An increasing number of municipal water and wastewater treatment facilities are using “green” power—that is, renewable energy sources with the highest environmental benefits, such as solar and wind—as part of their energy mix. The treatment industry is not alone in its adoption of renewable energy. Overall, demand for electricity in the U.S. is anticipated to grow by 40% over the next two decades, with demand for renewable fuels expected to grow from 13% in 2011 to 16% in 2040.
The reason water and wastewater facilities are turning to renewable energy may be logistical (e.g., the plant is located in a remote location), practical (e.g., to address power intermittencies) or financial (e.g., reducing overall energy costs or providing an additional revenue stream by selling energy back to the utility).
Unfortunately, our nation’s increasing appetite for power is constrained by its aging infrastructure. Power outages are a significant problem in the U.S. A 2012 Ernest Orlando Lawrence Berkeley National Laboratory study titled, “An Examination of Temporal Trends in Electricity Reliability Based on Reports from U.S. Electric,” analyzed 10 years of electricity reliability information collected from 155 U.S. electric utilities (accounting for roughly 50% of total U.S. electricity sales). The study reported “reliability is getting worse, on average, over the [past] 10 years.”
Intermittency is an even greater problem with renewable energy. Wind power is affected by wind speeds and temperature, air density, and turbine characteristics, among other factors. Intermittency also affects solar energy. Solar output varies throughout the day and by season, as does cloud cover. The greater the reliance on renewable energy, the greater the impact of such intermittencies.
Decreasing reliability and increasing outages cost consumers and businesses money. According to the U.S. Environmental Protection Agency (EPA), “Cost of a service interruption varies by customer and is a function of the impact of the interruption on the customer’s operations, revenues and/or direct health and safety.” In one study, Pacific Gas & Electric Co. (PG&E) estimated the total annual cost of power outages to its customers at $79 billion per year.
Power outages or service interruptions can impose direct costs on customers through loss of data, equipment damage, extra maintenance, replacement or repair of damaged components, permanent loss of revenue from downtime, costs for idle labor and liability for safety and health.
Energy storage is one of many tools available to help address renewable intermittency, as well as the integration of renewable energy by addressing transmission constraints. Commercial-scale batteries are becoming an increasingly popular choice due to their reliability, affordability and scalability.
Design Considerations
Correctly designing a power-and-backup system and selecting the right storage batteries for the renewable application can have a significant impact on overall performance, efficiency and longevity.
Renewable applications are characterized by deep discharge-and-recharge cycles intermixed with partial state of charge (PSOC) cycles. As such, the batteries for these applications should exhibit the following performance characteristics:
- • Long cycle life;
- • Cycling in state of discharge;
- • Low rate of self-discharge; and
- • Large electrolyte reserve.
Often, other factors such as cost, space and maintenance are given higher priority, complicating the purchasing decision.
It is important to select a correctly sized battery when setting up backup storage for renewable energy. Improper battery sizing is a common mistake. There are a number of factors impacting battery life that need to be considered, including:
- • Cycle life. A battery completes a cycle when it is charged and discharged once. Different battery technologies are rated for different cycles. For example, a typical absorbed glass mat or flooded battery may have a life of 300 to 700 cycles.
- • Depth of discharge. This is a measure of how hard the battery is working. The higher the depth of discharge, the shorter the cycle life.
- • Temperature. Temperature affects battery performance, shelf life, charging and voltage control. At higher temperatures, there is more chemical activity in the cell. Therefore, warmer temperatures may shorten battery life. Conversely, cold temperatures may negatively impact battery performance.
- • Recharge voltage and rate. The closer a battery is to being fully charged, the slower it should be charged in order to retain optimum pressure. Excessive overcharging may dry out the battery and lead to premature failure.
These factors must be considered to ensure that the “energy in” is sufficient to compensate for the “energy out.”
Unfortunately, like everything else, renewable energy systems must be built within budget guidelines. When the project exceeds the budget, the easiest way to reduce costs is to seek savings from the largest-ticket items—in this case, the renewable energy source (i.e., solar panels or wind turbines). If the renewable energy source is inadequate, then the batteries also will become depleted.
Continuously discharging lead acid batteries greater than 80% will cause them irreversible harm. Therefore, the more cycles anticipated, the lower the depth of discharge should be designed into the battery system. For maximum investment, it is best to not discharge the battery more than 40% to 50% in a diurnal system.
Battery Chemistry & Design
Batteries have evolved over the past few decades to meet the specific needs of utilities and other industries. Renewable energy applications require batteries that provide excellent reliability and maximum cycling capability.
In general, the batteries marketed to the renewable energy industry are manufactured with either flat or tubular plates.
Flat plate designs are the principal battery designs used in stationary utility and switchgear applications throughout North America. They consist of a grid structure of negatively and positively charged plates made of alloyed lead—either lead calcium or lead antimony—or pure lead in an electrolyte. The flat plate structure has proven to be a robust, flexible design in which the plate characteristics, such as thickness, metal alloys, wire radius and placement, can be adjusted to create application-specific batteries delivering optimum performance in terms of float service, cycle service, duration and high rate.
Tubular positive plate designs are widely used in renewable energy applications in which maximum cycling is key. The current-carrying lead metal in tubular designs is entirely surrounded by active material. This keeps the active material tightly against the spine and helps ensure long life. Tubular batteries have either round or square tubes. In general, the square tubes provide the most surface area on the positive plate, exposing more positive plate active material to the electrolyte. The square tube construction also prevents active material from dislodging away from the grid. This combination of greater positive surface area and better paste adhesion allows for excellent cycling capacity.
Lead vs. alloy plates. Over the years, manufacturers have experimented to find just the right materials to use to produce the positive plates in traditional lead acid batteries. By itself, pure lead provides excellent performance; however, it is also malleable and requires special handling during manufacturing to ensure that the plates, grids and elements maintain their integrity. To address these issues, manufacturers have tested various alloys. Alloying lead with calcium greatly facilitates manufacturing but results in higher corrosion rates. Other materials like cadmium, antimony and selenium improve cycle life and help facilitate manufacturing, but may pose difficulties with recyclability, heat buildup and efficiency. For this reason, use of these alloys is limited to applications such as solar systems, in which the demand for frequent charge-discharge cycles overrides the disadvantages.
Over the past 20 years, manufacturing equipment also has evolved to the point that thin plate pure lead (TPPL) batteries are now a viable and available option. They are ideal for backup applications because they are a smaller, lighter solution with a longer shelf life, longer service life and a higher rate of efficiency than traditional alloyed technologies. They also have a low gassing rate, which reduces water usage and maintenance costs. However, TPPL battery manufacturing requires a high capital investment and considerable knowledge. Therefore, they are primarily marketed as a premium solution for challenging applications ranging from submarines to telecommunications systems.
Flood vs. valve regulated. Lead acid batteries are available in two types of containers: flooded or valve regulated. Flooded batteries require rewatering, making them more costly to maintain. They also require larger containers for the flooded electrolyte, making them less energy dense. Flooded batteries also allow for visual monitoring of the cells, enabling operators to identify issues and preventive maintenance needs more readily than sealed, valve-regulated lead acid (VRLA) batteries.
On the other hand, VRLA batteries weigh less, require lower maintenance and are more cost-efficient. To accommodate for corrosion in lead alloy batteries and prolong life, manufacturers increase grid thickness at the cost of reduced energy density and increased weight.
Environmental Considerations
Many renewable energy sources are located in areas with extreme temperatures. These extremes—especially high heat—can be damaging to batteries. While it is important to protect batteries from the effects of external temperature variations, it is even more important to monitor the more critical internal core temperatures of the batteries.
TPPL batteries, with a recommended operating range of -40˚F to 122˚F, outperform most other batteries in extreme temperatures. Tubular lead-antimony batteries, with a recommended operating range of 5˚F to 113˚F, are moderately tolerant of temperature variations. Lead alloy batteries such as these, however, can fail more quickly in an overcharge situation than TPPL batteries. For example, when a lead alloy battery warms up, it draws more current, which generates more heat. This can produce a vicious cycle that causes the battery to reach a critical stage in a matter of hours. TPPL batteries exhibit a more gradual increase in temperature, taking a longer amount of time for a battery to reach a critical stage in the event of a malfunction of thermal protection circuitry. This offers more time for problems to be discovered and remedied before the battery fails.
Many designers overlook the opportunity to use the ambient cold to their advantage. A venting system can use the night air to cool batteries, yielding more positive effects than negative ones while temperatures get much hotter during the day.
In general, battery cells should be maintained in a clean, cool and dry environment that is free from water and dirt. They should be positioned so that there are minimal temperature variations between the cells. For example, battery lines should not be located near HVAC ducts, exhausts, heating sources or direct sunlight. Temperature variation will cause irregular core temperatures amongst different cells and imbalance in state
of charge.
Adequate ventilation in the battery compartment also is important in order to prevent hydrogen accumulation from exceeding 2% of the total volume of the battery area. Pockets of trapped hydrogen gas, such as near the ceiling, can be extremely dangerous, as hydrogen gas is highly combustible. Monitoring is essential, especially at remote locations, where batteries can go unattended for long periods of time. These concerns can become more exaggerated in climates where extreme temperatures can impact battery performance at least part of the time.
While it may be tempting to cut budgetary corners when selecting a backup battery for a renewable energy system, it can yield catastrophic results down the road in terms of poor performance, shortened battery life, outages or worse. For this reason, it is best to invest smartly from the beginning, recognizing the honest needs of the system and choosing generation and backup products that are best suited for the needs of the water and wastewater treatment environment.
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