Many of the most exciting 21st century renewable-energy technologies are based on engineering concepts that date back decades, if not centuries. Wind turbines apply principles from one of humankind's oldest elements-harnessing technologies, while fuel cells employ concepts pioneered in the early days of the Industrial Revolution in 19th century Great Britain. Joining this list is a less well-known and still nascent renewable energy source with huge potential: tidal power. One innovative approach to ocean flow-harnessing technology is based on the Venturi tube, a pre-World War II invention that uses pressure differentials to create an energy flow through an enclosed space. The tidal energy creates the flow within the tube that powers a turbine, but it is actually air, not water, that drives the turbine blades in this form of tidal power. "It's not science-fair technology," says Dave Olson, a consultant with MCC Global Energy Advisors, which helps energy firms seeking private equity funding. "It's 60- year-old technology being used in a modern application." Olson himself used Venturi technology in industrial flow meters while working in Honeywell's controls business some years ago. Among MCC's clients is U.K.-based HydroVenturi, which is working with the city of San Francisco to turn the tidal flows of San Francisco Bay into electricity. The city is aiming to complete a proof-of-concept test this summer, leading to a $4 million pilot project in the next two years that is projected to generate a peak of two megawatts of energy and a daily average of one megawatt. Project champion Peter O'Donnell, senior energy specialist with the city's department of the environment, says fully harnessing the currents of San Francisco Bay could produce 2,000 MW. That's clearly not ecologically or economically feasible, but even a small percentage of that amount could figure in the mix for the city's energy needs, currently about 850 MW at peak. "Our goal is to give the city another option as it expands the use of renewable sources," says O'Donnell. "And the tides are so predictable, we can tell the peakers when to turn on and off." Like wind turbine farms in some areas, tidal power generation faces environmental concerns - in this case possible impacts on bay sedimentation, phytoplankton supply, and fish spawning patterns. That makes ecologically conscious San Francisco the perfect proving ground, says O'Donnell. "If we can show a minimal impact from tidal power with the level of environmental scrutiny we get here," he says, "that bodes very well for the rest of the world." HydroVenturi also has projects to harness the power of river flows, mostly in Europe. The other major emerging tidal-power technology is a system of buoys that utilize the up-and-down motion of waves to power small generators linked to an undersea transmission cable. Its leading purveyor, Ocean Power Technologies in Pennington, N.J., is testing its PowerBuoy system in a U.S. Navy project off the north shore of Oahu in Hawaii, a Mecca for surfers. But you won't see PowerBuoys riding 40-foot waves on the covers of surfing magazines; slow and steady wave action is what works, and the system actually shuts down if the waves are too big and potentially damaging. Ocean Power Technologies says it can produce power for 3 to 4 cents per kilowatt- hour in a primary (100 MW) power operation and 7 to 10 cents per kwh in a 1 MW secondary power scenario. O'Donnell says the San Francisco project could be similarly cost-competitive. The hunt for tidal power projects and company funding is on, as it is for most clean- energy projects. But tidal faces some unique challenges. "Marine engineering and power generation issues are very different, but they will have to be integrated," says O'Donnell, to minimize environmental impacts and maximize generating efficiency. Funding expert Olson notes that tidal power is in the Catch 22 position of needing both project funding and equity backing for startups like HydroVenturi. So sources like municipal bonds and private foundations may be tapped, as well as more traditional investors. What will help loosen the purse strings are creativity and innovation -- the same things that got people thinking about turning ocean waves into electricity in the first place. Huge challenges remain, but a successful demonstration project this year in San Francisco or elsewhere could help propel tidal power into a realistic clean energy option. Clint Wilder is Clean Edge's contributing editor. E-mail him at wilder[at]cleanedge[dot]com.