Cooling the Nutrient Solution

[GpsFrontier]

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.

[omerizm]

Hello Gpsfrontier,

Well I am a mechanical engineer, and I am "impressed with" formulas eheh You need to consider the max amount of cooling you need in the hottest season, and the max. reservoir size. Your cooler should be able to work at that extreme design point and you can use a thermostat to adjust to more favorable conditions by simply turning the cooler on and off, keeping your NS temp. at the desired level automatically.

You do approximations and put efficiency factors in your formula to simulate real life conditions. You don't need to consider every single physical effect and all the details for that. It will give you a basic idea to start your design. My point is, a vapor compression refrigerator can be 60% efficient tops, so you calculate how many calories you need per second (for the extreme condition) and divide it by 0.6, and you know the least amount of power you need. It is reasonable to expect that your cooler will be even less efficient than that so expect to use more power.

If you calculate this, you will know the minimum cost you can realistically expect and then maybe you will eliminate the option of vapor compression cycles.

The length, wall thickness and thermal conductivity of the tubing has no effect on efficiency. That only effects the heat removal rate from the NS passing through the tubing. Efficiency is all about the percentage of energy you waste. By using a shorter tube you don't waste anything, you only draw heat slower and it will take a longer time to cool down the same amount of solution but you will use the same amount of energy to do that.

Major energy waste is in the compressor and heat absorbtion of the fridge from the surrounding medium, that is air. So what effects your power consumption is mainly how efficient your compressor is and how well your fridge is insulated. It is also important to insulate the tubing between the reservoir and the fridge.

[GpsFrontier]

Hello omerizm,
I think you misinterpret what I'm talking about when I use the term "efficiency". I'm speaking of how cool the nutrient solution gets, not how efficient the compressor is. I know that adding any amount of tubing to the inside of the frig is not going to change how well the compressor works. I'm only concerned with how cool I can get the nutrient solution, and how much it costs to get it there. The key to making it efficient (cooling the nutrient solution) is in getting the warm nutrient solution to transfer as much heat with the cool air inside the frig as possible. Then it will depend on the size (volume of water) of the reservoir and the outside temperature (I grow outside) weather it can cool down the entire volume of water (nutrient solution) to a sufficient temperature, and be able to maintain that water temp all day long. That is what it takes to be efficient to me. Of coarse another factor in how efficient it will be in cooling the nutrient solution is how well insulated the reservoir and outside tubing will be. I didn't really mention that because it's easily enough to do, and I already have planed on doing that regardless.

And although the thermostat can help adjust the water temp, I can assure you my water temp in my reservoir will be no ware near the temp on the thermostat in the fridge (unless it's winter, maybe early spring). The thermostat will need to be set to its coldest setting and remain there all summer. Considering that the fridge will be outside during the hottest parts of the summer that will be 120-125 degrees fahrenheit during the day, and between 100-110 at night. There is no way to bring it inside and just run the lines outside, there is simply no place to put it inside, other than the garage and it's hotter in there than it is outside. Also I have no set plan of what I will be growing this summer, but suffice it to say as many things as I can. That would include more than one reservoir needed. With the amount of money I have for materials being the largest deciding factor in how much I will be able to grow. With reusing as much as I already have, but I want to try some new setups.

Like I said the key to making it efficient is getting as much heat out of the nutrient solution as possible. I have already designed a cooling box witch is pretty much just like a large ice chest. With the reservoir inside here and ducting the cold air from a small refrigerator or window AC unit, that will make the heat transfer more efficient. And kind of like keeping the entire reservoir inside the refrigerator, instead of only running some coils through it.

I designed this cooling box to work with the heat sinks that I was working on as the cooling source, but it's very adaptable and functional. It will even be able to work with the geothermal coil trench design I have as the cooling source. How well and how efficient it works with that cooling source at cooling the nutrients depends mostly on how deep the trench is, how long the coiling tube is, volume of the reservoir, and how well the tube transfers heat. I even have another cooling device design that I'm planing to test this summer, with or without using the cooling box. I'm looking forward to seeing how efficient that will be. Building on what the original person built I can make some improvements, but it's supposed to work well as is in our climate.

P.S. Also another thing to consider is using air stones in the solution. Obviously pumping hot air into the reservoir is not going to be beneficial. So the placement of the air pump, lines and how long the lines are is all going to make a difference.

[one800fish]

Is there any such thing as a thermostat with a probe that can be placed in the reservoir and be set for say 72 degrees and only run the pump until it cools?
I don't suppose running the solution thru a fan cooled radiator (like on a auto only smaller) would work ?
If I could find a thermosat like that the pump and the fan would'nt be running constantly.
Idunno, just asking

[omerizm]

Originally Posted by one800fish

Is there any such thing as a thermostat with a probe that can be placed in the reservoir and be set for say 72 degrees and only run the pump until it cools?
I don't suppose running the solution thru a fan cooled radiator (like on a auto only smaller) would work ?
If I could find a thermosat like that the pump and the fan would'nt be running constantly.
Idunno, just asking

Hello one800fish,

Of course there is. The one I use to control my cooler works exactly as you described. Here it is:

Enda

And this is the sensor that I connect to it:

Enda

Don't worry about the brand and model. These type of devices are pretty much all the same so you can search and find a manufacturer in your country and purchase one. You don't need to buy it and have it shipped from accross the world because it will probably be more expensive and you will find it hard to get it fixed in case it brakes.

These devices work basically like this: You put the sensor in the medium you want to control, in this case you dip it in the nutrient solution inside the tank. Beware that there are different types of sensors for liquids and gases and also for different temperature ranges. You need to get one suitable for water. My controller works between -60 to 150 degrees Celsius and my sensor measures up to 150 degrees Celsius and it is for measuring liquid temp, so its good for me. By the way, some thermostats are used for cooling and some for heating but nowadays almost all digital thermostats can be used for both. You have a power input in the thermostat, connect it to your power source with a cable. And you have an output, there you want to connect the device you want to control, in this case a pump or fan or whatever you like. There should be a manual that comes with the device that explains the connections. Also from that manual you can learn how to set the controller to a desired temperature and adjust hysteresis and stuff like that. In cooling mode, it basically powers up the fan or pump or whatever when the temperature exceeds a certain point and it cuts the power when it drops below a certain point.

Say that your set temperature is 72F and hysteresis is se to 3F. it will power up the pump when temp exceeds 72+3=75F and it will stop the pump when temp. drops to 72-3=69F. You want a few degrees of hysteresis because you don't want to turn your device on when temp hits 72 and off when it drops to 71, in which case your pump would break from a non stop on/off abuse.

Cheers