Does the Water-Energy Nexus Matter?
As an energy efficiency professional moving into the water sector, I've obviously been interested in the concept of the "water-energy nexus." It's taken some time, however, to think through why the concept is strategic rather than descriptive. Ultimately, the usefulness of looking at how our water and energy needs intersect requires a good deal of planning effort. I hope this post helps to highlight where and why that effort might be necessary.
Water-Energy Nexus as a Description
Getting water requires work, work requires energy, and many forms of energy these days require water. In my specific case, my family uses (on average) about 40 gallons per capita per day (GPCD) of water. With five of us, that's 200 gallons per day. If I were to drive to the headwaters of Barton Spring every day (let's say upstream from the pool, rather than downstream where the dogs hang out), that's a 10-mile drive each way. Relying on one of my favorite websites to play around with units, that adds about 50% to the weight of my car. The trip would require about 1.2 gallons of gasoline (and at least two Advil). I would then need to treat the water to remove bacteria and other contaminants, and distribute it across the various end uses in my home. This doesn't even begin to address treating water after its use. All of these steps require electricity.
I get a better deal from Austin Water, whose efforts often go unappreciated. It requires less than 1 kWh of electric energy for me to receive drinking-quality water at a high enough pressure to distribute it throughout my home and to prevent water contamination. My surface water is relatively close by (and the Highland Lakes are full again, for now). Were I to live in a state with a large central water project, such as in California and Arizona, electricity requirements would be much higher.
On the other side of the equation, thermoelectric generation is the largest end use for water withdrawals (but not water consumption) in the United States. According to the U.S. Geological Survey, in 2010 thermoelectric generation accounted for 45 percent of water withdrawals (see chart, below). Water consumption by thermoelectric power can be lower than 2% of water
withdrawals, depending upon the type of thermoelectric plant (e.g., nuclear, coal, natural gas) and the type of cooling system (e.g., once-through, cooling pond, recirculating, dry cooled). [Source: Union of Concerned Scientists]
This creates an interesting, iterative approach to assessing the impacts of energy efficiency or water conservation projects. For example, the installation of a new, Xeros polymer-bead commercial clothes washer at a 500-person hotel could reduce hot water use by 1.4 million gallons per year and 64,000 kWh per year (assuming a heat pump water heater). Assuming 10% real water loss, those washers reduce electricity requirements for a water utility by about 6,200 kWh per year. And they reduce water withdrawals by thermoelectric plants by about 1.5 million gallons per year and water consumption by 30,000 gallons per year.
Such projects both manage water scarcity risk (by lowering demand) and mitigate climate change (by lowering energy requirements). But aside from describing these interactive effects, what more have we done?
Because water is heavy, it's a local resource that's been pooled (above or below ground) by gravity and/or pressure. It's at the local level local level where this intersection needs to be addressed, as articulated by many others, in my personal experience by Michael Webber at the University of Texas, Kate Zerrenner of the Environmental Defense Fund, and David Sedlak of the University of California at Berkeley.
The map at the top of this post, developed by the Government Accountability Office, identifies states where water managers expect water shortages - statewide, regional, or local - by 2030. These shortages will occur where multiple end uses threaten to overdraw available resources.
A specific example of this can be seen in the draft 2017 Texas State Water Plan, below. Here, Region D (in Northeast Texas) projects growing demand for manufacturing, municipal supply, and thermoelectric power. [Units are in acre feet per year at each decade.] The "need" for water (the gap between demand and supply) starts at 24% in 2020 and grows to 43% by 2070. These are average, annual numbers - it is possible that "need" is a higher percentage of demand in hot summer months.
At the current moment, energy efficiency or alternative power generation strategies are not part of the State Water Plan. The projected growth of manufacturing and thermoelectric water requirements in Region D suggest that energy efficiency in manufacturing could play a significant role in lowering thermoelectric water requirements. Additionally, alternate sources of electricity, including renewables or hydropower in gravity-fed water supply pipes, could lower the water intensity of electric generation.
The effectiveness of energy efficiency or alternate forms of electric generation to serve as water management strategies depends upon the local context. It might even be possible that the Regional perspective itself is too large - "hot spots" with water and energy scarcity might be more localized, both geographically as well as seasonally. "Brownouts" could ensue, both on the electric grid and in water availability, at the summer peak.
Planning is key
Hopefully, the next (2022) Texas State Water Plan will be able to integrate energy management strategies as water management strategies, especially in those Regions where thermoelectric power represents a large proportion of demand and is also a large driver of the need. It might be appropriate to set energy efficiency or alternative generation targets in specific Regions.
In any state that identifies at least local water shortages by 2030, it seems important that this level of water and energy planning should be taking place, and the incorporation of energy efficiency or alternate energy targets to address potential shortages.