No one knows how many of the vast array of PV technologies currently under development will prove to be viable. Especially if commercial realities hit home and funding dries out before feasibility has been demonstrated. Some will be spun out into start ups…and then fail. And then a few more will survive, and begin the difficult journey towards commercialisation.
But regardless of the prospects, enthusiasm and ambition for this technology knows no bounds, as the following roundup of what's hot in research demonstrates.
Dr. Seth B. Darling is a Scientist at the Center for Nanoscale Materials at Argonne in the USA. His team are developing a new type of solar concentrator that is based on inorganic semiconductor quantum dots.
Quantum dots are a small chunk of a semiconductor – on the order of 2-10 nanometer (nm). These hold some advantages: They are highly stable and have a high quantum yield so they can be very efficient at absorbing and re-emitting light. The inorganic quantum dots don't have the re-absorption problems of organic dyes, and as a bonus, Darling says, they are very tunable – i.e. the dots can be tuned to work through all parts of the solar spectrum.
“This new quantum dot allows us to make a greater separation between absorption and emission properties,” he says: “We can tune those two independently to absorb light from virtually any part of the spectrum, then tune the emission so that it doesn't overlap with the absorption”.
Darling's team is doing a Monte Carlo ray tracing simulation, where the path of photons travelling through the slab can be tracked in a computer. Using this method, the team can see how many photons get captured by the PV cell at the end. This supports the best optimisation of all parameters to find the optimal system design and get the most light to the cell.
“Creating quantum dots is all chemistry,” he adds. “Precursor inorganic materials can be purchased in fairly large quantities, and they are mixed together with various organic molecules in solution. There they grow very small nanoparticles of inorganic materials – that have a core of inorganic material capped with organic molecules. This is a very low cost process, and yet highly scalable,” he adds.
The quantum dots are mixed with a liquid polymer before it is set. Then it is poured into a mould in the desired shape. “Not only is this very low cost, it works in diffuse light, and there is no tracking needed,” Darling says.
“We can take this idea and go further with it by making it into a multi-layer stack. Then you can split up the light spectrum into different pieces. Transparent concentrator slabs cover the same area that expensive PV panels would, and only smaller solar panels on the side are needed. What you're really getting at is the geometric gain ratio:
“How much area are you collecting light over vs. how many PV panels? Roughly we are talking about a ratio of 20 to one.”
|“This quantum dot allows…a greater separation between absorption and emission properties…”
|- Seith B. Darling, Argonne Lab
Optimising CdTe at CSU
Dr. Walajabad Sampath, Mechanical Engineering Professor at Colorado State University, teaches classes like any other professor, but he also leads a double life. He developed the research and technology that led to the creation of Abound Solar.
Today, he is still hot on the scientific research trail to develop better and more efficient Cadmium Telluride (CdTe) thin-film cells: “A CdTe cell has a layer of cadmium sulphide and a layer of cadmium telluride, but there are compounds in this family that [have three and four elements] and so forth,” he says. “If you think of making layers with these, and not just the simple binary types, then there are many opportunities to improve efficiency,” he explains.
Sampath's scientists are currently working on a machine to deposit these new materials for testing. “The materials are not something we need to do extensive research on. It's more the processing step for final testing,” he says. “If we succeed with this it will [add] a simple modification to [an existing] CdTe production line. And it could increase cell efficiency by up to 30%, and bring down manufacturing costs down by 50%.”
Nanotubes at GIT
Jud Ready, Ph.D. Sr. Research Engineer & Adj. Professor at the Georgia Institute of Technology is developing a 3D PV cell that uses carbon nanotubes as a support scaffold and back electrical contact for the cell: “This structure offers significant ‘light trapping’ capabilities and can be used in various PV systems including CdTe, CdSe [cadmium selenide], a-Si [amorphous silicon], and CIGS [copper indium gallium selenide]. The benefit of this 3D structure is that it ‘orthogonalises' carrier extraction and light absorption to benefit both – whereas planar systems must optimise one, typically to the detriment of the other,” he says.
Ready says that textured PV cells have been around for many years, but previous technology used an etching procedure to remove material from the wafer. “Our technology was the first to demonstrate this texturing and light-trapping structure in an additive manner.
“Due to the light trapping structure,” he adds, “I see [that] this [could be] used in numerous PV markets; for example transportation where a fixed or tracking array…would offer benefits but is not fiscally viable in terms of cost [residential], or in terms of weight/maintenance [aerospace]”.
|A few will survive, and begin the journey towards commercialisation…
Solar window on the world
New Energy Technologies, based in Columbia in the U.S., develops alternative and renewable energy technologies. It believes it may have a winner on its hands with SolarWindow. Why? Because all of the well-known PV technologies today have one basic thing in common – they all require sunlight to produce electricity.
But SolarWindow technology does not. It is a coating which produces electricity in both natural and artificial light; sunlight or incandescent and CFL type lighting.
“We're going to let all the other people who make all the other types of modules fight it out on the roof, we take our technology [for] the windows”, says John Conklin, president and ceo. “If you look at large commercial buildings in big cities, most have very limited roof space and often there are other fixtures such as HVC and electromechanical installations taking up much of the space that is available. But they also have a tremendous amount of glass in their structural design.”
According to Conklin, the material is bi-spatial. The coating on one side of a window will collect power from sunlight, and on the other side it will collect power from interior lighting. This two-faced power generation offers a whole new method to offset electricity costs. It hooks up to an inverter the same as any other PV installation, wired through the walls to the building power supply.
“We are in the process of evaluating performance and testing various environmental and basic life cycle conditions. So, we will be reporting on the data as we get it in. But the film is covered. It is protected from direct contact after the active layer is applied. We will be better able to report when we have the data,” he adds.
Solar 3D, from Santa Barbara in California, is a company with a new technology borrowed from the Fibre Optics industry. According to Jim Nelson, president and ceo, the current 2-dimensional technology is simply not well engineered either for efficiency or for cost effective production: “Our 3 dimensional technology is developed to get greater efficiency out of the solar cell, as well as make it more production friendly”, he says.
|“If we succeed with this it will only be a simple modification to any CdTe production line, not a whole new thing."
|- Walajabad Sampath, Colorado State University
Nelson says that what the company is working on is effectively a new type of solar cell: “We are really reengineering the whole thing,” he says, “not starting from scratch, but taking current PV technology and making it much more robust in terms of its ability to absorb energy from the sun,” he says.
Nelson says that the technology is essentially materials agnostic. In other words, he claims the company will be able to work with any basic format:
“There are two ways of getting better at absorbing the power of the sun. One is the material itself, and there are great strides being made by many companies in this area. The other side is creating a better structure. That is what we are doing.
We eliminate reflection off the solar cell, and also create more space in a defined area. It is basically a total light trapping technology. Once the radiation enters the cell, it can't get out. It is all used to create power. It happens in the same footprint and essentially enhances the power that the footprint can absorb,” he adds.
The basis for this technology comes from the company's scientific adviser, Dr. Nadir Dagli, one of the pioneers in optic technology. He developed a method to use fibre optics for this light management technique for solar cells: “Using his technology, we can manage the direction we take in the light, and capture it within the structure,” Nelson says. The company is developing a prototype, and in the meantime key milestones have been set to guide the efforts of the development team in 2011 including:
- The design of the light-trapping element of the solar cell;
- The determination of its expected efficiency;
- The design of the 3-dimensional micro-photovoltaic structure of the cell;
- Fabrication of the prototype.
Jerry Buchanan, Honeywell's global business manager, says that the challenge to the module industry is to reduce a module's cost and improve its performance. He believes that the company's PowerShield PV270 starts to address some of those issues. However, he says that his long-term view is that total PV packaging plays a critical role in the performance of any module: “Once a module is put together, it's the packaging that protects it for 25 to 30 years.”
The biggest challenge when looking at packaging, he adds, is finding the right solar modules to work with. “We supervise and are very careful regarding how we select companies and what we work on,” he says. “Crystalline is the long term contender, but we haven't lost sight of CIGS, CIS [copper indium selenide], CdTe, and we think there are some real challenges there.”
This extract is taken from a longer article published in the November/December 2010 issue of Renewable Energy Focus U.S. magazine.
Joyce Laird has an extensive background writing about the electronics industry; semiconductor development, R&D, wafer/foundry/IP and device integration into high density circuit designs.
Renewable Energy Focus, Volume 12, Issue 1, January-February 2011, Pages 48-50