There are of course a lot of different technologies around to utilise low-grade heat with temperatures of 80 to 150°C. Some are commercial, but the majority is still experimental. Some very interesting and unusual concepts and ideas out there.. This list is by no means complete., I just want to give some examples of the hugely different principles employed to generate electricity or mechanical energy from low-grade heat.
The Organic Rankine Cycle (ORC)
This is the furthest developed technology for low-temperature energy generation. In principle it is a standard steam turbine cycle, only it employs fluids which evaporate at temperatures well below 100°C. ORC systems operate under pressures of 6 to 20 bar.
NIFTE
The NIFTE (“Non-Inertive-Feedback Thermofluidic Engine”) or thermofluidic oscillator is a slightly more complex thing.
Thermoelectric generators – the Seebeck effect
This hermoelectrc effect is created when a temperature difference exists between two dissimilar conductors. hermoelectric elements are widely used in the oil and automotive industry, they are however limited to small power outputs (several watts) and requier temperstures of 200C or more to be effective.
Shape memory machines
Newcomen – shape memory
The Condensing Engine Project 