Australian National University - CECS
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- Offer Profile
- Research into solar energy
technologies at CSES is aimed at increasing the uptake of environmentally
benign solutions by making solar energy cheaper and the conversion process
more efficient. Some of the research looks at new technologies and other
research looks at news ways to increase the efficiency of existing
technologies, much of this effort has lead to technology transfer to
commercial solar energy projects.
Product Portfolio
Hybrid systems
- In concentrator photovoltaic (CPV) systems, sunlight
is focussed using optics such as mirrors and lenses to form a point or a
line. The photovoltaic (PV) cells are placed at the focus of the optics
where they receive concentrated light. These systems offer several
advantages over flat plate photovoltaic systems, including the replacement
of costly solar cells with inexpensive optics (hence reducing overall system
costs). Due to the increased concentration of light onto the cells, cells
must be either passively or actively cooled to ensure optimum performance.
However, this heat introduces the option of creating a hybrid CPV-thermal (CPV-T)
system which can generate both electrical and thermal energy from a single
integrated system. At CSES a range of linear, single-axis tracking hybrid
systems (and their components) are studied.
Microconcentrator
- The microconcentrator (MCT) system is a linear,
single-axis tracking CPV-T system being developed in partnership with
Chromasun Inc., based in the U.S.A.. The system operates at concentration
ratios of up to 30X.
Most CPV and CPV-T systems are significant structural installations. Hence,
unlike conventional flat plate PV systems, they are generally unsuitable for
domestic and commercial rooftop applications. The microconcentrator system
reduces the size and weight of all components, resulting in a system which
is suitable for rooftop installation. In addition, hybrid receivers allow
the option of producing both electrical and thermal power from a single
unit. The target performance of each box is to simultaneously produce 2 kW
of thermal power and 500 Wp of electrical power.
One of the key design features of the MCT is the sealed enclosure. This
enclosure is 3.0m long, 1.2m wide, and 0.3m deep and isolates all the
functional components of the system from external environmental influences
such as wind loading, humidity, and soiling. The removal of the effect of
wind loading on the optics allows the use of a Fresnel array of ultra
lightweight reflectors which require no structural support other than
tensioning at mounting points at each end of the enclosure.
Spectral splitting
- A limitation of many conventional CPV-T systems is that
the desire to have the circulating fluid cool the cells conflicts with the
desire to achieve a high temperature fluid. CSES is now working with UNSW,
CSIRO, Chromasun, and NEP to develop a hybrid CPV-T system that can very
efficiently deliver both 150˚C heat and solar electricity. The key technical
goal of the project is to use spectral splitting to thermally decouple the
solar cells from the 150˚C circulating fluid.
Silicon solar cells waste most of the energy of the solar spectrum. When
illuminated by the whole solar spectrum, silicon linear concentrator cells
have a solar conversion efficiency of 20%. However, when illuminated by
monochromatic light of wavelength around 1100nm, the conversion efficiency
of a silicon concentrator cell approaches 50%.
The linear hybrid CPV-T
receivers being developed will split sunlight into several bands based on
wavelength. The near infrared light (700-1120nm) will be directed to PV
cells. The cell efficiency when illuminated by this wavelength range is
35-40%. The balance of the solar power is converted to heat, and is used to
pre-heat the thermal fluid. Since the conversion efficiency for this
spectral range is high, the cells have substantially reduced cooling
requirements. UV and visible light (<700nm) and FIR (far infrared) light
(>1120nm) will be absorbed by a thermally insulated absorber. About two
thirds of the solar power is in this wavelength range, allowing heating of
the thermal fluid to 150 ˚C. Since the thermal and the PV absorbers are
decoupled, the thermal fluid can reach high temperatures while the solar
cells remain cool.
There are many possible spectral split options,
utilizing selective absorption, reflection and refraction. The most
promising configurations and designs are being selected based on a cost
benefit analysis.
Combined heat and power solar (CHAPS) systems
- CSES, in collaboration with the Solar Thermal Group, have
developed large-scale, linear CPV-T systems based on parabolic trough
mirrors. Demonstration systems have been constructed at Rockingham in Perth
and at ANU’s Bruce Hall.
HIGH PERFORMANCE SILICON SOLAR CELLS DESIGNED FOR CONCENTRATION
APPLICATIONS
High-performance silicon solar cells were designed and manufactured for
systems operating in the range 10-60X. These cells operate at high
efficiencies (20-24%) and can be obtained at moderate cost using an elegant
process sequence. Cells are integrated into concentrator receivers which
feature thermal cycling stress relief, bypass diodes, and encapsulation. The
receiver can have either a light weight aluminium fin heatsink, or water
cooling.
PARABOLIC TROUGH MIRRORS
Mirrors are constructed using an elegant glass-on-metal-laminate technology
in which thin rear-silvered glass mirrors are bonded to a coated steel
sheet. Stamped tab ribs are fitted to the end of the sheet to produce the
correct parabolic profile. This construction produces a lightweight, durable
mirror.
RECEIVERS ARE MOUNTED AT THE LINE FOCUS OF THE MIRRORS
Nanostructures for photovoltaics
- Nanotechnology is the science of the small. It
involves using new scientific understanding and new fabrication techniques
to achieve much greater control over the properties of materials and
devices. Nanotechnology has the potential to allow solar cells to be more
efficient and thinner, both of which would make solar cells cheaper. In this
project we are focusing on nanophotonics, that is, using nanotechnology to
improve the optical properties of solar cells.
Nanostructures for solar cells
- Conventional solar cells are made from silicon wafers,
which are around 300 microns (1/3 of a millimetre) thick. The top surface of
these cells is covered with pyramids around 10 microns high. The pyramids
reduce the reflection from the top surface and trap the light inside the
solar cell (see figure A).
To reduce the cost of solar cells, thin film solar cells are being
developed. These are only a few microns thick and have the potential to be
much cheaper than conventional solar cells, because they use much less
silicon. Pyramid textures can’t be applied to these cells, so new structures
are needed. Unlike pyramid textures, which can be described with geometrical
optics, we need wave optics to predict the behaviour of these new
structures. In this project we are investigating how metal nanoparticles and
nano-structured surfaces can be used to increase the amount of light
absorbed in a solar cell, and hence increase the amount of electrical
current generated. Figure B shows the intensity of the electromagnetic field
around a nano-scale metal cylinder as it scatters light into a thin silicon
solar cell. Because the metal particle is nanoscale and because of the high
refractive index of the silicon, instead of reflecting the light the metal
particle actually scatters the light into the silicon.
The metal particles are formed using a simple self-assembly process of
evaporating a thin layer of metal and then annealing it at low temperature
to form metal particles. Pictures C and D show the metal particles, only
about 100nm across, taken with a scanning electron microscope.
Silicon Materials
- Our research aims to understand the effects of defects
and impurities in silicon solar cells, and to develop practical ways to
reduce their impact, or remove them. This is particularly important when
using low-cost silicon materials for solar cells, such as multicrystalline
silicon wafers, or solar-grade silicon feedstocks, since these materials
always contain significant quantities of unwanted impurities. The unwanted
impurities of most interest are: dopants (such as B, P and Al), which are
very difficult to remove during purification; metals (such as Fe, Cr, Ni
etc), which can create strong recombination centres; and light elements
(such as O, C and N), which may create defects that cause recombination or
shunting. Crystal defects such as grain boundaries and dislocations also
play an important role, and can interact with the impurities, for example,
by acting as preferred sites for precipitation of metals.
In particular, our research focuses on the following
topics:
- - The effect of transition metal impurities such as Fe, Cr, Ni and Cu on
solar cell performance.
- Development of sensitive, spatially resolved methods for detecting dopants
and impurities in silicon wafers.
- The relative impact of defects and impurities in n- and p-type silicon.
Many impurities are less problematic in n-type silicon, which has led to a
renewed interest in this material for photovoltaics.
- Studying the relative recombination activity of dissolved or precipitated
metals.
- Removal of metallic impurities such as Fe by heavily doped surface
diffusions – a process known as ‘gettering’. We are especially interested in
gettering of metals by boron diffusions, which are required for the
development n-type multicrystalline silicon solar cells.
- Studying the impact of hydrogenation on dissolved and precipitated metals,
and on crystal defects such as dislocations and grain boundaries.
- The use of Photoluminescence imaging and Photoluminescence Spectroscopy to
study defects and impurities in silicon wafers.
- The study of intrinsic recombination processes in silicon - Auger and
radiative recombination.
- Accurate measurement of the distributions and concentrations of impurities
in solar-grade silicon feedstocks.
- The effects of dopant compensation (the simultaneous presence of p-type
and n-type dopants) on carrier recombination and mobilities.
- Understanding the structure and evolution of the boron-oxygen defect,
especially in relation to compensated silicon wafers.
We collaborate with a large number of institutes and companies in this area.
Our major partners are: the Institute for Solar Energy Research Hameln (ISFH,
Germany), the Energy Research Centre of the Netherlands (ECN), the
Fraunhofer Institute for Solar Energy Systems (Fh-ISE), The University of
New South Wales (UNSW), BT imaging, FerroAtlantica and CSG Solar.
Surface passivation
- The surfaces of solar cells are critical in
determining the overall performance of the devices. Typically, surfaces are
covered with one or more dielectric materials which serve to optimize both
the electronic and optical properties of the surfaces. They may also serve
other functions.
The interface properties depend on many factors, such as the deposition
conditions and resulting detailed properties of the dielectric, the doping
profile of the silicon in the near-surface region, the electric charge
density in the dielectric, the surface crystal orientation and surface
roughness, creating a large matrix of parameters for investigation.
Several dielectric materials are being studied in detail. These include
silicon dioxide, silicon nitride, and titanium dioxide. Of particular
interest are multilayer stacks in which two or more dielectrics are used to
allow more flexibility in optimizing the electronic, optical and other
required properties of the surface and interface regions.
We utilize a range of measurement techniques to obtain a good understanding
of the material systems we are studying. These include charge carrier
lifetime, capacitance-voltage, electron paramagnetic resonance, Kelvin
probe, and ellipsometry measurements. New Projects
- New projects include an ARC Linkage project with partners
Spark Solar and Centrotherm to study aspects of surface passivation, and an
ARC Linkage project with partners Spark Solar and BraggOne to study the
application of spray-on films for the hydrogenation of the surface as well
as the bulk of silicon wafers.
Silicon Solar Cells
- SLIVER solar cells are a new type of solar cell with
the potential to revolutionise the global solar power industry. They were
developed at the Centre for Sustainable Energy Systems with funding
assistance from Origin Energy. SLIVER technology uses a revolutionary
process to achieve high efficiencies while significantly reducing the amount
of expensive silicon in solar cells. Solar modules made from SLIVERs can be
lightweight, flexible and transparent and offer imaginative opportunities
for building integration and other applications. By substantially improving
the cost competitiveness of photovoltaics compared to electricity derived
from fossil fuels, slivers have the potential to revolutionise the
photovoltaics industry and simultaneously address the critical environmental
issue of global warming.
SLIVER Technology Research
- A one-square-meter solar panel using SLIVER Cell
technology needs the equivalent of two silicon wafers to convert sunlight to
140 watts of power. By comparison, a conventional solar panel needs about 60
silicon wafers to achieve this performance. By dramatically reducing the
amount of expensive pure silicon, the largest cost in solar panels today,
this new technology represents a major advance in solar power technology.
The unique attributes of SLIVER Cell technology could open many new SLIVER
Cell applications, in addition to conventional rooftop and off-grid uses,
including:
- Transparent SLIVER Cell panes to replace building windows and cladding
- Flexible, roll-up solar panels
- High-voltage solar panels, and
- Solar powered aircraft, satellite and surveillance systems.
Flexible SLIVER modules
- Sliver cells have several properties that make them
ideally suited for use in flexible modules. Firstly, because they are
fabricated from mono-crystalline silicon they have high and stable
efficiencies. In addition, the elongate form factor of the cells means that
when connected in series, system voltage can be rapidly built, at a rate of
5 to 10 V/cm. Hence, battery voltage can be generated in a small area,
allowing cells to be incorporated into small portable electronic devices.
The dimensions of the cells mean they are naturally flexible, particularly
about their long axis, and do not require any post processing for them to be
incorporated into flexible modules. The cells are also lightweight, allowing
high power to weight ratios to be achieved. Also, because of the
non-destructive reverse breakdown properties of Sliver cells, bypass diodes
can be avoided, significantly reducing the complexity of the module design.
CSES is developing flexible photovoltaic modules based on SLIVER cells. The
performance of these module is over 130 W/m2 with a power to weight ratio of
greater than 150 W/kg, a radius of curvature of 5 cm or smaller, and
operating temperatures between -40˚C and +65˚C. Modules can be either
unifacial or bifacial depending on the desired application.
Linear trough concentrator
Semiconductors and devices