As a result of the Clean Water Act, the U.S. Environmental Protection Agency is proposing a new rule limiting the intake of water pulled from lakes, rivers, estuaries, or oceans that will be used for cooling. If the rule passes in July, it will affect more than 1,200 power plants and manufacturing facilities that each withdraw at least 2 million gallons of water per day to dissipate waste heat.
Under the new rule, existing facilities adding new electrical generating capacity would be given eight years to implement closed-cycle recirculating cooling to reduce the amount of water used. According to the EPA, more than half already employ technologies likely to comply. But some power plants desiring to increase generating capacity will opt to shut down rather than install closed-cycle cooling.
Many large plants will soon be interested in options to use their waste heat to offset the costs of compliance. Charles Bowman, president of Chuck Bowman Associates, Inc., Knoxville, TN, a mechanical engineering consultancy serving the electric power generation industry, proposes a cost-effective "waste heat park" in his paper "Electric Power Plant Waste Heat Utilization," which will be presented at the ASME 2012 Summer Heat Transfer Conference.
CCW is discharged into the environment through cooling towers.
Condenser circulating water (CCW) is used to condense the low-pressure steam, thus removing the "waste heat" from coal-fired, oil-fired, combined cycle gas-fired, and nuclear plants that operate on the Rankine cycle. The CCW is heated while passing through the plant's condenser and discharged into the environment. In open systems, the heat in the CCW is discharged to lakes, rivers, or oceans, and in closed systems, it is discharged directly into the atmosphere, typically through cooling towers. Closed-system CCW is discharged from a plant with a temperature that is warm enough to be very usable.
Waste Heat Park
The concept of co-locating a business near a power plant to use its waste heat or water is not new, but building dedicated systems to deliver hot water can be prohibitive with a single user. Bowman's proposal is to combine multiple users who have different needs. Some would "consume" the waste heat. Others would use the water after it is cooled. This not only can minimize the cost of the distribution system but also provide a thermal efficiency benefit to the power plant by lowering the heat sink temperature of the CCW returned to the plant.
Bowman's proposed energy park is based on a two-unit nuclear power plant that was approved during the 1980s, but the second generating unit was never built. It is a closed-loop system for a portion of the CCW, flowing first from the power plant to a greenhouse user, where the temperature is ideal for floor heating in the winter or useful for evaporative cooling in the summer. The flow could cascade from greenhouses with a need for higher temperatures to those requiring lower temperatures. Using the waste heat, greenhouse operators could lower costs through reduced use of natural gas for heating fuel and potable city water for cooling.
The CCW would then be sent through a spray aeration system where it would be cooled to a temperature suitable for its next use at an aquaculture facility. Oriented spray cooling systems, currently used at a few plants, have a performance similar to cooling towers but are not dependent on ambient wind, have a simpler design, use less power, and require lower capital and maintenance costs, Bowman says.
After aeration, the CCW would flow down a raceway to an aquaculture facility, where it would be changed a number of times per hour, allowing fish that benefit from warm water, such as catfish and tilapia, to be raised. Being located near a power plant could be an economic advantage for aquaculture facilities, since it could supply fresh domestic seafood year round from a central location near cities. Sharing the infrastructure in a park such as this allows the aquaculture facility to operate with no additional aeration. According to Bowman, the small amount of ammonia produced by the fish waste in the returned CCW would either evaporate in the cooling tower or be removed by dilution when additional water is added in the plant.
The water would then be gravity drained into a settling pond and sent back to the power plant's cooling tower, where, in Bowman's example, it provides lower temperature water to the condensers. This increases the thermal efficiency of the turbine cycle and ultimately increases electrical output of each generator.
Synergy Is Key
Bowman says it is easy to envision adding other businesses to maximize the heat used and minimize the power plant's cost. Many others could potentially use the CCW, such as facilities raising pigs or chickens, drying lumber, tanning, or processing switchgrass for ethanol production.
"To be economically attractive, there needs to be a synergy among the users," he notes. "Everyone who has ever worked in this area has focused on one waste heat application. When a utility agreed to put in a 12-inch line to a greenhouse, it was run to capacity. No one thought: 'What else can we do with that water?' No one else has looked at the possibility of a synergistic effect. The better mousetrap here is the idea that in order to make this economically viable, both power plants and waste heat users should gain the maximum benefit from the capital costs of the distribution system."
Debbie Sniderman is CEO of VI Ventures LLC, a technical consulting company.
No one thought: 'What else can we do with that water?' No one else has looked at the possibility of a synergistic effect.
Charles Bowman, Charles Bowman Associates
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