Optimizing window solar heat gain coefficient for energy and carbon performance in a changing context of electrification and decarbonization

Abstract

Traditional building energy code requirements and design best-practices encourage limitations on the passive solar heat gain that can be admitted by window glass. This requirement exists even in cold climates, residential buildings, and equator-facing windows, where winter solar heat gain may be especially beneficial. This study hypothesizes that such solar heat gain limitations may not always lead to optimal operational energy and carbon performance, especially when considering new trends, including solar electricity generation and space heating with air-source heat pumps, which need to work harder at the extreme winter temperatures experienced in cold climates. To investigate, we simulated 13 window solar heat gain coefficient (SHGC) permutations in prototypical south-facing apartments with heat pumps in 19 US cities from mixed to very cold ASHRAE climate zones to compare energy performance, carbon emissions, and peak electricity loads. We calculated the optimal SHGC of the windows and compared these values to the limit set by prescriptive building energy codes. In colder ASHRAE climate zone cases, a higher SHGC than allowable by prescriptive codes improved performance for every metric tested. Optimizing SHGC for annual heating, cooling, and lighting electricity use in the six coldest and cloudiest cities, resulted in savings of 1–6 % annual electricity use, 3–11 % peak-hour heating, cooling, and lighting electricity use, and 6–19 % long-run marginal carbon emissions. These findings suggest that window SHGC limitations, and perhaps other design decisions that include a compromise between heating and cooling performance, may warrant further investigation in this changing context of building electrification and grid decarbonization.