title: Efficiencies and coefficients of performance of heat engines, refrigerators, and heat pumps with friction: a universal limiting behavior.
abstract: For work-producing heat engines, or work-consuming refrigerators and heat pumps, the percentage decrease caused by friction in their efficiencies, or coefficients of performance (COP's), is approximately given by the ratio W(fric)/W between the work spent against friction forces and the work performed by, or delivered to, the working fluid. This universal scaling, which applies in the limit of small friction (W(fric)/W </~ 20%) and when the engine's figures of merit (FOM's, either efficiencies or COP's) do not come too close to unity (no higher than, say, 0.5 in the case of heat-engine efficiencies), allows a simple and quick estimate of the impact that friction losses can have on the FOM's of thermal engines and plants, or of the level of those losses from the observed and predicted FOM's. In the case of refrigerators and heat pumps, if W(fric)/W </~ 20% is not ensured (actually a condition that can be largely relaxed for heat engines), the COP percentage decrease due to friction approaches asymptotically (W(fric)/W)/(1+W(fric)/W) instead of W(fric)/W. Estimates for the level of frictional losses using the Carnot (or, for heat engines and power plants only, the Curzon-Ahlborn) predictions and observed FOM's of real power plants, heat engines, refrigerators, and heat pumps show that they usually operate in domains where these behaviors are valid.
authors.affiliation: Associação Euratom-IST, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal. bizarro@ipfn.ist.utl.pt
authors.forename: João P S
authors.initials: JP
authors.lastname: Bizarro
authors.forename: Paulo
authors.initials: P
authors.lastname: Rodrigues
mesh_terms: D003198:Computer Simulation; D004735:Energy Transfer; D017276:Friction; D006361:Heating; D008962:Models, Theoretical; D012034:Refrigeration; D012212:Rheology / Q000379:methods*; D013816:Thermodynamics