Water for Biomass-Based Energy on Maui, Hawaii

Biomass-based energy has characteristics that could help Maui Island meet multiple long-term goals, including decreasing reliance on oil for electricity and transportation fuels, increasing use of local resources that do not need to be shipped long distances, and diversifying the island economy beyond tourism by preserving agriculture. Biomass can be used for liquid fuel production and for electricity production. On Maui, sugarcane has been grown at plantation scale for over a century. Accordingly, sugarcane-derived ethanol and combustible sugarcane bagasse have long been of interest as energy sources for the island. State and county level focus on increasing renewable energy utilization on Maui have renewed study of potential crops and available land, with a special emphasis on sugarcane. However, there is some concern about the water requirements associated with biomass-based energy. A primary motivation for using local, renewable energy sources is that Maui is an island with limited resources, fresh water among them: thus, exploring ways to increase energy sustainability without compromising water availability is of interest to many. This work examines the water needs associated with growing sugarcane for ethanol and combustible biomass on Maui Island. Virtually all sugarcane on Maui is irrigated because soil and sunlight resources do not generally coincide with natural precipitation patterns. Growing sugarcane for energy represents a large water demand that is limiting under certain development scenarios on Maui, such as a scenario where environmental streamflows are highly prioritized. By comparing the irrigation demand of Maui’s currently grown sugarcane with published figures for ethanol yield from cane, this work finds that 700 to 1,500 gallons (gal) of irrigation water are needed per gallon of sugarcane-based ethanol (from fermentable sugars and fiber; 0.7 to 1.5 cubic meters, m3 , per liter, L). More water is needed for processing. However, combustible waste streams could provide additional energy return per unit of water. This paper discusses how water demand for sugarcane-based energy interacts with other island water demands, given that about 37,000 acres (150 km2 ) of sugarcane land are potentially available for bioenergy production. Though seawater cannot be successfully directly used for irrigation, sugarcane can tolerate some salinity and other contamination, so this paper also considers brackish water and treated wastewater — for which there is little other demand — as potential irrigation resources. Notably, the range of tolerable water quality expands significantly when sugarcane is not intended for human ingestion or when biomass yield, not sugar content, is targeted (as for cellulosic ethanol or combustible biomass production).Copyright © 2011 by ASME