Solar Power

With the project progressing steadily towards lock-up, I've spent a lot of time this week researching solar power with a view to engaging an electrician in the next few weeks to begin the electrical install.

As may be evidenced by the fact that I'm building most of this place myself, I'm not the kind to go shopping for a "package deal" solar installation - I prefer to understand the intricate details, and then design and build a system myself. I know I'll have to use an electrician to install it, but I prefer to do the thinking myself for free, rather than pay someone else to think about our design for us.

With that in mind, I've been looking at the various components we'll need for a completely independent off-grid setup. I can't remember whether I've mentioned it here before, but we're probably 1.5km at least from the nearest power pole through dense, steep forest, and so the cost of connecting mains power to our site is prohibitively expensive - and completely contrary to the spirit of our project, which is to build a completely (or as close to completely as possible) self-sustaining home.

To say this research has been bewildering is an understatement. When you go looking, you find available solar panels of a thousand different types, batteries of a thousand different varieties, inverters (grid-connect or otherwise), panel inverters, inverter/chargers, charge controllers (diversion or disconnect), dump loads, battery chargers and generators. There's AC coupling, DC coupling and grid connect.

What's worse, nearly all the literature you find in Australia is relevant only to grid connected solar installations, since these are by far the most common - evidently there are only about 12,000 off-grid solar installations in the country. So finding my way through this mess has left my head spinning just a little bit.

But to paraphrase the wise and ancient Yoda: found my way, I have.

In a stand-alone solar power system (SAPS, or remote-area power system, RAPS) you basically need solar panels (duh) to charge a big-ass battery, and an inverter to take the DC power generated by the panels and stored by the battery, into AC power for household loads. In the simplest (and ironically, most complex to design) case you need just that, and a charge controller which is a device whose basic role is to prevent the batteries from being over-charged. This is a DC-coupled system - the panels, which generate DC power, are connected (via the charge controller) directly to the battery, and the inverter takes that low voltage DC power and converts it to 240V AC (in Australia and most of the civilised world ;). I have a miniature version of this exact setup in our caravan: one panel, a dinky charge controller, a deep-cycle car battery and a 150W inverter. It works.

Where this simple architecture becomes complex however, is that in order to maximise the efficiency of the system you have to very accurately match the characteristics of the solar panels to those of the battery array, taking into account factors such as temperature (which affects the panel output), loss in the low-voltage / high-current wiring, and the expected decay in energy production (and storage) over the panels' and batteries' lifespan. With so very many different panels and batteries on the market, finding a perfect match at a great price is quite literally like finding a needle in a haystack.

Too hard.

This is where the professional solar installers make their money, I think - they've already invested time in this thinking, and pass that cost in $$ on to buyers.

There is an easier way, however - the AC-coupled system. In this design, you need panels, batteries (again, duh), but two inverters; one of which is a common (and therefore relatively cheap) grid-connect type, the other an inverter/charger. The grid-connect-type inverter is called a panel inverter in this architecture, because its sole role is to take the low-voltage DC power generated by the solar panels and turn it into 240V AC to directly power the household loads. Any excess power generated is then used to charge the batteries via the inverter/charger but if there's a deficit of solar power, the inverter/charger draws power from the batteries and supplies it to the household as 240V AC. Many inverter/chargers also have the capability to automatically start a backup generator as well, if the available battery power is too low.

The beauty of an AC-coupled system is that you don't have to match the solar panels to the batteries, because they're isolated from each other by the 240V circuit. You do need to select panels appropriate for your panel inverter (and likewise batteries to an inverter/charger) but these are far, far easier to match and therefore easier to find at a good price. An AC-coupled system is potentially slightly less efficient than its correctly matched DC equivalent, but when you take into account high-current DC transmission losses in long wiring runs, it probably evens out and you could make up for this inefficiency by simply adding another inexpensive solar panel.

So it's no surprise then which architecture I'm leaning towards :)

In my digging around online without actually calling anyone, I've found a couple of component suppliers who seem to have the right gear at the best price. What I've arrived at (so far) is:

1. 250W, 49V solar panels - $249 ea, via Low Energy Developments
2. Surrette 6V, 600Ah batteries - $703.92 ea, via Apollo Energy
3. SMA Sunny Boy 3kW panel inverter - $1609.00, via Low Energy Developments
4. SMA Sunny Island 48V, 5kW inverter/charger - $4477.00, via Low Energy Developments

There certainly seems to be a trend developing here in supplier :)

With our projected energy budget of 6.5kWh/day, I've arrived at a 48V system comprising two strings of 8x6V batteries (for a total of 1200Ah capacity), coupled to 2.5kW of solar capacity (10x250W panels) which should hopefully give us 7.5kWh/day of generation on average. If we can keep our energy use within our budget, this system should give us at least three days of autonomy with effectively zero solar generation, before we'd need to start the generator (based on depleting the batteries only to 50% charge, which keeps them healthy).

Based on these prices (which I may well be able to beat) we're looking at nearly $20,000 not including installation, wiring and ancillaries such as switchboards. Certainly better than the $30-$40k I've seen advertised for 6.5kWh/day systems...

Just don't get me started on the Australian Government's rebate schemes and carbon pricing (STCs)...