Peltier Fridge project launching on Indiegogo - Page 2

# Thread: Peltier Fridge project launching on Indiegogo

1. A 60W peltier consumes 60W of electricity and being around 10% efficient, only provides about 6W of cooling capacity. "Rapid processing of air" will not increase that 6W, it will howver increase the heat transfer rate from the cold air to the body of the "fridge". SO rapid processing of air will actually make it less efficient, not to mention that a fan is expending energy which is dissipated into the air inside the fridge. Say your fan uses 6W. Now you have 0W effective cooling. Good luck with that.

2. The challenge with peltier refrigeration is not "letting in hot air", it is the "temperature differential" from inside to outside.

The concept is explained here, as well as the significantly lower efficiency compared to a compressor driven system.

Thermoelectric cooling - Wikipedia, the free encyclopedia

You can think of a peltier cell like an alkaline battery. Each "cell" produces 1.5 volts of "differential". If you want 150 volts, you need 150 / 1.5 = 100 "cells" in series.

The analogy in peltiers is that each "layer" is good for a certain "temperature differential. Here is a link to an example (no idea how good)

12709 TEC Thermoelectric Cooler Peltier Plate 136.8W

This particular one is made of 127 cell layers and produces a "maximum" temperature differential of 67 F. This is pretty typical of what you see in the market, and explains the challenge of this type in a hot car.

130 F (in the car) - 30 F (goal temp) = 100 F differential needed, compared to only 67 F maximum possible with one of these plates. That means you really need two of them in series to be useful under real conditions.

Using that example system, it will take 250 watts just to operate at this temperature differential, not yet considering the "amount" of heat that needs to be moved.

This type of system needs approx 3 watts of electricity for every watt of heat moved (per unit). This means this 125 unit will move around 40 watts of heat - heat coming in from the outside into the refrigerator, or in the case of needing 2 in series like I need, that would be 250 watts to move 40 watts.

3. To give you an idea of what this means, it would take this 250 watt setup 1.5 hours to cool down 1 can of soda from car temperature to drinking temperature. Moving the air around fast inside the cooler / refrigerator, would have negligible effect on this rate.

The same 250 watts with a modern freon based refrigerator would take 20 minutes. Using the "old" (and less toxic) types of freon, the refrigerator would actually weigh 25 % less (because the pressures are lower) and would only take 12 - 15 minutes.

I don't discount that older freon refrigerants did affect the ozone layer. What is less obvious, is that it was not so much the refrigerant use, but the solvent use of freon and SF6 that was the real problem. Use in Japan to make hard drives (especially Sony) and modern use in China ( to make teflon like polymers ) are far higher release rates than the total world wide refrigerant use.

If you really care about the ozone layer, CO2 releases, and pollution in general, don't buy stuff made in China. There is not a nation in the world worse for pollution per item made than there.

4. Personally, I think the biggest driving force for a peltier small refrigerator is if you can eliminate the volume taken up by the compressor. Otherwise, there is nearly no use.

BTW - I don't think GE actually makes their own refrigerators any more, it just slaps its brand on them.

5. I wanted to do refrigeration with a pelteir for of all things to chill rhubarb plants that grow hydroponically in the tropics.
I'm not an engineer, a student that will become an engineer, or an electrician but the efficiency of compressor based refrigeration surpasses peliters by many folds.

The only reason I'd want a peltier fridge is if I were off grid and didn't want to mess with an inverter(Ei charge controller, battery, and fridge), or if I wanted a small fridge I'd be using in portable applications. But literally if you ran anything with a peltier for an extended period of time when you could easily be running a normal fridge, you will be paying for it in electricity. That's a big deal when electricity is \$.50 per KWH, although still something to consider for others who pay 1/5 that.

6. “A 60W peltier consumes 60W of electricity and being around 10% efficient, only provides about 6W of cooling capacity. "Rapid processing of air" will not increase that 6W, it will howver increase the heat transfer rate from the cold air to the body of the "fridge". SO rapid processing of air will actually make it less efficient, not to mention that a fan is expending energy which is dissipated into the air inside the fridge. Say your fan uses 6W. Now you have 0W effective cooling. Good luck with that.”

Your argument is rather ridiculous. That 6W of cooling power is enough to turn water to ice in under 30 seconds.
See Here: http://www.youtube.com/watch?v=XrK3EsjhvZc]Thermoelectric Peltier Cooler Making Ice Very Fast - Ice Maker, Sub Zero Cooling. - YouTube

“The challenge with peltier refrigeration is not "letting in hot air", it is the "temperature differential" from inside to outside. The concept is explained here, as well as the significantly lower efficiency compared to a compressor driven system.”

I assume that what you are saying is a confusion of this wikipedia line: “Peltier (thermoelectric) cooler performance is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load”

A peltier can easily produce freezing temperatures, even in an exceptionally hot environment.

“To give you an idea of what this means, it would take this 250 watt setup 1.5 hours to cool down 1 can of soda from car temperature to drinking temperature. Moving the air around fast inside the cooler / refrigerator, would have negligible effect on this rate. The same 250 watts with a modern freon based refrigerator would take 20 minutes. Using the "old" (and less toxic) types of freon, the refrigerator would actually weigh 25 % less (because the pressures are lower) and would only take 12 - 15 minutes.”

Your argument here seems sound, and clearly you're especially knowledgeable on the subject. I look forward to seeing how the results compare to your assertion once I've done testing.

Thank you all for your input, even if it was exceptionally critical :P

7. Originally Posted by guunhan
Your argument is rather ridiculous. That 6W of cooling power is enough to turn water to ice in under 30 seconds.
You do the math from the video and you will see that 6W is about right to freeze 0.5g (estimate) of water.

specific heat = 4.186 J/kg*K
heat of fusion = 334 J/g
temperature change = 23 C
mass of water = 0.5g (estimated, probably much less in the video)

energy to cool water from 23C (stated in video) to 0C = 4.186*.5*23 = 48.14 J

energy to freeze water = 334*0.5 = 167 J

total energy = 48.14+167 = 215.14 J

total thermal power = 215.14J/30s = 7.17W

So you expend 33.6-38.4W (stated in video) of electrical power to get 7.17W (calculated based on a rough conservative guess on the amount of water) of thermal power. Best case that is 21% efficiency.

I am not trying to change the laws of physics here. You fail to realize that freezing a drop of water is much different from keeping 4.7 cubic feet of space cold. Assuming a cube, that is a minimum of 16.7 square feet of surface area. If you cared to do the calculation, you could use that surface area and a guess on your thermal power (6W?) and find the required "R-value". You will find that it is IMPOSSIBLE to achieve your stated performance specifications for anywhere near your intended cost projection.

As I said, good luck with that. I'll refrain from further comment as I don't want you to feel I am being "exceptionally critical".

Matt

8. The reason I took the time to run the calculations is that I am wanting you to succeed with your project. I have been personally wanting such a cooler / small refrigerator for my min van and could not find anything reasonable on the market. Obviously inside of a vehicle, space efficiency is a big deal, and the solid state approach to cooling is attractive for that aspect.

I guess what I am trying to say, is that I am still interested, in spite of the limitations of the technology.

In the US, we can easily buy "coolers" with Peltier based cooling, but I have not been very impressed with what they can do. The issue was exactly what I described - they work great when it isn't very hot outside. The example calculation helped me focus on what is actually possible with this approach, and honestly, I appreciate that your project incentivized me to run these numbers.

Now that I can see first hand what the real technical challenges are, I can see a technical path going forward to meet my needs. (two of that type of module in series)

Edit - It takes a lot less energy to keep the contents cold than to cool them down. If you have access to ice or ice water, that can actually help quite a bit.

9. There actually are some other paths to consider for your project. If the goal is to be able to run off of 12 VDC, then perhaps focus on that goal vs "how to get there". Perhaps there are 12 VDC compressors around.

Since you live in a warm climate, perhaps it makes sense to consider an adsorbtion type refrigerator. Those do not use a compressor at all, but rather heat (often from propane or solar)

Absorption refrigerator - Wikipedia, the free encyclopedia

They are commonly used in portable applications in locations with limited or high cost electricity. I built one when I was 14 for a science project, so they don't have to be super high tech.

10. Thanks for the link, that looks like an interesting project

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