TEXT OF ARTICLE
Water and solar energy enable trees and plants to grow today, just as hundreds of millions of years ago. Creation of the deposits of biomass that became fossil fuels depended on these two resources. The technological advances that have changed human life profoundly over the past decades and centuries have altered, but not resolved, this close coupling between water and energy. Current technologies for power generation, with some exceptions such as wind turbines and photovoltaic solar cells, rely heavily on the availability of large amounts of water. Primarily this water is needed for cooling thermoelectric plants and supplying fluid pressure and flow for hydroelectric power generation. But in a changing climate, it is not clear whether sufficient volumes of water will continue to be available where needed. And if, in an attempt to combat climate change, petrol is replaced by biofuels at a significant scale, more water will be needed for irrigation. Furthermore, clean fresh water is a basic necessity for human health and development. Large quantities of water can be provided — as long as sufficient energy supplies are available to reach deep aquifers, treat dirty water or desalinate the oceans. But a scarcity of energy implies a scarcity of water, just as constraints on water availability threaten the supply of energy, at least with the current infrastructure. Because of this interconnection (Fig. 1), water and energy cannot be treated in a disaggregated fashion, as is common today with both
markets and policy makers.
THE NEED FOR WATER IN POWER GENERATION
Large power plants present a strain on water resources. In the
Water is obviously central to power generation in hydroelectric dams. In the
THE NEED FOR ENERGY IN WATER PRODUCTION
The relationship goes the other way too, in that energy is necessary for producing and delivering fresh and potable water, just as water is necessary for generating energy. For example, energy is needed to convey, heat and treat fresh water and waste water. Heating water in homes and businesses for cooking, cleaning and other municipal and commercial uses consumes 3.6 quads, or 3.6%, of total
Supply and conveyance of water is one of the most energy-intensive water processes, estimated to consume over 3% of total
Recent news articles illustrate these competition between water resources and power generation: the debate of whether to use water from a reservoir to serve municipal needs for drinking water versus generating hydroelectric power arose with Uruguay’s Salto Grande dam and the US Colorado River lakes; a natural gas power plant and private landowners argued over groundwater rights in Texas; and hydroelectric dams were taken offline in response to drought in Georgia, among others2–5.
As freshwater supplies become strained, many have turned to water sources once considered unusable, including brackish ground water and sea water. Although use of these water sources mitigates constraints on drinking-water supplies, treatment of brackish ground water and sea water requires as much as 10–12 times the energy use of standard drinking-water treatment. However, usually, untreated saline water can be used to cool the thermoelectric power plant that may be required for desalination.
Partly because of the high energy requirements, proposed desalination water treatment plants in
Wastewater treatment also requires large amounts of energy, which will increase as discharge regulations in the
Estimates range from 250 kW h Ml–1 for trickling filter treatment, which uses a biologically active substrate for aerobic treatment, to 350 kW h Ml–1 for diff used aeration as part of activated sludge processing, and 400–500 kW h Ml–1 for advanced wastewater treatment that uses filtration and the option of nitrification6. Sludge treatment and processing alone can consume energy in the range of 30–80% of the total energy used in a wastewater plant; other physical and chemical treatment processes use much of the remaining percentage.
A THIRST FOR TRANSPORTATION FUELS
Two of the earliest fuels were wood and dung. They are still the major primary energy fuels for many regions of the world, providing 8.5% of the primary energy globally10. Trees require water for growth, as do the animals that supply dung. But modern liquid fuels such as gasoline, ethanol and diesel also require water for their extraction, farming, processing and refining. This ‘embodied water’ is not directly used in a vehicle, but rather indirectly required to make the fuel.
Water use for transportation can be considered in a similar fashion as for power generation in the form of withdrawal — that is, the amount of water that is necessary, but may eventually be returned to the system — and consumption, for example, through evaporation. However, in the context of transportation, consumption can additionally be associated with irrigated farming and as a feedstock. Generally, while driving light duty vehicles using current petroleum-based gasoline (assuming an average fuel economy of 20.5 miles per gallon (mpg) = 8.7 km l–1) and diesel (28.2 mpg = 12.0 km l–1), embodied water is withdrawn and consumed from nature at rates of up to 1.5 l km–1 and 0.3 l km–1, respectively.
Using liquids converted from other fossil fuels (coal, oil shale and tar sands) means that the rates of water consumption and withdrawal are 2–4 times and 1–2 times higher, respectively, and they are much more concentrated in regions where the fossil fuel resources exist.
Propelling vehicles using hydrogen and electricity also has substantial water impacts when drawing from the average
Additionally, competing water demands oft en make the siting of ethanol plants difficult because they require large amounts of water. Ethanol processing plants consume water for cooling processes, including the exothermic fermentation reaction, requiring 1–3 million litres per day to produce 250,000–750,000 litres per day of ethanol13,14.
LOOKING TO THE FUTURE
We see trends towards more water- intensive liquid fuels, more energy-intensive water sources and a growing population that will require more of both. These challenges will be exacerbated by climate change, which may cause geographic and temporal changes in the amount, annual distribution and form of precipitation (for example, as rain or snow). Because cities and their infrastructure are built on the basis of past precipitation patterns, such climatic changes may require substantial adjustments. In regions where water becomes scarcer, people must weigh the pros and cons between moving the people to the water and moving the water to the people (the latter of which requires continuous additional energy).
Regional climate projections will be needed to inform planning and policy. When siting new power plants, governments and power generators need to consider water availability over the plants’ lifetimes, normally on the order of several decades. Policy for energy and water resources should be integrated to consider less-water-intensive options in agriculture, such as forestry, and in electricity generation, such as wind, photovoltaic solar and air-cooling technologies. In the fuel sector, water consumption and withdrawal need to be included in environmental impact analyses such as those dictated by the Energy Independence and Security Act of 2007, which requires life-cycle analysis for understanding the greenhouse-gas impacts of renewable fuels. Like diversified long-term financial investment strategies, future water and energy infrastructure should also be diverse and multiscaled in order to create resilience in an uncertain climate and energy future. For example,
Distributed energy systems provide smaller-scale systems and add resilience to electric grids dominated by large centralized power plants, but they are usually considered more expensive owing to conventional financial and appraisal systems that account for capital but not operating expenses. For new power plants at
We also need policy that allows the operating costs and energy consumption of buildings and homes to be integrated into the construction, sales and finance phases of development. More energy efficient buildings within larger cities require less bulk city services. We must ask ourselves why we require no energy return on investment for crown moulding yet claim photovoltaics do not pay back fast enough. Fortunately sustainable concepts such as LEED (Leadership in Energy and Environmental Design) and projects such as the China EcoBlock18 guide and demonstrate integrated water and energy infrastructure via whole system design.
Water and energy cannot be separated. With an unlimited supply of available energy, we would be able to supply as much clean water as the world needs. In the real world of resource constraints, we need to simultaneously conserve water and energy. Thankfully, water conservation and energy conservation are synonymous with each other, so we have the opportunity for swift progress.