Selecting the right battery for commercial solar projects can mean the difference between success and failure
By: Michael R. Kulesky is the director of commercial marketing for telecommunications, utility and new technologies at EnerSys
As renewable installations increase worldwide, so does the need for reliable cycling batteries for energy storage. Correctly designing a power-and-backup system and selecting the right storage batteries for renewable applications can significantly affect overall performance, efficiency and longevity.
Solar 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
- Large electrolyte reserve
Often other factors such as cost, space and maintenance are given higher priority, complicating the purchasing decision.
Selecting the right-sized battery is one of the most critical decisions in setting up backup storage for a commercial solar project. When factors such as cost play a big role, however, proper sizing sometimes takes a back seat.
It’s no surprise then that one of the most common mistakes in battery selection for solar applications is the improper sizing. This occurs when the installer miscalculates the number of days of autonomy and the size and number of solar panels needed to support the load and charge the batteries.
When sizing the solar panels, it is important to estimate the system load accurately, available sunlight, average power output, daily discharge and days of autonomy.
The most important rule is that “energy in” must be greater than “energy out.”
Unfortunately, like everything else, solar installations 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 solar panels).
In a solar installation, however, the solar panels are the only power-generation source. Installing too few or improperly sized panels can have a direct effect on the life, as well as the short- and long-term performance, of the system components — especially the batteries.
Without enough power generation in the system, the batteries become depleted. With no recharge period, they plateau and discharge again, creating a downward “stair-step” cycle pattern. Continuously discharging lead-acid batteries more than 80% causes irreversible harm. Therefore, the more cycles anticipated, the lower the depth of discharge (DOD) 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 And Design
Batteries have evolved over the past few decades to meet the specific needs of utilities and other industries. Solar applications require batteries that provide excellent reliability and maximum cycling capability.
Grid and Plate Design: In general, the batteries marketed to the solar industry are manufactured with either flat or tubular plates.
Flat-plate designs are the principal type 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 (PbCa) 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 solar and other demanding 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 to ensure long life.
Tubular batteries have either round or square tubes. In general, square tubes provide 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 — a common failure mode in flat-plate designs that can lead to early failure due to sediment shorts. This combination of greater positive surface area and better paste adhesion allows for excellent cycling capacity.
To Alloy Or Not To Alloy?
Over the years, manufacturers have experimented to find the right materials 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 facilitate manufacturing but cause various negative attributes, such as issues with recycling and increased float current that causes internal heat buildup and lowers 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 has also evolved so that thin-plate, pure lead (TPPL) batteries are a viable and available option. They are ideal for backup applications because they are a smaller, lighter solution with a longer shelf life, 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 a wealth of knowledge to manufacture. Therefore, they are primarily marketed as a premium solution for challenging applications ranging from submarines to telecommunications systems.
Flooded vs. Valve Regulated
Lead-acid batteries are available in two containers — flooded or valve regulated. Flooded batteries require rewatering and so are costly to maintain. They also require larger containers for the flooded electrolyte, making them less energy dense. Also, flooded batteries allow for visual monitoring of the cells, enabling operators to identify issues and preventative maintenance needs more readily than sealed, valve-regulated lead-acid (VRLA) batteries.
On the other hand, VRLA batteries are lighter weight, require lower maintenance and are more cost-efficient. To accommodate for corrosion in PbCa batteries and to prolong life, manufacturers increase grid thickness at the cost of reduced energy density and increased weight.
Climate And Environmental Considerations
Many solar farms are located in desert locations with high daytime temperatures and dramatically cooler evening temperatures. Temperature extremes — especially high heat — can damage batteries. While it is important to protect batteries from the effects of external temperature variations, it is even more important to monitor the critical internal-core temperatures of the batteries.
TPPL batteries, with a recommended operating range of -40 to 122˚F, outperform most other batteries in extreme temperatures. Tubular lead-antimony batteries, with a recommended operating range of 5 to 113˚F, are moderately tolerant of temperature variations. Lead-alloy batteries such as these, however, 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 longer for a battery to reach a critical stage in the event of a malfunction of thermal protection circuitry. This offers more time for the 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 the batteries, yielding more positive effects than negative ones, while temperatures get much hotter during the day.
In general, battery cells should be maintained in clean, cool and dry environments that are 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 among different cells and will cause imbalance in state of charge.
Adequate ventilation in the battery compartment is also important 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 because it is highly combustible. Monitoring is essential, especially at remote locations, where batteries can go unattended for long periods. These concerns can become more exaggerated in climates where extreme temperatures can affect battery performance at least part of the time.
Following the manufacturer’s guidelines for monthly, quarterly and annual maintenance will ensure long battery life and strong performance. In addition, here are several key areas on which to focus to reduce irreversible harm:
- Charging (charger output, torturing bolts, dirty solar panels, etc.)
- Watering flooded batteries (a must for life and safety)
- Voltage (low voltage cells will eventually cause harm to good cells)
- Visual inspection for bulging, leaking, cracking, etc.
While it may be tempting to cut budgetary corners when designing commercial solar systems, it can have catastrophic results down the road in terms of poor performance, shortened battery life, outages or even worse. For this reason, it’s best to invest smartly from the beginning, recognizing the true needs of the system and choosing generation and backup products that are best suited for the rigors of the solar-farm environment. SPW