A　single-screw　expander　(SSE)　is　a　type　of　volumetric　expander　that　can　be　applied　to　low-grade　heat　and　waste　heat　recovery　utilizing　the　organic　Rankine　cycle　system.　In　this　paper,　a　new　thermodynamic　modeling　of　SSEs　considering　the　friction　power　consumption　is　carried　out　to　examine　the　performance.　The　mathematical　model　was　verified　by　experimental　results,　and　the　calculation　results　have　a　good　agreement　with　the　experimental　results.　The　clearance　height　has　great　influence　on　the　expander　performance.　The　influence　of　clearance　height　on　the　leakage　flow　rate,　friction　power　consumption,　volumetric　and　isentropic　efficiency　were　investigated.　Results　show　that　path　L-8,　L-7　and　L2+4　are　the　three　main　leakage　paths,　and　path　L-8,　L-7　and　L-9　are　the　three　main　friction　power　consumption.　Compared　with　the　meshing　clearance　height,　the　fitting　clearance　has　a　more　significant　effect　on　the　performance　of　SSEs.　Setting　a　higher　fitting　clearance　and　meshing　clearance　is　an　effective　measure　to　reduce　friction　power　consumption　and　prevent　gaterotor　wear,　respectively.　It　would　be　favorable　if　the　fitting　and　meshing　clearance　height　is　no　less　than　0.04　mm　and　0.02　mm,　respectively.　(C)　2018　Published　by　Elsevier　Ltd.