Energy Efficiency Improvements of Tumble Dryers : -Technical Development, Laundry Habits and Energy Labelling

Abstract: Tumble dryers are becoming more and more common in ordinary households as a complement to the washing machine. Tumble dryers offer a fast drying cycle independent on weather conditions and require small space. They do, however, considering the large number of units use a large amount of electricity. The main objective in this thesis is to identify possibilities in order to reach a reduced electricity use for domestic tumble-drying of clothes. This involves an investigation of the condensing tumble dryer in order to point out possible energy efficiency improvements. The purpose of the energy label, which indicates the energy efficiency of the tumble dryer, is also studied, whether it matches the actual laundry habits. Finally, suggestions for technical development of the tumble dryer are made in line with today’s consumer behaviour. The performance of the condensing tumble dryer has been studied using a design of experiments to create a statistical model in Paper I. This model was used to find the best settings for the power supply to the heater, the internal airflow and the external airflow in order to reach a high specific moisture extraction rate (SMER) and a low leakage ratio. A low external airflow and high power supply to the heater gives the highest SMER. To reach the lowest values for the leakage ratio, a low internal airflow should be applied together with a high external airflow. The use of a statistical model gave valuable information of the performance of the existing tumble dryer. For further improving the energy efficiency of the dryer, the amount of leakage and its location was investigated in Paper IV. By studying energy and mass balances from experiments, pressure measurements and modelling, the effects of leakage on the process were evaluated. As the location of the leakage is so important for the energy efficiency, the worst-case scenario where leakage is located between the heater and the drum is used as a start point in the study. It was determined that there is a large leakage of air between the heater and the drum leading to a significant loss in energy recovery. The drying loads used by consumers are getting smaller, often less than 3 kg dry load, while the maximum capacity of the dryers are increasing, up to 7 or 8 kg. In Paper II, tests were made with different loads in order to investigate if the energy label serves its purpose as today’s standard is set at the dryers’ maximum capacity. The results from this study show that the energy efficiency when drying a small load is significantly lower than for a large load. In order to encourage a production of tumble dryers with higher energy efficiency for small loads, where the dryer is most frequently used, the standards for the energy label should be revised. Today, manufacturers do not gain any benefits by improving the performance for partial loads. A mathematical model over a venting tumble dryer was established in Paper III with the aim of testing different control strategies in order to improve the energy efficiency of the tumble dryer for partial loads. The ideas behind the different strategies were to minimize the heat losses during the drying process and to increase the residence time for the air in the drum and thereby increase the moisture content of the air leaving the drum. Using such a control strategy it is possible to reach an improvement of SMER by approximately 4% when drying small loads. In order to reach larger improvements, however, a more extensive product development will be necessary. Finally, the results in this thesis points at the necessity of including not only the technical development of the tumble dryer, but also the policy tools involved and the consumers’ habits in order to reach a reduced electricity use for drying clothes in households.