Its products have found a niche market in industries where extended downtime (to recharge a battery) isn't an option: Mostly warehouse forklifts and airport safety vehicles. Unfortunately for Plug, batteries remain the dominant alternative vehicle fuel, and with gasoline prices so low right now, the economic argument for switching to alternative-fuel delivery vans is likely to take a hit. As for Plug, it's tough to assess its valuation, because the company has never posted an annual profit in its more-than-20-year history, so price-to-earnings ratio can't be calculated.