Thanks to Greg Daigle for pointing out the following patent by Garrett Moddel and Bernard Haisch, published in 2005 and granted in 2008.
Quantum Vacuum Energy Extraction. US Patent US7379286
Greg notes that this patent is also cited in Andrea Rossi’s paper “E-Cat SK and Long Range Particle Interactions” (it is reference 23 in the endnotes). https://www.researchgate.net/publication/330601653_E-Cat_SK_and_long-range_particle_interactions.
The paragraph in Rossi’s paper referencing this patent is as follows:
Within this conceptual framework Puthoff has explored  the idea that, in principle, it’s possible to extract energy and heat from electromagnetic zero-point radiation via the use of Vacuum pressure. A device that may reach this goal has been proposed in the US patent US7379286 , where the authors consider the possibility of a local energy extraction that is replenished globally from and by the electromagnetic quantum vacuum.
Here’s the abstract from the Moddel/Haisch patent with some emphasis added. It seems to me that working systems that can extract useful useable energy from the Quantum Vacuum/Zero Point Energy field would be extremely useful and beneficial, compared to the dominant forms of energy production used today, which each have their own set of drawbacks and limitations.
A system is disclosed for converting energy from the electromagnetic quantum vacuum available at any point in the universe to usable energy in the form of heat, electricity, mechanical energy or other forms of power. By suppressing electromagnetic quantum vacuum energy at appropriate frequencies a change may be effected in the electron energy levels which will result in the emission or release of energy. Mode suppression of electromagnetic quantum vacuum radiation is known to take place in Casimir cavities. A Casimir cavity refers to any region in which electromagnetic modes are suppressed or restricted. When atoms enter into suitable micro Casimir cavities a decrease in the orbital energies of electrons in atoms will thus occur. Such energy will be captured in the claimed devices. Upon emergence form such micro Casimir cavities the atoms will be re-energized by the ambient electromagnetic quantum vacuum. In this way energy is extracted locally and replenished globally from and by the electromagnetic quantum vacuum. This process may be repeated an unlimited number of times. This process is also consistent with the conservation of energy in that all usable energy does come at the expense of the energy content of the electromagnetic quantum vacuum. Similar effects may be produced by acting upon molecular bonds. Devices are described in which gas is recycled through a multiplicity of Casimir cavities. The disclosed devices are scalable in size and energy output for applications ranging from replacements for small batteries to power plant sized generators of electricity.
I get the feeling that this are of research is going to grow in interest and significance over the next few years, providing that working devices can be demonstrated to be rea.