Heat pumps may obtain low-grade heat from the outdoor air in mild climates; in areas with average winter temperatures well below freezing, ground source heat pumps extract residual heat stored in the ground at a more constant temperature.

The energy obtained is in the form of low-grade heat of 70 to 80 C compared to an assumed 20 C ambient temperature.

A key feature of the technology is its use of low-grade 'waste' heat from a co-located power station to provide the evaporative drying energy.

This is commonly referred to as waste heat or "secondary heat", or "low-grade heat".

"It's about the cost of access, and about the value of low-grade heat."

So heat engines always produce a surplus of low-temperature waste heat, called "secondary heat" or "low-grade heat".

Efficiency is further increased when used in conjunction with a power plant or other source of low-grade heat that would otherwise be lost to the atmosphere.

In order to recover low-grade heat, the fluid generally has a lower boiling temperature than water.

Asimov described how energy flows through a life-support system from a low entropic state from which it moves all material resources, such as air, water, and food with the rejection of low-grade heat as the final energy output.

Near atmospheric operation permits corrosion free and scale-resistant polypropylene construction, and also allows the use of low-grade heat to drive the process.