SOLAR UPDRAFT TOWER-A TRULY SUSTAINABLE SOURCE OF ENERGY

1S.P Singh, 2Varun Pratap Singh, 3Neelanchal Dixit1Assistant Professor, Lord Krishna College of Engineering, NH-24 Ghaziabad, UP, India

2&3Student, Lord Krishna College of Engineering, NH-24 Ghaziabad, UP, IndiaEmail;1spsinghr@gmail.com, 2rayvarun@gmail.com, 3neelanchal90@gmail.com

ABSTRACTA solar updraft tower plant, utilizes 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. The enhancement of updraft plants results in ozone generation and CFC reduction which increases the efficiency of plants to maximize the power output and work for the damage control of ozone layer (protection from UV rays) which is essential for the survival of living organism. The real objective of the above innovation is to work for the well fare of mankind at large and drastically reduced global warming.

KEYWORDSSolar updraft tower, Green House Effect, Cold corona, CFC, Kyoto protocol, Montreal protocol.

INTRODUCTIONThe generating ability of a solar updraft power plant depends primarily on two factors: the size of the collector area and chimney height. With a larger collector area, a greater volume of air is warmed to flow up the chimney; collector areas as large as 7 km in diameter have been considered. With a larger chimney height, the pressure difference increases the stack effect; chimneys as tall as 1000 m have been considered. 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. Turbines can be installed in a ring around the base of the tower, with a horizontal axis, as planned for the Australian project and seen in the diagram above; or—as in the prototype in Spain—a single vertical axis turbine can be installed inside the chimney. Carbon dioxide is emitted only negligibly while operating, but is emitted more significantly during manufacture of its construction materials, particularly cement. Net energy payback is estimated to be 2–3 years. A solar updraft tower power station would consume a significant area of land if it were designed to generate as much electricity as is produced by modern power stations using conventional technology. Construction would be most likely in hot areas with large amounts of very low-value land, such as deserts, or otherwise degraded land. A small-scale solar updraft tower may be an attractive option for remote regions in developing countries. The relatively low-tech approach could allow local resources and labour to be used for its construction and maintenance.

MONTREAL PROTOCOLThe Montreal Protocol on Substances That Deplete the Ozone Layer (a protocol to the Vienna Convention for the Protection of the Ozone Layer)is an international treaty designed to protect the ozone layer by phasing out the production of a number of substances believed to be responsible for ozone depletion. The treaty was opened for signature on September 16, 1987, and entered into force on January 1, 1989, followed by a first meeting in Helsinki, May 1989.

KYOTO PROTOCOLThe Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change (UNFCCC or FCCC), aimed at fighting global warming. The UNFCCC is an international environmental treaty with the goal of achieving "stabilization of greenhouse gas concentrations in the atmosphere at a level that would minimize dangerous anthropogenic interference with the climate system.” Under the Protocol, 37 industrialized countries (called "Annex I countries") commit themselves to a reduction of four greenhouse gases (GHG) (carbon dioxide, methane, nitrous oxide, sulphur hexafluoride) and two groups of gases (hydro fluorocarbons and per fluorocarbons) produced by them, and all member countries give general commitments. Annex I countries agreed to reduce their collective greenhouse gas emissions by 5.2% from the 1990 level. Emission limits do not include emissions by international aviation and shipping, but are in addition to the industrial gases, chlorofluorocarbons, or CFCs, which are dealt with under the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer.

OZONE GENERATOR

CORONA DISCHARGE METHODThis is the most popular type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube. They are typically very cost-effective and do not require an oxygen source other than the ambient air. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.

CFC ARRESTOR CFC arrestor is an arrangement of continues water (chemical can be used but with low acid formation) flow on fiber blades supported by stiffening rings and middle tower arrangements. Recycling of continues water flow with same temperature as tower inlet air have can be maintain by using of several unites at different height of feeding system which contend pump, filter, reservoir and many accessories on it.

COLLECTORHot air for the solar tower is produced by the greenhouse effect in a simple air collector consisting of a glass or plastic glazing stretched horizontally several meters above the ground. The height of the glazing increases adjacent to the tower base, so that the air is diverted to vertical movement with minimum friction loss. This glazing admits the solar radiation component and retains long-wave re-radiation from the heated ground. Thus the ground under the roof heats up and transfers its heat to the air flowing radially above it from the outside to the tower

STORAGEIf additional thermal storage capacity is desired, water filled black tubes are laid down side by side

on the radiation absorbing soil under the collector . 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

TOWER TUBEThe tower itself is the plant's actual thermal engine. It is a pressure tube with low friction loss (like a hydro power station pressure tube or pen stock) because of its favorable surface volume ratio. The updraft velocity of the air is approximately proportional to the air temperature rise (ΔT) in the collector and to the tower height (cf. equ. 8). In a multi-megawatt solar tower the collector raises the air temperature by about 30 to 35 K. This produces an updraft velocity in the tower of (only) about 15m/s at nominal electric output, as most of the available pressure potential is used by the turbine(s) and therefore does not accelerate the air. It is thus possible to enter into an operating solar power plant for maintenance without danger from high air velocities.

TURBINESUsing turbines, mechanical output in the form of rotational energy can be derived from the air current in the tower. Turbines in a solar tower do not work with staged velocity like free-running wind energy converters, but as shrouded pressure staged wind turbo generators, in which, similarly to a hydroelectric power station, static pressure is converted to rotational energy using cased turbines. The specific power output (power per area swept by the rotor) of shrouded pressure-staged turbines in the solar tower is roughly one order of magnitude higher than that of a velocity staged wind turbine. Air speed before and after the turbine is nearly same. The output achieved is proportional to the product of volume flow per time unit and the pressure differential over the turbine. With a view to maximum energy yield, the aim of the turbine control system is to maximize this product under all operating conditions. To this end, 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 pressure drop 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 80% of the total pressure differential available. The optimum fraction depends on plant characteristics like friction pressure losses.

Figure 4: Solar Chimney Project Arabia: Turbine house

TYPICAL DESIGN CONSIDERATION

Figure 5 Typical Design Considerations

PROTOTYPE IN SPAINIn 1982, a small-scale experimental model of a solar chimney power plant was built under the direction of German engineer Jörg Schlaich in Manzanares, Ciudad Real, 150 km south of Madrid, Spain; the project was funded by the German government.

The chimney had a height of 195 meters and a diameter of 10 meters with a collection area (greenhouse) of 46,000 m² (about 11 acres, or 244 m diameter) obtaining a maximum power output of about 50 kW. However, this was an experimental setup that was not intended for power generation. Instead, different materials were used for testing such as single or double glazing or plastic (which turned out not to be durable enough), and one section was used as an actual greenhouse, growing plants under the glass. During its operation, optimization data was collected on a second-by-second basis with 180 sensors measuring inside and outside temperature, humidity and wind speed. For the choice of materials, it was taken into consideration that such an inefficient but cheap plant would be ideal for third world countries with lots of space - the method is inefficient for land use but very efficient economically because of the low operating cost. So cheap materials were used on purpose to see how they would perform, such as a chimney built with iron plating only 1.25 mm thin and held up with guy ropes. For a commercial plant, a reinforced concrete tower would be a better choice.• tower height: 194.6 m• tower radius: 5.08 m• mean collector radius: 122.0 m

• mean roof height: 1.85 m• number of turbine blades: 4• turbine blade profile: FX W-151-A• blade tip speed to air transport velocity ratio: 1 : 10• operation modes: stand-alone or grid connected mode• Typical collector air temp. increase: ΔT = 20 K• nominal output: 50 kW• coll. covered with plastic membrane: 40'000 m²• coll. covered with glass: 6'000 m²This pilot power plant operated for approximately eight years but the chimney guy rods were not protected against corrosion and not expected to last longer than the intended test period of three years. So, not surprisingly, after eight years they had rusted through and broke in a storm, causing the tower to fall over. The plant was decommissioned in 1989.Based on the test results, it was estimated that a 100 MW plant would require a 1000 m tower and a greenhouse of 20 km2. Because the costs lie mainly in construction and not in operation (free 'fuel', little maintenance and only 7 personnel), the cost per energy is largely determined by interest rates and years of operation, varying from 5 eurocent per kWh for 4% and 20 years to 15 eurocent per kWh for 12% and 40 years.

Figure 6: Manzaners Solar Tower

In Fig. 7 the main operational data, i.e. solar insulation, updraft velocity and electric power output, are shown for a typical day. Two things shall be pointed out: First that power output during the day correlates closely with solar insulation for this small plant without additional

storage. But, second, there is still an updraft during the night, which can be used to generate power during some hours of the night (Fig. 8).

With increasing collector size, i.e. generally speaking with increasing thermal inertia of the system, this effect increases, as will be seen later from the results of simulation runs for large scale plants (Fig. 10). In order to arrive at a thorough understanding of the physical relationships and to evolve and identify points of approach for possible improvements, a computer simulation code was developed that describes the individual components, their performance, and their dynamic interaction. This program was verified on the basis of experimental measurement results from Manzanares. Today, it is a development tool that takes all known effects into account and with the aid of which the thermodynamic behavior of large-scale plants. Meteorological conditions can be calculated in advance (Haaf, 1984; Weinrebe, 2000). From mid 1986 to early 1989 the plant was run on a regular daily basis. As soon as the air velocity in the tower exceeded a set value, typically 2.5m/s, the plant started up automatically and was automatically connected to the public grid. During this 32month period, the plant ran, fully automatically, an average of 8.9 hours per day. In 1987 there were 3067h with a solar global horizontal irradiation of over 150 W/m² at the Manzanares site. Total operation time of the plant with net positive power to the grid was 3157 hours, including 244 hours of net positive power to the grid at night. These results show that the system and its components are dependable and that the plant as a whole is capable of highly reliable operation. Thermodynamic inertia is a characteristic feature of the system, continuous operation throughout the day is possible, and for large systems even abrupt fluctuations in energy supply are effectively cushioned. Using the custom-made thermodynamic simulation code based on finite elements that solves the equations for conservation of energy, momentum and mass, the theoretical performance of the plant was calculated and the results compared with the measurements obtained. The code includes simulation of collector performance based on standard collector theory (Duffie and Beckman, 1991), extended by an integration of thermal storage effects of the natural collector ground and – if required – additional thermal storage by water filled bags into the model . Fluid dynamics of

Figure 9 shows a comparison between the measured and calculated average monthly energy outputs, showing that there is good agreement between the theoretical and measured values. Overall, it may be said that the optical and thermodynamic processes in a solar tower are well understood and that models have attained a degree of maturity that accurately reproduces plant behavior under given meteorological conditions.

CONVERSION RATE OF SOLAR ENERGY TO ELECTRICAL ENERGYThe solar updraft tower has power conversion rate considerably lower than many other designs in the (high temperature) solar thermal group of collectors. The low conversion rate

of the Solar Tower is balanced to some extent by the low investment cost per square meter of solar collection.

According to model calculations, a simple updraft power plant with an output of 200 MW would need a collector 7 kilometers in diameter (total area of about 38 km²) and a 1000-metre-high chimney. One 200MW power station will provide enough electricity for around 200,000 typical households and will abate over 900,000 tons of greenhouse producing gases from entering the environment annually. The 38 km² collecting area is expected to extract about 0.5 percent, or 5 W/m² of 1 kW/m², of the solar power that falls upon it. Note that in comparison, concentrating thermal (CSP) or photovoltaic (CPV) solar power plants have an efficiency ranging from 20-40%. Because no data is available to test these models on a large-scale updraft tower there remains uncertainty about the reliability of these calculations

FINANCIAL FEASIBILITYThis section discusses only the classical design of a solar updraft tower: more exotic variations are not considered.

A solar updraft power station would require a large initial capital outlay, but would have relatively low operating cost. However, the capital outlay required is roughly the same as next-generation nuclear plants such as the AP-1000 at roughly $5 per W of capacity. Like other renewable power sources there would be no cost for fuel. A disadvantage of a solar updraft tower is the much lower conversion efficiency than concentrating solar power stations have, thus requiring a larger collector area and leading to higher cost of construction and maintenance.

Financial comparisons between solar updraft towers and concentrating solar technologies contrast a larger, simpler structure against a smaller, more complex structure. The "better" of the two methods is the subject of much speculation and debate.A Solar Tower is expected to have less of a requirement for standby capacity from traditional energy sources than wind power does. Various types of thermal storage mechanisms (such as heat-absorbing surface material or salt water ponds) could be incorporated to smooth out power yields over the day/night cycle. Most renewable power systems (wind, solar-electrical) are variable, and a typical national electrical grid requires a combination of base, variable and on-demand power sources for stability. However, since distributed generation by intermittent power sources provides "smoothing" of the rate of change, this issue of variability can also be addressed by a large interconnected electrical super grid, incorporating wind farms, hydroelectric, and solar power stations

ESTIMATED RETURN Based on current retail rates generation of electricity over the 60 year life this technology is expected to return 20.34% p.a. for 60 years. *

SUITABILITY Suited areas must be flat, hot, essentially uninhabited wasteland. The US, Australia, China and India all have large areas where similar conditions exist to those at Mildura. Due to the large quantities of glass panels and steel supporting framework, along with the materials to build the tower, a manufacturing base will need to be established with a ready supply of raw materials. Again, there are areas in each of these countries which meet these criteria.The erection of the solar panel array will require a lower level of construction expertise than

is required on similar installations so in developing areas the local population could become involved in this.

CHALLENGES NEED TO BE METThe tower itself will be the major challenge due to its proposed height of 1km. However, similar height structures are now being built around the world in Dubai, Taiwan and China. The engineering and construction knowledge is available now.

C02 EMISSIONSOnce the plant is built there will be no need for the ongoing depletion of natural resources. The net advantage of the plant will be equivalent to reducing green house emissions by 1,752,000 tonnes of carbon dioxide per year. To put this into perspective this is equivalent to taking 640,000 cars off the road, or supplying 284,000 of Australian households with completely green power.

SUMMARY & CONCLUSIONSThe updraft tower works on a simple proven principle, its physics are well understood. As thermodynamic efficiency of the plant increases with tower height, such plants have to be large t become cost competitive. Large plants mean high investment costs, which are mostly due to labour costs. This in return creates jobs, and a high net domestic product for the country with increased tax income &reduced social cost(=human dignity, social harmony), &in addition no costly consumption of fossil fuels. The reduction dependence on imported oils & coal, which is especially beneficial for the developing countries releasing means for their development. There is no ecological harm & no consumption of resources, not even for the construction, as solar towers predominantly consists of concrete &glass which are made from sand & stone plus self generated energy. Consequently in desert areas-with inexhaustible sand & stone- solar towers can reproduce themselves. A truly sustainable source of energy. REFERENCES

1. Design of Commercial Solar Updraft Tower Systems – “Utilization of Solar Induced Convective Flows for Power Generation” (2008-09) international report, Jörg Schlaich, Rudolf Bergermann, Wolfgang Schiel, Gerhard Weinrebe Schlaich Bargeman und Partner (sbp gmbh), Hohenzollerns. Stuttgart, Germany.

2. Solar Engineering of Thermal Processes, Duffie J.A. and Beckman W.A. (1991), 2nd edn. Wiley Interscience, New York.

3. Meteotest (1999). “METEONORM 4.0”, Swiss Federal Office of Energy, 3003 Bern 4. Energy principle of thermal collection and storage, S.P. Sukhatme, Tata McGraw-

Hill, 1996.5. Performance studies on solar concrete collectors, J.K. Nayak, S.P. Sukhatme, R.G.

Limaye and S.V. Bopshetty, Solar Energy Journal ,Volume 42, Issue 1, 1989, Pages 45-56

6. Advances in solar energy and technology collection and storage system, Garg HP, Springer, 1987

7. Solar thermal energy storage, H.P.Garg,Springer, 19858. Solar and wind technology, S.P. Sukhatme, Tata McGraw-Hill, 19969. Glass roof of the prototype plant at Manzanares, US patent 4275309-system for

converting solar heat to electrical energy, solar energy system

10. System performance studies on a photovoltaic/thermal (PV/T) air heating collector, H. P. Garg and R. S. Adhikari, Renewable Energy Journal,Volume 16, Issues 1-4, January-April 1999, Pages 725-730.

11. The Solar Chimney, J. Schlaich, (1995), Edition Axel Menges, Stuttgart, Germany.12. Solar Chimney Simulation, Weinrebe, G. (2000), Proceedings of the IEA

SolarPACES Task III Simulation of Solar Thermal Power Systems Workshop, 28th and 29th Sept. 2000, Cologne.

13. Greenhouse Gas Mitigation with Solar Thermal Power Plants, Weinrebe, G. (1999), Proceedings of the PowerGen Europe 1999 Conference, Frankfurt,Germany, June 1-3.

14. The potential of solar chimney for application in rural areas of developing countries, Frederick N. Onyango, Reccab M. Ochieng, Fuel 85 (2006) 2561–2566.

15. Solar thermal engineering systems, GN Tiwari, S.Sangeeta. New Delhi: Narosa Publishing House; 1997.

16. Solar energy: fundamentals and applications, H.P.Garg, J.Prakash, New Delhi: McGraw-Hill; 2000.