Experiment: holding a temp with peltier cooler

In another thread the participants were thinking about how useful peltier refrigeration might be in a 'dweller context.  We know peltier devices are thirsty for power, that's a given.  There were a couple of related ideas that we might be able to test:

Do peltier coolers have to be run constantly, or do they use less power when switched on/off at a given temperature?

I could not test in ambient sub-70F temperatures, because my house is old and drafty.  I tested at ambient 78F and held a few deltas below ambient to find the duty cycle at each delta.  I suspect this is a decent stand-in for the original question but I lack the thermodynamic background to know.  Comments requested.

== The method  ==

hold at N degrees Fahrenheit below ambient.  Note whether it can hold the temp at that delta.  Note the time the devices is running vs the time it is off;  this ratio is the duty cycle.   Adjust setpoint by ~5F and repeat.

Additionally, allow the cooler to run to max to see what the delta is when the duty cycle is 100% (ie, cooler maxxed out).


== The hardware  ==

• empty Igloo 3392 32qt peltier cooler run off 110v for this experiment.  The 110vac adapter actually converts to 12vdc so results should be similar on van 12v.
• lerway stc-1000 110v digital temp controller.  Celsius only.  Available in 12v also. 
• kill-a-watt meter to watch power requirements. 

== running the experiement ==

Here is a sample run holding at 17.2C, which is 15F below ambient. Time lapse at one frame per minute. 

1. From 0-14 seconds or so the cooler is rising from a previous test setpoint (15C) to rise.  
2. At 17.5C (setpoint + 0.3C) the controller turns on the cooler and it runs until 17.2C is reached and it turns off again.  Repeat.

I apologize in advance for the potatocam quality.  My modern android phablet phones wouldn't fit into tripod mount so I used an ancient smaller phone.

[video=youtube]

The controller itself uses ~0.75W when idle, and ~1.5W when a relay is engaged (either cooling or heating).   In this case only the cooling side relay has a load.  You can see the 1.5W relay draw active at the beginning while the cooler is warming up.  At that point the controller is sensing a temperature colder than 0.3C from the setpoint and so it closes the heating relay.  It doesn't know there is no heating load.


== observations ==




My peltier cooler may be dying.  The maximum delta it could hold was 19.36F below ambient.  When last used about 4 years ago it cooled 30-40F below ambient and held my groceries without spoiling until I got a refrigerator.  I don't think empty v. full could make that kind of difference but I didn't test that.  I did try increasing ventilation on the exterior hot side of the peltier unit but it made no measurable difference. 

The duty cycle vs. 5F delta steps charted in a predictable, linear fashion until we approached maximum delta.  In the last 0.36F duty cycle shot up from 75% to 100%.  This is not obvious on the chart because the temps on the bottom are spaced evenly. The two points on the left should be very close together; this would show the sharp rise correctly. Couldn't figure out how to make OpenOffice map them realistically.

Power consumption while running is fairly stable.  Upon starting each cycle the controller + cooler pulls ~68-69W and it drops to 67-68W before shutting off.

The empty cooler temp rose 0.3 to 0.5C after the cooler was powered on before the cooler was able to reverse the trend.


== conclusions ==

1. A peltier cooler does not need to be run constantly unless the desired temp is at the edges of the cooler's performance.
2. Running intermittenly to hold a given (non-maximum) setpoint can save a considerable amount of power (over 50% when ambient is close to internal temps)
3. a user may want to config the setpoint to be ~0.5c below the desired temp to allow for thermal inertia
4. frater secessus is a geek and his alleged girlfriend is either exceptionally tolerant or, more likely, inflatable.

so at 12 volts you would be pulling about 7A and not even to be able to hold a safe temp for food storage. I would say it's an improvement but not being able to hold a safe temp for food storage, it's still a poor choice. I vote for number 4. seriously though I appreciate you going to the trouble and expense to perform this experiment. it's better to have facts then assumptions. highdesertranger

Bought one of these one summer while working on the road during a very hot summer. Leave plugged in overnight, yup, dead battery next morning. Saw its limitations right away. Cool in the morning. Not so much in the afternoon heat when you needed it most. However, did boost the cooling by dumping ice in there and succeeded in keeping drinks cool and for alot longer, mostly throughout the day. Ice was cheap and free from hotels. 
So the question that begs an experiment...
How much ice will boost the cooling effect by how much for how long? And economically justifiable with the purchase price then?
Or was it just the effect of the ice, cooler effect only upon ambient air?

the ones I have seen and used say don't put ice in them. highdesertranger

highdesertranger said:

the ones I have seen and used say don't put ice in them.  highdesertranger

Click to expand...

Could use frozen water bottles, gel packs, ice in Ziploc, etc.... Just curious if there is a value gain to be had there over just a cooler.

highdesertranger said:

so at 12 volts you would be pulling about 7A and not even to be able to hold a safe temp for food storage.  I would say it's an improvement but not being able to hold a safe temp for food storage,  it's still a poor choice.

Click to expand...

That wasn't what the experiment was testing; only whether or not it had to be run constantly or could be cycled to save power.

I don't know if they are ever worth purchasing new, but mine was $10 at a thrift store. It worked great for a couple of years and now isn't working great.

50% less power consumption when internal temps near ambient.....

I have a cooler that keeps ambient while not using any power.

Weight said:

I have a cooler that keeps ambient while not using any power.

Click to expand...

Me too!

frater secessus said:

 . . . I could not test in ambient sub-70F temperatures, because my house is old and drafty.  I tested at ambient 78F and held a few deltas below ambient to find the duty cycle at each delta.  I suspect this is a decent stand-in for the original question but I lack the thermodynamic background to know.  Comments requested.

My specialty was not in Thermodymanics or Heat Transfer and classwork in the field was 40+ years ago, but I think that your assumption of holding outside temperature and varying inside temp is a good first order approximation of the opposite.

 . . . I did try increasing ventilation on the exterior hot side of the peltier unit but it made no measurable difference. 

Fan blowing on hot side?

The duty cycle vs. 5F delta steps charted in a predictable, linear fashion until we approached maximum delta.  In the last 0.36F duty cycle shot up from 75% to 100%.

I didn't expect linear.  I would have expected a mild exponential curve.

 . . . The empty cooler temp rose 0.3 to 0.5C after the cooler was powered on before the cooler was able to reverse the trend.

I think this would be because you are dumping watts into the system (that would heat the air up) and it would take a little while for the peltier effect to ramp up?  Air changes temperature very quickly (very low heat capacity).

Click to expand...

Interesting experiment, Thanks.

Yeah, I couldn't tell if the exterior fan was pushing enough air so I augmented it. No difference.

Yeah, the inertia could be ramp up time. The probe was also about 10" away from the cool-side fins, so it could also have been a function of distance. Or maybe how weak the peltier was.