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As long as owners avoid energy-gobbling
fridges, solar can supply 4WD and camper trailers' electrical needs.
Workable alternatives include three-way units running on gas
whilst camping, conventional40-litre chestopening fridges, or
larger eutectic units.
It is possible to run conventional 60- to 80litre fridges
and even fridge/freezers from solar, but it needs a lot of solar
capacity, particularly up north. With still-low solar efficiency,
and 80-130 Ah/day plus draw of such fridges, it makes sense to supplement
solar with a generator - or use only the latter.
With 12-volt systems, modules produce 65%70% of that seemingly
claimed. A further 0.5% cent is lost for every degree above 5° C.
(See p.30 for full explanation.)
In temperate climates a typical120-watt module produces 85-90 watts
and 80-85 watts at 35°C. For true output in amps, divide claimed wattage
by 16-17 (ie. not 12).
At mid-2005 most 12-volt solar modules produce about 110 watts (9 .1
amps) per square metre, weigh about 10 kg per 100 watts and cost $8-$10
per generated watt.
Amorphous modules (eg., Uni-Solar) are not affected by heat, but are
twice the size.
The solar industry uses 'Peak Sun Hours' (PSH) to quantify solar irradiation.
A PSH can be seen as the contents of a barrel full of sunlight of known brightness.
That barrel may fill in 45 minutes in Marble Bar, or 6 hours in a Hobart winter.
When full, that's one 1 PSH.
The PSH maps (p.29) show average irradiation. Multiplying PSH, for area and time
of year, by actual output of a solar module gives Wh/day and Ah/day
respectively. The maps show mid-summer and mid-winter. The change is more or
less linear in between.
Two by 120-watt modules should provide a minimum
of 475 Wh/day (37.5 Ah/day) and a conservative maximum of 1000 Wh/day
(83 Ah/day) at midwinter and midsummer
respectively in most parts of Australia except the lower south in mid-winter.
Those modules will run a 50-60 litre fridge, two or three halogen or compact
flurosa, and a TV.
| Nominal Watts |
Actual Watts (amps) |
| 32 |
21.7 (1.89) |
| 64 |
43 (3.76) |
| 80 |
54 (4.70) |
| 100 |
68 (5.9) |
| 120 |
81 (7.0) |
| Table 10a: Typical real-life module outputs, see also p.30. |
Solar modules work from sunlight, not heat. Cloud cover will cut input by half
or so, rain even more. It's rare to have none.
The highest input is often on bright days with broken white cloud. Then, the
sun is reflected back and thence down again to reinforce the direct rays.
The highest average input is with modules facing the sun. Mounting them close
to flat loses 20%-30%. Rather than attempting to track the sun, add a bit more
solar capacity to compensate. In this book such allowance has been made by adjusting
the PSH figure.
The solar mounting used by Trak Shack and shown
in Fig. 1. 14) is simple and effective but
is heavy and necessitates the
trailer being in the sun, although the modules provide shade.
Mounting modules on the roof of the towing vehicle enables the vehicle
to be left in the sun, with the trailer in the shade.
My own preference is to have one or two modules on the towing vehicle
and one on the trailer; the voltage regulator and one auxiliary battery
also to be in the trailer, and a second battery in the towing vehicle.
Solar regulators control the output from solar modules. They
ensure batteries charge quickly and deeply but are not overcharged,
even if permanently connected. Some people do without a regulator
but the risk of damaging batteries and equipment is very high,
except where < 5 watt modules are used to float
charge batteries of at least 100 Ah.
Budget regulators ($50-$100) are primarily voltage-sensitive on/off switches.
As a battery gets close to full charge, they switch off. Then battery voltage
falls and they switch on again - sometimes cycling many times a minute. They
are cheap, reliable and more or less effective - but lack energy monitoring.
The more up-market regulators provide more efficient charging and have many monitoring
functions. It is worth spending the extra money for these functions. Unless you
have prior experience of solar you are unlikely to realise how necessary
they are.
Functions may include: battery voltage; voltage across the solar modules; energy
coming in now; energy being used now; total incoming energy so far today; total
energy used since midnight; remaining charge (as a percentage of battery capacity);
details of the present charging cycle and all or any of the above for the past
30 days.Some turn things on/off automatically at preset times.
Regulators from about $275 upward have programs
for conventional deep-cycle batteries, gel cells, AGMs, and
often sealed calcium batteries. Regulators will need setting for
time, battery voltage and battery capacity. This is not hard once
the manual has been read a few times. Teenagers are good at doing
this - but not at explaining how. Alternatively, ask the vendor how
easy it is. When being told it's simple - demand to be shown how.
A 12-volt module that produces 120 watts should, by definition output
10 amps. But it doesn't, it produces about 7.1 amps. And 7.1 amps
at 12-volts is 85 watts - not 120 watts.
To determine output, the vendors measure volts and amps separately.
They then plot whatever combination gives peak watts regardless
of whether the voltage at which that is measured is 'usable'.
Most modules produce maximum power around 17.1 volts, and as that
times 7.1 amps is 119.7 watts a module that does this is rated at
120-watts.
With a 17.1-volt system (which is as rare as a sardine singing Tosca),
that module will produce what is claimed - on a really cold
day with a bright sun. But into a typical 12-volt system it will
produce 20%-25% less.
Sample illustration
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Figure 9.2 Peak sun hour contours (top) July, bottom January.
Multiplying true module output by the relevant number of Peak
Sun Hours results in the module output for one day. There is
no need to correct for changes as the sun moves across the sky.
These redrawn maps are based on Australian Bureau of Meteorology
data. |
Click here for a
screen size image |
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