Sion Power has developed a lithium sulfur battery with and energy density of 350Wh/kg, and they claim that a density of 600Wh/hg will be achievable relatively soon. They do not specifically cite a cycle life for this battery but if I attempt to interpret the cycle life information given in this presentation I conclude that Sion Power’s Current lithium sulfur battery has a cycle life at full depth of discharge considerably less than 1000 cycles. Cycle life is currently the Achilles heel of lithium sulfur batteries. Unwanted side reaction of the lithium sulfide formed in the sulfur cathode as well as mechanical damage due the the growing and shrinking of the sulfur cathode conspire to limit the cycle life of this battery technology. Like Oxis Energy Sion Power’s applications page does not mention the obvious market of consumer electronics, making be suspect that the cost of the batteries is fairly high compare to lithium ion batteries. The high theoretical energy density of lithium sulfur batteries makes them an attractive alternative to lithium ion batteries, but I do not think that this technology is ready for prime time.
Lithium sulfur batteries have a much higher theoretical maximum energy density than lithium ion batteries (3 to 5 time according to this article). To date, however, practical implementation of this battery chemistry have have achieve only a fraction of the theoretical maximum energy density and have relatively short cycle life because of undesirable side reaction of the lithium sulfide formed in the sulfur cathode and because of physical degradation of the cathode due to swelling and shrinking during the charge/discharge cycle. Lots of fundamental research is being done trying to overcome both of these problems, but commercialization still seems to be in the indefinite future.
It is not clear whether Oxis’ technology represent to leading edge of commercialization for this technology or not. Cost is a key in addition to performance. On their applications web page Oxis mentions electric vehicles, grid energy storage for renewable, and mobile power for military applications. The fact that they do not mention consumer electronics makes me suspect that their technology is probably expensive. Why would they neglect this large existing, performance hungry market if they had costs competitive with lithium ion batteries? I think that I will not hold my breath waiting for Oxis’ batteries to revolutionize energy storage markets.
This abstract of an article published in the Dec 2012 issue of Nature describes a method of growing crystalline GaAs (gallium arsenide) nanowires in gold nano-particle aerosols at high throughput rates compared to traditional molecular beam epitaxy (MBE) or metalorganic chemical vapour deposition (MOCVD) methods of synthesis. The hope is that a high growth rate will translate to lower costs.
The company Sol Voltaics is hoping to utilize the technology in conjunction with traditional silicon solar cells produce a tandem device with higher efficiency than can be achieved using silicon by itself. GaAs is a direct band gap semiconductor which means that the probability of the absorption of an impinging photon whose energy is near the band gap is much higher than for an indirect band gap semiconductor such as silicon. This high probability of absorption means that GaAs solar cells can be very thin and still effectively absorb solar energy, whereas silicon solar cell have to be much thicker to effectively absorb all of the photons that are capable of being converted to electrical current.
The band gap of GaAs is different than that of silicon so that it absorbs a different part of the solar frequency spectrum. That fact that the GaAs film can be made very thin means that the photons of a frequency that it is not capable of using for the production of electricity can easily pass through and enter the silicon cell where the photon near the band gap of silicon can be absorbed and converted to electricity as well. The combined cell has substantially higher energy production than a silicon cell alone.
The key to the economics of such a tandem cell the the production cost of the GaAs layer. Triple junction cells with two of the junctions based on GaAs films have achieved solar conversion efficiencies in excess of 45%. However the enormous costs of producing crystalline then films of GaAs have prevented the commercialization of this technology except in specialized niche applications where low weight and small surface area is more important than cost (e.g. remote power for military application, solar power for space missions, etc).
Whether or not the cost/performance characteristics of PV cells produced by this technology can enable mainstream commercial application of GaAs remains to be seen.
I am not really a believer in the hydrogen economy (i.e. a seamless transition from fossil fuel powered transportation to renewable hydrogen powered transportation within the context of growth based financial capitalism). However, I still follow developments in fuel cell powered transportation since a variety of mobile power application will eventually have to be weened off fossil fuel, and it is not clear that storage batteries will be effective in all of these applications.
Green Car Congress recently published a news item about a partnership between Alstom Transport and Hydrogenics to develop fuel cell powered commuter trains for the European market. Alstom Transport is a rail transport company based in France which a focus on ‘sustainable’ transportation. I put the word sustainable in quotes since what level of economic activity which is going to going to be reasonable stable on long time scales is highly uncertain, though certainly fossil fuel powered transportation cannot be included in this category. Hydrogenics is a company headquartered in Canada with manufacturing sites in Germany and Belgium which specializes in PEM fuel cell systems and in hydrogen production by electrolysis. Hydrogenics originally came into existence in 1948 as an electrolyzer company but has since added PEM fuel cells to its product line.
Whether or not the proposed system of hydrogen powered commuter trains will use hydrogen produced from Hydrogenics electrolyzers or whether the cheaper (but environmentally friendly) option of hydrogen produced from steam reforming of natural gas will be used is not clear. The announcement of this partnership talks about the delivery of several hundred engine systems and heavy duty fuel cells, but does not mention electrolyzers.
Fuels cells powered by reformed natural gas is the most likely entry point for a commercially successful entry of this technology into the transportation market. However, carbon emission point of view this option is not very attractive. Amory Lovins and others argue that a system based on fuel cells hydrogen from reformed natural gas will produce less CO2 emissions per passenger mile than transportation based natural gas powered internal combustion engines. However given the long dwell time of CO2 in the atmosphere we need to leave a significant fraction of fossil carbon reservoirs in the ground to have any impact on climate change, the economics of electrolytically produced hydrogen is still highly questionable.
This brochure on Hydrogenics PEM fuel cells for transportation application they cite an expected operation lifetime 10,000 hours. Ten thousand hours at an average speed of 30mph comes out to 300,000 miles which is pretty good durability. Fuel cell cost are dropping and reliability is rising, but hydrogen costs still stand in the way widespread adoption.
A reasonable standard of consumption is a prerequisite of an economic system with long term productive stability. Many people now recognize this truth, and calls for voluntary simplicity and the end of consumer society meet a positive response from many people. However, the social systems implications of a reasonable standard of consumption have not been clearly thought through, partly because social systems thinking is a difficult discipline, and partly because a strong bias in favor of certain cultural norms strongly discourages any thinking which would challenge those norms. I call this bias the Social Will. The ability of the Social Will to produce conformity of thought on key social institutions is quite powerful.
The french historian Jean Michelet in writing about the Maillotin tax rebellion (named after the mallets or Maillots that were carried by the leaders of the rebellion) under the reign of the mad king Charles VI in late fourteenth century France explains how the leaders of the rebellion demanded audiences with the Dauphin (the future Charles VII) and lectured him about how a good king should treat his faithful subjects. Michelet explains that it never crossed the minds of the rebels that they could do without kings altogether, because the religion of royalty was still in full flower. If anyone had suggested to the Maillotins the elimination of royalty, they would have rejected this advice as the ravings of a radical mad person who wanted to destroy the natural social order and bring chaos and anarchy down upon the world. The Social Will of the France of that time had ordained a hereditary monarch as an essential element of good social organization. The suffering bourgeoisie wanted a good king who would rule wisely, but the idea of replacing monarchy by some other form of government was not within the event horizon of fourteenth century France.
Similarly many people are dissatisfied with the performance of private credit markets in wake the great recession of the last decade, but the percentage of these people who recognize that fundamental structural problems exist in this system which require new forms of credit is still quite small. The Social Will of modern global society has declared private credit markets to be an eternal and perfect form. John Mavrogordato summarized this resistance to new thinking on economic organization in the following terms in his 1917 book The World in Chains: Some Aspects of War and Trade:
“The distribution and exchange of commodities are necessary to the existence of the State; so necessary that it might be supposed that their regulation would be one of the primary functions of government. Proper systems of distribution and exchange correspond to the digestive processes of the body, on which depend the proper nutrition of all the parts and the real prosperity of the State as a whole; yet any comprehensive plan for their control is still regarded as the most unattainable dream of Utopia, and they are left to carry on as best they can in the interstices of private acquisitiveness.”
Unless we are planning to go all the way back to neolithic villages as the fundamental social unit credit cannot be dispensed with. Any time someone proposes to spend a large amount of resources in the present in order to produce economic value over long period of time in the future (e.g. building a bridge, a university, a semiconductor manufacturing plant, a solar PV farm, a nuclear power plant etc.) someone must evaluate the likelihood that this present expenditure of resources will produce in the future a sufficiently large flow of goods and services and a small enough amount of negative externalities to justify the expense. This evaluation of the future consequences of spending on infrastructure development is the process of granting credit.
To me it seems clear that leaving this process primarily in the hands of private credit markets whose raison d’etre is to turn money into more money is inconsistent with ecologically sound economic development. The people making decisions about credit should be public servants whose purpose is to create and maintain useful infrastructure rather than to turn money into more money. These community financiers should be paid salaries for services rendered rather than making money in proportion to the amount of debt they create. This not to say that community financiers should not be concerned with monetary flows and the economic viability of the enterprises to which they grant credit. Quite the contrary. But a significant difference exists between trying to create and maintain valuable infrastructure and trying to create as much debt as you can because your personal wealth is proportional to the size of this debt.
Of course if ‘public servant’ and ‘Marxist vampire’ are inescapable identities in this context then no useful reform of the credit system can be made, and we should quit wasting our time yakking about ways to save a form of social organization which is an evolutionary dead end.
The most widely used battery technology for multi-hour grid energy storage is NGK Insulator’s high temperature (300 to 350C) sodium sulfur (NAS) battery. The high operating temperature is required to obtain adequate mobility for the sodium ions through the solid ceramic beta-alumina electrolyte. The largest installation of such batteries is 204MWhr installation supporting a wind farm in Rokkasho Japan. This installation opened operations in 2008. The sodium sulfur chemistry is attractive from a scalability point of view since both sodium and sulfur are earth abundant elements.
High temperature operation creates safety concerns, and, in fact, a battery fire occurred on September 21, 2011 at one of NGK installation’s in Japan, leading to a shutting down of all NAS battery storage installations world wide for a long period of time. In June of 2012 NGK Insulator’s published a report describing the causes of the fire and safety enhancements which they felt provided sufficient operating margin to justify reopening their battery factory. This presentation mentions two California installations (One in San Jose and one in Vacacville) of NGK’s NAS batteries which have occurred since the safety enhancement were announced. This technology is not dead, but my sense is that enthusiasm for it has waned in spite of the enhanced safety features. These batteries are too expensive ($320/KWh) to truly revolutionize grid operations but they have found some niche markets.
From time to time I stumble upon research work exploring options to lower the operating temperature of NAS batteries. If these ideas introduce rare elements to the battery chemistry they may not have the same long term scalability as the current chemistry. For example this abstract claims that adding cesium to the liquid sodium cathode will allow the operating temperature of sodium sulfur batteries to be lowered to 150C. Unfortunately cesium is a rare element which cost about $40,000/Kg. The effect of the cesium is to enhance to wettability of the sodium on the solid beta-alumina electrolyte. Conceivably some other lower cost metal can be found which produces the same effect, but such an option awaits future research.
Recently an interesting low temperature NAS battery design has emerged from scientists (Dr. Gao Liu, Dr. Dongdong Wang, and Dr. Kehua Dai) at the Lawrence Berkeley Laboratory. They claim that this battery can operate at a temperature of 80C. This temperature is below the melting point of pure sodium (97.7C). A liquid metal anode avoids the problem of dendrite formation by solid sodium and is a key enabler of the long cycle life (4500 cycles) of current NAS batteries. Liu et al maintain this feature of the NAS battery by using an alloy of sodium and potassium which has low melting temperature. The solid beta alumina electrolyte is replaced by a solid polymer which conducts sulfur ions (rather than sodium ions) at low temperature. Their battery design also includes a new cathode which is a mixture of sulfur and a conductive polymer. Some details of the cathode and electrolyte chemistry and structure can be found in this patent
As this application for Cleantech to Market funding shows, Liu et al seem to believe that this battery architecture is not just a laboratory curiosity but has the potential to be manufacturable. I believe that the manufacture of the tubular solid ceramic electrolyte for the current high temperature version of the NAS battery is an expensive process, so that the replacement of this component by a polymer electrolyte potentially represents a significant cost savings. Some information about projected development time lines manufacturing costs for of these batteries is given in this presentation. The numbers in the presentation are probably highly speculative. Furthermore the LBL scientists themselves will probably not follow this technology to maturity but will attempt to license it to a private company. I would not hold my breath waiting for these batteries to start rolling off the assembly line in large quantities, but I am always interested in technologies which have potential long term scalability.
The free charge carriers in so called intrinsic semiconductors such as silicon are the result of the thermal excitation of electrons from a valence band into a conduction band. The excitation of the electron leaves behind a positively charge hole in the valance band which can also act a charge carrier. Because the free charge carriers are created by thermal excitation, the colder a semiconductor becomes the less free charge carriers are present and the electrical resistance become higher. This behavior is in contrast to metallic conductors which have free electrons in the conduction band at all temperatures and which actually have lower resistance at lower temperature because thermal lattice vibration do not interfere as much with the free motion of the conduction electrons. The density of free charge carriers in pure silicon at room temperature is low compared to metallic conductors, and the electrical resistivity is high.
The number of free charge carriers in silicon can be greatly increased by adding a small amount of another element called a dopant. Silicon has four electrons in its out shell. If the dopant atom has 5 electrons in its outer shell (e.g. phosphorous) then one of these electrons can easily be excited into the conduction band of silicon. The positively charge ionic core left behind is bound to the site the dopant atom and therefore cannot act a current carrier. Thus silicon doped with phosphorous has an excess of negative charge carriers and is therefore called an n-type semiconductor. If an atom with 3 electrons in is outer shell (e.g. boron) is added to silicon then one of the electrons in the valence band can easily become attached to the atom leaving behind a positively charge hole which can act as current carrier. The electron attached to the dopant atom is immobile and cannot act as a current carrier. Thus silicon doped with phosphorous has an excess of positive charge carriers and is therefore called a p-type semiconductor. Both of these types of doped silicon are called extrinsic semiconductors. Extrinsic semiconductors have much higher densities of free charge carrier and much lower resistivities than intrinsic semiconductors.
Pure silicon is capable of a absorbing sunlight and exciting electron/hole pairs, but no electromotive force exist which can drive the free electrons through an external circuit as an electrical current. In order to produce such a current a P-N junction is required. A P-N junction is an interface between two volumes of silicon one of which has an excess of negative charge carriers and the other of which has an excess of positive charge carriers. The most common method of producing such a junction is to take a wafer of p-type crystalline silicon (most commonly doped with boron) and exposing one surface of the wafer to phosphorous at high temperature. The phosphorous diffuses into the silicon and forms a thin layer of n-type silicon. The boundary between this layer and the bulk of the wafer is the p-n junction.
When electron/hole pairs are excited by light absorption in the vicinity of the p-n junction the electron moves to one side of the junction and the hole to the other side and they can recombine only after the electron has moved through an external circuit an back into the other side of the junction. This movement of the photo excited electrons is the source of the electrical current produced by PV cells.
Silicon PV cells can be produced starting with either p-type silicon or n-type silicon. It turns out that controlling the electrical resistivity of n-type silicon crystals is more difficult than controlling the resistivity of p-type silicon thus making n-type silicon with will controlled resistivity more expensive to produce. However PV cells utilizing n-type silicon wafers can achieve higher light to electricity conversion efficiency than PV-cells utilizing p-type wafers. P-type solar cells still dominate the market, but high efficiency n-type cells have found a niche in applications where space is at a premium so that the higher efficiency allows more energy production in a give area of rooftop.
Silevo is a player in the high efficiency n-type PV solar cell field. The typical method of producing a p-n junction in a n-type wafer to expose is one surface to boron gas at high temperature. The boron diffuses into the silicon and converts a thin layer of the wafer into a p-type semiconductor. Silevo creates an ultra thin p-type layer at one surface of the wafer not by doping but by allowing one surface of the wafer to become oxidized and then depositing thin film layers on top of the oxide. The actual physics by which by which the so-called inversion layer with a majority of p-type carriers is created is a mystery to me, but it obviously works. I believe that the high temperature boron diffusion process is fairly expensive so that Silevo’s thin film deposition process may represent a significant cost reduction.
PV cells of this type have a lower temperature coefficient of performance (% loss of efficiency per degree increase in temperature). The Silevo cells have a temperature coefficient of -0.22%/C compared to -0.45 to -0.50%/C for diffusion based solar cells. Silevo claims that this lower temperature coefficient leads to 5% to 12% increase in energy production under real world conditions. Obviously hot desert conditions will get the most benefit from the lower coefficient.
Silevo does not manufacture n-type silicon wafers itself. They are depending on general industry progress for lower costs in this aspect of the solar modules.
Silevo has garnered media attention recently because it was acquired by Solar City, a major player in the residential PV market. This attempt by Solar City to become a vertically integrated company who manufactures it own solar panels is somewhat risky, but if Silevo can really deliver a high performing low cost product the payoff could be very substantial.
As I have mentioned before several different companies are developing near isothermal gas compression systems which inject water vapor into the compression chamber in order to remove heat from the compressed gas. I have recently read about another company called Carnot Compression which has developed an isothermal gas compressor which injects gas into water. The compression fluid is a gas/water emulsion (i.e. a mixture of gas bubbles in water matrix). The emulsion is contained inside a spinning cylindrical chamber where the gas bubbles are compressed as they migrate to the outer surface of the cylinder. The water absorbs the heat generated in the gas by compression which the compression nearly isothermal. Exactly how the gas gets out of the rotating compressor chamber into the pressure vessel is not clear from the limited description given on Carnot’s web site. Apparently only gas gets out and not water, since Carnot claims that the compressed gas is very dry and does not require further drying steps as do other varieties of gas compressor.
I am not really sure how this technology differs from that of a company called Oscomp which has also developed a multiphase (i.e. water + gas) rotary compressor. Oscomp claim that their compressor can achieve 60 compression ration in a single stage while Carnot claims that they can theoretically achieve a 200 to 1 compression ratio. I assume that the use of the word theoretical in this claim implies that the real world implementation of this technology will not actually reach this compression ratio.
In discussing possible applications of their compressor Carnot does mention energy storage, although this use is at the bottom of their list. Carnot’s view is that they have developed an efficient, low energy use, low cost gas compressor which is suitable for a wide variety of existing applications, so they are chasing obvious markets and obvious money making opportunities. If a compressed air energy storage market comes into existence they will be positioned to support it, but they themselves are not pursuing this rather risky business opportunity.
As far as I can tell their compressor is not reversible, so that a compressed air energy storage system based on their technology might require a separate expander stage, which might be a disadvantage compared to the technology of energy storage companies like Lightsail Energy and SustainX. Nevertheless, Carnot’s technology is new in the field of gas compression and could conceivably play a role in commercializing compressed air energy storage.
Silicon photovoltaic cells are capable of capturing and converting to electricity only a fraction of the available solar frequency spectrum. This limitation is part of the reason that the light energy to electrical energy conversion efficiency is limited to a little over 20%. More efficient solar cells can by manufactured from multi-layer thin film semiconductors (typically gallium arsenide compounds). Different layers are optimized to capture different part of the solar spectrum. Triple junction cells have achieved sunlight to electricity conversion efficiencies of more than 40%. In spite of the high efficiency compared to silicon PV cells the triple junction cells have made very little market penetration because of very high manufacturing costs.
An alternative method to capture a larger part of the solar spectrum is to make a combined heat and power system where silicon PV cells convert part of the spectrum to electricity and part of the spectrum is used to heat up a heat transfer fluid which can be used for hot water, space heating, or industrial process heat.
A relatively low tech version of such systems called PVT panels are already available from a number of manufacturers. These systems take advantage of the fact that PV panel naturally heat up (up to 70°C to 80°C under optimal conditions) when exposed to the sun. The solar panels are installed on top of a solar collector which contains a circulating fluid which absorbs heat from the solar panel and stores it in a tank where it can be used later for water or space heating. These systems cool down the solar panels to about 30°C and thus increase the efficiency of electrical generation by about 6%. Since residential hot water systems deliver water in the temperature range of 50°C to 60°C a heat pump is required to make this system useful for providing hot water. A heat pump can provide an energy lift of four or five. That is the amount of heat delivered is four or five times greater than the energy consumed in running the pump.
Higher temperature for the thermal collector could be achieved by using concentrating optical systems such parabolic troughs, but unfortunately the high temperature would degrade the performance and reduce the life expectancy of the silicon solar cells. I recently came across an article published on the web site of SPIE (the International Society for Optics and Photonics) describing a design for concentrating solar PV/thermal hybrid system which overcomes the silicon cell heating problem. This design uses a spectral beam splitter which splits the incoming sunlight into two frequency bands one of which is fed to the silicon solar cells and the second of which is fed to a thermal receiver which is insulated from the solar cells.
The frequencies which fall on the PV cells are ones which can be successfully converted to electricity so that their energy is converted to electrical current which does not significantly heat up the silicon cells. The concentrated light which falls on the thermal absorber can produce temperatures of 150°C or higher without degrading the performance of the solar cells.
Spectral beam splitters have existed for a long time but they are fairly expensive devices which require costly thin film processing. The inventors of this particular solar pv/thermal hybrid system claim that they have developed a low cost beam splitter based on bulk materials.
The the real economics this solar design remain to be seen. 150°C temperatures are not really need for domestic energy needs, so this system will be aimed at industrial roof tops. Also motorized parabolic troughs do not seem like a good match household installations. The advantages of this system are lower silicon use due to the solar concentrator and the extra value stream of thermal energy. The disadvantage is the cost and complexity of motorized parabolic troughs.
Solar concentrators do not work well with diffused sunlight, so these kinds of systems will have the most attractive economics in locations with lots of clear skies (e.g. the American Southwest, the Mediterranean, Australia, etc.). This design is certainly not a miracle system which will transform renewable energy use, but it might find some effective economic niches.
The most common negative electrode or anode used in lithium ion batteries is made of graphite, the same material used for pencil lead. The electrodes are comparatively cheap, have high round trip charging/discharging efficiency, and have very good cycle life. The positive electrode or cathode is substantially more expensive and has a shorter cycle life. A new class of electrochemical storage devices has emerged called lithium ion capacitors. They are not true capacitors because the anode is the same graphite anode used in lithium ion batteries and undergoes a chemical reaction with lithium. However, the cathode is replaced by the same high surface activated carbon used in ultracapacitors. This electrode does not interact chemically with lithium but stores energy in the electric field between two layers of charges.
Ultracapacitors using high surface area activated carbon have very high power density (they can be charge and discharged very quickly) and very long lifetimes. Maxwell Technologies, a prominent manufacturer of ultra-capacitors claims that their capacitors can last up to one million duty cycles. Lithium ion capacitors are intermediate in performance between lithium ion batteries and ultra-capacitors. They have 4 to 10 times higher energy density than ultra-capacitors (at the high end the approach the energy density of lead-acid batteries) and they have higher power density and longer cycle life (100,000 duty cycles) than lithium ion batteries. They also have a higher fully charged voltage than ultra-capacitors (3.7V as opposed to 2.7V) which is an advantage for some applications. In addition lithium-ion capacitors have much lower self discharge rates than ultracapacitors; Even after 2000 hours nearly 98% of the original charge remains in the capacitor.
Among the manufacturers and developers of lithium ion capacitors are Taiyo Yuden, and JSRmicro.
A key question relating to the application potential of these devices is cost. If the higher energy density relative to ultra-capacitors translates into lower cost then the field of application might be very large. I looked on line for lithium ion capacitor and ultracapacitor prices. I found an offer for a 350F 2.7V Maxwell technologies capacitor for $10.75. I also found an offer for Taiyo Yuden 200F 3.8V lithium ion capacitor for $46.09. The energy stored on a capacitor is ½CV². Therefore the energy storage capacity of the Maxwell Technologies capacitor is somewhat less than the Taiyo Yuden capacitor in spite of the higher capacitance. If I scale the price of the Taiyo Yuden capacitor by the ratio of the energy storage capacities I get a price of $40.72 for equivalent energy storage.
There may be certain application for which the higher voltage, the smaller size, or the longer charge retention time make lithium ion capacitors the preferred solution, but if the price quote I found above is reflective of real costs then lithium ion capacitors then the field of applications which they can take over from ultracapacitors may not be that large.