When sustainable design pioneer William McDonough discusses the abundance of opportunities for innovation in his field, he often stresses the need to first acknowledge the limits of technological wizardry and the enormous blindspots in human ingenuity — the need, in other words, for humility. “If anybody has trouble with the concept of design humility,” McDonough likes to say, “reflect on this: it took us 5,000 years to put wheels on our luggage.”

When we talk about innovation, we tend to focus too much on the raw material — the new technologies, the gee-whiz gadgetry of it all — and not enough on novel implementation and creative repurposing and unique application. The integration of technology, in other words, is often as important (maybe more important) than any given technological breakthrough. The kind of innovation I intend to focus most of my attention on in this blog is McDonough’s kind. There are plenty of reinventions of the wheel out there and lots of places to gawk at them; I’m more interested in how any given set of new wheels helps us with our luggage.

I’ve developed a sort of equation to help illustrate the difference between innovation and raw technology. It goes like this: INNOVATION = TECHNOLOGY + NEED + WILL.

In the case of our suitcases, the technology at work behind a literal reinvention of the wheel was, of course, the lightweight tubeless polyurethane wheel. The need was one as old as civilization — the burden of heavy baggage. But the will to make wheeled luggage arose much later, as air travel exploded (both in the volume of passengers and in the frequency of trips) at the same time the availability of human labor at airports was vanishing. We had bags that needed carrying — frequently — and not a porter in sight, so we finally got around to putting wheels on them.

That’s a somewhat silly example. Here’s how McDonough’s axiom goes when it’s applied to the cleantech game: It took us about 200 years to find a use for the Stirling engine.

Invented by a Scottish cleric named Robert Stirling in 1816 — less than half a century after Watt’s original steam engine — the Stirling engine is best understood as a highly efficient energy amplifier. Instead of using a fuel source to boil water and make steam, it uses the heat to create an imbalance in air temperatures between two chambers and generates energy out of the movement of air back and forth. And unlike Watt’s engine, the Stirling didn’t necessarily require combustion — any heat source could be used to get the pistons pumping. Clever as it was, though, the Stirling engine was a footnote to the fossil-fueled industrial revolution, a curiosity for engineering geeks. And there it would remain until the shifting needs (and fuel sources) of the renewable energy boom came along.

Now, Stirling engines are at the core of multimillion-dollar cleantech plays worldwide. A couple of lab rats at the University of Canterbury in New Zealand figured out how to use one to power a household-scale combined-heat-and-power (CHP) plant, and they dubbed it the WhisperGen and partnered with the Spanish manufacturing giant Mondragon to make tens of thousands of these hyper-efficient, electricity-making furnaces for the European market. Other tinkerers (including Segway inventor Dean Kamen) have begun to toy around with Stirling engines in electric vehicles, cooling systems for computers, even space travel. And perhaps most significantly, Stirling engines are the power plants at the center, literally, of some of the most ambitious new Concentrated Solar Power (CSP) plants, particularly the ones being built at industrial scale in the southwestern U.S. by Stirling Energy Systems and its partners. (In Stirling-powered CSP plants, parabolic mirrors concentrate the sun’s rays on the Stirling engine to create the heat differential it needs to generate electricity.)

Stirling’s CSP plans are, to my mind, all about innovation. But note that they are not entirely about new technology. The engine design itself is two centuries old, and the fuel source is as old as time; what was missing, until now, was the need — a burgeoning demand for emissions-free power at industrial scale — and the will, in the form of a booming global market in green power. Like a wheeled suitcase, the Stirling-powered power plant had to wait a good long while for its moment to arrive.

The exciting thing about green innovation right now is that there are so many low-hanging-fruit ideas (or is that rotting-on-the-ground ideas?) like the Stirling engine just beginning to find their niches. Consider another of McDonough’s favorite anecdotes, this one about the radical design innovation that first got him coverage on the front page of the Wall St. Journal: “operable windows.” That is, windows that open. In particular, the ones he installed in the hyper-efficient headquarters he built for the Gap in California back in 1997, which put him on the map as an architect.

Here’s how McDonough likes to explain it: “We thought that was a low point in Western civilization, when a window that opens is news.” Fair enough. But now, at least, things are looking up.