Money is flowing into alternative energy companies so fast that “the warning signs of a bubble are appearing,” according to a report on investment in clean technology by a New York research firm, Lux Research.

The report also suggests that companies that make equipment to cleanse air or water, or that process waste, have been overlooked by investors.

The initial public offerings were primarily in companies involved in solar power or biofuels, according to the report, to be released today.