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.
The German company Siemens is developing an energy storage system that will convert electrical energy to sensible heat storage in hot rocks at a temperature greater than 600°C, and then reconvert the stored energy to electricity using a steam turbine. I stumbled on this energy storage concept in an article published by energydigital.com. The advantage of such an energy storage system is low capital costs and very long cycle life. The major disadvantage is low round trip efficiency. Siemens claims that its preliminary prototype system is running at 25% efficiency (i.e. 25% percent of electrical energy input to the system is retrieved as electrical energy at the of the storage/steam generator system) but that it believes an efficiency as high as 50% can eventually be achieved.
One can question whether 50% efficiency is really obtainable. In this Mitsubishi Heavy Industries, LTD. presentation on advanced turbine technologies for high efficiency coal-fired power plants Mitsubishi claims that advanced steam turbines can obtain a net LHV efficiency (i.e. percentage of chemical energy converted to electrical energy) in the range of 43% to 45%. The upper end of the range requires operation at 625°C whereas the lower end correspond to the more usual operating temperature of 565°c. This reference claims that typical steam boiler efficiency (i.e. the percentage of chemical potential energy converted to energy in super-heated steam is 85%. One can use this number to calculated the efficiency of the rest of the steam generator system (=ηS) according to the following equation:
For for 43% total efficiency we find ηS=50.5%, and for 45% total efficiency we find ηS=52.9%. Since electrical energy can be converted to heat at close to 100% efficiency it is technically possible to obtain 50% total efficiency if heat can be transferred in and out of the storage system with very low parasitic losses. The required efficiencies of heat transfer (=ηS) can be calculated using the equation:
ηS×.505/.529 = 0.50
For the low efficiency case we find ηH = 99.0% and for the high efficiency case we find ηH = 94.5%. Keeping the parasitic losses in the heat transfer process under 1% seems unlikely to me, so that the most advanced steam turbine technology will be required to obtain 50%. Even then there is no guarantee that the heat transfer efficiency obtain in a practical implementation will meet the required limit.
Of course even 50% is not very high efficiency. Any electrical energy coming out of this system will cost twice as much per kwh as the electrical energy going in based on efficiency consideration alone. And on top of this efficiency cost must be added the cost of the energy storage and reconversion system. However, this extra cost may be fairly low. Rocks are cheap and the cycle life of this system could be extremely long. Steam turbines are a mature technology with a life expectancy of thirty years. Furthermore there is good reason to believe that the low costs of this energy storage system would survive a major volume scale up since it uses earth abundant materials.
On question of interest is whether the cost of this storage system is low enough to allow storage times of more than a few hours which seems to be the limits for currently popular but still expensive lithium ion battery storage systems. In their press release Siemens talks about building a full system capable of delivering electrical power continuously for 24 hour from a fully charged rock reservoir, which is a time period far beyond what anyone is proposing for lithium ion batteries.
Siemens is a major manufacturer of wind turbines and they are specifically proposing using this storage system for storing wind energy.
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.
A Bergen-based Norwegian shipowner Rederiet Stenersen has ordered a pair of 17,500 metric ton chemical tankers outfitted with lithium ion batteries from a Finnish company called WE Tech Solutions to improve the efficiency of its diesel generator set which provides on board electric power. The batteries will act as a spinning reserve to handle peak loads, thus allowing the generators to run for most of the time at steady high efficiency operating point. Of course more efficient use of diesel fuel is not really a world changing innovation if you are concerned about lowering GHG emissions to zero, but this use of lithium ion batteries is another sign of the progress being made in performance and cost of these high energy density long-lived batteries.