The following is the first of a series of posts on recent developments regarding the “water/energy nexus.”
The term “water/energy nexus” came into widespread use over a decade ago to reflect the fact that the provision of water and wastewater services tends to be highly energy intensive, while most types of power generation tend to be highly water intensive. The term is often used to characterize the challenges confronting policy-makers in both sectors as they seek to increase capacity in what historically has been essentially a “zero-sum” game. Investment in new or expanded water and wastewater infrastructure projects almost invariably has increased the demand for power (with a commensurate increase in costs), while the establishment of new or expanded power generating capabilities almost always has resulted in the increased consumption of water, long the preferred cooling medium for most power plants, regardless of the technology they employ. This post provides an overview of the conceptual framework underlying the “water/energy nexus;” subsequent posts in the series will discuss how that concept has driven the development of new energy recovery and energy efficiency technologies, how the traditional water and wastewater utility model is changing in response to the need to address both water scarcity and high energy costs, and will provide a number of case studies that illustrate the changes taking place in the water/wastewater sector to reduce energy costs while at the same time expanding capacity.
To understand why the operations of water and wastewater utilities are so energy intensive, one need only look at how water utilities allocate the substantial amounts of electricity they consume. As a general rule, water utilities use between 75% to 80% of the power they purchase simply to move water from point A to point B. As a result of investment decisions made by prior generations, at a time when energy was inexpensive by current standards, the water distribution system in many parts of the country is highly inefficient, in the sense that water supplies are located far from where the water is ultimately consumed and must be pumped long distances..
Until very recently, new water projects typically involved investment in massive “grey” infrastructure –think huge dams, long-distance aqueducts and, more recently, large-scale seawater desalination plants, which often were located in remote areas far from the cities and industries that used the water. Particularly in the arid West, these highly-engineered, large volume water supply systems and structures long ago became the favored approach to domestic water management. Not only were these massive water infrastructure projects extremely costly and energy-intensive to construct and maintain but, more importantly, they depended on a distribution system that requires the pumping of large volumes of water very long distances. Because of the inefficiencies inherent in that model, when the cost of energy increased exponentially in later decades, so did the cost to the utilities of distributing water to the ultimate consumers – typically urban areas or areas of high agricultural usage such as the Central Valley of California that often were located hundreds of miles from the “source” of the water..
For a time, it was hoped that advances in the cooling efficiency and generating capacity of the current generation of “conventional” renewable energy technologies, many of which are now being utilized in utility-scale power plants, might provide the solution to the dilemma posed by the water/energy nexus. However, of the current array of market-ready alternative energy technologies, only a couple, most notably solar PV, can provide power to consumers without an intermediate cooling step or are able to rely on air-based cooling systems such that they need not consume water as part of the power generating process. At present, there remains substantial doubt as to whether any of the renewable alternatives that do not utilize conventional water-based cooling technologies could be scaled up to such an extent that they could power a high-volume desalination plant or large-scale regional wastewater treatment plant.
The current thinking regarding the “solution” to the water/energy nexus is that the greatest promise lies with two recent trends — (1) the emergence of a new generation of energy efficiency and energy recovery technologies that will reduce water utility energy costs and even generate revenues from the sale of excess electricity by the utilities, and (2) a “re-thinking” of and movement away from the current water distribution model to investment in more localized, smaller scale water projects which do not require long-distance pumping of water to end users. With respect to the former, studies have shown that wastewater can contain two to four times the energy that is required to treat it, and consequently the ability to harness that energy through biogas capture and other emerging technologies will be key to addressing water utility energy costs. With respect to the latter, increasing public acceptance of more localized solutions to the issue of water scarcity, such as reclaiming brackish groundwater and recycling “grey” sewer water, offers the promise of water supply models that do not entail the need for energy intensive and costly long-distance pumping of water.
Both of these topics will be discussed here at a later date.