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Help needed: Water cooling problem


Ronald Lao

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Background:

 

A few months ago I decided to start a small aquaponics (growing fish plus vegetables together) farm in the Philippines. I was hoping to make my production as efficient as possible. One way is to make the plant root temperature lower than 24 degrees celsius. I'd like to find the most cost effective solution to do this.

 

IMG_20150306_093832.jpg

 

 

 

Situation:

 

A recirculating system with about 25,000 Liters of water needs to be cooled to around 24 degrees Celsius. A part of the water system is outdoors but still insulated by 2 inch styrofoam from direct sunlight. Typical ambient temperature during the day can reach up to 32 degrees celsius during summer. Water Temperature is measured to be usually around 2 to 3 degrees cooler than the ambient temperature.

 

Question:

What is a the more cost effective solution to cool the water system? My idea is to use TEC/Peltier devices if possible to use very small wattage that can be turned on during daytime using solar cells as power since it's cooler at night anyway. Please include the computations to support as well as your assumptions.

 

 

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TECs are not very efficient (and their efficiency gets smaller in the very situation you want them to work); their utility is really for size and weight, for low-power applications. The amount of power you need is going to depend on how much water needs to be cooled.

 

 

Details matter, but one solution might be burying the pipe and exchanging heat with the ground, which generally cooler and more stable than the air. But then you won't have control over the temperature.

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...

Question:

What is a the more cost effective solution to cool the water system? My idea is to use TEC/Peltier devices if possible to use very small wattage that can be turned on during daytime using solar cells as power since it's cooler at night anyway. Please include the computations to support as well as your assumptions.

Perhaps an evaporative cooler.

 

What is Evaporative Cooling?

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Probably not a viable candidate in the Philippines. You need low humidity to make that work.

The lower the humidity the more efficient it is I agree. However, unless the humidity is 100% you can get some evaporation. Since Ronald is already circulating water he would only need to power some fans and a small pump with his solar panels to run secondary water over cooling coils.

 

Large power plants using evaporative cooling continue to run regardless of humidity.

Edited by Acme
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Large power plants using evaporative cooling continue to run regardless of humidity.

 

Right, but that's probably a case where you have very hot water and you are trying to cool it down. At 32 ºC and 60% RH the dew point is 23.3 ºC. That's your cooling limit — you aren't going to get any net evaporation below that. It's going to take a while to cool down to 24º

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Ronald says typical temp is 32ºC and the water temp. usually 2 or 3 degrees lower; call water temp 30ºC. He wants to cool the water to 24ºC, so 30-24=6º.

 

Taking this similar climate example:Evaporative Cooler @ Wiki

...The same equation indicates why evaporative coolers are of limited use in highly humid environments: for example, a hot August day in Tokyo may be 30 °C (86 °F), 85% relative humidity, 1,005 hPa pressure. This gives dew point 27.2 °C (81.0 °F) and wet-bulb temperature 27.88 °C (82.18 °F). According to the formula above, at 85% efficiency air may be cooled only down to 28.2 °C (82.8 °F) which makes it quite impractical. ...

So at highest temp and humidity he would only get 2º cooling. But, it is cooling. Moreover, according to this source those extremes of temp. and humidity in theTokyo example only occur for a couple months in Manilla, Phillipines.

Manilla Climate Data

The average annual relative humidity is 73.8% and average monthly relative humidity ranges from 64% in April to 82% in August.

Ronald can use the performance equations at the Wiki page to work out his own specifics & determine if evaporative cooling is workable for him.

Performance

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Good afternoon, Ronald, have you abandoned your other thread?

 

Meanwhile here at water cooling central I think swansont has shown his great depth of physics knowledge in his replies, I had been considering an evaporative solution. I also worried about the water loss needed to remove the heat.

 

So here are some questions:

 

First location, are you on the coast or near a river or lake that you could use as a heatsink?

Second what is the source of your water and what is its inlet temperature?

 

You may need to resort to a water storage (re)cycle over two or more days so that the water can be cooled as far as practicable and then returned to use.

This could also help isolate your garden against supply interruptions as well as saving water costs.

The cold water would need underground storage so the capital cost could be significant.

There are several possibilities to cool the water in your cold store, depending upon local site conditions.

A heat pump may be part of the solution, but you will not find a magic box that you can just pump significant quantities of water through hot to leave cold.

Edited by studiot
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...Meanwhile here at water cooling central I think swansont has shown his great depth of physics knowledge in his replies, I had been considering an evaporative solution. I also worried about the water loss needed to remove the heat.

Maintenance of the nutrient concentration would be more difficult if the water was encouraged to evaporate. Mess that up and you might lose any gain from having cooler roots

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Maintenance of the nutrient concentration would be more difficult if the water was encouraged to evaporate. Mess that up and you might lose any gain from having cooler roots

There is no need to evaporate the nutrient water. It would remain in a closed system and separate cooling water/air run across a heat exchanger.
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Ronald says typical temp is 32ºC and the water temp. usually 2 or 3 degrees lower; call water temp 30ºC. He wants to cool the water to 24ºC, so 30-24=6º.

 

Taking this similar climate example:Evaporative Cooler @ Wiki

 

So at highest temp and humidity he would only get 2º cooling. But, it is cooling. Moreover, according to this source those extremes of temp. and humidity in theTokyo example only occur for a couple months in Manilla, Phillipines.

Manilla Climate Data

Ronald can use the performance equations at the Wiki page to work out his own specifics & determine if evaporative cooling is workable for him.

Performance

 

My point was that you will never cool below the dew point (net evaporation stops), and that is really close to the target temperature (24 ºC) even under the better (60% RH) conditions. That's a hard limit for the system. As your numbers show, you're going to end up a few degrees warmer than that. You'll get some cooling, but not enough. You either need a second system (of another kind), or another approach.

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My point was that you will never cool below the dew point (net evaporation stops), and that is really close to the target temperature (24 ºC) even under the better (60% RH) conditions. That's a hard limit for the system. As your numbers show, you're going to end up a few degrees warmer than that. You'll get some cooling, but not enough. You either need a second system (of another kind), or another approach.

I agree my proposal is not enough cooling for Ronald's stated goal of 6ºC drop. He may however judge for himself if an approx. 2º cooler is better than none. As Studiot hinted at, running some piping underground would get some additional cooling. Here's a page that's helpful for judging that idea: Ground Temperatures as a Function of Location, Season, and Depth It's not specific to Phillipines but gets the idea across.
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25,000 liters - if we assume 1000L/hour, and we need to cool by as much as 5º. It's 4.18 J/g (or per ml), so 4.18 MJ in an hour, per degree of cooling, or 5.8 kW for 5º. The good news is that it is possible to move thermal energy around with less energy than you are actually removing - you just need to dump it somewhere else.

 

Maybe a heat pump would be the solution

http://en.wikipedia.org/wiki/Geothermal_heat_pump

 

 

Also, can you use the solar panels to shield some of the water from direct sunlight?


As Studiot hinted at, running some piping underground would get some additional cooling. Here's a page that's helpful for judging that idea: Ground Temperatures as a Function of Location, Season, and Depth It's not specific to Phillipines but gets the idea across.

 

I think that's the solution we're converging on, unless there's a deal-breaker we haven't seen yet that precludes it.

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I didn't actually say (necessarily) run the pipes underground.

 

Firstly and most important would be the proximity of a nearby large body of water.

If this were the case this body will be cooler than either the land or the air, and also easier to exchange heat to.

 

If this is not the case then heat needs to be pumped either to air or ground.

 

Also I mentioned a cold store.

 

This is because it would be more efficient to dump heat at night than midday.

Also a smaller heatpump would be required than for instantantous cooling.

Further a cold store would even out water and cooling flows in time.

 

This is similar to using hot water tanks in cold countries rather than flash boilers.

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...I think that's the solution we're converging on, unless there's a deal-breaker we haven't seen yet that precludes it.

Acknowledged.

I didn't actually say (necessarily) run the pipes underground.

Ergo my saying "hinted".

 

Firstly and most important would be the proximity of a nearby large body of water.

If this were the case this body will be cooler than either the land or the air, and also easier to exchange heat to.

Agreed. As any scuba diver will tell you there is a steep temperature gradient as one descends. [Edit: See Thermocline ]

 

Also I mentioned a cold store.

 

This is because it would be more efficient to dump heat at night than midday.

Also a smaller heatpump would be required than for instantantous cooling.

Further a cold store would even out water and cooling flows in time.

 

This is similar to using hot water tanks in cold countries rather than flash boilers.

The only issue I see with the storage is cost.

 

I saw Ronald signed in earlier, but he didn't post. Perhaps he will return & favor us with some feedback. :)

Edited by Acme
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What does the temperature difference do for the roots if it is lower?

 

According to literature I have read, Lettuce will be bigger at 24 degrees celsius at the root temperature. Probably the same reason why lettuce is usually a "cool weather" plant.

Uh... please forgive my newbie ways. I have not been actively in forums for over 13 years and I have to relearn the etiquettes and direct quotes. Thank you for the overwhelming responses and ideas, let me try to give a few feedback.

 

1. No large body of water near me. I live in the city and space is a big issue. This project is really targetting an urban setting.

2. I have thought about burying pipes underground... from literature and actual small testing, my ground temperature is around 25 degrees celsius. I have actually used this to some advantage as well when I designed the system to have my sump buried 75% on the ground. Burying pipes underground to cool the nutrients would probably be my last option since space is a major issue and the pipes needed are not cheap either.

3. I'm no expert on evaporative cooling, however, my gut feel is that it's probably not going to be very effective because the temp difference is really very small. Example, yesterday the water temp was at 27.9 degrees.

4. 24 degrees celsius requirement is actually for the plant roots because according to research, you can grow lettuce really big if you keep the root temperature at such temps. For a commercial venture this is very important.

5. I know that peltier is a "lossy" device. But one major disadvantage that I have here in the philippines is the cost of electricity. I believe we have one of the world's most expensive electricity. So since Peltier can be operated at low power (i.e. solar panels) I thought it might just do the trick. Power on during the day only.

 

6. There was another idea I was toying with but I'm not sure if it's uh... legal here. Somebody told me that in some remote areas where their is no electricity they actually use a refrigerator that is powered by a "candle" called an ammonia cooler. It was cold enough to make ice so I guess that could be another solution for me. Barring the legal issue of operating an ammonia based cooler maybe there are other "refrigerants" with the same idea I could use. A solar panel with heating element would replace the candle obviously.

 

Hope these feedback will keep until Monday. I'll be off volunteering as a councillor in youth camp for the weekend. TTFN and thanks again!

 

tiglao-jan-10.jpg

Electricity rates in Manila, Philippines where Meralco is the provider.

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...6. There was another idea I was toying with but I'm not sure if it's uh... legal here. Somebody told me that in some remote areas where their is no electricity they actually use a refrigerator that is powered by a "candle" called an ammonia cooler. It was cold enough to make ice so I guess that could be another solution for me. Barring the legal issue of operating an ammonia based cooler maybe there are other "refrigerants" with the same idea I could use. A solar panel with heating element would replace the candle obviously.

 

...

These are called absorption refrigerators and they can be powered by a variety of heat sources including solar. They are common in recreational vehicles in the US and typically powered by both AC and propane gas.

Dometic is an international supplier of these refrigerators. >> Dometic

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It seems root-chilling is quite important in the tropics for good yields and avoiding certain problems in temperate-climate plants. This article called Chilling The Root Zone by Dr Atomic Leow Chuan Tse goes into the details. He has also written a book called A Guide To Hydroponics published by Singapore Science Center that's probably worth acquiring as he will understand your needs and potential problems in your part of the world. It might be worth checking out other literature he's done as well.

Edited by StringJunky
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5. I know that peltier is a "lossy" device. But one major disadvantage that I have here in the philippines is the cost of electricity. I believe we have one of the world's most expensive electricity. So since Peltier can be operated at low power (i.e. solar panels) I thought it might just do the trick. Power on during the day only.

 

 

I think you're missing an important issue. A peltier operated at low power won't do very much cooling. They are useful when you don't need much cooling, because they can be scaled down, while other devices can't. But even something with the capacity of a small refrigerator, a peltier is not going to be the cheapest option.

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I'm building a small greenhouse or personal use, and have been looking at chilling water for it. Absorption chillers aren't made small AFAIK, the smallest I can find are 5 Tons. The ones used for hydroponics all run on electricity, which is expensive to buy, even if you put up a PV array.

 

A time before CO2 levels increased to 300ppm, it was possible to cool via radiating heat into the night sky, which is about 3 degrees Kelvin. At that time infrared from temperatures as low as 0C would radiate into the sky and in some cases freeze shallow pools of water, even if ambient air temperature was above 0C. See: http://en.wikipedia.org/wiki/Radiative_cooling.

 

With CO2 levels as high as they are today, radiative cooling must be less effective; however, I do not know how much.

 

A refrigeration unit cools most when the exhaust temperature is cool; although, too cold might adversely affect the refrigerator. I am currently considering using a chiller.

 

My preference would be water to water, but those are hard to find in a small size, as are absorption units. If I can find a water to water chiller, then I might cool the water exhausting the heat via evaporation and radiation. Lacking this, I may use an evaporative air cooler to blow air through the water chiller (to cool air being used to exhaust heat from the chiller). One might also, cool the water circulating through the air cooler using radiators exposed to the night sky. If night sky cooling is used, one must have a large tank of water to cool at night, for use during the day to cool the hydroponic fluid and perhaps the greenhouse.

 

I've suggested a complex system with several inefficiencies and unknowns. Just how much of this idea is viable, I don't know. I do know that commercial systems have more options than residential, for several reasons. First, residential systems are driven primarily by low cost installation. Unfortunately, savings on a low cost refrigeration unit may be consumed by operating expenses. Individuals don't know how to calculate such tradeoffs, and may not have the time or desire to evaluate and install a system with higher initial cost and lower operating costs.

 

Consequently, commercial systems have more options than residential systems, including absorption units, and units that operate at night to freeze water in an insulated container, that cools commercial property on hot days. Refrigerators run more efficiently at night and electricity may be less expensive.

 

I will have a 3000 gallon tank in my atrium+greenhouse to store heat in winter and cool in summer. I hope to run it all with PV electricity. I haven't decided how to cool the water in summer, yet. I'll probably start with misting units in the greenhouse and really good ventilation. That's likely to allow temperatures in the greenhouse to rise above 38C, which may be too much.

 

I intend the atrium to be an enclosed-outdoor environment, which compared to outside is warmer in winter and cooler in summer. However, I don't intend to keep it anywhere near as comfortable as people like inside their houses. I want it to be friendly to plants all day and night, and a few hours each day great for people.

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I'm building a small greenhouse or personal use, and have been looking at chilling water for it. Absorption chillers aren't made small AFAIK, the smallest I can find are 5 Tons. The ones used for hydroponics all run on electricity, which is expensive to buy, even if you put up a PV array.

See the links I gave Ronald above on absorption refrigerators. They do make them small; I have one in my travel trailer. Moreover as I said and my links show, they run on electricity or propane. They are dual use so when you're in a park with electricity they run on electricity and when your traveling or parked with no electricity they run on propane. The units run around $1200 US dollars, and cost to run is as cost to run does.

 

If you by chance meant 'evaporative coolers', they come small too; search for 'swamp coolers'. swamp coolers @ Yahoo

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The issue is when you want to cool something you need to "throw out" the redundant energy and it is costly. The trick then is to find some use for this energy. IOW to use the energy to heat something else. I don't know what that could be in the Philippines. Maybe you could cultivate in parallel other plants that need more heat than usual? or sell boiled lettuce? :) Or use the heat for another process like pasteurization? Or make beer? Something like that. In these cases, heat pumps are very efficient. the cost of electricity would be split in two because you would have 2 products, one that need cooling and one that need heating.

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See the links I gave Ronald above on absorption refrigerators. They do make them small; I have one in my travel trailer. Moreover as I said and my links show, they run on electricity or propane. They are dual use so when you're in a park with electricity they run on electricity and when your traveling or parked with no electricity they run on propane. The units run around $1200 US dollars, and cost to run is as cost to run does.

 

If you by chance meant 'evaporative coolers', they come small too; search for 'swamp coolers'. swamp coolers @ Yahoo

A chiller is designed to cool water that flows into and out of it through pipes; thus, is different from a refrigerator. swamp cooler = evaporative cooler. I mentioned both.

The issue is when you want to cool something you need to "throw out" the redundant energy and it is costly. The trick then is to find some use for this energy. IOW to use the energy to heat something else. I don't know what that could be in the Philippines. Maybe you could cultivate in parallel other plants that need more heat than usual? or sell boiled lettuce? :) Or use the heat for another process like pasteurization? Or make beer? Something like that. In these cases, heat pumps are very efficient. the cost of electricity would be split in two because you would have 2 products, one that need cooling and one that need heating.

Yes, if you have two uses, your overall process is more cost effective. It is possible to make hot water for industrial or residential use and cool the hydroponic fluid. Often, the exhaust heat from refrigeration is discarded into the environment via a cooling tower.

 

If you need 5 Tons or more of chilled water, you might use an absorption chiller and concentrated solar as a heat source; although, you probably cannot buy such a rig off-the-shelf.

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