As alternative energy technologies advance, companies are cutting costs and improving their bottom line, which has translated into continued strength in related green sector exchange traded funds.

“The business model is getting better and better,” Ulf Moslener, a professor at the Frankfurt School of Finance & Management, said in a Bloomberg article. “The value-for-money trend for renewables is improving – and in the traditional sources, it’s deteriorating.”

Moslener points out that while some investors have pulled back from the sector, given misgivings about an end to government subsidies, some investors are still taking advantage of improving technologies and financing. For instance, costs for solar panels has declined 60% over the past three years.

The booming alternative energy sector has helped clean energy stocks outperform, with the PowerShares WilderHill Clean Energy Portfolio (NYSEArca: PBW) up 8.3% and the First Trust NASDAQ Clean Edge Green Energy Index Fund (NasdaqGS: QCLN) up 12.0% year-to-date. In comparison, the S&P 500 index only gained 1.4% so far this year. [Time Again for Tesla ETFs]

Photovoltaic, or solar, companies still lead the clean energy sector. The Guggenheim Solar ETF (NYSEArca: TAN) has increased 24.9% year-to-date. [Solar ETFs Shine As Sector Shows Added Strength]