A Different approach to Waste not/ Want not

[tecnodave]

I am using my excess power in a rather unique way , I am remote and off grid , my refrigeration is adsorption cycle heat pump powered by generator / solar / propane. I use a large R.V. Refrigerator/Freezer which is triple power source made by Dometic . It has a logic board which will automatically switch power sources as available. It is 120 volt AC 250 watt , 12 volt DC 250 watt,  and propane, used in that order of priority. It has 12 volt DC input from battery plus a signal wire hooked to ignition of R.V. to detect engine running. I use a auxiliary logic board to detect when I have excess power from my solar array i.e. When batteries are near full charge and p.v. voltage is raising above MPP to provide the "ignition run signal" to the refrigerator causing it to switch from propane to solar. When sun sets PV voltage goes down switching off logic and frig reverts to propane, all automatically.  These units need 12 volts at all times no matter what the source of refrigeration power is to run the logic board and for the gas ignite assembly. Logic board power is 50ma. or so depending on model and igniter is 100 ma. 12 v.
Adsorption cycle refrigeration is not as efficient as compression refrigeration but in small scale solar with no inverter it is more cost effective.   These units are expensive at around $1900 new but they can be found in auto wrecking yards for $200  Look for the model numbers which have AES (Automatic Energy Selector) in the model number. I am using Dometic 2800AES, 3500AES and 763AES units in my cabins. The models that you have to manually switch between gas and electric do not have the logic board to do the switching but can be used as well, but you will be the logic board. Also note that the older ones switch between low flame and high flame to regulate refrigeration whereas the AES models extinguish and relight the flame to regulate temperature and are therefore more efficient In gas use.

//edit//. I failed to mention that units only use 12 volts while the engine is running so that the battries are not drained while camping. There is a voltage detect function in the logic board as well so that the refrig will not run down the batteries if the key is left on.   Simple run time meters on the gas valve and heater elements tell me that solar use time is 45-55% but the propane on time is less that heater on time for the same temperatures. I haven't calculated thermal gains solar vs propane but my gas use is significantly. less

[ClassicCrazy]

So it must just be a resistance heater coil . Does that draw 250 watts all the time even if the refrigerator is satisfied for cooling or does it cycle the heater on and off ?

[tecnodave]

It is just a resistance heating coil, two of them, one for 120 volts and one for 12 volts. Logic board powers relay for heaters  on and off to regulate cooling. Does not use freon gas, boils ammonia vapor from ammonia/ water solution which is then absorbed by hydrogen gas to provide cooling, high pressure @ about 500 psi, closed cycle, very common in R.V. refrigeration and also in off grid (propane , natural gas, or kerosene) Servel refrigerators, also used on ships powering refrigeration from engine waste heat. Known as Absorption or Adsorption cycle. I use it to save my scarce solar power in foggy season. I rigged my own auxiliary board to sense battery state, p.v. Output to run refrigerators own logic board. System was built before I had the Classic when I was using EP Solar Tracer controllers,  Classic 150 does not do this function although it could do that.

My system is 24 volts with 24 to 12 volt DC to DC converter ( high quality SAMLEX brand, very pure power) to power 12 volt LED lighting, TV, & radio, etc also refrigerators logic board and igniter and a second 24 to 12 volt DC - DC converter (Pyle brand, lower quality) to run heating elements, which is switched on and off by refrigerators logic board which is controlled by thermostat.

Run time on 12 volt DC approx 20 % total day more when hot out! run time! propane, < 10% of total on time, 5 gallon bottle good for 2 weeks, about 12 cubic foot refrigerator and freezer

Refrigerator does not have compressors so is totally silent.

This will not work on the more common Freon Gas compressor type refrigerators.


tecno