I know that the airconditioners and refridgerator work on this principle, but these gadgets use costly parts and high voltage to achieve this.
That's the best current technology can do. If you'd like a measure of efficiency of doing this you can use the EER for air conditioners, for example. The number tells you how many BTU's of cooling is achieved for each watt of power consumed. An EER of 12, for example, means for each watt of electricity you pay for, you get 12 BTU's of cooling...pretty good these days.
If you look at typical window air conditioning units in Home Depot for example, maybe 5,000BTU to 12,000 BTU range (approximately) you'll see EER's range from about 9.8 to around 10.8. Central air conditioning units in homes might be about 13, much, much more efficient.
Exterior air conditioners can be used for efficiently when kept in the shade, say under a large tree which blocks direct sun, but allows for free airflow.
I wonder why they don't use the same measure for home refrigerators?? Instead, they utilize an "energy use" measure reflected on the yellow energy tags.
One way to make home refigs more efficient would be to exhaust heat in the summer and retain that heat inside in winter. But I'm guessing the cost of ducting and automatic shutters and heat loss and gain of the ducting itself means it's a long,long,long,payback.
"For example, I saw in one science museum that there was one thin copper pipe kept, over which air was blown fast from one end. In a few seconds, the temperature at one end of the pipe dropped considerably."
That sounds like a heat exchanger...if the pipe was already hot or the air cooled, sure that's what happens, but the heating is what takes most energy, not the movement of air. Pressure changes alone do not produce cooling, it's a means of achieving temperature gradients.
If you search THERMOACOUSTIC REFRIGERATION you'll find experimental work being conducted at Rutgers and Penn State.
