Calculating energy-saving potentials of heat-island reduction strategies

We have developed summary tables (sorted by heating- and cooling-degree-days) to estimate the potential of heat-island reduction (HIR) strategies (i.e., solar-reflective roofs, shade trees, reflective pavements, and urban vegetation) to reduce cooling-energy use in buildings. The tables provide estimates of savings for both direct effect (reducing heat gain through the building shell) and indirect effect (reducing the ambient air temperature).

In this analysis, we considered three building types that offer the most savings potential: residences, offices, and retail stores. Each building type was characterized in detail by Pre-1980 (old) or 1980⁺ (new) construction vintage and with natural gas or electricity as heating fuel. We defined prototypical-building characteristics for each building type and simulated the effects of HIR strategies on building cooling- and heating-energy use and peak power demand using the DOE-2.1E model and weather data for about 240 locations in the US. A statistical analysis of previously completed simulations for five cities was used to estimate the indirect savings. Our simulations included the effect of (1) solar-reflective roofing material on building (direct effect), (2) placement of deciduous shade trees near south and west walls of building (direct effect), and (3) ambient cooling achieved by urban reforestation and reflective building surfaces and pavements (indirect effect).

Upon completion of estimating the direct and indirect energy savings for all the locations, we integrated the results in tables arranged by heating- and cooling-degree-days. We considered 15 bins for heating-degree-days, and 12 bins for cooling-degree-days. Energy use and savings are presented per 1000 ft² of roof area.

In residences heated with gas and in climates with greater than 1000 cooling-degree-days, the annual electricity savings in Pre-1980 stock ranged from 650 to 1300 kWh/1000 ft²; for 1980⁺ stock savings ranged 300–600 kWh/1000 ft². For residences heated with electricity, the savings ranged from 350 to 1300 kWh/1000 ft² for Pre-1980 stock and 190–600 kWh/1000 ft² for 1980⁺ stocks. In climates with less than 1000 cooling-degree-days, the electricity savings were not significantly higher than winter heating penalties. For gas-heated office buildings, simulations indicated electricity savings in the range of 1100–1500 kWh/1000 ft² and 360–700 kWh/1000 ft², for Pre-1980 and 1980⁺ stocks, respectively. For electrically heated office buildings, simulations indicated electricity savings in the range of 700–1400 kWh/1000ft² and 100–700 kWh/1000 ft², for Pre-1980 and 1980⁺ stocks, respectively. Similarly, for gas-heated retail store buildings, simulations indicated electricity savings in the range of 1300–1700 kWh/1000 ft² and 370–750 kWh/1000 ft², for Pre-1980 and 1980⁺ stocks, respectively. For electrically heated retail store buildings, simulations indicated electricity savings in the range of 1200–1700 kWh/1000 ft² and 250–750 kWh/1000 ft², for Pre-1980 and 1980⁺ stocks, respectively.