The Energy‐Water Nexus: Why Should We Care?

Water “will be to the 21st century what oil was to the 20th.” “That's a big no. The president believes … that it should be the goal of policymakers to protect the American way of life. The American way of life is a blessed one.” Water and energy are essential for human livelihood and the large-scale capture and use of these resources have brought many economic, social, and health benefits to humans across the globe. Both energy and water belong to the so-called critical natural capital category, which means that they are essential for human survival. As supply becomes scarce, they exhibit high price inelasticity of demand, so that a small reduction of supply leads to a huge increase in price. As a result the total value (price times; quantity) rapidly increases as total quantity declines (Farley and Gaddis 2007). This is true for any resource that is essential and non-substitutable. As there is less water or energy available, their price quickly increases towards infinity. This can create havoc in markets and stress the whole economic system, as during the energy crisis of the 1970's. Diminished water supplies may lead to direct conflict and violence. When energy and water supplies are abundant, their value is low. It may seem that we have an infinite supply and there is no need to worry. However, as we approach depletion, even small perturbations due to unforeseen climatic events, sharp increases in demand or technical malfunction results in disproportionate changes in their values and prices, if the market is allowed to work. Humans are quickly depleting non-renewable fossil fuels to produce electricity, heat homes, power vehicles and for other purposes. Likewise, in many areas we are “depleting” (over utilizing or degrading) our fresh water resources to the extent that we must compensate for shortages by either increasing energy use to import water from other basins, desalinate salt water or reuse waste water, or by depleting non-renewable water by over-pumping water from fossil aquifers. These practices are unsustainable and they may leave future generations with fewer options and more risks. Therefore, we will likely have to meet many of our future water and energy needs via increased efficiency and conservation. Water and energy are intimately intertwined. For example, in biofuel production water is essential for growing, harvesting, and processing the biomass, while additional energy is needed to allocate and utilize water in the process (Figure 1). For a comprehensive, valid assessment of these resources, we need to conduct life cycle assessments that track both the water and energy flows and storage reservoirs for the entire system and estimate both the inputs and outputs for each aspect of the system. As water is required in production of almost all forms of energy, energy is also essential for water supply, treatment, desalinization, disposal, and other uses. The water sector, including treatment and conveyance, is presently one of the largest users of energy, comparable to the paper and refining industries (Anderson 1999). The demand for energy in the water sector will likely substantially outpace growth in other high-energy use sectors. The most obvious use of water in the energy sector is hydroelectric generation; however, the amount of water withdrawn to generate thermal electric power from fossil fuels is roughly equivalent to the amount of water utilized by irrigated agriculture at about 40 percent of U. S. withdrawals each (Hutson et al. 2004). The life cycle of energy from biomass production. For both energy and water, it is not just the quantity that matters, but quality as well. For water, quality is measured by the concentration of impurities, constituents dissolved or suspended in the water, as well as by its physical characteristics, such as temperature. Water quality can be significantly affected by energy-related projects. For example, water used for cooling purposes in power stations is returned to the river with a higher temperature, which may prove detrimental to some fisheries. Hydropower that requires dams can also significantly affect physical and chemical parameters of water. Mining may destroy whole landscapes, including streams. For energy, quality means efficiency, reliability, and continuity of supply. We use one kind of energy to produce other kinds of higher quality. Say, low quality solar energy is abundant but hard to use. In photosynthesis solar energy is accumulated in woody biomass that can be then be used as a source of energy of higher quality. With photovoltaics we can convert solar energy directly into electricity, which is of higher quality energy than biomass. The efficiency can be measured utilizing the Energy Return on Energy Invested index, discussed below. If we need to use energy to produce a different type of energy of the same quality, we lose efficiency. The higher the quality, and the more efficient a water or energy supply is, the more reliable and the easier it is to provide to end users. As demand for water grows, there will be more competition with regard to water needed for energy production. If water becomes as limiting as energy, there will be more pressure on water-intensive energy producers to seek alternative supplies. Energy Return on Water Invested is a useful indicator to compare various methods of energy generation. Ideally, it can be estimated for a given technology by applying the life cycle assessment methodology (International Standard Organization 2006, Guinée 2002) to calculate the energy produced per unit of fresh water used (megajoules/liter, MJ/L or kcal/gal, 1 joule = 0.24 cal = 0.00028 watt/hour) for a given technology. Variations of the life cycle assessment methodology are already generally used to calculate the Energy Return on Energy Invested for a technology (see Spreng 1988 for an overview) and the application of life cycle assessment to estimate Energy Return on Water Invested is analogous. However, things become complicated since water does not necessarily have to be consumed to produce energy. Much of the water withdrawn for energy production is returned back and can be reused. From a basin perspective, the only consumption occurs when water is either lost through evapotranspiration (in which case it may also reappear in the basin, but at a different place when rainfall occurs) or degraded through contamination that changes its chemical properties (e.g., toxic additions, including nutrients, pesticides, herbicides) or physical properties (e.g., water temperature, oxygen content), to such an extent that it is no longer usable. Energy Return on Water Invested can be calculated in a way similar to Energy Return on Energy Invested. If, eout is the amount of energy produced, and ein is the amount of energy used in production, then, Energy Return on Energy Invested, e = eout/ ein. In some cases net Energy Return on Energy Invested is used, which is the amount of energy we need to produce to deliver a unit of net energy to the user: Similarly Energy Return on Water Invested would then be ew= eout/ win, and net Energy Return on Water Invested, Energy Return on Energy Invested is usually criticized for not taking into account all the other resources (including social and environmental ones) that are required to produce energy. One could assume that as long as a technology has an e > 1, it can then be chained as many times as needed to produce infinite energy. This is certainly not the case, because other resources may be depleted along the chain (say, land, or pollution absorption capacity, or certain metals or minerals that are needed to run the operations). Therefore, Energy Return on Energy Invested is a good indicator to make comparisons between technologies (Cesar et al. 2007); however, we must always keep in mind the other limiting factors that can play a crucial role (e.g., availability of land, environmental carrying capacity, carbon dioxide and other greenhouse gas emissions). Water is one such limiting factor, so Energy Return on Water Invested is a good supplement to Energy Return on Energy Invested, taking into account the water needs for energy production. Mulder et al. (2007) estimate Energy Return on Water Invested and net Energy Return on Water Invested at various technologies showing that it can range from 0.025 MJ/L for electricity production from biomass up to 285.3 MJ/L for petroleum diesel. Net Energy Return on Water Invested for the same technologies was 0.02 and 228.4 MJ/L respectively. The best net Energy Return on Water Invested for biofuels (sugar cane ethanol) is 0.903, over two orders of magnitude lower than the most water-efficient fossil energy sources. Indeed, the study by Kannan et al. (2004) for a petroleum power plant in Singapore shows that even electricity production, one of the least water-efficient forms of fossil energy production, can be made very water-efficient when necessary. Singapore has perennial shortages of fresh water and the petroleum power plant studied there has an Energy Return on Water Invested seven times higher than typical power plants with water recirculation. This is because direct water withdrawals are reduced to less than 0.02 L/MJ, a number dwarfed by the lower-bound water withdrawals of 13 L/MJ for biomass electricity production (Berndes 2002). This implies that there are water-efficient fossil electricity sources that yield almost 600 times as much energy per unit of water invested, as does the most water-efficient biomass source of electricity reviewed by Berndes (2002). Ethanol refining currently consumes 4-8 liters (1-2 gal) and uses ∼ 130 liters (34.34 gal) of water for each liter of ethanol (Dominguez-Faus et al. 2009), so we need 480 million to 4 billion m3 (127-1056 billion gal) of water to provide for the Presidents' 2025 goal of producing 120-240 billion liters (30-60 billion gal) of ethanol. Some new technologies used to convert cellulose to fuel use even more water per liter of fuel gained than converting corn to ethanol. Irrigating seed and field corn needed for ethanol adds another 4 to 7 liters of water for each liter of fuel (Mubako and Lant 2008). Irrigating marginal land may need many times more water. It should be also noted that various of the production cycle have different water For example, in biofuel production requires orders of magnitude more water than ethanol (see 1). the of water consumption can be much higher for of of water are to be withdrawn on the significantly and additional to provide that water. Water required to produce one liter of between liters to liters on the used for the process to liter per or and For a that means billion liters billion gal) of water per water to meet the needs of to from U. S. of Energy more than 4 liters of water are needed for liter of fuel produced, the water supplies that it needs to production, because of over pollution and the of water more it was that the “will and is to convert million of per into one million of oil such as for will use and water from to supply the million it will need each In some of the water was the it is the because the is used in an unsustainable will to the between water and energy. of the water used in energy production is used for cooling and the the the more efficient the cooling This means that there are on the of water that is Therefore, as at the there is a demand for water to provide the same cooling with on the water temperature. plants have or other that their to their In the run means that they will less a in less efficient and less electric energy for the same amount of energy many plants have on that could more in In environmental usually on the of water back into the and As the of energy demand will for more technologies and of In in the sector, demand for the is likely to since is low in energy and total This means more in and requires much new which has high and long lead it is that the system will to in energy supply in the if the energy is and over longer we will likely more and a higher of due to an increased number of and more on the whole system. In of are by on As energy production and economic more on there will be even more because of or could have that may through the whole economic system. As we will can only the for the system. If there energy, we would have a with water for use. For example, there are resources of water that could be to provide for all the for water if there is energy to run and then water to it is is certainly not the Energy is an as a and energy efficiency is a Water supply in case will need to with many other energy and the Water Return on Energy Invested may become a useful to compare various water supply projects. Water Return on Energy Invested can be calculated also in a way similar to Energy Return on Energy Invested. that index, Water Return on Energy Invested does not into account any other resources energy (including social and environmental ones) that are required to produce water of the required quality. when Water Return on Energy Invested to compare always keep in mind the other limiting factors that can into it is one of to use in are many that energy is required for production and of water. and the huge capital and energy This energy for and water including water Water and Energy is needed to water to required quality and for water through and which is the means for water supply and This also for and of and waste water. water In to would energy for and so Some water is in and The water is on energy to and water across huge to are one billion of water a in and in the a equivalent to The amount of energy for is way to Water Return on Energy Invested is to at the of water from various sources. some of the of water at It also an of what the of different technologies of energy to is a way to increase efficiency. most of the for energy and water shortages are on the supply The approach is to the growth for demand, and then seek resources either through new and or efficiency or through new resources new water resources, or new oil or water or fuel from In most the are to provide the supply to meet or future For example, the of Energy is to a supply … of and The of and of are to provide water. of the and are on increasing the supply. 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