Venting the attic space during summer to cool the house
Given that hot air raises, have you considered a Passive Stack Ventilation system.  Basically:


A tall pipe coming out of the top of your loft so the hot air can raise
You must not have bends in the pipe or it does not work as well
The topping on the pipe must not add much to the flow resistance
The sizing of the pipe is important (sorry can’t help on this)

Too wide you don’t get fast enough upwards air movement. 
Not wide enough, you get to much air resistance

The pipe must come out of the top of the loft and be high enough above the roof
Soffit venting of some type so the colder air can get in at the bottom


The system will only tent to pull air though the loft when the outside air is colder than the air in the loft (just what you need).  

Given the correct conditions and design, a Passive Stack Ventilation system will work forever with no running cost or maintenance.  Get anything wrong and it will not work at all.

Also check that your ceilings are well sealed, otherwise all the expensive cold air from your AC will just go up into the loft and be vented out.