A Treasure Trove of (Potential) Energy is Hidden in Plain Sight within Urban Heat Islands. Urban Dwellers are virtually “Walking on Sunshine”
Energy Recovery along with Cooling Could be Realized by Installing Atmospheric Vortex Engines.
by Jerry J. Toman, MS, ChE
Summary
A problem that worsens each decade for southern cities such as Houston or Phoenix is an effect called the Urban Heat Island, for which inner city temperatures have been observed to exceed temperatures measured in nearby rural areas by amounts now approaching 20 F. Sass has shown that an 8-9 F reduction in average temperature could be achieved in such cities if ~ 1.0 kwh/m2/day of solar radiation (heat equivalent) were somehow to be removed from a large (urban) area (see above figure).
To mitigate the UHI, it is proposed to install a straight-forward technology, called the Atmospheric Vortex Engine. It has been herein estimated that, by installing AVE facilities that could continuously elevate 1000 m3/s of air per square kilometer of surface from the inner city into the mid troposphere, during the summer, approximately 0.3-0.5 kwh/m2/day of heat (~ 65% via evaporation) would be removed. As per Sass, a mean temperature reduction of 3-4 oF could thereby be achieved as cooler, drier air from rural areas is pulled in to replace the warmer, wetter air that would be ejected from the “hot zones”, closer to the city center.
As a result of the air exchange, water use above current levels due to higher evaporation rates in each km2 during the warm, dry season (April-June, Phoenix) could be as high as 10 grams per m3 of air (10 kg/s or 860 m3/d). However, this still represents a relatively low fraction of total water use. During the monsoon season, much of it would be spun-out of the vortex at low altitude, and fall back to earth as an evenly-distributed, cooling rain. Also since any electricity produced (see below) would not require cooling towers, a net water savings should result in the long run. The amount “saved” would be especially large when compared to the water needs associated with solar (trough-based) power installations, often touted as low-carbon solutions.
It has been further estimated that, if each 1000 m3/s stream of air were aspirated into a duct, and expanded through a turbine located near the end of each duct before being discharged into the AVE arena, approximately 1-2 MW, (day/night year around) of electricity could be generated, with the higher generation rates occurring during summer afternoons and evenings, when demand for power is greatest. For 50 ducts, this corresponds to a power output varying between 50 and 100 MW; these could discharge into a single arena which would cool an area of 50 square kilometers (~ 20 sq. mi.).
From each AVE arena, a vortex would ascend to altitude through a hole in the roof, some 50-60 m across. The pressure at the base of the vortex and at the turbine outlet would be slightly sub-atmospheric (~ 4%). This is physically possible, due to heat released by condensation of water vapor at elevation, where the resulting liquid would be spun-out of the vortex. The warming makes the vortex air less dense than ambient air when averaged over 8-12 km of column height, creating a vacuum at its base.
Other benefits of AVE installations would be in health-care costs and power saving, as well as a reduction in the city’s carbon footprint. In addition, by draining away “thermal potential” from a city, it would enjoy a reduced likelihood to suffer from the worst consequences of damaging winds, floods and other weather-related events.
Nature of the Vortex Engine and Urban Heat Mitigation
Click here for an up to date technical description of the Atmospheric Vortex Engine given in the magazine Energy Manager published in India. Read about several previous configurations of the AVE.
The AVE may be viewed most simply as a collection of horizontal air ducts, arranged in a geometric pattern, and of nominal length. Each of these would take in air at a surface location, and discharge it uniformly, not far from ground level into a common circular enclosure or arena (D =75-125 m). Each duct (D = 4-6 m, L= 10-25 m) would transect the wall of the arena at a constant fixed angle (say 45-60 deg.) from the radial vector. At the point of discharge, the duct’s aspect ratio (H/W) could be 2-3. (See figure below)
Control surfaces, serving much the same purpose as “flaps” do on airplane wings, or the A/C registers do in a car, would be provided in the form of vertically-oriented airfoils, distributed across the entrance of each duct into the arena. During operation (air inflow velocity = 30-60 m/s), they could be adjusted to direct (deflect to left or right) incoming air either tangentially, which (up to a point) would increase air flow through the system, or toward the opposite, or radial direction (i.e., center). The latter adjustment would tend to decrease air flow capacity by increasing friction and decreasing net vorticity.
For an UHI mitigation application, urban “hot air” gathered from near the surface, would be expelled upward as a buoyant vortex (exhaust plume). Dynamic columns thus created would ascend at a higher velocity, and rise to an altitude much greater than would a simple updraft, devoid of vorticity emitted from any similar device. The superior performance of the “spinning updraft” results from its ability to resist the admixture of cooler, dryer air from the surroundings during its ascent, which otherwise would result in greater drag, slowing the process.
Creation of the vacuum by the lighter air column is what induces the surrounding air to flow over the surface toward and through the device. The surface air temperature is reduced as a result of heat being removed from the zone; surface air from the surroundings is not only cooler than the air it replaces, it is also dryer. Thus, not only can the temperature difference be sensed; its capacity to evaporate water (by virtue of its “dryness”) actually plays an even greater role to increase comfort for city inhabitants.
When installed and operated in an UHI application, the convection cooling by the AVE would be amplified enough to become a significant fraction of the (net) long-wave (thermal) radiation emitted by the surface. The latter heat removal mechanism is the one that now predominates during periods of “slow cooling” within built-up areas, where little wind penetration occurs to carry away heated air.
By increasing air circulation through urban areas through enhanced convection, pollutants, along with heat, are removed from the “breathing zone” of city inhabitants. These risk factors are known to cause adverse health effects; by reducing mean daytime temperatures, the rates of formation of many of them would be reduced.
By initiating a process to install AVEs in a location like Phoenix, Arizona, at a rate of two per year over the next ten years, an average day/night heat removal rate of 0.3-0.5 kwh/m2/day (when also accounting for diminished anthropogenic heat from A/C units), would be achieved for 20 units (arenas), and the summer temperature could be directly reduced by 3-4 oF over a wide swath (400 square miles) of the Phoenix metropolitan area.
As a result of such an intervention, the corresponding minimum (5 AM) temperature difference between the city’s central core and rural areas (10-20 km away), could be reduced by about one-third. By starting the day 4 oF cooler and with cleaner air, the maximum temperature reached in cities would be a few degrees lower and be achieved about an hour later during the day. After sundown, both the cool-down rate the pollutant removal rate from the evening rush hour would be more rapid than now experienced.
Power Production By-Product of AVE Cooling
As the removal of heat accumulated within the urban canopy during the day occurs by facilitating the buoyancy-driven ascent of heated air, not only would surface cooling be enhanced, a significant fraction of its heat content would be convertible into electricity.
While performing the electrical generation function via turbo-expansion, resistance to air flow would be provided by turbine blades which would also cause the air to cool during the expansion. Together, they would act as a control mechanism or brake on both the air flow volume and its rotation rate. Trim control would be provided by adjusting the position of the airfoils located at the outlet of the tailpipe (arena inlet).
The power turbines would be situated inside the ducts at the end of a convergent section and could generate 1-2 MW each (40 oC entrance temperature, DP= 2.5 kPa), for an airflow of 1000 m3/s, before the air is released into the arena via the tailpipe. Without such control, the volumetric rate could exceed design parameters, endangering equipment, or worse, enabling vortex escape. However, careful design (arena siting and rotational direction choice), along with installation of a back-up system (e.g. cold water quench) could easily prevent this.
Accrual of Renewable Capacity Enables EPA Targets
By installing two AVEs per year (200 MW), the amount of additional summer electric generation capacity would increase by 2000 MW over ten years, which is equivalent to the largest of nuclear or coal-fired power plants, at half the investment cost. Meanwhile, power demand within the cooled zone could be reduced by ~ 500 MW due to a reduction in the A/C load. In combination, this amount would be far greater than current trends for renewable generating capacity by the year 2020. From the AVE contribution alone, penetration of renewable energy capacity for the region would be well on its way to meeting proposed national goals of 20% (of ~ 30,000 MW, AZ) by 2020.
Health Benefits
AVEs, by virtue of their air renewal capacity, would also facilitate the meeting of EPA standards for fine particles in urban environments, emitted mostly by diesel engines operating in the streets. This is a known source of asthma in children, as is ozone created by mobile sources. The health benefits obtained from just a 4 oF temperature reduction could be substantial, reducing the frequency of cases of heat stress or stroke.
In some cities, the cost of building the AVE devices could be justified based on their health benefits alone. The benefits would especially be felt among the poor, many of whom have little or no means to reduce the temperature of their environment by increasing air conditioning.
Other Benefits and Costs
As a result of the reduced temperatures, undesired water losses from swimming pools, would also be reduced, with the savings distributed to new parks where the cooling would have a more beneficial effect. The loss of trees and other vegetation from heat stress would be reduced.
On the periphery of an AVE, energy intensive businesses could be built and eject their “waste heat” directly into a duct, including heat from condensers of large A/C units. Also, businesses in need either of venting or drying capacity could be installed nearby. Urban greenhouses or “engineered” solar collectors could also feed into an AVE duct, increasing power output.
Due to lower daytime temperatures, PV capacity losses due to high temperature would be reduced. As mean urban temperature is reduced, worker productivity would increase and the schedules of municipal workers working outside (now during the night) could be normalized.
The cost of the AVE devices would be similar, or possibly somewhat lower per KW of capacity, than are current combined-cycle natural gas plants, with one major difference—the “fuel” would be free. Furthermore, if generating capacity that is now met by burning coal (~35% in AZ), were to be replaced, the state’s carbon footprint could be reduced. If current proposals for “cap-and-trade” are passed, the reduced carbon emissions could form the basis for a significant new municipal revenue stream.
During the mid-winter months, small additional net costs for home heating might occur if the AVE units continue to operate during these periods (at reduced throughput) to provide enough ventilation to remove pollutants during atmospheric inversion episodes.
The AVE Devices Are Made of Commonly Available Components and Would Create Jobs During Construction and Operation
Common AVE components such as turbo-expanders, control items, and power units are all made in North America, and don’t require the importing of expensive and technically-complex, engineered equipment items made of exotic materials, such as would be the case for many nuclear reactor components. Of great importance to economic recovery is sustainability–construction, operation and maintenance jobs would be created for core city inhabitants.
SPX Cooling Technologies is an example of a company that makes round cooling towers that would be suitable for the arena plan. Of course they would need to be built to support a slight vacuum, not internal pressure.
AVE Sentinal Function for Protection Against Severe Weather
It is also worth mentioning that, in many, if not most locations, the beneficial result of installing AVEs would not end with decreased urban temperatures, increased power production, pollution abatement and water savings. By rapidly draining off a significant portion of the UHI “temperature potential”, for large cities which may be prone to exogenous episodes of severe weather, such as St. Louis, MO, a “partial immunization” from the consequences of a direct hit of a severe weather system impinging on the city would likely be achieved.
For example, rather than running the risk of having an F-1 or F-2 tornado be converted into an F-3 or F-4 monster by accessing UHIs in the 10-15 oF range (say in Kansas City at 7 PM on June 1), such intensification in an “AVE protected” city might be limited to just one level (for an UHI elevation in the 5-10 oF range, say).
In the author’s opinion, “if” such an event would occur–is not in question–just “when”.
A stitch in time, saves nine (proverb)
Conclusions
Bold solutions will be needed to supply the energy needs, increase livability as well as to provide new employment opportunities in southern cities, which are among the fastest growing in the country.
The atmospheric engine is an inexpensive and conceptually simple technology, the installation of which could provide a mltiplicity of benefits to all people who live in a medium to large urban environment either in a southern or, in some cases, a mid-latitude location, without regard to their economic class.
AVE Projects are not proposed to supplant current valid proposals to mitigate the UHI effect, involving increased planting of trees and other vegetation within affected zones, or ones which apply the “white roof” concept to increase albedo, but rather as ones which could work symbiotically with such projects.
For all of the above reasons, the author encourages the initiation of demonstration projects which could ultimately pave the way for the implementation of AVE-based, UHI mitigation strategies in cities.
Appendix: The Urban Heat Island Effect
The Urban Heat Island Effect is a growing problem that is facing so many metropolises around the world and will only become increasingly severe as global warming exacerbates the intensity and duration heat waves. A review from NASA is available at http://rsd.gsfc.nasa.gov/912/urban/background.htm
A depiction of the varying degree of the urban heat island effect as a function of land use.
Quotations
“Hot air rises, but while endeavoring to do so it is inhibited by mixing with cooler overlying air. The timely and felicitous departure of the hot air can and should be encouraged by inducing the air to spin”–Louis M. Michaud, P. Eng., Inventor of the Atmospheric Vortex Engine, AVE
“To waste, to destroy our natural resources, to skin and exhaust the land instead of using it so as to increase its usefulness, will result in undermining in the days of our children the very prosperity which we ought by right to hand down to them amplified and developed.” — Theodore Roosevelt
“When life gives you a lemon, make lemonade”—Dr. Norman Vincent Peale
