Metal Wire Structures as Heat Transfer Surface Area Enlargement - Design Study and Potential Analysis for Air-to-Water Heat Pumps

It is common practice in air-to-water heat pumps to use fin-and-tube heat exchangers as evaporators. The typically used heat exchanger concept is based on round tubes which are connected by non-adhesive joints. The surface area enlargement for tubes in the range of 15mm down to 5mm is about 3.5-20. Other concepts like the so-called microchannel heat exchanger uses typically adhesive joints reaches similar surface area enlargements for stationary equipment. In order to increase the surface area further on, without using more material, wire structures can be used as fins instead of metal sheets. In addition fluid flow along wire-based structures experiences a very high heat transfer coefficient due to the repeating interruptions and small dimensions. New textile developments enable the fabrication of adapted structures with non-regular grid sizes purpose-built for gas to liquid and gas to gas/liquid heat exchanger application. At Fraunhofer ISE a variety of textile structure heat exchanger samples has been manufactured and evaluated. In this paper a simulation study for different geometries of a flat tube wire structure heat exchanger will be performed and the integration into air-driven evaporators of residential heat pumps will be discussed. The results will indicate the potential of wire structures with respect to performance enhancement, material usage and size.