I do not think you need to provide 24 hour humidity to the orchids. These plants appreciate a nice cool down at night. It becomes critical in the daytime when transpiration goes faster with the extreme heat and intense sun. Relative humidity increases at night and what you will need is adequate ventilation by then.
There are so many types of orchids. you did not mention what types you are actually growing. There are temperate ones which can take the extremes of temps as long as they also get good lighting/shade, good air circulation and watering as needed.
You can also try using solar powered fountains, that will run the water and provide humidity, as long as the panel receives good direct 8 hours sun.
