Fed up with constantly having to recharge or replace batteries in your ever-expanding electronic devices? The solution may be just a few steps away. "Energy harvesting" promises to power countless consumer devices, often with nothing more than your body's movement or heat. Dozens of companies around the world already offer such products, but many experts believe the market for the technology could explode due to electronic devices being developed for the Internet of Things. "It's huge," said Graham Martin, CEO of the EnOcean Alliance, a San Ramon-based group of businesses that promotes wireless energy-harvesting technologies. With the Internet of Things expected to combine billions of devices, "if they are all battery-powered, we'll have a problem because there's not enough lithium   in the world," he added. "So a lot of them will have to use energy harvesting." Among the most basic forms of the technology is body power. When certain materials are squeezed or stretched, the movement of their atoms creates an electrical charge. Automatic watches have employed the concept for decades, for example, by winding themselves when their user moves their arm. Now, the concept is being considered for a number of other devices. In a contest seeking visionary ideas for wearable technologies, Intel awarded $5,000 for a concept to change the temperature difference between a person's body and a special piece of clothing they'd wear into electricity for mobile devices.  Using sound to power devices is another energy-harvesting variation. Stanford University engineers are testing smart microchips   that create electricity from ultrasound   to power implantable   devices that can analyze a person's nervous system or treat their diseases. A textile   research association in Spain is proposing to obtain electricity from radio waves that flow around everyone to power sensors sewn into clothes, which can monitor a person's heartbeat or other vital signs. Research firm IDTechEx has estimated that annual global sales of energy-harvesting products could hit $2.6 billion by 2024, while WinterGreen Research predicts sales of $4.2 billion by 2019. Obtaining stable energy from devices can be complex, however. For one thing, the motion that generates the electricity has to be constant to be useful. Moreover, the amount of power the devices produce depends on the person using them, according to a Columbia University study. It determined that taller people on average provide about 20 percent more power than shorter ones when walking, running or cycling. It's also unclear how eagerly consumers might welcome energy-harvesting products. While such devices are expected to cost less than battery-powered alternatives when compared over many years, experts say, people may continue buying ones with batteries merely because those would be cheaper in the short term. It can be learned from the passage that   _  .
energy-harvesting products save money in the long run taller people can surely produce a larger amount of power automatic watches harvest energy from the users' body heat two ways of harvesting energy are mentioned in the passage