High Efficiency Heat Exchanger For Ice Energy Storage and Beyond Grady Iliff, Levon Atoyan and Mitchell Ishmael Active Energy Systems
is a new HEX design that can produce up to 6 times more ice per cycle than existing HEXs with the same surface area. The new HEX design is ice-phobic: the adhesion stress of freezing water is negligible.
Freezing ice on coils is a slow and transient process Here we present our observations of this new phethat hinders the deployment of ice thermal energy nomenon and the associated freezing HTC. storage systems for large commercial buildings. Here, we share observations of a novel interfacial phenomExperimental Setup enon that was used to engineer an ice-phobic heat Heat Exchanger setup exchanger. Water drops that were frozen on the icephobic surface demonstrated extremely low interfaThe ice adhesion experiments were initially done on cial adhesion, allowing the frozen drops to be shed a much smaller scale than the HTC experiments. A off by body forces alone. Heat transfer coefficient small piece of aluminum was placed at the bottom of measurements of this steady-state freezing process a glass container on a cold plate cooled by a chiller. Mitchell Ishmael indicates improved performance over the traditional The aluminum stage had a small hole drilled into it ice-on-coil process. Increasing the heat transfer coefficient reduces for a thermocouple probe. Adhesive PTFE (polytetrafluoroethylthe necessary heat exchanger size, which decreases an ice thermal ene) tape was applied to the aluminum surface. The glass container storage systemâ€™s footprint, important for high-density urban envi- was then filled with 5 cSt silicone lubricating oil. The density of the ronments. These factors combine to substantially lower costs and silicone oil was nominally reported to be ~930 kg/m3 @ 0 ÂşC, which increase applicability of ice thermal energy storage systems. would place it at an intermediate density between ice and liquid water. The camera was then setup so the PTFE surface was in focus at Introduction approximately 40x zoom and recording at 10 fps. Ice thermal energy storage is a process in which ice is produced using off-peak, low-cost electricity as a means of storing energy in the form of latent heat. The ice is used to provide cooling for buildings during the warmer, on-peak hours of the day (avoiding chiller operation during these hours). Ice thermal energy storage is used by commercial buildings and campuses to lower energy costs all over the country [4,6]. Current shortcomings of ice thermal energy storage technology can be attributed to poor heat transfer during the ice making process [4,6]. Ice-on-coil heat exchangers (HEX) grow ice radially outward from the cooling surface. As ice grows on the surface, the thermal resistance between the water and the HEX surface increases, and thus the heat transfer coefficient (HTC) decreases. Eliminating the ice layer would increase overall efficiency and lower costs. The major challenge in developing an ice-phobic heat exchanger is the adhesion of ice onto other surfaces. It is a notoriously strong force to overcome, even on superhydrophobic surfaces [2,3,7]. The current state of the art in ice-phobicity is either removing condensate before freezing with either a superhydrophobic surface or SLIPS (slippery lubricant impregnated porous surface) or removing the ice after it has formed by using some thermal or mechanical energy [1,8,9]. Our approach to the problem of ice-phobicity was inspired by SLIPS which uses a combination of hydrophobic surfaces and lubricating oils. Whereas a traditional SLIPS uses a thin film of lubricant to enhance the superhydrophobic characteristics of the surface, our approach was to totally submerge water drops in oil so that frost and ice build-up cannot suppress with the freezing process. Active Energy Systems has developed a new method of making ice that passively sheds ice as it is produced, eliminating the inefficiency of thermal resistance from ice-on-coil systems. The key innovation 58
Figure 1. Heat exchanger experimental setup.
We designed a benchtop-scale test bed for proof-of-concept testing of our heat exchanger prototype. A basic diagram of this setup is illustrated in Figure 1. A chiller was used as the source of cooling for these experiments (1). The heat exchanger we used is made of three aluminum plates with imbedded copper tubes (4). The plates are fastened together using a bolt and the spacing between the plates is 0.5 in. The plates were spray coated with a 5 Îźm layer of PTFE. The inlets and outlets of each plate is connected to a header that leads back to the chiller inlet and outlet (2). The refrigerant in the system is a glycol mix. This heat exchanger is submerged in oil (5) inside of CTI Journal, Vol. 40, No. 2