By Sean McKenna, PhD.
In heavily populated regions of the world, the available water is fully subscribed. That is, claims have been made on all the available water for different uses including recreation, drinking water supply, industry, agriculture, and energy production.
As global energy consumption continues to rise (estimated 56 percent growth between 2010 and 2040 – US Energy Information Administration, 2013), additional water will be needed to increase energy production.
Finding water that is not already claimed is becoming difficult. However additional efficiencies can be obtained through improved management of the existing water resource. Generating energy through less water intensive means is another approach, but even Solar PV and wind turbines require water to mine and manufacture the materials that comprise them.
Electric power production is particularly sensitive to the quantity and temperature of water in rivers and lakes. Between 2006 and 2012, 27 power plants in the US, both nuclear and coal, had significant issues with cooling water (Union of Concerned Scientists, 2012). Either the temperature of the intake was too high for cooling or the discharge water temperature was too high to return to the environment, or there was not enough water for cooling.
As older existing power plants that utilize “once-through” cooling are replaced with re-circulating cooling systems, these issues with water temperature will decrease, but where once-through systems return the majority of water to the local source, re-circulating systems consume the majority of the cooling water. This changes the water use for cooling from simple extraction to consumption and problems of not enough water for cooling will become more prevalent.
The other side of the coin is the energy needed for water. As an example: 10 percent of global electricity is used to pump fluids, mainly water; 90 percent of all electricity used on California farms goes to pumping ground water for irrigation (NRDC, 2004); Water utilities typically spend 30-60 percent of their annual operating budgets on electricity.
Traditionally water system operators have benefitted from day/night electricity pricing such that they can fill storage tanks at night when electricity is cheap and then let gravity do the work during the day when electricity is expensive. However, electricity pricing is becoming more complicated as increasing amounts of renewable energy enter the grid.
Here in Ireland, large consumers of electricity may opt for dynamic pricing where the variable price of electricity over the next 24 hours is provided as a forecast on half-hour intervals. The cost of electricity no longer set at fixed rates for periods defined by consumer demand; energy is now cheaper when the sun shines and/or the wind blows. Taking advantage of these new pricing schemes is not simple, and we develop and apply optimization tools that can deliver optimal pumping schedules to minimize electricity costs and/or greenhouse gas emissions.
Defining a pump schedule that is robust to deviations from the forecast is necessary. This approach is a bit like portfolio management in the stock market. The goal is to find the sweet spot in the trade-off between achieving the largest expected savings and minimizing the risk of tying the pump schedule to an inaccurate price forecast that results in running the pumps during a time of extremely high electricity costs. The physical infrastructure comes into play here as well. Water utilities must meet minimum and maximum tank levels as well as maintain pressures within their distribution networks. Any schedule has to avoid excessive on/off switching of the pumps that can cause increased wear on the equipment.
Energy for water also plays a role in new sources of drinking water. Desalination is a proven technology to generate pure water from saline water, but it is also a very energy intensive process. In order to push saline water through a membrane and into a more dilute solution, osmotic pressure must be overcome and again, pumps are called into action. The energy consumed by pumps to create pressures greater than osmotic pressure for typical seawater works out to about three kilo-Watt-hours per cubic meter of fresh water and is very sensitive to the salinity of the seawater.
The bottom line is that there are no simple solutions. As water resources become increasingly constrained, the nexus between water and energy becomes tighter and the path forward is more complex. Analytic tools including big data, data-driven analytics and predictive modeling can be brought to bear to deploy creative solutions. Sensors that can continuously monitor continuously monitor the quantity and temperature of water in rivers and lakes provide data to analytics that optimally allocate water to power plant cooling while meeting environmental constraints.
Predictive models of ocean salinity variations lead to improved forecasting of desalination energy requirements. These solutions improve water resource management and extract more efficiency from existing resources.
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