Partially evacuated spaces with small volumes of HTF (Heat Transfer Fluid) Phase Change Materials (PCMs), called thermal diodes, can minimise heat losses in ICSSWHs. However, the collection and retention performance of thermal diode ICSSWHs is material dependent as well as influenced by environmental conditions. To investigate this condition, three laboratory scale thermal diode 1CSSWH prototypes were experimentally evaluated with different component materials and volumetric capacity. The units were tested indoors under constant solar simulator irradiance for 6 h to determine heat collection, followed by an 18-h cooling period to determine heat retention. In addition, the water temperature in storage was raised to desired levels using a refrigerated/heating circulator and prototypes left to cool overnight in stable ambient air conditions. ICSSWH 1 with 16.7 L of storage capacity, had absorber and evaporator components of aluminium and stainless steel, respectively whilst ICSSWHs 2 and 3 had vessels made from stainless steel with 16.7 and 27.7 L storage capacity, respectively. The mean 6-h collection efficiencies for ICSSWHs 1, 2 and 3 were 47.4%, 51.6% and 48.0%, respectively. Normalised water temperature profiles, retention efficiencies and thermal loss coefficients suggest that the performance of ICSSWH 2 and ICSSWH 3, are preferable compared to ICSSWH 1. (C) 2019 Elsevier Ltd. All rights reserved.