Specifications
AT 51
to produce its best output.). For this example,
we’ll assume the system needs to provide
power for three hours per day, every day (i.e.
autonomy = one day). And we’ll also assume
a minimum of four hours of peak sunlight
per day for recharging, which is achievable for
most residents of Australia throughout the year
(according to average hours of sunlight statistics
from the Bureau of Meteorology).
The information necessary to assemble the solar
power system is summarised below:
Power: 150W
Autonomy: 3 hours per day
Peak sunlight: 4 hours per day
A battery’s ability to provide power is measured
in Amp/hours (A/h); a figure describing the
amount of current it can provide for one hour
before going flat. Amps and hours are inversely
proportional, so doubling the current would
halve the hours and vice versa. Therefore, a
battery rated at 10A/h can provide 10A for
one hour, 20A for half an hour, 5A for two
hours, 1A for 10 hours, or any other realistic
combination of Amps and hours that equals 10
when multiplied together.
For the example above, the battery must provide
150W of power. To determine the appropriate
A/h rating, the first thing we need to know
is the battery’s output voltage – from this we
can calculate the current required (current =
power/voltage). Solar power systems typically
use either 12 or 24V battery systems; for the
purposes of this exercise we’ll use a 12V battery.
To produce 150W from a 12V battery requires a
current of 150 / 12 = 12.5A. A 12V battery that
can supply 12.5A for one hour will therefore be
able to provide 150W for one hour.
Now that we know the current required per
hour, we can multiply it by the number of hours
the system will be used between recharges (i.e.
the autonomy). If the system was going to be
used for three hours per day, every day, between
recharges, then the total A/h requirement of the
battery would be 12.5 x 3 = 37.5A/h.
This figure assumes the battery is going to be
fully discharged between charges, which is not
healthy for the battery! So we have to factor
in the depth of discharging. If we want the
battery to have no more than 80% of its energy
discharged between charges, its A/h rating will
be 37.5 / 0.8 = 46.88A/h, which we’ll round up
to 47A/h.
In other words, a 12V battery rated at 47A/h
can provide 150W of power for three hours, and
still have 20% of its charge remaining. This is
the minimum battery capacity required to run
the system in the example for three hours. If
the budget allows it, a larger battery is a better
choice because it requires less discharge depth
and will therefore have a longer life. It would
also allow greater autonomy in the event of an
overcast or rainy day with less than sufficient
sunlight.
The solar panel’s job is to replace the power
taken from the battery. To determine the
appropriate solar panel, we need to know a) how
much power has been taken from the battery, b)
how many hours of peak sunlight we can expect
per day for recharging, and c) the battery’s
efficiency.
Because the voltage and current produced by a
solar panel varies with the amount of sunlight,
we cannot use Amp/hour figures reliably.
Instead, we use Watt/hours (W/h).
In the example above, the audio technology
drew 150W from the battery for three hours,
making a total of 150 x 3 = 450W/h. We chose
a conservative figure of four hours per day of
peak sunlight, which means the solar panel
must produce 450/4 = 112.5W per hour. In
other words, a solar panel rated at 112.5W
would recharge the battery after four hours
of peak sunlight – but only if the battery was
100% efficient, which it is not. Factoring in
a typical battery efficiency of 90% means the
solar panel must produce 112.5/0.9 = 125W per
hour. A 125W solar panel would recharge a 90%
efficient battery with 450W after four hours of
peak sunlight. A more expensive 250W panel
I’ve done quite a bit of
field recording, and I’ve
always been interested in
getting decent multitrack
recordings rather than
the good old ethnographic
stereo mic standard. My
recording rig is currently
built around an Apple
Macbook (white model)
and the very versatile
Metric Halo 2882 MIO,
which can be powered
from anywhere between
+9V to +30V, provided it
can draw 16 Watts. It can
be powered directly off the
Firewire bus (taking power
from the laptop’s internal
battery) or from a separate
supply.
I decided to put together
a solar power system for
a recording expedition
I had planned to the
Solomon Islands. I pieced
things together from
on-line ideas, and from
calculations suggested
by engineers working at
appropriate shops. The
best response I got was
from energymatters.com.
au. They have a useful
online calculator, and the
people I spoke to didn’t
baulk when I said I was
trying to put together a
laptop-based recording
studio powered by the sun!
My system contains two
20W solar panels, a charge
controller/regulator and a
28A/h battery; total cost
was about $800. Power-
wise it went well, and we
managed a solid five hours
or so of recording in a day
without any problems. A
slightly larger solar panel
(say, 60W or 80W) with
a bigger battery would
allow some pretty solid
recording time.
It is vital to be organised,
so that when the machine
is on, you are recording,
not fiddling around
setting up patches or
troubleshooting. Setting
up templates in the DAW
application is important
– inputs, monitoring
buses and so on should be
ready to roll as quickly as
possible. The 2882 was
amazing in this respect – it
sounds great and allows an
impossibly flexible array
of routing possibilities. It’s
like having a console in the
bush, but more flexible!
– Denis Crowdy
Regular readers will
remember Denis from
issue 47, where he
spoke of his adventures
recording stringbands
in Papua New Guinea
amidst the fallout from
an active volcano (‘Songs
of the Volcano’). For more
information about his solar
power system, go: www.
motekulo.net/solar.html