Design and Performance Evaluation of a Pressurized Closed-Loop Flat-Plate Solar Thermal Collector for Urban Domestic Hot Water Supply

Abstract

   Domestic water heating accounts for a substantial share of urban household energy demand in hot, sunny regions, yet conventional open-loop solar water heaters in such climates suffer from evaporative losses, scale formation, and stagnation-overheating. This study reports the design and three-day field characterization of a pressurized closed-loop flat-plate solar thermal collector intended for rooftop urban deployment. The unit comprises a 2 m² aluminum-copper absorber with a selective coating, anti-reflective safety glazing, a sealed glycol-water primary loop maintained at elevated gauge pressure by an expansion vessel and safety relief valve, a brazed-plate heat exchanger to a domestic storage tank, and a low-voltage circulation pump that may be driven by either grid alternating current or a dedicated 24 W photovoltaic module for off-grid operation. Three sensors monitor inlet, outlet and tank temperatures, while a turbine flow meter records the mass flow rate. From the measured temperature rise and mass flow, the useful heat gain Qu, instantaneous thermal efficiency η and daily energy yield were computed under clear-sky conditions in northern Iraq during summer. Across three test days, the average outlet temperature peaked at 44 °C around solar noon, the corresponding peak useful heat gain reached 1.26 kW, and the daily-integrated thermal efficiency varied between 0.61 and 0.64. The instantaneous efficiency in the steady solar window from 10:00 to 16:00 averaged 0.50 to 0.55, in agreement with published flat-plate collector data. A propagated relative uncertainty of 11% to 16% is reported for the efficiency, depending on the temperature rise. The contribution of this work is therefore not the invention of a new collector but a transparent, reproducible characterization of a pressurised flat-plate unit operating under Iraqi urban summer conditions, together with a clarification of the boundary between the collector itself and the steam-cycle prime mover that it could conceivably feed. The system is not a steam power plant; it is a domestic water heater whose thermal output is quantified here for the first time at this site.