What is the difference between topping and bottoming cycle CHP?

These terms are used to describe two different types of combined heat and power (CHP) systems which utilise waste heat to increase the efficiency of the power generation process.

Topping Cycle CHP

Topping cycle CHP is the process of using fuel to generate mechanical energy which is used to create electricity using a generator. The resulting waste heat is captured from this process to heat water for use as hot water or steam throughout the facility. 

Topping Cycle CHP - image US Energy Information Administration
Topping Cycle CHP - image US Energy Information Administration

The topping cycle style CHP systems are the most popular due to their versatility. Inoplex cogeneration systems utilise topping cycle CHP style power generation.

Bottoming Cycle CHP

Bottoming Cycle CHP, sometimes called Waste Heat to Power (WHP), is where fuel is combusted to produce useful heat to carry out an industrial process of some sort. Some of the waste heat from this process is recovered via a heat exchanger, producing thermal energy which is used to power a turbine generator to produce electricity. 

Bottoming Cycle CHP - image Energy Information Administration
Bottoming Cycle CHP - image US Energy Information Administration

Currently a bottoming cycle CHP system requires a heat source of sufficiently high temperatures (usually over 500 °F or 260 °C) to be practical.

Energy intensive industries have the most to gain from bottoming cycle combined heat and power systems. Ongoing research and development in the use of working-fluids which can utilise lower temperature waste streams further boost the opportunities for industry to use bottoming cycle CHP.

Both topping and bottoming cycle CHP produce either locally used or grid exported electricity.

Ref - US Energy Information Administration https://www.eia.gov/workingpapers/pdf/chp-Industrial_81415.pdf