Hi rosneft, welcome!
Do you know more about the temperature fluctuations? Because, if the temperature is guaranteed to pass by 26°C at some time of the day or the night all the year, then the cheap solution is to have enough water, thermally insulated enough, and send it to an exchanger with air only when the air has an adequate temperature.
Or could you find deep seawater colder than 26°C, and if your project is big enough, pump this cold water to a heat exchanger when needed?
More generally, do you mean cheap procurement cost, or energy-efficient? What's the size? Do you have good sunlight everyday for sure?