Staying Cool in the Sun

Alta builds highly efficient solar material.  To be exact, we’ve set world records for single junction cells at 28.8% conversion efficiency and dual junction cells at 30.8% efficiency under one sun (non-concentrated light).  This means that over a quarter of the light that lands on an Alta solar cell is converted into electricity.  However, efficiency measurements are made in a controlled environment.  We wanted to understand our true outdoor performance and asked the National Renewable Energy Laboratory (NREL) to help us.

The biggest factor that can impact true performance is temperature.  When you put something out in the sun, it gets HOT.  Solar panels can run up to 40°C hotter than the ambient temperature.  And when the air temperature is high (like in Phoenix or Las Vegas) you start to deal with extreme heat.  Silicon solar modules don’t like heat.  They lose 4.0% of their performance for every 10°C of excess heat (over 25°C) due to a poor temperature coefficient.  Therefore, when the sun is the brightest, and it’s 40°C outside (104°F), a silicon module will be operating at 80°C (176°F) and be generating 22% less power than its rated performance.

On the other hand, Alta Device’s solar material has much better thermal performance. In a real world experiment, NREL found that an Alta Devices solar module lost only 0.8% of performance per 10°C over 25°C, and was almost entirely offset by a different positive effect — changes in the sun’s spectrum due to water vapor in the air.  If we go back to our theoretical hot day in Phoenix, while the silicon module loses 22% of its performance, the Alta module continues to operate nearly at its full rated performance number.

NREL saw another interesting effect.  Not only did the Alta technology harvest more energy in these hot environments, it stayed cooler.  This is because Alta’s material does a better job at converting solar energy into electricity instead of wasted heat. NREL found that the silicon module operated up to 10°C hotter in the same environment as Alta’s module.  You can download the paper here: http://www.nrel.gov/docs/fy13osti/57902.pdf

Significant for Roof-Integrated Applications

So what does all this really mean?  We, at Alta, are excited about putting solar in places that are truly portable: directly onto planes, cars, consumer devices, and in people’s hands.  Consider the roof of an automobile.  A midsized sedan can easily support solar cells on a 3 ft by 3 ft area (or one square meter).  If we were to fill the area of this roof with high performance silicon solar cells (at 20% efficient), the roof would generate 200W (in a lab environment).  With Alta’s mobile power technology, the same area would generate 240W (in the same lab environment).  We know that car roofs get HOT in the summer.  At an ambient temperature of 40°C (104°F) and assuming the roof is 40°C hotter than that, that silicon module gets de-rated to 156W. An Alta-based roof, on the other hand, will stay ~10°C cooler, and continue to produce 240W, a 54% improvement.

Not only will more electricity get produced over the course of the day, the automobile roof stays cooler reducing the overall load on air-conditioning the interior of the car.  In addition, the auto manufacturer can receive off-cycle credits from the National Highway Traffic Safety Administration (NHTSA) toward continuously more stringent Corporate Average Fuel Economy (CAFE) standards for installing a solar roof.  It’s a win-win all the way around.

In summary, we now have real-world evidence that Alta’s technology is more efficient, stays cooler, and generates more energy throughout the day when exposed to high heat.  This finding is significant for most outdoor solar installations and particularly beneficial for roof-embedded applications.  We believe this will change the way solar is used.

Why Gallium Arsenide?

We are often asked why we use gallium arsenide (GaAs) to build our solar cells. It’s because GaAs naturally performs better at converting the sun’s energy into electricity than other materials under normal conditions. Further, GaAs solar cells deliver more energy in high heat or low light, two of the most common real-world conditions for solar cells! At Alta, we’ve developed a way to manufacture thin, flexible layers of GaAs that utilize only tiny amounts of material but retain all of the performance benefits of a traditional GaAs solar cell. This allows the solar cells to be cost-effectively incorporated into a wide variety of end products, bringing our vision of “Solar Everywhere” even closer to reality.

While the “layman” may not be familiar with GaAs, it’s been used to build solar cells for over four decades. That’s because the chemical and physical properties of GaAs make it the undisputed choice for high efficiency solar cells. For this reason, the space program has used GaAs solar cells for over 25 years, starting with the Mir space station and continuing to this day, with virtually every space bound vehicle incorporating GaAs solar cells. These types of cells are also used in terrestrial solar concentrating systems because of their ability to efficiently convert the sun’s energy into electricity.

At Alta, we use GaAs for our solar cells because this material has the ability to deliver the highest energy conversion efficiencies, which is a measure of how much of the sun’s energy is converted to electricity. In fact, all of the world records for high efficiency solar cells are held by some form of a GaAs solar cell. The unique properties of GaAs that lead to high efficiency include a direct band gap for efficient conversion of photons to electron-hole pairs. In turns out that the best GaAs solar cells operate very much like an LED, being almost equally capable of converting electricity to light as they are of converting light to electricity. As one of our founders is fond of saying, “The best solar cells also make great LEDs!”

The high efficiency of GaAs is not an artifact of the artificial conditions established under standard test conditions (STC). Under real world conditions, where changing levels of illumination and temperature are common, GaAs is a true standout compared to other materials. In fact, GaAs operates near its full efficiency at levels of illumination that are only one tenth of a sun, a level where most semiconductor materials have long since stopped operating as efficient solar cells. GaAs also has a temperature coefficient that is a mere one fifth of silicon and only one third of CIGS or CdTe. That means that at high temperature, GaAs continues to deliver energy at near its rated output, while the energy output of a silicon cell declines by 30% or more.

The only problem with GaAs is that the material itself is expensive. But another unique capability of the material comes to the rescue. It’s possible to grow extremely thin layers of GaAs that use just miniscule amounts of material, keeping the cost down. At the same time, these thin layers of semiconductor material actually get more efficient as they get thinner. The opposite is true for most other solar cell technologies. And a side benefit of these thin cells is that they are completely flexible and can be incorporated into any of today’s commercially available encapsulating materials. Even better, the flexible nature of these cells opens up the potential for a whole new generation of innovation in solar cell form factors that can dramatically reduce the cost of solar electricity.

We’ve recently released a pair of videos that describe the advantages of GaAs solar cells. Please watch them below:


 

Solar in Any Form

What if you needed a wrench and when you tried to buy one, you found that they were all the same size? How many problems can be solved with a wrench of only one size?  Eventually, you would have to find other ways to solve problems and relegate the single-sized wrench to the class of projects that happened to be compatible.  Much the same is true of today’s solar solutions: they come in one size (large, flat plate glass modules) and are suited primarily to the task of capturing the sun in large open fields.  But what about all of the other places where energy is either unavailable or not cost-effective?  Solving that problem requires “wrenches” of different sizes, and even different shapes.  That’s how we think about solar solutions at Alta.  The “one size” mentality does not fit all problems, and in fact, a wide variety of alternative forms are required to really address the potential for solar energy to address the world’s energy needs in a cost effective manner.

Reducing the cost of the electricity generated by solar starts by building high efficiency solar cells that can deliver high energy levels throughout the course of an actual year.  The most expensive part of a solar energy system are the fixed costs like steel frames and racks to hold the modules, wires to connect them, labor to install the modules, and things like land, permits and electronics.  There are two ways to reduce these costs.  One way is to generate more energy from the same fixed costs.  We do that by using gallium-arsenide solar cells that have higher energy density than any commercially available solar cells.    This essentially divides the fixed costs over more kilowatt-hours generated each year, reducing the cost per kilowatt-hour of the generated electricity.

A second way to dramatically reduce the cost of solar is to eliminate some of these fixed costs.  For example, why not embed solar energy generation capability into the roofing materials of buildings?  Most buildings that are built today require some level of energy efficiency capability such as insulation in the walls and double paned windows to stop energy losses throughout the year.  If solar roofing materials can be made sufficiently cheap, why wouldn’t every building have some ability to offset it’s own energy consumption by converting some of the sun’s energy into electricity?  In fact, incorporating thin, flexible, high energy density cells into roofing materials can eliminate all of the extra hardware, labor and wires that would otherwise be required to install solar modules on a rooftop, dramatically decreasing the cost of the electricity that is generated.

But what about the problem of needing electricity where it simply isn’t available?  Electric vehicles are a good example.  The batteries of today’s electric vehicles must be maintained at a specific temperature in order to maintain their maximum storage capacity and overall lifetime.  What happens when an electric vehicle is left parked in the hot daytime sun?  The cooling system for the battery system will be drawing energy from the same battery it’s trying to cool!  In some cases, this could result in a catastrophic discharging of the vehicle’s battery, destroying the battery.  Why not integrate some flexible solar film into the roof of the electric vehicle and solve this problem?  A car rooftop can hold one to two square meters of solar material.  Using Alta’s gallium-arsenide technology, flexible sheets of solar film can be molded directly into the car’s glass roofing material.  That provides over 500 watts of generation capability on the top of every car roof!

The same technology can be integrated into the wings of unmanned aerial vehicles (UAVs) increasing flight times dramatically.  How about charging that cell phone and iPad we carry everywhere with us?  Flexible sheets that can be right-sized to fit our portable devices could provide tens of watts of power everywhere we go.  And no plug is required!  In the developing world the need for energy is extreme.  In fact, many researchers believe that economic and social prosperity starts by ensuring a ready and adequate supply of energy.  The sun shines all day long – why not build flexible charging mats that can provide kilowatts of energy in an area the size that is no bigger than a small patio?  A ten foot by ten foot mat could be easily rolled up and stored, but provide over two kilowatts of energy generation capability when unfolded and exposed to the sun!  Even a solar matt the size of a sheet from a queen sized bed would provide a kilowatt of generation capability.  In much of the developing world, the primary source of energy generation is from either centralized coal power plants or from decentralized diesel generators.  Why not deploy hybrid diesel systems that use the sun to generate power during the day and only consume diesel at night?  The problem of intermittency goes away completely, while the consumption of costly diesel fuel would plummet.

The solar technology we are developing at Alta is designed to address the solar energy problem by focusing on providing energy where it does not exist today.  Rather than provide a solution that can only be used in one way, we aim to enable a wide range of energy generation solutions that are “right sized” for the problem being solved.  The need might be for mobile energy to power cars, UAVs or portable devices.  Alternatively, the need could be to offset the on-site energy consumption of a building, or to provide cost effective off-grid generation.  In every case, the solution starts with high energy density solar cells, crafted into sheets that can be made into the size and shape needed to solve the problem.

That’s what we call “solar in any form”.