Heat Transfer in the Kitchen: Cutting Energy Costs and Thermal Management
Understanding Energy Costs
This blog will further explore the appliances that have been discussed previously, as well as offer ideas and explanations on how they operate and how to be more energy efficient when utilizing them. With energy costs continually on the rise, being efficient will not only save energy but money as well. We give a number of examples of thermal management at play in the kitchen.
One of the ways that many households are saving electricity is by purchasing High Efficiency Energy Star appliances. These appliances are backed by the Environmental Protection Agency (EPA) and the Department of Energy (DOE), because they are able to consume much less energy than their non-high efficient counterparts. However, the high efficiency products do have some minor drawbacks. For example, a HE washing machine will have longer cycles because it uses less water and power than a traditional washing machine. While high efficiency machines may be more expensive, in the long run they will save you money on utility bills.
Over the course of a year a high efficiency refrigerator will save a certain amount of energy. For those of you who do not wish to calculate the savings using his/her engineering skills, there is a convenient web site that lets you calculate how much energy your energy star refrigerator will save you, found at the following link: http://www.energystar.gov/?fuseaction=refrig.calculator.
The refrigerator is essentially a heat pump operating on the vapor compression cycle. It removes heat from the refrigeration zone, and pumps it into your kitchen. See Figure 1. It operates on the well known vapor compression refrigeration cycle. The cold coils (evaporator) in the refrigeration compartment draw heat out of the refrigerator, and it is expelled at the back of the refrigerator through the condenser coils. A fan typically operates in the refrigerator to force cold air over the food, in an example of forced convection cooling.
FIGURE 1 The Vapor Compression Cycle and a Refrigerator
The working fluids of the immediate past, Freons™, are green house gasses, chloro-fluoro-carbons (CFC). These have been replaced by modern working fluids for refrigeration that are more environmentally friendly, and have a low carbon dioxide equivalent (CO2E) value.
In order for a refrigerator to function at its maximum efficiency the refrigeration coils (the condenser in the vapor compression cycle) must be exposed to the natural convection of the room. If they are too close to the wall, or covered in dust, they will not work efficiently and end up costing more money and electricity to operate. Also, ensuring that the refrigerator has a tight seal on it when it closes is important to maintaining efficiency. Even a small break can lead to a large increase in electricity use.
When it comes to cooking appliances in the kitchen, it is important to remember that smaller is usually better for saving energy. Why use the oven when you can use the toaster oven; this may be obvious for toast, but it is true for other items as well.
The source of energy is also important. A microwave oven is a better option than a conventional oven for simple warming. A microwave oven is very efficient at exciting the oxygen hydrogen bond in water. Cooking that involves the heating of water or steaming, e.g. vegetables can be both efficient and palatable when performed in a microwave oven. In fact a microwave oven uses about 20% less energy then a conventional stove or oven due to its shorter usage times. Smaller appliances will draw less power than large appliances, and therefore use less energy. When using the range for heating water, for pasta for example, cover water when bringing it to a boil.
Pressure cookers offer a unique way to save energy. Pressure cooking beans takes only 20 minutes, compared to two-three hours for regular boiling. The same is true for many other foods. By boiling water under pressure, the water achieves a higher temperature. The higher temperature is efficient in at least a few ways. The lower the pressure, the lower the boiling temperature. Most people know that things take longer to cook in the mountains; this is partly because the pressure is lower in the mountains, and water boils at a lower temperature. The boiling temperature versus pressure is given in steam tables, an essential part of any mechanical engineers library.
When an oven is necessary, it is a smart option to utilize the convection option. A convection oven has a fan that blows the hot air around the inside of the stove, thereby increasing the rate of heat transfer form the hot air to the cold food by the method of convection. This is known as forced convection. Free convection occurs when gravity does the work; hot air rises and cold air falls. An example of forced convection is blowing on your food to cool it, which is something that most people understand at an intuitive level. Convection ovens require less energy, usually because one may cook for a shorter time, compared to the conventional oven, often reducing time by about 30%. However, one usually does not bake bread in a convection oven, because it can dry it out too much. This method, like most, is not appropriated for all foods or preparation styles.
When using an oven, whether convection or conventional, it is a good idea to remember the thermal mass property. Using a pan that is preheated on the stove top before putting it in the oven allows one to both brown meat or vegetables, and preheat the pan to working temperature. This reduces cooking time considerably, and increases the quality of preparation for certain foods. The food may then be finished in the oven, which is a standard practice in professional kitchens. Opening the door when cooking can be a significant energy waster. See our previous blog where we tracked how much a BBQ grill cooled when it was opened.
Awareness will allow you to utilize your kitchen in an efficient and cost saving manner. Knowing and understanding the engineering methods that are used in designing each of these appliances, such as convection, heat transfer, and thermal mass can enable you to be much more efficient when using them.
Lastly, if you are cooking for one, instead of heating up the kitchen, consider going to the local pub and enjoying some gastro-pub cuisine.