Concentrated Photovoltaics

Recent advances in solar cells for concentrated photovoltaic systems may push solar electric production to new heights and lower costs in the near future.

Background

For years I've wondered why I haven't seen PV solar panels that use a lens to concentrate the sun's energy onto itself.  Solar cells have a very low efficiency overall (25% at best for silicon solar cells), and they produce more energy when exposed to more solar radiation, so why not magnify the solar radiation that each cell sees?

In the near future, I will perform experiments to demonstrate the effectiveness (or lack thereof) of doing this with conventional solar cells, but I can tell you now that it's not as simple as it may seem.

Problems

• Solar cells become less efficient as they heat up, and magnifying the amount of solar radiation that falls upon them will heat them up beyond what is normal.
• Concentrating optics can be heavy and expensive.

To try to overcome this, many researchers are experimenting in a specialized subset of the photovoltaic (PV) field; Concentrated Photovoltaics (CPV).

Concentrated Photovoltaics (CPV) 

CPV is the technology of using optics, like lenses and mirrors, to concentrate our sun's energy onto solar cells in order to increase their energy yield.  A properly-shaped lens or mirror can gather extra light and concentrate it into a smaller space than it would have naturally illuminated.  But the expense of the optics and extra cooling hardware has kept this from being an economically viable option for use with standard solar cells.

An Alternative Approach

So researches have turned their attention to the idea of doing more with less.  If smaller, more efficient solar cells could be made, then combining them with small optics may just surpass the energy production of current cells and overcome the heating issues.

This is exactly what a company called Semprius Inc. set out to do, and it appears they may have succeeded.

Semprius Inc.

Image 1: Semprius Solar Panel PCB (side-view).  Glass-bead lenses
concentrate solar light by 1,100x onto the tiny surface of the gallium
arsenide solar cell below them.  Credit: Semprius

During the winter of 2011/12, the startup company Semprius Inc. broke a record in the solar industry by building a solar panel that can convert light into electricity with an efficiency of approximately 34%.  They did this by using many, very tiny solar cells made of gallium arsenide, instead of silicon, with tiny glass lenses attached to them (see Image 1).

Gallium arsenide (GaAs) is much more expensive than silicon (Si), but is also more efficient at converting light to electricity.  The industry has not seen wide use of GaAs solar cells because the expense of it makes it impractical at the size of existing Si solar cells; leading to their use only in specialized systems such as space satellites and solar-powered vehicles.  GaAs solar cells have been widely used in the space industry because of their higher conversion efficiency, a higher resistance to radiation damage, and ability to operate at a higher concentration of solar radiation; as much as 2,000 suns.


Semprius Inc. realized that by making GaAs solar cells very small (see Image 2) and including lenses that increase natural light intensity by 1,100 times (Image 1), they can produce more electricity per panel than with Si solar cells, and all without the need for cooling systems typically required for CPVs.

Image 2: Semprius gallium arsenide (GaAs) solar cells.   The black square in the middle of each
yellow package is the solar cell, measuring 600 x 600 micrometers; < 1 mm².  That's smaller
than an ink dot made with a ball-point pen.   Credit: Semprius

The solar panel PCB looks and works much differently than standard solar panels.  The solar cells themselves are tiny and, for heat dissipation reasons, are placed far apart (≈1.5 cm between them), so only 1/1000th of the face of the panel PCB actually captures and converts solar energy.  For this reason, the glass on the front of the panel module is formed into lenses and is placed at a specific distance from the panel PCB to direct widespread solar radiation onto the small glass beads that sit atop each GaAs solar cell.  For comparison, a cross-section of a traditional Si panel versus the Semprius GaAs panel is shown in Image 3.

Image 3: Cross-section comparison of a traditional Si PV panel (left) versus the Semprius GaAs PV panel (right).  Traditional panels use large Si-based
solar cells and bathe them in solar radiation through a strong plate-glass face.  Semprius' panels use tiny high-efficiency GaAs-based solar cells fitted
with glass-bead lenses and placed deeper below a face formed into multiple inexpensive lenses.  The multi-lens face plus the glass-bead lenses work
together to concentrate and intensify solar radiation (up to 1,100 x) onto the tiny surface of each solar cell.   Credit: Semprius

Semprius Inc. projects that the panel price for power utility-sized projects may produce electricity at less than the current U.S. average retail price for fossil fuel generated electricity.

The Future

Semprius Inc. was named one of MIT Technology Review's Top 10 Emerging Technologies of 2012 and has raised over $40 million from venture capitalists, Siemens (a power-plant builder), and a U.S. federal stimulus grant.  Semprius Inc. is dedicated to transforming their R&D efforts into production panels from now through 2013.

Sharp Corporation

More recently (May 2012), Sharp Corporation developed a compound solar cell that, when used in a CPV system, achieved a conversion efficiency of a record 45.5%.  A compound solar cell uses two or more layers of photo-absorbing material, with each layer being naturally sensitive to certain solar radiation wavelengths, and passing other wavelengths through to the layer below it.  In this case, Sharp used three layers, with indium gallium arsenide (InGaAs) as the bottom layer.

The size of Sharp's solar cell is over 6x that of Semprius', leading to the likely necessity for special cooling hardware to be employed within the panel.  While Sharp's solar cell is a great advancement in research efforts, it may not prove as practical as Semprius' efforts.

The Downside

Image 4: Semprius solar panels on 2-axis solar tracker.  Credit: Semprius

Unfortunately, there's still a big disadvantage plaguing every concentrated PV system; sensitivity to angle of solar radiation.  The concentrating lenses work best when the panel is facing the sun directly.  Light rays that deviate from the perpendicular (like those when the sun is not directly above the face of the panel, or those that are diffused through cloud cover or other airborne particulates) are nearly useless for CPV systems, dropping the energy output to almost zero.  For this reason, all CPV systems are mounted to solar tracking equipment (see Image 4) that constantly repositions the panel array, on 2 axes, to face the sun as directly as possible throughout the day.  Additionally, a dry environment that yields many days of cloudless access to the suns rays is critical.

I would think that this could be overcome by using parabolic mirrors to realign stray solar radiation, but I haven't seen much evidence of successful experiments in this area.

What This Means For Us

It's too early for such advances to benefit most individual PV system owners/builders, but it's clear that R&D is continuing to push the boundaries of PV technology.  While it's possible this advancement will eventually lead to even cheaper electric rates for the common person (due to lower-cost solar power plants), I think it's just as likely this may be a stepping stone on the way to providing even more affordable, more efficient PV equipment to individual system owners/builders.  All things considered, I'm hoping for the latter outcome.

Resources

For additional reading on this topic, review these resources:

• Technology Review: 10 Emerging Technologies - Ultra-Efficient Solar
• Wikipedia - Concentrated Photovoltaics
• National Renewable Energy Laboratory - Tiny Solar Cell Could Make a Big Difference
• Semprius Inc.
• Sharp - Sharp Develops Concentrator Solar Cell with World's Highest Conversion Efficiency of 43.5%
• The Berkeley Energy Review - Semprius' New Ultra-Efficient Solar Cell Marks a Breakthrough in Solar Technology
• Next Big Future - Sharp Develops Concentrator Solar Cell with World's Highest Conversion Efficiency of 43.5%
• Science in the Triangle - Unleashing the power of 1100 suns
• greentechsolar - CPV Startup Semprius Adds $7.5M to Its War Chest