Solar Updraft Tower

Seminar Report 2008-12 TKM College Of Engineering Kollam

Solar Updraft Tower Mechanical Engg Dept

SOLAR UPDRAFT TOWER



1. ABSTRACT

The solar updraft tower is a renewable energy power plant. A solar updraft tower power plant

– sometimes also called 'solar chimney' or just ‘solar tower’ – is a solar thermal power plant

utilizing a combination of solar air collector and central updraft tube to generate a solar

induced convective flow which drives pressure staged turbines to generate electricity. It

combines three old and proven technologies; the chimney effect, the greenhouse effect and

the wind turbine. The tower is constructed with a material that sits 3-15 feet off the ground.

Light can shine through the material, heating the air underneath the material and funnelling it

towards the only escape for the hot air, a tower at the centre of the circle. In the tower is a

turbine, that turbine is turned by the heat rising upwards. . Heat can be stored inside the

collector area greenhouse to be used to warm the air later on water with its relatively high

specific heat capacity, can be filled in tubes placed under the collector, increasing the energy

storage as needed.



2. INTRODUCTION Sensible technology for the wide use of renewable energy must be simple and

reliable, accessible to the technologically less developed countries like India that are sunny

and often have limited raw materials resources. We are also in seek of additional power

sources which are environmental friendly It should not need cooling water and it should be

based on environmentally sound production from renewable or recyclable materials.

The solar tower meets these conditions. Economic appraisals based on experience and

knowledge gathered so far have shown that large scale solar towers ( ≥ 100 MW) are capable

of generating electricity at costs comparable to those of conventional power plants

(Badenwerk and EVS, 1997). This is reason enough to further develop this form of solar

energy utilization, up to large, economically viable units. In a future energy economy, solar

towers could thus help assure the economic and environmentally benign provision of

electricity in sunny regions.

The solar updraft tower’s three essential elements – solar air collector,

chimney/tower, and wind turbines - have been familiar for centuries. Their combination to

generate electricity has already been described in 1931 (Günther, 1931). Haaf (1983, 1984)

gives test results and a theoretical description of the solar tower prototype in Manzanares,

Spain. Transferability of the results obtained in Manzanares is discussed by Schlaich et al.

(1990). The same author provides an overview (Schlaich 1995). For Australia, a 200 MW

solar tower project is currently being developed (http://www.enviromission.com.au).

Conditions in Australia are very favourable for this type of solar thermal power plant:

Insulation levels are high (http://www.bom.gov.au), there are large suitably flat areas of land

available, demand for electricity increases, and the government’s Mandatory Renewable

Energy Target (MRET), requires the sourcing of 9,500 Gigawatt hours of extra renewable

electricity per year by 2010 through to 2020 (http://www.mretreview.gov.au).

Here we discuss about the working and the economies of large scale solar updraft power

plants.



3. FUNCTIONAL PRINCIPLE

The solar tower’s principle is shown in figure 1: Air is heated by solar radiation

under a low circular transparent or translucent roof open at the periphery; the roof and the

natural ground below it form a solar air collector. In the middle of the roof is a vertical tower

with large air inlets at its base. The joint between the roof and the tower base is airtight. As

hot air is lighter than cold air it rises up the tower. Suction from the tower then draws in more

hot air from the collector, and cold air comes in from the outer perimeter. Continuous 24

hours operation can be achieved by placing tight water-filled tubes or bags under the roof.

The water heats up during day-time and releases its heat at night. These tubes are filled only

once, no further water is needed. Thus solar radiation causes a constant updraft in the tower.

The energy contained in the updraft is converted into mechanical energy by pressure-staged

turbines at the base of the tower, and into electrical energy by conventional generators

(Schlaich and Schiel, 2001).



4. POWER OUTPUT The fundamental dependencies and influence of the essential parameters on power

output of a solar tower are presented here in a simplified form: Generally speaking, power

output P of the solar tower can be calculated as the solar input Qsolar multiplied by the

respective efficiencies of collector, tower and turbine(s):

The power output of the plant is dependent on two physical properties of the plant,

the area of the collector and the height of the tower. The solar input to the plant is

proportional to the area of the collector, while the efficiency of the tower is dependent on its

height. The other efficiencies of the tower are not dependent on the conceptual design of the

tower.

The solar energy input into the system is dependent on the area of the collector and

the solar insulation onto the collector, where G is the normalized solar insulation:

The efficiency of the tower is dependent on its height. The tower efficiency can

be described by

The power from the flow is dependent on the pressure drop in the tower. The

power contained in the flow is

The pressure change in the tower is related to the buoyancy change in the

heated air. The air column in the tower creates the pressure difference driving

the flow.

Without a turbine in the tower, all the pressure difference in the tower is

converted to velocity. The power contained in the flow is then

We can equate the two expressions for the power in the flow to find the velocity Sin

the flow.



Using the Boussinesq approximation and the ideal gas law, the expression for the

maximum velocity simplifies to

Combining this with our second expression for the power contained in the flow, we

can find that the efficiency of the tower is

Theoretical Power Output

Based on the equations developed in the previous sections, the total power generated

by the tower is

Thus, the power generated by the tower is proportional to the area of the collector and

the height of the tower. An easy way to think about this is that the power is proportional to

the volume of the cylinder with a base the size of the collector and a height equal to that of

the chimney, as shown in Figure 1.

The ability of the tower to convert the input solar insolation is the product of the

efficiencies of each of the components and the efficiency of the tower overall. Assuming that

the efficiencies of the individual components are high, the efficiency of the solar updraft

tower is directly tied to the tower efficiency. As stated in the previous section,

Evaluating for typical conditions, where cp=1012 J/kg-K, T0=300 K, and g=9.8

m/s2, we find that the tower efficiency is roughly 0.000032 H. With the other efficiencies

near one, this gives

A very tall tower is required to achieve even a modest tower efficiency. For example,

a 1000 meter tall tower is required to achieve an efficiency of about 3%.



5. SOLAR UPDRAFT TOWER COMPONENTS

As we see that the solar updraft tower consist of a solar collector, chimney and

turbine and rest of the component i.e. generator, transmission is as same in other power

plants. The main components in solar chimney power plant are as follows.

1. The solar collector.

2. The chimney

3. The wind turbine.

THE SOLAR COLLECTOR

By means of an absorber, a collector can be used for space heating. Solar collector

transforms about 80% of radiation energy into heat. Hot air for the chimney is produced by

greenhouse effect in a simple air collector consisting only of a glass of plastic film covering

stretched horizontally 2 to 6 m above the ground. Height increases only adjacent to the

chimney base, so that the air can be diverted to vertical movement without friction loss. This

covering admits short wave solar radiation component and retains long-wave radiation from

the heated ground. Thus, ground under the roof heats up and transfers its heat to the air

flowing radially above it from the outside to the chimney, like flow heater. The air

temperature rise could be 350C in a well-designed collector. The total radius requires for

5MW, 30MW, 100MW is 500, 1000 and 1800 m respectively. Peripheral area of the collector

can be used as greenhouse or drying plants, at no extra cost and without significant

performance loss. A collector roof of this kind is of long span and continuous maintenance

can give service up to 60 years or more. Collector efficiency is improved as rise in

temperature decreases. Thus, a solar chimney collector is economic, simple in operation and

has a high-energy efficiency level.



THE COLLECTOR

The chimney itself is the plant's actual thermal engine. It is a pressure tube with low

friction and loss (like a hydroelectric tube) because of its optimum surface-volume ratio. The

up-thrust of the air heated in collector is approximately proportional to air temperature rise

dT in collector and volume (i.e. height and diameter of the chimney). In a large solar chimney

the collector raises the temperature of air by dT=350C. This produces an up-draught velocity

in chimney of about V=15 m/s. The efficiency of the chimney (i.e. conversion of heat into

kinetic energy) is practically independent of dT in collector and determined by outside temp.

To (lower the better) and height of chimney (higher the better). Power = K. (Hc/To)*(Solar

radiation at location)*(Area of collector).

Thus, solar chimneys can make particularly good use of the low rise in air temperature

produced by heat emitted by the ground during the night and even the Meagre solar radiation

of a cold winter's day!

However, compared with the collector and the turbines, the chimneys efficiency is

relatively low, hence the importance of size in its efficiency curves. The chimney should be

as tall as possible e.g.: at 1000m height can be built without difficulty. ( Let it be remind that

T.V. Tower in Toronto, is almost 600m height and serious plans are being made for 2000 m

skyscrapers in earthquake- ridden Japan.)



THE TURBINES

Mechanical output in the form of rotational energy can now he derived from the

vertical air-current in the chimney by turbines. Turbines in a solar chimney do not work with

stepped velocity like a free-running wind energy converter, but as a cased pressure-stepped

wind turbo-generator, in which, similar to a hydroelectric power station, static pressure is

converted into a pipe. The energy yield of a cased pressure-stepped turbine of this kind is

about eight times greater than that of the same diameter. Air speed before and after the

turbine is about the same. The output achieved is proportional to the product of volume flow

per time unit and the fall in pressure at the turbine. With a view to maximum energy yield the

aim of the turbine regulation concept is to maximize this product under all operating

conditions.

The turbine regulates air speed and air flow by means of blade tilt. If the blades are

horizontal, the turbine does not turn. If the blades are vertical 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. If

the air stream is throttled the air takes longer to heat up. This increases the rise in temperature

in the collector. This in its turn causes increase ground storage and thus enhanced night

output, but also greater loss from the collector (infrared emissions and convection). Turbines

are always placed at the base of the chimney. Vertical axis turbines are particularly robust

and quiet in operation. The choice is between one turbine whose blades cover the whole

cross-section of the chimney or six smaller turbines distributed around the circumference of

the chimney wall, here the blade length of each turbine will a sixth of the chimney diameter.

The diversion channel at the base of the chimney is designed for one or six turbines as

appropriate. But it is also possible to arrange a lot of small turbines with horizontal axes (as

used in cooling tower fans) at the periphery of the transitional area between canopy and

available technology. Generator and transmission are conventional, as used in related spheres.

In a solar chimney there are no critical dynamic loads on blades, hubs and setting equipment

of the kind met in free-running wind energy converters due to gustiness of the natural wind as

the canopy forms an effective buffer against rapid pressure and speed changes. This makes

these components structurally simple and cheap to manufacture, and they also have a long

life span.



6. ENERGY STORAGE If additional thermal storage capacity is desired, water filled black tubes are laid

down side by side on the radiation absorbing soil under the collector (Kreetz 1997). The tubes

are filled with water once and remain closed thereafter, so that no evaporation can take place

(Fig. 2). 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 characteristics (Fig.3).

At night, when the air in the collector starts to cool down, the water inside the tubes

releases the heat that it stored during the day. Heat storage with water works more efficiently

than with soil alone, since even at low water velocities – from natural convection in the tubes

– the heat transfer between water tubes and water is much higher than that between ground

surface and the soil layers underneath, and since the heat capacity of water is about five times

higher than that of soil.





7. SOLAR UPDRAFT TOWERS IN VARIOUS AREA

MANZANARES, SPAIN Detailed theoretical preliminary research and a wide range of wind tunnel

experiments led to the establishment of an experimental plant with a peak output of 50 kW on

a site made available by the Spanish utility Union Electrica Fenosa in Manzanares (about 150

km south of Madrid) in 1981/82 (Fig. 4), with funds provided by the German Ministry of

Research and Technology (BMFT) (Haaf et al. 1983, Schlaich et al, 1990).

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. The main dimensions and technical data for the facility are

listed in table 1.

The tower comprises a guyed tube of trapezoidal sheets, gauge 1.25 mm, corrugation

depth 150 mm. The tube stands on a supporting ring 10 m above ground level; this ring is



carried by 8 thin tubular columns, so that the warm air can flow in practically unhindered at

the base of the tower. A pre-stressed membrane of plastic-coated fabric, shaped to provide

good flow characteristics, forms the transition between the roof and the tower. The tower is

guyed at four levels, and in three directions, to foundations secured with rock anchors. The

tower was erected at ground level, utilizing a specially developed incremental lifting method

proposed by Brian Hunt of SBP: First, the top section of the tower was installed on a lifting

ring on the ground, and then it was raised onto the supporting ring by means of hydraulic

presses. Subsequently the other sections were assembled on the ground, connected to the

already installed top tower section(s) and then the whole assembly was lifted. So the

complete tower was built in 20 shots of 10m each.

The turbine is supported independently of the tower on a steel framework 9 m above

ground level. It has four blades, which are adjustable according to the face velocity of the air

in order to achieve an optimal pressure drop across the turbine blades (Fig. 5). Vertical wind

velocity is 2.5 m/s on start-up and can attain a maximum of 12 m/s during turbine operation.





The collector roof of the solar tower not only has to have a transparent or translucent

covering, it must also be durable and reasonably priced. A variety of types of plastic sheet, as

well as glass, were selected in order to determine which was the best – and in the long term,

most cost effective – material (Fig. 6). Glass resisted heavy storms for many years without

harm and proved to be self-cleaning thanks to the occasional rain showers.

The plastic membranes are clamped to a frame and stressed down to the ground at the

center by use of a plate with drain holes. The initial investment cost of plastic membranes is

lower than that of glass; however, in Manzanares the membranes got brittle with time and

thus tended to tear. Material (temperature and UV stability) and design improvements (e.g.

membrane domes) achieved in the last years may help to overcome this particular

disadvantage.

Completion of the construction phase in 1982 was followed by an experimental phase,

the purpose of which was to demonstrate the operating principle of a solar tower. The goals

of this phase of the project were (1) to obtain data on the efficiency of the technology

developed, (2) to demonstrate fully automatic, power-plant-like operation with a high degree

of reliability, and (3) to record and analyze operational behavior and physical relationships on

the basis of long-term measurements.

AUSTRALIA In Australia at Melbourne the world's tallest Man made structure could soon be

towering over the Australian outback as part of a plan to capitalize on the global push for

greater use of renewable energy. Mainly the team at Manzanares works with this plant

combining Enviro-mission.

Australia power company Enviro-mission ltd. hopes to build a 1,000 meter (3,300 feet)

solar tower in south west new South Wales state, a structure that would be more than twice

the height of Malaysia's Petron's Towers, the world's tallest building. The plant having seven

kilometer roof diameter and 1 km chimney height, and a 3 meter distance at outer periphery

and 25 m distance at inner periphery of solar collector roof. And which it allows to sucked

hot air through 32 turbines which generate power 24 hrs a day having output expected to 650

GW/yrs.



INDIA A 200 MW power plants is being built at Thar (Jaisalmar ) by a consortium of

SriLanka and Germany at the cost of Us $ 450 billion which is going to commissioned in

year 2015 according to Rajasthan Energy Development Agency (REDA).

CHINA In December 2010, a solar updraft tower in Jinshawan in Inner Mongolia, China

started operation, producing 200-kilowatts of electric power. The 1.38 billion project was

started in May 2009 and its aim is to build a facility covering 277 hectares and producing

27.5 MW by 2013. The greenhouses will also improve the climate by covering moving sand,

restraining sandstorms

NAMIBIA The Namibian government started operation for a 400 MW solar updraft tower called

the 'Green tower'. The tower is to be 1.5 km tall and 280 m in diameter, and the base will

consist of a 37 km2 greenhouse in which cash crops can be grown.



8. TYPICAL DIMENSIONS AND ELECTRICITY OUTPUT

The typical dimensions and electricity output for the solar updraft tower is shown

below. The solar updraft tower is built by considering these typical dimensions and the

electrical output in MW is obtained according to the dimensions. If dimensions are more the

electrical output will also be more.



9. ADVANTAGES The advantages of solar updraft tower are,

1. It provides electricity 24 hrs a day from solar energy alone. At night, heat absorbing

or other sources in the "green house" would slowly release the thermal energy built up

during the day, maintaining the indoor-outdoor temperature differential The solar

chimney can operate around the clock, instead of depending on environmental factors

such as the wind needed for wind farms.

2. No fuel is needed, it needs no cooling water and is suitable in extreme drying regions,

it is practically reliable and a little trouble – prone compared with other power plant.

The material concrete, glass and steel necessary for the building of solar chimney

power stations are everywhere in sufficient quantities.

3. It does not exhausting poisonous gases or smoke as in thermal power plant.

It does not utilizes the sources of energy it does not unbalance the natural phenomenon

4. As in hydroelectric power plant due to storage of water the lack of water to

agricultural land may arise problem of agricultural fields. This type of problem is not

arises in solar chimney power plant.

5. It can use the infertile land for the construction such as desert land which will cause

to progress in that area.

6. The peripheral area of collector is used for the greenhouse cultivation for drying

plants.



10. DISADVANTAGES

The disadvantages of solar updraft tower are

1. High construction cost

It requires a large amount as initial capital for the built of a solar updraft tower.

2. Efficiency & Production Cost

Cost/kWh is higher than traditional forms of natural gas energy production.

3. Requires large amount of land

Not suitable for areas with high cost/acre and most suited to be built in non

Populated areas.



11. CONCLUSION From the above discussion this paper would like draw following conclusions.

1) The collector of solar chimney plant can use all solar radiation both direct and

diffused. So, this plant technique is also helping hands to those countries where the

sky is frequently overcast.

2) There are many regions in country which are deserts and soil don't bear any crop. And

thus no contribution to mankind. But installing plant there give excellent results.

3) The technology and the material to build such plants are available in the country.

Hence, such power plants are very attractive in India for bulk power generation even

in deserts. The capital cost is high, nearly 7 crore/MW, which can be reduced.

However, the cost of generation could be as low as Rs.1.62 per KWH in long run.

Hence due to various advantages now most of the country are attracting towards the

generation of power by using solar chimney power plant techniques.



12. REFERENCES

[1] Schlaich J, Bergermann R, Schiel W, Weinrebe G, The Solar Updraft Tower - An

Affordable and Inexhaustible Global Source of Energy, Bauwerk-Verlag, Berlin, 2004

[2] Dos Santos Bernardes M.A., Voß A., Weinrebe G. (2003). “Thermal and technical

analyses of solar chimneys” Solar Energy, 75, 511-524.

[3] Weinrebe, G. (1999). “Greenhouse Gas Mitigation with Solar Thermal Power Plants”,

Proceedings of the PowerGen Europe 1999 Conference, Frankfurt, Germany, June 1-3

[4] Stinnes, W W “Greentower: performance guarantees through insurance policies”. 2004.

Industrial and commercial use of energy conference 2004. (http://active.cput.

ac.za/energy/web/icue/papers/2004/32_W_ Stinnes.pdf) [Accessed May 2008].

[4] www.wikipedia.org

[5] www.sciencedirect.com

[6] www.solarserver.de/lexikon/aufwindkraftwerk.jpg