As the manufacturer says “It’s one mean battery”! |
Below is Dennis's second post in the continuation of the Solar Installation. Cue music here.
The Solar Installation Part Two – Batteries and Battery Bank
In the first installment, we discussed how to site the solar
panels on the roof of a new cabana built so that the roof was oriented
optimally. In this installment, we are
going to go over the steps to estimate the electrical usage; how many storage
batteries are needed and other concerns about the batteries. We will save consideration for how many solar
panels are needed and how much sunlight reaches your panels (insolation) for a
subsequent blog. J
How Much Electricity
Do You Use? The next step in setting
up your solar system is determining what you use electricity to power, how long
you use it, and ultimately how many watts are used. This is the dreaded (i.e., pain in the butt)
“electricity inventory”. You can find forms for this on-line that vary in
complexity – search for “battery calculator
solar”. I opted for simple. We itemize
all the items we are likely to use on a regular basis. There are tables of
values for electric appliance wattage on-line, but I felt more comfortable
using a Kill-a-Watt meter to quantify the electricity usage where I could. Plug the Kill-a-Watt meter into an electrical
outlet and plug the device/appliance into the meter. Set for Watt.hrs and collect the data. It will sum the amount of wattage used over
the length of the collection period.
Some would argue this is another aspect of my compulsiveness – ahem. (Note from Wilma: some would be right!) For those items we could not quantify with
the meter, we did use average values found online.
Push the "Amp" button to make it easy! |
We got some surprises however. We were using more electricity than we
thought, so would have underpowered the solar system by solely using the values
published online. For example, the very efficient (aka expensive) refrigerator
we bought was actually using 50% more electricity than claimed. Hrumph! (Note from Wilma: Maybe because our ambient temps are around 85F instead of the 70F used in the test setting?)
Here are the results from the dreaded “electricity inventory”.
We excluded power tools used in construction since these
would be used on an intermittent basis and would be powered by our gas
generator if used for more than a few minutes.
I decided to round up the daily usage estimate to 6000 watts to allow
for devices and appliances we do not have on site currently: a television and a microwave. (Note from Wilma: also a sewing machine, washing machine, maybe
a small toaster oven.) These will
ultimately be here and used, but not for some time. We have a good collection of DVDs too. We will not be connecting to satellite TV
service however.
One item to note is how you get Amps, in the first column,
from watts AC in the second column. It
is calculated from a rearrangement of Ohm’s Law. Basically, Watt.hours =
Amp.hours X Voltage, or Amp.hours = Watt.hours/Voltage. Luckily there is an easier way to get the
Amp.hours however - just push the Amp button on the Kill-a-Watt meter!
The Battery Bank. You now have an estimate of how many
Watt.hours of solar electricity you need to generate and capture for use. Now you need to determine how much battery
capacity you need to store sufficient electricity for daily use, keeping in
mind a few other items, however. You
want your usage of electricity to be a small amount of the total capacity of
the battery bank, referred to as the Depth of Discharge. If you have a smaller amount of usage, the
batteries last longer. If we use only 5
to 10% of the battery capacity, our specific batteries may last around 20 years. In other words, the battery capacity is not a
measure of the total that you can use because you want to avoid discharging the
batteries more than 10% of their charge carrying capacity. Unless, of course you plan to replace your
batteries frequently – not a good financial decision!
Redrawn from: http://support.rollsbattery.com/support/solutions/articles/8136-cycle-life-vs-depth-of-discharge-5000-series |
You also want to make certain that the solar system will be
able to fully charge the batteries, otherwise the batteries will be
consistently undercharged which may lead to premature failure due to sulfation
of the plates in lead-acid batteries.
So what type of
battery do you want to use? We had some
bad experiences with deep cycle cheapo lead acid batteries and more expensive
AGM (Absorbed Glass Mat) batteries when we were relying upon our diesel
generator for charging, and were not encouraged. But we had seen a battery installation in
Western Belize at DuPlooy’s Jungle Lodge and Botanical Garden in 2010. They had installed twenty-four 2V batteries wired
in series to produce a 48volt DC system.
These batteries are manufactured by Surrette Batteries in Canada, a
Rolls company and are the top of the
line batteries for renewable energy applications. I looked at space, weight, price, and decided
we could do something similar with Surrette 4V batteries, requiring twelve 4V
batteries wired in series to get the 48 V system.
Here is how batteries are wired in series using 12V
batteries as an example:
This is what we did for our original battery bank and is
similar to how we would wire the 4V batteries together. The negative pole of
the first battery is connected to the positive pole of the second battery … and
so on. While the voltage is additive,
“The Amps Remain The Same”.
According to the manufacturer, the capacity of the 4V Surrette battery is ~1900 Amp.hr at a
slow discharge rate of 100 hours. The Watt.hours for the bank of 12
batteries would be 1900 Amp.hours X 48V = 91,200 Watt.hours or 91
kilowatts. With numbers like this it
seems we are becoming our own electric company! (Note from Wilma: Well, that is actually the idea, isn't it?!)
We now have enough information to perform a battery bank
calculation to determine how many batteries are needed. Here is the actual analysis, but this
calculator does not appear to be online any longer.
You should use no more than 50% of the battery bank
capacity, to improve battery longevity, which gets us to having 45 kilowatts of
storage and at least 5 days of using electricity without reducing usage and not
having to run the generator (by the morning of day 3 of overcast skies, we
would turn the generator back on). If we
are careful and reduce our usage during overcast periods, then we could go much
longer. By keeping the depth of
discharge to between 5 and 10%, the batteries should last around 7000 charging
cycles or approximately 20 years before (anticipated) failure. Poor maintenance can decrease the time to
failure – ahem. We certainly don’t aim
to be guilty of that!
To summarize, we have planned to have the capacity to use
7500 Watt.hours of solar power for at least 5 days of autonomy, which will not
draw the battery bank down to less than 50% of capacity, while normally keeping
the daily depth of discharge to between 5 and 10%. We are going to wire twelve 4volt batteries
together to produce a 48V system.
Now a practical issue:
each battery weighs 315 pounds when filled with battery acid (34 liters
of acid, BTW). There are 12 in series
plus a spare battery. How do you support
this weight 10 feet off the ground since the combined weight of the batteries
is 4095 pounds? You make floor joists
composed of two 2”X8”X8 ‘pieces of treated pine lumber with plywood in between,
glued with Liquid Nail, a construction adhesive in the USA, and nailed from
both sides, spaced 16 inches on center, supported on reinforced concrete
beams. Then you lay a plywood subfloor,
glued and screwed down on the joists, with the edges epoxied to prevent water intrusion
and delamination. And finally nail
tongue and groove flooring on top of that. The box enclosing the battery bank weighs
about 1060 pounds (more on this in a subsequent post on Insurance). The total weight
of batteries and box is 5055 pounds, distributed over a 27.5 square foot area (2.75X10
feet). This reduces to 188 pounds per
square foot. Whooh!
This is overbuilt support for the batteries, done
intentionally since you not only have to support the weight of the batteries
and box, but you have to consider there is an earthquake slip fault line about
130 miles away off the coast of Honduras that moves Guatemala eastward every
now and then. A million years from now
Guatemala may have some nice seashore and a decent East Coast! See the very bottom of the Magnetic
Declination Map in the first installment for where Guatemala is moving eastward.
The last significant earthquake was a hefty 7.1 on the Richter Scale in late
May, 2009 (see here for images of the aftermath, originally linked to Trip Advisor
by our neighbor Sue Harris). It
shows the destruction in Monkey River Village, with cabanas sinking to ground
level from the effects of liquefaction of sediment, concrete buildings cracked
asunder, and mud geysers erupting from formerly solid ground.
We would like to ensure that the batteries and box remain
where they are and not fall through to the ground below during the next “Big
One”. Our original cabana and this addition include
features to mitigate damage from liquefaction as shown in a previous blog post.
This is getting to be pretty thick reading, so I will stop
here for now.
The 3rd installment will deal with how many solar panels you
need, how much sunlight you have, how many watts produced, and does this meet
your needs.
It takes a fair amount of planning and effort to “go green”
or off-grid. Cash too, as will be explained in a
future installment on costs and safety.