The air conditioning and refrigeration industry has been increasing the efficiency of its equipment dramatically.
One of the biggest contributors to these efficiency gains in the past has been the modification of heat exchangers to use enhanced surfaces, such as attached/integral fins, porous coatings, reentrant cavities and internal grooving. Today, a radically new method of increasing heat transfer efficiency, called the electrohydrodynamic (EHD) technique, is being investigated. Some early results have shown more than a 1000 percent increase in heat transfer coefficients of R-134a.
What is EHD? How does it work?
The EHD technique works by applying a high-voltage electrostatic potential field across a heat transfer fluid, such as a refrigerant or refrigerant mixture. The applied electric field serves to destabilize the thermal boundary layer, increasing boiling or condensation of the fluid near the heat transfer surface, and producing better mixing of the bulk fluid flow. The net effect is to increase the heat transfer coefficient, sometimes by several hundred percent. Although the EHD technique can be applied to both single phase and phase change heat transfer, it is more effective when applied to phase change processes (boiling and condensation).
How is the electric field applied?
A common method for using the EHD effect is to suspend a charged electrode (for example, a straight wire running parallel to the tube) in the fluid medium and to electrically ground the heat transfer surface. The reverse case (charging the surface and grounding the wire) is also possible. The applied electric field can be either direct or alternating. The field polarity in most cases has little effect on the enhancement mechanism, particularly for the phase change processes. For shell-and-tube heat exchangers, the tube and shell sides can be simultaneously energized by placing electrodes in both the tube and shell sides. For plate heat exchangers there is no need for an external wire electrode, as the plates themselves can serve as the electrodes by charging one plate and grounding the other.
What are the benefits of EHD?
Initial studies have been done with several refrigerants, including R-123, R-134a, an R-11/Ethanol mixture, and R-404A. The improvements in heat transfer are dramatic, especially at lower refrigerant qualities (more fluid and less vapor). This may allow manufacturers to produce highly compact heat exchangers with less complicated surfaces without sacrificing heat transfer efficiency.
Another potential application is to use continuously EHD enhancement with smaller, less costly heat exchangers. The EHD effect would offset the loss of heat transfer capacity experienced from the smaller heat exchanger. Similarly, EHD could be used in place of, or in conjunction with, enhanced surface heat exchangers.
What are the obstacles?
To use EHD, an electrical voltage needs to be added to the heat transfer device. Depending on the application, anywhere from a few volts to thousands of volts are used. However, because the heat transfer fluids are typically dielectric (low electrically conductive) materials, very little current is generated, despite the high voltage. This low current helps keep the power (voltage times current) and the associated energy penalty small. The electronics needed also represent an increased material cost.
More significantly, applying the electronics would require a more complicated manufacturing process. Some manufacturing techniques have been described (see for example, the March 1994 issue of Tech Update); however, this may be the biggest current obstacle to employment of EHD technology to air conditioning and refrigeration equipment.
Another important question is the long-term effects of the EHD equipment on the system. For instance, will the application of large voltages cause the refrigerants and lubricants to break down? Will the electronics used accelerate chemical reactions of the refrigerant and lubricant? Furthermore, the long-term reliability of the EHD equipment needs to be assessed. Whatever electronics are used will need to last for the llfe of the air conditioning system, or will need to be easily repaired by a service technician.
Research efforts addressing these issues have already begun in the U.S. and Japan. A recent study by Toshiba on an EHD condenser involved thorough testing of the unit after one thousand hours of continuous operation. It is reported that this research showed that the EHD did not leave any side effects on the refrigerant or heat exchanger components. However, to achieve this, careful design of the electrode was essential. The U.S. Air Force has also initiated a project to study the reliability and long-term performance of EHD heat transfer for use in aircraft environmental control systems.
Research is continuing to address these issues as well as to gather additional basic design-oriented data on the effects of EHD on heat transfer. If results continue to show success, EHD may become viable for use in mass-produced air conditioning and refrigeration equipment. But whether or not it is employed will depend on manufacturer’s analyses of its benefits and costs as compared to other means of increasing efficiencies.