SOLAR CHIMNEY Phys 471 (Solar Energy) 2001-2002/2 Instructor: Prof. Dr. Ahmet Ecevit Presented by: Ebru Koç, Özlem Çiçek.

SOLAR CHIMNEYSOLAR CHIMNEY

Phys 471 (Solar Energy)2001-2002/2

Instructor: Prof. Dr. Ahmet Ecevit

Presented by: Ebru Koç , Özlem Çiçek

Table of contentsTable of contents

1. Introduction ...................................... 5

2. How does a solar chimney work?...... 6

3. The technology................................... 8

3.1 The Collector.................................... 8

3.2 The Energy Storage.......................... 9

3.3 The Chimney....................................11

3.4 The Turbines....................................12

4. A Hydroelectric power station for the desert....... 144.1 A summary of “How it work?”.......................154.2 Some similarities between them......................16

5. The Prototype in Manzanares................................186. Designing large solar chimney..............................22 7. Energy production cost..........................................298. Physical principles of solar chimney.....................33

8.1 Approach calculating efficiency......................349. Advantages of solar chimney ...............................4510. Disadvantages of solar chimney .........................4811. Conclusion...........................................................4912. References............................................................51

SOLAR CHIMNEYSOLAR CHIMNEY

ELECTRICITY FROM SUN

Fig.1. Working principles of Solar Chimney[1]

INTRODUCTIONINTRODUCTION

Man learned to make active use of solar energy at a very early stage: greenhouses help to grow food, chimney suction ventilated and cooled buildings, wind mills ground corn and pumped water[2].

HOW DOES A SOLAR CHIMNEY HOW DOES A SOLAR CHIMNEY WORK?WORK?

Incident solar radiation heats the air under a large transparent collector roof. The temperature difference causes a pressure drop over the height of chimney resulting in an upwind which is converted into mechanical energy by the turbines and then into electricity via conventional generators[3].

Fig.2. Principle of the solar chimney: glass roof collector, chimney tube, wind turbines[4].

Thus the solar chimney combines 3 well-known technologies in a novel way[5].

the glass roof hot air collector

the chimney

wind turbines with generator

3. THE TECHNOLOGY3. THE TECHNOLOGY

3.1 THE COLLECTOR:Hot air is produced by the greenhouse effect. The collector consisting of plastic film or glass plastic film. The roof material is stretched horizontally 2 or 6 m above the ground. The height of the roof increases adjacent to the chimney base, so that the air is diverted to the chimney base with minimum friction loss[2].

Fig.4. The collector Fig.3. Principle of the solar chimney[6].

3.2 THE ENERGY STORAGE: Water filled black tubes are laid down side by side on the black sheeted or sprayed soil under the glass roof collector. They are filled with water once and remain closed thereafter, so that no evaporation can take place. The volume of water in the tubes is selected to correspond to a water layer with a depth of 5 to 20 cm depending on the desired power output [2].

The water inside the tubes stores a part of the solar

heat and releases it during the night, when the air in

the collector cools down [2].

Fig.5. Principle of heat storage underneath the roof using water-filled black tubes [3].

3.3 THE CHIMNEY:The chimney itself is the plant's actual thermal engine. It is a pressure tube with low friction loss because of its optimal surface-volume ratio. The upthrust of the air heated in the collector is approximately proportional to the air temperature rise .Tcoll in the collector and the volume, of the chimney. In a large solar chimney the collector raises the temperature of the air by about 35°. This produces an updraught velocity in the chimney of about 15m/s. It is thus possible to enter into an operating solar chimney plant for maintenance without difficulty [2].

Fig.6. Solar Chimney[1]

The chimney height is affected by collectors’ glass.

1. If glass is cheap and concrete expensive then the collector will be large with a high proportion of double glazing and a relatively low chimney.

2. If glass is expensive there will be a smaller, largely single-glazed collector and a tall chimney.

Fig.7. Solar Chimney Prototype at Manzanares (Spain)

3.4 THE TURBINES: Using turbines, mechanical output in the form of rotational energy can be derived from the air current in the chimney. Blade pitch is adjusted during operation to regulate power output according to the altering airspeed and airflow. If the flat sides of the blades are perpendicular to the airflow, the turbine does not turn. If the blades are parallel to the air flow and allow the air to flow through undisturbed there is no drop in pressure at the turbine and no electricity is generated. Between these two extremes there is an optimum blade setting: the output is maximized if the pressure drop at the turbine is about two thirds of the total pressure differential available [2].

4.4. A 'HYDROELECTRIC POWER A 'HYDROELECTRIC POWER STATION FOR THE DESERT:STATION FOR THE DESERT:

Fig.8. Toledo Bend Dam Hydroelectric Power Plant

A solar chimney is a kind of Hydroelectrıc Power Station for a desert

4.1 A Summary of How It Works:

Water from the reservoir enters the intake (1) through the open intake gates (2) the water flows down the power tunnel (3) through the wicket gates (4) which can be controlled automatically or manually. It then continues past the turbine blades (5) which turn the generator (6) at a constant 100 revolutions per minute (RPM), changing the changing the mechanical energy into mechanical energy into electrical energyelectrical energy [7].[7].

Fig.9. Hydroelectric Power Cycle [7].

Solar chimneys are technically very similar to hydroelectric power stations - so farthe only really successful large scale renewable energy source: the collector roof is the equivalent of the reservoir, and the chimney of the penstock. Both power generation systems work with pressure-staged turbines, and both achieve low power production costs because of their extremely long life-span and low running costs. The collector roof and reservoir areas required are also comparable in size for the same electrical output. But the collector roof can be built in arid deserts and removed without any difficulty, whereas useful land is submerged under reservoirs.

4.2 Some similarities between 4.2 Some similarities between themthem

Solar chimneys work on dry air and can be operated without the corrosion and cavitation typically caused by water. They will soon be just as successful as hydroelectric power stations.

Electricity yielded by a solar chimney is in proportion to the intensity of global solar radiation, collector area and chimney height [2].

5.The prototype in Manzanares

Fig.10. Prototype of the solar chimney at Manzanares [8].

The aim of this research project was to verify, through field measurements, the performance projected from calculations based on theory, and to examine the influence of individual components on the plant's output and efficiency under realistic engineering and meteorological conditions.

Fig.10. Prototype of the solar chimney at Manzanares [8].

To this end a chimney 195 m high and 10 m in diameter was built, surrounded by a collector 240 m in diameter. The plant was equipped with extensive measurement data acquisition facilities. The performance of the plant was registered second by second by 180 sensors.

Fig.10. Prototype of the solar chimney at Manzanares [8].

A realistic collector roof for large-scale plants has to be built 2 to 6 metres above ground level. For this reason the lowest realistic height for a collector roof for large-scale technical use, 2 metres, was selected for the small Manzanares plant. (For output, a roof height of 50 cm only would in fact have been ideal.) Thus only 50 kW could be achieved in Manzanares, but this realistic roof height also permitted convenient access to the turbine at the base of the chimney. During the 32 month period, plant reliability was over 95 % [2].

Measurements taken from the experimental plant in Manzanares and solar chimney thermodynamic behaviour simulation programs were used to design large plants with outputs of 200 MW and more. This showed that thermodynamic calculations for collector, tower and turbine were very reliable for large plants as well. Despite considerable area and volume differences between the Manzanares pilot plant and a projected 100 MW facilities, the key thermodynamic factors are of similar size in both cases.

6. Designing Large Solar Chimneys [2]

It includes thermodynamic calculations by computer simulation and an analysis of technical feasibility as seen in the table I.

Power Block Size MW 5 30 100

Temperature rise in Collector

oK 25.6 31.0 35.7

Updraft Velocity in Chimney

(ful load) m/s 9.1 12.6 15.8

Total Pressure Difference Pa 388.3 767.1 1100.5

Pressure Loss by Friction

(Collector And Chimney)

Pa28.6 62.9 80.6

Pressure Drop at turbinePa

314.3 629.1 902.4

Table. I: Thermodynamics Data

With 2300 kWh/m2y global radiation

The overall performance of the plant, by day and by season, given the prescribed climate and plant geometry, considering all physical phenomena including single and double-glazing of the collector, ground storage, condensation effects and losses in collector, and turbine, can be calculated.

Reliable statically and dynamic calculation and construction for chimney about 1,000 metres high

(slenderness ratio=height/diameter <10) is possible without difficulty today (Figure.11)

Figure (11): Wall thickness of a chimney tube 1.000 m high and 170 m diameter and 1.000m chimney tube under construction.

0,25m

0,25m0,32m0,41m0,53m0,68m0,87m0,99m

1000m

840m

660m

500m

With the support of a German and an Indian contractor especially experienced in building cooling towers and chimneys, manufacturing and erection procedures were developed for various types in concrete and steel and their costs compared. The type selected is dependent on the site. If sufficient concrete aggregate materials are available in the area and anticipated seismic acceleration is less than 9/3, then reinforced concrete tubes are the most suitable.

There is no physical optimum for solar chimney cost calculations, even when meteorological and site conditions are precisely known. Tower and collector dimensions as seen table 2 for a required electrical energy output can be determined only when their specific manufacturing and erection costs are known for a given site.

Power Block Size

MW 5 30 100 200

Collector Diameter Dcoll

m1110

2200 3600 4000

Chimney Height HC

m 445 750 950 1500

Chimney Diameter DC

m 54 84 115 175

Annual Energy

ProductionGWh/y

13.9

87.4 305.2 600

Dimensions With 2300 kWh/m2y global radiation

Table. 2: Typical Dimensions for Solar Chimneys With Different Power

With the support of construction companies, the glass industry and turbine manufacturers are rather exact cost estimate for a 200 MW solar chimney could be compiled. We asked a big utility "Energie in Baden-Württemberg" to determine the energy production costs compared to coal- and combined cycle power plants based on equal and common methods.

7. Energy Production Costs [2]

Table 3: Comparison between the energy production costs of a 2 x 200 MW solar chimneys and 400 MW coal and combined cycle power plants according to the present business managerial calculations.

Purely under commercial aspects with a gross interest rate of about 11 %and a construction period of 4 years during which the investment costs increase already by 30 %(!) Electricity from solar chimneys is merely 20 %more expensive than from coal. By just reducing the interest rate to 8 % electricity from solar chimneys would become competitive today.

No ecological harm and no consumption of resources, not even for the construction. Solar chimneys predominantly consist of concrete and glass, which are made from sand and stone plus self-generated energy.

Consequently in desert areas with inexhaustible sand and stone solar chimneys can reproduce themselves. A truly sustainable source of energy.

Fig. (12 ) Energy production costs from solar chimneys, coal and combined cycle power plants depending on the interest rate.

Precise description of the output pattern of a solar chimney under given meteorological boundary conditions and structural dimensions is possible only with an extensive thermodynamic and flow dynamic computer program. This includes the equations which reflect the effect of heat transfer between the ground and air in the collector, friction loss in the collector and chimney, heat storage in the ground, the turbine and its power control [9].

The power output of a solar chimney are presented here in a simplified form:

8. Physical Principles of the Solar Chimney

8.1 Approach Calculating Efficiency

The Collector

A solar chimney collector converts available solar radiation G onto the collector surface Acoll into heat output. Collector efficiency ncoll can be expressed as a ratio of the heat output of the collector as heated air Q and the solar radiation G(measured in W/m2) times Acoll.

.

Q: Heat output of the collector

m: mass flow

Cp: Specific heat capacity of the air

ρcool: Specific density of air at tempereature To + ΔT

Vcoll= Vc : Air speed at collector outflow/chimney inflow

.

.

For collector efficiency this gives:

α: Effective absorption coefficient of the collector

β : Loss correction value (in W/m2K), allowing for emission and convection loss

Thus collector efficiency can also be expressed like this:

The link between air speed at the collector outflow Vcoll and the temperature ΔT can be expressed:

The simple balance equation is independent of collector roof height because friction losses and ground storage in the collector are neglected

G=1000 W/m2

ΔT=300C

= 0.75-0.8

β = 5-6 W/m2 Thus, with radiation of 1000 W/m2 a collector efficiency of 62% is established.

The chimney converts the heat flow Q produced by the collector into kinetic energy and potential energy (pressure drop in the turbine). Thus, the density difference of the air is caused by temperature rise in the collector works as a driving force.

The Chimney:

.

HC

in differential form

And

g : acceleration due to gravity

HC : Chimney height

ρ : density

Fig. (13) Chimney

ΔPtot =ΔPS+ΔPd

The static pressure difference drops at the turbine, the dynamic component describes the kinetic energy of the air flow. soΔPS= O ΔPtot=ΔPd

The power contained in the flow: Ptot= ΔptotVC,max AC

Efficiency of the chimney :

Maximum flow speed:

The wind turbine fitted at the base of the chimney converts free convection flow in to the rotational energy. The pressure drop across the turbine can be expressed in a first approximation by the Bernoulli equation:

The Wind Turbine

The pressure drop:

The appropriate charesteristic curve is expressed by:

Thus mechanical power taken up by the turbine is:

Powerwt,max = (2/3)ncoll nc Acoll G

Powerwt,max = (2/3)ncoll(g/CpTo)HcAcollG

It is recognized that the electrical ouput of the solar

chimney is proportional to Hc * Acoll, i.e to the volume

included within the chimney height and collector area.

The dimensions of a 30 MW lant listed in the table 2 [9].

Chimney Height HC: –––––––– 750m

Collector Diameter Dcoll: –––––––– 2200m

Solar Irradiation G: –––––––– 1000W/m2

Mechanical Efficiency nwt : –––––––– 0.8

Collector Efficiency ncoll : –––––––– 0.6

Heat Capacity of the Air CP : –––––––– 1005j/kgK

Ambient Temperature T0 : –––––––– 200C

Gravity Acceleration g : –––––––– 9.81m/s2

Pelectric: (2/3)(0.8x0.6)[9.81/(1005x293)]x750x3751000x1000

Pelectric: 30 MW

Solar chimneys operate simply and have a number of advantages.:

9. Advantages [2]1. The collector can use all solar radiation, both

direct and diffused. The other major scale solar- thermal power plants, which apply concentrators and therefore can use only direct radiation. 

2 Due to the heat, storage system the solar chimney will operate 24h on pure solar energy.

5. The building materials needed for solar chimneys, mainly concrete and glass, are available everywhere in sufficient quantities.

3. Solar chimneys are particularly reliable and not liable to break down, in comparison with other solar generating plants.

4. Unlike conventional power stations, solar chimneys do not need cooling water.

6. Solar chimneys can be building now, even in less industrially developed countries. No investment in high-technology manufacturing plant is needed.

7. Even in poor countries, it is possible to build a large plant without high foreign currency expenditure by using their own resources and work force; this creates large numbers of jobs and dramatically reduces the capital investment requirement and the cost of generating electricity.

1. Solar chimneys can covert only a small proportion of the solar heat collected into electricity, and thus have a ‘poor efficiency level’. However, they make up for this disadvantage by their cheap, robust construction, and low maintenance costs.

10. Disadvantages [3]

2. Solar chimneys need large collector areas. As economically viable operation of solar electricity production plants is confined to regions with high solar radiation, this is not a fundamental disadvantage; as such, regions usually have enormous deserts and unutilised areas.

11. CONCLUSION

Why do we use solar power?

Current energy production from coal and oil is damaging to the environment and non-renewable. Inadequate energy supplies can lead the poverty, which commonly results in population explosions. Solar energy is the answer.

Sensible technology for the use of solar power must:

-Be simple and reliable,

-Be accessible to the technologically less developed countries that are sunny and often have limited raw materials resources,

-Not need cooling water or produce waste heat,

-Be based on environmentally sound production from renewable materials.

 THE SOLAR CHIMNEY MEETS THESE CONDITIONS

REFERENCESREFERENCES1. www.argonet.co.uk/users/bobsier/sola6.html

2.wire0.isses.org/wire/publications/Research.nsf/00a329276ae7f371c125680e003falf3/0DED34BF3EB9A985C12569840055F09E/$File/SolarChimney_short_version.pdf

3. Schlaich J. Engineering structures 21, 1999, pp 664-668

4. http://www.solarserver.de/lexikon/aufwindkraftwerk.jpg

5. Schlaich J. The Solar Chimney, Edition Axel Menges, Stuttgart, 1995, pp.18

6. Schlaich J. Renewable Energy Structures, Structural Engineering International 1994; 4(2), pp.76-81.

7. http://www.toledo-bend.com/toledo_bend/index.asp?request=tolbenddam

8. Schlaich Bergermann and Partner; Structural Consulting Engineers; Stuttgart

9. Schlaich, J. (1995). Solar Chimney: Electricity from the Sun. Stuttgart; Edition Axel Menges, pp.54-55.