High-Efficiency System for Dry Heat Rejection

Dry cooling systems are the only option for industrial and utility power plants that are unable to obtain cooling water supplies or permits, and can be applied to both nuclear- and fossil-fueled plants. Available dry cooling systems are less efficient than wet systems, so significant improvements in efficiency are needed to make them economically viable. Previous attempts at using foam as a cooling fin material for power generating systems have focused on high thermal conductivity graphite foams, but because those materials had very high pressure drops and hence low permeability with respect to air flow, their internal volume and surface area were not effectively used and they performed poorly.A more open foam would increase the permeability, greatly enhance air flow through the bulk material, increase the rate of heat transfer, and enable the material to outperform traditional fin structures. In recent work for DOE, Ultramet developed high-performance, air-cooled graphite foam heat exchangers for use in conventional power plants located in desert environments where cooling water is not available. Whereas graphite foams made via conventional processes have high densities and high pressure drops, graphite foams made by chemical vapor deposition (CVD) have pressure drops that are orders of magnitude lower. This enables the cooling air to permeate the entire foam volume, thus taking full advantage of its high internal surface area for heat transfer. Various foam architectures have been developed and tested, and at an air-side pressure drop of 1 in H2O, the heat transfer rate is nearly four times greater than that of high-performance aluminum fins.The heat transfer and pressure drop models developed in previous work at Ultramet will be used to develop foam architectures specifically for use in a Brayton power cycle system. A flow sheet will be developed, the heat exchanger duty and operating conditions will be quantified, foam architectures will be developed specifically for those conditions, and a series of subscale heat exchanger cores using those architectures will be tested. The resulting data will be used to refine the models (if necessary) and to scale up the heat exchanger design.Commercial applications and other benefits: Large-scale commercial applications include industrial and utility power plants (both nuclear and fossil fuel), petrochemical refineries, and chemical process installations. Smaller applications include heat exchangers used by the military in desert environments (e.g. air conditioners, environmental control units, heat exchangers for radars).