NETL has assessed the impact of cooling water intake regulations on power plant efficiency

EPA is developing regulations to implement section 316(b) of the Clean Water Act (CWA), which addresses cooling water intake structures. EPA proposed regulations for new sources in August 2000 and will finalize them later this year. Under the proposed rule, most new facilities could be expected to install closed-cycle cooling systems, presumably wet cooling towers. Further, the environmental community and several States have supported the use of dry-cooling technology as the appropriate technology for addressing adverse environmental impacts.

Although the final outcome of the new source rule is not yet known, that rule may have a role in shaping the rule for existing facilities scheduled to be proposed in February 2002. Recognizing that over 50 percent of the existing coal-fired power plants employ once-through cooling systems, a decision to require these plants to install dry- or wet-cooling tower systems could have serious impacts on electricity availability. Further, it could increase greenhouse gas and other atmospheric emissions due to decreased efficiency associated with the different cooling options. Additionally, power plants using other fuel types could also be affected by the upcoming proposed rule which will exacerbate electricity availability and emissions impacts.

To evaluate potential impacts of a new 316(b) existing facility rule, NETL is working with EPA and industry to develop estimates of the loss of electricity output (the energy penalty) and increased air emissions associated with retrofitting two types of cooling water systems. The types of cooling water systems to be evaluated are:

1. Wet Cooling Tower - The condenser is cooled with water recirculated to a mechanical draft cooling tower. Because there is a direct air/water interface, heat transfer is controlled by the wet bulb temperature of the air. Some of the circulating stream is evaporated into the atmosphere. Additionally, some of the recirculating flow is discharged as blow down water. Makeup water to replace these losses is withdrawn from a surface water body or municipal water source.

2. Dry Cooling Tower - The condenser coolant is enclosed within a piping network with no direct air/ water interface. Heat transfer is based on the dry bulb temperature of the air and the thermal transport properties of the piping material. While water loss is less for dry cooling towers than wet cooling towers, some makeup water is required. Cooling efficiency is lower for dry cooling systems than wet cooling towers due to the higher dry bulb temperature.

Retrofitting a once-through cooling system with cooling technologies that produce higher condenser inlet water temperatures will result in lower net plant efficiency. This in turn will produce unfavorable energy and environmental impacts since lower efficiency would require more fuel to generate replacement power and more emissions from combusting that fuel. Additional negative environmental impacts would also be observed due to fuel extraction and transportation requirements to meet the added fuel demands.

NETL is working with Argonne National Laboratory (ANL) to study the energy penalty resulting from converting existing plants with once-through cooling to wet towers or indirect dry towers. Five different climate locations- Delaware River Basin (Philadelphia), Michigan/Great Lakes (Detroit), Ohio River Valley (Indianapolis), South (Atlanta), and Southwest (Yuma, AZ)- were modeled using the ASPEN process simulator. The model evaluated the performance and energy penalty for hypothetical 400 MW coal-fired plants. The modeling was done to simulate the hottest time of the year using temperature values that are exceeded only 1 percent of the time between June and September, at each modeled location. This corresponds to the time of year when the highest power demands are observed. To the extent possible, estimates of national energy losses and increased air emissions will be developed. The results of this study will be available soon, and will be used to aid decisions regarding what is the best available technology to address the impending cooling water intake structure regulations.