From the pen of Fabrizio Gallelli, directly on the italian language site www.ingegneri.info, here’s an interesting article on the topic of Heat pumps, explaning how this kind of thermal machines work, achieving energy efficiency using a combination of a compressor, a condenser, an expansion valve and an evaporator.
The topic is mostly referred to civil and residential conditioning and HVAC, more than to chillers for industrial refrigeration. Anyway it helps in a clear way on the kinds of heat pumps existing, and working principles of this coolers/heaters.
Heat pumps provide heat from a cold source, ground, water or environment air, absorbing heat from the cold source employing electric power and transferring thermal energy to a heat sink. A heat pump can be three to four times more efficient than simple electrical resistance heaters.
The cycle can also be reversible obtaining residential conditioning and cooling during summer.