National Handbook of Recommended Methods for Water Data Acquisition

Figure 10. Generalized diagram of thermoelectric power generating plant.

The thermoelectric power generation category includes water-use activities, such as: withdrawals from ground and surface water; deliveries from water suppliers; consumptive use from cooling towers, cooling ponds, and steam venting; water and wastewater treatment; and return flow (figure 11). The water in the steam cycle usually is treated at the power plant before use to reduce impurities that would cause build-up of mineral residue inside the boiler. The cooling water, which comes from lakes, rivers, or oceans, always is separate from the boiling water/steam (non-contact cooling water) and can be (1) discharged directly to the ocean, lake, or major river (once-through cooling), (2) discharged to a canal, cooling pond, or cooling tower before returning to the river, or (3) sent to a cooling pond or cooling tower before being recycled. Although most of the water used in thermoelectric plants is cooling water for condensing the steam, water is required for makeup water to replace the water lost as steam, blowdown (purging) of boilers, washing of stacks, plant and employee sanitation, water and wastewater treatment, and in nuclear plants, to keep the nuclear fuel from overheating and melting. Water from non-cooling uses goes to either a public wastewater treatment facility or the plant's onsite wastewater treatment facility. Storm water from roof drains and area storm sewers also may be treated at the wastewater facility and may cause discharge values to be higher than expected.

Figure 11. Diagram of thermoelectric water use.

The volume of water required in the thermoelectric power generation and the rate of consumptive use are dependent on whether cooling towers are used. Some plants built on an ocean or large river simply pump in large amounts of water to cool the steam. This cooling water then is returned back to the ocean or river. The water is somewhat warmer than when it entered the plant. If the cooling water is discharged to a significant water body, such as an ocean, lake, or major river, the water can be discharged directly. However, if the water is discharged to a river, its return to the river may be delayed by routing it through a canal or cooling pond first. Some plants recycle cooling water. Cooling ponds and towers are used to transfer the heat in the cooling water to the air.

A cooling pond is a shallow reservoir having a large surface area for removing heat from water. The surface area exposed to the air may be increased through the use of spray nozzles. Cooling ponds are used where land is relatively inexpensive, cooling water is scarce or expensive, or where there are strict thermal loading restrictions in place. If cooling ponds are used, water in the pond can be reused, thus reducing the overall water-withdrawal requirement.

A cooling tower is designed to remove heat by pumping water up into the tower and allowing it to fall down inside the tower. Air comes in from the sides of the tower and passes by the falling water. As the air passes the water, it exchanges some of the heat and evaporates some of the water. This heat and evaporated water flow out the top of the tower is in the form of a fine cloud-like mist. The cooled water is collected at the bottom of the tower and pumped back into the plant for reuse. Cooling towers are used where land and (or) water are expensive, or regulations prohibit the return of once-through cooling waters.

There are two primary types of thermoelectric plants--fossil fuel plants and nuclear plants. Although there are many similarities between them, there are some important differences that affect how each plant uses water. In fossil fuel plants, coal, natural gas, or oil are burned to provide the heat necessary to turn the water into steam. Biomass or solid waste fuel types are included with fossil fuels. Water is required to take care of the ash waste created during combustion. This includes both maintaining the stacks and carrying the waste ash away from the plant (fig.11). In nuclear plants, water is not required for ash disposal but is needed to keep the nuclear material from overheating and melting. Geothermal plants are only in California, Nevada, North Dakota, and Utah (Solley and others, 1993).

Power generation is measured in watt-hours, which is an electrical energy unit of measure equal to one watt of power supplied to, or taken from, an electrical circuit steadily for one hour. Most power generated from power plants are reported in megawatt-hours, or a million (1,000,000) watt-hours.

The thermoelectric power generation water-use data are the rate of (1) withdrawal by source, (2) deliveries from public water supply, (3) evaporation, (4) return flow, and (5) recycled water. The power-generation values also are collected for quality assurance.

11.J.2. Sources of thermoelectric power generation water-use information

11.J.3. Measurement, estimation, and data-collection methods for thermoelectric power generation water-use

11.J.3.a. Secondary data acquisition

Information for each power plant should be requested from (1) the EPA-State agency on name, location, and monthly discharge rates by pipe; (2) the USDOE, EIA on name, location, type of fuel, type of cooling system, and monthly rate of power generation; and (3) the State agency regulating withdrawals on the name, location and monthly rate of withdrawal of water. The rate of deliveries from public water suppliers can be obtained from (1) the plant, (2) the public water supplier, or (3) the NPDES permit application.

Gross power generation figures should be obtained, not net generation figures. Gross power is the total amount of power generated by the plant. Some of the power generated by the power plant is used to run the plant so that the power available to the grid is net power generated. The net power generated is equal to gross power generated by plant minus the amount of power needed to run the plant.

11.J.3.b. Derived data

The information from the above sources should be integrated. A coefficient relating water use (withdrawals and public water supply deliveries), and return flow to megawatt-hours generated can be developed for each plant. Unless there are major changes in the plant operations, estimates of water use and return flow can be made from power generation data for future years.

11.J.3.c. Quality assurance

There is a general relation between energy generated and water used (withdrawn and delivered from public water suppliers), discharged and evaporated that is primarily dependent on whether cooling towers are used. This generalized relation should primarily be used to ensure that the data set received from the above agencies are reasonable, and not to estimate the rate of water use, discharge, and evaporation.

Withdrawals can be estimated for plants with a once-through cooling system, using the general ratio of 1 Mgal/d withdrawn to 1 megawatt hour generated. Consumptive use is between 0.0005 and 0.001Mgal/d per megawatt hour generated, or between 0.05 and 0.1 percent. For plants with cooling towers, the general ratio is 0.01 Mgal/d withdrawn to 1 megawatt hour generated. Consumptive use may vary from 0.0033 to 0.001 Mgal/d per megawatt hour generated, or between 33 to virtually 100 percent. The low end may represent a mix of once-through cooling and cooling ponds/towers. These relations are based both on published coefficients (found in the selected references) and reported values for New England and Colorado. Significant variation from this relationship should be investigated by contacting the individual plant. For instance, frequent starting up and shutting down a plant can result in large consumptive use because steam is vented to the atmosphere when the plant is started up and shut down. In some instances, pump intakes may continue to run after the plant has been shut down.