At first pondering over this seems pointless. After all, if the space had energy in it how can we call it empty space? But it all does make sense.
Empty space, in a quantum mechanical sense, is not empty. It is a brewing mixture of all kinds of particles which pop in and out of existence for very small time scales. Heisenberg’s Uncertainty principle allows for this.
An electron-positron pair may pop into existence and then annihilate each other before anyone notices. These changes in the state of empty space are called Quantum Fluctuations.
Another way of looking at it is in terms of Quantized fields. A quantized field is a field which can take only a certain set of values. Every point of space in the universe contains various fields like electromagnetic, gauge and fermionic fields. These fields transmit the forces of nature such as gravity and electromagnetism. So, is it right to call empty space empty space then? It’s your call!
And we know that fields and particles carry energy, some less some more, but they do carry some energy. So we can conclude that empty space contains some amount of energy.
It does feel like a violation of the laws of thermodynamics, but it is not. To understand it let’s talk about the Casimir Effect.
For simplicity let’s take the case of electromagnetic waves alone. Now, we know that photon pairs are being created and destroyed because of the Quantum fluctuations outside the plates, and also in the separation between the plates.
And we also know that a photon which pops into existence can have any wavelength.
Because the separation between the plates is very small, photons with wavelength longer than the separation between the plates do not have enough space to fit between the plates and, therefore, cannot exist between the plates as shown in the diagram below.
This means that there are more photons popping in and out of existence outside the plates than between the plates. This excessive density of photons outside the plates pushes the two plates together and hence produces mechanical energy.
There are several other mechanical setups which can be used to harness energy from the vacuum of space-time itself like the one designed by former director of Nuclear Power Corporation of India, Paramahamsa Tewari. You can read more about his machine here.
These systems provide us with an alternative method of producing energy without causing harmful emissions while supporting a sustained growth and prosperity for the human race.