Some costly high-tech solar power projects aren't living up to promises their backers made about how much electricity they could generate.

Solar-thermal technology, which uses mirrors to capture the sun's rays, was once heralded as the advance that would overtake old fashioned solar panel farms. But a series of missteps and technical difficulties threatens to make newfangled solar-thermal technology obsolete.

The $2.2 billion Ivanpah solar power project in California's Mojave Desert is supposed to be generating more than a million megawatt-hours of electricity each year. But 15 months after starting up, the plant is producing just 40% of that, according to data from the U.S. Energy Department.

The sprawling facility uses "power towers"--huge pillars surrounded by more than 170,000 mirrors, each bigger than a king-size bed--to capture the sun's rays and create steam. That steam is used to generate electricity. Built by BrightSource Energy Inc. and operated by NRG Energy Inc., Ivanpah has been advertised as more reliable than a traditional solar panel farm, in part, because it more closely resembles conventional power plants that burn coal or natural gas.

Turns out, there is a lot more to go wrong with the new technology. Replacing broken equipment and learning better ways to operate the complex assortment of machinery has stalled Ivanpah's ability to reach full potential, said Randy Hickok, a senior vice president at NRG. New solar-thermal technology isn't as simple as traditional solar panel installations. Since older solar photovoltaic panels have been around for decades, they improve in efficiency and price every year, he said.

"There's a lot more on-the-job learning with Ivanpah," Mr. Hickok said, adding that engineers have had to fix leaky tubes connected to water boilers and contend with a vibrating steam turbine that threatened nearby equipment.

One big miscalculation was that the power plant requires far more steam to run smoothly and efficiently than originally thought, according to a document filed with the California Energy Commission. Instead of ramping up the plant each day before sunrise by burning one hour's worth of natural gas to generate steam, Ivanpah needs more than four times that much help from fossil fuels to get plant humming every morning. Another unexpected problem: not enough sun. Weather predictions for the area underestimated the amount of cloud cover that has blanketed Ivanpah since it went into service in 2013.

Ivanpah isn't the only new solar-thermal project is struggling to energize the grid. A large mirror-powered plant built in Arizona almost two years ago by Abengoa SA of Spain has also had its share of hiccups. Designed to deliver a million megawatt hours of power annually, the plant is putting out roughly half that, federal data show.

NRG and Abengoa say their plants will reach power targets once the kinks are worked out.

In contrast, incremental improvements to traditional solar panels have allowed SunPower Corp. to get more electricity that it originally thought it could from its 1,500-acre solar farm. California Valley Solar Ranch was designed to produce 600,000 megawatt-hours a year in 2013 when it started operating, but today it can generate up to 4% more.

"It's years of learning from experience," said Tom Werner, chief executive of SunPower, adding that his employees have been building large-scale solar farms for more than a decade.

Solar-thermal developers including Abengoa and BrightSource continue to build new plants in South Africa, Chile and China. But Lucas Davis, an economics professor at the University of California, Berkeley, says it is unlikely more U.S. projects will gain traction as utilities opt for cheaper solar farms that use panels.

"I don't expect a lot of solar thermal to get built. It's just too expensive," he said.

American solar farms generate 15 million megawatt-hours of electricity each year. That satisfies less than 1% of U.S. electricity demand, but six times the amount of power that solar-thermal plants currently produce. And the vast arrays of solar panels that blanket the ground cost roughly half as much to build as new mirror-powered plants, according to the U.S. Energy Department.

Electricity prices from new solar farms average around 5 cents a kilowatt-hour, according to GTM Research, which tracks renewable energy markets. That compares with between 12 and 25 cents a kilowatt-hour for electricity generated by the Ivanpah power plant, state and federal data show.

It is unclear how much power would cost from a brand new solar-thermal plant, but it would be more than 5 cents a kilowatt-hour, said Parthiv Kurup, an analyst at the National Renewable Energy Lab in Denver.

The cost of solar panels has plunged in recent years amid a world-wide glut of equipment as China, Taiwan and other countries rapidly developed solar manufacturing centers. A few years ago solar-thermal technology was heralded as a better way to deliver carbon-free renewable energy; some utilities even predicted the technology would replace traditional solar farms.

Even if solar-thermal developers could offer the same power prices as their solar-panel rivals do, solar-thermal plants face environmental hurdles in the U.S.

The Ivanpah plant was delayed several months and had millions of dollars in cost overruns because of wildlife protections for the endangered Desert Tortoise. Once built, U.S. government biologists found the plant's superheated mirrors were killing birds. In April, biologists working for the state estimated that 3,500 birds died at Ivanpah in the span of a year, many of them burned alive while flying through a part of the solar installment where air temperatures can reach 1,000 degrees Fahrenheit.

Bird carnage combined with opposition by Native American tribes to industrial projects on undeveloped land has made California regulators wary of approving more. Last September, Abengoa and BrightSource abandoned their quest to build a solar-thermal project near Joshua Tree National Park when the state regulator told them the plant's footprint would have to be cut in half.

In March the Board of Supervisors of Inyo County, a sparsely populated part of California that is home to Death Valley National Park, voted to ban solar-thermal power plants altogether. "Ivanpah had a significant effect on the decision making," said Joshua Hart, the county's planning director.

Write to Cassandra Sweet at cassandra.sweet@wsj.com

Access Investor Kit for NRG Energy, Inc.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US6293775085

Subscribe to WSJ: http://online.wsj.com?mod=djnwires