Potential of ultrasonics for energy saving in the household washing process

The main parameters that affect cleaning performance in household washing machines are detergent action, mechanics of drum rotation, time, and temperature (Sinner, 1960). Depending on the combinations of the parameters, different programs are designed to suit different kinds of fabric.

Several factors are responsible for the energy consumption in the washing process. While the heating of the suds constitutes the lion’s share of energy consumption, drum rotation and the short spin-drying process have a lower impact. Heating of suds can be accomplished by using hot water from the mains or by heating cold tap water in the machine. When the water supplied to the machine is cooler than the machine and its surroundings, some heat energy is also obtained through radiation and heat conduction from the machine’s parts. However, this contribution is usually low as the time between the water inlet and the beginning of heating is short—between 5 and 10 min—and usually not calculated and reported in studies for washing machines. In this study and the majority of studies, the machine’s heating was achieved by electricity. Water pumps, valves, displays, sensors, and controls play a minor role in energy consumption. As all these devices are also electrically controlled, in this report energy consumption is identical to the measured electrical energy consumption.

The cleaning of durable cloth, such as cotton textiles, is usually done in horizontal axis washing machines, which are the European market standard. The main contribution of the machine to the cleaning effect is the rotation of the drum during the main wash phase for about 30 min to 3 h. A higher temperature allows achieving a certain cleaning performance in a shorter time or a higher cleaning performance. The detergent helps to achieve a high level of cleaning.

For the cleaning of delicate items, in particular wool, these machines have special programs that mimic washing by hand and reduce the movement of the textiles to a minimum. Wool becomes felted and shrinks when its fibers rub against each other in the suds. This can be prevented through a chemical modification of the fibers. However, as wool is traditional cloth and one of the ongoing fashion trends is to use unaltered natural fibers, wool shrinkage is still a problem.

Wool programs are more or less soaking processes with nearly no drum movement. Some do only a partial rotation, and some do only one complete rotation at intervals of several minutes. Cotton programs have the highest drum movement, at about 50 rotations per minute. At the end of both programs, a short period with a much higher rotation speed takes place to spin-dry textiles. Centrifugal forces are so high in this period that the clothes are pressed onto the walls and no friction occurs between textiles. Thus, no fiber damage is witnessed in this last stage of the process, but no additional cleaning effect takes place either.

Cotton programs often have a lower water-to-textile ratio than wool programs. The first reason is that washing machines, by design, have a dead volume under the drum that must be filled before the textiles can be sufficiently saturated with the suds. Because of this, programs such as the wool program, for which a low load is recommended, have a higher water-to-textile ratio than programs with a high load such as the cotton program. The second reason is that a high water level is generally preferred in wool programs because the buoyancy of the textiles in the suds and the greater distance between them reduces friction. On the other hand, the energy consumption of wool programs is not subject to EU regulations and is rarely examined in consumer tests. Therefore, increased energy consumption due to a high water-to-textile ratio is hardly noticeable.

Heating of suds to higher temperatures dominates the consumption of energy. For example, in the Bosch WCM69 washing machine, measurements showed that the energy consumption for a wool program changes from 0.01 kWh at 20 °C to 0.23 kWh at 40 °C because suds have to be heated.

The mean temperature of washing programs in Europe is about 41 °C (International Association for Soaps 2013). The average washing temperature, however, has decreased over the past few decades for the following reasons:

On the other hand, consumers also expect a high cleaning performance, so they do not use only cold or 20 °C programs, especially when it comes to washing woolens with a low mechanical cleaning action. According to a survey in Finland, more than 75% of consumers use temperatures of 30° or more for wool washing (Laitala & Klepp, 2016).

Ultrasonics are used in many applications to clean hard surfaces immersed in a liquid solution. Above a threshold of about 0.5–6 W/cm², ultrasound cavitation occurs, which is the main mode of cleaning (Ensminger & Bond, 2011). When steam bubbles formed in this process collapse near a surface, adhering soil particles are removed by local fluid currents. As the local effect is very strong, some degree of surface degradation is usually observed. Nevertheless, this process is a standard application for cleaning metal parts, for example.

For soft surfaces, the application is more difficult because cavitation occurs predominantly at phase boundaries with a high difference in density. For metal-water boundaries, cavitation is a common effect, whereas the textile-water surface does not show this effect. In this case, cavitation has to be enabled throughout the liquid bath, requiring a high-pressure field. Another effect is that textiles trap some amount of air, which dampens the ultrasonic field (Moholkar & Warmoeskerken, 2004).

Not many household washing machines claim to use ultrasonics, and those that do are mostly top loaders with a vertical drum axis. Vertical axis machines use more water than European horizontal axis machines; therefore, it is easier to apply ultrasound because textiles are immersed in water. Around the year 2000, Sanyo introduced a washing machine, ASW-HB 700D, which claimed to use ultrasonics and a no-detergent washing cycle. It was compared in a study to a standard washing machine; but the study lacked details and comparability as a vertical axis machine with ultrasonics was compared to a horizontal axis machine without. Also, the study failed to characterize the ultrasonics that were used during the operation (Wüstemann et al., 2006). The system was discontinued in the following years.

The washing machine, WVMD1208AHG, from Whirlpool is said to speed up detergent dissolution by ultrasonics. The one examined in our laboratory had no ultrasonic transducer but only a membrane pump to introduce air into the suds. No ultrasonics could be measured during the program. X10 from TCL had only an additional ultrasonic bath at the top of it, and it was not intended to clean textiles but to clean items like glasses or jewelry. A realistic use of ultrasonics for laundry was only found in WT EON 650 from Godrej (Godrej appliances announces launch of new washing machines, 2013). It had an ultrasonic transducer at the top of the appliance, which could be used to do a manual pretreatment, but it did not claim to use ultrasonics in the main wash process in the drum.

There are many laboratory experiments about cleaning textiles with ultrasonics. They were usually conducted in devices used for other purposes, such as ultrasonic cleaning baths for cleaning metal parts (Wang et al., 2021). However, most of these studies lack suitable references to typical washing programs, a reasonable selection of stains, and the ability to measure ultrasonics.

The effects of different frequencies of 40, 60, 80, and 100 kHz were examined in Wang et al. (2021). However, neither the ultrasonic efficiency of the transducer nor the temperature was reported. Only beverage stains were used to test the cleaning efficiency. Thus, there are still some uncertainties about the best-suited frequency.

The damage caused to different fiber types by ultrasonics was examined in Wang et al., 2021). No change in tensile strength (Wang et al. (2021) and Muhammet (2013) or abrasion resistance (Muhammet, 2013) was observed, but there was an alteration in the thermophysical properties of the fibers (Muhammet, 2013). The effect of ultrasonics on delicate textiles such as wool (Gotoh et al., 2015) and silk (Ma et al., 2014) was studied and considered less damaging than traditional washing of wool by hand (Hurren et al., 2008) or machine washing of silk (Ma et al., 2014).

In Kimmel et al. (2022), devices for the pretreatment of stains were examined. If a pencil-shaped ultrasonic device is used for a manual 30 s pretreatment of 20 different consumer-relevant stains of 5 × 5 cm² each, followed by a 20 °C cotton program, the average stain removal level achieved is the same as a 40 °C cotton program without pretreatment.

In wool programs, the energy-efficient cleaning effect due to the drum rotation cannot be used. This leads to very low cleaning performance compared to, for example, cotton programs. The reduced drum movement is also reflected in the energy consumption of wool programs at 20 °C that do not have to heat suds and in which the motor is the main reason for energy consumption.

As the cleaning effect of the wool program is limited due to the reduced friction, this is also the reason why consumers often choose higher temperature levels in wool programs. Thus, for this kind of program and other similar programs with a low mechanical impact, such as the ones for delicates, the use of ultrasonics could provide an opportunity for making eco-efficient improvements in overall cleaning performance.

In recent years, many small devices claiming to use ultrasonics for washing have appeared on the market, which are designed for cleaning small loads. In this study, a selection of these devices was tested. Two typical devices were selected that can be immersed in a tub as an alternative washing process outside the washing machine. Conventional washing processes, on the other hand, are usually not very energy efficient for small loads as they need a minimum amount of water and the program structure is not very flexible in regard to the load. One pencil-shaped device was additionally tested, which was not intended to be immersed. It was originally intended to be used for a pretreatment. However, in this study, the device was not used as a pretreatment but as a method to get rid of visible stains, for example, by only treating single annoying spots and omitting the need to do a complete conventional washing process.

The aim of the study was to test whether these new devices show a sufficient cleaning effect at a lower energy consumption level compared to conventional washing programs. The test conditions were selected so that the highest possible cleaning effect could be achieved without the interference from additional load due to effects like dampening of ultrasound.

The order of effectiveness of cleaning for the stains sebum/pigment and olive oil/pigment is almost the same (Figs. 3 and 4).

The smallest cleaning effect was observed during soaking in treatment 5; in this process, neither mechanical forces nor ultrasound was used. This value can be regarded as a reference for the lowest possible cleaning effect when doing manual washing by using a detergent in an aqueous solution. For a household washing machine, the lowest possible washing performance is the wool program at 20 °C in treatment 7 because the process resembles soaking and the drum movement is minimal compared to all other washing programs.

The cleaning effect of the two ultrasonic devices 1 and 2 was on the same level, according to ANOVA and post hoc Tukey test with \(\alpha = 5\%\). They were at a higher level than that of soaking but at a lower level compared to the wool program at 20 °C.

To see if the effect was too small due to the limited use of energy by the ultrasonic devices 1 and 2, a test under the same conditions with device 4 was made, a typical ultrasonic bath. In this case, the cleaning level is higher than that of the 20 °C wool program but lower than a 20 °C cotton program. It is, however, closer to the 20 °C cotton program. Thus, the effect of ultrasonics in a water bath can be enhanced to a higher level if devices with a higher ultrasonic intensity are used.

The use of device 3 is a completely different approach, as it was originally intended for manual pretreatment. For the olive oil/pigment stain, it showed the same cleaning level as the cotton 20 °C program, and for sebum/pigment, even a higher cleaning level than cotton 40 °C.

As expected, the cotton 40 °C program shows the highest cleaning performance of the washing programs, but the step between cotton 40 °C and 20 °C is not as high as that between cotton 20 °C and wool 20 °C.

For the soaking process, a value of 0 kWh was applied, as in this study, the temperature increase of the tap water to room temperature was not taken into account.

The energy consumption of device 3, originally intended for a pretreatment, is very small for the treatment of four stains with the size of 5 × 5 cm² each. As most of the washing loads in typical households have only a few visible stains and consumers tend to keep the time for pretreatment short, it is not expected that a manual treatment will contribute a big share to the energy consumption. Even if a bigger area corresponding to stains with the size of 10 × 10 cm² was treated, the energy required would still account for less than 0.5% compared to the energy consumption of a cotton 20 °C program.

Device 1 needed half the energy and device 2 more energy than that for the wool 20 °C program, but both showed a lower cleaning level than that of the wool program. Device 4, the ultrasonic bath, showed a much higher cleaning level than the wool program. Although the bath needed fourfold energy of the wool program, the cleaning effect was still less than 25% of a cotton 20 °C program. The relationship between energy consumption and cleaning effect for all treatments is summarized in Fig. 5.

The ultrasonic devices showed a very broad spectrum of cleaning levels at a low energy consumption level, as they used neither drum rotation nor suds heating.

Devices 1 and 2 had a slightly better effect than soaking, but it seemed unlikely that they would meet consumer expectations, as the cleaning effect was even lower than that of a wool program at 20 °C. The ultrasonic bath, which was used as a test to see whether intensification of ultrasonic could yield a higher cleaning effect, had a cleaning level between the wool program at 20 °C and the cotton program at 20 °C. As this was a test with a very low load, the results need to be verified in household appliances to determine under what circumstances and at what level of textile load this effect could be achieved.

A manual treatment of stains with device 3 only makes sense if visible stains can be perceived. Lightly soiled items are defined by the German industry association IKW as laundry without visible stains, and even medium soiled is defined as having only a few light stains (Industrieverband Körperpflege- und Waschmittel e. V, (2017). Therefore, treatment would be possible for medium or more heavily soiled textiles. As it can be expected that consumers tend to keep the time for treatment short, it is not expected that the treatment will contribute a big share to energy consumption. An area of application of device 3, as it was used in this study, would be to remove stains from single textiles with an additional manual rinse or a rinsing process in a washing machine. In this case, the use of a full washing program for a single textile would be avoided.

The recommendation in the manual of the manufacturer was to pretreat single stains in heavily soiled laundry that could not be removed without a pretreatment. Another application is to use the pretreatment to change a mixture of textiles that would be rated as heavily soiled to medium soiled by the pretreatment. In this case, a more gentle washing process as a second step can be chosen, which will enable, in combination with the low energy consumption of the pretreatment, an overall lower energy consumption.

Critically, it should be noted here that the washing load in processes with ultrasound in this study was not chosen to reflect typical loads in consumer households, but to see the maximum effect of the new devices 1 and 2 under very favorable conditions for cleaning performance. The stained textiles have only a negligible fraction of the weight of typical cloths in the household. One stained textile with the size of 10 × 10 cm² has a weight of about 2 g, whereas a typical T-shirt has a weight of more than 100 g. The amount of dirt may be comparable for slightly soiled T-shirt, but the weight and dimensions of the textile are not comparable.

To reach a conclusion about the potential of ultrasonics for new washing programs, we will first look at programs for tough textiles and high loads, such as the cotton program. In (Kimmel et al., 2022), it was shown that it is possible to use ultrasonic pretreatment for a low temperature program. This enables the consumer to use a 20 °C program instead of a 40 °C program for a textile load that contains only a few visible, hard-to-remove stains. However, this depends solely on consumer behavior.

The use of ultrasonics directly in the drum during a cotton cycle with a high load of textiles still seems unfeasible (Gallego-Juarez et al., 2010). There are no such appliances in the market, and in the past, machines with this claim were discontinued. The main argument is that the mechanical drum rotation has a very high cleaning effect for low-to-high loads and does not need that much energy compared to suds heating. If a comparable cleaning effect has to be achieved only by using ultrasonic sound, the energy input will have to be at a very high level, which will have to be determined in separate studies.

Programs like cotton allow the biggest loads and require the least amount of liquid. Modern horizontal axis machines in Europe need only about 2–3 L of liquid per kg of textiles in the main wash process. So textiles are not immersed in a water bath like in the experiments in this study. Thus, ultrasonic propagation is hindered, and additionally, strong dampening occurs because of several layers of textiles and trapped air in the textiles and the suds. One method to reduce trapped air in the suds would be to lower drum rotation. However, as drum rotation in cotton programs is optimized to achieve a high cleaning performance, this would reduce the combined cleaning performance.

Device 3 showed that the direct contact of a transducer surface with textiles had a high cleaning performance. Up to now, there are no washing machines using this principle in the drum. Analogous to a manual treatment, an ultrasonic-sound-emitting surface could, for example, be fixed into the door as proposed by different patent applications (Frucco & Sburlino (1987)); Chindyasov (2013)). In this case, the textiles would accidentally come into contact with the surface when laundry is moved in the drum. In this case, the direct contact with stains on textiles would be comparable to device 3. However, only a small part of the textile surface of the whole load will accidentally come into contact with the ultrasonic-sound-emitting surface for a short time, and cleaning is expected to be uneven as the effect is usually limited to only a few layers of textiles and requires some time to be noticeable.

For delicate textiles like wool, which cannot be washed by using high mechanical forces from drum rotation and for which smaller batches than cotton are recommended, ultrasonics seem to be an alternative option of operation. These programs typically have a higher water-to-textiles ratio and a lower absolute textile load when compared to cotton programs. The appliances used in this study had a water-to-textile ratio of approximately 5–8 L per kg of textiles in the main wash phase.

In this study, we showed that the cleaning level of the tested devices 1 and 2 was, even at very favorable conditions, lower than that of a wool program at 20 °C. However, in experiments using an ultrasonic bath with a higher ultrasonic input, the cleaning level could be enhanced. For an application of ultrasonics in the washing process, either as a separate device or integrated in a washing machine, the next step would be to determine how much energy would have to be used to balance the dampening by a consumer-relevant load to get a sufficient cleaning performance.