As one key economic engine after another — housing, finance, autos, retail — sputters and stalls out, the fledgling eco-economy is purring right along, fueled in no small part by venture capital firms hungry for new opportunities in industries that promise outsized returns on their investments. In the first three quarters of 2007, VCs poured $2.6 billion into alternative energy and clean-tech firms, more money than they invested for the whole of 2006. The new year promises to be another record breaker.

And it’s not only the Silicon Valley sharpies that are on the prowl: GE is promising to plow $6 billion into renewable energies by 2010, double what they were projecting only last year; Germany’s Schott Solar is plunking down $100 million to build a plant in New Mexico, and predicts its investment will grow to $500 million when the facility is completed; and as 2007 drew to a close, Morgan Stanley made a $190 million investment in a clean-tech venture. Morgan, by the way, estimates the global renewable energy industry has a market cap in the neighborhood of $170 billion.

Certainly not all is rosy in the clean-tech patch. Tesla Motors and Imperium Renewables, once considered high fliers, have been dealt setbacks — and as a result, have trimmed employee rolls. And alternative energy stocks are starting to look positively bubble-ish to some on Wall Street (the subject of a future post).

Recessions don’t play favorites, for the most part. When U.S. consumers snap their pocketbooks shut, it creates a drag on the overall economy and everyone — including governments that depend on tax receipts — feels the pinch. The eco-economy probably won’t be immune. But with the hundreds of millions of dollars streaming through the doors almost weekly, it’s not a bad place — and better than most — to ride out the storm.