Molecular solar thermal storage integration for domestic hot water co-heating: experimental discharge characterization and numerical feasibility assessment

Journal article, 2026

Molecular solar thermal storage (MOST) systems represent an emerging closed-loop technology with energy
storage densities up to 1.6 MJ/kg, more than double that of typical phase change materials. MOST systems store
solar energy in the chemical bonds of photoswitchable molecules via photoisomerization, releasing heat on
demand during molecular reversion. Despite extensive research, their potential for domestic hot water co-heating
remains unexplored. This study investigates the potential integration of MOST systems into a conceptual
laboratory-scale co-heating device by combining experimental discharge characterization with numerical thermal
simulations. Laboratory experiments evaluated the discharge characteristics of a norbornadienequadricyclane
MOST system, demonstrating thermal activation for the first time under varying activation temperatures
and residence times. A numerical thermal simulation model was developed to assess the performance
of two norbornadiene-quadricyclane systems designed for short- and long-term storage. Parametric studies
analyzed the impact of molecular properties and device design parameters on discharge and water heating
performance. Laboratory results demonstrated 95–100% back conversion of quadricyclane to norbornadiene at a
molecular concentration of 1.3 mol/L (290.2 g/L) in toluene. Simulations indicated activation-normalized waterheating
ratios of 43–86% for the long-term system and 106–204% for the short-term system, assuming perfect
insulation and current molecular and device constraints, representing idealized upper-bound estimates. Among
the MOST molecular properties evaluated, thermal enthalpy had the strongest sensitivity effect on discharge and
water-heating performance. Additional gains were achieved through increased energy storage capacity, optimized
molecule concentration, and adjusted device operational temperatures. Future research should prioritize
molecular enhancements, catalytic strategies to accelerate back-conversion, and device design optimization to
minimize heat losses and improve scalability.