Air Conditioning a Greenhouse - uncommon but not unheard of ....
Air conditioners (or heat pumps) are expensive to operate but they may be appropriate for some greenhouses depending on the use. Most of the heat gain / heat lost is through the roof so it's more common to see air conditioners on sunrooms with solid roofs.
Singapore's National Orchid Garden has an air conditioned greenhouse for mountain orchid's. It wasn't open when I was there but maybe soon?
I haven't seen any btu calculators for greenhouse AC but maybe the supplier of the equipment can provide an estimate. It may
not get it down to 75 degrees but 85 or 90 is a lot better than 140. Make sure the structure is tight and invest in some shade cloth or insulated panels for the roof!