Aqueous Accumulator

The Aqueous Accumulator works if it is placed with two or more sides touching water source blocks (simply in a pool of water). As soon as that requirement it met, it will start filling up with water (it requires no power).

As soon as there is water inside the machine and at least one Waterproof Pipe or Liquiduct connected to any of the 5 possible outputs, it will automatically start pumping water out. It will eject the water at a rate similar to a BuildCraft Pump being powered by a couple of Redstone engines. Approximately one bucket every two seconds.

The Aqueous Accumulator will not function in the Nether, since there's no water there.

Source
Thermal Expansion wiki