The little engine that could

Coffee may power your mornings, but did you know that it could soon power your cell phone battery too? Epiphany Labs recently finished a Kickstarter fundraising project for a little gadget called the onE Puck. It is a small hockey puck-sized Stirling engine that can produce enough power to charge a smart phone battery. But this begs the question: What the heck is a Stirling engine and how does it work?

It all started with Scottish minister and inventor, Robert Stirling, who developed and patented the engine back in 1816. It preceded the first commercially successful internal combustion engine by 44 years, but because Stirling’s design was less efficient the engine slid into relative obscurity in comparison to its fuel-burning competitor. Stirlings are zero-emission because they are closed-circuit systems; the air trapped inside the engine is never released. Now, nearly 200 years later, the push for climate-friendly technology has brought the Stirling engine back into focus.

Watch this video to see a Stirling engine in action. [Video Credit: Michael S]

Although Stirling engines come in a bunch of different shapes and sizes, they all run on the same basic principle. The simplest configuration has four components: a hot cylinder connected to a heat source, a cold cylinder connected to a cooling source and a regenerator. Each cylinder has air space and a moving component called a piston that shifts back and forth as the air trapped in the cylinder moves around. The regenerator is the key connecting component. It usually sits between the hot and cold cylinders and acts as a heat exchanger for the air that moves through it.

To get the engine started, you have to connect an external heat source, like a cup of hot morning coffee, to the hot cylinder. Then the air inside the hot cylinder will heat up, causing it to expand like a balloon. As the air expands, it pushes the piston into motion. It’s during this step that the Stirling Engine makes its power. The moving piston then pushes the expanding air from the hot cylinder through the regenerator to the cold cylinder. By the time the air reaches the cold cylinder it has already been cooled since the regenerator removes heat from the air traveling through it and stores it.

Once inside the cold cylinder, the air continues to chill down because it is near the cooling source. As it gets colder, the air begins to compress, and this change in air pressure moves the piston in the cold cylinder, propelling the cooler air back through the regenerator toward the hot cylinder. During its return trip to the hot cylinder, the moving air picks up the heat stored by the regenerator. It heats up further inside the hot cylinder, thanks to the adjacent heat source, until the air starts expanding again. This causes the piston to move back — and the cycle begins again.

As long as there are hot and cold sources touching the respective cylinders, a Stirling engine will keep running and keep producing power.

The neat thing about Stirling engines is that it only takes a small temperature difference between the external heat source and the external cooling source to make the process work. In the case of the onE Puck, a hot coffee can heat the air in the hot cylinder and the countertop can cool the air in the cold cylinder.

Robert Stirling’s invention was originally intended to be a safer alternative to the steam engine; in his day, steam explosions were common. (In fact, they still occur sometimes; the 2001 blast at the Medina County Fairgrounds in Ohio is one recent example.) But today its basic technology has been adapted to many other uses. The regenerator, the most important part of a Stirling engine, is the reason why we can produce so many industrial metals today. Regenerators are used in blast furnace technology, which has been widely used since the 19th century to smelt metals. The Stirling engines have found niches in some very specialized applications, like being used as power sources in submarines.

Recently, engineers have even explored using Stirling engines in cars and hooking them up to solar energy collectors. In fact, the Ripasso Energy Company, based in Sweden, claimed in a press release earlier this year that its project in South Africa has produced the highest “solar-to-grid-quality electricity” efficiency of any solar technology in the world, at 32 percent efficiency. On average, solar technology has only 15 percent efficiency when it produces energy on the grid.

With Stirling engines starting to appear in more technologies, maybe it’s not so hard to believe that a Stirling engine might one day sit underneath your morning coffee and charge your cell phone battery. All it takes is a little heat, a little cold and a clever 197-year-old invention that may finally be hitting its prime.