Thermal battery cost scaling analysis: minimizing the cost per kW h

Abstract

Thermal energy storage technologies have many applications, from grid-scale energy storage to building space cooling and heating storage. When packaged into a device, these “thermal batteries” contain a storage material, heat exchangers to supply and extract the stored heat, and insulation to prevent the stored heat from escaping. Energy can be stored through sensible, latent, or thermochemical heat, with latent heat “phase change materials” (PCMs) being among the most common. While much ongoing work focuses on reducing the cost of either the PCM, the heat exchangers, or the insulation, herein we evaluate the cost scaling analysis wholistically to consider the entire system cost. We show how costs scale with certain characteristic lengths and the tradeoffs thereof. Our analytical framework reveals that the optimal PCM thickness (which minimizes the $ per kW h cost of the thermal battery) is often on the order of cm and depends exactly on the PCM properties and operational parameters. For example, improving the thermal conductivity of n-tetradecane by adding graphite filler reduces the thermal battery cost from $155 per kW h to $69 per kW h, and further improving the properties (density and latent heat) to the Department of Energy aspirational target reduces the thermal battery cost to $24 per kW h (for a C-rate of C/4). Our methodology applies to thermal storage systems of many geometries, requiring only knowledge of how the geometry affects the device state of charge. We provide a cost regime map with three regions: one in which the PCM cost dominates, one in which the heat exchanger costs dominate, and one in which insulation costs dominate. From the regime map, we also derive figures-of-merit for PCM thermal storage materials corresponding to the three different cost-dominant regimes.