Steam engine
Steam engines are used to produce electricity. They do this by taking in hot water and converting it into electricity. Thus the hotter the water the more electricity can be produced. If there is an overabundance of electricity then steam engines will reduce their production to match consumption.
Hovering the mouse over a steam engine will display information for that particular steam engine: current production as well as maximum productivity.
It is possible to chain multiple steam engines together as water will pass through them. The current optimal ratio is 1 offshore pump : 14 boilers : 10 steam engines
Steam engines themselves don't produce any pollution. (Boilers do!)
Using other liquids?
The steam engine can use any type of liquid to generate electricity. This can be used to get rid of unused oil-products. Just fill unusable oil products into a steam engine and it will destroy the liquids. It works the same as for cold water; you don't need to warm it. If you heat the liquid before that, the hot oil will produce electrical power, just like with hot water.
But of course you should convert the unusable oil products into useables, instead of just making electricity out of it!
See also Oil Refinery RANT
Examples
