To drive any heat engine, you need a temperature difference. Usually the hot end vaporises a liquid increasing it's volume, and the cold end condenses it again.

However, the laws of thermodynamics are very clear. The greater the temperature difference, the more energy can be converted. A low temperature difference will have a very low efficiency and very little of the heat can be used to make electricity, movement, etc.

The Carnot Theorum give the formula:

Maximum efficiency = 1- T cold/T hot. Temperature of hot and cold sides are expressed in degrees Kelvin.

In practice the efficiencies in industry is usually about half the theoretical. For a power station producing steam at 650 deg C, the maximum theoretical efficiency is 66%. The average around the world is 31%.

With more expensive materials the steam can be heated more and the efficiency can reach 43%.

Sources of  low grade heat are:

Ocean thermal

Geothermal

Waste heat can be recovered in the following:

Recovered Waste Heat (RWH), in the following industries:
- Oil and Gas
- Petrochemical and chemical
- Mining and minerals processing
- Ferrous and non-ferrous metallurgical processing
- Wood, pulp and paper
- Food, beverages and sugar
- Power Plants
- Energy from waste
Solar-Thermal Power
 

Source Granex