Author(s): Mustafa Abdulhussein Lafta

Abstract: This study reports a combined experimental and numerical investigation of a hybrid solar thermal system in which a single-glazed flat-plate collector (aperture area 1.5 m²) supplies a plate heat exchanger that serves as the evaporator of a small organic Rankine cycle (ORC) driving a micro-expander, while the condenser delivers low-grade heat for domestic use. The system is therefore analysed as a combined heat-and-power (CHP) unit rather than as a pure power plant. A complete thermodynamic model is presented, comprising the Hottel–Whillier–Bliss collector equation with an explicitly evaluated heat-removal factor, an effectiveness–NTU description of the evaporator, a second-law formulation of the expander, a hydraulic model of the circulation loop, and a component-resolved exergy balance referred to the Petela radiation exergy. Experiments were conducted outdoors under steady-state conditions for mass flow rates of 0.01–0.05 kg s^?1 and irradiance levels of 600–1000 W m^?2, with a collector inlet temperature held at 60 °C. A three-dimensional conjugate heat-transfer model was solved in ANSYS Fluent using the k–? SST closure with y+ ? 1 and was verified by a formal grid-convergence study (apparent order p = 2.00, GCI = 1.19 %). Propagated measurement uncertainties, evaluated by the Kline–McClintock method, are ±2.0–2.3 % for useful heat gain, ±2.6–2.8 % for energy efficiency, ±2.8 % for exergy efficiency and ±3.1 % for net mechanical power. Predicted and measured outlet temperatures agree within an RMSE of 2.35 % and a maximum deviation of 2.97 %. The measured collector thermal efficiency rises from 59.1 % to 63.5 % as the flow rate increases from 0.01 to 0.05 kg s^?1, while the system energy efficiency rises from 56.1 % to 60.2 %. Net mechanical power and exergy efficiency, in contrast, exhibit a clear interior maximum at 0.03 kg s^?1, where 29.34 W and 3.11 % are obtained; beyond this point the cubic growth of pumping demand outweighs the marginal thermal gain. The exergy balance shows that 93.25 % of the incoming radiation exergy is destroyed in the collector through optical absorption and radiative degradation, whereas the expander (48.3 %) and the evaporator (41.6 %) dominate the component-level, technologically avoidable destruction. These findings identify the source temperature — not the collector thermal efficiency — as the binding constraint on thermo-mechanical conversion in flat-plate-driven solar CHP systems, and quantify the operating point at which the trade-off between heat collection and mechanical work extraction is optimal.

DOI: https://www.doi.org/10.22271/27078043.2026.v7.i2b.144

Pages: 120-131 | Views: 23 | Downloads: 10

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