The problem with solar power is that it only works when the sun is shining. Once the sun goes down, there’s no more power. SolarReserve hopes to change that, with a design it says can store solar power at 99 percent efficiency.

SolarReserve’s basic technology is similar to that of other companies, including Ausra, Brightsource and eSolar. Called solar thermal power, it uses mirrors to reflect sunlight onto a centralized structure, in this case a tower. But where there’s usually a mix of water or oil absorbing the sun’s energies, SolarReserve uses molten salt.

Salt’s useful characteristic is that unlike water, which will expand to steam, it will remain in a liquid form even when brought to extremely high temperatures. This allows SolarReserve to easily store it in a reservoir, using its heat to generate electricity during hours when the sun is low.

Using melted salt has been one of the storage ideas that renewable energy proponents have been suggesting for years, but this is one of the first times that it has been put to practical use: SolarReserve intends to use its funding to help build out 5,000 megawatts of capacity, with its first plants coming online in a couple years.

It’s starting to look like energy storage for intermittent renewables (wind and solar) will be viable, taking the $20 million funding that Energy Storage and Power just received for its idea to compress air into caverns to later drive wind turbines alongside the SolarReserve funding. Both technologies offer a fairly economical way to mimic the benefits of conventional energy generation — 24 hour-a-day power.

The $140 million funding was led by Citi Sustainable Development Investments and Good Energies. They were joined by US Renewables Group, PCG Clean Energy & Technology Fund, Nimes Capital and Credit Suisse Customized Fund Investment Group. SolarReserve, based in Santa Monica, Calif., was spun out of United Technologies’ Rocketdyne division earlier this year.