The Low Carbon Energy Group consists of two programs – the Capture effort carried out by Bob Williams with colleagues Tom Kreutz and Eric Larson, and the Energy Storage program led by Craig Arnold. The Capture group focuses on carbon capture from large-scale fossil fuel and biomass sources, including electric and synfuels plants. The Energy Storage group seeks improved strategies for energy storage that are compatible with renewable energy sources.
Highlights for 2012 include participation in a National Coal Council study of CCS for coal via enhanced oil recovery requested by US Energy Secretary Steven Chu, a new study of CO2 “activation” to make low-carbon synthetic transportation fuels, and a study that suggests lithium iron phosphate batteries have the potential to significantly reduce system costs for off-grid renewables.
Enhanced Oil Recovery (EOR) for Early Applications of CO2 Capture Technologies
- The Capture group played a leading role in a study prepared by the National Coal Council for Energy Secretary Chu on CO2 EOR applications of capture technologies; a major Capture group contribution is the finding that synfuel plants and plants making synfuels + electricity as a major coproduct are likely to be competitive even in distant EOR markets if an adequate CO2 pipeline network is in place.
- In studying the relative profitabilities of coal/biomass plants making diesel and gasoline synfuels and plants coproducing such synfuels plus electricity, it was found that coproduction plants are almost always more profitable.
- A public policy proposal is being developed to use federal tax revenues from new crude oil production via CO2 EOR and synfuel production to help finance several early plants that sell captured CO2 for EOR - as a subsidy mechanism for capture technology cost reduction through experience (learning by doing) that would not aggravate the federal deficit.
Studies for the National Energy Technology Laboratory (NETL)
- NETL-supported studies were completed of coproduction of electricity and light olefins from coal and from coal/biomass and coproduction of electricity and ammonia from coal.
- A new one-year project supported by NETL will identify coal/biomass strategies for making synthetic jet fuel in the Ohio River Valley.
- An analytical framework has been developed for investigating the climate-mitigation implications and economics of a strategy termed “CO2 activation,” in which low-carbon H2 and fossil CO2 are combined to make synthetic liquid transportation fuels.
- Publication of The Global Energy Assessment: Toward a Sustainable Future, the Fossil Energy Chapter of which was co-authored by Princeton and Tsinghua University researchers.
- Contributions to the ongoing NETL project on making low-carbon jet fuel by a visiting scientist from SINOPEC’s Research Institute for Petroleum Processing in Beijing.
- Participation in meetings convened by the Shenhua Corporation (the world’s largest coal company) to discuss creation of a Strategic Research Institute (SRI) and a new magazine (Cornerstone) for the World Coal Association that will communicate hopeful coal strategies to world leaders in the public and private sectors.
- Collaborations with researchers at the Energy Conversion Systems Group at Politecnico di Milano and Politecnico di Torino led to the publication of several papers.
Energy Storage Technologies
- Multiple competing mechanisms were identified that gradually increase mechanical stress and the rate of cell degradation in lithium-ion cells over time.
- Higher levels of mechanical stress were found to accelerate the rate of capacity fade in lithium-ion cells, highlighting the importance of stress management in these cells.
- A study of the fundamental transport properties of lithium-ion battery separators is the first step in developing separators that can sustain deformation without inhibiting battery performance.
- The dependence of battery charge and discharge efficiency is modeled and experimentally verified in lithium-ion batteries.
- Lithium iron phosphate batteries are shown to have much better longevity and charge acceptance than lithium cobalt oxide, lithium nickel manganese cobalt oxide, and lead-acid batteries when aged with a variable power profile typical of an off-grid wind system.
- Energy storage used in complex variable power systems, like renewable energy and electric vehicle applications, is modeled in the frequency domain to provide a better metric for characterizing and designing hybrid storage systems.
During 2012 Williams investigated prospects for early deployment of CO2 capture technologies for which storage is carried out in conjunction with CO2 enhanced oil recovery (CO2 EOR). In so-called miscible flooding, injected CO2 dissolves in oil, reducing its viscosity and facilitating oil flow to the well bore. When the crude oil is recovered, the CO2 released from the oil is captured and re-injected to extract more oil. After repeated CO2 recycle, nearly all of the purchased CO2 ends up being stored in the reservoir from which the crude oil is recovered.
Two of the CO2 EOR-related projects gave focused attention to the relative economics of alternative CO2 capture technologies for mature technologies—so-called Nthsup>-of-a-kind (NOAK) plants. The third explores an innovative subsidy mechanism for capture technology cost reduction from first-of- a-kind (FOAK) cost levels through experience (learning by doing) in CO2 EOR applications.
In October 2011 Energy Secretary Chu asked the National Coal Council (NCC) to prepare for him a report on CCS for coal via CO2 EOR. The NCC invited Williams to participate in this study, and he was given the lead responsibility for the report’s analyses of synfuels and synfuels/electricity coproduction, and analyses comparing the economics of all capture options. Williams persuaded the NCC study team to consider natural gas as well as coal in considering synfuels and coproduction systems and to consider co-processing modest biomass amounts (< 10% on an energy basis) with both feedstocks in early-mover projects.
Important findings of the NCC report based on Williams’ internal rate of return on equity (IRRE) analyses for NOAK versions of present-day technologies are (for an assumed oil price of $90/bbl and no price on greenhouse gas emissions):
- The only power-only option offering attractive an IRRE is a post-combustion capture retrofit for an existing coal power plant when such a system is located near a CO2 EOR site.
- Synfuel and coproduction plants located practically anywhere in the US could compete in CO2 EOR markets (even distant markets) if an adequate CO2 pipeline infrastructure were in place.
The latter led the NCC study group to its major finding that it is technically and economically feasible to increase US crude oil production via EOR using CO2 captured at coal plants from the current level of 280,000 bbls/day to 3,600,000 bbls/day by 2035.
The NCC study stressed the strategic importance of deploying first-of-a-kind (FOAK) projects for promising capture technologies as needed first steps toward realization of the identified opportunities. However, because FOAK capture projects are already going forward for pre-combustion, post- combustion, and oxycombustion capture technologies, the only new FOAK project recommended by the NCC study is for a plant selling CO2 for EOR that coproduces synthetic transportation fuels and electricity based on coprocessing a modest amount of biomass with coal. The NCC study argued that the coal industry and the Energy Secretary should work together to find a way whereby such a plant could be financed, built, and demonstrated at commercial scale.
The Capture group has previously analyzed synthetic fuels production for NOAK plants based on both: (a) the Fischer-Tropsch liquids (FTL) process to produce diesel fuel and gasoline from coal (CTL) and from coal/biomass (CBTL), and (b) the methanol to gasoline (MTG) process to produce gasoline from coal (CTG) and from coal/biomass (CBTG) via methanol as an intermediate product. System configurations that recycle unconverted syngas to maximize liquid fuel output were analyzed, as well as system configurations that provide electricity as a major coproduct.
During 2012 Williams extended these analyses to CO2 EOR applications for synfuel options involving CO2 capture, to explore relative profitabilities of recycle and coproduction configurations. The expectation when the work started was that at sufficiently high oil prices recycle will be the more profitable; the analysis is aimed at identifying breakeven oil prices for both FTL and MTG systems, and for both coal only and coal/biomass options.
When only coal is a feedstock: When only coal is the feedstock, it makes sense to consider plants of equal capital cost in comparing recycle and coproduction configurations. For FTL it was found that recycle is more profitable than coproduction at crude oil prices higher than about $100/bbl (with no price on GHG emissions)—but also that profitabilities are comparable over a wide range of oil prices (e.g., $80 to $120/bbl). In contrast, for MTG there is no oil price at which coproduction is more profitable than recycle.
When biomass is coprocessed with coal: Because biomass is typically more expensive than coal, coprocessing biomass can be cost-effective only under a carbon mitigation policy. It makes sense to compare systems with equal inputs of scarce biomass and offering equal carbon-mitigation benefits as measured by the greenhouse gas emissions index (GHGI)1. Here IRRE values are estimated for two FTL options and two MTG options that coprocess enough biomass to reduce GHGI to 0.17, assuming a $50/t GHG emissions price - see Table 1. The modeled biomass is switchgrass, the growing of which does not require good cropland.
For MTG systems, recycle is more profitable than coproduction for crude oil prices greater than about $80/bbl. For FTL systems, coproduction is always more profitable. This finding arises from the greater percentage of feedstock carbon stored as CO2 for coproduction (65% vs 54%). The corresponding greater negative CO2 emissions from photosynthetic CO2 storage implies a smaller biomass input percentage (24% vs 38%) to realize the targeted GHGI, which in turn implies a loweraverage feedstock cost. Also, coproduction enjoys scale economy benefits as a result of the assumed equal biomass rates for recycle and coproduction. The FTL finding is especially important in light of the greater need for diesel than for gasoline - which implies that much more FTL capacity than MTG capacity is likely to be built. Table 1 presents IRRE values for these four coal/biomass options when the crude oil price is $90/bbl.
Because, in the absence of a price on GHG emissions, required levels of subsidy for launching CO2 capture technologies in the market are likely to be so high that it would not be feasible to use general federal funds (GFF) to fill the “cost gap,” Williams has been exploring an innovative strategy for government support of early launch of CO2 capture technologies for systems that sell CO2 into enhanced oil recovery (EOR) markets. The strategy involves using federal tax revenues from new crude oil production via CO2 EOR and synfuel production to help finance (via competitively-bid grants proportional to the captured CO2 amount) early plants selling captured CO2 for EOR. The aim is capture technology cost “buydown”—i.e., cost reduction through experience (“learning by doing”).
The Capture group has focused on estimating NOAK costs for energy conversion systems. For this CO2 capture technology cost buydown analysis, it is assumed that: (a) FOAK costs for capital and operation and maintenance (O&M) costs for new technology are 2.0 X NOAK2 costs (b) capital and O&M costs decline with experience at the same rate as the learning-by-doing (LBD) rate for SO2 scrubbers (11% for each cumulative doubling of production), and (c) the subsidy must be large enough to reduce the cost of generating electricity to that for a natural gas combined cycle power plant venting CO2, currently the technology of choice for new power plants in the United States. To the extent possible, subsidies would be financed with the new federal revenue streams associated with new domestic liquid production. Required subsidies in excess of what can be provided by these new federal revenue streams would be paid for out of GFF.
The analysis is being carried out for four systems that make only electricity and four systems that coproduce transportation fuels and electricity. Findings are presented here for one of the power options (NGCC: a natural gas combined cycle plant) and for two coproduction options that coprocess biomass (an FTL system for natural gas/biomass and an MTG system for coal/biomass—see Table 2). It is assumed that the FTL plant is located at the natural gas wellhead but that the MTG and NGCC plants are located at average power-plant sites. The coproduction systems investigated were designed with just enough biomass (7% in the natural gas-based FTL case and 5% in the coal-based MTG case) to realize GHGI = 0.50.
For NGCC the required subsidy declines slowly with the cumulative number of plants built (see Figure 7); the Nth (59th) plant still requires a $30/tCO2 subsidy, and GFF contributions to the subsidy are needed until after 44 plants have been built. In sharp contrast, for the coproduction options the required subsidy declines rapidly. No subsidy is required for the 12th plant, and GFF subsidy contributions are needed only for the first CBTG plant and the first three GBTL plants. Moreover, new federal revenues (net of required subsidies) from domestic liquid production for the first 12 plants amounts to $17 billion for CBTG and $2.6 billion for GBTL. This suggests that market launch for both coproduction technologies and the associated biomass supply logistics technologies can be accomplished in the absence of a price on GHG emissions. Moreover, this can be done in a way that contributes significantly to federal deficit reduction rather than to its buildup, despite huge subsidies for early projects (e.g., $1.8 billion for the first FTL project and $2.9 billion for the first MTG project).
In October 2010 the Capture group was awarded a grant from the National Energy Technology Laboratory (NETL) to investigate Energy, Environmental, and Economic Analyses of Design Concepts for the Co-Production of Fuels and Chemicals with Electricity via Co-Gasification of Coal and Biomass. Led by Larson, the early research program explored the coproduction of gasoline and electricity via co-gasification of coal and biomass in a single oxygen-blown entrained-flow gasifier.
The NETL grant co-supported additional work (beyond what was reported last year) that is described here: research on the co-production of electricity and bulk chemicals – ethylene/propylene and ammonia. This analytical extension is aimed at understanding implications for carbon mitigation and economics of systems providing, as coproducts of electricity, liquids that have higher market values than transportation fuels.
In the coproduction of light olefins and electricity, coal and torrefied biomass are first co-gasified in a dry-feed, entrained-flow gasifier. The synthesis gas from the gasifier is then converted to methanol, and the methanol is subsequently processed through a commercially-available methanol-to-olefins synthesis reactor. Both coal to olefins (CTO) and coal/biomass to olefins (CBTO) options were analyzed. Process configurations with different biomass feed percentages [HB = high (30%) biomass; LB = low (5%) biomass] and electricity output percentages (EF25 = 25% electricity; EF50 = 50% electricity) were simulated using Aspen Plus software.
Figure 8 shows some of economic findings (in $2007) from an electricity generator’s perspective. The figure shows that the options providing a 25% electricity fraction outperform the options for which electricity accounts for 50% of energy output. Moreover, the levelized cost of electricity (LCOE) for both CTO-EF25 (GHGI = 0.41) and CBTO-EF25LB with 5% biomass (GHGI = 0.30) are lower than for a new supercritical coal plant with CO2 vented (PC-V, with GHGI = 1.00) at all GHG emissions prices. For CBTO-EF25HB, which has a strong negative GHG emission rate (GHGI = - 0.32), a GHG emissions price of only $20/t CO2 is needed in order to offer the same LCOE as PC-V. The negative GHGI arises from the assumption that carbon in the olefins is sequestered permanently when products made from them are land-filled at the end of their lives.
Because neither electricity nor ammonia contain carbon, it is possible in a system with CCS to achieve near complete decarbonization for such systems without biomass. For this reason, only coal inputs were considered for the ammonia/electricity coproduction analyses. Two sets of coproduction cases were analyzed: one considered steady-state plant operation and a second examined potential impacts of diurnally-varying production rates aimed at exploiting higher electricity values during peak electricity demand periods.
In contrast to the findings for coproducing olefins and power, it was found that LCOE values for the steady-state ammonia-electricity coproduction cases are higher than those for PC-V plants, even assuming prices for natural gas and hence values for the ammonia (since the primary feedstock for commercial ammonia production today is natural gas) that are far higher than prices prevailing today in the U.S. and even when assuming a $100/t tCO2 greenhouse gas emissions price.
In process simulations for plants designed to be able to vary the electricity/ammonia output ratio (producing more power when electricity is highly valued) various practical challenges were ignored— such as those associated with rapid up-and-down ramping of plant components, efficiency penalties that might occur with such ramping, and long-term maintenance and equipment fatigue issues with repeated cycling. It was found, nonetheless, that the internal rates of return for co-production were not sufficient to make this alternative plant design and operating strategy an economically viable one.
In late 2012, the Capture group launched a new one-year project with co-support from a new grant from the National Energy Technology Laboratory. The goal of this work, which is still in its early stages, is to identify coal/biomass-to-liquid (CBTL) system implementation strategies for the Ohio River Valley that might increase the viability of constructing and operating such plants there in the next 5 to 10 years. The emphasis is on assessing technical and economic viability of alternative plant designs for producing primarily synthetic jet fuel that, when blended 50:50 with conventional jet fuel, will meet or beat current and potential future regulatory requirements.
Tom Kreutz is leading the design and simulation of process configurations that will include separate coal and biomass gasifiers feeding syngas to cobalt-catalyzed Fischer-Tropsch synthesis reactors. The latter produce primarily heavy paraffins that are hydrocracked and refined to aviation fuel. Designs that co-produce different levels of electricity will be assessed, and plant designs with different biomass/ coal input ratios will be analyzed. Byproduct CO2 will be captured and sold for use in enhanced oil recovery (EOR), resulting in the carbon ultimately being stored permanently underground.
The Capture group is collaborating in this project with engineers at Booz Allen Hamilton, who are contributing feedstock supply analysis and detailed lifecycle greenhouse gas accounting to complement detailed process design/simulation that will be carried out by the Capture group. The work is benefiting from a one-year visit to Princeton by Dr. Qiang Li, whose home institution is Sinopec’s Research Institute of Petroleum Processing in Beijing.
Working with Robert Socolow, Tom Kreutz has developed an analytical framework for investigating the economics and climate mitigation implications of making transportation fuels from CO2, termed “CO2 activation” (CCA). The research has focused on a prototypical system that combines low carbon H2 (i.e. H2 produced from renewable or nuclear energy sources) and CO2 to make synthetic liquid transportation fuels via the reverse water gas shift reaction followed by Fischer-Tropsch synthesis. Detailed simulations of thermodynamic performance have been carried out as a basis for economic analysis.
Kreutz found that high oil prices favor CCA over the two alternatives: CO2 venting [Vent (BAU)] and CCS (Figure 9). At low CO2 emissions prices (below the price P* needed to induce CCS), CCA mitigates GHG emissions by capturing CO2 en route to the atmosphere and re-using the carbon to make synthetic transport fuels. In this way the carbon is “used twice” before entering the atmosphere as CO2. This climate benefit is reflected in Figure 9 by a breakeven oil price (BEOP) between CCA and Vent (BAU) which falls with increasing CO2 price; in other words, a rising carbon tax makes CCA increasingly competitive. At CO2 emissions prices above P*, however, the BEOP for CCA becomes flat; compared to the alternative option, CCS, CCA provides no benefit to the climate.
CO2 EOR is much less expensive than CCA, which requires costly capital equipment and vast quantities of low carbon H2. As a result, the oil prices required for profitable CO2 EOR (blue lines) are much lower than those needed to induce CCA (black lines), and thus CO2 EOR is the economically preferred method of “converting” CO2 to transportation fuels. The climate benefits of CCA and CO2 EOR are also quite similar.
This work confirms a key result from a previous paper by Kreutz (presented in 2010 at the 10th International Conference on Greenhouse Gas Control Technologies), that CCA does not significantly reduce CO2 emissions when CCS at power plants is an economically viable option for CO2 disposal. Moreover, the present analysis shows that, if CO2 EOR is available as an option for providing additional liquid fuels, EOR will be far more profitable than CCA while providing roughly comparable carbon mitigation benefits.
The China-related activities of the Capture group are expanding. Collaborations with Chinese colleagues are especially important in light of the high level of interest in China in coproduction technologies that are the focus of much of the Capture group’s recent work. The coal chemical process industry in China has extensive experience with modern coal gasification technologies (more than all the rest of the world combined), and there is much interest in extending this industrial experience from niche chemicals markets to the much larger fuels and electricity markets that require very similar energy conversion technologies.
Zheng Li. A highlight of the Capture group’s long-term collaboration with Zheng Li at Tsinghua University was the publication in the fall of 2012 by Cambridge University Press of the Global Energy Assessment: Toward a Sustainable Future - an 1865 page (5 kg!), 25-chapter IPCC-style study involving several hundred authors and reviewers globally. The GEA describes technologies and strategies for addressing the major societal challenges related to energy. It is anticipated that the GEA will be regarded as “essential reading” for public- and private-sector decision makers worldwide who are interested in advancing energy toward sustainable development goals.
Eric Larson and Prof. Zheng Li were the Co-Convening Lead Authors for the Fossil Energy Chapter of this report. Williams was a Lead Author, and Guangjian Liu was one of the contributing authors. The Fossil Energy chapter highlights the importance of co-processing coal and biomass with CCS and natural gas and biomass with CCS for meeting sustainability goals. In addition Larson was a Lead Author of the Renewable Energy Chapter for which he made contributions relating to biomass.
Guangjian Liu. The collaboration with Guangjian Liu, established when he was a post-doc with the Capture group during 2008-2010, has been sustained since he returned to Beijing to a faculty position at the North China Electric Power University (NCEPU).
During 2012 the continuing collaboration with Prof. Liu included i) research on olefin/electricity coproduction systems (described above), ii) Williams’ visiting NCEPU in October 2012 as part of the continuation of the Capture group’s participation in the Coal Conversion and Utilization Research and Education Project led by NCEPU (see 2011 CMI annual report), and iii) Liu’s contributions to a new undergraduate engineering course at Princeton, “The Energy-Water Nexus,” taught by Larson.
Dr. Qiang Li. Dr. Xiangbo Guo, a scientist at SINOPEC’s Research Institute for Petroleum Processing (RIPP) in Beijing, spent calendar year 2010 as a visiting research fellow with the Capture group, where he contributed his understanding of refining processes to the Capture group’s coal/biomass- to-liquids work.
In 2012, Dr. Guo introduced the Capture group to his colleague Dr. Qiang Li, who had recently been awarded the same prestigious award by SINOPEC that allows him to spend a sabbatical year at a U.S. university. The Group invited Dr. Li to Princeton, and he is making contributions to the ongoing Capture group project on the production of low-carbon jet fuel from coal and biomass.
Advising Shenhua. Williams was invited by the Shenhua Corporation (the world’s largest coal company) to a brainstorming session in Chicago in July 2012 as well as to a followup meeting in Beijing in October 2012 to discuss two initiatives that Shenhua Chairman Xiwu Zhang is pursuing in his new capacity as Chairman of the World Coal Association - creation for the WCA of: (a) a Strategic Research Institute (SRI) for coal that will deal with strategies for the future of coal worldwide, and (b) a new Cornerstone magazine that aims to communicate hopeful strategies to world leaders in the public and private sectors who are interested in coal issues.
In these meetings Williams stressed the strategic importance for coal of strategies such as those articulated in the Global Energy Assessment that would enable coal to make important contributions to sustainable development goals for global society. Subsequently, Williams recommended a strong candidate for editor of Cornerstone, who was hired for that position.
In 2012 the Capture group continued its longstanding collaboration with the Energy Conversion Systems Group at Politecnico di Milano (POLIMI). The research used the novel bottoming cycle optimization methodology of Prof. Emanuele Martelli (former Capture group visiting researcher) to re-analyze and understand more deeply the efficiencies of advanced energy conversion facilities that produce both electricity and liquid fuels. By generating both theoretically optimal and more realistic (i.e., economically viable) plant configurations for waste heat recovery, Martelli’s software provides important context for previous Capture group work, highlighting the difference between sub- optimal designs versus fundamental concessions necessary for improved operability and economics.
The research of Dr. Andrea Lanzini (in residence with the Capture group in 2010 as a Fulbright scholar and currently a researcher at Politecnico di Torino) on solid oxide fuel cells and fuel cell/gas turbine (FCGT) hybrids was the focus of another Italian collaboration. Lanzini applied the Capture group’s systems analysis methodologies to advanced power plants, coupling coal gasification with FCGT hybrids, focusing on strategies for “methanating” synthesis gas upstream of the SOFC in order to improve overall conversion efficiency and reduce plant costs.
Energy storage is playing an increasingly important role throughout the energy infrastructure, from powering hybrid and electric vehicles to offsetting the inherent intermittency of renewable energy generation. Unlike batteries for electronic devices, which can be charged using a pre-determined protocol simply by plugging them into the wall, many of these applications are characterized by highly variable charge and demand profiles. The Energy Storage Group headed by Craig Arnold is working to characterize how such variability in charging powers affects battery behavior in order to improve overall system efficiency and lifespan.
This year, the Energy Storage group has identified several sources of mechanical stress within lithium- ion cells and investigated the impacts of this stress on battery life and performance, identifying several potential areas for improvement.
Stress evolution in Li+ cells. Lithium-ion cells typically operate under some level of compression applied by a rigid constraint, for example a cylinder battery cell. The level of initial stress is fixed by the manufacturer, but this stress is not constant over the life of the cell owing to electrode expansion during charging/discharging as well as other effects. Knowledge of the stress evolution over the cell’s entire useful life is important for understanding and predicting battery cell degradation, which is heavily influenced by stress state.
Mechanical stress evolution was found to be a function of three competing mechanisms: viscoelastic stress relaxation of the polymeric battery components, volumetric changes of the battery active material due to lithiation, and growth of the SEI (a solid film produced by side reactions). The relative importance of these mechanisms were found to change depending on the initially applied stack pressure, with viscoelastic stress relaxation becoming a more dominant mechanism with increasing levels of stress. At long time scales, SEI growth became a dominant mechanism, increasing stress in all cells. (Figure 10) Future plans include development of more mechanically robust cells through improvements in materials selection and design.
Effects of stress on capacity fade. It is well known that mechanical stress (on the order of hundreds of MPa) that builds up within the electrode particles as a result of particle expansion during lithiation is linked to capacity degradation through particle failure. However, little attention has been given to the relatively modest pressures (tenths of MPa) that are applied to the entire cell during manufacturing and that build up during normal operation. It is commonly believed that these modest pressures have no negative effects on cell operation. However, due to the nature of the soft materials employed in some of the battery components such as the separator, these applied stress levels can result in major deformations over time which could potentially impact battery degradation over the lifetime of a cell.
Arnold and colleagues investigated the effect of applied stack pressure on the electrochemical performance and capacity retention characteristics of lithium-ion batteries. It was found that higher levels of stack stress resulted in higher rates of capacity fade. However, it was also shown that very small amounts of stress (on the order of hundredths of MPa) are beneficial to capacity retention through the prevention of electrode layer delamination. Upon disassembly of the cells it was discovered that growth of a surface film on the electrodes had occurred in the stressed cells, with highly stressed cells showing more film growth. This coupling between stress and chemistry had not been anticipated and will be a subject of future investigation.
Effects of stress on ion transport. High power battery operation requires very fast transport of lithium-ions through a liquid phase electrolyte between anode and cathode of a battery cell. In a real cell, this liquid phase is contained in a porous polymeric separator which is placed between the two electrodes to keep them from coming into contact and creating a short circuit. During battery manufacturing and operation, applied stresses build up which result in compression of the separator, which is relatively compliant compared to the battery electrodes. This separator deformation results in pore closure which ultimately restricts ion transport between the battery electrodes. Knowledge of how the impedance associated with this transport restriction varies with deformation is critical for predicting performance in high power cells.
Arnold’s group measured the impedance as a function of deformation of commercial separators by compressing a pouch cell containing separator wetted in electrolyte but no active battery material while simultaneously measuring impedance. Wet-manufactured, dry-manufactured, monolayer, trilayer, polypropylene, and polyethylene separators were tested. A relationship between deformation and impedance using the Bruggeman tortuosity-porosity relationship was derived and verified by curve fitting experimental data (Figure 11). Using the derived relationship the empirical Bruggeman parameters could be determined by curve fitting the experimental data, yielding fundamental information about transport in the separators. Future work will focus on development of separators that can sustain deformation without restricting transport.
The variability of wind, solar and other similar power sources necessarily means that batteries in these systems are charged over a range of different powers. Discharge efficiency is known to have a dependence on the discharge power; in Krieger and Arnold (2012) a similar effect on charge efficiency is modeled and experimentally confirmed for charging power. Both models have been expanded to account for an additional limitation to battery capacity as power increases: significant undercharging and underdischarging due to voltage limitations. As power increases, the charging voltage is offset higher and discharging voltage offset lower, leading to premature voltage cutoffs; this effect is more pronounced on charging due to the non-symmetric shape of the voltage curves. (see Figure 12) Energy-power relationships in battery charging and discharging are therefore found to be highly dependent on both the efficiency of charging and voltage limitations at any given power, which must be taken into account when designing battery systems operating over a variable range of powers.
Battery degradation in off-grid renewable applications
The stresses of highly variable and incomplete charging in off-grid renewable energy systems result in rapid degradation of the lead-acid batteries typically used for these electrification projects, incurring large replacement costs over the lifetime of the system. To identify more promising energy
storage technologies, Arnold and colleagues compared aging rates and mechanisms among constant- charge and wind-charged lead-acid, lithium cobalt oxide (LCO), LCO-lithium nickel manganese cobalt oxide composite, and lithium iron phosphate batteries.
Accelerated aging studies conducted over the course of a year find that while constant-charged lead- acid batteries last longer than wind-charged cells, lithium cobalt oxide cells last longer under variable and incomplete charging conditions than constant charging, and lithium iron phosphate cells show only 1-3% degradation under all charging protocols. While these last cells are more expensive per installed kWh than the lead-acid cells, their consistently good power and voltage performance and ability to withstand deep discharge and incomplete charging allow the systems to be sized smaller. Combined with their long lifespan in variable power conditions, these results suggest significant potential for lithium iron phosphate batteries to reduce system lifetime costs for off-grid renewables.
Variable power energy storage requirements may be best met by a suite of energy storage technologies instead of a single device. Applications like electric vehicles require both slow delivery of energy and rapid absorption of power during regenerative braking. Variable powers, as seen in the previously described projects, affect the efficiency and lifespan of energy storage devices to different degrees. Batteries may be better at providing bulk storage, whereas ultracapacitors are good at handling high-power bursts. The metrics to understand these complex systems are limited, however. Energy storage devices may be characterized by energy or power density, or discharge time.
Arnold and colleagues are working instead to re-frame energy storage in the frequency domain. In systems where power supply demand has both rapidly changing and slowly changing components, this power profile can be translated into the frequency domain to quantify the energy contained in high, medium or low frequency oscillations. Energy storage devices are also classified by their ideal frequency range – e.g. high frequency for ultracapacitors, low for compressed air energy storage. Frequency analysis is performed using wavelet transforms, which can accommodate the non- stationarity characteristic of many of these systems. This classification of energy storage systems in the frequency domain allows for improved understanding of complex and hybrid systems.
Cannarella, J., C. B. Arnold. “Ion transport restriction in mechanically strained separator membranes.” Journal of Power Sources, 226 pages 149-155. doi: 10.1016/j.jpowsour.2012.10.093. 2013.
Chiesa, P., M. C. Romano and T. G. Kreutz. “Use of Membranes in Systems for Electric Energy and Hydrogen Production from Fossil Fuels.” In: Handbook of Membrane Reactors, Volume 2: Reactor Types and Industrial Applications, Angelo Basile (ed), Woodhead Publishing Limited, Philadelphia, PA, 2013.
Kreutz, T. G. and R. Socolow. “Prospective Economics of CO2 Capture and Activation to Transportation Fuels.” Submitted to The 12th Annual Carbon Capture, Utilization and Sequestration Conference, Pittsburgh, PA, 13-16, May 2013.
Krieger, E. M. and C. B. Arnold. “Effects of undercharge and internal loss on the rate dependence of battery charge storage efficiency.” Journal of Power Sources, 210, pages 286-291. 2012.
Krieger, E.M., J. Cannarella and C. B. Arnold. “A comparison of lead-acid and lithium-based battery behavior and capacity fade in off-grid renewable charging applications.” Submitted 2013.
Lanzini, A., T. G. Kreutz and E. Martelli. “Techno-Economic Analysis of Integrated Gasification Fuel Cell Power Plants Capturing CO2.” GT2012-69579, ASME Turbo Expo 2012, Copenhagen, DK, June 11-15, 2012.
Lanzini, A., T. G. Kreutz, E. Martelli and M. Santarelli. “Energy and Economic Performance of Novel Integrated Gasifier Fuel Cell Cycles with Carbon Capture.” Submitted to the International Journal of Greenhouse Gas Control, Jan. 2013.
Larson, E.D, R.H. Williams, T.G. Kreutz, Lanzini, A., Hannula, I., and Liu, G. “Energy, Environmental, and Economic Analyses of Design Concepts for the Co-Production of Fuels and Chemicals with Electricity via Co-Gasification of Coal and Biomass.” Final report to National Energy Technology Laboratory under DE- FE0005373. June 30, 2012.
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Larson, E.D. and Z. Li (Co-Convening Lead Authors), T. Fleisch, G. Liu, G. Nicolaides, X. Ren, and R.H. Williams. “Fossil Energy Systems.” In: The Global Energy Assessment, Chapter 12. Cambridge University Press, Cambridge, UK, 2012.
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