Solar School

Words: Allan Whiting

Solar cells have been with us for the past 40 years and are the most reliable, low-voltage electricity generators ever invented. Many solar panels carry a 25-year warranty and you don't get that with any other product we've come across. Solar cells have no moving parts, resist weathering and hailstone impact, and require almost no maintenance.

So why doesn't everything run on solar power?

If you've seen a large-scale solar array you'll have a pretty good idea of the solar panel area you need to replace fossil fuel power generation. In addition to the solar cells a full system needs storage batteries for when the sun isn't shining, a regulator to ensure the batteries aren't improperly charged and an inverter if you want to run 240V appliances, all of which introduces the question of cost.

Total solar power probably isn't a viable option for a house that has access to mains electricity supply and is mostly used at present by installations away from the power grid. However, a caravan or motorhome is an entirely different question.

Three 80-watt solar panels, feeding three deep-cycle batteries can make a typical caravan electrically self-sufficient, providing power to run a fridge, TV, microwave, lights and a water pump. (Protracted, heavy cloud cover may demand some external generating assistance from mains power, a petrol or wind generator or a vehicle alternator, but for average weather conditions the solar system will suffice.)

Modern solar panels are more efficient than those of 40 years ago and are packaged in more user-friendly sizes. Oz-made BP Solar panels are available in 5W to 170W sizes, but the most popular caravan sizes are 80W and 125W. There's also a fold-up double 40W panel that's designed to be positioned directly in sunlight. The cells of the BP Solar panels are protected from the elements by 3mm tempered glass that is rated to withstand a 25mm hailstone. Panels rated at 40W or more have an 80 per cent power guarantee for 25 years.

Uni-Solar panels are different in construction, having their cells embedded in UV stabilised polymers and with triple junctions that give better performance in shady conditions.

The Uni-Solar cells are deposited on a continuous stainless steel roll cut to size.Other brands include Sharp and Kyocera.

Selecting a solar system
To view the latest solar systems and for some guidelines on system selection CW visited NSW's Solar Online Australia factory (www.solaronline.com.au). "The most common mistake buyers make when selecting solar systems for their caravans is to undersize - too small a solar panel area and too little battery capacity," said managing director Brett Sutherland.

An undersized system is up against it right from the start, because the batteries get a proper charge only at times when there's full-strength sunlight and little power demand.

In marginal sunlight conditions the batteries drop below 50 per cent charge and, while they'll cope with deeper discharge, their efficiency soon drops off and battery life is reduced.

"A correctly-sized system has all the components working in harmony and although the initial cost is higher there's a long-term efficiency and lower running cost payoff. If people look only at solar panel wattage ratings and battery amp hour (Ah) figures, it's easy to finish up with an undersized system," Brett explained.

Factors that have to be taken into account include the average sunlight hours for a given location at different times of the year - for instance in the Hunter Valley in NSW, the average daily wattage produced by an 80-watt panel in June would be only 320Wh (watt-hour), compared with the same panel's likely daily output of 500Wh in December.

Contrary to many people's impressions, solar panels produce more power when they're cool, not when they're hot. Another variable is output in shady conditions, with some panels producing more power in these conditions than others.

Battery capacities are nominal and depend on variables such as the charge level, the type of charger, the battery temperature and the rate of power consumption, so there's more to consider than just the Ah rating.

Adding it up
A typical smallish caravan is likely to be fitted out with two 15W 12V DC fluoro lights, a 60W 12V DC water pump, a 75W 12V DC fridge, a 50W 240V AC TV and a 600W 240V AC microwave oven. (Note that a "600W" microwave actually consumes approximately 900W of power.) The process of selecting the correct solar system begins with a load calculation.

two hours lighting (2 x 15W x 2hr = 60Wh);
15 minutes water pump (1 x 60W x ¼hr = 15Wh);
four hours fridge (1 x 75W x 4hr = 300Wh);
two hours TV (1 x 50W x 2 hrs = 100Wh) and
15 minutes microwave (1 x 900W x ¼hr = 225Wh/day).

The latter two 240V appliances need to work through an inverter, for which the efficiency level is 85 per cent, so the total 240V load of 325Wh needs to be divided by 0.85 to give the actual load on the solar system, which is 382Wh. The 12V DC of 375Wh/day plus the 240V AC load therefore totals 757Wh/day.

The next step is to consider the likely power required each day from the solar panel. If a conservative daily sunlight estimate of 5.5 hours is adopted, the theoretical daily input to the system from the solar panel is 757Wh divided by 5.5hrs, giving 137.6W/day.

However, experience has shown that this calculation needs to be multiplied by a constant - the figure of 1.4 - which gives us a total of 192W/day.

(In winter the sunlight hours should be reduced to 4.5 average.)

So, the minimum daily panel input wattage required for this system is 192W, but increased capacity, where possible, is desirable. Three 80W panels would be ideal (75W panels used to be available, but the new rating is 80W), providing a total of 240W/day, a good margin.

Regulating and inverting
The charge coming from the solar panel to the batteries needs to be regulated, so that the batteries aren't overcharged. Pulse-width modulated shunt controllers that charge the batteries optimally are considered a good option.

The regulator needs to have sufficient capacity to handle not only the panel's rated input, but also the 'spikes' that can occur in such circumstances as a cold panel suddenly being exposed to full sunlight.

The rated short circuit current of an 80W solar panel is 4.8amps, so the total current capacity of our three-panel installation is 14.4amps. Allowing for a possible 25 per cent current spike the regulator needs to have at least an 18amp capacity, so the obvious choice is a 20amp unit.

The ideal regulator for a solar system is the latest design that comes with an integrated charging circuit. The unit comes with a self-learning algorithm that adjusts charge rate to the battery's age and capacity.

Appliances designed for household use need an inverter to change the current from 12V DC to 240V AC. The inverter for our hypothetical caravan should be able to operate the TV and the microwave at the same time - a total of 950W. The choice would be a 1000W inverter as a minimum and preferably a pure sine wave type, rather than a cheaper modified sine wave unit.

Most 240V electrical appliances will operate happily on a modified sine wave inverter, but it's not uncommon for motors and power supplies to run warmer and less efficiently. Some sound systems and fluoro lights emit a 'buzz' when powered by modified sine wave inverters.

Volt boxes
The last link in the solar system chain is the batteries. If you calculate battery size based on the total daily consumption of 757W, you simply divide the power needed by 12V, to get a figure of a 63Ah battery rating. However, that battery would be dead flat in a day, so to guarantee no more than 70 per cent discharge the Ah figure needs to be divided by 0.7 to give 90Ah.

Batteries are only about 90 per cent efficient, so the real-world Ah figure is actually 10 per cent higher - 99Ah.

A 100Ah-rated, deep-cycle battery would power the caravan without solar panel input for one day, but it's obviously best to have back-up capacity of two or three days. The optimum installation is three 100Ah batteries.

The preferred battery types for installation in enclosed spaces and living areas, such as caravans, are gel or absorbed glass mat (AGM) designs.

These batteries won't leak if tipped over or cracked and the AGM-type doesn't even need to be mounted flat. In an AGM battery the hydrogen and oxygen gases produced during operation are retained inside the casing and recombined back into water. Unlike lead-acid deep-cycle batteries, AGMs will accept a fast charge. Top-shelf AGM batteries have spiral-wound plates, so the battery looks like a 'six-pack'.

High quality deep-cycle batteries aren't cheap, but with correct charging should last you for at least five years.