A water capacitor is a device that uses water as its dielectric insulating medium.
Theory of operation A capacitor is a device in which electrical energy is introduced and can be stored for a later time. A capacitor consists of two conductors separated by a non-conductive region. The non-conductive region is called the dielectric or electrical insulator. Examples of traditional dielectric media are air, paper, and certain semiconductors. A capacitor is a self-contained system, isolated with no net electric charge. The conductors must hold equal and opposite charges on their facing surfaces.
Water as a dielectric Conventional capacitors use materials such as glass or ceramic as their insulating medium to store an electric charge. Water capacitors were created mainly as a novelty item or for laboratory experimentation and can be made with simple materials. Water exhibits the quality of being self-healing; if there is an electrical breakdown through the water, it quickly returns to its original and undamaged state. Other liquid insulators are prone to carbonization after breakdown and tend to lose their hold off strength over time. These characteristics, along with the high dielectric constant, make water an excellent choice for building large capacitors. The drawback to using water is the short length of time it can hold off the voltage, typically in the microsecond to ten microsecond (μs) range. Deionised water is relatively inexpensive and is environmentally safe. If a way can be found to reliably increase the hold off time for a given field strength, then there will be more applications for water capacitors. Water has been shown not to be a very reliable substance to store electric charge long term, so more reliable materials are used for capacitors in industrial applications. However, water has the advantage of being self-healing after a breakdown, and if the water is steadily circulated through a de-ionizing resin and filters, then the loss resistance and dielectric behavior can be stabilized. Thus, in certain unusual situations, such as the generation of extremely high voltage but very short pulses, a water capacitor may be a practical solution – such as in an experimental X-ray pulser. A dielectric material is defined as a material that is an electrical insulator. An electrical insulator is a material that does not allow the flow of charge. Charge can flow as electrons or ionic chemical species. By this definition, liquid water is not an electrical insulator, and, hence, liquid water is not a dielectric. The self-ionization of water is a process in which a small proportion of water molecules dissociate into positive and negative ions. It is this process that gives pure liquid water its inherent electrical conductivity. Because of self-ionization, at ambient temperatures, pure liquid water has a similar intrinsic charge carrier concentration to the semiconductor, germanium, and an intrinsic charge carrier concentration three orders of magnitude greater than the semiconductor, silicon; hence, based on charge carrier concentration, water can not be considered to be a purely dielectric material or full electrical insulator but to be a limited conductor of charge.
Experimental The discharge of a platinum parallel-plate capacitor placed in a vessel filled with ultrapure water has been measured. The observed discharge trend could be described by a Modified Poisson-Boltzmann Equation only when the voltage was very low and the system capacitance showed a dependence on the spacing between the two platinum plates. The permittivity of water, calculated considering the system as a plane capacitor, appeared to be very high. An examination was made of the effect of applying voltages from 0.1 to 0.82V on pure water between metal electrodes. The movement of hydronium ions away from and hydroxide ions towards the anode was followed. This movement resulted in the formation of an ion double-layer with a steeply rising electric field and a maximum pH of approximately 12. At the cathode, the opposite occurred and the pH reaches a minimum of approximately 1.7. Thus pure water in a static electric field is not a homogeneous substance but may be considered to have three zones: (i) a zone containing excess positively charged aqueous hydrogen ions through which the electric field strength changes (ii) an intermediate zone containing pure water in which there is no significant electric field (iii) a zone containing excess negatively charged aqueous hydroxide ions through which the electric field strength changes. The transition from conductive to dielectric screening of electric fields by a tube of pure water has been investigated using a parallel plate capacitor that was used to generate a uniform electric field. Two concentric acrylic plexiglass tubes passed perpendicularly through the electric field generated between the plates. The region between the tubes was filled with air or water. An electrode, suspended within the inner plexiglass tube, was used to sense the electric potential at its location. The sensor was designed so that it could be rotated to measure the potential at a second symmetric position. From the difference in the two potentials, the frequency dependence of the magnitude and phase of the electric field could be determined. With deionised water between the tubes, the magnitude and phase of the interior electric field was measured from 100 Hz to 300 kHz. The high-pass filter frequency response expected for a dielectric tube with non-negligible conductivity was observed. Fits to the data yielded a very reasonable experimental value for the ratio of the water's conductivity to its dielectric constant. The model also predicted that at zero frequency (a static electric field) pure water would be expected to behave as a Faraday cage, which is natural given water's capacity to conduct electricity.
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