Ian Choveaux

In order to facilitate Canada’s recently announced ban on the sale of internal combustion engine vehicles by 2035, the need for reliable, low-cost, low-emission charging infrastructure is more pressing than ever (Clean Energy Canada, 2021; Scherer, 2021). Previous research has shown that solar photovoltaic (PV) plus battery storage is not feasible in the Alberta context, requiring additional storage capacity or energy drawn from the grid in order to meet charging requirements (Lefebvre, 2018), providing an impetus to determine if there were a lower-cost option available for producing on-site generation for electric vehicle (EV) charging infrastructure.

My research focused on the use of thermal energy storage, particularly a novel phase change material known as miscibility gap alloys (MGAs). MGAs are a type of binary metallic phase change material (PCM) composed of two thermodynamically stable, immiscible metals in which “discrete, fully encapsulated particles of a lower melting point metal are contained within a dense matrix of a higher melting point metal” (Kisi, et al., 2018) – analogous to a chocolate chip muffin in which the cake provides a matrix for the hot chocolate chips which retain their latent heat. MGAs operate under a narrow temperature range (±50°C, heat) to store energy as a combination of the “latent heat of fusion of the lower melting point metal and 100°C of sensible heat storage” (Kisi, et al., 2018). The narrow temperature range of the latent heat of the miscibility gap alloys enables precise control of system parameters.

MGAs provide benefits over existing phase change materials as they have high thermal conductivity (50 – 200 times greater than the majority of installed thermal storage materials) and high energy density and employ conductive rather than convective heating, which enables rapid, uniform heat distribution, and allows for greater energy storage in smaller volumes, thus reducing plant footprint and associated system costs (Kisi, et al., 2018; Reed, Sugo, & Kisi, 2018). Owing to the higher energy storage density per unit volume, and greater thermal conductivity, miscibility gap alloys exhibit decreased time delay between discharge-recharge cycles than other phase-change materials(Sugo, Kisi, & Cuskelly, 2013).

Operating across a broad range of temperatures (232℃ to 1414℃), MGAs are versatile and adaptable, making them suitable for many different applications, including space heating, industrial processes, waste heat recovery, concentrated solar thermal energy generation, and energy storage  (Sugo, Kisi, & Cuskelly, 2013).

As MGAs remain and behave as a solid during operation, the storage unit can be composed of modular blocks. The modular, scalable nature of the blocks enables opportunities for re-use and recycle as the blocks can be re-configured for new or different applications, and, as MGAs are composed of immiscible metals, they can be readily separated by melting and recycled at end of life (Kisi, et al., 2018). As MGAs are thermodynamically stable, they are expected to function for decades with minimal maintenance, providing additional benefits over battery storage systems which are prone to degradation through multiple charge-discharge cycles.

Additionally, lower conversion losses for thermal energy storage systems provide for more efficient storage than thermochemical, chemical and mechanical storage means, suffering only from environmental losses, which can be on the order of a few percent per day (Sugo, Kisi, & Cuskelly, 2013).

The US National Renewable Energy Laboratories (NREL) has utilized MGAs to create a modular CSP system known as the solar thermoelectricity via advanced latent heat storage (STEALS) system.

The STEALS system is a novel, fully-integrated solar electricity-generating technology that includes the solar receiver, phase-change material (PCM) thermal storage (in the form of miscibility gap alloys (MGAs), heat pipes, thermal valve, thermoelectric generators, and heat rejection in a single module, providing cost-effective, dispatchable power at a variety of scales, ranging from 10kW to 20MW  (Olsen, et al., 2016; Glatzmaier, et al., 2017; Rea, et al., 2018).

The design of the system, as a solid-state device that incorporates latent heat thermal energy storage combined with a thermal valve, eliminates the need for piping, valves, and pumps associated with circulating heat-transfer fluid as part of conventional concentrated solar power system designs, thus reducing operation and maintenance costs while enabling the STEALS system to deliver near-constant power generation at times shifted from peak sunlight hours, to peak demand hours, providing dispatchable electricity on demand  (Olsen, et al., 2016; Glatzmaier, et al., 2017). The solid-state design has the added benefit of making the system inherently modular and scalable, overcoming the challenges faced by traditional CSP steam turbine plants, which require large scale deployments (minimum 50MWe) to be economically viable  (Olsen, et al., 2016; Glatzmaier, et al., 2017).

An array of heliostats (reflective mirrors) that use two-axis tracking reflect concentrated sunlight through an aperture in the bottom of the STEALS device, which is located atop a central tower. The concentrated sunlight is converted to heat via a solar absorber containing the phase- change material, which acts as both the solar receiver and thermal energy storage medium (Olsen, et al., 2016). Thermal gradients within the STEALS device are minimized as heat pipes with extremely high thermal conductivity (10,000W/m/K) are embedded within the MGA, providing for a high thermal- conductivity pathway through the MGA. The heat pipes work in tandem with a thermal valve (valved thermosyphon) controlling the rate at which the heat is then delivered to the thermoelectric generator (TEG) module where it is used either for direct electricity generation, or to charge the MGA for thermal energy storage, enabling subsequent generation during off-sun hours, or both for simultaneous electricity production and energy storage (Olsen, et al., 2016; Glatzmaier, et al., 2017; Rea, et al., Experimental demonstration of a latent heat storage system for dispatchable electricity, 2018).

The STEALS module has an operating temperature of 650°C and the heat valve is 90% efficient, resulting in less than a 50°C temperature drop between the MGA and hot side of the TEG when the system is operating, which is crucial as the thermoelectric generator is reliant upon a stable temperature range for optimum performance (Olsen, et al., 2016). The narrow operating range of the latent heat of the MGAs, provide a precise temperature range during system operation providing for ideal system design and compatibility of the two technologies  (Kisi, et al., 2018). Having no moving parts, these solid-sate devices have been shown to work for decades without the need for maintenance, reducing overall system cost (Olsen, et al., 2016).

Excess heat is vented through the top of the STEALS module through a finned, air-cooled heat exchanger (Glatzmaier, et al., 2017). The configuration of the system limits convective losses of the STEALS module at the receiver and allows for improved heat flow throughout the system by enabling gravity-assisted liquid return through the embedded heat pipes (Olsen, et al., 2016). By directly integrating the thermal energy storage and power block components together with the solar receiver, the STEALS system reduces the length of pathways for heat, thereby reducing thermal losses, making the system nearly isothermal with a combined receiver optical and thermal efficiency of 95%, and reducing system cost compared to traditional concentrating solar power designs, thus enabling a modular system design that allows for dispatchable solar electricity generation at a lower levelized cost of energy (LCOE) than traditional CSP or solar PV plus battery systems (Olsen, et al., 2016; Glatzmaier, et al., 2017; Rea, et al., Experimental demonstration of a latent heat storage system for dispatchable electricity, 2018). Previous techno-economic analysis of a 100 kW system located in Daggett, CA showed favourable LCOE for STEALS systems when compared to a solar PV plus battery; 11.7- 11.9 cents/kWh versus 15-25 cents/kWh for solar plus battery (Glatzmaier, et al., 2017).

Findings & Results

The Tesla Model 3, with an 82kWh battery pack, was used as the proxy vehicle to model system requirements. Estimates were made for a throughput of 3 vehicles per hour, at an 80% depth of discharge of the battery pack, operating over a span of 15 hours daily, for the duration of the year at 100% utilization rate. System requirements of 6 hours of daytime charging and 9 hours of storage were used to determine system component sizing for each of the respective system designs; solar PV + battery storage & solar PV plus STEALS storage.

NREL’s System Advisor Model (SAM) system modeling software was used to model a solar plus battery energy storage system located in Medicine Hat, Alberta, with a nameplate capacity of 810 kWdc and a 1,771kWh battery that would meet the power demand required for charging, providing for a total LCOE of 18.77 cents/kWh, assuming a battery replacement after 10 years of operation.

NREL’s SAM system modeling software was also used to model the solar photovoltaic component of the solar PV plus STEALS storage system, requiring a 300kW solar array and a 450kW STEALS system. A literature review was conducted to produce proxy estimates for capacity factors and system costs for the STEALS component of the system for the given location of Medicine Hat, Alberta, leading to a total LCOE of 17.35 cents/kWh for the solar PV plus STEALS storage system.

As a recent Bloomberg white paper indicated, thermal energy storage offers the potential for decarbonization of some of the hardest to abate industries (Bloomberg New Energy Finance, 2021), potentially offering an additional tool in the path towards decarbonization for Canadian oilsands producers, amongst other potential industrial use cases. When combined with cogeneration facilities, the increased efficiency offers the potential for significant emissions reductions, while providing for additional resilience for Alberta’s grid, providing for multivariate solution for industrial processes.
 

^References

^{Bloomberg New Energy Finance. (2021). Hot Spots for Renewable Heat - Decarbonizinf Low- to Medium-Temperature Industrial Heat Across the G-20. Retrieved September 13, 2021, from https://assets.bbhub.io/professional/sites/24/BloombergNEF-Hot-Spots-for-Renewable-Heat-Sep-2021.pdf}

^{Clean Energy Canada. (2021, June 29). Canada’s updated 100% zero-emission vehicle sales target points to a clear solution. Retrieved from Clean Energy Canada: https://cleanenergycanada.org/canadas-updated-100-zero-emission-vehicle-sales-target-points-to-a-clear-solution/}

^{Glatzmaier, G. C., Rea, J., Olsen, M. L., Oshman, C., Hardin, C., Alleman, J., . . . Ginley, D. S. (2017). Solar thermoelectricity via advanced latent heat storage: A cost-effective small-scale CSP application. AIP Conference Proceedings 1850 - SolarPACES 2016 (pp. 030019-1 - 030019-8). AIP Publishing. doi:https://doi.org/10.1063/1.4984362}

^{Kisi, E., Sugo, H., Cuskelly, D., Fiedler, T., Rawson, A., Post, A., . . . Reed, S. (2018). Miscibility Gap Alloys – A New Thermal Energy Storage Solution. World Renewable Energy Congress XVI. Murdoch: ResearchGate. doi:DOI: 10.1007/978-3-319-69844-1_48}

^{Lefebvre, N. (2018). A Comparison of Grid Power and Solar Power for Electric Vehicle Fast Charging. University of Calgary. Retrieved from https://prism.ucalgary.ca/handle/1880/108761}

^{Olsen, M. L., Rea, J., Glatzmaier, G. C., Hardin, C., Oshman, C., Vaughn, J., . . . Ginley, D. S. (2016). Solar thermoelectricity via advanced latent heat storage. AIP Conference Proceedings 1734 (pp. 050035-1 - 050035-8). AIP Publishing. doi:https://doi.org/10.1063/1.4949133}

^{Rea, J. E., Glatzmaier, G. C., Oshman, C., Parilla, P. A., Siegel, N. P., Ginley, D. S., & Toberer, E. S. (2018). Techno-Economic Analysis of a Small Scale Solar Power Tower at Varied Locations. AIP Conference Proceedings 2033 - SolarPACES 2017 (pp. 040034-1 - 040034-8). AIP Publishing. doi:https://doi.org/10.1063/1.5067070}

^{Rea, J. E., Oshman, C., Hardin, C. L., Singh, A., Alleman, J., Glatzmaier, G., . . . Toberer, E. S. (2018). Experimental demonstration of a latent heat storage system for dispatchable electricity. AIP Conference Proceedings 2033 - SolarPACES 2017 (pp. 090022-1 - 090022-8). AIP Publishing. doi:https://doi.org/10.1063/1.5067116}

^{Reed, S., Sugo, H., & Kisi, E. (2018). High temperature thermal storage materials with high energy density and conductivity. Solar Energy, 307-314. doi:https://doi.org/10.1016/j.solener.2018.02.005}

^{Scherer, S. (2021, June 29). Canada to ban sale of new fuel-powered cars and light trucks from 2035. Retrieved from Reuters: https://www.reuters.com/world/americas/canada-ban-sale-new-fuel-powered-cars-light-trucks-2035-2021-06-29/}

^{Sugo, H., Kisi, E., & Cuskelly, D. (2013). Miscibility gap alloys with inverse microstructures and high thermal conductivity for high energy density thermal storage applications. Applied Thermal Engineering, 1345-1350. doi:https://doi.org/10.1016/j.applthermaleng.2012.11.029}