Xcel Energy is working with scientists at the National Renewable Energy Laboratory in Golden, Colo., to work out details of a proposed concentrating solar power (CSP) plant, according to a report in the Rocky Mountain News.

Concentrating solar power uses reflective mirrors that focus sunlight to run a heat-engine. The 200MW project — Colorado's biggest — would cost between $300 million and $500 million and, if approved by state regulators, would go online in 2015, the company says.

The advantage of CSP is that utilities can, at a relatively small cost, incorporate thermal storage, the company said. Having such storage allows the solar plant to keep producing electricity even after the sun sets. Arizona Public Service Co. announced a 280MW CSP project earlier this year that would store heat up to six hours, serving 70,000 customers. Nationwide, CSP projects totaling 4,000MW are on the books.