According to literature I have read, Lettuce will be bigger at 24 degrees celsius at the root temperature. Probably the same reason why lettuce is usually a "cool weather" plant.
Uh... please forgive my newbie ways. I have not been actively in forums for over 13 years and I have to relearn the etiquettes and direct quotes. Thank you for the overwhelming responses and ideas, let me try to give a few feedback.
1. No large body of water near me. I live in the city and space is a big issue. This project is really targetting an urban setting.
2. I have thought about burying pipes underground... from literature and actual small testing, my ground temperature is around 25 degrees celsius. I have actually used this to some advantage as well when I designed the system to have my sump buried 75% on the ground. Burying pipes underground to cool the nutrients would probably be my last option since space is a major issue and the pipes needed are not cheap either.
3. I'm no expert on evaporative cooling, however, my gut feel is that it's probably not going to be very effective because the temp difference is really very small. Example, yesterday the water temp was at 27.9 degrees.
4. 24 degrees celsius requirement is actually for the plant roots because according to research, you can grow lettuce really big if you keep the root temperature at such temps. For a commercial venture this is very important.
5. I know that peltier is a "lossy" device. But one major disadvantage that I have here in the philippines is the cost of electricity. I believe we have one of the world's most expensive electricity. So since Peltier can be operated at low power (i.e. solar panels) I thought it might just do the trick. Power on during the day only.
6. There was another idea I was toying with but I'm not sure if it's uh... legal here. Somebody told me that in some remote areas where their is no electricity they actually use a refrigerator that is powered by a "candle" called an ammonia cooler. It was cold enough to make ice so I guess that could be another solution for me. Barring the legal issue of operating an ammonia based cooler maybe there are other "refrigerants" with the same idea I could use. A solar panel with heating element would replace the candle obviously.
Hope these feedback will keep until Monday. I'll be off volunteering as a councillor in youth camp for the weekend. TTFN and thanks again!
Electricity rates in Manila, Philippines where Meralco is the provider.