Solar Cell Concentrators for HarnessingElectrical Energy Prof. (Dr.) R. S. Rohella,

Director (R&D)Hi-Tech Institute of Technology, Khurda, Bhubaneswar-752 057

rsrohella@yahoo.comAbstract:

The earth receives 2.9X1015 kW of energy every day in the form ofelectromagnetic radiation from the sun, which is about onehundred times the total energy consumption of the world in ayear. The solar energy falling on earth has been quantified as“Sun” and is approximately equal to 100 watts/ft2 or 1000watts/m2. The first pn junction solar cell had an efficiency ofonly 4 to 5%. The development of solar concentrators wasnecessitated due very low current and voltage capabilities of asolar cell and it all started only after 1970s. The developmentof different types of solar cells like; point focus, line focus,and other types developed subsequently have been discussed indetails particularly the parabolic concentrator by Spectrolab,having an efficiency of 36.9% with 236 suns. The development of anew multi-junction GaSb solar cell is another big achievement inthis direction. Development of short-focal length concentratorjointly by Pyron Solar and Boeing-Spectrolab to produce 800 timesmore electricity than conventional non-concentrating cells of thesame size is a quite significant progress in this direction. Withthe target of achieving an efficiency of 40% in the near futureby HyperSolar, the harnessing of solar energy through solarconcentrators or new types of solar cells, the feasibility oflarge-scale solar power stations installation in the near future.A compilation of world-wide solar concentration with all detailshas also been included.

Solar Energy on Earth:

The Sun has been producing energy for billions of years. Thesolar energy is the solar radiation that reaches the earthand isa renewable source of energy. On a cloudless day, the solarenergy falling on an area equal to the size of tennis court per

day is roughly equal to the energy obtained from 35 litres ofpetrol or 80 kg of coal. The earth receives 2.9X1015 kW of energyevery day in the form of electromagnetic radiation from the sun.This is about one hundred times the total energy consumption ofthe world in a year. The solar energy falling on the earth can beconverted to electricity by means of solar or photovoltaic cellsand the process is called photovoltaic.

Photovoltaics:

Photovoltaic (or PV) systems convert light energy intoelectricity. The term "photo" is a stem from the Greek "phos,"which means "light." "Volt" is named for Allessandro Volta (1745-1827), a pioneer in the study of electricity. "Photo-voltaics,"then, could literally mean "light-electricity." Most commonlyknown as "solar cells," PV systems are already an important partof our lives. The simplest systems power many of the smallcalculators and wrist watches we use every day. More complicatedsystems provide electricity for pumping water, poweringcommunications equipment, and even lighting our homes and runningour appliances. In a surprising number of cases, PV power is thecheapest form of electricity for performing these tasks.

 Photovoltaic cells convert light energy into electricity at theatomic level. Although first discovered in 1839, the process ofproducing electric current in a solid material with the aid ofsunlight wasn't truly understood for more than a hundred years.Throughout the second half of the 20th century, the science hasbeen refined and the process has been more fully explained. As aresult, the cost of these devices has put them into themainstream of modern energy producers. This was caused in partby advances in the technology, where PV conversion efficiencieshave improved considerably.

  French physicist Edmond Becquerel first described thephotovoltaic (PV) effect in 1839, but it remained a curiosity ofscience for the next three quarters of a century. At only 19,Becquerel found that certain materials would produce smallamounts of electric current when exposed to light. The effect was

first studied in solids, such as selenium, by Heinrich Hertz inthe 1870s. Soon afterward, selenium PV cells were convertinglight to electricity at 1% to 2% efficiency. As a result,selenium was quickly adopted in the emerging field of photographyfor use in light-measuring devices. Major steps towardcommercializing PV were taken in the 1940s and early 1950s, whenthe Czochralski process was developed for producing highly purecrystalline silicon. In 1954, scientists at Bell Laboratoriesdepended on the Czochralski process to develop the firstcrystalline silicon photovoltaic cell, which had an efficiency of4%.

Solar Cells

A solar cell is any device that directly converts the energy inlight into electrical energy through the process ofphotovoltaics. The development of solar cell technology beginswith the 1839 research of French physicist Antoine-CésarBecquerel. Becquerel observed the photovoltaic effect whileexperimenting with a solid electrode in an electrolyte solutionwhen he saw a voltage developed when light fell upon theelectrode [1].  "What is often considered the first genuine solar cell was builtaround 1883 by Charles Fritts, who used junctions formed bycoating selenium (a semiconductor) with an extremely thin layerof gold. These early solar cells, however, still had energy-conversion efficiencies of less than 1 percent. This impasse wasfinally overcome with the development of the silicon solar cellby Russell Ohl in 1941. Thirteen years later three other Americanresearchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller,demonstrated a silicon solar cell capable of a 6-percent energy-conversion efficiency when used in direct sunlight."

Solar Concentrators:

A concentrator captures a large area of solar energy and focusesit onto a small area, where the solar cells are mounted and theratio of the two areas is called as concentration ratio. A

typical solar concentrator unit consists of a lens or mirrors tofocus the light, a tracking system to collect the solar energyfrom dusk to dawn and a cooling mechanism to dissipate excessheat produced by concentrated sunlight on the solar cells. Thisprocess leads to greater power falling on the area of focusthereby increasing efficiency of conversion. The unit of thesolar energy falling on earth is called “Sun”. One sun is definedas solar radiation that on a cloudless day falls over an area ofone sq. foot and has been approximately quantified as100watts/ft2 or 1000watts/ m2. Further, when the light from 100cm2 is focused onto 1 cm2, the intensity of the light is definedas 100 Sun. Spectrolab., a subsidiary of The Boeing Company hasachieved an unprecedented 34% conversion efficiency for aterrestrial concentrator solar cell at 236 suns with a furthergoal set 40% cell under concentrated sunlight [2].

Development of Solar Concentrators:

The solar concentrators use lens and or mirrors to focus thesolar radiation at a point or on line with a wide range ofgeometrical conversion or concentration ratio. Most of theconcentrators follow the sun as it crosses the sky, eitherthrough single-or dual-axis tracking. The solar cell with aninitial efficiency of 4 to 5% in 1954 when discovered, to thedevelopments of solar cell concentrators with an efficiency of38-40%, the development of solar concentrators has travelled along journey through the development of different types ofconcentrators discussed below.

1. Point Focus Fresnel lens Solar Cell Concentrator:

The most prominent optical lens is the Fresnel lens. It wasdeveloped in 1822 for use in lighthouses and can achieve highconcentration ratios [3]. Newer lenses such as Aspheric lensesand TIR (Transmission, total Internal Reflection, Refraction)lenses having a concentration ratio of over 300 while being only2 cm thick have been found to be quite useful in solarconcentrators. In the early 1970’s, Sandia National Laboratories,USA developed 1 kWP power developed first modern Point Focus

Solar Concentrator using Fresnel lenses constructed of castmoulded acrylic material. This required a number of modificationsfor better performance.

2. Line Focus Parabolic Reflective Trough Concentrator:

The Australian National University (ANU) has developed a 20 kWPV-Trough concentrator with solar cells mounted on the undersurface. The system comprised foundations, mirrors & support andaluminium passive heat sink-receivers. The above 20 kW solarconcentrator featured two-axis continuous tracking. All troughmodules are mechanically linked so that one motor actuates thetilt the other actuates the roll. A time based open loop centralprocessing controller via a driver interface and positionfeedback system controlled both the motors. This allowed theconcentrator to take full advantage of the daylight from dawn todusk [4]. The concentrator performed very well with an oversystem efficiency of 13% while the cell efficiency was reportedto be 22%. The salient features and performance of the 20 kW ANUSolar Trough Concentrator is given below in Table-1

Table-1

S. No.

Description Data

1. Mirror Aperture 1200 x 1600 mm2. Number of Mirrors 803. Total Reflector Area 154 m2

4. Concentrator Factor (Geometric) (Actual)

30:122:1

5. Cell Efficiency 22 % under concentration

6. Power Output per Trough

250 Watts (peak) SOC

7. Power Output of the System

20 kW

8. Tracking Mechanism 2-Axis Accurate to within 0.5o

9. Total System Efficiency

13%

3. Euclides Photovoltaic Concentrator:

The world’s largest PVconcentration grid connectedpower plant, the EUCLIDESTM-THERMI plant has been installedby BP Solarex in the south ofTenerife in Spain. The plantrated at 480 kWp and shown inFig. 1 is composed of 14parallel arrays each 84 meterslong. The arrays areNorth/South oriented and closeto the ground. Each arraycarries 138 modules and 140 mirrors. The modules are seriesconnected in each array. The geometric concentration ratio isx38.2. The mirror technology is based on metallic reflectivesheets shaped with ribs to the parabolic profile. Three differentmaterials have been tested as reflective material. The modulesare cooled with a passive heat sink. Every two contiguous arraysare connected, in parallel, to one inverter sized 60 kVA. Theoutput voltage at standard operating conditions is 750 Volts. Theconcentrating optics is mirrors instead of Fresnel lenses usedpreviously in all PV concentration developments. The trackingsystem is one axis and horizontal since it is cheaper than thetwo-axes tracking ones. The concentrating schemes present a moreconstant output than the flat panels. The system is costeffective and thus might present some advantage in the value ofthe electricity produced [5].

4. Parabolic Reflective DishConcentrator:

In the recent years another two typesconfigurations are possible for thesolar concentrators. Solar Research

Fig.1: A 480 kWpEuclidesPV Concentrator at

Fig. 2: A 22 kW DenseArray Parabolic DishConcentrator in South

Corporation has developed reflective dish concentrator and theSolar Systems Pty Ltd, an affiliated company of The SolarResearch Corporation has installed two such Concentrator systemin Australia made up of parabolic dishes each producing 22 kW.One of them with 10 parabolic dishes and will be installed in theAnanguPitjantjatjara Lands (South Australia) and the system ispowered by a 130 m2 parabolic dish and can generate approximately20 kW of AC power. The other with 42 parabolic dishes will beinstalled at a 20-hectare solar farm at Broken Hill. Fig. 2shows such a Parabolic Dish Concentrator installed in Australiaby Solar Systems Private Ltd. [6].

5. Dense Packed Array Photovoltaic Concentrator by Amonix:

The use of individual cells, eachwith its own concentrator optics,was the way in which relativelylow-concentration CPV systemswere first built. But no furtherlarge-scale CPV projects wereundertaken until recently. AmonixCorp. has designed and installeda large-scale set-up usingindividual cells but under highconcentration (260X) and is shownin Fig. 3. The manufacture hassealed flat modules of 5 kWrating and 5 of which can bemounted on a single two-axis tracker to form a rectangular arraywith total aperture area 182 m2. The most important advantages ofthis type of individual-cell concentrator system are: (1) Most ofthe optics can be controlled at the time of manufacture so thatfewer items need to be accurately aligned in the field. (2) Nofailsafe provisions need be included to prevent concentrated fluxfrom causing damage in the event of erroneous sun tracking. (3)The modules are sealed, flat panels, rendering cleaning acomparatively easy task. (4) The cells are passively cooled. Thus

Fig. 3: A 25 kW DensePacked Array Concentratorhigh concentration (260X)installed by Amonix forArizona Public Service

there are no fluids to handle, and no failsafe provisions need beincorporated to prevent damage caused by loss-of-coolant events.

6. Dense Packed Array Photovoltaic Concentrator:

Spectrolab. USA designed,fabricated and tested twodense packed modules usinghigh efficiency multi-junction cells. One of theSpectrolab modules is shownin Fig. 4. Test results werevery encouraging with cell area based efficiencies over 25%.Further improvements in the new designs are possible for higherefficiency [8].

7. Photovoltaic Cavity Converter:

A concentrating photovoltaic module isprovided which provides a concentrationin the range of about 500 to over 1,000suns and a power range of a few kW to 50kW. A plurality of such modules may becombined to form a power plant capable ofgenerating over several hundred megaWatts. The concentrating photovoltaic module is based on aPhotovoltaic Cavity Converter (PVCC) as an enabling technologyfor very high solar-to-electricity conversions. The use of acavity containing a plurality of single junction solar cells ofdifferent energy band gaps and simultaneous spectral splitting ofthe solar spectrum employs a lateral geometry in the sphericalcavity (where the cell strings made of the single junction cellsoperate next to each other without mutual interference). Thepurpose of the cavity with a small aperture for the pre-focusedsolar radiation is to confine (trap) the photons so that they canbe recycled effectively and used by the proper cells. Passive oractive cooling mechanisms are employed to cool the solar cells. Aphoto cavity converter developed by United Innovations is shownin Fig.5. Test performance is excellent [8].

Fig.4 Spectrolab Dense PackedModuleof low Solar Concentrator

Fig.5: UnitedInnovations

8. Multi-junction Cells for Higher Efficiency:

The non-silicon cellshave not achieved thesame degree ofefficiency as Si cells.Emcore, USA hasdeveloped multi-junctionsolar cells using GaAsand GaSb. A multi-junction solar celltechnology employs threesolar cells in seriesand each cell is tuned to absorb a different colour of light.This technique converts more sunlight to electricity and themulti-junction cells with x1000 concentration thereby can operateat much higher efficiency of the order of 38% as compared 28% forthe single junction cells. Fig. 6 shows the configurations ofmulti-junction and single-junction cells. Emcore’s entry intothe space market effectively eliminated the use of silicon solarcells in high power GEO satellites. Till mid 90’s, silicon cellsgenerated 80% of satellite power. However, today the multi-junction cells generate 80% of satellite power requirements andare being used in high power generation also even on the land[9].

9. Multi-junction Solar Cell and Panels for Space Communication:

Emcore, Albuquerque, USA a basedcompany reputed with very highefficiency solar cell technology interrestrial applications is claiming abig success in solar power for spaceapplications. With an overallefficiency of 28.5%, the multi-junctiontechnology has completely changed themode of the solar power generation in

Fig.6: Configuration of Multi-junction and

Fig.7. Multi-junctionSolar Cell and Panel for Space Application

space applications. The solar cell and panel designed, fabricatedand fitted in the satellites for terrestrial and spaceapplications by the above company are shown in Fig. 7. Thecompany is presently concentrating on with multi-junction solarcells for PV systems for higher utility scale in the range of(10-100 MW) power production with continuously increaseefficiencies executing a road map to 40% by 2008 [9].

10. Hyper-Powerful Inexpensive Solar Concentrator:

A company in California, Pyron Solar, is making the bold claimthat its solar power system can compete head-to-head withconventional power plants. The system, developed with Boeing-Spectrolab, is very compact, and uses short-focal-length lensesto concentrate direct sunlight to photovoltaic cells. The companysays these cells produce 800 times more electricity thanconventional non-concentrating cells of the same size. Theirfirst prototype, which is 23 feet in diameter and 16 inches high,produces an astonishing 6.5 KW of electricity enough to power sixhomes. The Pyron Generator costs around $18,000 as compared witharound $32,500 for flat-panel systems. Pyron Solar has futureplans for large-scale production, with 30 kW and 50 kW units[10]. With multi dimensional approach and R&D work going on allround the world, the present investment versus efficiency andeconomics of the power generation using solar cell concentrationis given in Table-2.

Table-2Investment versus the Efficiency Gains for a 100 MW/Year

Manufacturing Plant for Solar Cells

Investment Requiredfor 100 MW Year Manufacturing Plant

Efficiency Gains

Technology

Cost (US$) Million)

Year Efficiency

Crystalline

150-300 2004 Spectra Lab., Sylmar announced 36.9% efficiency [9].

Silicon PVThin PV 150-300 April

2005Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg,Germany, announced ‘European’ record 35.2% efficiency foe a cell measuring 0.031 cm2. Consists of gallium indium phosphide, gallium arsenide and germanium. Suitable for use in terrestrial concentrator.

Concentrating PV

30-50 May 2005

NREL, Golden Colorado, announced a solar cell efficiency of 37.9% at 190 suns.

June 2005

Spectra Lab, Sylmar announced 39.0% efficiency at 236 suns [10].

Multi-junction Solar Cell

6-8 February 2006

Emcore Photovoltaic Division, Albuquerque, New Maxico, USAEfficiency reported as 38% and the capital cost of electrical power production 6-8 US$ per watt(with no subsidy).

11. Concentrator and Space Applications of High-Efficiency SolarCells – Recent Developments

The two-junction (cascade) Ga0.5In0.5P/GaAs cell was invented inNovember 1984 at the National Renewable Energy Laboratory (NREL)(1). Over the next few years, the growth and basic properties ofGa0.5In0.5P (hereafter, GaInP) were studied. As the purity of thesource materials was improved and the device optimized, theefficiencies climbed: 4% in 1985 (2), 10% in 1987 (3), 21.8% in1988 (4), and 27.3% in 1990 (5). When the two-junctionefficiencies passed the efficiency of single junction GaAs, the

cascade cell became attractive for space applications. Thecascade cells provide a higher efficiency, lower temperaturecoefficient, improved radiation resistance, and reduced series-resistance losses, and they were recognized with an R&D100 Awardin 1991 (see Fig. 8).

Fig. 8:History (Efficiency Records, indicated by *) and future projections for high efficiency GaInP/GaAs cells

11. New Solar ConcentratorBeing Developed atHyperSolar Inc.

A new version of solarconcentrator is beingdeveloped by a Santa Barbara,CA company, HyperSolar Fig. 9.If successful, “the world’sfirst thin and flat solarconcentrator for directplacement on top of existingsolar cells.” could revolutionize solar energy industrypricing.In a press release, HyperSolar reports it uses innovativephotonics and low-cost manufacturing processes to develop “theworld’s first thin and flat solar concentrator for standard solar

Fig.9. New Solar Concentratorsin Development at HyperSolar

cells. Applied on top of solar cells, the low-costHyperSolarconcentrator can increase solar cell power output, allowingmanufacturers to use fewer solar cells in the production of solarpanels and dramatically reduce the cost-per-watt of solarelectricity.IfHyperSolar’s concentrators are able to magnify thesun’s rays by 300 to 400%, the company states that aconcentrator-equipped panel will function with 75 percent lesscells than one without.Tim Young, HyperSolar CEO, said, “Ourultimate goal is to develop an inexpensive and thin solarconcentrator for use in replacing expensive solar cells inconventional flat solar panels. After a year of intense researchand development, we are excited to report that we have finallyachieved a prototype design that we believe can be refined into acommercial product.”HyperSolar’s sheets incorporate a photonicsthermal management system that keeps unusable parts of the solarspectrum from reaching the cells-keeping   the cells fromoverheating and becoming less efficient.

12. Advantages and Drawbacks:

Although solar cells have many advantages, they have alsonegative effects and disadvantages. One of major drawbacks in thekind of technology is the absolute volume of land that is neededto put up the solar plants. Most of the time, a large area isneeded, and a typical value of a plant cell for 9.5 square milesof land area for its house operation. Depending upon the cost ofthe electricity rates, and the installation, the pay off can be14 to 20 years. Although the modules have 20 years warranty, theinvestment will be lost if one decides to move.

Besides increasing the power and reducing the size or number ofcells used, concentrators have the additional advantage that cellefficiency increases under concentrated light. How much theefficiency increases depends largely on the cell design and thecell material used. Another advantage of the concentrator is thatit can use small individual cells, an advantage because it isharder to produce large-area, high-efficiency cells than it is toproduce smaller-area cells.

There are, on the other hand, several drawbacks to usingconcentrators. The concentrating optics they require, forexample, are significantly more expensive than the simple coversneeded for flat-plate modules, and most concentrators must trackthe sun throughout the day and year to be effective. Thus, higherconcentration ratios mean using not only expensive trackingmechanisms but also more precise controls than flat-plate systemswith stationary structures.

13. Performance of Solar Concentrators

There are numerous projects regarding the implementation of thesolar concentrators. Research centres, universities and companiesto investigate and analyze the reliability and the performance ofthe concentrator have done these projects. Table 3 shows some ofthe projects which have beenconducted throughout the world,showing the principal investigator’s name and the location of theproject.It presents the estimated output obtained as well as theoverall efficiency of the system.

TABLE 3: Worldwide projects related to solar concentrators [11]S.No.

Name Location Concentra-

torType

Focus(Point/

Linear)

Output

(kW)

Sunconcentratation(X)2

Tracking(yes/no)

Efficiencyofthe

system

Ref)

1. Alpha Solarco

Pahrump,Nevada,USA

fresnellens

Point 15 n/a yes n/a [13][14]

2. AMONIX andArizona PublicService

Arizona,USA

fresnel lens

point 300 250 yes 24.0% [15]

3. Australia Spring parabol linea n/a 30 yes 15.0% [16

nNationalUniversity

Valley,Australia

ictrough

r ]

4. PETAL SedeBoqer,Israel

parabolic

dishes

154000

400 yes 16.5% [17]

5. BP Solar and thePolytech-ncicalUniversity ofMadrid

Tenerife,CanaryIsland, USA

parabolictrough

linear

400 38 yes 13.0% [23][18]

6. EntechInc Ft. Davis,Texas, USA

fresnellenses

linear

100 20 yes 15.0% [19]

7. Fraunhofer-Institutefor SolarEnergy Systems

Freiburgh,Germany

parabolictrough andCPC

linearandpoint

n/a 214 yes 77.5% [20]

8. PolytechnicalUniversity ofMadrid

Madrid,Spain

Flatconc- centrationationdeviices (RXI)

point n/a 1000 no n/a [21]

9. PhotovoltaicsInternational, LLC

SacramentoCalifornia,USA

fresnellens

linear

30 10 yes 12.7% [12] [22]

10.

Solar Research

Australia

parabolic dish

point 0.2 239 yes 22.0% [23]

Corporation,Pty. Ltd.

11.

SolFocus Ben GurienUniversity,Israel

Paraboloid andhyperboloid

pointandpoint

0.25 500 yes 81% [24]

12.

SunPowerCorporation

USA fresnellens

point n/a 250 -400

n/a 27.0% [10]

14. Effect of Temperature on Solar Cells:

As the intensity of illumination increases, the solar cell heatsup and the efficiency of solar cells decreases as the temperatureof the cell increases. The loss in efficiency is about 10% forevery 25 K increase in temperature, although the exact loss inefficiency depends on the specific cell. The cooling of thephotovoltaic cell is therefore very essential for a consistentperformance of the solar concentration system.

15. Future of Solar Concentrators:

Japan is targeting to have 30% of its future residentialelectricity requirements supplied through new designs of solarcells and photovoltaic concentrators. Similarly the German targetis 10% of its energy needs by 2010. Other developed countries,including the USA, are aiming at meeting 15% of their futureresidential electricity needs through solar cells and all thiswithin a time frame of 25-40 years. Developing countries, whichlie in the earth's sun belt, consider PV in many instances as theonly means to provide rural electricity and these objectivesalone are compelling reasons to develop fully integrated solarconcentrator capable of competing profitably on the world stage.The parabolic dish or dense packed array solar concentrators

using highly efficient solar cells like GaSb by with an expectedefficiency of more than 40% do hold the key for economics of thesolar power generation and R&D work in many advanced countries inthese directions are going on. The terrestrial solar cellstechnology will also become the driving force to reduce the costof materials used in space and terrestrial applications. With thesize, price, and expected efficiency of the system, Pyron Systemshas put forth its claim that with a piece of land measuring 50square miles in the desert southwest it could provide all theelectricity consumed by the entire US [10].

16. Latest Developments in Solar Power Concentrator Installation(Department of US Energy commits $2B for Two Giant SolarPower Plants)

This week in June 2011, a conditional loan guarantee commitmentswere issued by Department Energy in US for two of the biggestcapacity solar power projects in North America: the Mojave SolarProject and the Genesis Solar Project. At 250 MW each, theprojects would double the United States' currently installedconcentrated solar power capacity.

17. Future Work: The main conclusions of this paper aresummarized hereunder.

1. Solar energy has vast potential, but its contribution to theworld’s energy market is still very limited. 2. Solar concentrators could bring down the total cost of thesolar cell, thus making the solar technology cheaper andaffordable, but at the same time does not compromise the overallperformance of the solar technology. 3. There are a lot of designs of solar concentrators. Each designhas its own advantages and disadvantages. 4. In spite of advance designs achieved so far, there are still a lot of improvements that can be done especially on the concentrator design [12].

18. Conclusions:

As seen from above, the technology is yet not mature enough foreconomically harnessing solar power on large scale even withsolar concentrators. It is still capital intensive and lacks longterms reliability. In 2002, more than 1900 MW of photovoltaiccells were installed worldwide. Japan has the highest installedcapacity, followed by Germany and the US. There are too manyvarieties of technical solutions and few experiments to determinethe best solutions. For this reason it is not possible to clearlydiscard any existing or new technical solution. For about adecade the developmental work for solar concentrators in manyadvanced countries has been in full swing. If the prototypes withmore than 40% efficiency perform well, the stand-alone or withco-generation medium solar power plants will become a realityfuture.

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