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.
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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!
