When you put a pot or pan on a hot stove, you might feel the heat in the kitchen even though the food isn't cooked yet. That's because not all the electrical energy is being used to cook the food. It's like the stove is wasting energy! This is called heat transfer efficiency. And in this article, we're going to talk about how to make sure your kitchen stays cool while you cook. We'll explain what heat transfer efficiency is and how it affects your cooking. So, keep reading to learn more! And remember, no process is ever 100% efficient!
Thermal energy is defined as the energy that is associated with the temperature of an object. Warmer objects will typically have a greater amount of thermal energy than cooler objects. Thermal energy can be transformed from other forms of energy via friction, electrical currents and even air resistance.
Heat transfer is the flow of thermal energy from high-temperature regions to lower temperature regions. That means that thermal energy is transferred from hot objects to cold objects that are near each other. You are probably already familiar with this phenomenon; we use ice to keep drinks cool on hot days. The liquid transfers thermal energy to the cold ice, which increases in temperature and melts. This process extracts thermal energy from the liquid, cooling the drink down.
The topic of this article is heat transfer efficiency. We can first state a general definition as follows:
Heat transfer efficiency is the ratio of the useful output heat energy transfer to the total input heat energy transfer. When energy is transferred between different forms, a proportion of the energy is usually lost to an unwanted form of energy during the conversion and is wasted in the surroundings. Due to the conservation of energy, the total energy output of the system includes both the useful energy output and the dissipated (lost) energy. The efficiency can be calculated as a percentage and cannot exceed 100% efficiency, as this would imply more energy came out of the transfer than went in! The formula used to calculate heat transfer efficiency is shown below, where either energy or power can be used. The higher the efficiency the smaller the proportion of wasted energy and vice versa.
We can write the definition for heat transfer efficiency mathematically as follows: We can note from this equation that the numerator and denominator are both quantities that are measured in the unit of joules. It is clear then that the efficiency does not have a unit and is simply a number. Using this equation we can identify how much heat energy is transferred usefully from one system to another. Recall that energy transferred per unit time is the power output of the system; it follows then that we can write the heat transfer efficiency in another way:
Heat transfer occurs through three main mechanisms: conduction, convection and radiation. Energy is transferred via each mechanism but the processes are different. The efficiency of heat transfer can be calculated irrespective of the mechanism responsible for the heat transfer, as in most real-life situations heat will be transferred using a combination of these mechanisms.
Conduction is the mechanism of heat transfer between two substances in direct contact. The substance at a higher temperature has more energetic atom collisions, which gradually transfer heat energy to the cooler substance. Conduction is the flow of thermal energy through one object and into another, and also the mechanism by which heat diffuses through a solid object. The image below shows the conduction of heat energy of a metal rod from a candle's flame.
Convection is a mechanism of heat transfer which occurs in liquids and gases that expand when heated. The hot particles become less dense than their surroundings causing them to rise, and colder particles to move to take their place. These cooler particles are then heated, and the process repeats. This creates a convection current that transfers heat from one particle to another.
Convection is a form of heat transfer that involves the movement of particles in a fluid, such as air or water. When a fluid is heated, the particles become less dense and rise, while cooler particles move to take their place. This creates a convection current that transfers heat from one particle to another. Convection can be seen in everyday life, such as when hot air rises and cold air sinks.
Convection is also responsible for the transfer of heat in the mantle of the Earth. The extremely hot layers of the mantle closest to the core move toward the earth's surface as they are less dense than the cooler layers of rock in the higher regions of the mantle. The cooler rock sinks closer to the core and then gets heated enough until the process repeats itself. A convection current is produced which moves the material back and forth in the mantle very slowly.
Radiation is a type of heat transfer that occurs when energy is emitted in the form of electromagnetic waves. This energy can be emitted by any object that has a temperature above absolute zero, and the properties of the radiation depend on the type of surface of the material and its temperature.
An example of radiation is the energy emitted by a fire. Infrared waves are emitted by the movement of electrons and protons in the surrounding air, creating thermal radiation. This phenomenon can be seen in the image below.
The principle of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. However, in practice, some of the energy transferred is almost always lost to the surroundings. This energy cannot be used by the system and is known as wasted or dissipated energy.
To increase the efficiency of a system, it is important to reduce the amount of thermal energy that the system loses. This can be achieved by using insulation or improving the design of the system. By reducing heat loss, more of the energy input can be used for the desired purpose, increasing the efficiency of the system.
Q. A kettle is used to heat water. Thermal energy ofis transferred from the heating element to the kettle, and of thermal energy is transferred from the kettle to the water. Find the efficiency of energy transferred to the water.
A. We can apply the equation for heat transfer efficiency as follows:
Efficiency = (Useful energy output / Total energy input) x 100%
The useful energy output in this case is the thermal energy transferred from the kettle to the water, which is . The total energy input is the thermal energy transferred from the heating element to the kettle, which is . The efficiency of the energy transferred to the water is:
Efficiency = ( / ) x 100%
Efficiency = 82%
In the end, only 82% of the heat energy of the kettle goes into heating the water. The rest has been lost to the surrounding air and the body of the kettle via conduction and convection.
Q. A motor receives of electrical energy from a power supply. 20% of this energy is wasted. Find the efficiency of the motor.
A. We can first write the equation for heat transfer efficiency as:
Efficiency = (Useful energy output / Total energy input) x 100%
The total energy input in this case is the electrical energy received from the power supply, which is . The wasted energy is 20% of the total energy input, which is . Therefore, the useful energy output is:
Useful energy output = Total energy input - Wasted energy
Useful energy output = - (0.2 x )
Useful energy output =
Finally, we substitute the useful energy output and total energy input into the efficiency equation.
Efficiency = ( / ) x 100%
Efficiency = 80%
Therefore, the efficiency of the motor is 80%.
Q. Find the energy wasted in a system if the useful heat output is while the total heat input . is the efficiency of the system?
A. To find the wasted energy, we subtract the useful heat output from the total heat input. This gives us:
Wasted energy = Total heat input - Useful heat output
Wasted energy = -
Wasted energy =
Next, to find the heat transfer efficiency, we use the formula:
Efficiency = (Useful energy output / Total energy input) x 100%
The useful energy output in this case is the useful heat output, which is . The total energy input is the total heat input, which is . Therefore, the efficiency of the system is:
Efficiency = ( / ) x 100%
Efficiency = 60%
Therefore, the efficiency of the system is 60%.
Factors Affecting the Efficiency of Heat Transfer
In different applications, we may want to design devices to improve or restrict the efficiency of heat transfer in a system depending on the objective. Below we describe some applications that improve heat transfer and some others that restrict heat transfer.
Improving Heat Transfer Efficiency
Cooling fins
Cooling fins use conduction and convection to transfer heat away from objects that generate heat and need to remain cool. They increase the surface area over which heat can be transferred to the surrounding fluid via conduction and convection and are an efficient method of improving heat transfer. The image below shows an example of cooling fins on a motorbike, used to keep the engine cool.
Copper-based saucepans are a great example of a method to improve heat transfer efficiency. Copper is an excellent conductor of heat, and by using it in the section of the pan that contacts the stove, there is less thermal resistance to the heat transferring from the stove into the pan. This means the pan can warm up faster and is better to cook with.
On the other hand, double-glazed windows are an example of a method to restrict heat transfer efficiency. The two panes of glass are separated by a layer of gas, usually argon, which is a poor conductor of heat. The double-glazing is intended to prevent conduction or convection from occurring between the inside of the home and the outside, helping to keep the house warm in the winter and cool in the summer.
When handling hot objects from an oven, we often use oven gloves to protect our hands from being burnt. These work by increasing the thermal resistance to heat transfer by conduction, which decreases the rate that heat energy is transferred through the glove.
In summary, heat transfer efficiency is the ratio of the useful output heat energy transfer to the total input heat energy transfer. Heat transfer occurs through three mechanisms; conduction, convection, and radiation. Improving or restricting heat transfer can be achieved through various methods, depending on the application.
What is heat transfer efficiency?
It is the ratio of useful heat energy transfer to total heat energy transfer.
How do you calculate heat transfer efficiency?
Heat transfer efficiency = Useful heat output / total heat input.
What are the factors that can affect the efficiency of heat transfer?
The factors include the type of materials used, the thickness of the insulation used, the thermal conductivity of materials and the temperature gradient between materials.
What is an example of heat transfer efficiency?
A kettle that requires 1000 J of energy to transfer 800 J of thermal energy to the water in it, has a heat transfer efficiency of 80%.
What is the heat transfer efficiency formula?
Heat transfer efficiency = Useful heat output / total heat input.
