News Ticker

More freshwater is now used for thermoelectric power production than for agricultural irrigation in the United States—41% to 37%, respectively (U.S. Geological Survey Circular 1344, 2005). Much of this generation operates according to the steam-driven heat engine process known as the Rankine cycle. In a Rankine cycle-based power plant, heat generated from the combustion of conventional fuels (e.g., coal, gas, oil) and renewable fuels (e.g., biomass, waste-to-energy), or through the use of concentrated solar energy and geothermal energy, or through nuclear fission is used to boil water to make high-pressure steam that is expanded through a turbine to generate power. The exhausted steam must be condensed by dissipating heat to the environment before being returning to the boiler to complete the cycle.

For many years, the most popular cooling option for thermoelectric power was once-through cooling (or openloop cooling) because it requires the lowest capital costs. In this system, the water is withdrawn from a body of water and diverted through a heat exchanger (typically called a condenser) where it absorbs heat from and condenses the turbine exhaust steam. The water is then returned directly to the water source with minimal water consumption. However, open-loop cooling does require large water withdrawals and returns water to the source at a higher temperature.

At the high flow rates utilized for open-loop cooling (~30,000 gal/MWh), water intake structures can remove aquatic organisms through impingement and entrainment, causing direct kills of fish and eggs at the intake; aquatic ecosystems can also be altered as a result of the elevated water temperatures near plant water discharge, according to the U.S. Environmental Protection Agency (EPA). These systems are no longer considered to be a viable design option for new plants, and existing installations are under increased regulatory pressure to switch to technologies with less environmental impact. For example, EPA is currently proposing to update regulations that would require implementation of a closed-loop cooling technology or another design change equivalent (e.g., reducing intake flow rates, installing fish deterrents, etc.) to the entrainment reductions associated with closed-loop cooling.

Closed-loop wet cooling systems recirculate water through a condenser and a cooling tower. The cooling tower rejects the heat from the steam to the atmosphere via evaporative cooling using mechanical or natural draft airflow. Although closedloop systems recirculate a majority of system water, evaporative losses need to be continuously replaced; therefore, significantly less water is withdrawn but more water is consumed (~750 gal/MWh) compared to open-loop cooling systems.

Water-based cooling is cost-effective and efficient, but lack of water availability frequently makes water-based cooling a contentious issue because cooling needs are often perceived to be in conflict with sustainability of water resources.

“Two EERC projects are focused on reducing water use for thermoelectric power,” according to EERC Director Gerry Groenewold. “One is a novel dry cooling technology that can eliminate the need for cooling water. The second project involves a new hybrid cooling system that will economically decrease water requirements.

“Optimizing cooling systems used in power generation to increase water use efficiency is key to long-term sustainable energy development and economic development,” said Groenewold.

Novel Dry Cooling
Dry cooling options reject heat directly to the atmosphere, but they are more costly than wet systems and do not work as efficiently or produce as much electricity during hot weather. Conventional dry cooling systems cost 3.5 to 4 times as much as a wet cooling system. Particularly in those hot, dry areas of the country where water is scarce, there is a unique need for a dry cooling alternative.

The EERC has developed a novel new dry cooling technology with support from DOE and the Wyoming Clean Coal Technologies Research Program that is applicable to all Rankine-based power plants and similar heat rejection loads. The project also has in-kind assistance from SPX Cooling Technologies. This unique system uses a hygroscopic fluid as a coolant, which is nonvolatile and does not evaporate. This eliminates the continual need for cooling water, making the technology most suitable for locations without adequate water.

“One of the most remarkable aspects of the technology, to me, is that the coolant absorbs moisture from the air at night when temperatures are lower and, as the day heats up, evaporates the excess water, which is a benefit for cooling and helps to moderate performance,” said Research Engineer Chris Martin. Martin invented the technology and serves as project manager for its evaluation.

In late 2011, the EERC designed and built an experimental validation test facility at its research complex in Grand Forks, North Dakota. Testing in early 2012 showed that the initial feasibility concerns can be overcome and that the process dissipates heat to the atmosphere efficiently. Evaluation testing will continue in  2012 to demonstrate that the EERC technology offers improved cost vs. performance compared to conventional dry cooling methods. Future developments could include a long-term outdoor demonstration at a power facility with industry support.

Improved Hybrid Cooling
Systems that integrate both wet and dry cooling components are referred to as hybrid cooling systems. Hybrid systems generally have a lower capital cost than a dry cooling system and use less water than a completely water-cooled system. There are various configurations to achieve hybrid cooling, including a system that consists mainly of 1) a wet stream–surface condenser plus a wet cooling tower and 2) a dry  stream–jet condenser plus a natural or mechanical draft air-cooling tower.

The EERC and GEA Heat Exchangers, Inc. (GEA), with DOE support, partnered to build and evaluate a first-of-its-kind hybrid condenser, which combines the operations of a jet condenser and a surface condenser. The hybrid condenser was designed by GEA to maintain the benefits of a hybrid system, while further reducing costs.

A prototype module of the hybrid condenser design was fabricated and tested at the EERC to validate its potential to more economically reduce water consumption in electricity generation applications. GEA conducted the testing with the EERC to ensure operations and data were representative of conditions seen in  commercial industry. Data and economic evaluation is currently under way.

“This project reflected, to me, the best of the EERC, such as working in partnership with commercial, global companies like GEA, as well as using our pilot-scale fabrication and system-testing capabilities,” said Research Engineer Kerryanne Leroux, who, as the project’s principal investigator, oversaw the initial testing in the summer of 2012.

There is a growing market for the hybrid condenser, specifically at existing power plants with water-based cooling where water availability is becoming limited. The ability of the hybrid condenser to be retrofitted into these power plants’ existing footprints and the resulting water savings realized during operation make the hybrid condenser an attractive solution.