Different　from　solid　optical　filters,　the　nanofluid　filters　display　various　optical　properties　and　can　also　be　easily　pumped　at　low　cost.　However,　finding　a　proper　nanofluid　meeting　the　standard　of　practical　application　is　a　great　challenge.　In　the　present　study,　we　propose　a　novel　magnetic　electrolyte　nanofluid　(ENF)　for　the　first　time　which　can　be　potentially　used　as　liquid　optical　filters　for　hybrid　photovoltaic/thermal　(PV/T)　application.　By　dispersing　optimized　amount　of　magnetic　iron　oxide　(Fe3O4)　nanoparticles　in　50%　water/50%　EG　solutions　containing　methylene　blue　(MB)　or　copper　sulfate　(CS),　we　obtained　two　stable　ENF　filters　satisfactorily　meeting　the　optical　absorption　of　two　typical　PV　cells,　Si　and　InGaP,　respectively.　Due　to　its　magnetism,　ENF　can　be　considered　as　a　smart　liquid　optical　filter　controllable　by　external　magnetic　field.　The　root-mean-square　errors　(RMSE)　of　the　absorptance　between　the　ENF　filters　and　the　corresponding　ideal　filters　are　both　smaller　than　that　of　water-based　nanofluid　filters　containing　typical　core/shell　nanoparticles　while　their　thickness　is　much　smaller.　The　stability,　particle　size　distribution　and　thermal　conductivity　of　the　optimized　ENF　filters　were　investigated.　It　was　found　that　both　ENF　filters　have　good　stability　and　higher　thermal　conductivities　than　their　base　fluids.　Performance　of　ENF　filters　in　PV/T　system　was　evaluated　based　on　a　reported　theoretical　model.　The　results　showed　that　the　values　of　merit　function　(MF)　for　the　two　types　of　ENF　based　PV/T　systems　are　both　higher　than　that　of　typical　core/shell　nanoparticle　nanofluid　filters.　Overall,　our　study　demonstrates　that　ENF　represents　an　efficient,　low-cost　and　smart　liquid　optical　filter　for　the　application　in　PV/T　system.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.