“It’s interesting that our electricity system has lasted 150 years without storage as a major component,” says Sam Jaffe, Research Manager in Distributed Energy Strategies at energy consultants IDC Energy Insights. “That’s due, in part, to the nature of electricity, which can be created and delivered and consumed instantaneously.” But while conventional utilities may have been slow getting on board, storage is critical for other aspects of energy delivery, such as UPS and in off-grid sustainable energy applications.
Although a range of options are in various stages of development, at present three energy technologies lead the market: lead acid batteries-long an industry staple-newly evolving lithium ion technology, and flywheels. Choosing what you need for a particular situation is a function of the dynamics of the specific application, including footprint, operating conditions, maintenance capabilities, and cost of ownership.
Lead Acid-The Current Leader
A battery is an electrochemical storage device that converts chemical energy into electricity by means of galvanic cells. Each cell has three essential components: an anode, a cathode and the electrolyte, typically 30% sulfuric acid and 70% water at full charge. When the anode and cathode are connected by an electrical conductor such as a wire, electrons flow between the two, creating an electrical current as the electrolyte conducts positive current in the form of positive ions. As chemicals in the cells of a battery change, electrical energy is stored or released, some of which is lost as heat and chemical reactions.
Deep-cycle lead acid batteries, so called because they’re designed to deeply discharge and then recharge on a regular basis, are widely used in renewable energy and grid backup applications for their long life, low cost of ownership, and what the industry calls their “nearly 100% reliability.” They come in two types, flooded lead acid (FLA) with removable caps to maintain the water level at optimum, and sealed or valve regulated lead-acid (VRLA), which require less maintenance but have a shorter life. In fact, the life cycle of a deep-cycle cell battery will vary considerably, depending on the quality of construction; how it’s used, maintained, and charged; and how well it’s sized according to its application. According to Trojan Battery Company, factors to consider when selecting and utilizing lead acid batteries include:
Capacity
The amount of energy stored in a battery, measured in ampere-hours (Ah) multiplied by voltage. For example, a 100-Ah, 12-V battery will contain 1,200 Wh or 1.2 kWh of energy, and a 200 Ah, 12-V battery will have twice that amount.
It’s important that a battery be properly rated for its application, because a battery with insufficient energy content will mean insufficient run time and a shorter life. (Although an oversized battery may seem alluring, it doesn’t justify the cost). Battery capacity is reduced when the temperature drops and increases when it rises, although high temperatures also shorten battery life. Battery capacities are typically rated by manufacturers at 77°F, and for every 15 degrees above that, battery life is halved.
Voltage
When batteries are combined in series and parallel wiring configurations to create battery banks, the battery bank voltage must match the overall system requirements in terms of total Ah capacity and voltage. In renewable situations that involve installing a direct current-to-alternating current (DC-to-AC) system, battery bank voltage is often determined by the inverter specifications. In a DC system alone, it is determined by load voltages.
Battery life is also related to how deeply the battery is regularly cycled. Particularly in renewable energy applications, a battery’s cycle life can be the most important design criteria, measured as the number of discharge/charge cycles a battery provides at a specific depth of discharge (DOD) before its capacity drops to a specified percentage of its rated capacity. A battery that’s capable of 1,000 cycles, for example, is effectively half the life cost of one capable of only 500 cycles at the same DOD level.
“Deep-cycle flooded lead acid technology is still the least-cost solution per unit of energy storage in off-grid renewable energy systems where batteries are used to store energy and provide the balance between the energy resource and the load for 24/7 service,” says Dean Middleton, Director of Renewable Energy Sales at Trojan Battery. But the most common mistake people make when sizing energy storage for off-grid PV systems is characterization of the load profile.
“Both the real energy demand and the duty cycle can have a dramatic impact on performance and can cause the system to lose power or require a replacement battery bank sooner than necessary,” he adds. If manufacturer’s data is not available for a particular load, actual power use should be measured before calculating the total energy storage requirement.”
Wayne Robertson, senior sales executive for a leading national manufacturer and supplier of photovoltaic solutions, agrees. “Being able to quantify specific loads is one of the hardest things to do when you’re designing a system, but it’s the most important.”
Robertson himself has lived off the grid for 30 years and generates power for his house, shops, water pump, and outbuildings with a 5.3-kW photovoltaic system and 12 Trojan Battery ND23 industrial batteries, which provide 1,233 Ah of storage at 48 V.
Robertson consults the weather to model systems for specific locations, building in autonomy for sunless days, but he says a frustrating challenge for residential applications is the systems are so successful, customers add load once they’re installed.
“As important as determining load is the need to truly understand the specific nature of the application and the expectations of the off-grid customer,” says Middleton. “How and where will the system be used? If you design an off-grid PV system based on manufacturer ratings, which are based on ambient temperatures of 77 degrees Fahrenheit, without realizing that, your customer intends to deploy the system in an environment with typical ambient temperatures of 20 degrees Fahrenheit, the result will be an undersized system that sooner or later will fail.”
Despite their reliability, to achieve maximum potential life, FLA batteries require regular care. Daily charging is an important factor affecting longevity because both under- and over-charging can reduce battery life. “While it’s important to optimize systems to meet the application’s load requirements,” says Middleton, “at the same time you have to take into account the specific nature of available charging sources.
“The more conservative you are in your design approach, the longer the battery will last. If I design my system so I only take my battery down to 50% DOD everyday and recharge it back to 100% it’s going to last much longer than if I take it down to 100 DOD. We recommend that the ideal design is discharging to 20 to 30% DOD. But if you plan for a shallow discharge, you will need to add more batteries to the system. That’s the tradeoff.”
Another critical factor in off-grid applications is maintenance. As FLA batteries charge, hydrogen is produced and vented, which reduces the electrolyte level and means periodic watering of the batteries with distilled water is required to insure maximum life. (Robertson checks the water level in his residential system’s batteries every month whether he thinks they need it or not.) To help in off-grid applications where cultural differences may make maintenance an issue, Trojan Battery has developed a low-technology battery watering system that can be automated and effectively provides some of the benefits of sealed VRLA batteries without the investment.
At Eaton Power Quality Solutions Operation at Eaton Corporation, the emphasis is on a much more high-tech battery management solution for UPS applications. “What we typically do in the industry is trickle charge batteries,” says Product Manager Ed Spears. “What we’ve found over the last 23 years is that this increases the service life of a typical VRLA battery in a UPS as much as 50%.”
The battery management system along with remote monitoring capabilities were key components in the system Eaton installed for the City of Chicago’s disaster recovery data center. “I travel between our two offices,” says Amy Niersbach, the city’s platform architect. “And remote monitoring and management is critical in allowing access from anywhere on our network.”
Critical Applications
“Amen” about designing to meet individualized conditions, says Ron Seridian at Falcon Electric Inc., which specializes in UPS systems for demanding applications where temperature or absolute performance are factors. Such as supporting a 42-foot-high bank of computers housed in a steel enclosure on a southern California toll road.
“The high temperature was really eating up the batteries,” says Seridian. “So we replaced the old batteries with batteries that are rated for higher temperatures. The lead acid mixture contains more-and purer-lead, and the surface area has been exponentially increased.” As additional temperature protection, the toll road batteries were also encased in plastic.
“The thing about battery life is that long before a battery actually begins failing, run time is being jeopardized. When it’s brand new off, the shelf a battery is going to give you your 35 minutes of run time. But if you’re subjugating it to shallow or poor charging techniques, or if the application is in a hot environment, it’s going to fold.”
For a wind turbine controller in Denmark that regularly operates in variable temperatures, Falcon designed a 30-minute energy bridge for the UPS to cover potential slow start-up times in the system’s backup generator. “Longer times are typical when you don’t have a generator or as insurance in case a generator breaks down in critical application like a research laboratory.
“What it boils down to is that you’re specing in autonomy and immunity from blackouts. Our installations run from a power company in Mongolia to a SCADA system for oil wells, where if the control system goes down, there’s an oil spill, to Texas tollway booths that don’t have backup generators. In situations like these, not only the batteries but the heat sinks and inverters have to be rugged.”
“Data centers don’t like failures,” says David Sonner, Senior Director of Product Marketing at Liebert AC Power, Emerson Network Power. “As a result, a lot of companies over-provision. But if you have excess capacity built into your architecture, you’re paying a cost in terms of lower efficiency.”
“If you have more modules available on the bus than you need for the load you’re carrying at a given time, you can actually turn them off,” explains Sonner, “but still have them be able to come online fast enough to protect the application.”
The newest UPSs can also run in what Sonner describes as ecomode. “In the traditional architectures, energy loss can be as high as 10 to 12%, and since the amount of energy that’s used in a data center is significant, there’s pressure from government regulators about saving operating costs and reducing energy waste. Instead of normal operation, where 100% of the energy passes through the two-stage conversion processes and hence the losses, a UPS in energy optimization mode runs in conjunction with the utility to where only a very small amount of energy flows through the UPS. This makes for a 97 to 99% efficiency in normal operation mode.”
Sonner also notes that EPA is in the process of establishing Energy Star ratings for UPSs that will likely provide benchmarks in energy efficiency improvement for manufacturers who include a utility grade energy meter in their UPSs.
Flywheels
Although lead acid batteries are a proven technology and have a long history of energy storage applications, they can also have liabilities. They’re bulky, for one thing, can be a challenge to transport, and, depending on the application, often require a large footprint where temperature control is critical. They can also be cumbersome or expensive to recycle. As an alternative, some UPS suppliers are offering flywheels, which function effectively as kinetic or mechanical batteries, spinning at very high speeds to store energy that’s immediately available when its needed. In typical UPS applications, these smaller, 1,500-pound versions of the heavier flywheels that were originally designed for the automotive industry usually offer 1520 seconds of bridge time.
At Eaton, Spears thinks the shorter run times are easily resolved. “Research shows that more than 95% of utility outages last just a few seconds. So, using a flywheel as a complement to batteries during brief power interruptions can save data center floor space and lower maintenance costs while extending the life of your lead-acid batteries by reducing how often you use them.”
“Customers use flywheels rather than batteries in UPS applications because of their reduced maintenance costs, much wider operating temperatures, and smaller footprint,” says Dann McKeraghan, Vice President of Sales at VYCON.
According to McKeraghan, 20% of the flywheels its sells are used in hybrid applications in combination with standard lead acid batteries. “The flywheel takes care of most of the outages, which are typically less than 10 seconds, and then hands over anything above that to the battery. And because the flywheels come on first, batteries can typically be sized much smaller.”
David Filas, data center engineer at Trinity Health, which operates a string of 50+ hospitals, has flywheels in six of its 26 data centers, and three more installations planned, says “Our loads are highly variable, and I was looking for a more reliable technology. We have had lots of batteries, single and multiple strings, and had innumerable failures, a lot of them due to defective batteries. Except for one installation, which uses two flywheels in parallel, we use the flywheels in parallel with a set of batteries. We sized them so that we get a good three to four minutes out of the flywheels before the batteries kick in, which give us an additional five minutes. So far the flywheel technology has handled it all, and we haven’t sustained any outages that have required the batteries.”
Lithium Ion
While many UPS suppliers are offering flywheels as an alternative to or in combination with traditional lead acid batteries, some industry observers think the future lies in lithium ion batteries, which have grown steadily smaller, lighter, and denser over the last decades.
“The best lithium ion batteries currently available deliver a 30% weight and footprint savings over lead acid batteries, while improving backup time by 30%,” notes Spears. And although he estimates lithium ion can be as much as four times more costly in terms of initial investment, he thinks in the next few years lithium ion technology will offer roughly the same total cost of ownership as lead acid batteries.
“Lithium ion is clearly the winner in this phase of the energy storage race,” says Jaffee at IDC. “Lithium ion technology offers much greater energy density, a much longer lifetime, and much better power characteristics. When you’re trying to pack as much storage as possible into relatively small units, lead acid can’t compete.”
“In a UPS, lead acid batteries are a great value for the money,” says David McShane, Executive Director of Business Development and Sales at International Battery, a lithium ion battery supplier. “But with data centers consuming so much power, batteries are taking up valuable space, and there’s an interest in a device that has a lot less footprint requirement for the same amount of energy storage. Lithium is still more expensive on a kilowatt-hour basis, but it has eight to 10 times the cycle life of lead acid batteries, and can support a much heavier discharge for a lot longer. If you’re working with a cycling load, you’re losing a lot less energy in charging and discharging.”
Lithium ion batteries were the technology of choice for energy storage in a US Marine Ground Expeditionary Energy Network System solar-powered installation that provides energy for communication, targeting, and computing devices at remote locations and forward operating bases. The 24-V, 1.5-kWh system consists of 1,600-W solar arrays in combination with International Battery’s 60 Ah cells and the company’s battery management system. The mobile renewable system, which can be transported on a Humvee and quickly assembled, provides AC and DC power while interfacing with existing diesel generators.
Large-format International Battery lithium ion batteries were speced for American Electric Power (AEP) Ohio’s Community Energy Storage project in Columbus. The concept of community energy storage relies on small packages of batteries, typically totaling 25 kW (enough for one to two hours of backup time) deployed in neighborhoods to store grid power, which is then locally dispatched to small microgrids. Using this kind of utility-controlled device at the edge of a grid allows for voltage control and service reliability while keeping the entire operation under the utility. The project is a component in AEP Ohio’s gridSmart Demonstration Project funded in part by the Department of Energy.
“Energy storage is an enabling tool for the smart grid,” says Jaffee. “Without something like energy storage or distributed generation that can be dispatched when needed, you’re stuck with all that information you’ve collected and no way of responding to. Storage functions like a lever. On one hand, distributed energy storage is hard to do without a smart grid, but a smart grid can’t realize it’s full potential without energy storage. The AEP project is a perfect example. The utility has centralized control of all those distributed units, and it can dispatch them according to the needs of the local homes or the needs of the grid as a whole. This provides a tremendous amount of flexibility.”
“A UPS that can provide return on investment,” says Jim Crouse, Capstone’s Executive Vice President of Sales and Marketing.
As has been noted before, Syracuse University’s new Green Data Center is considered a game changer. Using 12 hybrid 65-kW, gas-powered microturbines from Capstone Turbine Corporation in a combined cooling, heating, and power allowing the data center to be isolated from the utility but still draw on it for backup, the system also provides conditioned power to the data center and uses the heat off the turbines to make hot water to cool the data center as well as heat an adjacent building. The system includes a 40-ton battery bank that holds enough power to carry the maximum load for 17 minutes in case of a catastrophic event. IBM provided the computer equipment, which operates from the DC-powered distribution system, thus eliminating the traditional power loss associated with converting AC from the utility.
