Solar cells manufactured by Spectrolab have powered more than 500 spacecraft launched over the last 50 years, and the company is now starting to see greater dividends as its technology becomes more widely adopted for generating electricity on Earth.
Sylmar, Calif.-based Spectrolab is the world leader in solar cell production for satellites, with some 70 percent of the market share, according to David Lillington, the company’s president. Spectrolab provided the solar cells for the international space station and the Mars rovers Spirit and Opportunity, and recently the company delivered the panels for NASA’s Juno mission to Jupiter, which, if successful, will be the first solar-powered satellite to orbit an outer planet. The company is a subsidiary of Boeing Defense, Space & Security of St. Louis.
For the past 15 years, most space missions have used multi-junction solar cells, which are built with several layers of material that convert a different part of the sun’s light spectrum into electricity. Spectrolab since 2004 has been producing triple-junction UTJ solar cells for satellites that convert about 28 percent of the sun’s energy into electricity; the company is now taking orders for its next-generation XTJ cells that will enter production next year and feature nearly 30 percent conversion efficiency.
For the last decade, Spectrolab has been selling modified versions of its solar cells for use in terrestrial power-generation systems. These cells are used in concentrator photovoltaic (CPV) systems, which require sunlight to be highly focused and magnified onto the cells to work properly. The company now sells terrestrial cells that are 38.5 percent efficient, and in early 2011 will begin producing cells that are 40 percent efficient.
At 40 percent efficiency, CPV systems in sunny regions of the world will be able to produce electricity at a cost that rivals traditional methods of electricity generation such as coal and nuclear reactors, Lillington said. The more efficient cells have garnered great interest, and Spectrolab is projecting its terrestrial solar cell production to increase threefold next year.
Lillington spoke recently with Space News staff writer Turner Brinton.
What is your current level of production for solar cells?
The space business is about 70 to 75 percent of our total revenue. We manufactured between 220 and 250 kilowatts of space solar cells last year. If one assumes about 10 kilowatts per satellite, that’s 22 to 25 satellites a year. We are on track to produce a little more than that this year and remain flat until 2013 or 2104.
On the terrestrial side, we delivered about 22 megawatts of CPV cells in 2009, and we expect to deliver about 30 megawatts this year. The terrestrial demand could be as high as 100 megawatts in 2011, which is basically due to the increasing maturity of the CPV business as a viable product for utility-sized systems. Business was a little bit slow to mature in 2008 and 2009 because of the global recession, but we’re actually finding our customers are getting real traction in the utility business right now. A number of them are ramping themselves up to deliver products for utility-scale systems in the multi-megawatt range.
How do you see space solar cells evolving over the next decade?
All of our triple-junction cells have been evolutionary technologies, where we’ve made small improvements to the cell design to minimize parasitic losses and small changes to improve the radiation hardness of the product. We’ve just completed qualifications of our XTJ cell that we will start manufacturing at the beginning of 2011, and we consider the XTJ to be the end of the evolutionary technologies.
The technologies we’re working on beyond that are revolutionary and should take us all the way up to 37 percent efficiency by the end of the decade. For instance, there are a whole series of cells called inverted metamorphic structures. In simple terms, a metamorphic structure is one in which you intentionally change the lattice constant in some of the layers to make the optical properties more efficient in converting sunlight. There is further technology and manufacturing development that needs to be done to be able to make those cells reliably at high volume and affordable cost.
Why are terrestrial solar cells more efficient than space cells?
There are two reasons terrestrial cells are higher in efficiency than space cells. The spectral content of terrestrial sunlight is different from space. Sunlight is much bluer in space, and it’s a little bit harder to convert that portion of the spectrum into electricity. Therefore, at a one sun concentration, a terrestrial cell will always have a higher conversion efficiency. The other reason is that it’s a basic law of physics that as you increase the concentration ratio, or light density, falling on a solar cell, the voltage on a cell increases logarithmically with intensity, so the power output goes up. There are certain limitations to that. For example, as you increase the intensity, other parasitic losses such as resistive losses can happen, but if you design the cell correctly, as we do, one can actually increase the solar cell efficiency substantially if one operates at 1,000 suns intensity.
How are CPV systems different from other types of terrestrial solar cell systems?
There are many types of terrestrial solar cells. There are thin film modules that convert about 10 percent of sunlight into electricity. There are the crystalline silicone modules that you typically see on bus stops and rooftops that convert 15 to 17 percent of the sun’s energy. These are relatively mature technologies, and it’s hard for me to see how those will ever improve much in efficiency.
CPV systems require direct sunlight because you cannot concentrate diffuse sunlight. Our technology is put into systems that need to track the sun all day. So they’re less appropriate for rooftop applications, although we do have some customers with designs that are suitable for mounting onto a flat industrial rooftop. Generally our products go into large-scale systems on large areas of land. With CPV technology, there are opportunities to increase the number of junctions in the cell and benefit from a number of space technologies to increase the efficiency to 45 or even 50 percent.
What has prevented solar cells from becoming more widely adopted for terrestrial energy generation?
Ultimately, photovoltaic power generation has to be competitive on a standalone basis, not only against coal and gas and nuclear, but also against other renewable technologies like hydroelectric, geothermal, wind and even solar thermal. In the past, solar has been enabled by investment tax credits, feed-in tariffs and renewable portfolio standards. But at the end of the day the technology has to be affordable on an unsubsidized basis.
I do think we’re getting closer to that point than we were years ago, and particularly with our technology, there’s quite a lot of runway left for efficiency improvements. I think we’re coming to a point where in many parts of the world electricity from photovoltaics can be competitive on a standalone basis with competing technologies. The 40 percent cell we’re producing next year in conjunction with a cost-effective CPV module will really open up the market for CPV with electricity prices around 10 to 13 cents per kilowatt hour, and we’re actually seeing that in terms of anticipated demand for 2011. One has to remember the cost of our cell is only one-tenth of the overall cost of a system, but everything we do to increase efficiency results in a direct increase in power output from the whole system.
In space, you have only one primary competitor, Albuquerque, N.M.-based Emcore Corp. With so many nations now building and launching satellites, why do you suppose you haven’t faced new competition?
We always try and give the customer what they want and deliver the latest in technology. Although we’re owned by Boeing, we’re a merchant supplier to the industry, and we’ve always had a policy of giving all of our customers globally the same state-of-the-art product. Additionally, our whole strategy is based around technology turnover. We’ve been successful in being first to market with the highest-efficiency solar cells ever since the early to mid-1990s when we developed multi-junction technology. We’ve been first to market with the single-junction cell, the dual-junction cell, and a number of iterations on the triple-junction cell. We always try to provide best value to our customers.