Sunday, July 13, 2008

Nanosolar

I'm not always aware of which topics are hot at any given time since I don't own a television (hmm - a future blog topic?) although I do get the daily Dagens Nyheter. But I've been curious about a company called Nanosolar for a while and I've decided get some perspective about their technology and claims in fabricating CIGS solar cells, the same kind that I research. The suggestion to do so at this time came from Bengt Axmacher's comment to my post about building a solar charger. If I'm surmising correctly, he's wondering how a company such as Solibro that uses expensive vacuum deposition equipment in a relatively slow and materials inefficient manner to painstakingly produce solar modules one glass pane at a time could possibly compete with Nanosolar's cheap ink printing method using a metal foil roll-to-roll process with 100% materials utilization to produce solar cells that are - apparently - just as good.

Some fact gathering is needed here and I started at Nanosolar's website. Before getting into their technology, I first was curious about the end result: the efficiency of their solar panels (I'm one of those people who likes to find out how the story ends before reading the book). That's apparently a secret - I couldn't find it anywhere even in a broader web search. But they do advertise the efficiency of their absolute best cell as being 14.6% in a paper published March 2006. It was a half square centimeter in size and was made on a glass substrate. Incidentally, March 2006 was also when I fabricated my own record cell of 18.5% efficiency using a process identical to Solibro's. Unfortuneately, it can be very tricky to compare results between two different cells. In this case, the Nanosolar cell was actually measured to be only 13.95% efficient but was calculated to be 14.6 had it not had a top metal contacting grid on it. Similarly, my cell would have been 19.1% if I'd used the same trick of calculating away the contact area. Other things can also affect efficiencies such as antreflective coatings or whether the cells are encased for durability. When making large cells there will be more inherent losses and they are also more difficult to manufacture uniformly. Modules compound the losses due to the interconnects of the individual cells and problems in cell matching. Trying to calculate what the efficiency of a fully assembled solar panel might be based on the results of a weentsy little lab cell is not straightforward. If you take a look at my posting about a solar module you'll see it was 14.8% efficient, i.e. not even close to the 18.5% I got for the single cell. So here goes: I'm going to guess wildly and predict 8 to 9% efficiencies for Nanosolar's fully assembled panels.

On to some technology. The copper-indium-gallium-selenium ink that Nanosolar uses to print their solar cells is proprietary, but apparently it's made of nanoparticles of metal selenides which are printed onto a metal foil which forms one of the electrical contacts. It then goes through a heat treatment of 4 or 5 hundred degrees. Additionally, solar cells require a buffer layer and a top contact to form a structure that I described in a CIGS solar cell. I couldn't find out which method is used to deposit these layers, but Nanosolar emphasizes that vacuum deposition is not it. Hopefully, I'll get around to describing the vacuum deposition method someday since it's the way I make my solar devices.

I'm undeniably intrigued by Nanosolar's process. Vacuum deposition methods are indeed wasteful. Materials usage can only be minimized when huge glass panels are processed since the waste is mostly at the edges. Applied Materials makes thin film silicon on glass panes that are 5.4 square meters - I think it's the biggest in the industry. But vacuum equipment is terribly expensive, takes a tremendous amount of energy to run, corrosive selenium gets into everything and, in my experience, the heaters for the metals keep burning out - I don't just melt copper, I evaporate it at about 1500C. It's difficult to get uniform coating and control the respective quantities of copper, indium and gallium in the resultant films. And this is just for one of the layers needed to make cells. Modules also require patterning steps to form the interconnects between cells. Nanosolar doesn't require this in their process, but instead they have to assemble cells into modules and make interconnects. As far as esthetics go, I think Solibro has the upper hand. I suspect their sleek black panels are easier for an architect to place on buildings.

Back to the original question of whether Solibro can compete with Nanosolar: I conclude that it would be entirely premature to predict the demise of Solibro.

But neither company has yet made a product available to the public.

Thursday, July 10, 2008

The Carpool

I just realized the significance of the empty halls in the lab. Four very particular co-workers are missing, at least for a while. Ventsi has gone back to Bulgaria for the summer, Tomas with wife Ariana to Prague, Per-Oskar is somewhere on vacation and poor Uwe's vacation request has been denied and he's been sent to Solibro in Thalheim, Germany where he's probably the most important person right now to get high efficiency solar modules coming off the production line there. But I'm rubbing my hands with glee. Why? These four co-workers are also the other four members in our carpool. With no one here but me, the car is all mine!

I should explain a bit about how our carpool works. The five of us all bicycle as our main means of transportation but we jointly own a car, the red Skoda Octavia in the pic. In reality Per-Oskar is the official owner having been chosen because he was the one able to get the cheapest car insurance despite being the youngest at 27. He's also the only Swede (hmmm... this couldn't possibly be correlated to getting a good deal on insurance, could it?).

The way it works is fairly simple. We maintain a group on yahoo where we can book the car as needed. We have no quotas since any conflicts are easily resolved by a trip down the hall at the lab. In the car we keep a journal of our driving and we pay 16 crowns per mil (this is Swedish for 10 kilometers). Gasoline is paid for with credit cards drawn on the carpool bank account which is maintained by P-O. Then we share all other car costs such as maintenance, insurance and the very expensive Uppsala parking which allows us to park at the lab - you didn't think that employees were allowed to drive to work and park in the lot for free, did you? The two car keys are kept in my office to which everyone has their own key.

The car's upkeep is shared, of course. Tomas and Uwe are particularly good at fixing things on it. Tomas is actually not the official member having given that honor to Ariana. He's also very willing to let her do all the driving but I believe this is due to the fact that he has no license and has never, in fact, learned to drive. When it's been time to change summer/winter tires I have always managed to get out of helping by offering to cook dinner for the other members. Tire changing happens at my house since I store the tires there. P-O handles the book keeping and gets us to pay up whenever the account becomes empty. A quick look at the log book reveals that in the first six months of this year the car has been driven 6800 km. So, on average we've each driven 6800/5 = 1360 km. In reality, we're often teaming up with each other when we go shopping or go on weekend trips since we're friends and not just carpoolers.

Now that I've gleefully used the car all week for my 5.5 km commute, I'm realizing that I very much miss my bicycle. I'm now starting my fifth year as a carpooler and have become entirely used to bicycling in all kinds of weather. When the weather gets too extreme - my personal temperature limit is -15C in winter - there's usually, but not always, a bus. Otherwise a quick call to a fellow pooler to come rescue me from someplace usually solves the problem.

Saturday, July 5, 2008

Building a solar charger

It's July and this has special meaning for Swedes. They disappear. Where once the town was vibrant with the comings and goings of students, it's now quiet with a preponderance of people who are, well, over 40. Many shops have shut and signs of "sommarstängt" abound. At work, the doors that are normally open now require a passcard and code to enter. Many co-workers left on Friday with wishings of "trevlig sommar". So what about the rest of us who chose to stay on and continue working in the silent halls of the lab? While I don't know what everyone else is up to, I can describe Uwe's and my just-for-fun project: building solar chargers.

Previously I've described a solar module. I've made many modules during my years at Ångström, but almost all have been used for some very destructive tests to see what they can tolerate. Last fall, I decided to make a batch of 20 modules that I would keep for fun. I ended up destroying 5 or so while trying to sandblast the edges to remove areas where the modules would short-circuit themselves and while trying to glue protective cover glass on them. Some modules were duds, but I think there are about 10 left for fun projects.

To get started, Uwe and I needed to buy a couple of things. I headed to a store called "Gallerix" where I bought a picture frame. This will house the module and it also has a stand for support. Uwe went off to "PC City" to buy a cell phone/ipod charger kit designed to be powered by the 12 volt outlet in cars aka cigarette lighter. The output of the charger needs to be a regulated 5 or so volts. Uwe guesses the charger to be about 85% effective - in other words, it will sap 15% of the incoming power. Our financial outlay: 50 crowns for the frame, 100 for the charger kit and one stolen solar module .

Next, Uwe gutted the charger to get at the electronics inside. We measured the module under the solar simulator to see what kind of ouput it has: 11.8% efficient, with 12.3 volts and 76 milliamps at maximum power output. Uwe said we needed a larger capacitor on the output side to prevent the charger from going into short-circuit mode. I included a picture of the I-V curve with the charger electronics and the new capacitor to be installed for the benefit of my geekier friends.

With all the parts in place, the rest was assembly. Uwe replaced the capacitor, soldered a couple of wires from the solar module to the charger and soldered the connection cable that came with the kit on the output side. To house the electronics, I cut the soft plastic packaging that came with the kit and we glued it with the electronics inside to the back of the picture frame. We also stuck on a wire holder so that the cable can be neatly coiled when not being used. A quick test under sunlight revealed that, yep, the thing works. The voltmeter reads 5.4 volts and the little led charger light is lit.

The next day, today that is, I was eager to see whether the module was worth the effort. A quick check of the connectors that came with the kit revealed that I didn't have the one needed to charge my digital camera which takes a USB connector. Nevermind, I did get the connector to charge my cell phone, a Sony Ericsson K220i. The cell phone indicates battery life in fourths. Great - only one fourth left! I headed to my covered veranda to point the solar module into the sun while keeping the phone itself out of the sun and, possibly, out of the rain. The immediate indication from the phone was good - when I plugged it in, the display showed "charging". I spent the afternoon checking progress and repositioning the module while writing this post. The weather varied with a couple of hours of sun and then clouding up. I finally brought it in a half hour ago. It's completely cloudy now and the phone no longer indicated it was charging even though the little led on the back of the module was still lit. Result: the battery's 3/4 charged now.

I'm pleased.

Tuesday, July 1, 2008

Ocean waves and wind

On Friday I attended a symposium on water, wind and wave energy at the Ångström Lab. I have to admit I rarely set foot outside the Ångström Solar Center where we research CIGS thin film solar cells - although I did put in an appearance a few weeks ago at a barbecue hosted by the Grätzel aka dye sensitized solar cell group.

There are actually many interesting things happening in the Ångström building. A couple of floors up from me is a guy, Josh Thomas who is building a better battery. Incidentally, Daniele got to interview him as part of a physics course and came away with the opinion that scientists like to explain things in detail well beyond the interest of an ordinary listener (I believe that this advice was also directed at us in the solar cell group). Then there are the space researchers making microthings that went up with the Rosetta space probe, the nuclear and particle physics researchers, the astronomers and their observatory at the south end of the building (I'm at the north end), Scandinavian Biogas of which I know nothing and the electrochromics group and their spin-off company Chromogenics making things like visors to motorcycle helmets whose transparency can be electrically controlled.

The two groups of interest at the symposium both involved energy - which is why I attended - and there are two spin off companies involved. One is is called Seabased http://www.seabased.com/engelsk where they manufacture linear turbines that they've developed such that ocean wave energy can be collected. The turbines had to be developed for the low frequency motion of waves - there are about 15 waves per minute. The company is now building a pilot installation in the sea near Gothenburg to provide energy to 20 homes on the island of Lysekil. They have generated quite a bit of interest which translates into financing from the electric company giants. Essentially they install an array of buoys in the sea that are connected by a rope to generators which will collect and transmit energy to land. Waves move the buoys around causing the generators to produce electricity. Seabased also likes to point out that their wave energy farms can double as fish havens to protect fish from those evil fishing boats. Of interest to me as a solar scientist is that they directly indicate how superior wave energy is to solar or wind: they claim wave energy can be utilized 4 to 5 thousand hours per year as opposed to 2 for wind and only 1 for solar in Sweden. While it's nothing new that solar is a summer thing in Sweden, what was surprising for me was the need for non-solar alternative energy technologies to compete with solar. I had naively assumed we were all partners in seeking global energy solutions.

The other company is called Vertical Wind http://www.el.angstrom.uu.se/frameset.html?/forskningsprojekt/vind_E.html. They have a home built test wind turbine situated near Uppsala with 5 meter long blades. This is a vertical axis turbine and thus doesn't require pointing into the wind - most wind turbines in the news are horizontal. The generator is at the base of the tower and the turbined is designed to operate at lower frequencies than the traditional horizontal axis wind turbines. Not surprisingly, the turbines were developed by the same group responsible for the Seabased turbines.

There is actually another company under development along the same lines of thought as the first two: Current Power. They have what look like miniature versions of the wind turbine that are placed not in wind but under water to collect energy from water currents. I found a site that illustrates all three companies http://www.energypotential.se/portfoljbolag.html. Even though it's in Swedish, it's got a few diagrams of interest.

The discussion of water power I didn't follow much. I think it was included because the major investors are the electric companies Vattenfall and Fortum. They like to tout hydro since it generates much of Sweden's electricity.