High-Efficiency Compact Heat Exchangers and Condensers for Small Modular Energy Systems

Statement of the problem or situation that is being addressed: Regardless of the design type, small modular reactors used for power generation require small components and small footprints in order to be economically viable. This is particularly true for the heat rejection equipment, which can be quite large. Furthermore, if the plant is located in an area where cooling water is prohibitively expensive or simply unavailable, dry cooling will be required. Graphite foams derived from pitch have been tested for dry cooling, 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. Statement of how this problem or situation is being addressed: Foam with a more open structure will 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 both fossil and nuclear 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 measured heat transfer coefficient is 3.5 times greater than that of similarly sized high-performance aluminum fins. What is to be done in Phase I? Using heat transfer and pressure drop models developed in previous work, foam-based air-cooled condensers/coolers will be designed for near-term use with small modular reactors. Flow sheets will be developed for the different types of plants, the operating conditions and heat rejection requirements will be quantified, one system will be downselected, foam architectures will be developed specifically for that plant type, and a series of subscale heat exchanger cores using those architectures will be fabricated and tested. The resulting data will be used to refine the existing models and pave the way for scaleup. Commercial applications and other benefits: Heat rejection equipment for small modular reactors will be the primary market for this technology, but it can easily be scaled up to 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).