Switching to a compressor type Refer…

Bunkysdad

My truck camper is now up to 560w mppt solar and with two 200a/hr LFP batteries…Considering swapping out our 6 c/f Norcold refer with a 8.1 c/f Danfoss-Secop compressor-type refer rated nominally at 5.23 amps (so say, 125a/hrs over a 24 hr period)…Harvest under ‘ideal conditions’ might even be around 34-35 amps, but (per BattleBorn estimates) about a 3.75 amp hourly average over an 8hr harvest period, per each 100w panel, thus 168 net averaged recovered a/hrs…

Question here is, just wondering what experiences others might have had when using a compressor type refer??

Thanks, Bunkysdad 

Bunkysdad

Oops, (oddly, I don’t see an edit button?..), Imeant 240 net averaged amps instead of the aforementioned 168…Hope I got this right…

Bunkysdad

Sorry, Upon re-re-re-review, 168 net averaged a/hrs…sorry for me dummy  

BB.

You should see a "black gear" button to top right of post--You can edit clicking here".

What you really want to know is how many Watt*Hours or Amp*Hour (@ 12 or 24 volts) uses per day.

https://norcold.com/product/norcold-nrf-60-portable-refrigerator/ (???)

For example, say the fridge (compressor type) uses 5.3 Amp*Hours and guess that it has a 50% duty cycle in "normal" operation:

• 5.3 AH * 24 hours per day * 0.50 duty cycle = 63.6 AH per day (guess)
• 63.6 AH per day * 12 volts = 763 WH per day

For a small DC refrigerator, 763 WH per day (guess) is a fair amount of energy usage... 500-250 Watt*Hour per day would be "nicer" (could not find ratings with quick search--A full size home Energy Star Fridge can be found around 1,000-1,200 WH per day... A converted freezer to fridge can get down towards 250 WH per day).

For your setup, guessing Carson City Nevada, fixed array, mounted flat to roof:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Carson City
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.29	3.15	4.61	5.85	7.01	7.77
Jul	Aug	Sep	Oct	Nov	Dec
7.76	6.89	5.60	4.06	2.60	2.11

Say use camper Feb-October:

• 560 Watt array * 0.77 off grid DC Li Ion system * 3.15 Hours of sun per day = 1,358 WH per day (February "break even")
• 1,358 WH per day harvest / 12 volt battery bank = 113 AH per day

Lots of guesses here... But having a 24x7 base load that must run (fridge), using 50-65% of predicted harvest is "comfortable" (don't need to run a genset every few days to keep up with occasional cloudy sky).

• 1358 WH per day * 0.65 "base load fudge factor" = 882.7 WH per day for "base loads"

Using a Kill-a-Watt type meter for AC loads, and/or a DC AH/WH meter for DC loads. Measure the energy usage to be sure.

https://www.amazon.com/s?k=dc+digital+amp+volt+energy+meter&crid=25P8P1GNKJVVX

Also, ventilation around the fridge is important... Above 85F ambient, many refrigerator/freezers tend to "have issues". And they also take more energy to cool.

Your thoughts?

-Bill

papab

I think Bill's 50% duty cycle is reasonable, or a bit conservative.   For comparison, I have a 4.4 CF Novakool.    I turned the compressor down to the lowest speed & it takes about 2.2 amps, & very roughly 50% DC. From end of charging, to start of charging I might be down 20-30 ah.  This time of year that's about 16 hours.  The heater probly takes about 5 - 10 ah. 

In hot weather 90F+, even with the compressor speed turned down, it keeps up, but duty cycle is probly approaching 100%

I have no trouble keeping up with this with 350W solar.

I'm going to be in Moab in mid March, I should keep closer track of consumption.

Bunkysdad

Wow, Sorry for my delayed response (sidetracked by projects and life - lol) but this is some terrific data, especially the calc methodology, as well as the valuable ‘real world’ testament !! Awesome!

Update, I’ve since added another 100w panel (for a new total of 660w), but have not yet quite decided on the refer swap because of a unanticipated longer pay-back period due now to costlier fuel and likely fewer camping excursions, but nevertheless still actively considering!!

Many Thanks to All!!

Bunkysdad

Hey Bill, upon a re-read, just curious what is meant by 0.77 dc lithium? - is this a kinda presumed LiFePO4 efficiency??

please clarify…

Thanks

BB.

Bunkysdad,

I am assuming the Li Ion batteries are pretty much 100% efficient. The 0.77 is the derating of the solar panels and charge controller efficiency.

• 0.81 panel derating (hot panels, Vmp falls and MPPT output drops or using PWM controller) * 0.95 charge controller efficiency = 0.77 overall efficiency

More or less, the "marketing" numbers for Vmp and Pmp are when the solar cells are at ~25C/77F and tested under a flash simulated sun (to keep cells "cool" and near room temperature).


On a hot summer day under full sun, the cells can be upwards 40C over ambient.

MPPT (maximum power point tracking) charge controllers basically follow the equation:

• Power to Battery = Vmp * Imp * 0.95 controller efficiency

PWM (pulse width modulated) charge controllers basically take just the Imp of the solar panel and passes it to the battery. Example of a 100 Watt solar panel (to keep numbers simple):

https://www.solar-electric.com/solarland-slp100-12u-100-watt-12-volt-solar-panel.html
Maximum power (Pmax): 100 WattsNominal voltage: 12 Volts DCVoltage at Pmax (Vmp): 17.2 Volts DCCurrent at Pmax (Imp): 5.81 Amps DCOpen-circuit voltage (Voc): 21.6 Volts DCShort-circuit current (Isc): 6.46 Amps DCPower tolerance: +/- 5%
As long as Vmp is over Vbatt--Then Imp (current from solar panel) is passed to battery:

• 5.81 Amps Imp * 14.5 volts charging = 84 Watts
• 84 Watts actual / 100 Watts Pmp = 0.84 "derating" for panels
• 0.84 panel derating (on PWM) * 0.95 Controller eff = 0.80 over all system eff

So, ignoring Imp (which rises a small amount at higher cell temperatures) and overall just to keep things "simple", I use 0.77 as the overall panel+controller deratings for both MPPT and PWM type controllers (close enough for solar).

-Bill

Bunkysdad

Many Thanks Bill for the very detailed and revealing answer!!…Upon the first cut, it’ll likely take me a bit to fully wrap my mind around it, but you’ve shared some truly rational and invaluable information Sir, I appreciate your fine tutelage !!

Bunkysdad

BB.

You are very welcome Bunkysdad,

Feel free to ask more questions as they come up.

Take care,
-Bill