ARB 40 Qt Fridge On Solar
On a recent 3 week trip we did a "hybrid" system for our ARB 40 refrigerator/freezer. When driving we powered the fridge from our Tacoma's alternator via the starting battery. When stopped we powered it from an auxiliary battery. The auxiliary battery was charged via a pair of 55 watt Siemens solar panels (about 7 total amps at 12-14 volts with good sun) mounted on top of our ARE shell. The system worked flawlessly and I suspect would have gone on indefinitely. Here's a summary of the system and the power data:
Background
We started the trip with the fridge full of frozen stuff and we set the fridge on freeze for 12 hr (120v connected to an outlet in our garage) so that everything was cold at the start and the system wasn't starting out by freezing stuff. Our daily cold, but not frozen, food and drinks were carried in a small igloo cooler (maybe 15 quart size) with 3 quarts of frozen water in plastic jugs.
Every morning before breaking camp we would move the partially thawed three quarts of water from the igloo to the ARB fridge and take 3 quarts of freshly frozen water out of the fridge and put them into the igloo. We did this whether or not the water in the igloo was thawed or not.
Whenever we purchased new food that we wanted to freeze or that was frozen we would place in the ARB. If we wanted to keep food cool we placed in the igloo. We generally kept the ARB set at 3 to 5, but in the morning after placing the partially thawed water containers in the fridge we'd set it on freeze if we were driving. We'd leave it on freeze until about 3 or 4 in the afternoon, or until we stopped driving.
Power consumption
We monitored current and voltage with a Xantrex Link 10 (
http://www.xantrex.com/web/id/237/p/1/pt/5/product.asp) battery monitoring system. Depending on how late we drove before setting up camp and how hot the night temperatures were, the ARB fridge consumed between 15 and 26 amp/hours (settings between 3 and 5). Our shortest "night" was probably about 8 hours and our longest "night"was probably about 16 hours. The coolest night was around 40 and the hottest probably around 85.
Power Production
The two solar panels could easily fully charge the auxiliary battery during the following day, sometimes by as early as 1 pm and sometimes it took through the afternoon. That depended on the cloud cover (we had a few days with nearly complete cloud cover, but we usually had several hours of strong sunlight); whether we were in forest or the open; and whether or not we parked in the shade to keep the truck cooler. In general we simply drove normally and didn't try to park in favorable solar conditions.
We almost always produced at least 1 amp unless it was before 8 am, after 5 pm, or actually raining. Usually we were producing between 4 and 6 amps, occasionally slightly over 7. 9:30 to 3:30 were the peak hours when we'd see the highest output. One problem with monitoring "real time" like this is that the charge controller we use for the solar panels, maintains voltage and adjusts current depending on the batteries state of charge. Thus when the batteries are getting "full" the controller reduces the current and so you rarely see the full output of your solar panels unless your battery is really drawn down. We never discharged the battery below 50% (it was a 55 ah Optima Yellowtop).
Balance - Could we have run the system completely solar?
Remember that we were using the solar panels to recharge amp hours used at night and we used the alternator while underway in the daytime. We wanted to test the system before trying a completely solar trip (which we'll do next). However, I think that we could run the system almost completely solar now that we've upgraded the battery to a 100 ah Lifeline. If the fridge uses 60 amp hours in a day and the solar panels can produce 45 amp hours in a day, then there's a 15 amp hour deficit per day. with a 100 amp hour battery drawing it down to 45 or 50 amp hours (approximately 50%) is a good target. That would give us about three, maybe four days completely solar before we'd need to run the fridge on the truck in the daytime and allow the solar panels full output to go into the battery for a day or so to bring it back up to a full charge. If you worked at reducing the amp hours required by the fridge (keeping it cooler, open it less, etc.) you could extend that time.
Cost
Solar is more expensive that a generator to purchase. We were lucky because Heidi traded some work for the two solar panels and we didn't have to purchase them. I would guess that our 110 watt set up with the charge controller would probably cost about $800. I prefer two panels totaling 110 watts to a single panel of 110 watts. While solar panels are tough they can break if hit by a rock or really large hail. With one panel a single break takes down the whole system. With two, you will decrease your output by 1/2 if one panel breaks but you'll still have some current produced.
We're really happy with the solar system. Once we get the auxiliary battery wired into the truck's charging system then we'll have the best of both worlds since the battery will charge when the truck runs - we almost always drive some within a three to four day period which will top the battery back up.
Howard L. Snell