What do we usually mean by the fifth energy?

It's embarrassing to say it's energy saving. Regard energy conservation as "the fifth energy source"

In today's increasingly severe energy problems, energy conservation has become the "fifth energy" after coal, oil, natural gas and electricity. The concept of the fifth energy will fundamentally change the traditional concept of wealth and interpret the way of wealth generation from a brand-new concept. The application of the fifth energy will really make China embark on the development track of "saving society" and will have a far-reaching impact on our daily necessities.

At present, the fifth energy, a huge market of 500 billion yuan, is showing the tip of the iceberg. Perhaps, the next richest man on the mainland will come from the emerging industry of "the fifth energy".