CSPOD solves the barriers for terrestrial solar

The biggest hurdle to widespread implementation of solar power is the fact that the sun doesn't shine constantly in any given place, so backup power systems are needed for nights and cloudy days. But a novel system designed by researchers at MIT could finally overcome that problem, delivering steady power 24/7.

The basic concept is one that has been the subject of much research: using a large array of mirrors to focus sunlight on a central tower. This approach delivers high temperatures to heat a substance such as molten salt, which could then heat water and turn a generating turbine. But such tower-based concentrated solar power (CSP) systems require expensive pumps and plumbing to transport molten salt and transfer heat, making them difficult to successfully commercialize — and they generally only work when the sun is shining.

Instead, Alexander Slocum and a team of researchers at MIT have created a system that combines heating and storage in a single tank, which would be mounted on the ground instead of in a tower. The heavily insulated tank would admit concentrated sunlight through a narrow opening at its top, and would feature a movable horizontal plate to separate the heated salt on top from the colder salt below. (Salts are generally used in such systems because of their high capacity for absorbing heat and their wide range of useful operating temperatures.) As the salt heated over the course of a sunny day, this barrier would gradually move lower in the tank, accommodating the increasing volume of hot salt. Water circulating around the tank would get heated by the salt, turning to steam to drive a turbine whenever the power is needed.

The plan, detailed in a paper published in the journal Solar Energy, would use an array of mirrors spread across a hillside, aimed to focus sunlight on the top of the tank of salt below. The system could be "cheap, with a minimum number of parts," says Slocum, the Pappalardo Professor of Mechanical Engineering at MIT and lead author of the paper. Reflecting the system's 24/7 power capability, it is called CSPonD (for Concentrated Solar Power on Demand).

The new system could also be more durable than existing CSP systems whose heat-absorbing receivers cool down at night or on cloudy days. "It's the swings in temperature that cause [metal] fatigue and failure," Slocum says. The traditional way to address temperature swings, he says: "You have to way oversize" the system's components. "That adds cost and reduces efficiency."