Silicon is the most important raw material used in
photovoltaic devices in modern times. The demand for
the high purity silicon has greatly increased recently
due to the rapid development of the solar photovoltaic
(PV) industry.[1] Classically, silicon is fabricated using
the Siemens process.[2] In addition, metallurgical pu-
ri cation methods have begun to replace the Siemens
process in some PV wafer fabrication. All these exist-
ing techniques su er from the drawback of high electri-
cal energy consumption. In this Letter, a new method
described here reveals a direct transformation from
solar energy to solar electricity. Instead of using elec-
tricity in the process, we use concentrated solar rays
with a crucibleless process to upgrade metallurgical
silicon into solar-grade (SoG) silicon feedstock.
Attempts have been made in the past[3] to use clas-
sical solar furnaces to purify metallurgical grade sili-
con. The processes described there are dicult for
commercialization partly because of the high cost of
the classical solar furnace and partly because of the
issue of crucible contamination.
Recently, Chen and his colleagues[4,5] proposed a
new type of solar furnace consisting of a heliostat with
actively controlled facets and a very small secondary
concentrator as shown in Fig. 1. They also published
a spinning-elevation tracking formula that allows only
for the movements of rows and columns, realizing the
aberration corrections to rst order, in order to reach
concentration ratios of 10000 or higher. This new type
of solar furnace can harness solar rays with high e-
ciency and high cost-e ectiveness.
Experiments have been carried out to use the
above described solar furnace for incident concentra-
tion of solar rays into a solid rod, which comprises
a mixture of metallurgical-grade silicon powder and
metal oxides such as CaO, Al2O3, SiO2 and others.
The impurities, including boron, inside the silicon ox-idize instantly in the high ambient temperature (1700{
2000∘C) and are removed by some processes: they
may transit into liquid metal oxides or become directly
vaporized. Unlike the common thinking that such a
process would take a long time to complete, we nd
that the puri cation speed is rather fast, i.e. on the
order of a few seconds. This feature not only makes
natural solar rays a practical energy source for indus-
trial use but makes a crucibleless process possible. |
