This week, some misaligned mirrors at the biggest solar thermal energy plant in the US caused an electrical fire that took out one of the plant's three boiler towers. The fire comes as the plant has been under pressure from the California Public Utilities Commission (CPUC) for failing to meet its productions targets in the two years since the plant opened.

The solar thermal plant, called the Ivanpah Solar Power Facility, is distinct from photovoltaic solar plants in that it doesn't use solar panels. Instead, it relies on an array of 78-square-foot mirrors, known as heliostats, to direct the hot desert sun to one of three 459-foot-high boiler towers. The heat creates steam in the boiler towers, which powers generators to create energy, which is then sold to PG&E and Southern California Edison.

On Thursday morning around 9:30 am, according to the Associated Press, some poorly-aimed heliostats focused the sun's energy about two-thirds of the way up the tower instead of at the boiler at the top of the tower. This caused electrical cables to catch fire, melting and scorching steam ducts and water pipes on the tower. Even though the plant reported that its personnel put the fire out within 20 minutes, NRG Energy told the Associated Press that it did not know when the tower would be back online. A second tower is also down for maintenance, leaving only one boiler tower producing energy for Ivanpah.

The Ivanpah plant has been controversial. Back in early 2014 when the plant went online, pilots complained repeatedly of being blinded by reflections from the plant. NRG Energy mentioned at the time that the pilot-blinding may have only occurred because workers were still testing and adjusting the heliostats. Environmentalists are also torn about the project. On the one hand, the plant will theoretically be able to produce 448,000 MWh of clean energy annually. On the other hand, the reflected sun from the heliostats is so powerful that heat has killed “thousands” of birds, according to the Wall Street Journal, some of which are burned to death. Ivanpah is required to keep close track of these incidents and report bird deaths monthly.

In March, the CPUC called Ivanpah's future into question, noting that it had failed to deliver the amount of electricity it promised it would contribute to the grid since it came online in 2014. Energy from the plant has yet to become cost effective, running about $200 per megawatt hour during the summer and $135 per megawatt hour during the rest of the year. By contrast, solar power sold from traditional plants in 2015 went for about $57 per megawatt hour. According to the Wall Street Journal, the construction of the solar thermal plant was financed with $1.5 billion in federal loans, as well as investment from Brightsource Energy, NRG Energy, and Alphabet Inc., the parent company of Google.

The CPUC eventually granted Ivanpah six months to a year to meet production targets for two of its towers.