Much research has gone into technologies to mitigate urban heat islands by making urban surfaces cooler by increasing their albedos. To be practical, the benefit of the technology must be greater than its cost. This report provides simple methods for quantifying the maxima of some benefits that albedo increases may provide. The method used is an extension of an earlier paper that estimated the maximum possible electrical energy saving achievable in an entire city in a year by a change of albedo of its surfaces. The present report estimates the maximum amounts and monetary savings of avoided CO2 emissions and the decreases in peak power demands. As examples, for several warm cities in California, a 0.2 increase in albedo of pavements is found to reduce CO2 emissions by < 1 kg per m2 per year. At the current price of CO2 reduction in California, the monetary saving is < US$ 0.01 per year per m2 modified. The resulting maximum peak-power reductions are estimated to be < 7% of the base power of the city. The magnitudes of the savings are such that decision-makers should choose carefully which urban heat island mitigation techniques are cost effective.

We present a simple method to estimate the maximum possible energy saving that might be achieved by increasing the albedo of surfaces in a large city. We restrict this to the "indirect effect", the cooling of outside air that lessens the demand for air conditioning (AC). Given the power demand of the electric utilities and data about the city, we can use a single linear equation to estimate the maximum saving. For example, the result for an albedo change of 0.2 of pavements in a typical warm city in California, such as Sacramento, is that the saving is less than about 2 kW·h of electrical energy per modified m2 per year. This may help decision makers choose which heat island mitigation techniques are economical from an energy-saving perspective.

Lawrence Berkeley National Laboratory partnered with Mid-America Regional Council (MARC) to quantify the costs and benefits from the adoption of urban heat island (UHI) countermeasures in the Kansas City region (population 1.5 million), and identify the best regional implementation pathway for MARC. The team selected cool (high-albedo) roofs and increased vegetation as the two countermeasures to evaluate. For vegetation, there were two strategies: (1) planting new trees to shade building surfaces, and (2) increasing urban irrigation (a surrogate for the use of vegetation to manage stormwater) to increase evapotranspiration. Using the Weather Research and Forecasting (WRF) model we simulated selected weeks during summer time, across five years (2011 – 2015) representing a range of normal summer conditions. We also simulated six of the most intense heatwaves that occurred between 2004 and 2016. We found under typical summer conditions (non-heatwave) average daytime (07:00 – 19:00 local standard time) regional near-ground air temperature reductions of 0.08 and 0.28 °C for cool roofs and urban irrigation, respectively. We calculated the building electricity, electricity cost, and emission savings that result from the reduction in outdoor air temperature (“indirect” savings) and found maximum regional annual indirect electricity savings of 42.8 GWh for cool roofs and 85.6 GWh for urban irrigation—yielding maximum regional annual indirect electricity cost savings of $5.6M ($0.05/m2 roof) and $11.1M ($0.01/m2 irrigated land), respectively, and maximum regional annual CO2 savings of 43.4 kt and 80 kt, respectively. We next evaluated the building energy, energy cost, and emission savings from reducing direct absorbed radiation on the building surfaces using cool roofs and shade trees (“direct” savings). For cool roofs, we found regional annual direct energy cost savings of $10.9M ($0.15/m2 roof) with regional annual CO2 savings of 66.4 kt. For shade trees, the regional annual direct energy cost savings were $21M ($21/tree) with regional annual CO2 savings of 126 kt. We investigated cool roof cost premiums (the additional cost for selecting a cool roof product in lieu of a conventional roof product, estimated to be zero to $2.15/m2) and shade tree first costs (assumed to be $100 per tree). The regional cool roof cost premium was calculated using the regional roof area per roofing material type and the range of cool roof product premiums for each material type. The extra cost of selecting cool roofs across the region ranged from $4.33M to $87.1M, while the additional shade trees planted across the region were assumed to cost $102M. When we compared the regional annual direct cost savings to the regional cool-roof cost premium and the regional shade-tree first cost, we found regional simple payback times up to 8.0 years for cool roofs and 4.9 years for trees, respectively. Since this comprehensive assessment of UHI countermeasures is a valuable methodology for other local governments to apply, we developed a step-by-step guide for others to follow. Based on the benefits and costs of the UHI countermeasures, MARC will pursue the inclusion of these countermeasures in existing regional plans where they can complement other regional priorities for transportation, climate resiliency, clean air, and hazard mitigation. They hosted a local workshop in 2016 for stakeholders to introduce the topic and will continue to share these resources to further appropriate adoption of UHI countermeasures.