WATER-ENERGY NEXUS

 

Introduction

Water availability is vital for the energy sector; without it, little energy can be produced or consumed, be it in the extraction of fossil fuels and their processing, water for cooling in power stations, or driving the generators for hydropower. Water is also an important element of new renewable energy sources, such as bio-fuels and biomass, for the manufacture of equipment and for Concentrated Solar Power (CSPs). The recent massive expansion in production of bio-fuels has become a major source of demand for water. Just as water is needed to supply energy, energy is required for treatment and delivery of water for human use. . A volume of water which exceeds that carried annually by the Ganges River is needed by the energy sector each year.

REQUIREMENTS FOR ELECTRICITY GENERATION

Some renewable energy sources—such as photovoltaic (PV) and wind energy—require very little water. The largest quantity of water use in thermoelectric generation is for cooling and condensing steam. Power plants differ in the process used to cool the steam. Most thermoelectric power plants use variations of two different wet cooling technologies: once-through and wet-recirculating (or cooling tower) cooling systems. In general, more efficient combustion platforms require less water per kilowatt-hour (kWh) of generation. For example, coal plants that are operated at supercritical temperature and pressure are more efficient than subcritical plants and require less cooling. The type of cycle used also has an effect. For example, natural gas combined cycle plants and integrated gasification combined cycle (IGCC) plants have lower water consumption per kWh of generation because the majority of the plants ‘output comes from combustion turbines that require minimal water compared to steam turbines. Table 1 shows the water utilization in the different thermoelectric generation technologies. (Withdrawal designates any water diverted from a surface or groundwater source. ―Consumption designates withdrawn water that is not returned to its source, because it has evaporated, been transpired by plants, or incorporated into products).

Table: 1 Water Withdrawal and Consumption in operation phase In Thermoelectric generation

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Abbreviations: CC: Combined Cycle; CFB: Circulating Fluidized Bed; PC: Pulverized Coal; SC: Supercritical Pulverized Coal; IGCC: Integrated Gasification Combined Cycle; CSP: Concentrating Solar Power;

(source:Meldrum, J., S Nettles-Anderson, G Heath and J Macknick, 2013, “Life cycle water use for electricity generation: a review and harmonization of literature estimates.” Environ. Res. Let. 8.)

For extraction and processing of fuel like coal, natural gas and nuclear fuels, water is a necessity. Coal production uses water mainly for mining activities such as coal cutting and dust suppression. The amount of water needed depends on the characteristics of the mine, i.e. whether it is at the surface or underground, and processing and transport requirements. Washing coal increases its quality, but involves additional water. Conventional natural gas entails minimal water use for drilling and processing.The production revolution experienced in the U.S. oil and gas sector in recent years due to the wide application of horizontal drilling and hydraulic fracturing has greatly enhanced the requirement of water in oil extraction. When large quantities of water are required, water supply may become an issue, particularly in arid regions. Table 2 shows the withdrawal and consumption of water for the different fuels, on a gallon (gal) per megawatt-hour (MWh) basis.

Table. 2 Water Consumption and Withdrawal for Fuels used in Electricity Generation

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Values represent medians, ranges represent medians for competing technologies

Data source: Meldrum et al. 2013 Meldrum, J., S Nettles-Anderson, G Heath and J Macknick, 2013, “Life cycle water use for electricity generation: a review and harmonization of literature estimates.” Environ. Res. Let. 8.

REQUIREMENTS OF TRANSPORTATION FUELS

Refining crude oil into end-use products requires further water for cooling and chemical processes, the amount varying widely as per technologies employed (the cooling system, for example) and process configuration. Bio-fuels require water for irrigating feedstock crops and for fuel conversion. Irrigation needs can range widely depending on the crop, the region in which it is grown and the water efficiency of irrigation technologies used. Table 3shows the water withdrawal and consumption rates in Lit/tonnes of oil equivalent for different transportation fuels.

Table: 3   Water use in transportation fuel production (lit/Tons of oil Equivalent)

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(Extracted from World Energy Outlook, 2012, www.iea.org)

DISCUSSIONS

From a power generation perspective, hydroelectric power generation is the most water efficient as most used water is returned to the source. Geothermal power generation is also water efficient. Geothermal plants do utilize less water, however the availability of geothermal energy is localized and the capacity of a geothermal power plant to generate power is much lower than nuclear and fossil fuel thermoelectric power plants. Nuclear power generation needs more water for cooling than fossil fuel thermoelectric due to their lower efficiencies. Significant volume of water is required during the fuel production and purification processes. However with fossil fuel we have environmental issues like green house gases release, which is much less with nuclear power.

Understanding the extent and complexity of the interrelationship between water, energy, and other resources, as well as climate change, is an essential first step in creating future resource supply security. Without this, it will be impossible to measure the actual or likely total demand of each resource, the range of technical options available, the right incentives for rapid and transformative change, and the investment priorities needed to engage the private sector in what are often public sector projects. While many of the linkages between energy and water are clearly demonstrated, experiences have shown that ‘energy and water issues are rarely integrated into policy’, be it on the national or international level. On the global level, combination of changing weather patterns and trends in resource production and consumption are moving the water and energy security up their separate domestic and international political agendas.

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