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Author Topic: Energy required to maintain water temperature  (Read 6694 times)

Offline Red

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Energy required to maintain water temperature
« on: 10/02/2014 17:51:22 »
Hello, I've been trying to answer to this question without much success, I'm sure somebody here can answer it. How much energy would be required to maintain water temperature of 28 degrees C, flowing at 10.8m3/h with a suface area of 100m2?

Thanks


 

Offline chiralSPO

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Re: Energy required to maintain water temperature
« Reply #1 on: 10/02/2014 22:17:48 »
What temperature is the surrounding environment? The rate at which heat must be added depends on the difference in temperature between the water and the environment...
 

Offline Red

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Re: Energy required to maintain water temperature
« Reply #2 on: 10/02/2014 22:27:35 »
It would be 22 +/-2 degrees
 

Offline Bored chemist

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Re: Energy required to maintain water temperature
« Reply #3 on: 11/02/2014 21:24:56 »
Is there an exposed water surface?
If so, what's the local airspeed and the relative humidity.
Actually, even if you tell me those I still won't be able to answer the question.
It's very complicated.
There are probably tables which will give you a rough idea (and, of course, there may also be  losses from other surfaces to consider.

What system are you actually thinking of?



This sort of thing might help
http://www.elecro.co.uk/elecro_sphfaq_linkedlist.asp
« Last Edit: 11/02/2014 21:27:28 by Bored chemist »
 

Offline Red

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Re: Energy required to maintain water temperature
« Reply #4 on: 11/02/2014 22:59:03 »
It's for a fish hatchery. The surface will be exposed but they (pools) will be insulated and in a thermal polythene tunnel. As you have said it's a very complicated equation, we're looking at using air to water heat pumps and/or waste oil burners with the possible addition of a heat recovery dehumidifier. The oil burner puts out 500,000 btu's at 10.8m3/h. The heat pumps can be sized to requirement but use a lot more electricity.

The monthly running costs are far more important than the initial cost.

 

Offline Red

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Re: Energy required to maintain water temperature
« Reply #5 on: 12/02/2014 09:02:43 »
Btw, thanks for the link, it's a good starting point.
 

Offline Bored chemist

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Re: Energy required to maintain water temperature
« Reply #6 on: 12/02/2014 09:16:37 »
If the water is recirculated then it doesn't matter much that it's moving.
The swimming pool data should give a reasonable answer.
(If you are heating it "on the fly" then you are going to need enormous amounts of power)
 

Offline CliffordK

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Re: Energy required to maintain water temperature
« Reply #7 on: 12/02/2014 11:33:10 »
If the water is recirculated then it doesn't matter much that it's moving.
The swimming pool data should give a reasonable answer.
(If you are heating it "on the fly" then you are going to need enormous amounts of power)

I assume the goal is to bring in fresh clean water to keep the waste and toxin buildup down.

You could calculate how much it would cost to raise 10.8 m3/h by 6C.

I suppose I still like calories....  so that gives you  10.8 m3/h x 1003 cm3/m3 x 6C x 1 cal / C =   64,800 kcal/hr

64,800 kcal/hr / 860 kcal/hr / kw/h

And you get about 75kw/h to heat the flowing water.

BC's link suggests about a 36kw heater for a 30 x 30 pool, or a total requirement of about 110kw.

I would certainly look for methods to optimize your power.  For example if you put in a heat exchanger, you may be able to recover most of the heat from the outflow.



By setting your heat exchanger as a grid of pipes, you may actually get fairly high efficiency with the outflow near ambient temperature, and the inflow near the tank being near tank temperature.  You may need a bit extra piping to pipe your inflow/outflow together, then to separate them again in your tank, but the cost of the heating would be so high that it should easily pay for the heat exchanger and extra piping.  With a good layout, the heat exchanger could be part, or all of that extra cross-connect piping.  Do you also need to add turbulence to the water flow for heat exchange efficiency?

Some swimming pools are using solar hot water heaters.  You may have problems with night-time heat using a solar system unless you built some kind of a thermal reservoir, but it may also help reduce your costs. 

You may also consider some kind of a hydroponics system to recover the excess ammonia and waste, and perhaps supplement some of your fish food.

« Last Edit: 12/02/2014 11:40:27 by CliffordK »
 

Offline Red

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Re: Energy required to maintain water temperature
« Reply #8 on: 12/02/2014 15:17:22 »
Thanks for the input guys.

The water will be recirculating through filters with 5% top up each day. So we just need to heat up the new water, make up for the loses throughout the system and maintain a stable temperature of 28 degrees C.

Ideally I'd want to use the boiler as its has the lowest monthly cost, with a recovery heater taking advantage of what we lose through evaporation. What volume of water could the 500,000 btu's hold at 28C, if the air temp was 25C, suface area 100m2?
 

Offline CliffordK

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Re: Energy required to maintain water temperature
« Reply #9 on: 12/02/2014 20:42:05 »
There are too many unknown variables, and there is no substitute for some empirical data.  See if you can find another fish farm that is doing something similar to what you're doing and ask their energy requirements and system characteristics. 

You will need to size your system based on the worst case scenario, not the best case scenario.  Will you be heating when it is below freezing outside?  What about variability in the input water temperature?

Anyway, what you have:
  • Evaporation: Based on surface area, temperature, humidity, wind, or exchange rate.  Also, do you have some kind of a bubbler or aerator which could increase the evaporation rate?  Technically it is dependent on surface area and independent of the volume, although you may select aeration based on volume.
  • Thermal conduction from tank to surrounding air (above ground), or surrounding soil (below ground).  Dependent on the temperature of the air/soil, and R-Value of your insulation.  This would be dependent on the volume or outside surface area of your system.
  • Input of cold water.  Depending on temperature differential and volume of water per hour.  See my calculations above.  This may be one of your major energy consumption factors.


Based on the previous stipulated raising the temperature (of input water) by 6C, and the sizing suggested for a 30' x 30' swimming pool heater (4.5' deep), I came up with about 110KW (see calculations above).

Look online for formulas to convert between different units, Calories, KW, BTU's, and etc. 

Your 500,000 BTU heater should put out about 146KW equivalent, and may be sufficient for the  6C temperature increase, but with the moderate temperature increase, you are running it at a 75% capacity.

Notes also discuss a swimming pool cover.  While a pool cover may not be practical for fish, the whole tank may not need to be open-air on top.  And, you may consider some kind of a greenhouse cover with an air gap, and supplying the oxygenation needs with the aerator. 

Can you get away with just aerating just the cold input water, and not the warm tank?
 

Offline Red

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Re: Energy required to maintain water temperature
« Reply #10 on: 16/02/2014 17:05:07 »
Thanks for the input. Turns out you can only use a waste oil burner if you generate the oil yourself. Taxman misses out otherwise!

We'll end up using swimming pool heaters, the air to water type with a dehumidifier to recycle lost heat.

Cheers
 

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Re: Energy required to maintain water temperature
« Reply #10 on: 16/02/2014 17:05:07 »

 

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