Solar project: general ideas

Before I mentioned my solar panel project, in general. The general requirements were simple:

• Supply additional heating to reduce electricity cost
• No grid-tie

If I were to go to simple solution, I'd go for grid-tie system, as electric grid already reaches the cottage. That doesn't really make any sense, however, since energy cost of electricity is around 4c/kWh now, and transfer cost (which essentially pays maintenance the electric grid) is around 6c/kWh (for total of ~10c/kWh).  As the grid-tie systems typically go, you get the energy cost you deliver reimbursed, but either way, the transfer cost is billed from you. In essence I'd be paying 2c/kWh for any excess energy I generate! Talk about insane proposition! Granted, there might be some lowered transfer pricing involved somewhere too, but I'm not fan of the idea either way.

• No batteries

No batteries clause is of course quite simple. Batteries large enough to contain sensible amounts of energy are huge and expensive and require additional investment later on, in form of replacements and maintenance. That would be pointless, so no.

• Monitor the temperature and grid supply in cottage

This is pretty handy for one's peace of mind when you consider that winters can be damn cold around here (-30 centigrade and below), and water and pipes tend to make really ugly mess when they freeze (or, really, when they thaw again). So knowing that temperature inside keeps above zero even during coldest nights is very nice. This would be in form of daily reports, with possibility of emergency reports thrown in if there is a problem - temperature inside dropping quickly, grid power going out for extended periods or something like that. So internet - or more to the point, cell network connectivity - is needed.

• Control connected (solar powered) heaters
• Monitor voltage and current of solar panels.

Now we're getting to interesting parts. First item here kinda requires the latter. If system turns on 800w heater and voltage drops, it's pretty clear the panels don't supply enough energy for that, so it's better to turn that one off and try some of the lower-powered ones instead. Current measurement isn't required here, but it allows system to calculate rudimentary power production figures. Just because I am engineer and such information, in form of tangible numbers is nice, no other reason.

• Supply mains-compatible AC power if requested

This is kinda extra alternative output for heater system. DC-AC inverters exists, so why not wire one up to supply extra power when heaters aren't needed but other power is, like for lights, laptop, fridge and so on during summers.

• Control grid-powered heaters

This is kinda an extra. The cabin will be empty for extended periods, and one can never know how cold it will be in the mean time - -10 or -40. So ability to turn off heaters when they are not needed due to warm temperature outside will be nice energy-saving bonus feature.

And, of course

• Do everything above safely

Whatever happens, this systems must not electrocute anyone, burn the cottage, cause electrical faults or anything else nasty like that. Everything else above is fairly simple stuff (but then again, I've been thinking about this quite a lot already so I have pretty good idea what the system will look like already), really, this is the line where design requirements get somewhat tricky.

So that is, in nutshell, my ideas of this project. More about the actual design later on when I start moving the ideas from my head onto the paper. Or CAD, actually.