## 2 Overview  
The City of Surrey has committed to achieving net-zero emissions by 2050. In 2007, buildings in Surrey emitted 911,000 \(\text{tCO}_{\text{2e}}\), more than 40% of the City's total emissions [13]. The Surrey City Energy (SCE) is part of Surrey's plan to reduce emissions from buildings. It is projected for the network to service more than 1,600,000 m\({}^{2}\) of built area by 2040, reducing the per unit area GHG emissions (\(\text{kgCO}_{\text{2e}}/\text{m}^{2}\)) from its service area by more than 70% compared to the 2007 baseline [13]. The newly built facility uses high-efficiency natural gas boilers. The City plans to connect more than 20 high-density buildings in Surrey's City Centre to a downtown DEN which will run on a portfolio of renewable energy sources, including geothermal exchange, biomass, waste heat, and sewer heat recovery [14].  
In collaboration with the City of Surrey, our team has developed a model for thermal energy storage (TES), short-term storage, using a water tank for peak shaving, improving DEN efficiency, and reducing capital cost, as well as serving as a connection point between the proposed M-TES and the DEN [15]. To supply low-carbon heat to SCE cost-effectively and energy-efficiently, a new M-TES has been proposed to harvest and transfer freely available waste heat from industrial sites, where the low-grade energy (sources with temperature less than 90 \({}^{\circ}\)C) is released to the ambient unused, to Surrey's DEN. The M-TES system should be competitive with other available low-carbon sources, see Figure 1. Estimated levelized costs for other low carbon sources in this study were given by Surrey's DEN.  
The system energy storage density (ESD) of M-TES is a key performance indicator in determining M-TES feasibility and effectiveness. The system-level ESD is a function of the storage material ESD and the system efficiency in extracting and delivering the stored heat from the storage material. M-TES systems that are based on phase change materials (PCM) may not be suitable for integration into SCEs mainly due to their low ESD\(\sim\)0.25 MJ/kg [16]. Moreover, they require large, embedded heat exchangers to travel with the M-TES container, which further lowers the system-level ESD and increases transportation costs. Their low thermal conductivity is another challenge that increases the \"charging/discharging\" process and waste heat source temperature [17]. The economic and environmental feasibility of PCM M-TES systems has been studied in [12]. There is a need for the development of compatible thermal energy storage with high ESD and charge/discharge temperatures suitable for the industrial waste heat sources available for SCE. A comparison between the material-based ESD and the system level ESD for heat and cold storage extracted from the literature shows a wide range of ESD (0.260-1.603 GJ m\({}^{-3}\)) [18]. Thermochemical materials (TCM) are promising candidates [19]. TCM storage such as LiBr/H\({}_{2}\)O solution is based on the application of reversible chemical reactions. They are well-matched to our M-TES requirements due to their stable reversibility and high reaction enthalpies, resulting in high ESD. TCM solutions can be transported in tanks and can be readily pumped in and out without the need to transport heat exchangers. System-level ESD is always less than the material level ESD depending on the bulkiness and inefficiency of the system. The minimum system-level ESD that is required to make M-TES economically feasible is determined in this work.