Of fridges, solar power and getting the most out of them

Yesterday’s tour of Sydney’s Sustainable House was packed both with people and the interesting stories and questions they asked.  One email has come in from one of people on the tour, Peter, and here it is as it’s got useful information about using solar panels efficiently and meeting the energy demands of that gas guzzler, the fridge:

Hi Michael
It’s Peter, and together with son Simon and two friends, was in your clutches for the tour yesterday Saturday 6th.
Once again many thanks for putting the time and effort into sharing what you’ve done. Very impressive.
The doing of it is admirable but the sharing bit is really special, some of the seeds you are planting with each little tour will surely grow…
We spoke briefly about the solar side and it seems you haven’t changed a lot from day 1, that’s not a criticism, if it ain’t broke don’t fix it etc., but it did sound as if you were on the verge of updating and including battery storage, so for what it is worth and from a total amateur…
I’ve been living in a little bus, urban-camping around inner and Eastern Sydney and rely heavily on solar panels to run a fridge, induction cooktop, laptops, lights and vitally a cappucino machine. I did a bit of research when migrating from an older vehicle with 12 volt panels charging a 24 volt battery bank via a first generation controller. The system in the newer vehicle is way better, the solar panels are marginally more efficient but the major change is inserting an MPPT (Maximum Power Point Tracking, whatever that means) controller between the solar panels and the batteries.
MPPT is the generic name, there are heaps of manufacturers, they are all over eBay and prices vary according to capacity – the maximum number of amps a unit will handle. The thing is typically about the size of a brick, mine cost less than $200 and has been running trouble-free 24/7 for three years. No heat or noise and it’s designed for mobile use, bounces around on the road and so is very rugged. 
The MPPT should sit as close to the panels as possible to reduce voltage drop, but contains (I imagine) lots of electronics so needs to be indoors or heavily weatherproofed. Ditto the batteries should be as close as possible to the MPPT to reduce voltage drop. In a terrace like yours I would imagine the MPPT, the battery bank and the inverter to convert battery to mains voltage would all be happiest close together in the roof space beneath the panels.
To state the obvious, the inverter I refer to here is a totally different box to your current inverter which feeds your excess power to the grid, which I imagine needs to be exactly where it currently is, next to your street power box.  
The MPPT thing beats first generation controllers in two respects – firstly, it manages the power delivered by the panels more efficiently. Heresy to you I know, but in no way have I measured this scientifically. I am however totally confident that my current set-up, despite having less-efficiently located panels on the roof of the bus, starts to register power to the batteries much sooner in the day and maintains a flow until later in the day than the previous set-up.
Secondly, the MPPT magically sorts out any differences between the panel and battery voltages automatically. This makes it very easy to wire the panels in series so, if I wanted to, I could connect say three 12 volt panels together to produce 36 volts into the MPPT and add or subtract panels at will. Likewise, the batteries downstream of the MPPT can be configured as 12, 24, 36 volt, whatever.
There may be limitations to the range of combinations, I didn’t explore much beyond what I needed, which was two 24 volt panels on the roof wired in series to produce 48 volts down a single pair of cables to the MPPT and thence to two 12 volt 200 amp hour deep cycle batteries in the vehicle, connected in series to feed a 24 volt inverter which produces 240 volt mains power.
My fridge as it happens is a very efficient 24 volt drawer (not door, so cold air does not ‘fall out’ every time the thing is opened*) item that has been running 24/7 for the last three years, directly from the batteries. This eliminates any (albeit small) efficiency loss that would be incurred if I ran a domestic 240 volt fridge through the inverter.  
Connecting the solar panels ‘in series’ (in my case to produce 48 volts out of two 24 volt panels) is important, because as the voltage increases, smaller gauge copper cables can be used – or better - thicker cable can be retained to offer less voltage drop. Ditto between the MPPT and the batteries and between the batteries and inverter. Thick, short cables are the go.
Downstream from the inverter, 240 volt mains power is of course happy to travel through standard skinny mains copper cable with minimal voltage drop, and in my bus, to circumvent any need to have mains wiring certified, I simply run extension cables from the (insulated) inverter to wherever I need power.
There you go. I’m sure you are all over most of this stuff anyway, but I hope some of it may prove useful to you or a blogee.
Please feel free to publish/edit any of the above if you see fit but of course there is no such thing as a free lunch so in return, please keep me posted if you do anything interesting, especially on the battery side – as you pointed out in the tour, solar gear prices are on a downward trajectory, but lead/acid batteries are and always will be expensive and heavy. In my case the batteries amount to about 50% of the cost of the whole system and about 90% of the weight.
Finally, I don’t have the details to hand right now, but if you or any of your flock are interested, I’ll check if the people who supplied me with my gear and advice are still alive and in business. They operated out of Melbourne, have a real shop and could arrange for stuff to be shipped ex-Sydney to Brisbane at the best price I could find at the time. And importantly, their after-sales advice was every bit as good as their pre-sales spiel.
* as an aside, all fridges should be designed like this but very few are. Mine is a 24 volt Vitrifrigo, Italian, dear as poison, hard to find and possibly discontinued now. The only other alternative 24 volt drawer fridge/freezer I could find was manufactured by the sun-roof people Webasto for the marine market at an eye-watering price. 240 volt domestic drawer fridges are equally hard to find, but I did buy one a few years ago for a house I have in Qld, think it’s a Mitsubishi. You may consider that when you get around to replacing yours…. 
Peter, that’s a terrific lot of helpful information; thanks for your time and generosity.
2 Responses to “Of fridges, solar power and getting the most out of them”
  1. With the rise in cost of electricity it has become an important factor to find an alternate for it like using the renewable sources such as solar energy. Solar energy uses sunlight to generate electricity it is 100% renewable.

  2. Moz says:

    Drawer fridges are fairly avilable now, a quick online search showed up a few options. Mitsubishi do a range of two-drawer fridge-freers with a top door, if you’re willing to compromise that way. And quite a few of the bigger firdges come with a freezer drawer option. Unfortunately performance seems to be poor (2 stars) for the most part.

    I’m glad to hear there are other options than the SunFrost fridges, as we’re currently suffering with a second hand power-sucker that I’d like to upgrade but haven’t been able to find anything that seems much better for under ~$2000 for a 300-400 litre fridge/freezer.

    Also, Muchael, have you looked at ready-to-recycle traction batteries instead of new designed-for-solar? The idea is you buy a forkilift battery that has degraded to the point where it won’t power a forklift for a full day, so you can buy a cheap battery that’s down to, say, 20kWh of a rated 50kWh capacity. Then when it really dies you sell it back to the recycler and generally the cost is delivery. I did that ~25 years ago in NZ, and when we buy a house and put PV on the roof and have enough cash I’ll look at doing it again. It was a very cheap way to get batteries when I did it before, but obviously not everyone can do it…

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  • Michael Mobbs

    Michael is a former Environmental Lawyer who is uniquely placed to consult in four main areas:

    • Sustainability Coach and Speaker,
    • Sustainable Urban Farm Design greening, watering and cooling the cityscape, roads, parks, suburbs,
    • Major Projects Consultant Commercial and Industrial,
    • Residential Sustainability Consultant.
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