How do you create electricity from 'waste' heat generated by natural gas compression?

Utilizing technology developed and manufactured by Ormat, the waste heat units constructed at Alliance Pipeline’s four compressor stations in Saskatchewan recover the exhaust heat from natural gas turbines (which compress the gas to transport it through the pipeline,) and convert it into electricity. A refrigerant in the installed energy converter unit captures the heat from the compressor station exhaust and then flows through turbines as a vapour to generate electricity. This ‘waste’ heat would otherwise be vented into the atmosphere. Electricity generated from waste heat is environmentally responsible: no greenhouse gas or other emissions are produced.

For more information:
General Waste Heat Fact Sheet (PDF File - 60k)
Waste Heat Generation Animation (SWF file - 9.5 MB)