My previous post discussed the erroneous reasoning that has led some commentators to believe that there’s a Moore’s Law for solar; there isn’t. Electrons are a massive commodity industry, and the only way solar energy will gain any significant market share is if becomes cost-competitive with natural gas and coal power plants, and is able to scale into the hundreds of gigawatts. Clay Christensen echoes some of these sentiments in his article here. Let’s see what the prospects of that happening by the end of the decade are.
I will argue that no new science is required for this to happen. Science is hard and costly – as the Solasta story attests. We know how to make tiny, high-efficiency thin-film solar cells in the lab from compound semiconductors like CIGS and CdTe, but manufacturing them reliably requires years of tweaking and optimization. The two most promising CIGS companies, Nanosolar and Miasole, have had disastrous results. After a decade of work and over a billion dollars in funding, Nanosolar’s latest valuation is a paltry $50 million and Miasole was just sold for $30 million to a Chinese firm. Forget about third-generation quantum dots, polymer solar cells, nanowires (I’ve heard they’ve had reproducibility issues), and “pyro-nano-quantum-thingamajig“s. Even if we had a breakthrough, solar investors are risk-averse and will want to see 20 years of field data before any technology becomes truly “bankable.” Furthermore, new manufacturing technologies are just not cost-competitive against silicon, which has a massive scale advantage. We already know how to mass-produce ~20% efficient silicon solar cells and assemble them into modules at a price of 70 cents/Watt-peak. Silicon is probably the single most-studied element and material in the world, thanks to its half-century history role at the center of the trillion-dollar semiconductor industry. Not only is it a single, well-understood element (compared to a crazy phase diagram for CIGS), it’s one of the most common elements in the earth, and we’re already refining over 200,000 TONNES of it annually.
What is needed is engineering to improve the cost of traditional silicon solar technology by a factor of 2 – that’s it. Let’s begin by analyzing some up-and-coming technologies that can reduce the cost of silicon solar cells, and then look at balance-of-system costs to see whether we can achieve this target. All of the approaches mentioned here involve ways of reducing the kerf losses of the silicon wafer. The kerf is the “sawdust” created when bricks of silicon are put through a wire saw to cut them into thin wafers of about 180 micrometers in thickness. Because the wire itself is 120 microns thick, and the sawing results in micro-cracks in a layer of the wafer 10 microns thick on either side, you end up with losses of 120 + 20 = 140 microns, or close to (140/(140+180)) = 44%. The industry is moving to thinner wires coated with diamond abrasives, but as you make the wafer thinner you enter what I call the silicon wafer valley of death – a wafer with a thickness in the range of 160-60 microns is just to brittle to survive a typical fab process. At around 50 microns, the wafers are flexible.
A number of companies are pursuing kerfless wafers. 1366 Technologies, spun out of MIT, is one of the most interesting. Their “direct wafer” technology apparently uses just 3 grams of silicon per watt of output, compared to the current industry best, which is 5.3 grams/watt for SunPower’s cells (down from 13 grams/watt in 2004). Based on my estimates, though, reducing the silicon usage by half will reduce the module cost by 25%, and the entire system (and thus the LCOE) by only 12.5% (as each is typically half of the cost of the higher level system). only results in a previous ribbon solar throughput. Their capital expenditure looks good, though at about 33 cents/watt of capacity. Seeing as how Ely Sachs previously invented the string ribbon technology that never took off (likely due to low throughput), he’s hopefully taken the lessons learned from that and applied it to the new design. So how does it work? Despite multiple press stories where we learn that “company officials will not say just how they do that“, investigative journalism doesn’t seem to be the forte of the New York Times. For if they had bothered to do a bit of research, they would have uncovered the masters thesis of one of the company’s employees, along with two patents (here and here) that go into great detail about the process. In the first step, a square-shaped chuck makes contact with a melt of silicon, and pulls away after a thin wafer has been formed on its surface. To remove the wafer from the chuck, the chuck has tiny holes in it through which a pressurized gas is blown to release the wafer. Because the wafer was crystallized so quickly, however, it’s got extremely poor quality. In order to improve its quality and grain size, its recrystallized by encapsulating it in a thin layer of silicon dioxide or silicon nitride, and then sent through a furnace which re-melts the silicon and cools it down slowly. The encapsulant is then chemically removed, and voila – a kerfless silicon wafer! How they’ve managed to get carrier lifetimes of 20 microseconds from such a process where impurities at the interface would be contaminating the crystal is quite impressive. Oxidizing silicon wafers typically is not a very cheap process, as it requires high temperatures and quite a bit of time, but using silicon nitride deposited through CVD could be a cheaper route. Here’s a prototype of the recrystallization furnace from Eerik’s thesis:
The other really fascinating approach is that of exfoliation, which is being pursued by Twin Creeks Technologies (whose gigantic Hyperion machine is pictured below) as well as Silicon Genesis. The wafers are placed in a vacuum where they are implanted with high-energy protons which induce cleavage of the wafers. Not only have they managed to create a tool with a throughput of 6 MW/year, but they’ve figured out how to support a 20-micron thin wafer so it’s compatible with existing production lines and do light-trapping on it as well. If the expensive tool, which promises productivity gains of 12X, gets orders from large Chinese producers, this could be the future of the industry.
IBM and Astrowattare trying to cleave wafers as well, but are doing it through a wet chemistry approach that IBM calls spalling. They both seem to be doing electroless nickel plating, followed by an annealing step to incorporate hydrogen ions into the lattice, followed by a lift-off of the nickel layer (along with a thin layer of silicon).
A few other approaches are in the very early research stages, but are nonetheless interesting. Solexel has made some progress with porous silicon technology licensed from IMEC, which grows crystalline silicon from silane gas through CVD. Crystal Solar is trying a similar approach on etching silicon wafers to create pores, then doing epitaxy, depositing a ceramic slurry, and then exfoliating a thin layer – efficiencies are still low, though. Others include a liquid phase epitaxy approach being pursued by Ribbon Technology, a pretty unique continuous sheet crystallization method called horizontal ribbon growth underway at Varian, and a method analogous to float glass production where the silicon floats atop a layer of molten tin at CMU.
On the manufacturing side, there are a number of existing techniques that could be fine-tuned and developed to make incremental improvements in cost. These include replacing the costly silver paste used for metallization on the front of the cell, shifting production to “quasi-mono” ingots with higher efficiency, and automating the tabbing and stringing of solar cells.
The solar industry consumes a lot of silver – almost 11% of the world’s supply. It’s become extremely expensive recently, “adding $23.52 to the cost of each panel,” based on some estimates. There have been many attempts to replace silver with copper, typically by using a thin layer of electroless nickel as a diffusion barrier, with copper electroplated on top through a self-alignment process. Due to the low throughput of the process compared to screen printing silver paste, I believe SunPower is the only company that has adopted this so far.
It was surprising to learn that many of the largest Chinese manufacturers still have not automated the stringing of individual cells in series. Suntech must not have showed this part of the factory to Steven Chu, who said during a tour that “It’s a high-tech, automated factory. It’s not succeeding because of cheap labor.” Suntech claims that the machines break more of the fragile solar cells than humans do, but how can this process ever scale? Hopefully with metal-wrap-through solar cells where all the contacts are on the back, some manufacturers will be able to automate this process without breakage.
(courtesy Technology Review)
The third technology that is starting to be adopted is directional solidification of ingots, known colloquially as “quasi-mono” because of its larger grain sizes than multicrystalline ingots. This results in higher efficiency, but at a lower cost than if one had used CZ crystal pullers to make a single crystal. Also known as vertical gradient freeze, the technique extracts heat from the crucible so that the silicon freezes over time in one direction, at a rate of about 1-2 cm/hour. Some of these furnaces (from the likes of GT, AMG, ALD, PVA, ECM, Centrotherm, JYT, Jing Gong Technology, and JingSheng) can take in a charge of 650 kilograms at a time and achieve a throughput of 10 MW a year. The cost breakdown of one of these furnaces per run is about $176 for 4,000 kWh of electricity, $27.5 for 60 cubic meters of natural gas, $1,248 for a fused silica crucible (such as those made by Ceradyne, Vesuvius, Sinoma, and Saint Gobain), and $700 for 20 hours of labor. In the end you end up with quality close to that of CZ ingots ($24.5/kg) at the price of multicrystalline ingots ($10/kg).
If you look at the non-cell costs of a solar module and normalize them by area, you end up with a breakdown like this, totaling $31/square meter (left graph). Including the solar cell, you have a breakdown of costs like so (data is from 2012, right graph):
Finally, let’s take a look at balance-of-system (BOS) costs, which include the inverter, racking and mounting hardware, installation labor, and permitting. It’s clear that the industry has reached an inflection point, where the cost of the solar module itself is now less than 50% of the total installed cost of a system. For example, First Solar estimates that in 2014 its module cost will be $.63 and BOS costs at $.98/watt. Clearly there is more fat to be trimmed from the BOS side than the module itself, especially for smaller residential and commercial systems. It’s certainly possible, as permitting costs for installations in Germany at less than 1/10th of what they are in the U.S. While companies like Clean Power Financeare working on streamlining the red tape around structural permits in different counties, it would be much more helpful if states simply removed the need for permits and inspections for small installations, like the Vermont legislature has done.As an investor at NEA (investors in Qbotix, formerly known as Black Swan Solar) statedearlier this year:
If you look at solar, cell prices are plummeting upstream. That means the cost bottleneck is going to shift into the balance of system, not the [solar] cell and module, but all the other stuff: the tracker, the inverter, etc. That has been a real issue. They haven’t materially gone down. [Because] the cell and module costs have plummeted, a much bigger percentage cost is now balance of system costs
The DOE’s Sunshot program has espoused the “plug and play“ paradigm, as has the Rocky Mountain Institute’s study – but I believe more work is needed on lower cost racking and installation. Installing panels one at a time on solar farms is difficult to scale – a tiny solar farm of 45 MW in capacity requires almost half a millionpanels to be installed over an area of 500 acres. By hand. I’d also like to point out that the oft-cited graph of when we can expect grid parity in different countries is over-simplified because it does not account for the differing costs of installation labor in those countries.
Ultimately, the most significant impact on the adoption of solar may not come from technology at all, but new financing models such as securitization by banks and startups like Solar Mosaic. Even if we get to $1/watt installed costs, the world is not going to switch from coal to solar overnight – it’ll still take decades, as Vaclav Smil has pointed out. As with any energy technology, the factors limiting the speed of growth will be the cost of capital and the time involved in working though a thicket of regulations around power lines and land use. As Dick Swanson has mentioned, “For the large-scale systems, you can’t just snap your fingers and install hundreds of megawatts overnight.” It doesn’t sound like solar is going to be able to scale to meet the challenges of global warming anytime soon.