Aaron Hydrick

Advisor: Dr. Jalal Baghdadchi

Final Report to the

Center for Environmental and Energy Research at Alfred University

Summer Undergraduate Research Fellowships 2003

30 September 2003

Summary

The study of efficient conversion of electrical energy to heat has been the goal of this project. The goal is to determine if any improvement could be made to existing technologies to allow a more effective conversion of electrical energy to heat, which could then be stored as hot water for rural/personal use. This could of course further the cause of cleaner, more efficient energy sources by making the use of alternative energy sources more attractive to the average person. The conversion of electrical energy into heat by passing current through a resistive material is a well-known technique already near the maximum theoretical efficiency limits for the materials in existence today. This leaves the transfer of the heat to a storage media and the actual storage media as areas to improve upon. As water is likely to remain the most readily available and one of the more practical and economic mediums in which to store heat energy the most logical area upon which to improve is the transfer of the heat energy from the generating source to the storage medium.

The method of approach consists of taking un-insulated resistive elements and coating them with the thinnest possible layer of electrically insulative material which will provide both resistance to insulation breakdown under operating conditions, as well as maximum heat conduction through the material while maintaining a hermetic seal around the material. Teflon is being examined for the coating material, as it will be able to provide all of the requisite thermal and electrical characteristics as well as one potentially significant benefit. Hard water tends to leave hard scale mineral deposits on not only heating elements but on anything left in contact with the hard water. A Teflon coating will provide an added resistance to hard water scaling and should significantly increase the life expectancy of traditional hot water heating elements.

Present experimentation indicates that in the absence of Teflon, coatings of Polyurethane, RTV and other silicone-based materials will work sufficiently well to observe any improvement in heat transfer. Glass has also been suggested a s a coating material, Two basic types of resistive elements are being considered as well. A solid cylindrical body ceramic consisting largely of carbon and second, and a tubular geometry consisting of a would-wire resistor are being examined. The tubular geometry is the most promising since it would provide the larges surface area to dissipate the heat to the storage medium.

Introduction

The project was intended to study the effect of coating heating elements with either Teflon or a silicone-based sealant and then immersing the elements in water. By using Teflon, a relatively heat-conductive material that is also electrically insulative the objective of placing a heating element into virtually direct contact with the medium that it is intended to heat is achieved. Because of the nature of Teflon, cylindrical element geometry is optimum and allows the most even and consistent coating. The project intended to investigate two possible element geometries. Standard heating-coil resistor wire as would be used in a commercial hair dryer was considered, as was the possibility of producing a ceramic heating element here at Alfred. The ceramic element composition chosen for this project was a basic whiteware-style ceramic body with carbon black added to moderate/control resistivity. The geometry for this element would be limited to a rectangular solid by the equipment available to press the powders into a form that could be handled and fired.

Methods

Relatively thin coatings of silicone-based materials and polyurethane were first tested on basic wire resistors (simple coils of wire with sufficient length of wire to achieve desired resistances and produce observable heating when immersed). This allowed the evaluation of various materials’ suitability as electrical insulators/heat conductors and gave an indication of how well, in general, thin coatings on wire-type heating elements might work. The ceramic resistors were prepared using a ram press set at approximately 2500psi to press the powder into rectangular bars with dimensions of 6” x 1” x ³/₁₆”. The bars were then fired and coated with an electrically conductive paste to allow electrical connections to be made.

Results

The preliminary experiments performed to date are qualitative in nature and confirmed the feasibility of allowing the heating element to be immersed with only a thin coating to serve as the electrical insulator. In order to verify the effectiveness of the coatings’ ability to transfer heat, an additional battery of tests will be required. The pressed bar of ceramic proved to be very fragile in the green stage of production and many of the samples failed. Additionally the firing schedule burned out most of the carbon leaving only a ceramic insulator. Alfred University facilities are not equipped to apply Teflon coatings, so the possibility of having some samples prepared by an outside facility was investigated. This proved to be cost-prohibitive and as a result the decision was made not to coat any of the materials with Teflon. A possibility that has been recently made available is to coat a preconstructed raw cylindrical ceramic resistor with a thin layer of glass, which could serve as an alternative to Teflon. This is being pursued in conjunction with a project being supervised by Dr. Walter Schulze.

Discussion

Based on the results of the silicone and polyurethane coating experiments, the suggested approach of coating resistive elements and directly immersing them in water seems viable. The amount of any given polymer coating needed to provide adequate electrical insulation is minimal, and would therefore allow heat transfer to be maximized. The use of ceramic versus conventional metal wire as the heating element does not necessarily provide any significant advantages in terms of efficiency. However, in terms of coating life and especially in the case of a glass coating - the ceramic has the distinct advantage of rigidity, which minimizes mechanical stresses on the coating and thus maximizes the life of the coating.

Conclusions

The project was short-term and did not yield any quantitative analysis, but the results did indicate that this is an area of possible continued study. The ideal element geometry should definitely be investigated further – possibly a tubular element. The Teflon coating could potentially lengthen lifespans of hot water heaters and would certainly minimize loss of efficiency due to hard water scaling. The possibility of glass-coating a ceramic is currently being pursued and results are expected within the next 5-6 weeks as to the viability of this concept. There exist numerous potential areas of research related to these preliminary investigations and it is hoped that there will be pursuit of some of these related research topics in future CEER projects.