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.
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
Most discussions of electrifying our transportation system avoid discussing the category of heaven duty machinery which has very high fuel demands which makes the lower energy density of batteries look very unattractive relative to diesel fuel. In the long run we need an alternative to diesel. Even bio-diesel is probably sustainable only at consumption levels much below current usage. A Finnish company called Visedo is producing electric drive trains for construction, agricultural, forest and other off-highway machinery based on electric motors which use synchronous reluctance assisted permanent magnet technology (SRPM). Visedo claims that SRPM motors offer smaller dimenson, lower weight, and higher efficiency compare to tradition induction motor (IM) or permanent magnet motor (PM). Heavy off road machinery is typically powered from from diesel engines which both drive the wheels and run various kinds of machinery for lifting, crushing, chipping, etc. The processing machinery is often run through a hydraulic drive train which requires very high power. Diesel engines are often slow to react to these fast-changing demands in power, and they therefore commonly run on high revolutions in order to be able to provide the needed power to the hydraulics. Visedo SRPM motors which which can supply maximum torque in millisecond even from zero speed allows much more efficient use of energy. Visedo’s SRPM motors are extremely rugged and can hold up to the high vibration environment typical of off road processing machinery. The image below shows hybrid stone crusher powered by a Visedo electric drive train in operation:
Visedo typically builds serial hybrid system in which a diesel engine running at a near constant speed turns a generator, which charges up a bank of super capacitors, which then drive the electric motor. The super capacitors are also charged from regenerative braking of the wheels and of the processing machinery. The fuel savings of the hybrid system are quite large and in some cases are nearly 50%. The heavy duty machines which use the diesel engines are huge fuel gobblers so that the extra expense of these hybrid drive trains can pay for itself in a period of 1 to 3 years.
In the case of a hybrid stone crusher designed by Visedo and a stone crushing and recycling company called Rockster the hybrid machine not only saves fuel, but also increases productivity by 40% because of superior handling capabilities compared to the diesel/hydraulic version of the same machine.
Unfortunately, hybrid machine’s which merely cut diesel fuel use in the range of 10% to 50% is not a long term solution to the problem of either climate change or fossil fuel depletion. In the long run (or perhaps even the short run if the worst case scenarios of climate change consequences come to pass) diesel fuel made from fossil hydrocarbons has to be eliminated. In the case of heavy machinery operating near the transmission grid battery electric machines are technically possible, though at present high battery costs and the high required frequency of recharging make them economically unattractive relative to diesel powered machinery. Continued improvement in battery costs and energy density could eventually change this situation.
For true off road application of heavy machinery which are relatively far from the grid another solution will be required. One possibility is fuel cells operating off of carbon emission free hydrogen (or possibly ammonia). One does not necessarily need a fuel cell in each machine to make this option work. You could potentially take a large solid oxide fuel cell and tank full of hydrogen or ammonia to your remote site and recharge the battery powered vehicles using the fuel cell. Again such an option is almost undoubtedly economically inferior to diesel machines at current oil prices, but it at least represent a route to technical feasibility of such off road operations in a post fossil fuel world.
Visedo is also producing hybrid drive trains for transportation applications, but not for passenger cars. Instead they are concentrating on application requiring high power such as hybrid buses and marine transport. I am guessing that in the passenger car power range Visedo electric motors are more expensive than more traditional designs, but offer advantages for high power applications which make them competitive.
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.
In the early 1980s R. Buckminster Fuller proposed the construction of a global electricity grid as a means of transporting energy from solar and wind generators long distances over the surface of the earth. Fossil hydrocarbons such as coal, oil, and natural gas can relatively easily be transported in pipelines, train cars, and on ocean going vessels, but electricity generated from wind and sunlight cannot be transported by these methods. Such long distance electricity grids represent a very large infrastructure investment which would require unprecedented levels of international economic and political cooperation to bring to fruition.
Renewable Energy World recent published an article about representatives from Japan, China, Korea, and Russia signing a memorandum of understanding (MOU) to conduct technical and economic feasibility studies about creating an electrical grid which would allow large amounts of wind and solar energy to be transmitted between countries in the region of northeast Asia. This MOU arose out of the efforts of Masayoshi Son, a founder, chairman, and chief executive officer (CEO) of Softbank Group, a Japanese multinational telecommunications and internet corporation, who was energized by the Fukushima disaster in Japan to seek for carbon free energy alternatives to nuclear fission.
Whether or not the idea of distributing renewable energy over large geographical areas can be economically effective is not clear. High voltage DC transmission lines can transmit power of long distances with losses of 5% per 2000km and so is a feasible technology for Mongolia to southern China power transmission (The distance from Ulaan Baatar to Hong Kong is 2900km). However, a lot of physical and economic modeling will be required before anyone can be convinced to invest in such a huge transnational infrastructure project. The recent MOU is the first step down the road towards such modeling.