A Proposal for a new Renewable Energy Source: the Atmospheric Vortex Engine
Increased global demand for earth's fossil fuel resources in conjunction with dwindling oil and natural gas reserves is beginning to cause concerns for meeting our long-term energy needs. Options for replacing fossil fuels are limited. As a result of these growing concerns, there has been strong new interest in solar, wind energy and other renewable energy sources.
Traditional solar and wind energy are low intensity variable energy sources. The land area required to meet the energy needs of a country with a high population density using only solar or wind energy would be a significant part of the country. Meeting the present energy needs of a country such as Germany would require that over 10% of the land area be covered with solar collectors or wind turbines. Use of such a large land area for solar collectors or wind turbines is simply not a practical solution.
The solar chimney is another method which has been proven capable of generating electrical energy from the sun. The solar chimney consists of a very tall chimney surrounded by solar collectors. The collectors heat the air near the ground and funnel the air into the base of the chimney. A turbine in the base of the chimney generates electricity as the buoyant air rises in the chimney. The Manzanares solar chimney built in Spain in the 1980's operated for seven years with a peak output of 50 kW.
The solar chimney is an interesting solar energy alternative, but it also requires solar collectors plus a very tall chimney. The efficiency of a solar chimney is proportional to its height with the overall efficiency unlikely to exceed 2%. Increasing the height of the chimney could increase the efficiency, but there are practical construction limitations which restrict the maximum height of chimney that could ever be built. The collector area would be as large or larger than in more traditional solar technology. Enviromission is currently in the process of designing a large 200 MW solar chimney to be built in Australia.
A basic understanding of atmospheric science is required in order to explain the principle of operation. The atmosphere is warmed from by solar radiation which strikes the ground. Air heated at ground level by solar radiation becomes buoyant. Heated air is transported upward by natural convection to the level at which the heat is radiated heat back into space as infrared radiation. Mechanical energy is produced when heat is carried upward by convection because the work of expansion of a heated gas is greater than the heat required to compress the same gas back to its original pressure after it has been cooled.
The mechanical energy produced in a large hurricane can exceed the energy produced by humans in a whole year. The energy produced in an average size tornado is equivalent to the energy produced in a large thermal power plant. The solar chimney demonstrates that it is possible to capture the energy produced during atmospheric upward heat convection.
2. Atmospheric Vortex Engine
A method of producing a tornado-like vortex and thereby concentrating this mechanical energy where it can be captured is proposed. The Atmospheric Vortex Engine (AVE) would consist of a vertical axis cylindrical wall open at the top and with tangential air entries around its base. The heat source could be the natural heat content of warm humid air, warm seawater, or waste industrial heat. Heat could be transferred to the air in an optional peripheral cooling-tower located outside the cylindrical wall and upstream of the tangential air entries.
The Atmospheric Vortex Engine has the same thermodynamic basis as the solar chimney. However, the main differences are that the physical tube of the solar chimney is replaced by centrifugal force and that the atmospheric boundary layer acts as the solar collector. A vortex engine can have a much higher efficiency than a solar chimney because a vortex can extend much higher than a physical chimney.
The electrical energy would be produced in turbines located upstream of the tangential entry inlets. The intensity of the vortex would be regulated by restricting the flow of air with dampers located upstream of the deflectors. The vortex would be stopped by restricting the airflow to deflectors with direct orientation and by opening the airflow to deflectors with reverse orientation. The vortex would be started by temporarily heating the air with a startup heat source such as fuel or steam.
An AVE power station could have an electrical capacity of 200 MW. It is estimated that a 200 MW AVE power station would require cylindrical walls approximately 100 m high with a diameter of 400 m. The convective vortex could have a diameter of 50 m at its base and could extend to a height of up to 20 km. There could be 20 x 10 MW turbines around the periphery of the station to generate electricity.
The existence of tornadoes proves that low intensity solar radiation can produce concentrated mechanical energy. It should be possible to control a naturally occurring process. Controlling where mechanical energy is produced in the atmosphere offers the possibility of harnessing solar energy without having to use solar collectors. The unit cost of electrical energy produced with a vortex engine would be much less than the cost of the most economical alternative because no fuel or heat collector is required.
The heat to work conversion efficiency of the atmospheric process is determined by Carnot efficiency. Carnot efficiency depends on the temperature at which heat is received and given up. The temperature of the atmosphere decreases with height, the atmosphere receives heat at an average temperature of 15 C at ground level but gives it up at an average temperature of -15 C in the upper atmosphere. This results in a Carnot efficiency for the AVE process of approximately 15%.
3. Energy production potential
The average upward convective heat flux at the bottom atmosphere is 150 W/m2, one sixth of this heat could be converted to work while it is carried upward by convection. The average work that could be produced in the atmosphere is therefore 25 W/m2. The total mechanical energy that could be produced in the atmosphere is 12000 TW whereas the total work produced by humans is 2 TW. The energy that could be produced in the atmosphere is 6000 times greater than the energy produced by humans. Capturing a small fraction of the mechanical energy that could be produced during upward convection in the atmosphere could meet human energy needs.
Capturing the mechanical energy produced when heat is carried upward by convection in the atmosphere will require specialized mechanisms; the expansion must take place at mechanical equilibrium. Like in a hydraulic power plant, there must be a conduit to transmit the energy downward. Without either the conduit or the turbine, the work is lost and heat is produced instead of work.
Atmospheric scientists call the mechanical energy that would be produced when a unit mass of air were raised Convective Available Potential Energy (CAPE). CAPE during periods of insolation or active convection is typically around 2000 J/kg, which is equal to the mechanical energy produced by lowering a kilogram of water 200 m. The kinetic energy of updrafts is typically less than 2% of CAPE, mainly because the expansion is not carried out at mechanical equilibrium. The kinetic energy of horizontal winds is only a small fraction of the energy that could be produced through atmospheric upward heat convection.
The AVE process could provide large quantity of renewable energy, could alleviate global warming, and could contribute to meeting the Kyoto protocol. The AVE has the potential of providing precipitation as well as energy. There is reluctance to attempt to reproduce a phenomenon as destructive as a tornado, but controlled tornadoes might actually reduce hazards by relieving instability rather than create hazards. A small tornado firmly anchored over a strongly built station need not be a hazard; the process could reduce instability and alleviate natural storms. Controlling a natural tornado let alone a hurricane would be daunting. Starting and controlling an artificial tornado might not be difficult.
The process could capture the work not produced as a result of lack of mechanical equilibrium and not just the kinetic energy of horizontal wind. Operation would not be limited to the daytime because the AVE could use the latent heat of water vapor accumulated in the air at the bottom of the atmosphere or the sensible heat accumulated in the warm seas. The need for very large collector area would be eliminated because the earth's surface without any modification would be the heat collector. An AVE could work at nighttime and on cloudy days. The energy produced per unit area of turbine would be higher than in conventional wind turbines because the differential pressure across the turbine is much higher since the air is forced to go through the turbines.
It is estimated that under ideal conditions it would be possible to establish a self-sustaining vortex to demonstrate the feasibility of the process with a station 30 m in diameter. Learning to control large vortices will be a major engineering challenge. Developing the process will require determination, engineering resources, and cooperation between engineers and atmospheric scientists. There will be difficulties to overcome, but they should be no greater than in other large technical enterprises.
Additional information including drawings and thermodynamic basis is available on web site: vortexengine.ca