comparassion between heat engine and refrigerator
An ideal refrigerator works like this: 1- A refrigerant is compressed in a compressor, the fluid is compressed isentropically (entropy or s=constant) and is forced into the super heated range. The fluid then runs through the heat exchanger (evaporator?) to release the heat into the surroundings, this is done at constant pressure. The fluid then enters the evaporator, this stage give the fluid a quality (percentage of fluid and gas), this stage is done with constant entropy (h=const). The fluid in then run through the second heat exchanger. Once it is run through the expansion valve it drops in temperature. This cooled fluid is run though the second heat exchanger (condenser?) and EXTRACTS heat from the space you are keeping cold. The fluid is then run through the compressor and the cycle is complete. A heat pump works the same way EXCEPT the cycle is literally run in reverse and is used to keep an area warm instead of cold. These 2 cycles have a work input. An example of a heat engine is a steam power plant. This will have a pump, a boiler, a turbine, and a condenser. The fluid is pumped into the boiler where heat is added. The fluid is superheated and run through a turbine where the energy is extracted. The fluid is condensed down and run through the pump again to complete the cycle. A heat engine can also be used is piston form. where the warm fluid expands and moves the piston to create an output work. The efficiency of a refrigerator is expressed as COP (coefficient of performance) and is usually shown as BETA. COP=QL/Work=QL/QH-QL=1/(QH/QL)-1 where QL is the heat lost and QH is the heat gained. COP can be larger than 1 because of isentropic inefficiencies (heat lost during compressor and pressure loss in pipes)
