Wind and solar power have gotten so cheap that other forms of renewable energy have been relegated to science labs and the dustbins of history.

As wind turbines and solar farms sprout around the world, investments in other technologies that produce power — from waves, geothermal steam pockets, rivers, biomass and more — have dropped off a cliff. Even those gleaming mirrors in the desert that focus sunshine onto thermal generators can’t compete on price with photovoltaic panels, and they probably never will.

As the modern green energy industry was starting to take off, investors poured money into all kinds of ideas. Bets on wind and solar have paid off, but the rest have mostly fizzled. Turbines and panels received about half the spending on renewables in 2004, according to Bloomberg NEF. This year, the two technologies will suck up 93 percent of global spending.

“The reality is that wind and solar got to scale first,” said Gabriel Alonso, an executive with Quantum Energy Partners, a Houston-based private equity company. “Other renewables have been cornered by the now-permanent success of wind and solar.”

Wind and solar have become the largest sources of new power in the U.S. and parts of Europe. That growth is accelerating the retirement of aging carbon-spewing coal plants. Total spending is expected to decline this year, in part because of a pull-back in China, but also because prices have come down.

“With solar and wind so cheap, why should you bother with anything else?” said Jenny Chase, an analyst at BNEF. “There just aren’t a lot of opportunities for mass deployment for anything else.”

This is also creating new challenges because panels and turbines can’t generate electricity around the clock. There are competing forms of clean energy that can fill the gap. Marine energy systems produce electricity from the never-ending motion of waves and tides, but they’re not price competitive. Geothermal systems use underground steam pockets that can be tapped anytime, but are only found in specific locations. Batteries can store electricity until it’s needed, but they’re still costly, even though prices are coming down quickly.

“Solar and wind can get you to 80 percent, but not all the way,” said Chase. “To fully decarbonize, you have to look elsewhere. You have to start considering things that aren’t cheap.”