New patents related to renewable energy technologies are on the rise, a Massachusetts Institute of Technology study shows.

The MIT study, in conjunction with the Santa Fe Institute, found that from 1970 to 2009, Japan led the world in the number of patents related to photovoltaics, or solar cells, with the United States second and China third.

The number of solar patents worldwide increased by 13 percent per year, from 2004 to 2009, says the report, published in the journal PLoS ONE.

"China's really taking off," Jessika Trancik, an assistant professor of engineering systems at MIT and a co-author of the report, said in an MIT release Thursday.

While China has been accused of taking advantage of renewable energy technologies invented elsewhere, "understanding the nature of the technological development represented requires a close look at patent content," she said.

China's patents in the study included those filed by foreign companies or inventors.

The researchers created a database of energy-related patents issued in more than 100 countries between 1970 and 2009 and focused on about 73,000 patents for energy-related technologies. Further analysis indicated a clear correlation between a rise in patents and previous investments in research and development, along with growth in the markets for such renewable technologies, Trancik said.

Patents in solar energy and wind power experienced the biggest increase, while patents for fossil fuel technologies showed a more modest increase. Patents related to technologies for nuclear power were flat.

The study found there were fewer than 200 renewable energy-related patents per year in the United States from 1975 to 2000. Those patents increased to more than 1,000 annually by 2009.

By comparison, there were about 300 fossil fuel-related patents in 2009, up from about 100 a year in earlier decades.

Separately, a Citibank report released Thursday says renewable energy will account for more than 70 percent of worldwide investment in new power generation by 2025, the Houston Chronicle reports.

Of the nearly $10 trillion expected to be invested into the power sector over the next decade, Citibank projects that more than $2 trillion will be invested in wind, followed by $1.5 trillion in hydropower and $1.3 trillion in solar power.

Although natural gas is increasingly replacing coal-fired power generation in the United States, the report notes that in the longer term, the lower price of solar energy will represent an appealing alternative, especially during peak demand periods.