The demand for electric power is increasing worldwide as economies develop and economies begin to prosper. In unregulated markets the price of electricity increases along with rising demand. That higher cost encourages entrepreneurs to develop methods of generating electric power from technologies that would otherwise be considered uncompetitive. Over time improvements are made to these technologies that reduce the cost at which they produce power.

In its broadest sense, solar energy conversion has undergone and still is undergoing such development that began with waterwheels, windmills and water turbines. Wind energy and hydroelectric power are indirect forms of solar thermal energy. Solar chimneys, solar towers and the vortex engine are among the more recent proposals by which to generate electric power from solar thermal energy. A scale model solar chimney of 50-kW output has operated for several years in Spain, while a scale model of the vortex engine is being tested in Utah.

Solar energy is used to heat the tower or chimney as well as a skirt that is built around the base of the tower. Heated air rises inside the chimney and draws air through turbines that are located at its base. While the efficiency of solar towers is low, they can be built at competitive costs for their power output and rival the output of solar thermal steam power plants as well as photovoltaic technologies. It is a large-scale technology that uses air as its working fluid and that can greatly surpass the estimated power output at lower cost than other competing solar technologies that could occupy the same land area. Solar chimneys could be combined with certain types of photovoltaic power conversion.

There are several regions around the world where prevailing winds undergo very little change in direction with the change of season. In these regions solar towers can be combined with wind energy to increase power output. The skirt at the base of the solar towers collects solar heat and preheats air prior to it passing through turbines and going up the chimney. That skirt could be built in a semi-circular shape to capture wind and duct the wind toward the base of the tower.

A spiral-shaped skirt could capture a large cross section of wind energy and duct it toward the angled inlet vents at the base of the tower to produce a fast swirling air mass or vortex immediately inside the tower. An intake of a very large cross section can capture a large amount of wind energy that would accelerate to higher velocity in the decreasing cross section area of the spiral section. It would pass through the small cross sectional area of the turbines at high velocity, high efficiency and deliver higher power output that would be based on the cube of the wind velocity through the turbines.

The diameter of the base of a full-size solar tower can vary from 200 feet (60 meters) to 600 feet (200 meters). The solar chimney would generate a low-pressure zone immediately downstream of the turbines at the base of these engines. The tornado or cyclone that is ejected from the top of a vortex engine achieves a similar result in that it would draft the turbines to generate power. Directing wind energy into an intake of decreasing cross section would increase air speed through the drafted turbines and raise efficiency and increase power output.

Vertical-axis Turbines

A large vertical-axis turbine may be used to generate power if the solar skirt is of spiral design. On a mini scale, the spiral skirt would resemble the layout of the turbocharger of a truck engine in which the casing serves as a stator. It induces a swirl velocity to the exhaust gas prior to it flowing through the radial-flow turbine. A Russian engineer proposed a design of a giant sized vertical axis wind turbine that can ride on rails. That concept could be used inside the base of a solar tower equipped with a spiral intake. Each carriage could carry a turbine blade or airfoil as high as the mast of a yacht or tall sailing ship which is up to 200 feet (60 meters) in height. The spiral skirt would ensure that all vertical turbine blades would deliver power at all locations through 360 degrees of travel.

A spiral intake that leads to angled vents at the base of the solar tower could generate a swirling air mass inside the tower. The concept would achieve a similar result as the casing of the turbocharger in a railway locomotive engine that generates a vortex that flows into an axial-flow turbine. On a mega-scale the vortex inside the solar tower would flow into an axial-flow turbine mounted at about 330 feet (100 meters) above ground. Its weight would be supported by rails built inside the wall of the main tower as well as on the wall of a central inner tower of smaller diameter. The outer rails would carry both the vertical load as well as the tensile load or hoop stress that would result from the rotational velocity of each blade generating a centrifugal force against the rail. The rail wheels that carry the turbine may also drive electrical generation equipment.

Efficiency and Power

The peak isentropic efficiency of large vertical-axis (radial-flow) turbines and large axial-flow turbines could exceed 60% in moderate winds and rise to over 80% during strong winds. The cross section of the entrance to the spiral intake could be twice the cross section across the turbine blades. That decrease in cross-sectional area would cause the air speed to gently increase before passing through the turbines. The effect of doubling the air velocity in the spiral intake would increase turbine efficiency as well as raise power output eight-fold over a free-stream vertical axis turbine.

The solar-heated tower could serve as an exhaust chimney that would propel air from the turbines into the atmosphere. At some locations the heated tower may be used to produce a vortex or tornado so as to draft air through the turbine of up to 200 MW of output. That output was calculated by research groups such as the Solar Mission group of Australia, the Vortex Engine group of Canada and the Floating Solar Chimney group from Greece. The Vortex Engine group has even suggested that their design could generate up to 500 MW of output from a tower of 200 meters (656 feet) diameter. Research is underway to determine as to whether the vortex of the vortex engine would continue to operate during periods of high wind.

A tower of 600 feet diameter could have angled intakes that have a width of 1,000 feet at the base of the tower and a possible vertical height of 200 feet. It could draft air at 0.065 pounds per cubic feet through the turbines and exceed 140 MW output at 60% isentropic efficiency. That output could rise to over 330 MW at air speed of 40 feet per second through the turbines having 60% isentropic efficiency. An output of 1 GW could be possible after air speed exceeds 50 feet per second through turbines operating at 70% isentropic efficiency. That output would be the combined output of local solar heating of air and distant solar thermal energy that gave rise to the winds. Additional output would be possible after installing flexible solar panels from a company such as Daystar on the tower. Those panels would convert mainly UV light while heat from the infrared spectrum could heat the walls of the tower.

System Exhaust

There are geographic locations where a solar tower of 200 meters (656 feet) height could propel a swirling vortex high into the atmosphere and generate a powerful vacuum effect immediately downstream of the turbine. That vacuum effect would enhance the efficiency and output of the turbine. An exhaust stator may be needed immediately downstream of an axial-flow turbine to sustain the exhaust vortex.

There are other locations where a high tower would be used instead of the vortex. The appropriate exhaust would be the 1,500 meter-high floating chimney design developed in Greece. It could be attached to a tower of up to 200 meters height and made of reinforced concrete. Such a tower would house an axial-flow turbine at a height if 100 meters. Solar towers that are built to a very large diameter and that use an axial-flow turbine and a smaller inner tower could use a circular array of several floating chimneys to draft the turbine.

The height of the concrete tower could be reduced to less than 100 meters if a large radial-flow vertical-axis turbine were used to generate power. The floating solar chimney could be attached to such a tower and extend to a height of 1,500 meters. It would be made from lightweight material and be kept aloft by a series of air cells that contain a light gas such as Helium. It may be possible to attach flexible photovoltaic technology from Daystar on the floating chimney design to generate additional output.

Suitable Locations

There are numerous locations around the world where wind direction undergoes very little change with the seasons. Powerful winds blow from the mid-Atlantic Ocean across the group of islands known as the Lesser Antilles as well as along the coastal regions of such countries as Surinam, Guyana and Venezuela. Some of these winds blow across the Central American countries of Honduras, Nicaragua and Panama. Similarly powerful winds blow from the south Atlantic toward the Brazilian coast between 5 degrees south and the Tropic of Capricorn.

There are unidirectional winds that blow toward Chile south of Valparaiso as well as winds that blow north along the coastal regions of northern Chile and southern Peru. Similar northbound winds blow along the west coast of Africa between the Tropic of Capricorn and the equator as well as along the west coast of Australia near Perth. Mainly unidirectional winds also blow along Australia’s northeastern coast and southern coast, over Tasmania, across southern New Zealand and also across the southernmost tip of South Africa.

Unidirectional winds also blow from the northern Atlantic toward Ireland, Scotland, over a part of the southern UK as well as western France and northern Spain. Winds that blow south over North Africa and the Arabian Peninsula undergo very little change with the seasons. At locations where rainfall is frequent and abundant, drainage systems need to be included in the wind-supercharged solar towers. At other locations the local topography could enhance the performance of such engines.

Effect of Mountains

There are numerous locations around the world where unidirectional winds blow into narrowing valleys throughout the year. It may be possible to take advantage of such features at some locations as the walls of such valleys could serve as the outer walls for part of the air intake. Cables could be secured to the walls of the valleys to stabilize the solar tower against buckling and even carry part of the weight of the tower. A floating chimney could be placed on top of the stabilized tower and reach up to 2,000 meters between the turbine and the exit. A cable stabilized tower could diverge into multiple exists on to which floating chimneys may be attached to increase the draft on the turbines.

Some valleys lead to dead ends from which incoming winds would accelerate upward at high velocity. Such updrafts could provide a drafting effect at the top of the tower and to the turbines at its base. The updraft may also be able to sustain at vortex at the tower exit. At other locations the tower could exhaust downstream into a widening valley over which a cover could be built so as to draft the turbines in the tower. There may be scope to use other topographical features to enhance the performance of the wind-supercharged solar tower systems. Suitable mountains and winds may be found at several locations around the world that would include:

* Andes Mountains along the coast of Chile and Peru.
* Coastal Mountains of California and Central Mountains of Baja California.
* Central Mountains of Panama, Costa Rica, Honduras and Guatemala.
* Coastal Mountains of northern Spain.
* Southern Alps of New Zealand.

Conclusions

The power output and efficiency of such engines such as the solar chimney, solar tower and vortex engines can be improved using wind energy in regions winds are unidirectional year round. Performance could be further enhanced by using thermal energy provided by exhaust heat from thermal power stations, from geothermal energy or by concentrated solar heat. Solar reflectors may be placed at different elevations on mountain slopes to increase heating on the walls of shorter solar towers and chimneys. While the overall efficiency of wind-supercharged solar towers may remain comparatively low, they could still be competitive against many other renewable technologies in terms of output per unit cost.

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