No abstract

The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage.

Molecular solar‐thermal energy storage systems are based on molecular switches that reversibly convert solar energy into chemical energy. Herein, we report the synthesis, characterization, and computational evaluation of a series of low molecular weight (193–260 g mol−1) norbornadiene–quadricyclane systems. The molecules feature cyano acceptor and ethynyl‐substituted aromatic donor groups, leading to a good match with solar irradiation, quantitative photo‐thermal conversion between the norbornadiene and quadricyclane, as well as high energy storage densities (396–629 kJ kg−1). The spectroscopic properties and energy storage capability have been further evaluated through density functional theory calculations, which indicate that the ethynyl moiety plays a critical role in obtaining the high oscillator strengths seen for these molecules.

Red-shifting the absorption of norbornadienes (NBDs), into the visible region, enables the photo-isomerization of NBDs to quadricyclanes (QCs) to be driven by sunlight. This is necessary in order to utilize the NBD-QC system for molecular solar thermal (MOST) energy storage. Reported here is a study on five diaryl-substituted norbornadienes. The introduced aryl-groups induce a significant red-shift of the UV/vis absorption spectrum of the norbornadienes, and device experiments using a solar-simulator set-up demonstrate the potential use of these compounds for MOST energy storage.

As a larger fraction of energy is based on solar energy and other renewable energy sources, technologies for energy storage and conversion is becoming increasingly important. Molecular solar thermal (MOST) is a concept for long-term storage of solar energy in molecules and release of the energy as heat with full regeneration of the initial materials. The process is inherently closed cycle and emission free. No assessment of the fundamental efficiency limits of the technology has been made. In this report, efficiency limits and fundamental factors for molecular design of molecular solar thermal systems are discussed. Maximum efficiencies and potential temperature gradients are estimated using a number of basic assumptions on desired storage lifetimes and energy losses. The predicted maximum solar energy conversion efficiency is 10.6% at a S 1 −S 0 gap of 1.89 eV. At this S 1 −S 0 gap, the stored energy is able to create temperature differences of ∼300 °C. Several existing systems have an energy storage density in line with the predicted maximum one but do so at larger than optimal S 1 −S 0 gaps.