Maui

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).