[RE-wrenches] PV Assist White Paper

[William Miller]

  Friends:

 

I wrote a draft white paper to discuss the issues relevant to PV Assist
systems.  If you are up for along read, please feel free.  If you have
suggestions, I welcome them.

This idea is new to me so I am learning as I go.  Also, I could not easily
past the chart so I included a link to it.  Also, the table did not quite
line u, but I think you canfigure it out.

Thanks,

William Miller

 

　

　GENERATOR / BATTERY WITH PV ASSIST

August 19, 2013

DRAFT, PRINCIPLES DISCUSSED ARE UNDER RESEARCH

　

Introduction: Whenever we design an Off-grid Alternative Energy systems, we
usually design the system so that non-fossil battery charging sources
(solar, wind, hydro) provide a majority of the energy for battery charging.
Sometimes we come across systems that have loads that are so large that it
is not practical to rely solely on non-fossil sources. A large generator
will be needed to run on a daily basis. We call these systems Generator /
Battery Systems with PV Assist, or "PV Assist" for short. We use the term PV
to describe all RE charging sources since PV is most common in our area.

We have found that battery charging in PV Assist systems using traditional
settings may cause premature failure in battery arrays for reasons described
below. We want everyone to enjoy the longest battery life possible so we
have developed some alternative settings for battery charging that will
hopefully achieve longer battery life.

It has always been the goal of solar technicians to minimize generator run
time. In retrospect, this may not be the best idea in PV Assist systems. The
consequence may be reduced battery life.

Batteries wear out when they are "cycled". A cycle is defined as a discharge
and charge. Normally, an off-grid system will charge batteries during the
day and discharge them at night, resulting in one cycle per day. In the case
of a PV Assist, the batteries may cycle multiple times per day. Your battery
bank is rated for a finite number of cycles. If they cycle 4 times per day,
they wear out 4 times faster than if they cycled once per day.

In an attempt to preserve the longevity of a battery bank in a PV Assist
system, we are experimenting with alternative generator auto start and
inverter / charge battery charge settings. In general, we suggest that if
batteries are going to be cycled multiple times per day, that they be cycled
less "deeply." In other words, do not discharged the batteries to as low a
voltage as one would normally.

In order to achieve this goal, the generator will run more frequently than
in a traditional hybrid off-grid system. In these systems it is already a
given that generator run time will occur every day, even multiple times per
day. We are essentially embracing generator run time to preserve battery
life. We are trading off greater "wear-and-tear" on the generator for less
wear-and-tear on the batteries. The decision to do this belongs with the
system owner.

Battery DOD versus life span: See figure 1 below that compares the depth of
discharge (DOD) of a battery to battery life. Note that the maximum expected
cycles is 4,000. If you cycle 4 times per day at this DOD, expect 1,000 days
of life, or three years. If you cycle to 80% you can expect 1000 cycles, or
250 days. This is a very short life span and this scenario needs to be
avoided. The sweet spot is to be determined for each unique situation, but
for discussion we are assuming we want 3000 cycles, or about 2 years. This
is not long for a battery bank, but keep in mind this design assumes we have
undersized the battery bank significantly. Essentially you are buying two or
three small battery banks in the span of time one properly sized battery
bank would last.

To achieve 3,000 cycles, the DOD must be limited to about 70%. The lowest we
can allow the battery bank voltage to get, when measured at rest, is about
49.0 volts. Keep in mind that batteries being discharged will exhibit a
voltage lower than the at-rest voltage. Also keep in mind that surge loads
will cause the battery voltage to temporarily sag. The above described logic
drives our suggested system settings below.

http://www.trojanbatteryre.com/PDF/datasheets/L16REB_TrojanRE_Data_Sheets.pd
f

Generator Auto Start: We will require that generators in a PV Assist system
be able to start automatically based on both battery voltage and load. Most
common inverter/charger systems are capable of this. The two conditions for
auto start are described below.

Battery start: In the case of a battery start condition, the master inverter
monitors the battery voltage. Generally, if the voltage becomes slightly low
for 24 hours, or the voltage becomes moderately low for 2 hours or extremely
low for 15 minutes, the generator will start. When the generator starts in
these conditions, the system will complete a full three stage charge cycle.
An explanation of a three stage charge cycle is beyond the scope of this
paper. 

If we are assuming that the generator will start more than once per day,
there is reason to desire a short run time. Charge settings will be modified
to allow for a shorter battery charge cycle. One must be cautious to be sure
that if loads decrease, settings are modified to reflect this reduction. If
this is not done, there is a risk that the batteries will never recharge
adequately. It is also required that system operators manually equalize the
batteries every 30 days. This normally only applies to flooded batteries but
we will present modified EQ settings for sealed (VRLA) batteries. In the
case of VRLA batteries, this is not to actually equalize the batteries but
to ensure a full absorption cycle.

Load Start: For load start settings, the system senses how high the loads
are (current draw on the system). If the loads exceed a programmed amount
for a programmed time, the generator will run until the loads decrease below
a lower programmed value for a programmed time delay. We will use this
feature extensively for PV Assist systems.

Load Start Levels: For this program, we are assuming that we do not want to
subject a battery bank to any draw that would deplete it in less than 10
hours. If the draw exceeds this amount, the generator should start.
Calculations show that if the draw exceeds 5% of the battery watt hours, the
bank will deplete in 10 hours or less. Example: If a 48 volt battery bank
has an amp/hour rating of 1,000, the watt/hour rating is 48,000. The
generator should start if the load exceeds 2.4 KW. These calculations are
based on an available amp/hour capacity 50% of name-plate.

Load Start Delay: The excess load must be present for a programmable period
of time before the generator will auto-start. If this load occurs for long
enough the battery voltage will decline and the generator will auto-start
for voltage reasons. This will cause a charge cycle, which is usually not
necessary. Therefore we need to find a proper delay time that will start the
generator if the excess load is expected from more than a short duration. It
is up to the system designer to anticipate load values and load durations.
Some examples are included below.

Load Stop / Load Stop Delay: Load stop is the value below which the load
must drop before the generator will auto-stop. The generator must fall below
this value for a programmed period of time before the generator will stop.
These values are difficult to predict unless the particulars of a system are
known. The suggested starting point for Load Stop is 2.5% of bank watt hours
(a 20 hour depletion). The stop delay is determined by factors discussed
below.

Quiet Time: Most modern inverter / chargers have a quiet time setting. When
this is invoked the generator will not battery start for the 24 or 2 hours
volt settings described above. We use this not only to allow for reduced
generator run at night, but to disallow battery starting in the morning of a
sunny day where the PV charge might prevent a generator start. We will
embrace quiet time as valuable in PV Assist systems.

PV Charging: We recommend that PV and other RE charging sources be set at
traditional values. This will differ from the suggested settings below only
in absorption duration. This value should be a minimum of 2 hours, or a
taper setting that provides a similar absorption period.

Settings: Below are suggested settings for three stage charging systems and
for generator auto-start systems. These settings are for nominal 48 volt
systems and can be interpolated for other nominal system voltages. These
settings apply to flooded lead-acid and VRLA (sealed) batteries:

Parameter     FLA     VRLA

Absorb volts:     57.6     56.8

Absorb time:     0.5 hours 0.5 hours

EQ Volts             61.0     56.8

EQ Time:         2.0 hours 2.0 hours

24 hour start:        48.8     48.8

2 hour start:         47.8     47.8

15 minute start:     47.0     47.0

Load Start:             5% of battery array watt/hours

Load Start delay:     Minimum setting

Load Stop:             1% of battery array watt/hours

Load Stop delay:     Determined by load profile, see below

Load profile: We need to determine the number of cycles of loads that exceed
5% of battery array watt hours. This is the number of times the generator
will start on a daily basis.

There should be a finite number of times a generator starts each day to
avoid excess wear on the generator starting mechanism. If the load cycle
number exceeds the desired generator starts per day, then the load cycle
needs to be reduced or the delay time will need to be extended to exceed the
off cycle of the 5% load.

For example if the excess load comes on every two hours, the generator will
start 12 times per day. If this is acceptable, then use a minimum Load Stop
setting. The generator can shut down soon after the loading stops.

If this number of generator starts is excessive, the load cycle count needs
to be reduced or generator load start settings adjusted. For example, if the
excess load is an appliance, the user needs to be instructed to use the
appliance less often. Or, the load stop time needs to be increased so that
the generator stays on for several cycles of the load. For example, if the
uses of the appliance can be grouped so that most of the times it is used
there is no more than 20 minutes between uses, set the stop delay for >20
minutes and the generator will remain on until the grouped use cycle is
complete.

An example might be a water heater that is used in the morning and evening,
but not the rest of the day. The heater might come on three times in the
morning and three times in the evening, cycling off for 30 minutes. In this
case the load stop delay can be set at >30 minutes and the generator will
run for a few hours morning and evening. The consumer should be encouraged
to group other loading to occur during those hours. The cooking, laundry and
workshop activities could occur during those hours, making use of the
generator run.

Summary: This program is still under development and we welcome your
feedback on the results you achieve using these settings.

End of paper.

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