a new concept for integrating a thermal air power tube with solar

� Corresponding author. F

E-mail address: fdibella@

0960-1481/$ - see front mat

doi:10.1016/j.renene.2004.0

ax: +1-617-373-2501.

coe.neu.edu (F.A. Di Bella).

ter # 2004 Elsevier Ltd. All rights reserved.

4.015

Renewable Energy 30 (2005) 131–143

www.elsevier.com/locate/renene

A new concept for integrating a thermal airpower tube with solar energy and alternative,waste heat energy sources and large naturalor man-made, geo-physical phenomenon

Francis A. Di Bella a,�, Jonathan Gwiazda b

a Northeastern University, 360 Huntington Avenue, 120 Snell Engineering Center, Boston, MA, USAb Electrical Engineering NU, 147 Mt. Vernon Street, West Roxbury, MA 02132, USA

Received 21 January 2004; accepted 22 April 2004

Abstract

A method for power generation combining a solar concentration system and a pneu-matic power tube system in a large open pit is described. Solar energy is concentrated bya plurality of heliostat mirrors placed along the embankment of the pit, which tends tobe spherical in contour. The pneumatic tubes recover waste heat energy from the solarRankine power cycle system and from a variety of sources that originate from or are inclose proximity to the very deep, man-made open-pit mine or from other naturally occur-ring geo-physical chasms. The man-made or naturally formed chasms provide structuralsupport for the pneumatic power tubes. The air in the tubes is heated by the recoveredwaste energy, and in so doing, its density is sufficiently reduced so as to produce airdrafts from which mechanical power can be recovered from wind turbines and convertedinto electrical power by suitable electric generators. The deep chasms can be from a man-made phenomenon such as commissioned, open-pit mines or from naturally occurring fis-sures in the earth. The waste heat can be from solar energy, ground source energy orproducts of combustion from waste products that are to be mitigated or destroyed. Theconcept is novel in its integration of a solar powered heat engine with recoverable wasteheat via the proposed pneumatic power tube as well as in the means of structural sup-port that the geo-physical phenomenon provides and the modularity (for ease in manu-facturing and installation) that makes the pneumatic power tube economically viable. Thecomplete system uses state-of-the art wind turbine power recovery, solar reflective surfaces

F.A. Di Bella, J. Gwiazda / Renewable Energy 30 (2005) 131–143132

for solar energy collection, heat pipe arrays for ground source heat recovery, and air dif-fuser subsystems for enhanced wind turbine efficiency.# 2004 Elsevier Ltd. All rights reserved.

Keywords: Solar Rankine cycle; Air chimney; Pneumatic tube; Solar concentrator; Heliostat; Waste heat

recovery power generation; Bernoulli (venturi) tube; Open-pit mining; Land remediation

1. Background of relevant technology

The invention presented is the result of a series of studies conducted by theauthors that culminated in two papers: ‘‘A Novel Thermally Induced Draft AirPower Generation System for Very Tall, Man-made and Natural Geo-PhysicalPhenomenon’’ (e.g. [1]) and ‘‘A New Concept for a Thermal Air Power Tube Usedwith Concentrated Solar Energy Power Generation in Open-Pit Mines and LargeNatural Geo-Physical Phenomenon’’ (e.g. [2]).

In a preliminary study conducted by Di Bella and Garen (e.g. [3]), a variety ofenergy generating options were studied whose common thread was their use inskyscrapers and other free-standing structures that are over 1000 ft tall. One of themore radical ideas promoted in that paper was the use of elevator shafts to sup-port the controlled generation of air currents due to the natural draft or ‘‘chim-ney’’ effect caused by internal columns of air that are hotter and thus less densethan the cooler, outside air. In that paper, the use of elevator shafts as a ‘‘chim-ney’’ was intended as a means of reducing or eliminating the cost of constructingfree-standing structures that only had the purpose of thermally inducing aircurrents.

In a study conducted by Gwiazda (e.g. [4]), novel methods for focusing and col-lecting concentrated solar energy from open-pit mines were studied. The study con-firmed the feasibility of economically harnessing solar energies for a solar Rankinecycle power generation system that could then be integrated with the proposedinvention.

In a 2002 ASME paper, the authors suggested that the thermal energy from theinduced air chimneys could be derived from the rejected energy from the HVACand lighting energy loads required in skyscrapers, and also from solar energy con-centrated on free-standing columns. The results obtained from the second studyindicated that appreciable power generation (over 1 MWe) could only be obtainedif the columns were very large in diameter and in overall height. The recommen-dation was made by the present authors that a more detailed analysis be extendedto man-made and natural geo-physical phenomenon that is gargantuan in size inorder to take advantage of the scaling effect that size has on the generation of sig-nificant electric power.

This concept and subsequent invention (patent pending, 17 December 2003application) is the result of an analysis and a more detailed design that was pro-duced by the inventors in preparation of that ASME 2003 paper.

133F.A. Di Bella, J. Gwiazda / Renewable Energy 30 (2005) 131–143

2. Prior technology

It is well known that a very tall, high temperature air-filled conduit will have itstop opening at lower atmospheric pressure and temperature than the opening atground level.

This is commonly experienced and utilized to produce air drafts as an effectivemeans of removing exhaust gases from combustion boilers. The air draft is inducedby the differences in air density inside and outside the air column. Air density issignificantly affected by temperature (hence the easily created air drafts in hotchimneys). Air is also a very compressible fluid where the air’s density is greatlyaffected by pressure. The atmospheric pressure that is typically quoted as 14.696psia is only correctly stated for sea level. In fact, air pressure at elevations greaterthan 1000 ft or below sea level can be as much as 0.25–0.5 psi greater.

Increasing the air temperature decreases the density of the air column andachieves the desired effect: that of establishing more pressure differential across theinlet and outlet of the pneumatic conduit. The addition of heat certainly requiresenergy consumption, unless as the authors’ patent disclosure claims, it can berecovered from an otherwise wasted energy source. The recovery of wasted energyfrom a solar energy resource or wastes that are to be destroyed or ground sourceheat energy is particularly attractive given the typically high cost of solar energysystems, the cost or remediation of waste products and the enormous potentialenergies that remain untapped due to the lack of an economically viable waste heatrecovery system. Any attempt to generate more power without a proportionalincrease in cost would be beneficial to the economics of the solar energy system.

Of particular relevance to the present study is an interesting design concept sug-gested by Profs. Gutman, Horesh, Gueta and Borschevsky, who are faculty at theIsrael Institute of Technology. The basic concept for their power generation systemwas originally suggested by Mr. Peter Carlson in the 1970s (patent no. 3,894,393)and a recently published text by Schlaich [5]. The power tower, called the aero-electric power tower, is in the proposal development phase [4]. The aero-electricpower tower is an enormous (1200 m high, 400 m diameter), free-standing, verticaltube, in which dry air is cooled via water injection to induce a downdraft of dense,moist air. The draft is passed through turbines at the bottom of the air column togenerate electricity. The motive force for this air tower is the use of very dry(desert) air that can be continuously sprayed with water and thus saturated(humidified). The cooled, moist air flow flows downward through the tower andthe increasing kinetic energy of the air stream is converted into electrical power viaan array of turbines that are installed at the base of the tower where the airexhausts from the system. The inventors suggest that as much as 380 MW ofpower may be generated from such an arrangement. It is also important to notethat the present inventors proposed pneumatic power tubes work by heating the airand creating an updraft as opposed to the downdraft created in an aero-electrictower. Most importantly, the present invention eliminates the need for the air col-umn to be free-standing under its own, costly structural integrity. Rather, the sup-port for the proposed pneumatic tube is via the previously (man-made) constructs


and/or naturally occurring geo-physical phenomenon while recovering the renew-able energies that are available in that locale using the invention’s several uniquelyintegrated design features.

All previous attempts at producing power from an air chimney have failed dueto either the size constraint imposed by reasonable safe engineering practice andconsequently a significant reduction in power output (making the magnitude ofpower virtually insignificant) or the extremely high cost required for overcomingthe safety issues as well as the cost for gaining environmental permits and publicsanctions.

2.1. Proposed invention pneumatic power pit tube (PPPT) concept and applications

The proposed invention is most suitable, but not exclusively, for combining apneumatic power tower and a solar power tower in an open-pit (ore) mine (seeFig. 2).

An open-pit mine is essentially a big hole in the ground as shown in Fig. 3. Themining method extracts minerals by blasting, excavating, and processing. Theshape the footprint of the mine takes depends on the most economic removal ofthe mineral, but the holes themselves generally take on a parabolic bowl shape asshown in Fig. 3. The sides of the mine are terraced with berms to prevent land-slides and to provide level road surfaces that can lead up and out of the mine.

Solar energy is concentrated by the reflectors (similar to state-of-the-art fieldarrays of parabolic heliostat) on a receiver. Molten salt is used as a working fluidto transfer the heat from the receiver to steam generators. Steam turbines then gen-erate electricity. The rejected heat after passing through the steam turbines is ven-ted into the bottom of the pneumatic tubes creating an updraft, which passesthrough wind turbines in the tubes generating more electricity. During the lastthree decades, considerable effort has also been expended to design, build and dem-onstrate the feasibility and cost effectiveness of solar power towers. However, it isimportant to note that all the solar power towers that have been built (forexample: Solar One and Solar Two Projects in Barstow, CA, and others) have hadtheir feasibility demonstrated on large flat surfaces. No one has contemplatedinstalling SPTs in previously excavated or naturally formed, conically sectionedcavities or very deep vertical shafts.

3. Summary of integration concept

The invention consists of integrating a novel design for a pneumatic power tube(PPT) with a solar power Rankine (or other heat engine with available waste heatenergy) in an open-pit or other very large scale, man-made or natural occurringgeo-physical phenomenon that it can use for economical structural support forpurposes of mechanical and electrical power generation. The proposed pneumaticpower tube that can produce a continuous air flow to generate significant electricpower generation via waste heat recovery from a variety of sources include, but notexclusively, solar energy, by integrating it into an existing, massive man-made


object: an open-pit mine. The novel design can eliminate the cost of constructing afree-standing column while also providing an environment friendly solution to theproblems associated with decommissioning such mines. A schematic of the pro-posed pneumatic power tube for the open-pit mine application is shown in Fig. 1.Similar wind power applications using pneumatic tube designs for large, naturallyoccurring geo-physical phenomenon such as very deep mountainous chasms arealso feasible and are claimed.

The pneumatic tube design that can be used in this application is shown inFigs. 1 and 2. This novel design for the pneumatic tube provides many unique fea-tures that add to its usefulness in producing wind energy from rejected heat. (Num-bers refer to system components shown in Figs. 1 and 2.)

1. The integrated pneumatic power tube and Rankine (or other heat engine) isconceived to recover the wasted energy from the heat engine and the groundsource heat thermally stored and/or generated naturally from the earth’s inter-nal energy.

2. The pneumatic tube can be structurally supported by the sides of the open-pitmind and thus avoid the costly structural engineering and materials that wouldotherwise be required to make it a free-standing column.

3. The outside, top surface of the pneumatic tube is constructed using reflective(but also flexible) surfaces that enable that surface to be used as part of thereflectors that focus solar energy on to the receivers.

4. The pneumatic tubes are wider at their top than at the bottom thus forming adiffuser for the induced air flow that can recover static pressure at the inlet to

Fig. 1. Concentrated solar energy and pneumatic power tube energy recovery system for open-pit mining

operations.


the wind turbines while also controlling the air velocity to reasonable (lower)speeds.

5. The outlet of the pneumatic tubes can provide a cowling of the wind turbine(s)and thus improve the efficiency of the wind turbine.

6. The pneumatic tubes can be used to recover solar influx energy (i.e. transmittedenergy) that is not reflected to the receivers. This energy can thus be used toprovide additional heat to the air column and thus further induce airflow insidethe pneumatic tubes.

7. The exhaust outlet of the pneumatic power tube can be fitted with a converg-ing–diverging nozzle (venturi) to produce a Bernoulli effect at the exit of thewind turbine. This will help reduce the pressure at the turbine exhaust by utiliz-ing the wind velocities at the top of the open-pit mines.

Fig. 2. Individual pneumatic power tube design schematic.


8. The structural support of the pneumatic tubes can also enable the pneumatictubes to be made mobile in order to better focus or adjust the foci of the inci-dent solar energy as the sun’s inclination changes during the day or simply forsimplicity and least cost for manufacturing many of the same (modularity con-cept) units.

9. The pneumatic tube design also enables heat recovery from heat storage via thegravel and ground surfaces of the open-pit mine. The heat can be recoveredusing easily buried air conduits and allowing the recovered air to be directlyused in the pneumatic tubes. For example, heat pipes can be imbedded in theground, in deep hot water springs or in volcanic locales with chasms or fissuresin order to recover the natural heat energy available from these naturalphenomena.

3.1. Advantages of integrated concept

The proposed unique integration of a solar power tower and a pneumatic powertower (seen in detail in Fig. 2) installed in an open-pit mine is shown schematicallyin Fig. 1 with an actual open-pit mine shown in Fig. 3. This integration of systemshas many attractive economic incentives as well as power generation advantages.Consider:

1. An open-pit mine that is scheduled for decommissioning requires considerableexpenditures to secure safely the area for subsequent reclamation by nature.

Fig. 3. Typical open-pit (ore) mine with characteristic spherical contoured topography.


Such expenditure, however, includes considerable maintenance to prevent thesite from becoming hazardous due to the accumulation of contaminated water.

2. Open-pit mines are generally parabolic. A parabola has a higher focal powerthan a flat plane.

3. An open-pit mine that has been in operation for decades usually has an estab-lished infrastructure that can be utilized to support site workers who will main-tain the new solar power generation facility.

4. The use of an existing open-pit mine eliminates the need to purchase virgin landareas and/or petition the local citizenry for the establishment of a solar powersite at the mine. The acquisition of land is usually a very expensive and time-consuming activity if environmental permits must be obtained.

5. The PPPT keeps the mine property productive after the mining life cycle andcould potentially provide a means to clean the mine of pollutants. The reutiliza-tion of an open-pit mine for purposes of providing solar power, electric powergeneration is considered a more reasonable end-use of the land than allowingthe developed site to degrade.

The proposed pneumatic power tube applied to open-pit mines is seen to be aneffective application of several well-developed technologies: wind turbines, solarconcentrated energy via Rankine cycle power systems, thermal energy storageusing rock and gravel medium, and the transportation of electric power using AC–DC–AC converters and/or microwave power generation, and transmission for veryremote sites. All these technologies have been developed by a variety of researchactivities that have been supported by private and public funding and for whichremarkable strides have been made in improving efficiencies while reducing costs.The deployment of a pneumatic power tube in depleted and decommissioned open-pit mines is seen as a perfect match of environmental concerns with state-of-the-artengineered, power systems.

3.2. Thermodynamic analysis: integrated pneumatic power pit tube and concentratedsolar energy system for open-pit mines

An analysis was conducted to determine the power that could be recovered froma pneumatic power tube that has been installed in an open-pit mine and poweredby heat rejected from a solar concentrated, Rankine cycle system. This analysis isconsidered to be a first order estimate of the power recovery potential of the solarenergy—pneumatic power tower—open-pit mine installation.

The analysis proceeded in three steps:

1. Determine the amount of pressure differential that can be produced by a columnof air that is 1000–3000 ft below sea level (i.e. in the bottom of an open-pitmine).

2. Determine the amount of mass flow rate and the velocity of air that can beinduced by this pressure differential assuming that the flow is a simple Darcy-type friction flow in a large conduit.


3. Determine the power that is therefore recoverable assuming that the heat rejec-

ted by the condensers of a Rankine cycle system heats the pneumatic powertube columns. The Rankine cycle is powered by solar energy in a conventionalconcentrated solar heated system.

In order to determine the power recovery, a special spreadsheet thermodynamics

model of the concentrated solar Rankine cycle system was prepared and a para-

metric study conducted to determine the effects of mine height and diameter on the

recoverable power.The results of a parametric study are shown in the tables and graphs provided.

The baseline condition uses 120vF PPPT internal air temperature, a solar incident

flux of 300 BTU/h/ft2, a solar Rankine cycle efficiency of 22% (and thus a 78%

rejection efficiency to the PPT system), and an effective incident energy land usage

of 50%. The latter parameter is an attempt to determine the fraction of the pro-

jected land area or the fraction of incident solar energy that can be effectively

recovered. The results indicate a power generation capability from the PPT that

can range from 1 to 30 MWe (Fig. 4) which represents approximately a 2–3%

improvement in the solar Rankine cycle system efficiency (Fig. 5). This power is in

addition to the approximately 50 and 800 MWe of power that could be generated

by the solar Rankine cycle system using concentrated solar reflectors and receivers

of conventional design. It is interesting to compare this power improvement with

the 2% overall cycle efficiency that is expected for the OTEC systems (ocean ther-

mal energy conversion) that have been considered as a viable alternative solar

energy power generation system. The proposed PPPT system is thus seen to be

comparable to OTEC systems with far less fabrication problems and associated

costs (Figs. 5 and 6). Fig. 7 identifies the effect of the ‘‘incident energy usage fac-

tor’’ on the power generation capability of the PPPT system.

Fig. 4. PPPT power as a function of mine diameter and mine depth.


Fig. 5. Rankine cycle efficiency improvement using PPPT system.

6. Power generation from PPPT as a function of percent incident energy u
Fig. se.
ankine cycle power generation as a function of mine diameter and incident
Fig. 7. Solar R energy usage.


4. Conclusion

A typical open-pit mine (the Palabora mine in South Africa) is shown in Fig. 3.The enormity of this open-pit mine is not unusual for such mining operations.Using the thermodynamic model that has been developed for this study and thedimensions of the mine and its geographic location (and thus knowing the solarflux at the site), an estimate can be made of the amount of power generation thatcan be produced at the site if it were to have an integrated solar power and pneu-matic power pit tube system installed. Using present specifications for concentratedsolar energy power improvements from DOE supported research (approximately22% overall conversion efficiency) would enable 142 MWe of electricity to berecovered from the Palabora mine once it was converted to a concentrated solarcollector system using fixed (and not tracking), flat-plates (and not parabolicallyshaped) reflectors. Given that the site is already conically and perhaps even slightlyparabolically shaped, geo-physical reconstruction could be minimal.

In place of the traditional mirrored reflectors, the sloped surface of the open-pitmine could be lined with flexible membranes that have high reflectivity. The shapeof the mine may need some alterations for the focal point for the resultinggeometry to be made more precise, but the traditional high cost of tracking helio-stats, land acquisition and time consuming environmental permits (to satisfy ‘‘not-in-my-back-yard’’ neighbors), and the construction of power production and sta-ging building and facilities has been largely eliminated or greatly reduced. Thisshould result in a lower cost per kW, reduced number of permits and constructiontime, and a productive use of what otherwise may have been unused and unattrac-tive land area.

Similar large open-pit mines are in operation but will eventually be decommis-sioned in some manner. For example, a larger open-pit mineral mine such as theChuquicamata mine is 2 � 3 km wide and 810 m deep and could produce as muchas 734 MW of solar Rankine power plus an additional 18 MWe of power from thePPPT system. The deepest mine in the world is the Western Deep Gold Mine loca-ted in South Africa. Its depth of 4000 m would enable cold ventilation air from the

surface to be naturally heated by the earth to over 100vF (38

vC). The thermally

induced air draft from this mine could serve as the largest prototype of the nature-draft wind tower concept promoted in this paper.

Whether such enormous man-made or natural phenomenon uses the proposedwind towers as presented in this paper or one of many solar energy recovery meth-ods or even the neutralization of acidic, contaminated standing water via anode–cathode electric cells will be the subject of a third in this series of technical paperson natural energy resource recovery and generation. For example, air towers con-structed in extinct and voided volcanoes or mountains (i.e. abandoned mines withvertical air chutes that connect 2000+ ft deep chasms with the upper atmosphere)may serve as a naturally induced air tower.

The authors have formulated a design to enhance the recovery of concentratedsolar energy in high temperature receivers while also enabling land reclamation andeffectively re-utilizing decommissioned open-pit mines.


The proposed novel design effectively recovers the kinetic and pressure potentialenergies of the thermally induced air drafts by providing the thermal energy inputfrom the wasted heat of a solar Rankine cycle system. The proposed (nominal)600 þ m long � 15 m diameter pneumatic power tubes are supported by the floorof the decommissioned, open-pit mine; mines that are typically 1500 m in diameterand 600 m deep. The design utilizes the concept of wind power generation using athermally induced flow of air also known as a chimney or draft-induced airflow.Thermal induced drafts have been studied in the past by DOE and other research-ers, and have been found to successfully induce sufficient quantities of air and thusenable efficient power generation using wind turbines. This earlier method, how-ever, required that the chimney structures be enormously tall structures, typicallyover 2000 ft tall for the induced air drafts to be sufficient for useful utility-sizepower generation. However, the tall, free-standing air columns are expensive toconstruct and thus negate the economic viability of the solar power generation.

The proposed system is unique in the field of power generation. The proposedsystem effectively uses previously developed high temperature, solar energy receivertechnologies and improves the solar Rankine cycle efficiency by recovering therejected condenser heat while also providing a solution to the reclamation and uti-lization of depleted open-pit mines. In place of the traditional mirrored reflectors,the sloped surface of the open-pit mine will be lined with a pneumatic power tubeof original design that includes a reflective top surface for focusing the solar energywhile also diffusing the induced air flow to produce both kinetic as well as potential(pressure) energy recovery via high efficiency wind generators that have beendesigned with cowling to reduce air by-pass.

Appendix A. US Patent documents

3,048,005
8 /72 Goodman 3,436,908 4 /69 Van Dellie 3,720,840 3 /73 Gregg 3,894,393 7 /75 Carlson 3,936,652 3 /76 Levine 4,106,295 8 /78 Wood 4,182,124 1 /80 Kraus et al. 4,213,302 7 /80 Woinsky 4,373,339 2 /83 Sorenson 4,367,627 1 /83 Pretini 4,391,099 7 /83 Sorenson 4,475,342 1 0/84 Assaf 4,481,774 1 1/84 Snook 4,497,177 2 /85 Anderson 4,742,682 5 /88 Assaf 4,801,811 1 /89 Czaja 5,047,654 9 /91 Newman


5,284,628
2 /94 Prueitt 5,349,606 9 /94 Lovell et al. 5,395,598 3 /95 Prueitt 5,477,684 1 2/95 Prueitt 5,483,798 1 /96 Prueitt 5,527,494 6 /96 Weinberg et al. 5,648,983 7 /97 Kostic et al. 5,675,616 1 0/97 Hulbert et al. 5,692,006 1 1/97 Ross 5,764,688 6 /98 Hulbert et al.
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a new concept for integrating a thermal air power tube with solar

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