Plans to build the Big Stone II coal-fired power plant near Milbank, S.D. officially ended on Monday after one of the principal utilities backing the $1.6 billion plant pulled out of the project.

The project had faced a storm of protest over the use of coal to generate electricity from environmental groups. Ultimately, however, it was the uncertain cost of running on coal that drove financiers away. Developers still feel coal is a cost-effective way to generate power, but the impact of looming federal legislation has others wondering.

Opponents of coal say cleaner sources of energy, like natural gas, biomass and wind generation, are better sources for the future.

It's difficult for planners to look 20, 30 or 40 years down the road and try to pick the next great technology for power generation. Tried-and-true methods like coal and nuclear, while still reliable, are out of fashion because of the environmental concerns they raise. Natural gas may be cleaner, but the cost could be volatile and the supply depends on developing cheap ways to get it from oil shale. Wind and biomass are appealing for their cleanliness and renewability, but the technology is still developing.

It may be a while before the best power source becomes clear.