Buoyancy-Induced Columnar Vortices for Power Generation: The Solar Vortex
Thermally-stratified air layers over solar-heated ground are exploited for scalable, low-cost power generation by the deliberate formation of intense buoyancy-induced vertical columnar vortices. Such vortices entrain the ground-heated air layer in regions with high surface solar heating rates, and convert the (gravitational) potential energy into “solar-induced wind” with significant kinetic energy. Unlike naturally occurring “dust devil” vortices, these columnar vortices are deliberately triggered and anchored. Each vortex is sustained by continuous entrainment of ground-heated air through an azimuthal array of stationary ground-mounted vertical vanes. Electric power is generated by using the air motions within the columnar vortex to drive a vertical-axis turbine. Meter-scale laboratory experiments at Georgia Tech have demonstrated formation and sustainment of a buoyancy-driven vortex that was investigated in some detail. The investigation has focused on the fundamental mechanisms of the formation, evolution, and dynamics of the columnar vortex using stereo particle image velocimetry (PIV) with particular emphasis on scaling and assessment of the available mechanical energy. The strength and scaling of theses vortices can be significantly altered by adjustment of the flow vanes and the amount of global sensible heat absorbed by the air flow, or the “buoyancy flux”. The laboratory vortex was also coupled with a simple, six-blade vertical-axis rotor which extracted significant amounts of kinetic energy and interfered minimally with formation of the vortex. Outdoor tests of a meter-scale prototype have demonstrated autonomous start-up, formation, anchoring, and sustainment of a buoyancy-induced vortex generated solely by absorbed solar energy.
Power Generation Concept
Nearly one-third of the global land mass is desert, providing huge untapped regions for capturing solar heat (about 200 W/m2 averaged over a 24-hour day, and up to 1000 W/m2 peak). The available power is competitive in magnitude with worldwide power generation from fossil sources. The essence of power generation by buoyancy-driven, anchored columnar vortices is to provide a new thermomechanical link between solar energy and electrical energy by harvesting vast amounts of low-grade solar heat for the sustainable production of electricity. Electric power is generated by deliberately forming these columnar vortices and coupling them to vertical-axis turbines. Unlike naturally occurring dust devil vortices that are free to wander laterally, and are hence susceptible to cross wind, each anchored columnar vortex is rendered stationary by an azimuthal array of tangential flow vanes. The rising air plume, at the center of this system of vanes, forces radial entrainment of ground-heated air along the surface. Passage through the stationary vanes imparts tangential momentum to the air flow. Once the vortex is formed, it is directly coupled to a vertical-axis wind turbine which extracts kinetic energy from the flow. The turbine can be specifically designed (and vertically placed) to maximize power extraction from the tangential and axial momenta of the vortex-induced flow. Simple calculations show that for a vortex 5 meters in diameter with nominal tangential and axial wind speeds of 8 m/s and 11 m/s coupled to a 10 m diameter vertical-axis turbine, 50 kWe could be extracted, in principle. An estimate for the cost of energy of the technology, $0.066 per kWh, is competitive with current renewable energy technologies.
Supported by ARPA-E and GT