Backyard Aquaponics
A place to discuss aquaponics

Having previously done the calculations for how much ice I would require to cool my AP systems' water by a certain amount, I conducted an experiment a couple of days ago to demonstrate why dropping a couple of 2litre milk bottles full of ice in your FT will have minimal effect, unless your FT is tiny.

This is a small section of the week-long graph as posted in my main system threads, showing the period from 12:00 to 16:00 on 14th Jan 2016. Red line is air temp, blue line my large system FT water, ~9500l. Water was draining from each system follwing high tide at hh:07 due to the flood pump cycle. Temperature logger resolution is 0.1C.



At 13:20 I dropped in 21.1kg of ice, made up of ice blocks from 2 and 4l ice cream containers, and at 15:10-15:15 I added a further 13.6kg, made up of a large 7.2kg block and a few smaller blocks as per the previous addition.

Calculations suggest I'd have about 0.25C drop from 21.1kg of cold ice (well below 0C), and a bit over 0.1C from 13.6kg of it, and the experimental results are consistent with that- limited by the 0.1C resolution, and the ongoing heat gain by the system during the period of ice melting and temperature equilibration throughout the tank. The chiller was not running, and the tank circulation pump was running, which aided with the mixing (but also added heat as the water passes through (currently) uninsulated pipes and the coils of the chiller.


Rounding things off, the general rule is 1kg of ice will cool 1000litres by about 0.1C, assuming no heat gain is occuring. On a hot day if your system water, say an IBC + GB with 1000litres total water, gains 5C, you would need about 50kg of ice to counter that heat rise- but don't drop it all in at once!

In practice there are a number of factors affecting this ratio of ice to cooling. They include the thermal mass of the container and anything in good thermal contact with it- ie the ground if buried, the heat gain from the surroundings/how well insulated everything is, water circulation adding heat to the FT from GBs, towers etc, and cooling due to evaporation.

There is a bit more to it than just the phase change- the energy required to heat the ice from say -15C to 0C (small effect), and then from 0C to the water temp (not so small effect).

Looking at it from a thermodynamics viewpoint, rounded for convenience:

A water temperature rise of 1C in 1000litres represents almost 1.2kWh of energy (or, the energy in over half a kilogram of ice cream!)
The melting of 1kg of ice requires ~0.1kWh of energy to be absorbed from its surroundings

For those who want to use refrigerated air to cool the water:
To have the same cooling effect as 1kg of ice, you would need over 3300litres of air to warm by 20C, ie release very fine bubbles at 0C in 20C water and have the air at 20C as it reaches the surface- probabaly not doable, so you'd need even more air, probably >4000l ... just to cool the above system water by 0.1C!

"A water temperature rise of 1C in 1000litres represents almost 1.2kWh of energy (or, the energy in over half a kilogram of ice cream!)
The melting of 1kg of ice requires ~0.1kWh of energy to be absorbed from its surroundings"

Lost me on that one Gordon.

Good info, Gordon. I will refer the odd "I want to cool my system" member to this post. #bookmarked

I have been running a shower to help cool my water,i hope you dont mind me adding my observations to your thread.With my system of 12,000 litres i have a three tier shower placed over the sump, the sump is in a position where daytime temps are offset,an example today the tunnel touched 33c,the sump house 25c,air temp outside 29c and water left standing in the sump will stabilise around 25c, night time temps have got as low as 10c, water temp as of today where 17.5c rising to 21.5c this evening,so for me it is a very valuable way to achieve a lower water temp,our hot season will see night time temps stay up around 20/25c so it will be interesting to see where the water temps end up,last year we had 32c,so with the shower picking up the night time lows and the slowly increasing the temp during say time i am hoping its not going anywhere near as high.
Looking at your temp charts,by utilising a shower maybe you could offset your power usage by starting with cooler water in the morning as a by product of running a shower.

Sleepe, I'm referring to the kJ contained in the fat & sugar in I've cream, which accumulates around the stomach when you eat to much of it;)

1st Law of Thermodynamics - Energy can neither be created nor destroyed, you can only change the form

Dasboot- yes evaporating water is certainly a valid method of removing heat from the water, but only if you have the water available. Unfortunately I don't have enough to use it that way. I reckon I'd need to evaporate
100l or more per day to keep temps under control for trout, and it would be very difficult to impossible do in humid weather, when the dew point is above 20C

As much as they are nice fish to eat, it would seem clear from all that has been said that Trout are just not the right fish for Aquaponics if one is to use it as a sustaining food source without it costing too much.

nah, no way, trout are great in AP.
you can get them to size in a bit over 6 months. i would normally do yearlings to 1kg in about 7-8 months.
as long as you dont get rid of the solids, then for the few months over summer that you cant keep trout, there will still be enough nutes to power the plants until you get more trout in.... or have a separate tank with other fish too.

There doesnt always have to be fish in an AP system, same with plants.

i set up a few beds onto my mums pond which was quite dirty, the beds got filled with fish solids and algae, etc.
the bed design was bad and taken offline, but plants remained in there.
roughly 2-3 years later now and although growth was never great in those beds, nothing ever died, plants to this day are still surviving in there and growing. this is with 0 water circulation, no new nutes going in, no oxygenation per se and now when i dig in it all the solids have broken down and gone completely..... but things still live on.

dont under estimate how long systems can be left offline, etc.
trout which are heavily fed could potentially power a system for months and months of good growth after removal if enough solids are left to build up.

Can I take it then that you are buying in your fish on a regular basis each year. Now my first question would be how much is that costing you?
Because it is clear that you are not breeding. Here in France or for that matter in the uk they are asking high prices for Trout and as for fry they want to sell 1000 at a time.
But I take on board your point about not having the fish in the summer or replacing them with say goldfish, now that does make sense.

I don't want Aquaponics as a hobby although it is a good hobby to have, but I need to grow without the use of so much water and here in France the climate is changing fast, this is now the third year we have had very dry spells in the summer, I have plenty of land and two large Polytunnels but water is not free and my bills are getting larger each year.

So give us an idea what you are paying for each fish or even a picture of the size

Ps is that location Victoria NSW

Gunagulla there is something strange about your posts....the date posted how can you get them up here when they are posted tomorrow. Never noticed that before.

im in Victoria.

i cant give a photo sorry, but i buy yearlings each year (not this year) for $2 each if i remember correctly (could be $2.50), they charge about half for fingerlings.

the yearlings are about 10-15cm long (fingerlings about 7cm).

most people buy fingerlings as harvesting at around 500gm is enough for people here, but i buy bigger because i want to get the fish as big as possible, i would rather just kill and gut one fish for 2 feeds rather than 2 fish.

not sure what you mean exactly, but you may have read the dates wrong i think.

what a silly goose I am, good celebration last nite and then this morning reading this and see Gunagulla had date 16th but its January ..

think another laced coffee is in order

I put 40-90mm fingerlings into my system in Feb, and have already harvested a number of fish around the 1kg mark. The fingerlings cost me $1.10 each for 400. The cost goes up for purchasing later, so I make savings by buying them earlier, which in part goes towards paying off the chiller on my big system, which is required to keep them in the hottest months. However, my small system still has trout in it, and I've had a number of days in the high 30sC already- see the water and air temperature plots in my system thread. The small system still supports good plant growth during the period with no fish in it.