IAN LANG ELECTRONICS
A direct DC load powered by a solar panel will of course only work during the daytime, or, if the power demands are small, when a source of artificial light shines on it. The latter case is obviously not true of a domestic or industrial installation but will work for small devices such as calculators and portable radios.
In hot sunny countries, when the Sun shines it has the effect of well, getting hot. A solar installation in a house or small workshop can in fact drive a fan or fans to cool the environs down, and has the advantage of starting when the Sun's out and stopping when it's not automatically. A sufficiently large installation can store energy in batteries so that at night, if it gets cold, a heating system can be powered and /or you can provide hot running water.
Most Devices that are directly driven are small and low voltage and stand alone. Here's a garden feature powered directly by energy from a solar panel which turns a low-voltage water pump. It's sold by ukwaterfeatures.com and a link to their website is provided on the left.
Not that you can see the panel here as it's hiding behind the pot but what it's doing is providing the juice for a pump at the bottom which is sending the water up to the jug at the top, which, when sufficiently filled, tips into the bowl and which in turn tips to the next bowl down and so on until the water ends up back at the bottom again.
If it were driven directly by DC provided by the solar panels only, in the UK this would not work often in the Spring and Autumn, nor on a Summer's day that was cloudy as the panel couldn't provide much juice. My wife was bitterly disappointed by this fact in a similar one bought from B&Q, and turned it into an enormous plant pot instead. To combat this, some have battery storage capacity, the panel drives the pump and charges the battery unless there's not sufficient light to drive the pump in which case the battery takes over. Added complications do mean however added expense.
And that sentence brings me neatly onto the next topic about solar power, charging a battery from it, which is what you'll find in domestic and industrial illustrations. Referring back to a portion of our diagram:
The panels charge the battery, that's obvious, but there's something between- the regulator. In America it's called the charge controller, and for once they've managed to get a much better name for things than the British because that's exactly what it is and does. And if you don't believe me about American nouns consider the water faucet. What's a faucet? C'mon, Hank, it's a tap. It taps off the water supply. Tap. T A P.
And whilst we are at it it's aluminium, not aluminum. And stop spelling harbour without a U please. And colour.
Where were we? Oh yes the regulator. Here's a picture of one:
This regulator is sold by Sunshine Solar and the link below takes you to their website:
Just to confuse the issue the manufacturers have called it a solar controller. It's actually quite fancy, this one. As you can see along the bottom there in the middle there's terminals for the positive and negative downleads from your panel(s) and the output for not one but two batteries to be recharged. There's terminals to the left for a remote sensor for temperature which the regulator takes into account when outputting, and readings for the priority of charging, type of battery and frequency of the charging pulse. Nice.
What's it do though? Well, it takes the output from your panels, turns it into a 13.8V supply and feeds power to the battery or batteries. Your panels will put out more than 13.8V under optimal conditions and you don't want the lot tumbling straight to your battery or it will blow up. Moreover when the battery is fully charged you don't want to be attempting to charge it any more. Imagine a cess-pit that never gets emptied and then one day you do the critical flush after an evening spent in the local curry house. It's just as messy with the added complication that there's acid in the batteries. Cess pits are unpleasant when they explode or burst. Batteries are life-threatening. The regulator stops the charge when the battery is full.
On to the battery then. The technology behind this is known as deep-cycle, and thus the batteries are known as deep-cycle batteries. Don't use any other kind with a solar system. The technology behind them is in principle exactly the same as that of a car battery, but whereas a car battery is designed to give you a large discharge to get the motor running, and then tick over quietly, a deep-cycle battery is designed to discharge lesser quantities but more continously. Think of them as the sprinter and the marathon runner if it helps. The main mechanical difference is that deep-cycle batteries have much thicker plates on the inside than do car batteries.
What kind of battery do we need for a solar installation? There are three main types, wet cell, AGM and GEL. They are all kinds of SLA (sealed lead acid) batteries. Note that LEAD and ACID bit. Don't go poking around inside them.
You may well never see wet cell batteries unless you're a technician certified to work on them. Even if they are used in industrial applications, they'll be kept apart, locked up and access to them will be restricted to certain individuals. This is because in recharging they can produce fumes which are toxic and in sufficient build-ups highly explosive. Moreover, they degrade easily and require a lot of maintenance. They can be made in very big banks though, relatively cheaply, for very high voltage applications, in which case they weigh tons. If you drive or have access to an electric fork-lift, have a look but don't touch. That's powered by a bank of wet cells, each one providing probably 2V or less. Note how big the bank is. And they only work to 36 or 72 Volts. But they need a lot of current, and that's why the battery is so large.
For use in houses, boats, caravans etc a popular choice is the AGM, which stands for Absorbed Glass Mat.
I defy even the person the clumsy people describe as "really clumsy" to break an AGM battery. They do not give out fumes, you can't spill the acid, they don't need maintenance or watering and no equalisation settings are required. The self-discharge rate is low, so that even if they aren't drawn on for a while they remain fresh, they're resistant to vibration and can be stood up either way.
The second choice is GEL, also known as MVG, and I for one have no idea what that stands for but since they're the most popular choice by far with nautical types, there's probably the word "marine" in there somewhere. Sailors say things that they know other people won't understand just to tick us off. It's not nine o'clock on a ship, oh no, it's "eight bells in the forenoon watch" or something and they don't have left and right, just port and starboard. You can't go to the back end of the boat, you go to the stern, and the pointy bit at the front is either the bow, prow or fo'c'sle just to confuse you even more. Shiver me timbers indeed.
Anyway, GEL batteries do the same job as AGM and seem to be a little cheaper, but the drawbacks are that you've got to set the charging current correctly and you can't fast charge them without ruining the battery.
All deep-cycle batteries can be fully discharged without harming them, which is not true of all SLA batteries.
Batteries are rated in Amp-hours, and for a home system, a good rule is to have at least 3 times as much capacity as you need on an average day. Some people say five, and that's true if you live in a remote area and are using the solar system as a backup for power outages from the National Grid.
How long does the battery last on a discharge? It depends on the next component in our system, the invertor (or inverter if you're American. It's the aluminum thing again........)
Here's an embuggerance supreme. We've got a battery that puts out a nominal 12V, although if it's newly charged it might be 13.8. But the toaster wants 220-230 to work. No problem. Use a step up transformer, yes? Oh, hang on, what's the principle behind a transformer? As the wave reaches it's peak, it falls again, and the changing waveform induces an EMF in the other coil.
Doesn't the battery output DC? Yes it does.
DC is a flat waveform isn't it? Yes it is.
So it doesn't change? No it doesn't.
So if the waveform doesn't change it can't rise and fall, it can't do a peak? No it can't.
So if that's the case, no EMF gets induced in the other coil, surely? That's correct.
Never fear, though, because we know a way round that. What we do is to make that DC an alternating waveform by the use of something called an oscillator. We can in fact capacitively couple some power transistors together and by having them switch each other on and off we produce not a true sine wave but one that looks a bit like a sawtooth, but which is good enough to fool the transformer into thinking it's got AC. Huzzah.
Now, you need to calculate the power you need because invertors come in a range of power outputs and for the UK they output 220V. You calculate the power by looking at the Watt ratings of your appliance. Over the page, we look at how to do this and what it means for your battery.