Assessing the value of coupling thermal energy storage with air source heat pumps for residential space heating in US cities

<h2>Summary</h2> Widespread air source heat pump (ASHP) adoption faces several challenges that thermochemical salt hydrate energy storage can mitigate. We quantify the space heating value of four salts—<mml:math><mml:mrow>MgS<mml:mi>O</mml:mi><mml:mn>4</mml:mn></mml:mrow></mml:math>, <mml:math><mml:mrow>MgC<mml:mi>l</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:math>, <mml:math><mml:mi>K</mml:mi><mml:mn>2</mml:mn><mml:mi>C</mml:mi><mml:mi>O</mml:mi><mml:mn>3</mml:mn></mml:math>, and <mml:math>SrB<mml:mi>r</mml:mi><mml:mn>2</mml:mn></mml:math>—coupled with ASHPs across 4,800 households in 12 US cities by embedding salt-hydrate-specific Ragone plots into a techno-economic model of coupled ASHP-thermal energy storage (TES) operations. In Detroit, salt hydrate TES can reduce household annual electricity costs by 5%–8%. Cost savings from TES vary widely between households, salts, and climates. We identify the most promising salt in this study, <mml:math><mml:mrow>SrB<mml:mi>r</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:math>, due to its high energy density and low humidification parasitic load. Breakeven capital costs of <mml:math><mml:mrow>SrB<mml:mi>r</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:math>-based TES can reach $17/kWh, making it the only salt studied to reach and exceed the US Department of Energy's $15/kWh TES cost target. Sensitivities highlight the importance of variable TES sizing and efficiency losses in the value of TES.