The Inscrutable Thermostat works just great!
 

I switched over from my Teensy thermostat to the Chinese model I bought on ebay, and the inscrutable thermostat works just great!

Here is a log of operation:


The new thermostat works just fine. I have the hysteresis set to minimum (1) and the temp swings are as you see above.

I did a previous run, then adjusted the temperature set point up one degree, to 3 degrees C which would be 37.4 degrees F.

Previously, the lower swing got too close to freezing, which can be rough on produce, so now the adjustment is about right.

The temperature swings with the Teensy thermostat were smaller, and the compressor kicked on more frequently. I'll do a power calculation after a couple of days of running to see if there is any change in power use.


My goal was to see if an upright freezer, when converted to refrigerator duty, had any appreciable power savings over a comparable refrigerator. My tests indicate that the converted freezer will use roughly half the power of a comparably sized refrigerator. In this case, I used a rather unspectacular freezer candidate (NOT Energy Star), and it is performing much better than the same-sized Energy Star refrigerators. I would expect that if I had started with an Energy Star Freezer, the resulting Freezerator would have performed even better.

Doing this conversion with a cheap Chinese thermostat is really easy.

Now that I have some modest experience with Teensy/Arduino, making my own thermostat was really easy, too.

So, all in all, this has been a very successful project.

Best,

-AC_Hacker

Wrap-up
 

A final wrap-up on the Freezerator...

I did a calculation on the energy used over the last 24 hours and 58 minutes, and the daily power use, calculated for a 24 hour day, was 278.74 w-h/day. which sounds astoundingly good, but the weather has been on the chilly side, and since I don't heat my kitchen unless it is a byproduct of cooking, it has been cool in there, and as can be seen in the chart above, the regular temperature in the kitchen is in the low 60's.

I intentionally made sure that the Mulitlogger board, with it's own 1-wire temperature sensor was close to the top of the refrigerator so that the heat given off by the condenser would be clearly shown.

Again, it is clear that the work of the compressor is an inverse image of the temperature inside the Freezerator. Pretty good indication that if you have one of these refrigerators with internal condenser coils, you don't want to inhibit airflow to that surface. In the case of my Freezerator, the back, the sides and even the top is a heat-dump surface.

The discontinuity at the 50 minute mark was me getting food for lunch.

All is working well.

-AC

Very nice work AC! Thanks for sharing.

What was the roadblock to just using the freezer thermostat?
Maybe with some tweaking of the range...

That would be a good idea... if you can get to the range you want.

As an after-note, I am finding my particular thermostat to be problematic, as it only adjusts in whole degrees C.

I'm going to return to the Arduino thermostat... much tighter control, both of temperature and hysteresis.

It turns out that 'just right' and 'too much' are very close together.

-AC

How do you have it setup to run with the Arduino?

Not bad! My 15 year old Maytag full size freezer-on-top fridge takes around 1500 watt-hours per day.

The first post in this thread illustrated the controller.

I'm using the word Arduino interchangeably with the word Teensy. They are the same thing, with the exception of the reference number of some of the pins. The body of the code just the same. In the head of the code, the pins are defined, and you just reassign the pin numbers to the appropriate Arduino pin numbers.

I can get 'imperfect' Teensies for $10 each, plus I personally know the guy that designed and makes them. That's why the Teensy.

But, I like 1-wire temperature sensors way better that analog sensors, so in 'rev 2' I will be using 1-wire. This will require that I include a 1-wire library, the rest should be very similar.

Also, now that I am aware of how critical temperature adjustment is, I'll make sure that my temperature-relevant variables are floating point and not integer.

Best,

-AC

I mean what is your temperature setting and allowable range? A quick look at your code suggests 38 +/- 2 degrees F. But your latest posts seems to suggest you want the temperature held more steady.

OK, just to make sure that we're on the same page, I ordered a Chinese thermostat, and while I was waiting for it to arrive, I hacked together a Teensy (Arduino) thermostat that used an analog temperature sensor. Aside from the fact that it didn't have a temperature display, it worked pretty well.

Then when to Chinese version arrived I started using it (and as of this writing, still am). That's when I noticed the too-large swings and too-large steps in targeting the set-point temperature.

The reason this is important is that it looks possible to me that this setup may be capable of keeping food acceptably cool while avoiding most, if not all problems associated with frost buildup. This is because the chill area is larger than in a normal refrigerator, and the temps don't have to go as low in the evaporator coil to provide the cooling. It's the same thing as low temperature heating, only this is high temperature cooling. Low exergy, etc.

So, the behavior of the Freezerator seems to be such that if I can get right at the 'sweet spot' it should work better.

I have found that there is an optimum hysteresis swing amplitude, and that if the hysteresis is too tight, the compressor cycles too often, and that uses up more energy... if it's too loose, the system bounces between food-too-warm and frost formation on the coils.

Another issue connected to frost buildup is that making frost requires the refrigerator to supply additional energy that would not be required if I can avoid too-cold temperatures or too-low hysteresis swings.

A third issue with frost buildup is that it has a de-hydrating effect on produce, and shortens its storage life. Since I am becoming a vegetarian, this is a big deal. I have noticed that in general, the humidity inside the Freezerator is unusually high for a refrigerator, and my produce like it that way.

Another issue with the Freezerator Rev 1 is that my previous analog sensor was exhibiting irregularity that may have been due to humidity... I don't know for sure, but I do know that keeping the sensor dry is a good thing.

So in the next revision, I want to use a 1-wire sensor that is properly protected from humidity. And I want to make sure that I have precise control over temperature and hysteresis. And a display, and a proper interface that will give me the ability to change the set-point and hysteresis by using buttons rather than needing to hook up my PC to my refrigerator.

Hope this addressed your question.

-AC