Thank you all for the ideas & suggestions.
I had mixed comments about using copper and today I decided and bought stainless steel tube. 7.5 mm diameter, 0.40 mm thickness, 6 meter long. I guess it shouldn't be hard to bend since it is very thin. It cost... let me convert... 6.7 USD. Is life getting any better than this?
As I mentioned earlier, I am planning to use the water from the evaporative cooling unit to cool down the
nutrient solution, since it is a source readily available. If I did not have that, then I would consider using some sort of vapor compression refrigiration unit such as a 2nd hand aquarium cooler. Note that this would cost a lot more to buy and operate, since it would consume much more energy.
Now I need to bend the steel tube into a coil form. Also I need a small pond
pump to circulate the cold water through the tube. I will use a thermostat that I already have to keep the solution at 22-24 degrees celcius. I will post the results.