Water-energy nexus: Understanding the inter-annual water use of power plants

January 11th, 2016

Xiawei Liao, Oxford University, United Kingdom

Water is an indispensible input, both withdrawn and consumed, to the power industry. Approximately 43% and 50% of freshwater is abstracted for thermoelectric cooling, in the EU and US, respectively.1 Water-related risks can disrupt electricity generation, for example as experienced by the shut downs and curtailments of thermal generators in the US during the 2007 and 2012 droughts.

An appreciation of the increasing water demand by the energy sector has therefore grown considerably in recent years.2,3,4 A comprehensive review of water uses in power plants and a close look at the intra-annual variations of water intensity are carried out in this article.

Power plants’ water use

Previous literature5 has shown that, within the life cycle of energy use, power generation makes up the lion’s share of water consumption. Although most studies have focused on cooling water usage, which dominates the water use at power plants, water use of other in-plant processes remains relatively unknown. A comprehensive understanding of operational water use can shed light on in-plant water conservation work. The water-consuming processes of power generation can be categorized as below:

Industrial water: raw water which undergoes clarification and can then be used for different industrial purposes such as lime-ash removal and wet desulfurization, as well as to cool the steam that drives the turbine and other auxiliary engines, including induced air fans, forced draft fans, primary air fans and so forth. This is the major part of power plants’ water use.

Domestic water: water consumed to meet the living needs of power plant workers.

Chemical water: demineralized water of the highest quality is needed for boiler water make up, as salinity enrichment, means that boiler water needs to be blown down regularly.

Detailed water consumption factors of power plants in a city in eastern China are demonstrated in Table 1.

It is worth noting that water intensity decreases with increases in unit capacity. This is because power units of larger capacities employ more advanced technologies with higher efficiency, specifically boilers with higher pressure, which subsequently lowers the water intensity. An overview of the technologies used by power plants of different capacities in China are demonstrated in Table 2 according to China Electricity Council.6

Intra-annual variations

Many studies have discussed the factors impacting the water intensity of power generating activities (e.g. the energy source or the cooling technology used) based on annual statistics. However, as illustrated in Figure 1, water consumption varies through the year. For the power plants in this fieldwork, water use as found to be slightly higher in the winter and lower in the summer.

In addition to the fact that thermal power production differs by month, intra-annual variations of water intensity also contribute to the variations in water consumption, something which has often been overlooked by the existing studies.

There are various factors contributing to the intra-annual variation of water intensity. First, temperature is of the upmost influence. Higher inlet cooling water temperatures cause efficiency loss and thus increase the water intensity. For instance, Kim and Jeong (2013)7 pointed out that increasing inlet cooling water temperature by 15 degrees can lead to a 2% loss of efficiency and a 6% loss of power. Van Vliet et al. (2013)8 assessed the impacts of climate change on global river flows and river temperatures and suggested that projected river temperature increases and low flow decreases in southeastern U.S., Europe, eastern China, southern Africa and southern Australi are likely to have subsequent impacts on thermoelectric power generation.

Secondly, as some water uses (e.g. domestic water) are relatively inelastic, the more electricity generated, the lower the overall water intensity will be. As an example, the monthly electricity generation and water consumption intensity of a power plant in southwest China in the year 2014 are presented in Figure 2.

It can be seen that thermal power production peaks in the winter, presumably due to two reasons: first, precipitation in southwestern China mainly occurs in the summer, i.e. June through August, owing to the monsoon climate,9 and thus hydropower capacity reduces the need for coal power. Conversely, electricity supply in the drier winters primarily depends on thermal power. Secondly, electricity demand increases during winter due to the heavy use of central heating in the north and electric heat pumps in the south. The low power production seen around February is attributable to the industrial shut downs during the Chinese New Year Festival. Thus water intensity is higher in the summer and lower in the winter, which offsets the impact of power production on water usage to some extent.

As water availability is the lowest in the winter, the temporal pattern of the power sector’s water consumption can be problematic.

Conclusions

A growing awareness of the water-energy nexus has raised increasing concerns over power plants’ water usage. In order to cope with this emergent and multifaceted issue, institutional transformations have been suggested and discussed by many scholars looking at different areas in the world.4 However, a fact that is often overlooked by scholars but encountered as problematic by practitioners is that both water and energy are highly context-dependent issues in terms of their temporal and spatial variances.

This article highlighted the intra-annual variations of power generation’s water intensity using information from several Chinese power plants. It highlights that temporal variations need to be taken into account when estimating the power sector’s water uses to strengthen the coherence between water and energy policies. For instance, even if water demand by the power sector can be met by the water available on an annual basis, seasonal variability may cause problems. Theses implications can be applied globally. However, as the annual patterns of the impacting factors, e.g. electricity demand and production, temperature and so forth, differ by region, local characteristics need to be considered when policies are formulated and implemented.

Moreover, a more comprehensive understanding of how water is used in power generation besides cooling helps us to better identify possible water conservation measures.

References:

United Nations World Water Assessment Programme (UN WWAP). 2014. The United Nations World Water Development Report 2014: Water and Energy. Paris. Zhang, C. and Anadon, L.D.. 2013. Life cycle water use of energy production and its environmental impacts in China. Environmental Science & Technology. 47: 14459-14467. Byers, E. A., Hall, J. W. & Amezaga, J. M.. 2014. Electricity generation and cooling water use: UK pathways to 2050. Global Environmental Change. 25:16-30. Qin,Ying., Curmi, Elizabeth., Kopec, Grant M., Allwood, Julian M., Richards, Keith S. 2013. China’s energy-water nexus assessment of the energy sector’s compliance with the “3 Red Lines” industrial water policy. Energy Policy. 82: 131-143. Meldrum, J., Nettles-Anderson, S., Heath, G. & Macknick, J.. 2013. Life cycle water use for electricity generation: a review and harmonization of literature estimates. Environ. Res. Lett. 8: 015031. doi:10.1088/1748-9326/8/1/015031 China Electricity Council. 2012. Notification of energy efficiency benchmarking and competition data of 2012 national 600MW thermal power units. Available online: http://kjfw.cec.org.cn/kejifuwu/2013-04-07/99877.html B.K. Kim, Y.H. Jeong. 2013. High cooling water temperature effects on design and operational safety of NPPS in the gulf region. Nuclear Engineering and technology. 45 (7): 961-968. Vliet, Michelle T.H. van., Franssen, Wietse H.P., Yearsley, John R., Ludwig, Fulco., Haddeland, Ingjerd., Lettenmaier, Dennis P., Kabat, Pavel. 2013. Global river discharge and water temperature under climate change. 23: 450-464. Ye, Wenhu., Zhang, Yong. 2013. Environment Management (3rd Version). Shanghai Xiawei Liao, 2015, Fieldwork Communications. China.

Xiawei Liao is a doctorate student working at the Environment Change Institute of Oxford University with Professor Jim Hall and Doctor Nick Eyre. His work focuses on the future conflicts between water availabilities and water demands by China’s thermoelectric power sector.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.