Solar energy firm BrightSource Energy has proposed the addition of thermal energy storage systems to three planned solar power tower plants in California in a bid to reduce the cost of energy produced by the plants.

BrightSource has signed new power purchase agreements with Southern California Edison for the three proposed plants, which will be equipped with its SolarPlus thermal energy storage technology.

The thermal storage systems will increase the capacity factors of the three solar power tower plants by enabling them to continue producing electricity into later parts of the day compared to similar plants with no thermal storage ability.

Improved capacity factors mean that BrightSource will only need to build five solar thermal power plants in order to meet its contractual commitments with Southern California Edison, instead of the six it initially planned.

“With these agreements, we’re demonstrating that power tower technology is not only advancing the solar thermal industry, but that utility-scale solar generation can be both cost effective and reliable,” said John Woolard, President and CEO of BrightSource Energy.

The move illustrates the pressure that the solar thermal industry is under as prices in the solar photovoltaic (PV) industry fall.

A number of proposed solar thermal power plants in the US south west have been changed to incorporate PV technology because the costs of solar PV panels have fallen drastically in the last two years.

Under its contracts with Southern California Edison, BrightSource is to provide the utility with around 4 million MWh of electricity per year with initial deliveries starting in 2015. BrightSource had originally planned to build six solar thermal plants to meet these commitments.

It says that it will now only build five plants – three with thermal storage capabilities and two without.