Hydrogen and Water: An Engineering, Economic and Environmental Analysis

The multi-year program plan for the Department of Energy's Hydrogen and Fuel Cells Technology Program (USDOE, 2007a) calls for the development of system models to determine economic, environmental and cross-cutting impacts of the transition to a hydrogen economy. One component of the hydrogen production and delivery chain is water; water's use and disposal can incur costs and environmental consequences for almost any industrial product. It has become increasingly clear that due to factors such as competing water demands and climate change, the potential for a water-constrained world is real. Thus, any future hydrogen economy will need to be constructed so that any associated water impacts are minimized. This, in turn, requires the analysis and comparison of specific hydrogen production schemes in terms of their water use. Broadly speaking, two types of water are used in hydrogen production: process water and cooling water. In the production plant, process water is used as a direct input for the conversion processes (e.g. steam for Steam Methane Reforming {l_brace}SMR{r_brace}, water for electrolysis). Cooling water, by distinction, is used indirectly to cool related fluids or equipment, and is an important factor in making plant processes efficient and reliable. Hydrogen production further relies on water used indirectly to generate other feedstocks required by a hydrogen plant. This second order indirect water is referred to here as 'embedded' water. For example, electricity production uses significant quantities of water; this 'thermoelectric cooling' contributes significantly to the total water footprint of the hydrogen production chain. A comprehensive systems analysis of the hydrogen economy includes the aggregate of the water intensities from every step in the production chain including direct, indirect, and embedded water. Process and cooling waters have distinct technical quality requirements. Process water, which is typically high purity (limited dissolved solids) is used inside boilers, reactors or electrolyzers because as it changes phase or is consumed, it leaves very little residue behind. Pre-treatment of 'raw' source water to remove impurities not only enables efficient hydrogen production, but also reduces maintenance costs associated with component degradation due to those impurities. Cooling water has lower overall quality specifications, though it is required in larger volumes. Cooling water has distinct quality requirements aimed at preserving the cooling equipment by reducing scaling and fouling from untreated water. At least as important as the quantity, quality and cost of water inputs to a process are the quantity, quality and cost of water discharge. In many parts of the world, contamination from wastewater streams is a far greater threat to water supply than scarcity or drought (Brooks, 2002). Wastewater can be produced during the pre-treatment processes for process and cooling water, and is also sometimes generated during the hydrogen production and cooling operations themselves. Wastewater is, by definition, lower quality than supply water. Municipal wastewater treatment facilities can handle some industrial wastewaters; others must be treated on-site or recycled. Any of these options can incur additional cost and/or complexity. DOE's 'H2A' studies have developed cost and energy intensity estimates for a variety of hydrogen production pathways. These assessments, however, have not focused on the details of water use, treatment and disposal. As a result, relatively coarse consumption numbers have been used to estimate water intensities. The water intensity for hydrogen production ranges between 1.5-40 gallons per kilogram of hydrogen, including the embedded water due to electricity consumption and considering the wide variety of hydrogen production, water treatment, and cooling options. Understanding the consequences of water management choices enables stakeholders to make informed decisions regarding water use. Water is a fundamentally regional commodity. Water resources vary in quality and quantity from region to region, but because of its ubiquity, and because of the enormous volumes in which it is used, there is relatively little long-distance trade in water. As a consequence, water management policies are highly regionalized. Therefore, projecting the water footprint of a given hydrogen facility requires site-specific knowledge of available water resources. Only in the most constrained regions, water may be drawn from remote sources and conveyed long distances by pipeline or aqueduct. Regional water conditions can change over time as fresh and ground water levels change, competing demands become more acute, and shifts in historical climatic patterns take hold. For hydrogen plant stakeholders, a robust development and operating plan would incorporate these regional issues.