Direct recovery comes first
If a hot stream can heat a colder stream through a practical exchanger, direct recovery uses no compressor. It usually has lower capital cost, fewer moving parts and no refrigerant circuit. The usable temperature difference must still cover exchanger approaches and fouling margins.
Add a heat pump when temperature blocks reuse
A heat pump is valuable when the source is abundant and simultaneous but too cool for the sink. It may also recover latent heat from vapour or provide cooling and heating together. The electrical input and upgraded heat must be counted at the same boundary.
A common combined design
Use direct exchange for the first preheat step, then use a heat pump for the remaining lift. This reduces compressor work and may allow a smaller machine. A boiler can remain for startup, peaks or the final high-temperature step.
Sources
Primary and authoritative sources used for the material claims on this page.