By Arthur L. Chait, EoPlex
Batteries are everywhere, powering everything from laptops to greener vehicles. The first dry-cell batteries were mass-produced in 1896. Today’s batteries are vast improvements over these early products. Panasonic recently claimed the Guinness record for longest-life alkaline battery with the Evolta that powered a robot for seven hours as it climbed out of the Grand Canyon.
Unfortunately, batteries have two significant drawbacks: limited energy and fixed life. Limited energy is a problem in critical applications and fixed life is an environmental issue, since 15 billion batteries are used each year. Rechargeable batteries help, but they contain elements which are more toxic unless recycled properly. Battery limitations can be critical, but there is an alternative.
One example of critical battery limitations is providing power for tire pressure sensors (TPS). TPS became mandated after a series of fatal accidents in the 1990′s were linked to low tire pressure. All new cars in the USA must now be equipped with TPS. This saves lives and also saves billions of gallons of oil, since MPG drops with under-inflated tires.
TPS requires a way to get power to the sensor, as well as information from that sensor inside a spinning tire. Current systems use batteries to power both the sensor and the wireless transmitter. In mild climates with good roads, batteries may last as long as advertised, but with extreme climates and poor roads, some TPS will fail prematurely.
Even routine battery replacement may be a headache for consumers. TPS are sealed for protection and the entire unit must be replaced when the batteries die. TPS maintenance costs are estimated at $1000 over the life of a vehicle. A growing concern is that high costs and early failures will result in drivers choosing not to have sensors replaced. When this happens, the safety and fuel economy anticipated from the legislation will be lost.
An alternative to batteries is needed. One approach is to harvest the vehicle’s vibration energy. Piezoelectric materials transform vibrations into electricity and offer a low-cost way to harvest energy. The best piezoelectric is PZT, commonly used to generate sparks in lighters and gas igniters. Advantages of an energy harvester include:
Figure 1 shows a harvester consisting of layers of PZT and metal bonded in a bimorph structure. The bimorph is fixed at one end and vibrates like a tuning fork. Electricity generated is stored in a capacitor for several seconds until it reaches a level sufficient to power the TPS. The harvester needs to be rugged, light weight and small about the size of a penny. Manufacturing such a product is a challenge with conventional techniques and this has kept harvesters off the market.
Figure 1: Piezo harvester. (Source: EoPlex)
A second example is powering emergency radios. Events like 9/11 and Katrina showed that high-powered radios, required by first responders, need more energy. The military has a similar requirement, since most soldiers carry 20 lbs. of batteries along with their other gear.The only way to assure that enough power will be available is to carry spare batteries, adding to the load. A way is needed to carry more power without extra weight.
Some types of fuel cells can meet this need. Unfortunately, the power required demands a fuel cell that runs on hydrogen. Carrying hydrogen is dangerous and support systems are complex. This problem can be solved if a miniature chemical reactor, called a reformer, is built into each fuel cell. A reformer produces hydrogen from a water-alcohol mixture, which supplies five to ten times more energy than an equal weight of most batteries. It can also be carried and refueled safely.
Designing a miniature reformer is easy, but building it with current technology is either impossible or cost prohibitive. This is because the reformer requires complex internal channels and many different materials, and must be no larger than a soda cracker. The design shown in Figure 2 requires five different materials for all the internal components.
Figure 2: Minature refomer using printed electronics technology. (Source: EoPlex)
Both of these examples are difficult or impossible to build with current technology. However, a breakthrough proprietary manufacturing platform that can manufacture these types of products at low cost has been developed.* The first step is to create a design with a CAD system while taking advantage of the freedom provided by new design rules. This technology enables designs that are monolithic with all internal parts built concurrently. The models are then optimized and the final designs are sliced into the number of layers required. A series of printing plates or masks is generated for each layer.
Special materials are needed to make this process work, and the materials for these parts are selected from a catalog of proprietary printing pastes. These pastes have complex recipes and are designed to do all of the following simultaneously: