Coolant for power armour.

[Zork13]

I thought it would be a good idea for power armour to require coolant. Coolant would slow AP drain. Without coolant the armour will damage itself. As a hardcore addition if the armour gets too hot it hurts you, and or the core goes critical and ejects. The catch of the core ejecting is that you can't install a new core til it has cooled down, and to make it even more difficult make it so you have to exit your frame to reload cores.