Improve appliance design by using heat generated during normal appliance operation to drive Shape Memory Alloy technology.
Shape memory alloys (SMA) are metal alloys with unique crystalline structures that allow them to be deformed at a low temperature and to return to their pre-deformed shape when heated. Typical SMAs are Nickel-Titanium, which is referred to as Nitinol. Below the transition temperature, SMAs are highly malleable – similar to solder wire in mechanical properties. Above the transition temperature, the crystalline structure of the alloy changes and the metal becomes stiff and springy, returning to its “memory state”.
Using only the hot and cold temperatures generated by the sun rising and setting each day, shape memory alloys can be used to regulate the internal temperature of a structure. Currently in homes and businesses, temperatures are regulated by heaters and air conditioners that work to create a comfortable internal environment, despite the outside elements. Energy is used to run these systems while the natural energy created by the earth heating and cooling each day is wasted. With the use of SMAs, this natural energy can be utilized to increase or decrease heat transfer rates (whichever is favorable dependent on the season) through walls, vents, and windows in such systems as blinds, attic vents, and perhaps even “breathing walls” with SMA activated vents all along the façade of a building.
In one such example, a system of blinds was used to cover a large span of sliding glass doors in a residence. SMAs were created with transition temperatures that matched the climate of the area, and the desired effect of the blinds. The SMA mechanism was oriented exposed to the outside air, so that during the daytime the heat of the sun contracted the SMAs and closed the blinds. This created shade inside the space and kept the internal temperature at a comfortable level against the high heat of the outside environment. When the outside temperature dropped below a preset transition temperature programed into the wire, the wire was stretched back out again using a bias force mechanism, shifting the blinds open. This would let in the cold night air to cool the home. This type of mechanism could in fact work the opposite way in the alternate season if designed properly. In this example, the blinds were also multicolored, adding a unique design element to the façade of the home.
A similar theory can be used to vent attics. Physics tells us that heat rises, thus the attic is the warmest space in a house, often reaching temperatures well above 100°F in the summertime in hot climates. With no way for the hot air to escape, a heat sink is created at the top of a home, causing increased air conditioning usage simply to balance out the temperature of the home to a comfortable level. With an SMA based attic vent, the SMA mechanism would be placed inside the space utilizing the trapped heat. The SMA would heat up and contract when a predetermined temperature was reached, allowing vents to open and release the trapped hot air to the outside environment.
With the increasing focus on smart structures and green initiatives the future of architectural design is leading to passive and active systems to reduce energy usage.
Controlling Valves and Actuators in Appliances and other Systems
SMA technology can optimize performance of standard valves and other similar systems. Just by using the ambient temperature of the appliance, like hot oven air or hot water from a dishwasher, the SMA can react to perform a function.
When the hot fluid or air enters the valve, the temperature alone will force the Shape Memory Alloys to go through a phase change and perform work, depending on the design. An existing example produced today are anti-scald valves, and the potential extends to many other similar applications.
Other solutions are possible like an SMA valve that can activate a self-starting fire extinguisher in ovens if dangerously high temperatures are reached. SMAs could also turn off the gas to an oven or other appliance if exposed to these same dangerously high temperatures preventing often catastrophic fires. Another advantage SMA technology brings is the ability to sense temperature over a large area, or the whole oven versus one or two locations without any need for electrical controls or other sensors. Other applications like opening the soap dispenser or other controls when exposed to hot (or cold) water in dishwashing systems are also possible.
Ovens that are equipped with a self-cleaning feature commonly reach temperatures between 800°F to 1,000°F, which are maintained for hours at a time. Under these kinds of conditions, it is a fairly common occurrence for electronic control panels and fuses to burn out, possibly unlocking the oven door or preventing it from locking in the first place. SMAs create the possibility of locking based entirely on temperature near the system, removing electronics from the equation, and reducing the chances of lock failure.
Motor Overheat Protection Using SMAs
Overheating motors resulting from common issues such as overload, frequent stops and starts, and even environmental reasons can be addressed in new ways using the SMA phenomena. For example, simply wrapping SMA actuator wire around and/or throughout a motor or appliance, designed to contract and trigger a mechanical actuation, can be done reliably and repeatedly. Because SMAs can be made in a wide range of sizes and forms, they can be used in small and harder to get to places without adding significantly to weight and cost.
Using Heat to Accomplish Work, and Relocate Heat
Over the last several decades SMAs have been explored heavily as an alternative energy source, most recently in 2010 with the US Department of Energy ARPA-E program. The notion is that low grade waste heat (roughly defined as < 200°C and as little as 10°C temperature differences between the heat source and ambient temperatures) be used to move the material back and forth between its transition temperatures and through this process accomplish usable work and ideally generate electricity. The results from this work indicate that yes, it is possible; however, today the efficiency levels keep the cost greater than fossil fuel based solutions.
Although the thermo to mechanical efficiency of the SMA heat engine is relatively low (approximately 0.5% to 3%) the latent heat in the SMA element between phases demonstrated a unique ability to move heat from one location to another, with the potential to span larger distances than existing solutions today. So, in addition to turning a fan (to even air temperate for more controlled cooking) or some other mechanical work, moving heat from one location to another has evolved in new ways. For example in high power battery applications the ability to move heat away, physically separate batteries, and/or break a circuit within a battery or between batteries simply based on temperature is possible in new forms.
To conclude, shape memory alloys hold clear potential as actuation mechanisms in architecture. While it is difficult to achieve a tight transition at a specific temperature, the alloys can be actuated using electricity at a high power for a very short amount of time. In this case, the mechanism is still consuming low amounts of energy overall. We imagine these mechanisms could be implemented into louvered facades as a way to create adaptive facades that are passively temperature-responsive. Similarly, facades could be designed to adapt at highly specific temperatures using digital electronic control mechanisms that actuate the SMA. While the possibilities are seemingly endless, we are most excited about the possibility of maintaining thermal comfort of an interior without the need for energy-intensive mechanical systems. The autonomous mechanisms offer a sense of wonder and require one to reflect on environmental conditions in a way that is often overlooked in a fully climate controlled building. With this in mind, we see Nitinol not so much as a solution, but rather a critical component of a fully-designed system.