1. Key World Energy Statistics 2017. 2017. Available from: .
2. Annual energy outlook 2018. 2018. Available from: .
3. Henrik Carlsen, H.D. Kühl, Natural gas driven Vuilleumier heat pumps for residential heating, in: New application of natural working fluids in refrigeration and air conditioning, Hannover, 1994.
4. Hanfei Chen, Jon Longtin, Thermodynamic modeling of a free piston vuilleumier heat pump, in: 2nd Thermal and Fluids Engineering Conference, Begell House: Nevada, 2017.
5. William Goetzler, Robert Zogg, Jim Young, and Caitlin Johnson, Energy savings potential and RD&D opportunities for non-vapor-compression HVAC technologies. 2014. Available from: .