Optimal design of solar thermal systems with thermal storage for continuous industrial process-heat

Solar thermal systems offer a promising pathway for industrial decarbonisation, yet optimal design strategies for continuous process-heat applications remain insufficiently characterised. This study develops a pseudo state modelling and optimisation framework combining hourly thermal simulation with techno-economic analysis to determine cost-effective configurations of collector area, storage capacity, and control strategy. The model was verified against published literature (agreement within ±3%) and applied to a continuous-duty dairy-processing facility in Pune, India, requiring 1.68 MW of thermal power at 90 °C supply temperature. After a comprehensive optimisation across different system configurations, the optimiser identified an economically optimal design comprising an 11,000 m 2 evacuated tube collector-field and 1150 m 3 thermal storage (storage-to-collector ratio 0.105 m 3 /m 2 ), achieving 90% solar fraction with total annualised cost of US$0.818 × 10 6 y −1 , a 23% reduction compared to moderately-sized baseline configurations. The levelised cost of thermal energy was US$61.9 MWh −1 , approximately 56% of local electricity tariffs, demonstrating economic competitiveness against electric resistance heating. A three-stage collector-loop flow control — unrestricted, minimum-flow and floating outlet is shown to be essential for a realistic operation under variable irradiance. A stratified-storage sensitivity analysis confirms that the well-mixed assumption used in the main optimisation is conservative; under matched floating control, a properly stratified system achieves SF = 93% and LCOH = US$56.7 MWh −1 at the same configuration. The well-mixed and thermocline models are reported as bounding extremes; a real industrial tank operates between these two limits. The system avoids approximately 9225 t CO₂ y −1 against the latest Central Electricity Authority (CEA) grid emissions factor (0.738 t CO₂/MWh, FY 2024–25). The results show that proper sizing of collector area, storage volume, heat exchanger area and control strategy, rather than maximisation of solar fraction, is critical for economically viable industrial solar thermal integration; competitiveness against natural gas depends on local fuel and carbon pricing.