Energy Storage News

Zinc Bromine Flow Batteries For Large Scale Stationary Electricity Storage

As I have written previously sodium sulfur (NAS) batteries are currently the leading commercial technology for large scale electrochemical storage of electricity. Another battery type which is being explored for large scale energy storage is zinc bromine flow batteries. Here are some details about three manufacturers of these batteries. From the web site of ZBB Energy:

The ZBB Zinc Energy Storage System (ZESS) is a proprietary and patented regenerative fuel cell based on zinc/bromide technology, which is very different in concept and design from more traditional methods of energy storage such as the lead/acid battery. The ZESS technology is based on the reaction between two commonly available chemicals, zinc and bromide. The ZESS consists of a zinc negative electrode and a bromide positive electrode separated by a microporous separator. An aqueous solution of zinc/bromide is circulated through the two compartments of the cell from two separate reservoirs. The electrolyte stream in contact with the positive electrode contains bromide which is maintained at the desired concentration by equilibrating with a bromide storage medium (Figure 1.). The bromide storage medium is immiscible with an aqueous solution containing zinc bromide. All battery components are made from a bromide inert plastic (Figure 2.).

Unlike the lead acid and most other batteries, the ZESS uses electrodes that cannot and do not take part in the reactions but merely serve as substrates for the reactions. There is therefore no loss of performance, unlike most rechargeable batteries, from repeated cycling causing electrode material deterioration. During the charge cycle metallic zinc is plated from the electrolyte solution onto the negative electrode surfaces in the cell stacks. Bromide is then converted to Bromine at the positive electrode surface of the cell stack and is immediately stored as a safe chemically complexed organic phase in the electrolyte tank. When the ZESS discharges, the metallic zinc plated on the negative electrode dissolves in the electrolyte and is available to be plated again at the next charge cycle. In the fully discharged state the ZESS can be left indefinitely.

The ZESS offers 2 to 3 times the energy density (75 to 85 watt-hours per kilogram) with associated size and weight savings over present lead/acid batteries. The power characteristics of the ZESS can be modified, for selected applications. Therefore, the ZESS has operational capabilities which make it extremely useful as a multi-purpose energy storage option.

The ZESS technology is composed of a module or a series of modules for increased power. The ZESS 50 holds 50 kWh, which would roughly power a home for two days, and includes three cell stacks, each containing 60 cells. The module dimensions are 4’ x 4’ x 6.5’, weighing 3,200 pounds. The ZESS 500 fully charges in 4.5 hours and has continuous power of 250 kilowatts (kW), sustainable for two hours, with 200% peaking capability. The ZESS 500 holds 500 kWh, which is actually ten of the ZESS 50s strung together, and is used for commercial or utility applications. Exhibit 4 shows the ZESS 50 and ZESS 500. These storage devices can be located anywhere in ambient temperatures and transported to support the need of renewable energy to mitigate utility congestion. The “Turnkey” capabilities are beneficial based on the ease of simply plugging the batteries into the grid or other alternative source. Furthermore, these products are predominantly manufactured with recyclable plastics, allowing for low production costs and economical mass production.

The ZESS systems store energy from a number of sources and are highly compatible with global renewable energy markets: wind, solar, hydroelectric, biofuels, ocean power, and biomass. The ZESS 500 is easily transported on a 20-foot trailer and weighs approximately 30,000 pounds. The modular design aims to increase or decrease power needed by either adding or subtracting modules in the string, which is a valuable feature. Other products do not provide these transportable designs and modular capabilities for the added flexible benefits.

MILWAUKEE, WI – November 9, 2009 – ZBB Energy Corporation (NYSE AMEX: ZBB) today announced that it has received a new order for its ZESS 50™ unit with a ZESS POWR™ PECC to SEI Group Inc. of Huntsville, AL for a US government facility as a demonstration site.

MILWAUKEE, WI – September 25, 2009 – ZBB Energy Corporation (NYSE AMEX: ZBB) today announced it has shipped its ZESS 50TM unit to the Wallace Energy Systems & Renewables Facility in Oregon as a part of the previously announced contract with Oregon State University (OSU).

The above two announcements concerning the sale if 50kW battery systems are the only signs I have found that ZBB is actually shipping finished products. ZBB Energy does not give specific numbers for cost or cycle life. The original Australian company (ZBB technologies) from which ZBB Energy is descended was formed in 1982, so ZBB is far and away the oldest of the three companies covered in this post.

From the web site of Premium Power Corporation:

Founded in 2002, Premium Power Corporation manufactures the world’s lowest cost, grid scalable flow batteries based on the company’s proprietary Zinc-Flow® advanced energy storage technology. Our products are lower in cost than lead acid batteries up front and roughly equivalent to pumped hydro, at under 2 cents/kWh, on a long-term basis. Our Company is headquartered in North Reading, Massachusetts, USA.

The Zinc-Flow® 45 regenerative fuel cell is a modular, redundant, scalable DC power solution for wireless, fiber and broadband communications networks as well as electric utility substations. Premium Power’s patented Zinc-Flow® 45 regenerative fuel cell technology provides high-density energy storage in a footprint roughly half the size of comparable solutions based on lead-acid batteries.

Special Features

Energy ratings:

The PowerBlock® 150 is a cost effective utility-grade energy management system. It provides up to 100 kW of uninterrupted power and 150 kWh of energy storage capacity in a single turnkey enclosure that is 1/3 the size of a 20’ freight container. PowerBlock® systems are designed to integrate with utility, renewable or sustainable energy sources to ensure power quality for mission-critical electronic, computer network, communications, utility, healthcare and manufacturing environments.

Using Premium Power’s Zinc-Flow® technology, the PowerBlock® 150 is optimized to deliver 2-3 times more energy density than comparable UPS systems that employ lead-acid batteries. PowerBlock® delivers 10X depth of discharge without degradation, 2X operable life and an order of magnitude improvement in cycling capability, making it ideal for use in peak-shaving applications to reduce utility demand charges and energy costs.

The TransFlow 2000 is the lowest cost utility-scale energy storage system. It is a fully integrated system that comprises energy storage, power conditioning, system control and thermal management subsystems packaged into a portable, turn-key, building block to be placed wherever it is needed for immediately dispatchable on-line energy storage. Each TransFlow 2000 provides up to 500kW of power and 2.8MWh of energy storage capacity in a single enclosure that fits onto a 53' trailer for mobility.

Premium Power's Zinc-Flow technology enables systems with an operational life of at least 30 years and the ability to withstand an unlimited number of cycles, whether full- or partial-discharge event

December 9, 2009, North Reading, MA - Premium Power Corporation, an advanced energy storage company, was selected last week by the US Department of Energy to receive a $7.32 million grant as one of only 16 Smart Grid Energy Storage Demonstration Grants under the American Recovery and Reinvestment Act.

Premium Power and its utility partners National Grid and Sacramento Municipal Utility District (SMUD), along with teammate Science Applications International Corporation (SAIC) were selected to run a three-year project that incorporates the fleet control engineering, manufacturing and installation of seven 500-kilowatt/6-hour energy storage systems. The systems will be installed in Sacramento, California, Everett, Massachusetts, and Syracuse, New York beginning in the third quarter of 2010.

I find Premium Power's claims of unlimited cycle life and thirty year life less than convincing, and in fact they contradict the thirty year lifetime claim for their mid sized system which they advertise as having 2X the lifetime of lead acid batters which would put them in the more believable 10 to 15 year range.

From the web site of RedFlow Energy:

The electrolyte in a ZB battery is an aqueous (dissolved in water) solution of zinc bromide. This is circulated with small pumps over both electrode surfaces, which is why it is referred to as a "flow" battery.

The basic operation can be most simply described as an electroplating process. During charging, zinc is plated onto the conductive plastic electrodes. During discharging, the reverse reaction occurs and zinc is removed as electricity is generated. The ZB battery is designed to offer full 100% discharge with no damage, unlike many other battery systems..

Plastic is used in the construction of the electrodes and most other parts of the battery. These do not corrode or deteriorate over time. Therefore, the ZB battery is a non-perishable battery.

Many other batteries types have limited shelf life because of internal corrosion and other degradation processes that occur whether the batteries have been used or not.

RedFlow's ZB battery can be stored indefinitely, moreover, as the chemicals associated with the reaction are separated when the battery is charged, the ZB battery has the potential to maintain its charge indefinitely with no self discharge.

RedFlow's ZB battery modules are sealed systems that do not require any specific regular maintenance procedures. The pumps and control system are fully integrated, making the RedFlow module a "set and forget" energy storage system. An annual check of the general installation is recommended.

When compared with many other battery technologies, the ZB battery offers high energy density (the amount of energy contained for a given battery weight or size). This is critical for developing commercially viable energy storage solutions. It also has the benefit of being a repairable product, unlike lead-acid batteries.

In addition it operates at normal ambient temperatures, minimizing the need for external heaters or chillers, which add to the cost and reduce the efficiency of other battery systems.

The ZB battery uses many low cost materials and with RedFlow's planned large scale manufacturing, this will result in a high performance battery at low wholesale cost. The battery modules produced by RedFlow are configured for 48 volt operation, ideally matched for high performance inverters to generate AC power at household and light commercial scale and also for telecommunication applications.

When compared with deep cycle LA batteries, the RedFlow ZB battery has the following advantages:

RedFlow's Power+BOS® LA (lead acid) system is an integrated electricity storage system. It uses the best available lead acid batteries.

RedFlow designed these lead acid systems as an intermediate step in commercializing its zinc-bromine systems. These have been used in field trials since early 2007.

Like the zinc-bromine systems, RedFlow's Power+BOS® LA Systems have the following features:

The Power+BOS® LA system is available for commercial installation

From May 2009, these units are being installed in Ergon Energy's network under a trial as part of an Advanced Energy Storage Technologies grant from the Australian Federal Government.

RedFlow is installing thirty systems at various locations in regional Queensland. This roll-out is part funded by the Australian Federal Government's Advanced Electricity Storage Technologies Program, managed by the Department of Resources, Energy and Tourism. Each of the systems is fully integrated with a solar PV array.

Long Service Life

Thanks to the benign nature of the zinc-bromine chemistry, RedFlow ZB modules have no calendar life expiry date or shelf life. Testing to date has demonstrated well over a thousand full depth of discharge cycles. The energy storage modules can be serviced and refurbished to extend their service life to over four thousand full depth-of-discharge cycles.

The comment about servicing and refurbishing the batteries to extend their cycle life is interesting. I have read that flow batteries with different charged species in the two half cell become degraded over time because of slow migration of the wrong species across the ion exchange membrane separating the two half cells. Therefore the liquid electrolytes have a shorter lifetime than the electrodes and the ion exchange membrane. RedFlow claims that even with this necessary electrolyte replacement they can beat the lifetime costs of lead acid batteries by a factor of five, which would be an impressive achievement if they can deliver on their claim.

It is clear that RedFlow is initially attempting to enter the market of small scale off grid power which is currently served by lead acid batteries. The fact that they are already selling lead acid batteries into this market seems like a good strategy. When their zinc bromine batteries are ready for commercial sale they will already have a track record selling and maintaining the other components (i.e. inverters, control electronics, etc) of such off grid systems.

Large scale grid storage is a hard market to enter. The up front costs are high, and investors need to be convinced that the product will perform reliably over long period of time. NPK insulators is the only company I know of that has established themselves in this market (with NAS batteries) and it took them a long time from their initial installations in the mid 90's to get the point of receiving large multi-megawatt orders for their product. Entering an established market first in order to demonstrate field reliability strikes as a good strategy.

None of these three companies gives the round trip efficiency(=kWh Out/kWh in) of their batteries. I think the reason for this omission is that the efficiency of these batteries is not outstanding. I have seen various stories which quote efficiency numbers in the range of 70% to 75% without mentioning whether this efficiency is in DC (direct current) or AC (alternating current mode). NAS batteries operate at 85% efficiency in DC mode and at 75% efficiency in AC mode. If the numbers quoted for Zinc Bromine batteries are DC efficiencies then the AC values may be in the low sixties. If we assume that is low figure applies what is the cost implication relative to NAS batteries?

If a cost per kWh coming out of a battery is determined then one must add on the following efficiency cost:

Efficiency Cost = [(1-eff)/eff]× Base_Generation_Cost

where eff is the efficiency expressed as a fraction. Therefore if NAS batteries have AC efficiency eff=0.75 and zinc bromine batteries have efficiency eff=0.62 the efficiency costs are given by:

Efficiency Cost (NAS) = 0.333 × Base_Generation_Cost

Efficiency Cost (ZnBr) = 0.613 × Base_Generation_Cost

The difference in the efficiency cost is significant, so that if zinc bromine batteries have efficiency close to 60% then the cost of delivering a kWh from such batteries must be significantly lower than from NAS batteries in order for them to be economically competitive.

December 27, 2009

Energy Storage News

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