New building typology for solar chimney electricity

Transcription

1 Wang B, Adolphe L, Cot LD (2014). New building typology for solar chimney electricity. In Cavallo R, Komossa S, Marzot N, Pont MB, Kuijper J (Eds), New urban configurations (pp ). IOS Press: Amsterdam. New building typology for solar chimney electricity Abstract Renewable and clean energy, such as solar and wind energy, is indispensable for our future. This article tries to develop the concept of solar chimney electricity within the angle of architecture design, and find some basic building types suitable for equipping this technology. By focusing on building-integrated solar chimney types and discussing the possibility of optimising the cost-efficient electricity production, it was determined that high-rise buildings would be the main study object. The solar chimney can be taken as a ventilation shaft, supporting column, or an organic part of the staircase or elevator shaft. Design principles are given and an example of imaginary application of this technology is discussed. 1. Introduction With the exhaustion of fossil resources and, especially, the increasingly severe global climate change, which is greatly influenced by the emission of CO 2, developing renewable and clean energy such as solar and wind energy is indispensable for our future. This investigation focuses on the interaction between urban morphology and the wind in order to benefit from wind energy. The CFD flow simulation in the urban context is considered and the impact of wind on different kinds of urban form is evaluated before a good position for installation and type of wind turbines are chosen. This paper tries to develop the concept of wind energy in architecture: solar chimney electricity (also called solar updraft tower power ). For the solar chimney (SC) research so far, most studies in the architecture domain are for interior ventilation and cooling systems, while the studies on the electricity generation generally appear in the area of mechanical engineering, of gigantic solar power plants that are often far away from both city and architecture. This article tries to integrate the fruits of both sides of the discussion and contribute to electricity generation in architecture by SC. In fact, as a thermo-siphoning air channel in which the principal driving mechanism of air flow is through thermal buoyancy (Zhai et al, 2011 p.3757), SC utilises solar radiation to heat the air inside the channel, causing an increase in air volume as well as a drop in air density, which in consequence pushes the air flow upward. Usually, there are three basic design elements for an SC for a building: a) a solar collector, which is often located in the top part or the shaft body of the chimney; b) a ventilation shaft, which can be vertical or inclined to its location; and c) inlet and outlet air apertures. The ventilation and cooling system is the main benefactor so far for SC's contribution to our green building environment. According to its working theory, SC is more favourable for the solar-intense and windless climate, but for the hot and humid region, stack ventilation is inefficient due to the small temperature difference between the inside and outside of naturally ventilated buildings (Yusoff et al p.2297). In this sense, that is why in many hot, dry areas, like the Middle East or Africa, people often integrate a solar-chimney wind-catcher with an underground cooling system.

2 On the other hand, if the speed of flow is relatively high while the scale gets enlarged, electricity can be generated by implanted turbines in SC. Due to the safe, stable feature of the air flow environment and the simple, green technology applied, it has been chosen as a means to generate green electricity. In 1903, Isidoro Cabanyes, a Spanish colonel, was the first to propose the use of an SC to generate electricity (Cabanyes, 1903 p.61-65). In 1926 Prof Bernard Dubos (engineer) proposed the construction of a long chimney attached to a high mountain and claimed that air flow speeds of 50 m/s can be reached (Rugescu, 2010 p.179). In 1982, a prototype SC electricity plant was set up in Manzanare (Spain), to verify the theoretical performance through field measurements (see Figure 1 the typical working sketch). However, because of the gigantic amount of space and the huge investment needed, SC power plant projects develop slowly and most still rest in the research and experimentation phase. Due to the development of technology and materials used, though, as well as the resolution of related economical social problems by the many projects analysed and some original ideas like a soft or a floating solar chimney (Castillo, 1984, Papageorgiou, 2004), the bright future of the SC plant shines ahead. At the end of 2009, Wuhai Jinsha in Inner Mongolia, China finished the first phase of a 200 kw SC experimental power plant, which by the end of 2013, after its third construction phase (25.1 MW), will be the first SC power plant in the world (Wang, 2011 p.98). Figure 1. Typical working sketch for a solar chimney power plant, modified from Zhou et al (2007 p.1638) 2. Integration of solar chimney in architecture There are many different ways to integrate an SC into a building with various technical and material choices. Nevertheless, according to its construction and function in a building, an SC can be implanted in 5 general positions as explained below. 2.1 Integration with wall In order to optimise the surface for heat exchange with the air and enhance the convection efficiency, a number of buildings take a glass-curtain wall as the solar collector for the SC. By far the most common application of this type is the well known Trombe wall composed of a glass cover, an air gap and a concrete wall (Zhai et al, 2011 p.3758). However, in case of heat loss in the winter or on cloudy days, and extra undesirable input in summer, it is proposed to

3 add an insulation layer inside, or have the house heated rather than ventilated, by closing or restricting the outlet of the cavity. In addition, if the outside wall is not designed to be covered by a glass curtain but just normal concrete or bricks, some certain measures like a solar-heated cap can be helpful to increase the efficiency of the SC. A wall SC can be favourable to low-rise and multi-story buildings, but for high-rise buildings, it s of little use because of the frequent air pressure caused by outside wind on the high-rise part of the building, which would be much bigger than the upward airflow from pressure generated by radiation. 2.2 Integration with pitched roof Compared with a wall-based solar collector, one on the roof is more advantageous because there is less shade and more flexibility in configuration. However, the area of the roof surface is often limited and for SC a certain stack height needs to be guaranteed. In this case, the orientation, the angle of pitch, the extended solar collector size and the economy aspect are to be considered. With the help of CFD simulation, for maximum flow an optimum slope-angle of 67.5 from the horizontal is obtained, while the performance of 45 or 90 is 10% lower and 60 is 30% lower (Harris and Helwig, 2007 p ). And it was found that the air flow rate per unit area decreased with increasing length of the roof SC (Hirunlabh et al p.390), so a roof with small units of divided SC is favourable for a determined roof height. Unlike low-rise and multi-story buildings where the space under a pitched roof can be used and roof design can be very free and flexible, high-rise buildings have less benefit from this kind of SC, because of their more complicated roof design, which often prioritises other equipments like air-conditioning or emergency evacuation. 2.3 Integration with atrium With important roles of central communication and public rest, an atrium often has access to natural light by covering with a glass ceiling or dressing with one or two transparent walls. And for an open courtyard of some ancient low-story or multi-story building a solar atrium can be realised if a glass roof is added. In this case, the atrium is like a fat SC when exhausts are positioned at the top (while the suction of air is automatically from the interior space around the atrium). Usually, as the activities inside the building produce heat, the atrium SC still works even if it s cloudy or cold. Nevertheless, for its human use aspect, it differs from the normal narrow SC: an atrium with relatively low height generally needs some shading measures like plants or moving shutters on the roof, in order to prevent overheating in the summer. For low-rise buildings this is of little use because of their low heights, but for multi-story and high-rise buildings this is often used and is well known for its natural ventilation function. However, for normal ventilation, atrium SCs in high-rise buildings are often divided into several small parts, in consideration of fire-fighting and discomfort from the airflow for people inside, which makes it difficult to obtain electricity from the SC. 2.4 Integration with staircase In a form of stack tower, the staircase of a multi-story or high-rise building can be developed into a SC. Aside from open-air stairs, the vertical air flow in a normal staircase can

4 be clearly felt near the top floor if the bottom is open. Like atria, staircases can also absorb heat from the human activities inside the building to help pump the air out, and if the staircase is covered by glass above or on one or more sides, the pump effect is much more powerful. In this sense, a staircase for SC doesn t require more than covering its sunny faces with pervious materials like glass to keep it like a greenhouse, or that can absorb solar radiation well, like dark concrete. And the more stack height it has and the more radiation it gets, the better the performance of the staircase SC. Of course, closed escape staircases in high-rise buildings normally have a pretty attractive height that can serve well for natural ventilation, but in consideration of the huge fire risk, they are often cut down into several parts and this makes it hardly worth adding the lower inlet and top outlet openings. Therefore, the staircase SC concept can only be attractive for multi-story buildings where there is enough stack height and no strict limitation for fire precautions, even though there should be some control systems for comfort and fire emergencies. 2.5 Integration with shaft or column As mentioned before, for high-rise buildings, the staircase cannot be taken as a ventilation passage, but the shaft can. In fact, some columns are cavitated and taken as a shaft for rain drainage pipes, for example. The SC concept can be used here again if there is a well designed air inlet and outlet, as well as a radiation-collector cap applied to promote thermal air float. Of course, to adjust the consideration from ventilation efficiency to implanting a turbine electricity-generator, the shaft or the column (here it s better to be identified as supporting structure) needs careful design to be well integrated in the architecture. Take an elevator shaft for instance; the SC concept could be explored and expressed in this good carrier when there is a set of elevators gathered together, where the connected vertical cavity space could be abundant and this layout is very common for high-rise buildings, like the Figure 2 showed the core structure plan of Shanghai Jinmao Tower. As a matter of fact, the floating air flow in the shaft can both be energy-economised for elevator cabins going up and going down. Besides this, for high-rise buildings, apart from the elevator shaft there are many other sorts of shafts for utilities, like water supply and drainage pipes, electricity and electronic wires, smoke extraction and fresh air supply, etc. Among them some of the bigger ones can be used and connected by horizontal pipes to the elevator shaft to enlarge the chimney section area. Of course, in consideration of fire emergencies, safe (emergency) elevator shafts, like escape staircases should be independent and cannot be used for SC as it can promote the fire by accelerating the airflow rather than stopping it.

5 Figure 2. Jinmao Tower standard floor plan 3. Solar chimney electricity in architecture As mentioned before, the SC performance grows as the stack height increases and the radiation collector area expands. Therefore, the thought to integrate SC electricity into cities, especially within high-rise buildings or other towers for TV antennas or tourism would be favourable. Generally, for an SC tower in a city, it could be the elevator shaft or smoke-exhaust shaft inside the high-rise buildings, which are indeed often suffering a high expense of mechanical ventilation, while the extra charge for an enlarged shaft for SC electricity will be offset by the natural ventilation and electricity generated. As for a radiation collector, it could be the roof of a big surface, like a supermarket, commercial centre, cinema, theatre or gymnasium, or, if the project was well designed and feasible, a vast parking lot or a square can be roofed, or some big city glass-house garden (or park) could be another choice for the radiation collector. 3.1 Design principles To make SC electricity more practical and effective, some design principles could be drawn as follows. 1) Favourable site condition. A favourable site for SC electricity in cities has two requirements: natural climatic condition and eco-social availability. Firstly, it should be hot and dry, and a competitive solar radiation of 1950 kwh/m²per year, according to Schlaich, who researches the profitability and operation capability of SC plants (Schlaich, 1995), needs to be guaranteed. Second, a practical high-rise building or tower and a vast roof around at its bottom should be foreseen, as well as the possibility to change some part, like adding a grass roof for the radiation collector or the redesign of the elevator shaft, to meet the condition of a well-rounded city SC power plant. 2) Big bottom & small mouth. To ensure enough air-flow inlet volume and solar radiation, the roof of the radiation collector should be as big as possible. For the turbine position, the mouth, it usually needs to be small

6 and narrow in order to install a set of practical turbines and at the same time to increase the wind velocity, which is more sensitive than other aspects for the productivity of the turbine. 3) Practical detailed design. To ensure maximum productivity, several aspects need to be considered carefully. To improve the flow velocity, for example, vent-cap baffle systems need to be well designed to use the negative pressure draw rather than suffering the turbulence from seasonal winds; smooth-face concrete inside the shaft would be favourable for reducing the friction, and some softening measures would help reduce turbulence where the flow changes direction. For the performance of the turbine, aspects like capacity, expense, installation position, safety, vibration, noise and repairs, etc. must also be considered. While regarding to the promotion of natural ventilation of the interior, hollow concrete floor slabs instead of the vent pipe can be applied, which also help to enlarge the inlet for the SC. 3.2 Example analysis Figure 3 shows a case of SC design in a high-rise building. In this scheme, three types of solar chimney are applied: outside-wall SC, shaft SC and atrium SC. The outside-wall SC here, which benefits from the neighbouring staircase as an additional radiation collector and chimney, can run much better than the separated wall SC. Also, to make the SC function more effectively, every atrium space for different floors is connected like a whole, so all the small atria can better benefit from the SC ventilation effect. For the two shaft SCs, the set of elevator shafts are the main chimney, with some other small facility shafts connected for greater flow, which acts as a smoke engine to improve the ventilation of the interior offices and the whole underground garage, but also to drive the set of turbines to generate electricity. In addition, apart from the big roof of the skirt-building, which connects to the SC shaft by hollow floor slabs, some solar vent-caps are applied as additional radiation collectors for the shaft SC and atrium SC. Furthermore, because the seasonal wind direction is southeast, the southeast side of the cap needs a wind-baffle system to reduce the counteraction to the upward airflow. Figure 3. R&D office building, Wuhan Solar-valley Software Park

7 4. Conclusion and perspective Facing the problems of energy exhaust and environmental pollution, SC electricity in architecture would be a favourable choice, even though some gigantic SC power plant projects have been well studied and have their own advantages. In this paper, five typical SC types in architecture are discussed and the high-rise building shaft SC is most recommended for generating electricity. Some design principles for SC electricity in architecture are also given to encourage better application of this technology. This paper is a first step toward using wind energy in cities, like applying micro wind turbines on roofs or in tunnels underground. In the future, practical technique problems of wind energy in buildings will be considered, and the modelling of flow in various urban scale contexts will be studied. Bibliography CABANYES, I Proyecto de motor solar. La Energia Eléctrica - Revista General de Electricidad y sus Aplicaciones, 8, CASTILLO, M. A A New Solar Chimney Design to Harness Energy from the Atmosphere, in: NAGAI, M. & HEINIGER, A. E. J. (ed.) Spirit of Enterprise - The 1984 Rolex Awards. Switzerland: Aurum. HARRIS, D.J. & HELWIG, N Solar chimney and building ventilation. Applied Energy. 84, HIRUNLABH, J., WACHIRAPUWADON, S., PRATINTHONG, N. & KHEDAN, J New configurations of a roof solar collector maximizing natural ventilation. Building and Environment, 36(3), PAPAGEORGIOU, C. D Solar Turbine Power Stations with Floating Solar Chimneys. In: BOURKAS, P. D. & HALARIS, P. (ed.), Proceedings of Power and Energy Systems, EuroPES Rhodes: IASTED. RUGESCU, R. D. (ed.) Solar Energy, Croatia: InTech. SCHLAICH, J. ROBINSON, M. (trans.) The solar chimney Electricity from the sun. Stuttgart: Edition Axel Menges. WANG. W. R Enterprise leads the innovation of low-carbon, China Investment (Chinese), 01, YUSOFF, W. F. M., SALLEH, E., ADAM, N. M., SAPIAN, A. R., & SULAIMAN, M. Y Enhancement of stack ventilation in hot and humid climate using a combination of roof solar collector and vertical stack. Building and Environment, 45, ZHAI, X. Q., SONG, Z. P. & WANG, R. Z A review for the applications of solar chimneys in buildings. Renewable and Sustainable Energy Reviews, 15, ZHOU, X. P., YANG, J. K., XIAO, B., HOU, G. H Simulation of a pilot solar chimney thermal power generating equipment, Renewable Energy, 32,