Solar updraft tower

In a solar chimney power plant (sometimes also called thermal power plant) air is heated by the sun and rises due to natural convection in a fireplace. One or more turbines from the air flow generating electricity.

The principle was described in 1903 by the author Colonel Isidoro Cabanyes in his article " La energía eléctrica " and patented in 1929 by Bernard Dubos. In 1931 the author Hanns Günther described in his book " One Hundred Years " a solar chimney power plant. In the late 1970s the idea was picked up by Michael Simon, who designed a pilot plant together with the Stuttgart Civil Jörg Schlaich, in Manzanares ( central Spain ) was then implemented as a research project of the Federal Ministry for Research and Technology.

She showed several years the technical feasibility in practical operation, but only on a small scale.

Function

The operation of a solar chimney power plant is very simple. The sun shines through a large roof made ​​of glass or transparent plastic or foil (collector) and heats the ground and the air below it. The warm air flows to a fireplace in the middle of the resort and attracts more heated air in the collector itself. This results in a solar chimney ( thermal), ie, the lower density of the warm air in the column tower to the outside air produces a pressure differential that can be used mechanically. The pressure change through the turbine (s) can be the bottom of the periphery of the entrance to the chimney or tube (as in the prototype ), by a vertical axis turbine at the lower end of the tower tube. The turbines and generators to convert the mechanical energy into electrical.

The significant variables for the performance of a solar chimney power plant (abbreviated AWK, English SCPP for " Solar Chimney Power Plant" ), the surface of the collector to convert the radiation into heat and the height of the chimney to convert this heat into a pressure difference. The greater the covered area, the more the air is heated, and the higher the flue, the greater the useful differential pressure.

Dependencies

Solar chimney power plants are dependent on site conditions and local weather patterns. The large space requirement of solar chimney power plants and the most important of the weather parameters, the global radiation, put plants in arid zones (eg Sahara) no land acquisition costs close. In the table influences are listed by meteorological and geographical variables, also in comparison to other renewable types of power plants.

Waste management plan are an alternative to wind turbines and at sites with a high degree of coverage a possibility to use also the diffuse radiation components in contrast to concentrating solar systems on flat, low-wind locations. The suitability for solar power generation at sites with the typical combination "wind poor location / location with a high degree of coverage " is usually only covered by photovoltaics. This has towards waste management plan has the advantage that it can be a module basis expanded (starting with small units, with correspondingly low initial investment), but was until 2007 in the 200 MW range at least twice (up to a maximum of 8 times) as expensive as solar chimney power plants.

Special

Waste management plan can also generate electric power at night, as the soil warms during the day. On the night he gives this thermal energy again and can continue to heat air under the collector. Because of the cooling ambient air at the same time still creates enough lift to operate the power plant. With appropriate soil conditions or special measures to increase the storage capacity of the soil ( eg black water tank ) the day - performance profile can be made more balanced: although the increased buffering of heat in the soil during periods of high noonday sunlight reduces the power maxima at high sun, but increases for the shares night performance at Back supplying this heat to the working air. Daily variation in the supply of electric power by waste management plan is always flatter and wider compared to the diurnal production curve of other solar power plants that follow the evolution of the instantaneous sun offer, so in general have a pronounced midday peak and marked change in performance at sunrise and sunset out ( see diagram " diurnal variation of power and energy from a solar chimney power plant and a photovoltaic system ").

At the pilot plant in Manzanares except the night performance also another peculiarity of waste management plan could be observed: significant performance increases when approaching a cold or rain fronts. Moreover, it succeeded in the test facility, by generating a pre-swirl in the canopy to bring the air in the tower to the tornado -like rotation. A continuation of the vortex over the spire addition to greater heights as you want to use in hypothetical air vortex power plants for energy production, but could not be observed.

The method of how solar energy can be made ​​affordable and competitive, is waste management plan the same basic idea as in the concentrating systems: one replaces expensive high -tech components for the conversion of sunlight (eg solar cells) by very large, but extremely inexpensive panels (glass mirror, gutters, greenhouse ) and combine this with a centrally placed converter ( Stirling or tower receiver, oily absorber tubes, tower). The most striking feature of waste management plan compared to other solar power plant types is the easy connection of the physical efficiency of the geometrical dimensions of the equipment over several orders of magnitude performance. The ratio of collector-to - cost converter accepts in principle with growing geometric dimensions of the infrastructure (and not by serial cheapening or future development progress ) and performs with increasing power to lower specific investment costs. With a few key equations, the relationship between efficiency and geometry can be shown and thus the " happy " Interaction of construction vehicles and construction costs are made ​​transparent in a very simple way.

Performance

Referenced to the solar irradiation on the collector overall efficiency of the achievable electrical power is with values ​​below 1% very low - compared with the technical elegance and the high efficiency of photovoltaic systems seems to be subject to the principle hopeless " solar chimney power plant ." The fact that the solar efficiency but need not be a measure of the profitability of a power plant, showing the hydro power plants: they work economically, and no one would think of this to doubt because of their low solar efficiency - and yet even in them the sun of the original engine on the way for electricity generation, namely the evaporation, cloud formation, winds to the "on" transport of water to the reservoir, with a vanishingly small efficiency on the primary solar energy required for this purpose.

Essential for the viability therefore are obviously the construction and operating costs in relation to income. The fact that in their gross, überschlägiger formulation related to the efficiency of waste management plan provides an interesting feature that distinguishes it from all other solar power plants, will be shown shortly:

The peak performance of a solar chimney power plant can be expressed as the product of machinery, tower and collector efficiency times the incident to the entire collector surface global radiation:

The tower efficiency is that of a Joule- process that describes the isobaric heating of the collector, on the basis of the outdoor temperature Ta, the adiabatic expansion of the tower and subsequent discharge of the heat to the atmosphere when the temperature level Tah in the height h of the tower. For adiabatic stratification of the ambient air

,

And it results in the conversion ratio of won (mechanical ) pressure energy to heat energy collected in the collector with

The conversion efficiency of heat into pressure energy is independent of the temperature deviation in the collector. A non - adiabatic stratification of the outside air can be described by an empirical, weather and location- dependent correction factor s, which can vary the tower efficiency by 1-3% depending on weather conditions and time of day. The use of the (absolute ) virtual temperature Tvakelv takes into account the water vapor content of the ambient air. The same result is also obtained if one calculates the total available pressure difference:

Pressure difference = difference in height x density differential ( outside - inside) x gravitational acceleration, - With change of sign, the same formula as for hydroelectric power plants. The multiplication of the removed pressure component with the mass flow rate results in the performance of both types of power plants. The peak power is then

And the abbreviation

Is the annual peak performance thus with c0 as standortabhängigem coefficient, with the values ​​from the table above for the location of Barstow in the Mojave Desert in California / USA, which includes the midday peak of solar radiation in the selected case ( see table below). The same - namely, the formulation of the electric power by only three factors - also applies to the average power if the appropriate radiation is used instead of mean value of the maximum. For the geometry of a system as described in the table below is obtained with c0 = 0.0052, a peak power of 200 MW. With the same c0, but the dimensions of the prototype with a tower height of 195 m and collector radius = 122 m yields a power of 47.3 kW, an excellent agreement with reality when one considers that the absolute maximum of power in Manzanares of 51, 7 kW was achieved only once during the multi-year project period.

By a simple equation with only three factors, one is not able to describe the dynamic behavior of a AWK eg to optimize a canopy - height profile to simulate the soil storage behavior or test out variants of the turbine area model. For such tasks detailed and specially developed for waste management plan calculation models shall be applied, go down in the all mentioned in the table above weather parameters with a temporal resolution of 10 min or less than specifications.

Economy

The above example of a 200 - MW plant has an overall solar efficiency of only 0.5 %. Looking at the efficiency chain more closely, it quickly becomes clear that only one of which can be improved to a greater extent: the collector efficiency - it is in the example, only 25.2 %, which corresponds to the average achieved in the pilot plant in about. The maximum technically achievable efficiency is flat, upward collectors ( as you do with standard (water) flat plate collectors ) by about 80 %, which can be achieved when the thermal surface losses are minimized by good insulation at the same time the excessively high temperatures ( eg by means of a small temperature lift the working air and vacuum insulation of the cover ). The total losses would then be reduced to the optical reflection losses.

However, at the current state of development such panels would be too expensive, and efficiency improvements would be the cost advantage of waste management plan divulge - this stands or falls with the low collector - in relation to the remaining costs. A factor of 3 in the technical development potential for collector efficiency improvements, coupled with corresponding increases in yield or / and area savings certainly provides enough incentive for further research and development work.

On the other side should also be ensured that the efficiency is also obtained in large plants of this size and does not decrease as less than 25%. This could occur, for example, by too slender towers at the same height of the tower and the same collector same power with a smaller tower radius could only be achieved over a larger temperature change due to the lower mass flow rate - but this causes higher losses, so the slender tower always has the smaller power. Another critical factor is the height profile of the greenhouse to the back center: a too low canopy leads at high air velocities to friction pressure losses. In each severity level of waste management plan so certain similarities in the geometric aspect ratios, material properties and construction methods must be observed that according to (p. 285 plants in the 100 MW range) a cost approach by both collector and tower costs as proportional to the respective surface justify. To determine the total cost per installed " peak" watts ( Wpeak ) you can do so as follows.

The total construction costs consist of the cost of machinery (turbines, gearboxes and generators), the tower cost and the collector cost plus an engineering component. Dividing this by the electrical peak power from Eq. (6 ), it can be very clearly read the special nature of waste management plan in this simple context, namely the fundamental dependence of the specific investment costs of the geometry of the systems. This relationship should be derived shortly.

With the proportionality of costs to the respective surfaces of the following applies: and

The machine costs are proportional to its installed capacity to ( well engineering costs ), the maximum annual power needs to be covered:

Or with insertion of values ​​from the table:

The summands of the specific investments show with increasing tower height and growing collector radius - and therefore increasing power - the following behavior:

• The specific equipment and engineering costs remain constant ( and at a low level compared with the other two components).

• The specific tower costs in the second summand is independent of the tower height and inversely proportional to the collector, ie they are at increasing performance of small and insignificant compared to the cost collector.

• The specific collector costs in the third summand also fall in principle with growing plant dimensions: they are inversely proportional to the height of the tower.

This behavior of waste management plan is not to be confused with a cheaper component by serial effects or by large-scale industrial production: this latter effect generally occurs in all types of power plants and in waste management plan can - eg the construction of several plants in nearby locations - additionally added. This showed up as a function of cost, performance and geometry also be valid irrespective that the specific investment costs of waste management plan are not a good measure for comparison of solar power plants with each other because of the flattened diurnal profile of performance.

With the location value in the table is given by these formulas, the annual peak power of 200 MW and the specific investment costs to 3.27 EUR / Wpeak or 3270 EUR / kW peak. The average capital cost per kW of installed capacity of solar power plants amounted to 4,000 EUR / kW peak, where waste management plan were not considered. After this comparison waste management plan are in this size cheaper to create than any other solar power plants, and is even more true when the peak power in the denominator in unit costs for the other power plants has a considerably larger proportion to the average daily or annual performance than waste management plan. Therefore, the generation cost is just at waste management plan used for the assessment of the economy, rather than the cost of capital. Here, the authors of the research report in 1985 called still a value to (converted ) 13 cents / kWh. The planners of the current projects (see below) in 2007 mentioned value is 8 cents / kWh for a 200 MW plant (p. 71). For comparison, the electricity production costs of concentrating systems ( trough power plants, power towers, and Stirling dish systems) with 15 cents / kWh to 23 cents / kWh, depending on the locations of high and lower sun exposure, the photovoltaic systems with 16-54 cents / kWh mentioned.

It is common practice that when considering the cost of research and development and the initial special design of project components ( the AWK Manzanares: turbine, gearbox and two generators, Tower of trapezoidal plates with supporting trusses, canopy with 4 different test covers) the exceed the cost of mass production far and let the projection into the " promised" profitable future always seem a little questionable. Not so with the solar chimney power plant: one is the achieved peak power of 51.7 kW in the above formula, we obtain with specific investment cost of 67,307 EUR / Wpeak a construction cost of EUR 3.2 million - so even considerably less than the construction costs of the prototype that are specified for tower and canopy with DM 3.7 million. This is because that the specific cost of a concrete tower, as it is planned for large systems, not with those of a corrugated metal tower are similar as in the pilot plant.

Neither the performance nor the specific investment costs by equations ( 6) and ( 10a) should be clearly identified, even if you can weather and location uncertainties into account. This is because it was assumed that a low, fixed easily realizable collector efficiency. The reality is that constructional variants, but also already own modifications to the system geometry effects on the collection efficiency of the order of ± 30 % may have. The actual engineering task as it almost always is to find the most economical compromise between " structurally feasible" and " physically meaningful ". In particular, increased friction losses in porch and tower upwind speeds around 15 m / s and higher simulated by calculation in detailed flow models need to be. How serious example can also be the feedback of a modified tower radius on the temperature change in the collector to its efficiency, one can even try for a simple, programmed in Pascal "coarse model " (excluding the plant dynamics, however ).

So solar chimney power plants appear to be a promising alternative for our solar power supply - as they function optimally according to the above considerations in low wind, but sunny regions, they are there thus a useful addition to wind turbines. Because of the simple principle is little technical uncertainties, they are of the order of weather uncertainty. Even the prototype in Manzanares has been certified by the technical consultants of the Ministry of Research of the KFA Jülich demonstration character who make more elaborate research projects unnecessarily.

The fact that since the Spanish pilot project no further AWK been completed yet, though again was to start up in the speech and is, reveals the weakness of the principle: the need for efficiency minimum size of the plants in the 100 MW range, associated with correspondingly high capital costs. The step from a 50 kW system to a 200 - MW power plant, without any intermediate steps, many investors appear dared the existing large-scale systems such as Andasol with 150 MW after completion have a long development time with a strong know-how growth look back and could be expanded in stages - and this itself is not at waste management plan.

Land use and environmental impact

Solar chimney power plants must have an appropriate magnitude for efficient operation in both the collector radius and in the chimney height. This has a large space consumption. Although the area under the canopy could in principle be used for further usages available, eg as the greenhouse in the classical sense for heat vulnerable plants. However Agrartechnische tests on the prototype showed a rapid withering of the test plants, with appropriate irrigation performance degradation were by evaporation or by reducing the transparency of roof condensation fitting result.

Against the fear that a large number of waste management plan could have consequences for the climate, says that the natural thermals, which is used here, an integral part of any weather developments and all climate processes is at the same location at the same rate, only shifted in time, even without collector and chimney occurs. The strong dependency on the local weather as well as on the geological characteristics of the site rather suggest a strong environmental influence on the system and its performance as a reaction in the other direction. This includes in particular the influence of - depending on location - possible sandstorms. The decrease in transparency through dusting has proven at the pilot plant as non-critical, but it would need a closed deposition of sand active or even automates be removed. How this could be accomplished, and what impact they have on the construction and maintenance costs, has not yet been studied in detail. Were also not yet been investigated possible causes and effects of a heaping occurrence of tornadoes across the collector roof, as observed on the prototype.

Pilot project Manzanares

39.042777777778 - 3.2533333333333Koordinaten: 39 ° 2 ' 34 "N, 3 ° 15' 12 " W

Half a century after Hanns Günther Jörg Schlaich future design developed from Stuttgart its solar chimney power plant and was commissioned by the German Federal Ministry of Research in Manzanares ( central Spain ) a first pilot plant with a peak output of 50 kW. The pilot plant in Manzanares had a collector radius of 122 m and a chimney height of 194.6 m, so that they reached an output of 50 kW.

In the spring of 1981, the work under the guidance of engineers Schlaich Partner, five technicians and ten Spanish laborers of the Munich-based company Maurer Söhne, experienced in the construction of thin-walled chimneys, were executed began. On eight inclined tubes, a ring was installed at 10 m height, on the floor, another ring, which could be pulled up via a hydraulic to the support ring. From 1.2 mm thick, trapezoidal sheets, a pipe were piecewise composed of 10 m in diameter on the lying on the floor lifting ring. The first eight-meter high piece of the chimney has now been lifted up on the lift ring to the support ring, then the next eight-meter high piece was composed. In this "eight -meter clock " of the 250 -tonne chimney was built at intervals of four meters reinforce outer rings of the tube. At some of the gains access 4 cm thick bracing with allowable carrying capacities of 50 t on, and stabilize the tower in a star shape in three directions. For cost reasons, massive DYWIDAG rods were held for wire ropes are used, as are known of bridge reinforcement. They were anchored in concrete foundations and at the bottom " 1/10-Punkt " with perpendicular to the attacking Störabspannungen for vibration damping provided ( Photo: silhouette with foundation and bracing against the night sky).

In the summer of 1981, the Assembly of the films which were strung two feet off the ground to support scaffolds with fields of 4 mx 6 m and 6 mx 6 m began in the middle of the fields, an approximately 60 -cm plastic dish as tensioners plate with drain hole for rainwater ( Photo: intact fields in the foreground). Half of the plant consists of 0.1 mm thick particularly resistant polyester foil with UV protection Kalle / Hoechst. The other half is made of 0.1 mm thick Tedlar fluorine-based with a lower mechanical strength. After initial storm damage to films ( photo) also stable, yet cost- glass greenhouse covers have been successfully tested. In late summer the turbines and machinery was installed by the company Balcke-Dürr/Ratingen. The wind turbine consists of four glass-fiber reinforced plastic sheets that are completed over a 1:10 gear to a generator to produce electricity. When the wind speed reached to 4 m / s, the wind turbine starts, during mains operation, the rotational speed to 150 rpm is kept constant. During lunch time, the speed increases to more than 20 m / s, but is slowed down by the turbine at 12 m / s. After sunset, the ground has stored under the roof film so much heat that the operation in the best case goes on all night.

The plant was in 1982. Between 1983 and 1986, a variety of experiments and improvements to the system have been performed. The plant was designed for an experimental period of three years and should then be removed without a trace again.

From mid- 1986 to early 1989, the plant was 32 ​​months almost trouble-free continuous operation. This gave the power plant in 8611 operating hours ( about 8.9 hours per day) stream. The availability was at this time about 95 percent. 1987 44.19 MWh of electricity were generated. This could also be the theoretical calculations which assumed a yield of 44.35 MWh, are fully confirmed.

Overall, the planned life of this updraft tower, originally three years has been far exceeded. At the same time so that the hitherto often doubted technical feasibility of such a system on a small scale could be detected. In 1989 the plant was taken several days of storm victims.

Current Projects

On 10.12.2010 was in Jinshawan, Inner Mongolia, China, taken in a 200kW Aufwindkaftwerk in operation. With the final stage to be reached in 2013 a total capacity of 27.5 MW. The project was supported by the Ministry for Research and Technology, Inner Mongolia and developed by the University of Science and Technology of Inner Mongolia and the Technical University of Madrid. The construction costs of $ 208 million are supported by a local company.

The first planned commercial power plant, the solar chimney power plant Buronga, wanted the company " EnviroMission Limited" in Australia, realize near Mildura from 2005. The fireplace should be 1000 m high, have a diameter of 130 m and a 38 -acre collector (7 km in diameter ) to be surrounded. The maximum power was planned with 200 MW. The start of the project should take place as early as 2005, but the operator could not secure the necessary funding for the construction until 2007. The realization of the project has thus become unlikely.

2007 announced the physicist Wolfgang Walter Stinnes to want to build his company " Green Tower " at Arandis in Namibia a plant with almost 38 km ² greenhouse area (7 km in diameter ) and a 1,500 m high tower. With 32 turbines and a nominal output of 400 MW of the total electricity demand of the country (excluding large industrial customers ) should be covered. The Ruhr -Universität Bochum was involved in the development. Information about a realization are not known.