May - June 2010
Vol. 73 No. 3

  

Water and power: joined at the hip

The nexus between water and power, either mechanical or electrical, goes back to the earliest days of power generation. In view of the current drought conditions in the American Southwest and moves to expand power production of all kinds, it seems appropriate to take a closer look at this long and intimate relationship and what it bodes for the future.

Generating power requires water

Water wheels have been used to power mills of various sorts for centuries, and versions of the water wheel have been used to generate electricity since the late 19th century. Hydropower production, while responsible for the diversion of rivers and the construction of dams, is not generally considered to be a consumptive use of water, although evaporative losses from the surfaces of large lakes (think Lakes Mead and Powell) can be quite significant.

The advent of steam-powered generation of electricity from fossil fuels dramatically changed the relationship between electricity generation and water. Generating electricity with a steam-driven turbine requires water, whatever the heat source. This is why steam plants have historically been located adjacent to a river or lake. While water is obviously needed to generate steam, the major use of water is for cooling purposes. In a modern closed-system power plant, the thermodynamic efficiency of the system is directly related to the difference between the input and output pressures (or temperatures) of the steam through the turbine. Cooling is required to recapture the steam and to maintain the maximum pressure differential. This also allows re-use of the corrosion inhibitors and anti-scaling compounds that are added to the water.

Water is one of the best and cheapest heat-transfer agents available, and evaporation of water in cooling towers is a very effective means of transferring the waste heat to the atmosphere; water is the preferred means of cooling. Some of the newest steam-powered generating systems use a "dry cooling system" of air flows, much like an automobile radiator, only a million times larger. The efficiency depends on air temperatures. At ambient temperatures above 80° Fahrenheit, efficiency declines so much that on a normal summer day in the desert southwest, the dry-cooled plants produce about 10% less power than the equivalent wet-cooled power plants for the same amount of fuel.

Moving water requires power

Prior to the 20th century all systems used to move water over long distances were gravity powered. The development of electric motors and water pumps enabled the pumpage and transport of large volumes of groundwater for agricultural and municipal purposes. The first aqueduct from the Owens Valley to Los Angeles was (and still is) a gravity system. The second aqueduct, constructed in the late 1960s, is filled with groundwater pumped to the surface electrically.

The Colorado River supplies water for Las Vegas, Phoenix, Tucson, and the cities of southern California. All depend on electricity to move water uphill from the Colorado River to its point of use. Hoover Dam provides power for the movement in California. About 20% of all the electric power consumed in California is used to pump, treat, or heat water; movement of water connected with the California Aqueduct is a large fraction of that. If desalination on a large scale becomes a reality, there will be an accompanying need for new power sources to operate those plants, and those power sources will require water to operate.

In southern Nevada the largest single power use is to pump water from Lake Mead up to the Las Vegas Valley, an elevation difference of a thousand feet or more. At the same time, one of the major water users in the region is NV Energy (formerly known as Nevada Power Company), which uses large volumes of water to cool its steam-powered electricity-generating facilities.

The Central Arizona Project (CAP) which transports Colorado River water to the Phoenix-Tucson region, is a major consumer of hydropower generated at Glen Canyon Dam. The need for electric power to operate the CAP was a major reason for building that dam and its power-generating facilities.

At present there are applications filed with the Bureau of Land Management to use about a million acres of land in the deserts of California and Nevada for solar generation of electricity. Most of these proposals are for what are called "solar thermal" facilities, which concentrate sunlight to heat a transport medium that in turn heats water and generates electricity with a conventional steam-turbine system. Some of these proposals would use dry cooling while others amazingly, are looking at wet cooling. They also will need water to wash the solar-collector mirrors to keep them operating at peak efficiency. Even photovoltaic power systems need water to clean the panels.

The bottom line

Energy needs and water needs of our society are inextricably linked, especially in our desert areas where per-capita demands for both water and electric power are very high during the summer months. It behooves us to remember that every water project will require power and that every power project, excepting wind power, will require water at the site. In a water-short area this is a sobering thought. Conservation of both water and power will be critical for the desert southwest.

Water and energy

Water and energy are inherently linked, especially in California.

Although water generates approximately 33% of the state's electricity, according to the California Energy Commission (CEC), water-related energy use in California consumes approximately 20% of the state's electricity, and 30% of the state's non-power plant natural gas (i.e. natural gas not used in turn to produce electricity). Water-related energy use includes pumping, treating, and distributing potable water, groundwater pumping, desalination, heating and cooling processes, pressurization, and the collection, treatment, recycling, and discharge of wastewater.

Some water systems are net energy producers, for example, the federal Central Valley Project as well as San Francisco's Hetch Hetchy and the Los Angeles Aqueduct water systems. Others are net energy consumers, for example, Metropolitan Water District's Colorado River Aqueduct and the State Water Project. In fact, the SWP is the single largest user of electricity in the state, although the project produces about half of the energy it consumes."