Researchers from the Hong Kong University of Science and Technology have recently reported on the performance of a new commercial ion exchange membrane for Vanadium flow batteries. The new composite membrane consists of two layers: a microporous layer and a Nafion layer which is much thinner than conventional membranes made from Nafion only. The new membrane is substantially cheaper than the conventional Nafion membrane and at the same time improve the batter efficiency (71.2% ==> 76.3%) and the electrolyte energy density (54.1% ==> 68.4% electrolyte utilization).
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 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.
Various kinds of bacteria can evolve hydrogen from water using the enzyme hydrogenase. The evolution of hydrogen is a key step in bacterial fixation of nitrogen from the atmosphere. Chemists at the Pacific Northwest National Laboratory have developed a synthetic nickel based catalyst for electrolytic hydrogen evolution based on the known structure of hydrogenase. Physorg has published a story about this research and an abstract of a recent publication of the experimental results is available on line. Nickel based catalysts are already used in commercial alkaline water electrolyzers such those produced NEL Hydrogen (formerly a part of Norsk Hydro). The hope of using hydrogenase base catalysts is to speed up the reaction and thus to reduced the amount of electrode material required to produce a given amount of hydrogen. The PNNL group has succeeded in making a fast hydrogen evolution catalyst but the efficiency with which it turns electrical energy into chemical potential energy is low compared to more conventional catalysts. The PNNL group is now focusing on means for making their catalysts more efficient.
Recently the website insideevs.com published a story about an energy storage innovation award won by the company EVgo which is creating a network of fast charging stations for electric vehicles. In cooperation with UC San Diego they set up an electric charging station powered by PV panels on the station sunroof connected to used lithium ion batteries from BMW i3 electric cars.
The story claims that the solar energy falling on the station roof can charge approximately 15 cars per day, but does not say whether this is the yearly average or whether this represents the peak Summer charging rate.
EVgo is planning to open its first 350kW ultra-fast public charging station in June of 2017 in Bakersfield, California. This station will have four 350kW chargers and will also feature a sunroof connected to lithium ion storage batteries.
A casual reader might get the impression that this station will charge vehicles using only the solar energy which falls on the sunroof. However, it is pretty clear that such is not the case. The capital expense of these high power chargers will not be justified by charging a few tens of cars a day. Nor will customers be happy about their ability to charge up depending upon how many cars happened to use the station in the previous few hours. Including solar panels and used EV batteries may save some amount carbon emissions, but clearly the intention is to hook up this station to the grid and have year round day and night service.
A 100% solar powered transportation system will require a lot more solar panels than can be included on the sunroofs of charging stations. It will also require some means of dealing with long term variations (e.g. seasonal variations due to the the tilt of the earth’s axis and long term variations in cloud cover) in the solar energy flux. Insofar as the costs of lithium ion battery storage are driven down by a cycle life measured in the thousands, they are not a practical tool for dealing solar flux variations on these long time scales.
rogerkb at energyevolutionjournal dot com
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.
Green Car Congress recently posted a story about an article in the magazine Oceanographydetailing a proposal to obtain liquid hydrocarbon fuels and protein for animal feed from large-scale industrial cultivation of marine microalgae (ICMM). Analysis indicates that coproduction of food and fuel is needed in order for algal bio-fuels to achieve production costs comparable to liquid fossil fuels. The authors argue that cultivation of marine micro algae is potentially an order of magnitude more productive (presumably per square meter of ocean cultivated) compared to biomass production on land. They further argue that the fact that salt water is used rather than fresh water can help to manage the demand on this important resource. They particularly emphasize the comparison of algal proteins to soy protein, maintaining that not only is the production more efficient but that the algal protein is potentially of higher quality. If ICMM is as efficient as projected then it may be possible to reforest marginally productive agricultural land thus leading to the removal some amount of CO2 from the atmosphere as well as helping to preserve land based bio-diversity.
Presumably intensive cultivation and harvesting of select species of marine microalgae over a large area will have negative effects on ocean bio-diversity, although this issue is not discussed in the Oceanography article. The authors do discuss the nutrient requirements of ICMM as as a sustainability issue. To achieve high productivity marine microalgae require a higher ratio of phosphorus to nitrogen compared to land based agricultural systems. The authors freely admit that the current use of rock phosphates in agricultural production is not sustainable in the long term and that nutrient recycling will have to be pursued. However, they argue that ICMM is very compatible with nutrient recycling since nearly 100% of the phosphorous in waste stream can be taken up and used by the microalgal population. This fact is in contradistinction to soil based agriculture in which nearly 80% of applied phosphorous is quickly transformed into stable forms which plants cannot utilize. Land which has undergone years of regular phosphorus applications has lots of phosphorus in it in stable forms that plants cannot digest. We may eventually be able to develop more complex system of agricultural production which utilize this phosphorus is place, but today’s highly productive (per acre and per labor hour) corn and soy bean rotations are not such a system.
My own view of the ICMM proposal is that it makes sense only if it is part of a long term plan to reduce the total human impact on the biosphere which includes other important effects in addition to green house gas emissions. We need to produce food for human beings, and if ocean farming can help us to do so with lower total impact on the biosphere then such a proposal is worth evaluating. However, if ICMM is being proposed in the context of a world of 10 billion human beings who expect constantly increasing standards of consumption (including such things as getting a large percentage of our protein from high on the food chain and frequent rapid travel over long distances) as a normal part of the operation of the economic system, then farming the ocean may just be one more step on the road to ecological disaster.
Several month’s ago Green Car Congress published a story about the development of water electrolyzer designs which do not require an ion exchange membrane separating the two half cells of the device. Since ion exchange membranes are quite expensive such designs have the potential to reduce the costs of hydrogen producing water electrolyzers. Furthermore, the researchers who published the paper in question argue that the elimination of the ion exchange membrane can relax the design constraints of MEA (Membrane Electrode Assembly) based electrolyzers and can potentially lead to design configurations which have other cost reductions in addition to the membrane manufacturing cost.
As the diagram below indicates this form of electrolyzer does indeed appear to have a very simple design. It would appear that the most costly components would be the titanium mesh electrodes impregnated with the appropriate catalysts.
The researchers used platinum nano-particles attached to the titanium mesh as catalysts. They were able to operate prototype electrolyzers in both acidic and alkaline solutions but obtained the highest electrolysis efficiency (72.5% based on the HHV of H2 at a current density of 100mA/cm2) for with alkaline electrolyte. This efficiency is comparable to that of current commercial alkaline electrolyzers such as those produced by Nel (formerly a division of Norsk Hydro). Alkaline electrolysis is economically attractive because it does not require the use of platinum group metals as catalysts.
In spite of the interesting initial results, one should not get too excited yet about this electrolyzer design entering the market place in the near future. Although the overall electrolytic efficiency of 72.5% is comparable to commercial alkaline electrolyzers an additional hydrogen loss of 10% occurs during the gas collection phase of the separation process reducing the effective efficiency to 65.3%. This loss of efficiency may not be a deal breaker since it is cost/kg that matters. Furthermore the electrolzyer in question is a non-optimized preliminary prototype and substantial optimization may be possible. The electrode current density of 100mA/cm2 is a factor of three lower than the current density of commercial alkaline electrolyzers although again similar comments to that just made about efficiency apply.
Another problem is that some amount (probably more than 1%) oxygen crosses over to the hydrogen side to the electrolysis cell and contaminates the hydrogen gas at the collection point. PEM fuel cells require less than 5 ppm V of oxygen content. Although further optimization may reduce the amount of crossover, orders of magnitude improvement seem unlikely. Nel claims that their electrolyzer systems produce H2 with less than 2ppm of O2 after purification. Whether or not a fraction of a percent O2 contamination represents a significant economic barrier to the production high purity H2 is not clear from any information that I have located so far.
Science Daily recently posted an article about some Spanish scientists who have modeled a thermal energy storage system based on molten silicon at a temperature of 1410C. They refer to this system as a phase change energy storage system, implying that in the discharged state the silicon will a solid at close to the melting temperature and most of the stored energy will be used to convert the silicon the the liquid phase rather than to raise the temperature of the storage mass. Presumably the best use of such an energy storage system would be in a high concentration dual axis solar field. The thermal energy storage density would be 5 to 10 times higher than the molten salt systems currently used for energy storage in concentrated solar power (CSP) applications.
The scientists specifically model the use of thermo photovoltaic cells (TPV) as the thermal to electric power conversion technology. It is not clear to me that TPV would outperform a steam engine or a closed cycle gas turbine running off the same heat source in cost and/or conversion efficiency. It is far from clear that his idea is anything other a calculational curiosity, but at least it is a new idea in energy storage. One good thing about using silicon as an energy storage medium is that it is extremely cheap and abundant in the earth’s crust.
Fossil hydrocarbons (e.g oil, natural gas, coal, etc) are used as feed stock for synthesizing a wide variety of useful compounds. Since humanity is tearing through the available feed stocks at far higher than the natural replacement rates an alternative feed stock will some day be required. One possibility is to use CO2 captured from the atmosphere. However, in order to convert CO2 into useful products it must first be reduced to carbon monoxide (CO) which is an energy intensive process. Some amount of research effort has been directed at using sunlight to drive photo catalytic reduction of CO2 to CO. Rhenium based catalysts can accomplish this feat using ultraviolet photons. However, ultraviolet light is a small component of incident sunlight so that a practical solar driven process requires a catalyst than can use the lower frequency visible components of sunlight. Chemistry World recently published an article about an Chinese/French collaboration that has succeeded in altering rhenium based catalysts so that they can catalyze the reduction of CO2 when radiated with visible light with a quantum efficiency similar to the utra-violet driven version of the same complex. Whether or not this particular organometallic complex is good enough (e.g. cheap enough, long-lived enough, efficient enough) to from the basis of a practical industrial process is not yet clear.