Hello omerizm,
If you have not built it before that is fine, I was just hoping for come first hand information on how efficient it was. I have thought about modifying it that way myself, but think I can make it more efficient. I would need to do some disassembling of the refrigerator to see how I could make it work best. I don't have a spare one to work with.
To be honest I have never been impressed with formulas. Especially in cases like this where the needed cooling can change from day to day. Not to mention all the variables in it's design. Not all tubing will conduct heat exactly the same, metal tubing will be much more efficient than plastic, and different types of metal will conduct differently. It's not only about the total output of the compressor and the temp drop needed. Not all of that energy (cooling power) will make it to the liquid in the reservoir. So any formula will be flawed. The formula wont be able to take into consideration all the materials used and design, therefore the efficacy of the design.
Like you mentioned using 25 meters of tubing, but it will be more efficient if you used 50 or 75 meters. But no mater what length of tubing you use, plastic tubing wont be as efficient as metal tubing. The formula cant adjust for that, nor can it take into consideration the diameter of the tubing or even thickness. If it did it wouldn't matter what type or size it was, you would be able to use just 1 meter of tubing and receive the same results. All those variables will affect how efficient it is in the end. It may be useful as a guideline, if you have a working model to compare to. But I'm more interested in working practicality of any design.
Also I wouldn't be building it to use in only one system. I wouldn't even want to build it unless it was capable of functioning well for reservoirs of different sizes, and/or where different temperature drops will be needed. It is a lot hotter here in July than it is in May. But I have come to the conclusion that so far compressor based cooling units will probably be more expensive to run than it's worth. I cant justify spending $100 a month in electric bills to grow $50 worth of produce.
I have considered using heat sinks, I even contacted some company's about it. The same type used in portable coolers, I got some good information, but conflicting theories on how efficient it would be on the size I would need to build them, compared to using a compressor unit. But That could be because the company that said it would be too expensive to run only sold completed units. The other company only sold one part and gave me links to places where I could get the rest of them. Any witch way you look at it, I would need to build at least one working model of these and measure the cooling ability's as well as the electrical cost.
Not sure if you know but I have already built a geothermal reservoir, not to mention that I have designs for using geothermal energy to cool the
nutrients. So far that's the most cost effective option, even if it's the most inconvenient method. I do have plans to build a sort of revers swamp cooler to cool the
nutrient solution. I don't have any experience on how efficient it is but I'm told it works well in our claimant. From the design I can see ways to make it better, but until I have a working model who knows if it will be useful.