Solar Solutions

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 many other products - spanners, maybe. 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 the size of the solar array you need to replace fossil fuel 240-volt mains power generation for a typical house you'll understand the problems of using solar power as a general replacement. In addition to the solar cells a full system also needs storage batteries for above-average loads and when the sun isn't shining, a regulator to ensure the batteries aren't improperly charged and an inverter if you want to run 240-volt appliances.

However, a medium-sized boat is an entirely different question. Three 80-watt solar panels feeding three deep-cycle batteries can make a boat electrically self-sufficient, providing power to run a fridge, a TV, a 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 an engine-driven 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. Australian-made BP solar panels are available in 5W to 170W sizes and there's also a fold-up double 40W panel that's designed to be positioned in an optimum sunlight position. The cells of the BP solar panels are protected from the elements by 3mm tempered glass that's 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 that is cut to size.

Other brands include Sharp and Kyocera.

SELECTING A SOLAR SYSTEM
We visited NSW's Solar Online Australia factory to check out the latest solar systems and for some guidelines on system selection.

 "The most common mistake buyers make when selecting solar systems for their boats is to undersize - too small a solar panel area and too little battery capacity," said Solar Online's 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 for them to finish up with an undersized system.

"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 on the Central Coast of NSW the average daily wattage produced by an 80-watt panel would be only 320 watt hours (Wh), compared with the same panel's likely daily output of 500Wh in December.

"Contrary to most people's impression, 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 a lot more to consider than just the Ah rating," said Brett Sutherland.

Solar Online offers a complete service, including installation, or they can send packaged solar systems anywhere. The company's website - www.solaronline.com.au - has details of the process of solar system sizing, but for the electronically deprived we've quoted a typical example in this article.

SIZE DOES MATTER
A typical medium sized cruising boat may need to operate at the same time two 15W 12V DC fluoro lights, a 60W DC water pump, a 75W 12V DC fridge, a 50W 240V AC TV and a 600W 240V microwave oven. (Note that a 600W microwave will consume approximately 900W of power.)

The process of selecting the correct solar system begins with a load calculation: assuming two hours light operation (2 x 15W x 2 hrs = 60Wh/day), 15 minutes pump operation (1 x 60W x ¼ hr = 15Wh/day), four hours fridge operation (1 x 75W x 4 hrs = 300Wh/day), two hours TV use (1 x 50W x 2 hrs = 100Wh/day) and 15 minutes microwave use (1 x 900 x ¼ hr = 225Wh/day).

 The 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 - 382Wh.

The 12V DC and 240V AC loads total 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 5.5h. However Solar Online has found through experience that this calculation needs to be multiplied by a constant - the figure of 1.4 - which gives a total of 192W/day. If a winter calculation is required, sunlight hours should be reduced to 4.5 average.

The minimum panel input wattage required for this system is 192W, but it's wise to opt for increased capacity where possible. Three 80W panels would be ideal. (75W panels used to be available, but the new rating is 80W.)

REGULATING AND INVERTING
The charge coming from the solar panel to the batteries needs to be regulated, so that the batteries aren't overcharged. Solar Online recommends pulse-width modulated shunt controllers that charge the batteries optimally.

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

The rated short-circuit current of an 80W solar panel is 4.8 amps, so the total current capacity of our three-panel installation is 14.4A. Allowing for a possible 25 per cent current spike the regulator needs to have at least an 18A capacity, so the obvious choice is a 20A 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 the 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 cruising boat 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 boat without solar panel input for one day, but it's obviously best to have backup 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 boats, 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 AGM types 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 for at least five years.

RECHARGE ONLY
Most boaties use their vessels only at weekends, leaving five days between power demands during which a solar system can recharge the batteries. That means it's possible to use a much smaller solar panel if it's needed only to recharge the batteries during the week. For example, on our boat we have a small 40-watt panel that charges Monday to Friday, giving us fully-charged batteries for Saturdays.

However, we intend to upgrade to around 240W panel power and replace our engine-driven fridge with a 12V/240V unit that can run off mains power or battery power.