Originally Posted by Half Ton Heavy
Bottom line, plan on at least 80 ah a day, maybe 100ah for hot ones and higher use. Better have 300 ah of lithium on board to get through a couple of crappy days unless you want to be a slave to your generator.
I too agree. But, in true engineer-with-nothing-better-to-do fashion, I have a heavily modified Nova Kool 9.1 cu ft unit with bottom freezer and the compressor/condenser below the freezer (uses indoor air to cool the compressor and condenser coils).
I didn't read the FAQs on this fridge before buying and only later learned that the freezer is designed to run only 20F cooler than the fridge. That's a theoretical 18F in the freezer with the fridge running at 38F.
I didn't see 18F on a good day. In fact I rarely saw 20F inside the box formed by the freezer evaporator plate (to the left) and was seeing 27-28F opposite that box (to the right).
Part of this result was my own fault. The fridge comes with about 2" of insulation, presumably about R13 or a bit more. I added 2" of Rmax R13.1 on the sides and back and 6" on top with the idea of reducing energy usage. It certainly did that, but the insulation I added benefitted the fridge more than the freezer in both coverage and relative heat flow (higher for the freezer) so probably compounded the freezer temp problem. Ideally I would have added R26 to the freezer but there was no space to do that anyway.
The refrigerant flows through the evaporator plate in the freezer then up to the evaporator plate in the fridge and the single thermostat is in the fridge. I.e., the freezer is a slave to the fridge and gets cooling only to the extent the fridge calls for cooling.
So, I had a fridge that was freezing lettuce at the bottom when I set the fridge temperature lower to help the freezer and still a not cold enough freezer. The frozen raw meat catfood we use needs to be colder.
So the experiments started.
1) Ran the fridge as low as I could without freezing lettuce at the bottom (about 36F).
2) Added an 80mm box fan running at half speed in the freezer to circulate air thus extracting more "cold" from the freezer evap plate and making the freezer temp more uniform. This helped a lot in that it dropped the evap box by 2F and the right side by 6-7F.
3) Added some insulation around the fridge evap plate so the fridge was getting less cooling and requiring the compressor to run longer. This helped a little but made it difficult to find the right setting for the fridge thermostat. And seemed to require a different setting for a different ambient temperature. BTW, the thermostat controls fridge evap plate temperature, not fridge air temperature.
4) Added 2" of Rmax to the bottom of the fridge. This slowed heat flow from the fridge to the freezer below. The big benefit was much more uniform fridge temperature (about 1F difference top to bottom instead of 2 or 3F and freezing lettuce). But it also let me set the fridge at 34F without freezing lettuce at the bottom. The lower fridge temp and lower heat flow into the freezer picked up a couple of degrees in the freezer.
5) Added 2" of Rmax to the bottom of the freezer (inside). This reduces heat flow into the bottom of the freezer from the compessor and condenser coils that are under it. There was no room for insulation under the freezer (between it and the coils). This picked up a couple of degrees in the freezer.
6) Swapped in a quieter and more capable compressor/condenser fan. Quieter but no help temperature-wise and maybe hurt since there's more convection going on under the freezer floor.
7) Added 2" of Rmax to the inside of the freezer door. This was wasted space in any event and the insulation probably halved the heat flow through the freezer door. This picked up a couple of degrees.
8) Added 2" of Rmax in the side walls of the cupboards to the right and left of the freezer. This took those areas from 4" to 6" of insulation. probably helped a little, but the effect was not obvious.
9) Added some fixed air vanes in the compressor/condenser compartment to reduce the bit of re-circulation of warmed exhaust air that was occurring. Probably helped efficiency a little.
10) This one has not yet been implemented. I'm going to reverse the air flow of the compressor/condenser fan so that air flowing over the hot compressor does not flow inward and across the condenser coils before being exhausted by the fan. I'll also add fixed air vanes so most of the air will flow across the compressor as it leaves the compressor/condenser cavity and be deflected away from the air inlet (the condenser is near the front). This will reduce the convection heating of the freezer floor and improve efficiency a bit.
11) With these improvements, I do not need to run the Danfoss/Secor variable speed compressor at full speed, even at 100F ambient (the max design temperature). The compressor is up to 15% more efficient at its minimum speed. I hoped more running time at the lower speed would help in spite of the refrigerant being less cold. No freezer temperature benefit that I could discern. And the compressor is not audibly quieter at the lower speed, so no benefit there. I now usually leave the compressor at 3500 rpm.
Long story short
..... the freezer now stays under 15F to the left and under 16-17 to the right most of the time. After extra run time from door opening at meal time the freezer air may get down to 10F briefly. The freezer is probably cold enough for the cat food if it's kept only a month or less (we put it in the box formed by the evaporator plate).
But there's still one problem!!!!!!!!!!! When the ambient is low, say in the 40's at night, the fridge runs very little and the freezer warms up. The freezer contents can be in the low 20's by morning. With all the added insulation, this is probably better than it would otherwise be, but not great.
Any suggestions would be appreciated. I'd like the freezer to be down around 10F.
Nova Kool makes a dual compressor model. I didn't buy it because of cost and Nova Kool rates it at a higher current. I think their current rating is pessimistic, at least if the fridge and freezer temperatures are set the same as for the single compressor model. However, I'm betting that most users, including me, would set the freezer to a lower temperature and thus use more energy. I could live with that, solar isn't that costly (much of the fridge energy use occurs during the hot hours when there is plenty of sun). I'm currently trying to get my son to take the single compressor unit so I can order a dual compressor model.
As for the battery and solar requirements. I'd bet that Half Ton Heavy's estimates are right on for the typical installation. The manufacturer states that duty cycle is 40-50% at 70F ambient. I'd guess 50% is most realistic. With the BD35F compressor running at full speed (5.2 amps) that would be about 63 Ah over 24 hours. Assuming the compressor duty cycle hits 100% at 100F in a typical installation, the day time half of that Ah use could double to 63 Ah. Ergo about 100 Ah on a day that is 70F at night and 100F during the day.
With all of my added insulation, I'm well below those numbers, not half but well less than 2/3. But, I don't recommend the route I went. More solar and battery would be a lot easier.
And a dual compressor model is essential if you are fussy about freezer temperature. At least this is the case with the Nova Kool RFU9000.