Investigation of Alternative Water Heating Methods

  • Mohamed Sakr Fadl

    Student thesis: Doctoral ThesisDoctor of Philosophy


    Water heating contributes an important proportion of residential energy consumption all around the world. Internationally, domestic potable water heating contributes between 15% and 40% of energy consumption within residential dwelling, electricity and natural gas are the major fuels reported in use for domestic water heating.

    Different kinds of domestic hot-water production systems exist. The operational cost, environmental effect and performance of these systems differ according to various energy sources, climates, system types and system designs. Therefore design of water heating systems for household and industry are more subject to optimization, in order to improve thermal performance and minimize losses.

    One of the most common types of water heating technologies is TEWHs (Tankless electric water heater). These high-power water heaters heat water instantly as it flows through the heater. (TEWHs) are compact, will not run out of hot water and have no standby energy losses but have to be wired to the electric consumer unit.

    The research work carried out and reported in this thesis is divided into three main parts. Firstly, influences by employing different inlet–outlet port arrangements, heating tank shapes and electric coil configurations on the thermal performance of the tankless electric water heater were investigated experimentally and numerically. Results were obtained for four operation water rates of 0.06, 0.08, 0.10 and 0.12 L/s. The analysis and design optimization are based on the transient temperature profile of the outlet water, the transient temperature distribution of the water inside the heating tank, flow profile of fluid flowing inside the heating tank and heat transfer between the heating elements and fluid, and also the temperature distribution on the surface of the heating elements. The characteristic performance of the heater with different configurations is analyzed and the best one is identified and proposed to use in practice especially the heater with horizontally inclined inlet ports.

    It is found that, the heaters with horizontal inclined inlet angle ports are successful in promoting good thermal distribution inside the heating tank, however, the degree of flow mixing produced by each design is found to have a significant impact on the heater thermal performance.

    Meanwhile the new heater fixture provides more hot water at almost constant temperature in the first mean residence time, which is of prime concern for the user.

    In the second part of the simulation, eleven different heating tank shapes were simulated, included two different structures of heating elements coils (external and internal legs) both have the same heating power rating and length. The heating tank diameter varied from (58, 66 to 73.2 mm) and with different inlet port positions.

    The numerical results demonstrated that the shape of the heating tank has significant influence on the water mixing and circulation inside the heating tank. Furthermore, modification of the heating elements coils structure and geometry of the heating tank could be significant benefit to assist in smoothing the flow of fluid from inlet to outlet through the heating tank so that the heating elements do not interfere with the flow of fluid through the heating tank. In addition, improve both heating efficiency, reduce occurrence of heater failure and more likely to improve durability.

    Secondly, experimental investigations were carried out to observe the advantage of using PCM in thermal energy storage system. The experimental findings indicate that the use of PCM helps to stabilize the system temperature to an allowable working temperature of 40°C and extends the usage time. The results show that inclusion of a PCM in water tanks for domestic hot-water supply is a very promising technology. It would allow the user to have hot-water for longer period of time even without exterior energy supply. Additionally, further studies should focus on optimizing latent heat thermal energy storage with heat transfer enhancement methods.

    Finally, an attempt of utilizing Ohmic heating method to generate heat in water as an alternative heating method rather than using electric heating coils has been studied experimentally. The effects of applied voltage, electrode gaps, electrode types and electrical conductivity of water during Ohmic heating were investigated. Heat was generated within the water using alternating electric current at frequency of 60 Hz, the range of voltage gradients were 26.31–87.0 V/cm.

    One major output of this research is the assessment operating parameters during Ohmic heating which adds a new perspective to system analysis and design Ohmic heating system for water heating process.

    The results indicate that Ohmic heating provides rapid and uniform heating method within less heating time. However, more studies are needed for continuous Ohmic heating system, modeling for commercial production as well as economical evaluation of the system.

    Industrial relevance
    In the present study, innovative methods to improve the thermal performance of tankless electric water heaters were applied. Some experimental and numerical models were developed and validated in this work for the industry to apply them for producing more robust and efficient heating systems, also, this work also show that the quality attributes of Ohmic heating is better than those of conventional water heating in many aspects.
    Date of AwardDec 2014
    Original languageEnglish
    Awarding Institution
    • Coventry University
    SupervisorShuli Liu (Supervisor) & Peter Claisse (Supervisor)

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