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, is 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. This behavior may be explained by the theory of super dielectric materials. The theory of super dielectric materials and simple tests demonstrated that material on the outside of a parallel plate capacitor dramatically increases capacitance, energy density, and power density. Simple parallel plate capacitors with only ambient air between the plates behaved as per standard theory. Once the same capacitor was partially submerged in deionised water (DI), or DI with low dissolved NaCl concentrations, still with only ambient air between the electrodes, the capacitance, energy density, and power density, at low frequency, increased by more than seven orders of magnitude. Notably, conventional theory precludes the possibility that material outside the volume between the plates will in any fashion impact capacitive behavior.