20 March 2008—His best-known invention, a high-powered water pistol, is a fun solution to a hot day in the sun, but to Lonnie Johnson, the potential of solar energy is no laughing matter. ”The sun is the only source that will be able to meet future terawatt levels of power demand, as more and more countries become industrialized and seek to improve their standard of living,” says Johnson, who is also the founder of Johnson Electro Mechanical Systems, in Atlanta. Harnessing the sun’s energy, of course, is easier said than done. But Johnson has developed a new kind of device that converts heat into electric current. He says it has the potential to be the best-ever method of converting solar energy into a form that we can use.
Among the potential applications are at utility-scale solar thermal farms and for plug-in hybrid vehicles, in which the device would use waste heat from the car’s internal combustion engine to help power the car’s electric motor. Johnson even envisions a day when miniaturized versions will power consumer electronics. Imagine your laptop producing power from its own waste heat, your cellphone being charged as you hold the handset against your face, or an implantable medical device exploiting the difference in temperature between, say, your chest cavity and the skin on your arm.
Johnson, who made a fortune when he licensed his most famous invention, the Super Soaker water gun, to a toy company in the late 1980s, says a prototype of the heat engine, called the Johnson Thermoelectromechanical Energy Conversion System, or JTEC, will be ready in a few months. It will convert heat to electricity at rates reaching just under 40 percent of the maximum theoretical efficiency available in an engine operating between two temperatures—the Carnot efficiency. The former U.S. Air Force and NASA Jet Propulsion Lab engineer says his group’s aim is to produce a commercial version whose efficiency can approach 85 percent of the Carnot ideal. Such a device would be capable of converting 66 percent of the available thermal energy into electrical energy.
In contrast, photovoltaic devices have net conversion efficiencies in the teens and thermionic (or thermoelectric) chips reach only a little higher than 20 percent of Carnot when converting heat to electricity.
As in all other heat engines, JTEC’s conversion efficiency is dependent on the difference in temperature between its hot and cool zones. For example, if the hot side is raised to 1100 °Celsius—which Johnson says an eventual commercial version would be able to withstand—while the cool side remained at room temperature, 25 °Celsius, it could, ideally, be 78 percent Carnot efficient. But what sets JTEC apart is its all-solid-state design. The lack of moving parts such as turbines and pistons eliminates nearly all of the parasitic losses that, in machines like an automobile engine, greatly lower efficiency. The conversion efficiency achieved by the best combustion turnbines is about half of what a commercialized JTEC device would offer, according to Johnson.
The JTEC’s setup is similar to that of a fuel cell [see an animation of how the JTEC works here]. A proton-conducting membrane allows protons from a hydrogen molecule to pass from one zone to another while preventing electrons from crossing the barrier. The electrons are therefore forced to move through an external circuit, in the process delivering current to a load. But instead of consuming hydrogen as fuel and expelling water, the JTEC is a closed system. It uses hydrogen as a working fluid that is conserved within the device.
The path the hydrogen takes is an elongated loop reminiscent of a racetrack. At the start/finish line, near the high-temperature heat source, the hydrogen is in a hot, high-pressure chamber. It immediately encounters an electrode that breaks each molecule into two protons and two electrons and carries the electrons to an external circuit, where they power a device or do some other useful thing. The pressure in the chamber forces the protons through a proton conductive membrane, after which they encounter another electrode that completes the circuit. This second electrode reunites the protons with the electrons, reconstituting the hydrogen gas.
The gas, now at low pressure, then heads down the loop’s straightaway toward the device’s cool side. At the low-temperature end of the loop the gas runs into another membrane electrode assembly stack, which, like the one on the hot side, has a proton membrane sandwiched between two electrodes. The same process occurs at this assembly, but instead of creating a voltage, an applied voltage goes to building up the pressure that will propel the gas back up to the high-temperature, high-pressure chamber where the process begins again.
The device’s net energy output results from the fact that the voltage generated on the hot side is greater than the voltage applied to the cool side: the higher the temperature difference, the greater the net voltage.
An important efficiency-boosting design element is the regenerative heat exchanger located between the hot and cool zones. This allows the hydrogen gas exiting the hot side to transfer its heat to the hydrogen that, having just been reconstituted on the heat engine’s cool side, needs to be reheated in order to prevent a drop in the temperature differential that drives the process. This allows the JTEC to get more out of the heat input. It also ensures that less energy is needed to pump the hydrogen gas up to full pressure at the cold end of the loop.
”Johnson has opened up a fundamentally new pathway to generate electricity from heat,” says Paul Werbos, program director for power, control, and adaptive networks at the U.S. National Science Foundation (NSF). Werbos, an IEEE Fellow, says the NSF is funding Johnson’s heat-engine research because of the strong chance that it could cut the cost of solar power in half. ”We’re in big trouble,” says Werbos, referring to the possibility of a future without enough energy. ”This could be a way out.” Werbos acknowledges that the product’s development is still at an early stage where unforeseen problems might creep in. ”But I don’t see any showstoppers,” he says.
When will a commercial version appear? ”That depends on funding, which will determine how aggressively we can move forward,” says Johnson. He notes that the company has been seeking strategic partnerships with organizations that would benefit from using the device and that it continues to pursue government grants to aid in its development. For now, the focus is on optimizing the design and layout of JTEC’s proton-conducting materials and electrodes with the aim of creating a stack containing current generating cells, each less than a millimeter thick.