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I live in the tropics, and during the summer it is very hot here. Today the air temp was low-mid 30C with 80% Relative Humidity.

I live close to a stream, which I'd like to use for free-cooling. Today I went out to the stream to check the temperature, and the stream was running at about 20C.

By my back of envelope calculations I can see that with a flow rate of 0.1l/s I would have about 418W/K change in the water.

My question is, how close can I expect to be able to get my indoor air temp to the outflowing water temperature? How effective is my setup likely to be if I pull in the water and run the water through as a "once through cooling" system, through a fan coil? I am missing some parameters to effectively size the system. But I do know that I want to be able to draw in the order of 1hp (750W) out of my home at 25C indoor air temperature.

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    If we assume your 750W is what you need, your next step is to size a water-air Heat exchanger within the constraints you have. Google "sizing air coolers" for relevant literature. – mart May 26 '14 at 11:02
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The effectiveness you are asking for is the end result of a heat exchanger analysis (of the level engineering schools assign in the junior year--somewhat complex). You would have to provide much more information to even be able to approach this problem. This web page demonstrates some of the equations needed to solve a problem like this:
http://www.esru.strath.ac.uk/EandE/Web_sites/12-13/Domestic_flue_gas/technical-analysis/heat-exchanger-analysis/crossflow-air-hx.html.

Most likely, your design will end up being a cross-flow heat exchanger. Your best bet at finding out how well the air conditioner will work is to look at the results of someone else's prior design.

As for how close you can expect to get the air temperature to the water temperature in a cross flow heat exchanger: I would estimate your maximum cooling would be less than half the difference between cold water temperature and warm air temperature ((30 degC - 20 degC)/2 = 5 degC).

So, you could cool outside air to 25 degC at the coolest. More likely you would end up at 27 degC with a simple cross flow heat exchanger made with a fan and cooling coil of flowing water.

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It would be good to have a more detailed description of your HVAC equipment for a better answer in what would be free cooling for you.

If you have a system that mixes return air with outside air for cooling and ventilation, free cooling could be shutting off the cooling equipment while keeping the fans on. This could be done by simply opening windows as well. The excessive humidity (and the energy included in it must be considered).

As you have a stream near your house, a chilling beam could be a possible solution. The stream water would used in its natural state, or chilled before flowing into the chilling beams. The advantage of using chilling beams is that the heat lost though radiation is felt differently from the air temperature, and a slightly higher temperature would be considered comfortable. 28 degrees Celsius instead of 26 degrees Celsius (this number depends on your clothing and activity level though).

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    "Extra humidity added"? What do you mean? If the cooling water goes through a heat exchange, how does extra humidity get added to the indoor air? – Aron Jul 1 '14 at 14:17
  • @Aron it may be that Dionisio is referring to the increase in relative humidity that happens when air is cooled without being dehumidified (just a guess). – EnergyNumbers Jul 2 '14 at 9:48
  • -1 "simply opening windows" = free heating. The question clearly noted I was in the TROPICS. Also it should be noted that running a dedicated dehumidifier could still be energetically beneficial, especially if one were to run the it on solar power (using solar thermal to regenerate desiccants). Assuming of course you meant that relative humidity increases (no extra humidity is added). – Aron Jul 2 '14 at 10:34
  • @Aron, I replied to your answer on a mobile device and was not able to read your question properly, including your location. Sorry for that. – Dionisio Franca Jul 3 '14 at 4:59
  • @EnergyNumbers, yes, I was talking about humidity. – Dionisio Franca Jul 3 '14 at 5:01

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