By Chad Dozier, Caterpillar Inc.

In many areas of the world, traditional grid supplied power can be unreliable or even unavailable. An energy storage solution can keep a telecom site operational during periods of grid instability by discharging stored energy. One emerging solution is a zinc-air energy storage system.

Zinc-air cells store significant amounts of energy through charging when electricity is inexpensive or available and then discharging when electricity is normally expensive or grid power is unavailable. In addition to the proven capability for managing both long and short partial cycles at the lowest cost, other key fundamental advantages include repeated ultra-long discharge time capability with no impact on product life due to number of cycles and increased safety. Pair this with fully integrated and Internet-connected software controls to continuously capture, self-optimize, learn, and transmit information to a control center. This system enables exceptional performance management, lower energy costs, and high reliability.

Zinc-air Technology
At the core of this energy storage solution is a rechargeable zinc-air cell for use in telecom applications. It is the first system that has successfully commercialized a high-cycle-life, rechargeable metal-air battery, long regarded as the lowest-cost way to store an electron.

Zinc-air cells use oxygen from the ambient atmosphere to produce electrochemical energy. As the air-breathing cathode '“ the positive electrode '“ is exposed to air, oxygen diffuses through a hydrophobic barrier to the cathode's catalytic active layer, where the reduction of oxygen is promoted in the presence of an aqueous alkaline electrolyte, potassium hydroxide (KOH).

Once in contact with the electrolyte, zinc from the anode '“ the negative electrode '“ is oxidized, which generates electric current from the oxygen reduction and zinc oxidation reactions. The cathode is an irreducible electrode, which means its physical size and electrochemical properties remain unchanged during discharge, unlike most other closed-cell batteries. This contributes to the long calendar and cycle life of the zinc-air cells.

Advances in the continuous fabrication of thin, high performance gas electrodes in the 1970s enabled the mass production of the single-use, non-rechargeable zinc-air button cell. The most successful applications of zinc-air batteries in this small form factor have traditionally been in medical devices, such as miniature hearing aids. However, their use was limited to single-use, disposable applications, as is the case with alkaline batteries (AAA, AA, C, D).

Reaction reversibility is the key to the breakthrough in zinc-air energy storage technology, which led to the creation of an electrically rechargeable cell. Leveraging the proprietary aspects of an advanced anode architecture, smart electronic controls, proprietary ionic liquids, and robust gas electrode design has sparked the development of the world's first reliable, rechargeable zinc-air cell. Enabling the rechargeability of the zinc-air cell has lifted this advantageous battery chemistry to a new level of cost effectiveness, usability, and application reach. This fundamental performance has been proven by over 550,000 cycles in customer systems comprised of over 35,000 batteries.

Clean and Safe
Zinc-air technology is arguably setting a new standard for what's possible in environmentally responsible energy storage. But what's even more impressive is that it achieves this status without compromising cost, reliability, life, or performance. The emphasis on sustainable product attributes has now achieved a universal acceptance in most markets around the globe. A product's true environmental impact or life-cycle assessment (LCA) on natural resources, health, safety, and society in general are now being taken into consideration in most buying decision processes. There are two recognized standards that drive a product's life-cycle environmental impact: environmental sustainability and societal health and safety.

Environmental sustainability applies to products that can be recycled at the end of life, use sustainable materials, and are made from materials synthesized or extracted/mined with minimal impact. Fundamental to zinc-air technology is a reliance on sustainable materials in the construction of the cell and ancillary equipment. Raw materials such as zinc, carbon, water, potassium, and consumer-grade acrylonitrile butadiene styrene (ABS) plastics are readily available and, for the most part, easily reclaimable and recyclable at the cell's end of life (EOL). Due to the abundant nature of the zinc-air cell raw materials, the technology provides an extremely sustainable platform for rapidly scaling manufacturing. Analysts for Meridian International Research have recognized zinc as the only metal that can sustain large battery production in volumes. This is not the case with other battery types comprised of earth-scarce materials that are either hard to mine or found only in isolated reserves controlled by a few countries.

Societal health and safety relates to products that do not produce toxic or harmful emissions, contain no toxic constituents known to cause severe or irreversible environmental damage, do not pose adverse human health risks, and are inherently safe with respect to fire or explosion. The zinc-air cell architecture is fundamentally safe with respect to thermal runaway or fire hazards due to its fail-safe discharge dynamics. All reacting oxidant is stored outside the cell until the reaction is desired and, unlike all solid-state batteries, the system discharges using ambient air. This fundamental attribute makes zinc-air one of the safest energy storage products deployable today. In addition, the zinc-air aqueous alkaline electrolyte does not contaminate ground water or other sensitive natural resources.

Maximize Site Availability
Unpredictable backup energy solutions will impact the availability of the telecommunication equipment.   It is important that the energy storage system actively provides reliable power with demonstrable improvements to equipment availability. A remote monitoring package will provide real time status of equipment operation, indication of any faults, and long-term data analytics on site uptime and performance. It will also provide information to optimize system performance, provide insight into operating cost optimization, and support the service management of the product. With the zinc-air energy storage system, monitoring occurs down to the individual battery cell level. If any abnormal operation is detected, a central network operating center is notified and will take action in the event of any issues impacting the performance of the deployed units, providing the highest possible availability wherever these solutions are deployed.

Cat® Energy Storage System (ESS) Configuration
Cat ESS represents the latest breakthrough in safe, long-life, high temperature-tolerant, reliable, and cost-effective energy storage technology. It is ideally suited for backup mission-critical DC loads, load shifting, renewables integration, and microgrid storage. At the core of Cat ESS is the building block of energy modules consisting of rechargeable zinc-air cells. These energy modules have integrated intelligence and are self-governing to autonomously balance the ampere-hour (Ah) discharge of the contributing zinc-air cells.

Since the individual energy modules behave just like a large-capacity 52-volt battery string, they are essentially sub-systems that can be arranged in parallel to each other and added or removed from the system based on the power and energy needs of the customer. This innovative and effective means of self-governing battery management is core to the zinc-air system architecture and yields increased redundancy and reliability. Inside the standard IP55 outdoor enclosure, shown here, a dedicated shelf supports each of the energy  modules. Each energy module is controlled by a dedicated IPM electronics unit conveniently co-located inside the electronics bay, which is accessible on the side of the unit. Each energy cell can be removed individually for service and inspection. This configuration enables superior technician service access and controller interrogation simplicity. For efficient use of space and ease of service, all electronics are conveniently mounted on the in a single enclosure in a co-located arrangement.

Proven and Reliable
The core building block of the system is the energy module, which can range from 0.75 kW to 4.50 kW and deliver from 6.0 to 40.1 kWh of energy. The unique structure of Cat ESS will allow for future generations of energy modules to be arranged in parallel, stringing together systems with peak power capabilities scalable beyond 5 kW and more than 40 kWh of energy. Each energy module is controlled independently, and customers can easily add additional energy modules as loads increase over time.

Unlike most other new or advanced energy storage technologies, Cat ESS has been proven in the market in high volumes with multiple large commercial customers around the globe. Cat ESS is one of the first commercially successful implementations of rechargeable zinc-air technology, with more than 35,000 batteries used in customer applications today.

For more information, visit www.cat.com/microgrid.