(e.g., poplar and rapeseed), a much higher water footprint was calculated than for a food crop such as maize. Another remarkable result was that biomass water footprints are on average 70-400 times higher than those of other primary energy carriers, except hydropower. A shift toward biomass energy will therefore have a large impact on the use of freshwater resources and will compete strongly with other water uses (Gerbens-Leenes et al. 2008). This is in line with the findings of Lloyd and Larsen (2007), who derived water consumption rates for energy production processes in the same order of magnitude (Table
14.1) and calculated a water footprint of 51 m3/GJ and 81 m3/GJ for ethanol production from U.S. corn and Brazil sugar cane respectively.
De Fraiture et al. (2008) state that globally, 2630 km3 of water is currently withdrawn for irrigation purposes per year, of which 2% is used for biofuel crops. A fourfold increase in world biofuel production by 2030—replacing 7.5% of current gasoline use—would require 180 km3 of extra water extraction from rivers and groundwater compared to 2980 km3 for food. These are modest average figures. However, in those parts of the world where shortage of water already limits agricultural productivity and where most crops require artificial irrigation, there is not enough water to support government plans for expanded biofuel production (Pearce and Aldhous 2007).
Trends in Water Footprint for Energy Production in Europe versus U.S.
As discussed earlier, U.S. water consumption for power production could more than double by 2030 if new power plants are built with evaporative closed-loop cooling. This is due to the fact that they use nearly 200% more water than open-loop systems, although withdrawal requirements are only 1-2%. Freshwater withdrawals are thus projected to remain quite stable till 2025.
For the European energy sector, a 54% increase in thermal electricity production is expected from 2000 to 2030. Assuming that once-through cooling systems are gradually replaced by tower cooling, this would result in doubled water consumption by 2030, which amounts to 15 million m3/d or 0.2% of available freshwater resources. Water withdrawals would, however, drop significantly. Although water availability issues for energy production in general may not seem as dramatic as in the U.S., the situation in Mediterranean countries or in countries with a high water use per unit area may be quite different (DHI 2007). As climate change may lead to changes in spatial and temporal water availability and also water demands from other sectors increase, energy production cannot continue at the current rates of water consumption. However, due to the growing competition for water availability, the value of water may increase, impacting energy costs and providing incentives for developing and implementing approaches and technologies to decrease the water intensity of the energy sector. Wind energy has by far the lowest water footprint (Table
14.2). Providing significant percentages of energy through wind turbines could thus save a lot of water.
In addition to water quantity issues, energy production may also impact water quality, either directly or through deposition of trace quantities of air pollutants into water systems (NETL 2006, 2009).
Salinity gradient energy. Among the energy sources that are currently considered to be sustainable, several are water-related: hydroenergy, wave and tidal energy, and ocean thermal energy. A significant potential to obtain clean energy lies also in mixing water streams with different salt concentrations. Salinity gradient energy—also called blue energy—can be made available from natural or industrial salt brines, or from estuaries, where freshwater fl ows into the sea (Post et al. 2007). Among the various ways that exist to harvest energy from mixing fresh or river water with salt or seawater, the two most important ones are based on membrane technology: pressure-retarded osmosis (PRO) and r everse electrodialysis (RED). While reverse osmosis and electrodialy-sis are implemented on large scales for desalination, these techniques can be used for power generation from salinity gradients when operated in the reverse mode. Brauns (2008) mentions that the extractable energy value of seawater is around 1 MJ/m3. Model calculations from Post et al. (2007) indicated that PRO and RED each have their own f eld of application. PRO yields higher power densities (W/m2) and higher energy recovery, when using concentrated saline brines, whereas RED is more attractive for power generation using sea-water and river water. From a technical perspective, both techniques are still in a research phase; further developments on membrane and system characteristics are needed to achieve the potential performances, indicated by the model. One of the locations where demonstration tests for the blue energy concept are planned is the Afsluitdijk in The Netherlands (van den Ende and Groeman 2007).
Cogeneration of energy and potable water. In the original RED concept, energy is produced by feeding dilute and concentrated chambers with fresh-and seawater, respectively. Brauns (2008) states that power production can be further improved by feeding them respectively with seawater and with salt concentrations which are much higher than those of seawater. At sites where seawater is desalinated for potable water production, such solutions are available, either as the brines from the desalination technologies or as brines in which salt levels have even been further increased by evaporation through solar heat. Heating of seawater and brine would additionally increase the performance and power output of the salinity gradient power unit, provided that solar energy is abundantly available. This leads to a hybrid concept of seawater desalination, RED, and solar energy units (Figure 14.1). It combines the opportunity of sustainable energy production with potable water production. In addition to the production from the seawater desalination unit, potable water will also be obtained through condensation of the water vapor resulting from brine
evaporation with solar heat. This sample concept shows the potential benefits of combining water and energy production systems. From an economical point of view, the feasibility of salinity-gradient power techniques relies on a further reduction of membrane prices (Post et al. 2007). On selected locations, the clever combination with solar power could, however, lead to sustainable and simultaneous renewable energy and potable water production. Water footprints for these and other new energy production concepts can, of course, only be calculated and compared with existing concepts once these technologies are implemented at full scale.
Moving, processing, and use of water consumes large amounts of energy, mainly for pumping, transport, treatment, and desalination; hence there is potential for significant energy saving. Similar to the water footprint for energy production, an energy footprint for water applications can be defined as the energy consumed during conveyance, treatment, and application of water (DHI 2009). No overview data were found in the consulted literature. Therefore, only isolated energy consumptions for parts of the water cycle are presented here.
Water and wastewater treatment and distribution in the U.S., for example, is estimated to consume 50,000 GWh, representing 1.4% of the total national electricity consumption. Municipal water supply and wastewater treatment systems are among the most energy-intensive facilities owned and operated by local governments, accounting for about 35% of energy used (Elliott 2005; EPA 2008). It is therefore not surprising that the USEPA has recently published a guideline for energy management in water and wastewater utilities, including suggestions for energy saving.
Potable water production. Drinking water is prepared through a combination of conventional treatment processes from groundwater and surface water or may be obtained through the desalination of brackish water or seawater. A study in the Netherlands mentions that energy consumption for drinking water preparation amounts to 0.5 kWh/m3 (Frijns et al. 2008). Vince et al. (2008) presented values between 0.05 and 0.7 kWh/m3 for freshwater or brackish water treatment processes. In contrast, the average figure for seawater desalination plants was always above 3.5 kWh/m3 of potable water (Table 14.3). Since energy consumption was shown to carry the highest environmental burden in potable water production, the environmental performance of seawater desalination technologies is thus worse than that of freshwater treatment plants.
Potential mitigation measures to reduce the climate impact of potable water production plants are energy efficient production, optimal distribution, and methane gas recovery (Frijns et al. 2008). Taking the example of seawater desalination by reverse osmosis (RO), energy consumption is currently between 3-4 kWh/m3 (Singh 2008) and is the largest component in the operational costs. Due to the development of high flux membranes and the introduction of energy recovery devices, energy consumptions below 2 kWh/m3 are technically feasible today, and recent innovations will make desalination by RO even more competitive (Fritzmann et al. 2007). In hybrid systems, RO can be combined with other desalination concepts or power generation facilities. Already
Life cycle step
Electricity consumption (kWh/m3)
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.