Joule heating is sometimes also referred to as ohmic heating or electrical resistance heating. It refers to the method of heating electrolyte-filled water by exposing it directly to electrical current.
In this article, we'll discuss the significance of this method of water heating and how it is achieved.
A Brief History of Water Heating Devices
The necessity for hot water and its general availability is something often taken for granted, especially when one stops to think about heating technology and its history. Hot water from the tap is one example and was a luxury unavailable to the general public until the late 19th century.
The earliest embodiments included simple open fire kettles and pressurized steam boilers. Outside of the home, these devices also satisfied the requirements for numerous industrial applications, scientific processes, and service industries. As material technologies improved and the miniaturization of electronics became commonplace, additional hot water appliances made their way into the market. Hot beverage machines, dishwashers, clothes washers, and floor heating systems come to mind.
Today, hot water has become entirely commoditized and pervasive in our everyday lives. Amazingly, the core technology to generate it has evolved very little.
Heat via Resistive Heating Elements
The energy source to heat water can be divided into two categories: electricity and fossil fuels. The fossil fuel category relies on a burner and a heat exchanger to indirectly transfer the heat from combustion into the water. In the electrical category, water is also heated indirectly by cooling a resistive device that is dissipating power in the form of heat.
These “resistive heating elements” are typically constructed from a special alloy of wire (NiChrome) wrapped in a stainless steel tube and filled with magnesium oxide powder. The resistance of the wire, usually a handful of ohms, causes it to get extremely hot when a current is passed through it. This wire is electrically insulated by the magnesium oxide powder and the heat transfers through the powder to the outermost jacket of metal, which is in contact with the water to be heated.
Calculating the Temperature Rise of Water
The specific heat of water is a physical constant that dictates 4.186 Joules of energy are required to heat one cubic centimeter of water by one degree Celsius. Knowing the resistance of the heating element, one can calculate the dissipated power and calculate how much time it will take to heat up a certain volume of water.
In flowing water, the time component of the water exposure to heat is determined by the flow rate. In the derivation below, the final equation will tell you the temperature rise of flowing water for a given heating power applied to it.
In the aforementioned water heating discussion, the mechanism of heating water is fundamentally the same.
A heat source, either an electrical heating element or a gas burner, gets extremely hot relative to the final desired water temperature, and this heat energy is transferred to the water.
Interestingly, another paradigm of heating water exists, and it works in a completely different way.
Joule Heating/Ohmic Heating (AKA When Water Is a Resistive Component)
Joule heating, often referred to as Ohmic heating, heats water using electricity by passing electrical current directly through the water. No heating elements are used and, in fact, the equivalent electrical circuit would depict the water itself as the resistive component.
Pure water is a terrible electrical conductor. Luckily, all of the water we interact with on a daily basis has dissolved salts in it, making it an electrolyte.
These dissolved salts take the form of ions in the water and allow the water to support the conduction of electrical current. It is very important to remember that this electrical current is not like the typical conduction of electrons through a metal wire. It is based on the transport of ions and is a remarkably complex chemical process.
The critical parameters that determine the amount of conduction, and in turn the effective electrical resistance of the water, are the conductivity of the electrolyte and the amount of electrolyte exposed to the electrical potential.
To make the problem simple, assume that the voltage potential is applied to the water using two electrodes in the shape of flat plates. The effective resistance of the solution is, therefore, the distance between the two plates divided by the surface area of the plates and further divided by the conductivity of the electrolyte.
An Example of Calculating Joule Heating
As a quick example, begin with two electrode plates, each 5 cm by 5 cm, that are 10 mm apart and submerged in typical drinking water with a conductivity of 400 uS/cm. The effective resistance of this circuit is 100 ohms. If one were to apply 240 VAC to the two electrodes, the resulting current would be 2.4 A. The power dissipated into the water works out to 576 W, and all of it is converted to heat.
It is important to point out that the conductivity of the electrolyte can vary widely. Typical drinking water can range from about 50 uS/cm to 2000 uS/cm. At the high end, the example above would be using over 2.5 kW of power.
Once the power dissipated in the circuit is determined, the resulting temperature change can be easily determined again using the specific heat of water. In our example above, assume that the two electrodes were submerged in 1 liter of water. After the voltage is applied, 576 watts will be dissipated by the water continuously. In 60 seconds, this would amount to 34.5 kJ. Since there is 1000 cc of water, simply divide 34.5 by 4.186 to determine that the temperature will have gone up by about 8 degrees Celsius.
It is worth noting that water exhibits a second order effect wherein the conductivity actually changes with temperature. For every degree Celsius of temperature rise, the conductivity increases by approximately 2%. So as we heat the water, the current will actually increase and the water will heat even more than anticipated.
AC Potential vs. DC Potential
It is important to note that in the example, an AC potential was applied to the electrolyte. This is a critical detail in using this method to generate heat in the water. If DC had been used instead, a completely different process called electrolysis occurs. Various gasses including hydrogen and oxygen will be generated at the electrode interfaces, and the electrodes themselves may become part of the reaction in a detrimental way.
As can be seen, Ohmic heating is nontrivial and poses some interesting challenges from the control perspective. For this reason, it has been historically relegated to industrial and commercial applications, such as bulk heating of food for pasteurization. The controlled environment, known electrolyte, and constant monitoring makes for a highly efficient and predictable process.
Ohmic heating technology is maturing, however. New techniques for adapting dynamically to wide conductivities coupled with some clever control algorithms have greatly enhanced its robustness. As such, it is beginning to find applications in consumer products such as home water heaters and tea kettles. In the near future, it may very well replace resistive heating elements all together.