The Prospects for Grid Scale Energy Storage
The economic engine of the United States uses over 40 quadrillion BTUs of energy per year, with only a small part of this being supplied by renewable energy sources (RES). The low rate of adoption of RES’s is largely due to under-utilization of these energy sources caused by the highly variable power capability (sunlight for PV and wind currents for turbines) and a misalignment of that power capability with demand.
A solution to this problem is large-scale energy storage, which will allow RESs to store energy locally during peak generating periods and then reliably deliver stored energy on demand. These energy storage solutions must be safe, environmentally benign, efficient, use materials that are available to meet the total demand for the technologies, and provide the lowest cost alternative. ViZn Energy’s Zinc – Iron Flow Battery uniquely meets these requirements and offers safe, efficient, reliable and affordable grid-scale energy storage solutions.
Energy Storage and National Policy
Energy is the lifeblood of our civilization; when access to clean, safe, and abundant energy is limited, growth and sustainability of any nation is crushed. Energy is the new currency – manufacturing, food production, transportation, and even the information superhighway are all dependent on reliable sources of energy, and the ability to harness and transport that energy is a persistent challenge to our civilization.
Our energy needs are of an enormous scale: the world requires over 200 quadrillion BTU’s (1 quadrillion BTU’s = 1 Quad) of energy per year to meet its service needs, and the US uses about 20% of this, or about 40 Quads. At 0.05$ per kilowatt-hour, the US energy bill is about 600 billion USD per year; this is about $1700 for each US citizen, equivalent to what we spend on food. Furthermore, over the next 25 years the US need will grow to about 50 Quads per year while the need in China will increase to about 80 Quads per year.
Today our energy supply is serviced from non-renewable energy sources (NRES) such as coal, petroleum, natural gas, and nuclear fission, plus renewable energy sources (RES) such as hydro, wind, solar, biomass, and geothermal generation. In 2009, the United States spent about 95 Quads of primary energy to produce 40 Quads of service (useful) energy. Thirty percent (30%) of this energy was in the form of electrical power.
National energy policy is driven by both the effect of carbon emissions on the environment and the effect of energy dependence on national security. The carbon offset cost as a consequence of energy storage can be quantified through an assumption of 1 terawatt-hour being equivalent to 0.6 million tonnes of carbon dioxide per day, or 216 million tonnes per year. If we assume a cost of 100B$ per terawatt-hour installation and a 20 year life, the cost of carbon dioxide abatement using this method would be $23 per tonne. While the income derived from sale of carbon offsets may not cover this cost, the resulting carbon credit would still provide a value stream that would augment revenues from energy arbitrage or regulation.
With regard to energy security, 1 Quad of energy is equivalent to 172 million barrels of oil; by this metric, 1 terawatt-hour of storage costing 100B$ would reduce imports by 4.4 billion barrels over 20 years, worth about 375B$ at current oil prices. The normalized cost of a barrel of oil equivalent for this service (excluding the opportunity cost of the energy production) is $22, while reducing our national dependence on oil by up to 6% per terawatt hour of energy storage.
The concurrent needs of carbon reduction, energy security, and a strong economy are often described as countervailing; as one is promoted, at least one of the others is reduced. The widespread deployment of RESs has been encouraged to help close this gap, but without an enabling energy storage device, the value of the energy produced and the ability of the RES to deliver on demand are severely limited. The energy storage device, however, must meet key requirements of safety, natural abundance of critical components, and cost.
The Role of Energy Storage
There are many ways to generate electrical power, both in a non-renewable and renewable sense. With the current imperatives of environmental protection coupled with energy security, however, the ideal energy source should be safe and sustainable; that is, derived from a source that is essentially inexhaustible while not contributing to net greenhouse gases emissions. In this respect, hydro, biomass, wind, geothermal, and solar have become the energy sources most preferred. Distribution of some of these is limited by geography, while the demand capability of others is poor and would lead to gaps in grid delivery if they were the sole primary source of energy. In addition, the grid is not particularly stable or even predictable – the grid is continually perturbed by variation in supply and demand due to natural diurnal and seasonal cycles.
Current methods to balance the electrical grid are based on over- capacitation, either by increasing nameplate capacity of the plant or by opening peaker plants that come on line to meet peak demands. Both of these solutions entail significant investment and offer poor ROI since the additional capacity is used for only brief periods. Some improvement to the supply and demand balance is possible if there are favorable geographic features that allow both wind and solar installations; however, even such mixes are subject to regular gaps in supply and demand. Although peaker plants could be used to support the grid during these gaps, these plants are typically fossil-fueled and therefore reduce or even eliminate the benefits of the RES.
An alternative means to support the grid and facilitate the use of RESs is to provide large scale electrical energy storage solutions. With sufficient storage, single point RESs such as wind farms or solar arrays could provide a ready and predictable response to power demand without the need for additional generation facilities.
The scale of these facilities would of course be large; on one extreme, where we presume electrification of the United States to meet all energy needs using wind or solar, the storage need could be half of the total energy need to load-shift. Presuming a 24h cycle, the energy storage need for the US would be on the order of 25 terawatt-hours; globally this would exceed 100 terawatt-hours.
Although physical means of load-shifting electrical energy storage have been proposed, they often are limited by geography (pumped hydro storage or compressed air), or by capacity or cost (super-capacitor or flywheel). In order to meet the needs of a large scale solution, it is necessary to consider electrochemical means of storage since chemical energy offers the best opportunity for a controllable and effective means of energy storage.
Batteries are electrochemical devices that allow oxidation of a substance at one location (anode), reduction of substance at another location (cathode), a means to collect the resulting electrical charge and deliver it to an external device (the circuit), and a means to depolarize the charge transfer though the exchange of ions (the electrolyte).
There are many possible combinations of substances to provide the basis of a battery, but as one might expect, not all combinations make a good battery, let alone a commercially viable battery. A commercially successful storage battery must:
be safe and free from toxic materials
be constructed of globally available and in quantities sufficient to meet long term needs
be efficient, rechargeable, and make economic sense.
The first criterion, safety, has many implications when selecting an electrochemical couple. In particular, flammability is a chronic concern in battery chemistry. Moreover, as the cell strings become longer to meet the efficiency needs of the transformers, converters and inverters, the risk of a cell reversal or severe overcharge (a common source of ignition), becomes much greater. With these concerns in mind, a non-flammable electrolyte such as water is a prudent and tactical design decision. The use of an aqueous electrolyte carries with it a number of chemically-related restrictions, however:
Anything with an oxidation potential higher than the oxidation potential of water will reduce water to hydrogen spontaneously
Anything with a reduction potential higher than the reduction potential of water will oxidize water to oxygen.
both of which should be avoided. If we consider all commonly available materials, those with high oxidation potentials (such as lithium, sodium, and aluminum) and high reduction potentials (such as chlorine) are excluded due to the chemistry with water. This leaves a much smaller subset of suitable materials.
The next selection criterion is availability of the energy storage material. If we consider the commercial abundance and the distribution of metals in the US and the rest of the world, the possibilities are limited should we want to commit a national initiative to a particular energy storage method. For example, although lithium is already excluded due its high oxidation potential, even if we allow it to be considered it falls far short of the needs in the United States and is in fact barely capable of supplying the ultimate needs of the world. Exacerbating the issue is the localization of lithium deposits, of which 75% are located in Chile. In this view, even elements such as lead or nickel are deficient as they are not capable of meeting the needs for battery production sufficient to promote sustainability. Of the elements that are stable in water, manganese, chromium, zinc, and copper are sufficient on the basis of worldwide availability, but the distribution of all of these elements do not necessarily match the geopolitical boundaries; where there is a deficiency of these elements within the geographic boundaries of any major political power, reliance on those elements for matters of national security may not be a sensible option.
If we consider the political boundaries of the highest energy consumers, the US, China, and Europe (including Russia), neither lithium, nickel, nor lead are sufficient in quantity and appropriately distributed to meet the needs of the population. Of the down-selected electro-active metals, only zinc and copper meet the needs of safety and availability.
With zinc and copper as contender battery chemistries, we should now consider the chemical potentials of each. Based on the voltage of the half cells of each, it is apparent that the energy available from zinc (at 1.28V oxidation potential for the soluble form of zinc oxide in alkali) is substantially greater than that of copper (-0.34V), and so the lower oxidation potential of copper reduces the desirability of copper as an anode. When coupled with the metal prices, the cost of copper is over ten times that of zinc on an energy basis. The cost of zinc on an energy storage basis is in fact one third that of lead or lithium, and while the design of the battery is such that the cost of the metal may be a modest fraction of the overall cost of the cell, it is still a significant consideration in light of the scale of our national energy storage needs.
Batteries for grid energy storage may be designed for either high rate / lower capacity (for short time scale frequency response), for lower rate / high capacity (for longer timescale load shifting), or for balanced performance in between. However, RESs are most dependent on the availability of low cost energy storage at longer discharge rates that approach eight hours or more. The dependency on cost drives the battery design to smaller and thicker electrode area (which is expensive to produce) and larger but less expensive storage of the electro active materials. For this reason, flow batteries have dominated grid storage since the capacity of the cell is largely dependent on the amount of storage, which can be low cost with appropriate electrolytes — aqueous electrolytes have a particular advantage in that many of the materials of construction can be inexpensive polymers, so stock Roto-molded tanks and polypropylene tanks can be used for process equipment. Of course, the pump energy can become significant and lead to substantial efficiency losses, so the system as whole (the electrode design, the storage, the flow rate, and the mobile phase electrolytes) is typically optimized simultaneously.
Flow Batteries and the Zinc-Iron System
There are several competitors in the flow-battery field, but many of them have failed to achieve success due to cost and the toxic/corrosive nature of their chemistry. Zinc bromine, zinc chlorine, polysulfide bromide, vanadium redox and zinc cerium are currently being commercialized, but concerns for public safety and developmental setbacks have limited their success and appeal to date. Flow battery technologies using vanadium oxides have also seen limited commercial success due to not only technical issues, but also the cost of vanadium, which is used in steel manufacturing and has become a limited and strategic material that is largely controlled by foreign interests.
Iron – chromium ion flow batteries are in an early developmental stage but offer most of the positive attributes that flow batteries promise. However, the nature of the iron-chromium ion chemistry leads to low electrochemical efficiency that compromises the ability to meet commercial requirements.
ViZn Energy’s zinc – iron flow battery consists of a zinc / zincate anode, a cathodic iron anion complex in an aqueous alkaline supporting electrolyte, and proprietary high efficiency electrodes in a stack configuration that allows parallel electrolyte feeds without significant shunt losses. The selection of chemistry permits the use of common and low cost materials such as zinc oxide, iron salts, polypropylene, steels, and nickel coatings. The electrochemistry has round-trip energy efficiency comparable to lithium ion, yet as a water-based electrolyte, provides unsurpassed safety. The low cost, safety, and efficiency inherent to this system thereby provides a natural scalability to the megawatt-hour range and above.
The commercialization of the ViZn Energy Zinc – Iron battery has focused on three critical areas:
Electrodes: ViZn Energy has developed high efficiency (low polarizing at low flow rates and low gassing) electrode designs and alloys that improve the cost of operation of the battery. Consequently, these electrodes allow high power charge and discharge that makes the designs suitable for a variety of revenue streams.
Electrolytes: Low-cost aqueous electrolyte blends have been developed that improve the deposition quality of zinc and thus capacity and allows extended reliable service. In addition, these electrolyte blends allow higher concentrations of metal salts without the associated problems of precipitation.
Manifolds: A manifolding system has been developed that minimizes parasitic shunt currents without impeding flow significantly.
Using these innovations, ViZn Energy has developed the Z20-60 (60kW / 120kWh) energy storage system, which is scheduled for production in 2013. The Z20 is a containerized module, designed to be integrated into megawatt-sized systems. The Z20 is among the most cost effective solutions, is location independent, and yet still covers a large application range for flexible revenue options.
It is proposed in this presentation that the energy needs for the world can be reconciled with a combination of RESs and large scale storage solutions. Zinc is a logical cost-effective means to achieve this and will allow regional interests to generate their own grid infrastructure with the natural resources at hand. In particular, ViZn Energy’s Z-20 family of products can meet these needs with both high power and high capacity cells suitable for large-scale deployment on the electrical grid, and provides the lowest-risk path to stable carbon emissions, energy security, and the establishment of a safe means to develop our nation.
Dr. Ron Brost is Chief Technology Officer for ViZn Energy where he leads technical development. Dr. Brost has been active in battery and fuel cell development for twenty years and has held expert and supervisory roles with General Motors and Ford Motor Company that included development of lithium batteries, VRLA (AGM) batteries, nickel metal hydride cells, and hydrogen fuel cells. Dr. Brost is a Design for Six Sigma Blackbelt and is an expert in advanced modeling and computer aided design of electrochemical systems. He has authored over thirty patents, publications, and conference papers. Dr. Brost holds a PhD in Chemistry from the University of Victoria and a B. Applied Science (Chemical Engineering) from the University of British Columbia, and has been a professional engineer for over 20 years.