Solar energy technology is enjoying its day in the sun with the advent of innovations from flexible photovoltaic (PV) materials to thermal power plants that concentrate the sun's heat to drive turbines. But even the best system converts only about 30 percent of received solar energy into electricity—making solar more expensive than burning coal or oil. That will change if Lonnie Johnson's invention works. The Atlanta-based independent inventor of the Super Soaker squirt gun (a true technological milestone) says he can achieve a conversion efficiency rate that tops 60 percent with a new solid-state heat engine. It represents a breakthrough new way to turn heat into power.
Johnson, a nuclear engineer who holds more than 100 patents, calls his invention the Johnson Thermoelectric Energy Conversion System, or JTEC for short. This is not PV technology, in which semiconducting silicon converts light into electricity. And unlike a Stirling engine, in which pistons are powered by the expansion and compression of a contained gas, there are no moving parts in the JTEC. It's sort of like a fuel cell: JTEC circulates hydrogen between two membrane-electrode assemblies (MEA). Unlike a fuel cell, however, JTEC is a closed system. No external hydrogen source. No oxygen input. No wastewater output. Other than a jolt of electricity that acts like the ignition spark in an internal-combustion engine, the only input is heat.
Here's how it works: One MEA stack is coupled to a high- temperature heat source (such as solar heat concentrated by mirrors), and the other to a low-temperature heat sink (ambient air). The low-temperature stack acts as the compressor stage while the high-temperature stack functions as the power stage. Once the cycle is started by the electrical jolt, the resulting pressure differential produces voltage across each of the MEA stacks. The higher voltage at the high-temperature stack forces the low-temperature stack to pump hydrogen from low pressure to high pressure, maintaining the pressure differential. Meanwhile hydrogen passing through the high-temperature stack generates power.
"It's like a conventional heat engine," explains Paul Werbos, program director at the National Science Foundation, which has provided funding for JTEC. "It still uses temperature differences to create pressure gradients. Only instead of using those pressure gradients to move an axle or wheel, he's using them to force ions through a membrane. It's a totally new way of generating electricity from heat."
The bigger the temperature differential, the higher the efficiency. With the help of Heshmat Aglan, a professor of mechanical engineering at Alabama's Tuskegee University, Johnson hopes to have a low-temperature prototype (200-degree centigrade) completed within a year's time. The pair is experimenting with high-temperature membranes made of a novel ceramic material of micron-scale thickness. Johnson envisions a first-generation system capable of handling temperatures up to 600 degrees. (Currently, solar concentration using parabolic mirrors tops 800 degrees centigrade.) Based on the theoretical Carnot thermodynamic cycle, at 600 degrees efficiency rates approach 60 percent, twice those of today's solar Stirling engines.
This engine, Johnson says, can operate on tiny scales, or generate megawatts of power. If it proves feasible, drastically reducing the cost of solar power would only be a start. JTEC could potentially harvest waste heat from internal combustion engines and combustion turbines, perhaps even the human body. And no moving parts means no friction and fewer mechanical failures.
As an engineer, Johnson says he has always been interested in energy conversion. In fact, it was while working on an idea for an environmentally friendly heat pump (one that would not require Freon) that he came up with the Super Soaker, which earned him millions of dollars in royalties. That money allowed Johnson to quit NASA's Jet Propulsion Lab (where he worked on the Galileo Mission, among other projects) and go independent. His toy profits have funded his research in advanced battery technology, specifically thin-film lithium-ion conductive membranes. And that work sparked the idea for JTEC. Besides, he jokes, "All inventors have to have an engine. It's like a rite of passage."