2.2 Method 1: Charging Current Method (CCM)
2.2.1 Determine the number of solar modules
needed:
First, determine the system voltage, Vsyst. We can estimate this from the DED. Practice has established that
for DED less than 1 kWhr (or a sum of module wattage
of about 160 W), a 12 V system is required, 1–3 kWhrs,
requires 24 V, and 3–4 kWhrs requires 48 V, etc. This
can be used as a general guide but actual sizing must
be done since other factors like cable sizing may make
it necessary to adjust the system voltage.
2.2.2 The next step is to estimate the charging
current needed from the modules
This is done by diving the final DED in Whrs with the
system voltage, giving us the DED in Ahrs. The DED
in Ahrs is then divided by the PSH to give the charging
current required from the modules to meet the DED.
2.2.3 Choosing the module(s)
The information we have now can enable us to go to the
market and choose a suitable module size or module
sizes that has/have a maximum current rating equivalent or close to the charging current obtained in 2.2.2
above. If such a module is not available, then a combination of modules of the same rating whose total Imax
is equivalent or close to the charging current is chosen. Note that some manufacturers use Pmax, I max,
and Vmax while others use Pm, Im, and Vm to designate the maximum power point, maximum current,
and maximum voltage, respectively. Here, we use the
former designation. The module performance in the
field will be less than the indicated. We assume a loss
of about 20%, so our Imax will be (Imax) ×0.8 to get
the real field value, Ireal.
2.2.4 Calculating the numbers of module strings
The charging current is divided by the real maximum
current, Ireal, to get the number of module parallel
strings, Np, you will need in the system. Please note
that this is not the number of modules but the number of
parallel strings you will connect to give the calculated
charging current.
2.2.5 Calculating the numbers of modules per
strings
Next the number of series modules, Ns, connected for each parallel string is calculated. This is
achieved by dividing the system voltage by 80% of
the maximum power voltage (Vmax), i.e., system
voltage/(0.8 × Vmax).
2.2.6 Calculating the total number of modules
Lastly, we calculate the total number of modules, Nt,
we need by multiplying the number of parallel strings,
Np, by the number of series connected modules per
string, Ns.
2.2.7 Next is the determination of the battery bank
capacity needed
The battery bank capacity (Ahrs) (this is not the number of batteries) needed in a day (i.e., the total energy
we need from the batteries in a day) is now determined
from step 2.2.2 above. To calculate the battery bank
capacity, we multiply the DED in Ahrs by the days of
autonomy (the number of days we wish to have energy
supply from the batteries when there is no or minimal
sunshine) and divide this product by the depth of discharge (DOD) (how much energy you can safely use
from the battery to avoid excess draining), given as a
decimal. Some designers prefer a DOD of 50% (0.5)
and others 20% (0.2). We use 20%. Take note that the
lower the percentage of DOD, the larger will be the
battery bank.
2.2.8 Choosing the battery size
Next choose the battery size (rating) depending on
what is available in the market.
2.2.9 Choosing the number of battery strings
The number of battery parallel strings, Nbp is now
determined by dividing the battery bank capacity
(Ahrs) with the rating of battery chosen (Ahrs)
2.2.10 Choosing the number of batteries in a string
The number of batteries connected in series, Nbs, in
each string is obtained by dividing the system voltage
by the voltage of one single chosen battery.
2.2.11 Calculating the total number of batteries
The total number of batteries, Nbt needed is now calculated by multiplying the number of parallel strings
by the number of series connected batteries in each
parallel string.
2.2.12 Sizing the charge controller
The charge controller size is determined by the maximum current that can flow through it and the system
voltage. The total short circuit current (Isc) that all
the modules can generate is used. This is computed by
multiplying the Isc of a single module by the number of
module parallel strings. A margin of 25% is allowed so
that the total current to the charge controller becomes
the total current from all the modules multiplied by
1.25. A charge controller with a current rating higher
than this calculated current is chosen. It is important
that the charge controller is of the same voltage as the
system voltage. If the user has plans of future expansion of the system by way of adding new appliances or
another room, it should be factored into the controller
sizing.
2.2.13 Sizing the inverter
The total power needed on a daily basis provides a basis
for estimating the inverter size. So the sum of column
6 in Table 1 is used and a factor of 15% in inverter
conversion losses is factored. So 0.15 × the sum of
column 6 is added to the sum of in column 6 to get
the minimum power rating of the inverter. An inverter
in the market that has such or near rating or higher is
chosen. If the PV system is purely direct current (DC),
then an inverter is not needed.
232
2.2.1 Determine the number of solar modules
needed:
First, determine the system voltage, Vsyst. We can estimate this from the DED. Practice has established that
for DED less than 1 kWhr (or a sum of module wattage
of about 160 W), a 12 V system is required, 1–3 kWhrs,
requires 24 V, and 3–4 kWhrs requires 48 V, etc. This
can be used as a general guide but actual sizing must
be done since other factors like cable sizing may make
it necessary to adjust the system voltage.
2.2.2 The next step is to estimate the charging
current needed from the modules
This is done by diving the final DED in Whrs with the
system voltage, giving us the DED in Ahrs. The DED
in Ahrs is then divided by the PSH to give the charging
current required from the modules to meet the DED.
2.2.3 Choosing the module(s)
The information we have now can enable us to go to the
market and choose a suitable module size or module
sizes that has/have a maximum current rating equivalent or close to the charging current obtained in 2.2.2
above. If such a module is not available, then a combination of modules of the same rating whose total Imax
is equivalent or close to the charging current is chosen. Note that some manufacturers use Pmax, I max,
and Vmax while others use Pm, Im, and Vm to designate the maximum power point, maximum current,
and maximum voltage, respectively. Here, we use the
former designation. The module performance in the
field will be less than the indicated. We assume a loss
of about 20%, so our Imax will be (Imax) ×0.8 to get
the real field value, Ireal.
2.2.4 Calculating the numbers of module strings
The charging current is divided by the real maximum
current, Ireal, to get the number of module parallel
strings, Np, you will need in the system. Please note
that this is not the number of modules but the number of
parallel strings you will connect to give the calculated
charging current.
2.2.5 Calculating the numbers of modules per
strings
Next the number of series modules, Ns, connected for each parallel string is calculated. This is
achieved by dividing the system voltage by 80% of
the maximum power voltage (Vmax), i.e., system
voltage/(0.8 × Vmax).
2.2.6 Calculating the total number of modules
Lastly, we calculate the total number of modules, Nt,
we need by multiplying the number of parallel strings,
Np, by the number of series connected modules per
string, Ns.
2.2.7 Next is the determination of the battery bank
capacity needed
The battery bank capacity (Ahrs) (this is not the number of batteries) needed in a day (i.e., the total energy
we need from the batteries in a day) is now determined
from step 2.2.2 above. To calculate the battery bank
capacity, we multiply the DED in Ahrs by the days of
autonomy (the number of days we wish to have energy
supply from the batteries when there is no or minimal
sunshine) and divide this product by the depth of discharge (DOD) (how much energy you can safely use
from the battery to avoid excess draining), given as a
decimal. Some designers prefer a DOD of 50% (0.5)
and others 20% (0.2). We use 20%. Take note that the
lower the percentage of DOD, the larger will be the
battery bank.
2.2.8 Choosing the battery size
Next choose the battery size (rating) depending on
what is available in the market.
2.2.9 Choosing the number of battery strings
The number of battery parallel strings, Nbp is now
determined by dividing the battery bank capacity
(Ahrs) with the rating of battery chosen (Ahrs)
2.2.10 Choosing the number of batteries in a string
The number of batteries connected in series, Nbs, in
each string is obtained by dividing the system voltage
by the voltage of one single chosen battery.
2.2.11 Calculating the total number of batteries
The total number of batteries, Nbt needed is now calculated by multiplying the number of parallel strings
by the number of series connected batteries in each
parallel string.
2.2.12 Sizing the charge controller
The charge controller size is determined by the maximum current that can flow through it and the system
voltage. The total short circuit current (Isc) that all
the modules can generate is used. This is computed by
multiplying the Isc of a single module by the number of
module parallel strings. A margin of 25% is allowed so
that the total current to the charge controller becomes
the total current from all the modules multiplied by
1.25. A charge controller with a current rating higher
than this calculated current is chosen. It is important
that the charge controller is of the same voltage as the
system voltage. If the user has plans of future expansion of the system by way of adding new appliances or
another room, it should be factored into the controller
sizing.
2.2.13 Sizing the inverter
The total power needed on a daily basis provides a basis
for estimating the inverter size. So the sum of column
6 in Table 1 is used and a factor of 15% in inverter
conversion losses is factored. So 0.15 × the sum of
column 6 is added to the sum of in column 6 to get
the minimum power rating of the inverter. An inverter
in the market that has such or near rating or higher is
chosen. If the PV system is purely direct current (DC),
then an inverter is not needed.
232