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To be able to choose the energy storage best suited to your needs you must first understand a some basic battery terms that will have great effect the total overall cost and performance of your energy storage system. They are; Amp Hours, Rate of Discharge, Depth of Discharge, Partial State of Charge, and Life Cycles. Renewable energy storage batteries are generally rated by the total daily amp hour capacity that they are capable of holding. Both the daily usable reserve and the total number of life cycles possible vary directly according to the other factors, so all must be considered carefully.
In general a battery will have the greatest Amp Hour storage capacity when it is charged and discharged slowly. A figure commonly used when comparing the amp hour capacity of various brands and types of batteries is the 20 hour discharge rate. This works particularly well when considering energy storage in New Hampshire because our average daily isolation period (the time when solar PV is most effective) is just over four hours. Accurately calculating how many AH you will need to run your home or business during an average day is the first step that needs to be taken when figuring out how much storage you will actually need.
Depth of Discharge refers to the percentage of a battery’s total amp hours withdrawn during a daily cyle. If a battery is rated at 200 amp hours and only 100 amp hours is withdrawn during a cycle, this would be a 50% DoD. Taking the DoD below 50% on many battery types will greatly lessen the total number of cycles that the battery will be capable during it’s lifetime. As a consequence it is a good idea to use the 50% DoD when comparing different types of batteries. It is important to note however that many of the newer technology batteries can be drawn down to 100% of their amp hour capacity (ie, fully dead) without too much adverse effect, while doing the same to an older tech flooded lead acid battery will permanently diminish or even destroy its ability to store energy.
Partial State of Charge refers to the possibility of a battery not being fully recharged during a daily cycle. This is a common occurance for batteries used in both renewable and self consumption applications and thus has to be a major point of consideration. Batteries used for TOU peak shifting applications are generally able to be fully recharged during the nighttime off peak hours and a partial PSoC would be a rare occurrence and thus not that important.
The total number of daily cycles available has the greatest effect on a storage systems overall lifetime cost. A good choice might be quite a bit more costly initially but far less expensive over the long run. Again, all of the battery’s characteristics should be given careful consideration and the final choice made to suit your particular needs and habits. Your New Hampshire Solar advisor will gladly spend the time with you to ensure that you have a full understanding of the variables and are able to get the best possible individualized choice.
The least expensive batteries are flooded lead acid deep cycle batteries similar to those found in your automobile, golf carts, forklifts, etc. The main drawbacks to these batteries is that they are somewhat performance limited, require a significant amount of maintenance, and can potentially be very toxic if not handled and disposed of properly. They do not tolerate the accidental abuse of a deep discharging or PSoC well at all. Most should never be drawn below 50% of their capacity and this severely limits the total amount of power available for your daily usage. The initial cost of flooded batteries is admittedly very low in relation to the alternatives listed below, but so is the total amount of daily cyles they are capable of. NH Solar does not generally recommend them.
There have been some recent innovations however that have kept the lead acid format viable and still cost effective, Absorbed Gas Mat construction and carbonizing. AGM batteries are nearly sealed tight (they do have small vents). This greatly reduces both the dangerous off gassing of hydrogen and the required monthly maintenance to replace the evaporated liquid. Carbonizing the plates reduces the amount of sulphur crystals that can form internally by PSOC daily cycles, and this extends the cycle life of the battery. The Ouback Energy Nano Carbon battery exemplifies this new breed and is an excellent choice for a budget minded new battery bank installation. A 200NC Energycell can store 178 Amp hours at the 20 hour rate of discharge, and if limited to a 50% DoD has a life of 2,800 cycles.Trojan has a line of carbonized AGMs that can be used as a direct replacements for older tired flooded battery banks
A significant development in energy storage was introduced in 2015 in the form of the the handsome and very power dense Tesla Powerwall lithium-ion home battery. The introduction of the Powerwall has sparked an explosion of public interest in how we will use and store electricity. Within just months of Tesla’s introduction numerous other large companies also jumped in and began developing Li-ion batteries with ever increasing capabilities.The initial Powerwall home storage battery was almost immediately matched in performance by batteries from LG Chem and Sonnen Most recently Pika Energy and Panasonic have announced the Harbour battery system. For those considering a new solar/storage energy system, this deserves serious consideration as all of the controlling communications and parameters have been designed to work in perfect harmony. The competition continues to be fierce in the compact lithium ion battery market and in the long run this will definitely benefit the consumer. The initial price for Li-ion batteries is currently high, however that is offset by the very long life, compact size, and the ability for the battery to be nearly fully discharged with minimal adverse effects.
Two other energy storage technologies on the horizon are zinc- bromine flow and sodium ion (salt water) batteries . Most of the flow battery development in the US seems to be currently focused on larger utility scale projects, but a residentially scaled battery has begun to be distributed in Australia by Redflow. It is very promising but not yet available in the US. The Aquion AHI 2.2 kWh 48 volt batteries aren’t as power dense as lithium ion batteries (ie, they require a bit more space), but they do have some other very significant advantages. Aquion Aspen batteries feature a long cycle life similar to Li-Ion, they have ability to repeatedly go to a full 100% depth of discharge without ill effects, they are non toxic and easily recyclable. Another significant advantage is that AHI batteries self equalize much better than other types of batteries, which allows additional capacity to be added to an existing bank well after the initial installation. The only downside to bear in mind is that they do not tolerate either rapid rates of charging or discharging well and have a somewhat limited power rating per module (~14 amps). High surge appliances like a deep well pumps and high draw appliances like electric ranges and dryers of any type would require a bank of multiple batteries to sustain the high instantaneous loading. The long term cost per kilowatt hour of Aquion batteries is similar to lithium ion. Unfortunately Aquion filed for bankruptcy protection in early 2017 and they are not currently available, hopefully the technology will be acquired soon. This is a New Hampshire solar favorite, both for new dc coupled installations and for retrofitting established lead acid systems
Which battery to choose? For clients with small storage needs (<5 kWh) New Hampshire Solar would recommend the carbonized lead acid AGMs as the logical choice. Carbonized batteries are also an excellent choice as replacement batteries for existing lead acid installations.
The StorEdge inverter paired with a LG RESU battery is an excellent choice for newly installed grid tied solar systems, especially if the utility company employs discounted net metering. The two significant advantages to be had by adding energy storage are backup power when the grid is down, and a much higher degree of self consumption. A higher degree of self consumption means that more of the power coming from the solar array is now able to be stored and consumed directly by the home or business. This avoids the penalty of sending excess daytime power back into the grid at a discount, and having to buy it back in the evening at the full rate.
New off grid installations are best served through the use of a “smart” inverter such as the Outback Radian, Schneider XW+, or Pika Energy Island. Smart inverters feature much higher control capabilities and programmability and can control all aspects of a home’s energy production and consumption. New Hampshire Solar recommends installing a prepackaged and fully integrated power module rather than mixing and matching different manufacturers componants. Two excellent examples are the Pika Energy Island combined with the Panasonic Harbor battery and Sentinel Solar’s WAVE
Retrofitting energy storage to an existing solar system that wasn’t designed with batteries in mind requires very specific and careful engineering, please contact us directly to explore your options.