Comparative Emergy Synthesis for Green Engineering

While we struggle to consistently value ecosystem goods and services within our current economic metrics, green engineering initiatives harness these same goods and services for sustainable but often quantifiably unknown benefits. Ecologically engineered ecosystems, such as planted roofs and raingardens, have become a common strategy to earn green building certification through the United States Green Building Council’s Leadership in Energy and Environmental Design (LEED) program. LEED promotes integration of these living systems into the built environment because they presumably increase environmental sustainability by reducing energy consumption, mitigating greenhouse gas emissions, reducing storm flows, improving water quality, conserving natural resources, and providing wildlife habitats. Although the popularity of these green engineering initiatives has accelerated, they have not reached ubiquitous use. One reason for this inhibition may be that multifaceted ecological benefits, such as their contribution to basic life-support services (water supply, air filtration, etc.), are undervalued by our economic system as these benefits accrue far beyond the boundary of the property owner. Within the green engineering industry, to begin addressing this concern, we analyzed and compared three roof assembly systems including a modified bitumen (MB) as a traditional benchmark, a cool reflective thermoplastic polyolefin (TPO) and a planted roof design to assess their overall greenness. We found that planted roofs can require significant emergy investments when compared to traditional or cool reflective roofs. However, despite the typically large expense of importing this emergy from nonlocal sources, once installed, planted roofs offer a greater potential of emergy outputs or benefits to the local area. While the high emergy investments for planted roofs are identified and potentially can be mitigated, the benefits are harder to elucidate and calculate as they are extremely site and situation specific. If the planted roof materials can be designed on an emergetic cost basis to be similar to a cool reflective roof, the potential benefits of the primary production system will likely outweigh those of the cool reflective roof. These costs, benefits, and various ratios begin to foster a more reflective discussion comparing the overall sustainability of two green products and will help lay the foundation for additional product reviews in the rapidly expanding green industry.