Sphelar® cell array module
Kenichi Taira* and Josuke Nakata
Kyosemi Corporation, 385-31 Toiso, Eniwa, Hokkaido 061-1405, Japan
taira@kyosemi.co.jp
Abstract: Sphelar® is a spherical solar cell measuring a mere 1-2 mm across, which Kyosemi developed on its
own. Sphelar® cells can capture sunlight 3 dimensionally, thus improve its power generation capacity to maximum
potential. We present a novel concentrating PV module that incorporate an array of 12 Sphelar® cells. Usually,
there is a gap between the output voltage of the energy harvesting transducers like inductive generators, solar cells
and thermo-generators to supply and the minimum required input voltage of remote sensors and micro-systems. In
this paper, we demonstrate that the Sphelar® modules can drive two different applications directly without any
peripheral circuitries.
Keywords: spherical solar cell, solar energy harvesting
SPHELAR® TECHNOLOGY
Kyosemi provides new surface structuring
technology through our featured products of Sphelar®
to capture sunlight 3 dimensionally due to its unique
construction made up of spheres [1-4]. It offers
superior efficiency over traditional cells that have only
one planar surface to capitalize upon sunlight.
Sphelar® technology is based on tiny single
crystalline silicon spheres measuring a mere 1-2 mm
across. They are directly produced from molten silicon.
Recently, this spherical solar cell has attracted much
attention. Silicon substrates for flat solar cells are
currently obtained from the ingot by wire sawing.
Almost half of the silicon is lost during slicing due to
Kirf losses. Silicon consumption becomes a critical
issue when the production volume approaches the GW
range. Our tiny silicon solar cells significantly reduce
the energy used in silicon cell production and it also
maximizes raw material use.
Figure 1 shows photograph image of Sphelar® cell
(a), and its schematic (b), respectively. As shown, p-n
junction almost covers the entire outer surface. A
positive and a negative electrode, opposite to each
other, are formed in the centers of p-type and n-type
surfaces, respectively. This electrode arrangement
makes the cell non-directive and can realize an even
distribution of generated current. Since the tiny
spheres absorb light from all directions, we call this
development three-dimensional (3D) light capture.
The output performance of the Sphelar® cell and a
conventional flat cell were experimentally compared in
an integrating sphere, where uniform radiance is
available [4]. The obtained data indicates that on the
Sphelar® cell the incident light from almost all
directions can be captured to generate electricity,
which provides direct evidence for 3D light capture.
Global solar radiation has direct, reflected and diffused
components. Solar energy harvest has to be considered
of fundamental importance. Spherical nature of
Sphelar® cells allows harvesting both direct radiation,
which comes directly from the sun and indirect
radiation, which comes from the sky, clouds and the
ground.
The patented electrode arrangement also facilitates
a serial and/or parallel connection of the cell to other
cell and allows for a compact concentrating PV
module that incorporate an array of 12 Sphelar® cells
where each cell can be combined in different versions
of parallel and serial.
(a)
(b)
Figure 1: Photograph image (a) of spherical solar
cells and schematic of cell, (b).
12 CELL ARRAY
We present a novel concentrating PV module that
incorporate an array of 12 Sphelar® cells. Figure 2
shows 52-mm long version (a) and 24-mm long
version (b) of the Sphelar® modules, respectively. The
outer case is made of polycarbonate and its
encapsulant is epoxy. Although each cell can only
have about 0.6 V of open-circuit voltage and 2.5 mA
of short-circuit current, Sphelar® cells can be
connected in serial and/or parallel to form arrays by
using different shapes of metal frames, and
substantially higher voltages and higher currents can
be delivered. Irradiated light is concentrated x4 by the
lens effect, greatly enhancing the power output. The
currently available combination of serial and parallel
connections are 12serial (s)/1parallel (p), 6s/2p, 4s/3p,
3s/4p, 2s/6p and 1s/12p. Electrical characteristics of 6
different versions of serial and parallel connections are
obtained under standard test conditions (irradiance of
1kW/m2, spectrum of 1.5 Air Mass and cell
temperature of 25 degree Centigrade) and are shown in
Fig.3. The values of maximum power output and Fill
factor are also shown. All the modules have high and
30
1s/12p, 13.6, :0.755
20
2s/6p, 13.8, :0.777
3s/4p, 13.4, 0.781
10
4s/3p, 13.5, 0.784
6s/2p, 13.5, 0.798
12s/1p, 13.9, 0.805
0
0
2
4
6
8
10
Voltage (V)
Figure 3: Electrical characteristics of 6 different
versions of serial and parallel connections under
standard test conditions. The values of maximum
power output and Fill factor are also shown.
(a)
(b)
Figure 2: 24-mm long version (a) and 52-mm long
version (b) of the Sphelar® modules, respectively.
Figure 4: The 12 cell array can be used for different
colors of LEDs.
stable value of Fill factor regardless of the module
configurations. Please note that the modules cover
wide range of voltages from 0.6V to 7.2 V. So, the
modules can be used for different colors of LEDs (Fig.
4).
Figure 5 shows current-voltage characteristics of
the 12s/1p cell array at various cell temperatures. The
array operated at 60 degree C can deliver about 11.9
mW. At the maximum power point, the corresponding
Vm and Im is 5.6 V and 2.3 mA, respectively. For
lower-temperature operations, power output will be
enhanced.
Figure 6 shows maximum power output of 6s/2p
version at various irradiance levels. Light sources are
fluorescent lighting and incandescent lighting. Linear
dependence is observed over a wide range of
irradiance levels.
shaded by your body and is always in the sun. This
product will be commercially available soon. The
monitor has won the Australian Design Award, the
ABC TV’s New Investor’s Award, and the People’s
Choice Award.
3
60.3, 42.7, 20.4
2
1
0
0
2
4
6
8
10
Voltage (V)
Figure 5: Electrical characteristics of 12s/1p version
under various temperatures (60.3, 42.7and 20.4°C).
Figure 7: Personal ultraviolet ‘B’ radiation alarm dose
meter, U-B-SAFE 1 from Australian company
HEALTHTRONICS.
-2
10
Incandescent lighting
-3
10
-4
10
Fluorescent lighting
-5
10
-6
10
102
103
104
105
Illuminance (lx)
Figure 6: maximum power output of 6s/2p version at
various irradiance levels.
APPLICATION
The Sphelar® module is compact, light and
durable. We demonstrate that the Sphelar® modules
can drive two different applications directly without
any peripheral circuits.
U-B-Safe 1 [6]
In the first demonstrator, the Sphelar® module has
been designed to ensure U-B-SAFE 1, which is a
personal ultraviolet ‘B’ radiation alarm dose meter
from Australian company HEALTHTRONICS, it
always has power under the sun. It lets you know when
your skin has absorbed the maximum safe daily
amount of UV radiation for your skin type. As shown
in Fig. 7, it is designed to pin to your hat so it can’t be
Aimulet LA [7]
In the second demonstrator, the 52-mm-long
version has already been commercially applied in
Aimulet LA, an infrared light-activated handheld audio
communication device, which was designed by Japan’s
National Institute of Advanced Industrial Science and
Technology (AIST). To improve the freedom of the
design of the terminal, the Sphelar array was employed.
Dimensions of the terminal are 86 x 56 x 7 mm3. The
device receives pulse-width modulated infrared light
via Sphelar® array set into the bottom of the handset
and translates it into audio messages when you stand
over special infrared LED emitters set into the ground,
so it is wireless and needs no battery. As shown in Fig
8, signal light is launched to the user like a footlight.
Users put the terminal over their ear like a cellular
phone. Sound hole is placed in the center of dome-like
bulge on the terminal. The outer shell is a molded
bamboo sheet instead of plastic polymer for ease of
recycling. It should be noted that the components are
the 12 cell array, ceramic speaker, and shunt resistor
only (Fig. 9). The outer surface of Sphelar® module is
coated so as to block out visible light. The device was
adopted in the Walk Project installation for World
EXPO 2005 AICHI JAPAN. Visitors used the device
to receive audio messages as they wandered the site.
The 2006 Good Design Award for Ecology Design
went to Aimulet LA for its ingenious blend of new
technology and natural materials.
The technology at work in Aimulet LA can be put
to use in public spaces such as outdoor exhibits and
events, amusement facilities, train stations and parks.
Figure 8: Aimulet LA, an infrared light-activated handheld audio communication device and LED emitter.
REFERENCES
Figure 9: Internal aspect of Aimulet LA
CONCLUSION
In this paper, we present a novel concentrating PV
module that incorporate an array of 12 Sphelar® cells.
Spherical solar modules are more flexible and less
dependent on vertically falling light. Such features are
favorable in energy harvesting for any handheld or
other wireless sensor nodes.
[1] Nakata J Asia Electronics Industry (AEI)
October (2001) 44-46
[2] Kumagai N et al. Proceedings 19th European
Photovoltaic Solar Energy Conference (Paris,
France, 7-11 June 2004) 659-662
[3] Taira K et al. Proceedings 15th Int’l Photovoltaic
Science & Engineering Conference (PVSEC-15)
Shanghai China 2005 202-203
[4] Kogo N et al. Proceedings 15th Int’l Photovoltaic
Science & Engineering Conference (PVSEC-15)
Shanghai China 2005 206-207
[5] Biancardo M et al. Solar Energy 81 (2007) 711716
[6] http://www.sunsafe.ws/
[7] http://dspace.itri.aist.go.jp/handle/2111/2812