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How much energy from solar panels can you really use?

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Posted by Pete S on February 2, 2012, 6:36 pm
 
This question is for those who store their solar panel energy in batteries.

"How efficient is a modern battery storage system in storing solar power for
later use?"

I have heard that using batteries for storing electricity  is about 25%
efficient.
  That is:   If you charge your battery bank at the rate of  one kilawatt
hour/hr of energy into the battery bank, only about 25%  or 250 watt hours,
is actually useable due to heat losses, chemical reactions, or whatever.
  Maybe they meant the the whole making/storing process, including losses
from an engine/generation system is what's only 25% efficient?  I don't know
for sure and that is why I am asking.

An example of one way to answer would be as follows:

-On a given day , a solar array produces 5KW of electrical energy and it is
all sent to the battery bank, but no power is taken from that battery bank
during the test.
-Once the sun goes down,  read the specific gravity of the electrolyte in
the  battery bank and then apply a reasonable load to it.
-Continue to take and measure power being extracted from the battery bank
until the specific gravity of the battery bank is  back to the same reading
as it was when  loading began.
-Read the power consumption meter to obtain the total power that was drained
from the battery bank during the test.
-Divide output power by input power to determine the efficiency of the
storage/power recovery system.

If there's a better way to measure the efficiency, I'm wide open to it.

Pete Stanaitis
---------------

 


Posted by NotMe on February 3, 2012, 12:51 am
 

I expect your assumptions is that lead acid batteries are used.

Further you are not taking into account the charge or discharge rate
(typically measured as C/delta T)  The greater the delta T the more
efficient is the process.  I might mention that pure DC is less efficient
for charging than a system that allows a partial discharge as a cyclical
function (most are 50 or 60 Hz)

The issue is not how much energy or how efficiently it is stored/use but
that the system provides 'enough'.  We had a place in the W NC mountains
that was totally off grid.  We had a back up gen set but with the exception
of monthly maintance runs (~2 hours under load) it went unused for the
better part of 15-20 years.






Posted by Pete S on February 3, 2012, 2:09 am
 Thanks for your comments, NotMe.
  But no matter what kind of batteries are used, there must be some loss of
power in charging and discharging them, and that is at the heart of my
question.
For those who have to buy PV collectors, I would assume this would be really
important.
You have to buy enough collectors to offset the power lost in the
storage/recovery process, don't you?

So, what IS that loss?  And, since you mention lead acid batteries as being
my assumption, how does that loss vary with other types of batteries and
their input/output systems?

Pete Stanaitis
----------------
--------------------------------------------------------


Posted by Ron Rosenfeld on February 3, 2012, 3:09 pm
 

The calculations are more complex than just the battery losses, but you are on
the right track in assuming you have to adjust for system losses in deciding how
much PV panel you require to generate a given amount of usable energy.

But for typical lead-acid battery "round-trip" efficiency, you could guesstimate
about 80%.  The mfg should be able to supply more precise values.
You also need to account for losses in wiring, aging of the panels, real output
of the panels if they are not rated under the same conditions they are being
used at your site.  If you are converting your battery DC to AC so as to use
normal household appliances, there will be losses in the inverter also, which
will vary with the load on the inverter as well as the type of inverter -- again
these are details you can obtain from the mfg.

So, for your original question, using lead-acid batteries (the most common in an
off-grid household system), storage efficiency is probably about 80% (1 kWh in
--> 0.8 kWh out) but the mfg has more accurate figures.  

Posted by Mho on February 8, 2012, 3:08 am
 You need to get more familiar with energy and power terms. You have mixed
and confused the terms repeatedly to the point of total nonsense.

I can pump 1000 Watts into my batteries to charge them and remove 2,000
Watts anytime and not violate any laws of physics.


--------------
"Pete S"  wrote in message

This question is for those who store their solar panel energy in batteries.

"How efficient is a modern battery storage system in storing solar power for
later use?"

I have heard that using batteries for storing electricity  is about 25%
efficient.
  That is:   If you charge your battery bank at the rate of  one kilawatt
hour/hr of energy into the battery bank, only about 25%  or 250 watt hours,
is actually useable due to heat losses, chemical reactions, or whatever.
  Maybe they meant the the whole making/storing process, including losses
from an engine/generation system is what's only 25% efficient?  I don't know
for sure and that is why I am asking.

An example of one way to answer would be as follows:

-On a given day , a solar array produces 5KW of electrical energy and it is
all sent to the battery bank, but no power is taken from that battery bank
during the test.
-Once the sun goes down,  read the specific gravity of the electrolyte in
the  battery bank and then apply a reasonable load to it.
-Continue to take and measure power being extracted from the battery bank
until the specific gravity of the battery bank is  back to the same reading
as it was when  loading began.
-Read the power consumption meter to obtain the total power that was drained
from the battery bank during the test.
-Divide output power by input power to determine the efficiency of the
storage/power recovery system.

If there's a better way to measure the efficiency, I'm wide open to it.

Pete Stanaitis
---------------



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