Multi-junction solar cells based on gallium and germanium have achieved solar to electricity conversion efficiencies of 46% in highly concentrated sunlight and an efficiency of 34.5% in ambient sunlight. However, these types of solar cells remain extremely expensive, and even the using highly concentrated sunlight which reduces the amount solar cell area required by a large factor, these cells have been unable to complete with the falling costs of conventional silicon solar cells.
A collaboration between researchers at The MASDAR institute of the Unite Arab Emirates and MIT has developed a dual junction solar cell which consists of a gallium arsenide phosphide cell grown on a silicon germanium substrate which is then bonded to a silicon substrate which acts as the bottom cell in the tandem cell design. The silicon germanium underlayer gives good performance properties to the gallium based top cell but blocks light from the bottom silicon cell. In the step cell design a patterning process is used to etch away part of the top cell exposing the silicon beneath directly to incoming solar radiation.
A news articlehas been published by the MASDAR institute describing some aspect of this dual junction cell design. An abstract of a Journal of Applied Physics paper about the theoretical performance limits of this cell design (38.7% max efficiency) is also available. The article and the abstract leave many puzzles in my mind about the physics of this cell and about the sources of cost reduction relative to the more usual designs of multi-junction gallium based cells.
Nevertheless the researchers are sufficiently enthused about the economic potential of this cell design that they are planning to create a startup company to try to commercialize it. Presumably the market will be in the currently moribund area of concentrated photovoltaics (CPV), which uses concentrating optics and dual axis trackers to produce electricity from high efficiency PV designs.
A hope has long existed that photovoltaic cells produced by thin film deposition techniques might economically outperform PV cells based on bulk crystalline silicon which is an energy and capital intensive commodity. However, bulk crystalline silicon cells have continued to improve in cost and performance and have so far managed to keep far in front of the thin film pack in terms of production volumes (i.e. Crystalline silicon cells still constitute 97% manufacturing volumes). It is true that the thin film PV company First Solar which manufacture cadmium telluride PV cells was the first company to break the $1/watt cost barrier for PV cells. Cadmium telluride cells are the second most used PV technology behind crystalline silicon, having capture 5% of the total market. However, the supply of tellurium, which is an extremely rare element, will probably limit the total market share of CdTe cells. Furthermore the extremely toxic nature of cadmium is keeping CdTe cells out of the rooftop and building integrated PV (BIPV) markets.
CIGS (Copper, Indium, Gallium, Selenide) thin film solar cells have been under intensive development for several decades as a potential low cost, high performance rival to crystalline silicon PV, but have so far failed to deliver a cost effective product. A recent article in published on solarserver.com implies that this history of failure may be coming to and end. The article claims that CIGS cells have recently achieved manufacturing costs and performance efficiencies comparable to the Silicon PV and that a reasonable road map for near term cost reduction by another 25% to 40% exists. CIGS PV cells utilize the relatively rare element Indium, but this white paper claims that projected supplies of Indium can support low cost manufacturing at a level of 150GW per year. Since the total global manufacturing capacity in 2015 was about 65GW this is volume is quite significant. Nevertheless it is probably not sufficient to allow CIGS to surpass silicon PV volumes in the long term term. CIGS cells could conceivable dominate the developing BIPV market where its use of flexible substrates and its black matte appearance may give it distinct advantages over crystalline silicon other than efficiency and cost.
I recently stumbled on the website of a company called Stratosolar which is proposing to deploy floating PV platforms 20km above the earth’s surface. The platforms would be 300 meters in length (along the direction of wind flow) and would be tethered to the ground by kevlar straps. Power cables which would conduct solar generated electricity to the ground would be attached to the kevlar straps. StratoSolar claims that the maximum wind speed of 50m/s at 20km altitude will allow stable, secure floating platform deployment with only small variation is horizontal position (According to their caclulation a length of 300 meters is required to achieve this stability.). Weights would be added to the tethers as a form of gravity energy storage. At 20Km of altitude each kg of weight would store 54Wh of gravitational potential energy (compare 38wh/kg for lead acid batteries). Excess electricity would be used to elevate the weights into the stratosphere, and the descending weights would be used to generate electricity during during period of low PV electricity production.
Twenty kilometers is above the cloud deck as well as being above a large portion of the atmosphere. These two effect lead to larger average incident radiation. StratoSolar claims that their designed deployments will produce three times as much electricity per unit area of deployment. All other things being equal this extra production would translate into one third lower cost of electricity production. However, it is far from clear that all other things are equal for stratospheric PV platform deployment. StratoSolar mention that the platforms could be filled with either helium or hydrogen. Helium would probably be used for inititial deployments, but StratoSolar admits bringing a high energy lifestyles to nine billion people (and this option seems to be the universal goal of climate change techno-fixers) would require the use of hydrogen as a buoyancy gas. There are very significant safety concerns with this use of such a highly flammable gas, but on their FAQ page StratoSolar claims that the engineering problems of hydrogen safety are solvable.
Siting PV panel above the clouds leads to high predictability of electricity production profiles which would lower the need for energy storage and which would make demand management schemes which try to match electricity use to the natural production profile easier to carry out. Furthermore this high predictability of solar electricity output can be achieved anywhere, including locations close to areas of high human population because the variable of cloud cover has been eliminated.
Since the power cables travel through all levels of the atmosphere below 20km the issue of lightning protection is very important. The design of the lighting protection system is discussed on the FAQ page
Whether or not the proposed energy storage in elevated weights is immune to weather is less clear. The weights will be moving through the troposphere which have potentially experience much higher winds than the height where the floating platforms will resides. The robustness of this energy storage scheme in the presence of violent wind is not clear.
Floating PV platforms will cast a shadow on the earth. The shadow will not be in fixed position during the day, and the path of the shadow will vary with the season. StratoSolar seems to to feel that landowners will not mind the relatively small amount of time that shadows from these platforms fall on their land. Whether or not such tolerance will apply in practice remains to be seen.
StratoSolar claims that their proposed solar energy production and storage scheme solves all of the problems associated with solar energy variability. This claim is not true since the seasonal variation of solar influx requires a scale of energy storage which larger than can be achieved by the proposed gravity energy storage scheme.
As far as I can tell from StratoSolar’s website they have not progressed beyond papers studies to real engineering on this PV deployment concept. While the ideas of stratospheric floating PV platforms is intriguing, I am not holding my breath waiting for a real world installation of one of these platforms.
A company called SolarWindow Technologies is planning to harvest solar energy from windows by coating them with organic PV materials dissolved in a liquid. These coatings will be semi-transparent. That is the PV material will absorb light in a certain frequency range and convert it to electric current and the rest of the light will pass through to the interior of the building. The occupants of the building will still be able to look outside even though the window glass will appear colored rather than completely transparent.
The SolarWindow Technologies web site is high in promotional hype and low in technical detail. They give no details on the composition, the efficiency, or expected lifetime of their solar coatings. They emphasize the fact that on tall buildings with lots of windows they will be able to harvest a lot more solar energy than a roof top installation (up to fifty times more according their promotional material). This claim may well be true, but cost per kWh and expected lifetime will be the keys to whether this technology is economically practical.
According to this press release manufacturing readiness is still more than two years away. However, based on projected costs the company is estimating a one year payback time for their solar windows. I assume this estimate applies to the extra cost associated with adding solar coatings, wiring, inverters and so forth to the base cost of the windows. In this case the payback time would apply to new buildings or to already planned window replacements in existing buildings. Presumably the economics of replacing an entire set of existing windows with SolarWindows would be significantly less favorable.
The question of performance profile over time of this technology is also interesting. Ideally the coating would last as long as the windows themselves. If the expected useful lifetime of the coatings is significantly less than that of the windows, then the economics of this technology will be less favorable.
Organic solar cells have been a subject of research for a long time, but no products based on this technology have reached the market. It remains to be seen whether SolarWindow Technologies will be the first company to pass the mark.