Words and Images by: Collyn Rivers N8054

Auto electricians and battery vendors in the few major far northern towns have a seemingly increasing stream of doleful RV owners, almost flat auxiliary batteries, fridges full of decaying food and undrinkably warm anything. There are many causes of such woe, the most common being that more electrical energy is needed in tropical areas (mainly for the fridge) than generated by whatever is supplying it. The battery inevitably runs down.

Despite this, common requests are not ‘help me fix this problem’, but ‘I need more battery capacity’. This is akin to opening a second bank account to ‘increase’ the same money paid in - it can only further increase losses (in higher bank charges or, with batteries, by greater energy loss).

Battery vendors rarely ask why a product is needed - they sell whatever the customer asks for – in this case one or more costly batteries. Batteries are sold fully charged so the system then works well for a day or so. By the time the enhanced battery capacity is exhausted as before, the owner may be way out of town and faced with the worsened problem of recharging that now larger battery bank.

The Various Causes

Most such energy issues relate to solar-based systems used for extended camping away from a 230 volt supply. Often, the owner installed that advised, or thought sufficient, and the system worked well down south.

If the fridge is well installed (few are) the main cause is usually not understanding that the electrical energy available must always exceed the energy drawn - (plus some excess to cope with so-called energy transfer losses).

Solar Input

When we lived in our self-built 100% solar powered home outside Broome, southern friends would comment ‘you are lucky to have so much sun year round.’ This is not the case, but so widely believed that even some professional installers (who do not live up there) fail to do their sums.

The Peak Sun Hour Concept

Solar modules respond to sunlight, not heat. The daily average sunlight for any given locality is measured much like rainfall into a ‘standard bucket’ and noting how often it totally fills each day. That ‘bucket’ may fill six or seven times a day during a Hobart summer, but only once in a day in winter –whilst up north it fills five times a day, in midwinter, and a little over six times on most other days year around. Each full bucket (actually 1000 watt per metre2 ) is called a Peak Sun Hour (PSH).

The far north lacks ‘winters’ as such, but during the peak tourist season (June-August), the available sun is typically 5.0 PSH/day – some 20% less than September-March down south.

The highest northern input (September/ October) is typically 7.0 to 7.2 PSH/day. During the rest of the year the average up there is 6.2 PSH/day: about 5.7 PSH/day in midwinter and 6.5 PSH/day in midsummer: comparable with southern summers, but not, as many down south assume, any more than that.

Ambient Temperature

In the far north, day temperatures are 280 C to 300 C in June-August, 32.C to 34.C for the rest of the year (with some days well over 40.C).

The north is thus typically 15.C hotter than down south most year around. When it becomes hot it can be seriously so – sometimes for weeks on end. The ‘winter’ is warm by southern standards – but comfortably so.

Energy Loss

All but the now rare amorphous solar modules (e.g. Uni-Solar) lose about 5% output per 10.C increase in ambient temperature. Again, a common misunderstanding reinforced even in otherwise reputable magazine articles, and often Internet forum ‘advice’, is that the loss is from an ambient temperature of 25.C upward. Not so: it is the temperature of black glass covered solar cells under a hot sun. At 25.C ambient, those cells are typically 47.c -49.C. 

That solar heat loss reality really is as stated above (and in my other articles and books) and is also made totally clear in the solar industry’s  own Nominal Operating Cell Temperature ‘standard’. It is shown in the solar industry technical literature and on a data panel on the rear of most solar modules (see example on previous page). As can be seen in the third column the NOCT output of this typical high efficiency ‘120 watt’ monocrystalline solar module is 87 watts at 25.C ambient temperature.

In plain English this means that, on a typical comfortable summer day’s 25.C, a typical solar module has already lost over 10% of its rated output. At 32.c-34.C it has lost 15%-16%. And at the not uncommon 40.C plus, heat loss approaches 20%.

Whilst maximum output is obtained when the module faces directly into the sun, because the tropical sun is mostly overhead, the typically flat RV solar mounting causes next to no loss up north. In many cases it is ahead.

The most probable output of a top quality 120 watt mono-crystalline solar module at a true 25.C ambient temperature (87 watts) is clearly shown in the third column. The cell temperature at 25.C ambient is shown here as 47.C. The confusion does not stem from the solar industry - but clumsy and ongoing misreporting, plus camp fire and forum mythology. Pic: courtesy Kyocera.

Energy Usage Increases

Compounding less solar energy being available, the energy needed to drive a fridge increases by 10% for every 5.C above a rated ambient temperature of 25.C. During a northern June to August the energy increase is about 5%, but for most other months it is 10-15%.

Overlooked too is that northern coastal areas remain warm all night. The 24-hour fridge energy draw typically increases by 20-30%; and a probable 40% plus for those partial to cold tinnies.

Conversely, nights inland can be very cold, even during summer, but this only assists those travelling eastern inland roads up north, the Barkly Highway west from Mt Isa to the Stuart Highway to Katherine and Darwin, and the inland road from Port Hedland, via Newman, to Perth.

Thus there is not only less solar energy available than most  expected but energy draw is far higher. Some fridges may just cope with such temperatures, but gobbling energy to do so. Many however just gobble energy and run continuously to no avail.

(Shortage of editorial space precludes the completion of this article in this issue. The remainder, to be published next month, quantifies the probable discrepancy, shows how to know whether your existing system will cope - and what can be done to assist if it cannot.)

Category: Technology
Written: Sun 01 Sept 2013
Printed: September, 2013
Published By: