Tallinn University of Technology

Why have refrigerators, washing machines and other household appliances become more energy efficient in the last 20 years? And when will the maximum efficiency be achieved? Answered by Jaan Kalda, Physics Professor at TalTech:

Jaan Kalda

The article* was published on the Estonian news portal Novaator.

Each household appliance has its own peculiarities and there is no single answer here. As there are many different household appliances, we will consider only the two mentioned above.

In the case of refrigerators, the main factor influencing energy efficiency is as simple as thermal insulation: the better the thermal insulation of the walls and door seals, the more economical the refrigerator. In principle, thermal insulation could always be improved (if nothing else, then made thicker!), but if the opening of the refrigerator door becomes the main factor that brings warmth into the refrigerator, then it no longer makes sense.

The thermal conductivity between the interior of the refrigerator and the cooling gas circulating in the pipes also plays a role. The more easily the heat is transferred from the interior to the gas moving in the pipes, the higher the temperature of that gas may be and the higher the cooling efficiency of the refrigerator. Ice that accumulates on cooling surfaces prevents heat transfer, and therefore ice-free structures also play a very important role. Old refrigerators generally had a relatively small cooling surface, which was low in temperature and therefore eager to produce ice from the water vapour of the warm room air. As the refrigerator (much like a heat pump) is a so-called inverted heat machine, the maximum efficiency in principle is given by the coefficient of performance (COP) of the ideal heat machine (Carnot cycle), which depends on the room temperature and the internal temperature of the refrigerator. If the room is 22 oC and the refrigerator is 4 oC, an ideal refrigerator would use only 1 joule of electricity to remove 15 joules. In reality, this ratio of refrigerators ranges from one to three, meaning that there is still plenty of room for development!

Unfortunately, in the case of washing machines, a significant part of the energy is used to heat water, because water has a very high thermal capacity. It takes almost a whole kilowatt-hour to heat ten litres of water to 80 oC. While the operation of the motor and the washing blades can be optimised to some extent, the energy used for heating cannot be reduced. A significant advantage would be gained if washing machines used hot water from the water supply (whether household or regional), but usually they do not. Thus, there is less room for development in the case of washing machines, as energy can be saved primarily by reducing the consumption of hot water, which can be achieved to some extent by optimising the washing cycles.

What has been the reason for the recent rapid development?

The reason why great progress has been made over the last few decades is probably due to the fact that society has understood the need to reduce energy consumption and, among other things, household appliance manufacturers are put under constant pressure by means of relevant standards.

*As of October, the researchers of Tallinn University of Technology answer the questions of readers of the ERR science news portal Novaator on topics that require clarification or are of current interest.