ouR escaPOD mods

Originally posted by GlueGuy The installation looks very nice Stephen; good job! I do have a couple of questions though.  The interior looks pretty roomy. What is the volume comparison between the old absorption fridge and this one? Do you have a delta on the energy cost difference?

Re cost difference, let's assume the actual heat being removed is the same with the two fridges. That's probably pretty close (assuming the absorption one is working), they very have similar size and insulation levels.

The coefficient of performance for the Danfoss compressor's specs have it around 1.5 to 2 (better at cooler ambient temps, worse at higher). So it's using around 0.5 to 0.7 watt hours of electricity to remove one watt hour of heat from the fridge.

It's a lot more difficult to get a COP for the absorption fridge but it's much worse, and drops off much faster at high ambient temps than does the compressor. From what I could find, a COP in the range of 0.5 to 0.75 is about right. That's consistent with the units using around 10-11 amps continuous (240-270 amp hours/day when in 12V mode, (and only marginally cooling in that mode) or about 3x-4x the energy of the compressor fridge. Let's call it roughly 1 kwh/day compressor vs 3 kwh/day absorption.

So then the question is cost of that energy. If youre running both on electric shore power at a campground there is zero cost difference to you. Advantage: neither.

If you're running both on a small gasoline generator (electricity from which typically costs about $0.75 to $1 per kwh) then the cost difference is about $1.50-$2.00 per day. Advantage: Compressor fridge.

But why would anyone do that? They would just run the absorption fridge on propane, which you can't to with the compressor fridge. Then the advantage shifts. That's because if you use a generator to run an absorption fridge you are burning fuel to create heat, converting that heat into electricity (only about 20% efficient at best in a small genny) and then turning that electricity back into heat.

That 5x difference in efficiency between burning the fuel directly in the absorption fridge and running it through the conversion to electric for the compressor fridge more than compensates for the inefficiency of the absorption fridge itself.

So if you are boondocking using a propane genny like I do, you would be better off with a propane fired absorption fridge than the Danfoss compressor one, all else being equal. Advantage: absorption fridge.

Finally, let's look at the case where you are boondocking running the compressor fridge on solar, vs running the absorption fridge on propane. Since you already bought enough solar, but are paying for the propane, the advantage shifts back to the compressor fridge in terms of incremental costs. iOW, you might as well use the solar since you already own it.

I'm gonna stop here, if you haven't bought the solar yet and want to calculate the payback time on that investment then that is another chapter.....

100% agree the compressor fridge will do a better job in hot weather, not only does it's COP stay higher, but it has much more capacity. It will just use more energy.

Not sure what size solar module you have but a 100 watt one should provide about 5-6 amps into a nominal 12V battery when oriented perpendicular to the sun's rays on a clear day. If that is an average spring or summer day they you'll get about 5 sunhours in the East or roughly 25-30 amphours. If it's an average fall day then 15-20 amphours. Either way. not enough to run a Danfoss fridge. I typically would use about 200 watts for a Danfoss vaccine fridge, and those were never expected to cool things down, the vaccines always went in ready cold.

So you can certainly add a second solar module but I'd be inclined to add additional battery to start with. In our part of the country we get lots of cloudy days where the solar module(s) won't produce squat and the genny would have to run anyway. Doubling your battery capacity would get you through 2-3 cloudy days boondocking in hot weather which sounds like it should be enough for the way you use your pod most of the time, and it would also allow another day or two for the weather to clear and your solar production to average out. One thing with off grid solar system design is to try to always balance how much battery capacity you have with how much solar production.

Since the Li batteries each has a dedicated BMS I don't think paralleling two different age Li batteries is anything near the problem it is with PbA.

Since each battery has a dedicated BMS which is keeping all the cell voltages within a tight range of each other, the only issue you could have with two parallel batteries would be if one has significantly lower internal resistance than the other and so gets more charge current and provides more discharge current than the other. If you place individual amp hour/energy monitors on each battery you would know if that is happening. There are ways of compensating for that in the unlikely event that if becomes an issue.

As with PbA batteries of course you should take the pos output from one and the neg from the other so they're balanced as well as possible.

Okay. Here are some stats on refrigerator power use: