Additive, Manufacturing, Heat, Exchanger, HVAC&R
The Advanced Research Project Agency (ARPA) includes the Advanced Research In Dry cooling (ARID) program. The objective of the ARID program is to develop technologies that allow power plants to achieve high thermal-to-electric energy conversion efficiency with zero net water consumption. The most direct method of achieving this goal is to improve the thermal efficiency of dry-cooled condensers by reducing the air-side thermal resistance without significantly increasing either the capital cost or the fan power required to operate these devices. One approach to this is the application of additive manufacturing to dry cooled condensers in order to obtain a low cost, high performance heat rejection system. Conversations with air-cooled heat exchanger manufacturers have resulted in aggressive cost targets that must be met in order to make this technology commercially viable in relevant first markets. A leading manufacturer of air-to-water heat exchangers has indicated that their “gold-standard” heat exchanger for HVAC&R applications achieves economic and thermal performance of approximately $10/kWth and 7kWth/kg, respectively. This represents the limit of state-of-the-art air-cooled system performance that is the product of a many decades of engineering research; the limits of this technology are unlikely to change using conventional approaches since geometric constraints and costs are largely dictated by the manufacturing technology. The application of additive manufacturing to heat exchangers allows an unparalleled freedom in design. Fused Filament Fabrication (FFF) is a relatively low cost additive manufacturing process that involves depositing thermoplastic material layer by layer. The coupling of FFF with numerical models of air-side thermal fluid behavior has led to a novel, compact, high performance heat exchanger design that has been manufactured and validated experimentally. The heat exchanger features water and air in cross flow with an airfoil fin array on the air-side. Design paths towards cost and performance targets that will allow this technology to be competitive with industry targets have been identified and include: better understanding for manufacturing process constraints, filled material development, and air-side convection improvements. In the FFF process, physical geometrical constraints such as extrusion nozzle diameter and layer height have implications on thermal performance, overall resistance to heat transfer, and manufacturability. Typical thermoplastic thermal conductivities are 100 to 1000 times smaller than that of the aluminum and copper used in industry standard heat exchangers. Efforts to manufacture polymers that are filled with conductive materials such as carbon fiber, aluminum flakes, and graphite aim to decrease this gap by an order of magnitude. To be competitive with traditional air-cooled heat exchangers with material that is much less conductive, air-side convection optimization using Computational Fluid Dynamics (CFD) has been utilized to implement and validate advanced air-side geometries. An FFF manufactured heat exchanger that is cost and performance competitive with traditionally manufactured heat exchangers will open the door to a future in air-cooled systems that currently does not exist. This technology will allow heat exchangers to become lighter and fit in a smaller envelope as well as result in rapid replacement, high customization, fouling resistance and other secondary advantages.