In　this　study,　a　novel　solar-assisted　auto-cascade　heat　pump　cycle　(SAHPC)　operating　with　the　zeotropic　mixture　of　R32/R290　for　small　water　heaters　is　proposed.　In　the　SAHPC　system,　a　cascade　heat　exchanger　(CHEX)　associated　with　a　phase　separator　is　used　to　achieve　auto　cascade　cycle　and　enhance　the　overall　system　performance.　The　performances　of　the　SAHPC　are　evaluated　by　using　the　developed　mathematical　model,　and　then　compared　with　the　conventional　air-sourced　heat　pump　cycle　(CAHPC).　Simulation　results　show　the　SAHPC　has　4.23-9.85%　and　4.37-9.68%　improvements　in　COP　and　volumetric　heating　capacity　compared　with　those　of　the　CAHPC,　respectively,　under　the　same　operating　conditions.　However,　the　improvement　of　performance　of　this　novel　cycle　largely　depends　on　the　absorbing　heat　ratio　and　the　zeotropic　composition.　It　is　expected　that　this　new　cycle　will　be　beneficial　to　developing　dual-source　coupled　heat　pump　applications.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.