Collector　storage　solar　water　heater　systems　that　use　phase　change　materials　as　heat　storage　media　have　attracted　increasing　attention　because　of　their　clean,　pollution-free　properties　and　high　energy　storage　density.　The　design　and　application　of　these　systems　require　efficient　and　optimized　phase　change　thermal　storage　units　(TSUs).　This　study　aims　to　optimize　a　TSU　by　using　theoretical　analysis　methods　and　to　fill　the　research　gaps　attributed　to　the　lack　of　theoretical　analysis.　The　TSU　in　this　work　comprises　a　multichannel　flat　tube　and　rectangular　fins　as　heat　exchange　elements.　The　performance　levels　of　this　TSU,　including　its　effectiveness,　compactness　factor,　gravimetric　specific　power,　and　fin　efficiency,　under　different　structures　are　compared.　The　economic　viability　and　energy　efficiency　of　the　proposed　solar　collector　phase　change　thermal　storage　system　are　preliminarily　evaluated.　Results　indicate　that　the　structure　of　the　multichannel　flat　tube　TSU　is　relatively　reasonable　when　the　fin　aspect　ratio　(Hfins/x)　and　thickness　are　8.60　and　0.001　m,　respectively.　The　effectiveness　of　the　optimized　TSU　during　charge　and　discharge　processes　respectively　increases　by　52.0%　and　16.3%　relative　to　the　original　multichannel　flat　tube　TSU.　The　corresponding　recommendation　for　the　use　of　multichannel　flat　tube　TSUs　in　household　baths　is　four　groups/person.　©　2020　Elsevier　Ltd