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Water Worries

With new plants pending, cooling requires serious thought.

Fortnightly Magazine - December 2008

developing, testing, and deploying efficient advanced water-cooling technologies. Options include power-plant siting, meteorological impacts on air-cooled condensers, indirect dry cooling, hybrid tower designs, water-recovery options, wet surface air coolers, advanced bottoming cycles, and preserving once-through cooling options.

EPRI’s Goldstein sees four options as particularly appealing.

Implementing a hybrid system that uses a combination of dry (air) cooling and wet cooling.

Increasing the thermal conversion efficiency of the thermo-electric plant, so more of the heat is used to generate electricity and less is being rejected. “This reduces cooling requirements,” he says.

Replacing a fresh-water source with a non-traditional water source, such as saline water, sewage treatment effluent, agricultural runoff, water produced in association with the drilling of gas and oil, drainage from mines, etc. “Of course, there are costs associated with this, including pre-treatment and post-treatment,” he points out. “There may also be transport costs.”

Recycling water within the plant. For example, technologies currently are being explored to capture a certain percentage of the vapor that normally would be leaving wet cooling towers, condensing it, and then recirculating it.

The first option, hybrid cooling, strikes a chord with a number of people in the industry. However, while the technology is quite appealing for coal and gas-fired plants, it presents challenges for nuclear. “Fossil plants can use, and are using, more dry cooling, over hybrid wet-dry cooling,” points out John Maulbetsch, president of Maulbetsch Consulting in Menlo Park, Calif. “It’s unlikely that nuclear plants will be able to use direct-dry cooling, especially nuclear plants using boiling-water reactors.”

Dry cooling might be possible with pressured-water reactors, but even then, it’s unlikely. “Nuclear people don’t want a direct steam path from the turbine exhaust out to a large open structure, so their options are limited to indirect-dry cooling, where the steam is condensed, and then the hot cooling water from the condenser is taken to an air-cooled heat exchanger, rather than an air-cooled condenser,” he explains. It’s cooled, then returned. These systems are more expensive and less efficient than air-cooled condensers, and as a result “going dry” raises costs for nuclear plants.

Costs also are higher for nuclear plants using hybrid systems. These involve air-cooled condensers operating in parallel with wet-cooling towers. “You use dry cooling until the ambient temperature gets high enough that the dry-cooling system can no longer maintain the condenser back pressure at the necessary level,” he says. “At this point, you phase in the wet cooling.”

This works in nuclear plants and fossil plants. However, in nuclear plants, the dry portion probably needs to be an indirect dry system, rather than using an air-cooled condenser, so it will be more costly and less efficient than a comparable hybrid system in a coal plant. “It’s all doable,” Maulbetsch says. “It’s just more expensive.”

Plant siting also might need some serious dialogue in the industry, according to Sandia’s Hightower. “We may end up in a situation where a lot of the new plants have to be located along ocean and large lake shores,” he says. “A lot of people may