutting Areas” category. In this<br />
method, sunlight is concentrated by using mirrors or lenses, which are much cheaper than PV<br />
panels, and the concentrated light is focused onto the PV cells. The required cell area is there<strong>for</strong>e<br />
reduced by the concentration factor, which in present systems can be as high as 500; future CPV<br />
systems may be able to sustain even higher concentration ratios. We refer the reader to the <strong>Basic</strong><br />
<strong>Research</strong> Challenges <strong>for</strong> <strong>Solar</strong> Electricity and <strong>Solar</strong> Electricity Technology Assessment <strong>for</strong><br />
details <strong>of</strong> PV development. More details on CPV systems can be found in the <strong>Solar</strong> Thermal<br />
Technology Assessment in Appendix 1. Projections (Stoddard et al. 2005) put the long-term<br />
installed costs <strong>of</strong> CPV with multijunction cells currently under development at about $2/W. The<br />
present cost <strong>of</strong> systems provided by Amonix and <strong>Solar</strong> Systems Pty Ltd. are roughly $4/W; these<br />
systems use single-junction silicon cells and are in an early commercialization stage.<br />
Concentrated <strong>Solar</strong> Thermochemical Processes<br />
The concentrating component <strong>of</strong> these systems is identical to that <strong>of</strong> concentrated solar thermal<br />
processes <strong>for</strong> power generation, but the energy conversion is a thermochemical process<br />
converting radiation-to-heat-to-chemical potential. These systems provide an effective means <strong>for</strong><br />
long-term storage and transportation <strong>of</strong> solar energy (e.g., in the <strong>for</strong>m <strong>of</strong> fuel) and its utilization<br />
in motor vehicles and industrial applications. The basic concept is shown in Figure 16.<br />
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