CIBSE Case Study: Sustainable Air-conditioned Greenhouse Dome
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Article from the August 2012 edition of the CIBSE Journal written by Andy Pearson.
Design two giant greenhouses for Singapore and then try to cool them sustainably. That was the challenge facing engineers on the Garden by the Bay project. Andy Pearson reports.
If there were a prize for the least sustainable building project, then an air conditioned, giant greenhouse dome in Singapore would be a contender, you would think. But think again. Two such huge "biomes" that have been constructed as part of the city state's Gardens by the Bay development, may have strong claims to being very eco-friendly.
The Gardens by the Bay comprise three public gardens being created on reclaimed land at the mouth of the river in the south of Singapore. The 52 hectare Bay South scheme includes landscaped gardens, a function room, concert arena, shops and the two giant 20,000 sq m, scallop-shaped biomes that each has its own particular microclimate. The biomes have been christened the "Flower Dome" and "Cloud Forest".
The design team for the £400m Bay South project faced an unusual and exacting sustainability challenge in such a hot and humid climate: to air condition the biomes so that their projected carbon emissions would be no worse than those of a modern Singapore office in November.
The Bay South domes, which opened to the public on 29 June, are air conditioned by circulating about 10,000 litres of liquid desiccant, burning 17 lorry loads of forestry waste a day, and cooling thousands of square metres of pathways and pavements.
Gardens in the tropics are noted more for their luscious foliage than for their vibrant fl owers, so the Flower Dome has been designed to recreate the conditions of a Mediterranean spring climate. The Cloud Forest dome recreates the conditions of...well, what else but a cloud forest?
Conditions within the Flower Dome have to be maintained at 25C and 60% relative humidity (RH) during the day, and dropped to 16C and 80% RH at night. The Cloud Forest dome will be kept at a temperature of 25C, 80% or higher RH, during the day; dropped to 17C, 80% RH at night. Theproject team's task was to recreate these cool environments beneath the domes' glass skins to enable the alien blooms to flourish at a latitude just 1.4 deg north of the equator, where the temperature is a constant 24C to 30C for 95% of the year.
The challenge of cooling the domes is made all the more difficult by the need to simulate seasonal fluctuations in air temperature. This is achieved by dropping the temperature at night in the Flower Dome by an additional 4C and in the Cloud Forest dome by an additional 1C. This temperature reduction takes place every night of every third month to simulate an end-of-winter condition to encourage the plants to bloom. "The system has to be capable of going down to these very low temperatures at night because we cannot fight the solar gain during the day," says Atelier Ten's Patrick Bellew, a member of the design team.
The first task facing the design team in developing a solution was to establish the relationship between the horticultural lighting requirements, solar gain and cooling load. The brief from the client, the National Parks Board of Singapore, was to achieve a level of 45,000 lux for the same number of hours as the domes at Cornwall's Eden Project, where similar plant species had flourished. "The real challenge of this building is trying to balance the light levels and heat with the comfort of the visitors," says Bellew.
Achieving optimum daylight levels while minimising the heat gains for the glazed structures required extensive daylight modelling. The domes were positioned adjacent to the river estuary to avoid being shaded by the tall buildings planned for the surrounding area.
However, in addition to buildings, daylight analysis of the initial designs highlighted a problem of shadows cast in the mornings and afternoons by the proposed structural solution. Conversely the structure was also found to provide insufficient shading from the intense midday sun. "Being close to the equator means direct solar radiation is intense when the sky is clear. However, Singapore can also be quite cloudy for long periods and the luminance levels under these conditions can be lower than in a Mediterranean summer," says Bellew.
As a result of the modelling, the initial structural solution of a fin-shaped truss gave way to a self-supporting structural gridshell with additional lateral stability provided externally by a series of giant steel arches. The advantage of this solution is that the elements of the grid shell are relatively slender to enable sufficient daylight to reach the plants. "It gave us the best overall transparency to daylight so when there is no sunlight we still get very high light levels," Bellew says.
Glazing selection was also critical to the scheme's success. It needed to have a high degree of transparency to meet the target daylight requirements for times of high cloud cover but it also needed to be able to filter out the infrared frequencies to minimise heat gains from the intense tropical sun on cloud-free days.
"When the sun does come out it is 1,100 W/sq m, so it is pretty toasty," says Bellew. The team modelled a variety of options including ETFE pillows and single glazing, but these were unable to filter out sufficient infrared. In addition, the single glazing would also have been prone to condensation when not in direct sunlight.
Double-glazing with a low-e coating applied to the inner face of the units' outer pane was found to provide the best solution. It allows 65% of daylight frequencies to pass through with only 35% of solar heat transferred. "We wanted sufficient light but no heat" explains Bellew.
In addition, a series of 7m x 10m retractable, cable-tensioned, triangular blinds have been incorporated into the supporting structure outside the domes for use on sunny days. The blinds also provide added resilience in case of system failure.
Once the domes had been designed, the next task was to find a way of dealing with the heat loads. The design team's goal was to make the domes' servicing invisible to avoid the flower displays being surrounded by the clutter of louvres, chillers and air handling units.
For the Flower Dome, Atelier Ten opted for a displacement ventilation system combined with a network of chilled water pipework embedded in the concrete screeds of the paths and circulation areas. The displacement ventilation system is the primary source of cooling. Conditioned air is supplied at low level within the occupied zone. This solution limits the volume of air needed to cool the large glazed space and to keep the visitors comfortable. Even so, 110cu m/s of conditioned air is supplied through ducts which measure up to 5m x 5 m, which have cleverly been incorporated into the landscaping. The chilled air is supplied at a temperature of 18° C, 80% RH, (or dryer) through grilles concealed in the edges of flower beds and through diffuser bins concealed within the planting.
The displacement air helps to pressurise the dome to prevent infiltration. As the air heats up it rises to the top of the dome where a proportion is allowed to disperse through openable vents. The remainder of the air is extracted, mixed with up to 20% fresh air and recirculated, or used to regenerate the desiccant that forms part of the cooling system.
In addition to the air system, the dome is cooled by chilled water pipework cast into the hard landscaping and pathways at a temperature just above the dew point of the air within the space. While the vegetation removes solar radiation by converting the heat into chemical energy, this simple solution works by absorbing the solar radiation as soon as it strikes the ground, and before it has had the chance to re-radiate as heat into the air.
"The chilled floors are about preventing heat gain," says Bellew. As a result displacement ventilation is kept to a minimum while the chilled surfaces of the paths help to keep the dome's 2,000 visitors an hour comfortable.
The environmental solution in the Cloud Forest dome is similar to that of the Flower Dome, with a displacement ventilation system and chilled pathways but, in addition, this dome has jet diffusers and evaporative misters to increase humidity and air movement. The climate will simulate a mountain cloud forest - a fact emphasised by the large man-made mountain contained within the space. This 40m high construction is home to the world's highest indoor waterfall and a series of exhibition spaces cast into its slopes and aerial walkways to take visitors through the tree tops.
This dome was originally intended to be cooled using displacement ventilation alone. However, computational fluid dynamics analysis showed that warm air would have collected near the top of the mountain, making it too hot for the plants and too uncomfortable for visitors. As a result displacement diffusers at the mountain's base and peak have been enhanced with the addition of jet diffusers concealed in the mountain's slopes to deliver 110 cu m/s of conditioned air. The jet diffusers blast cooled air into the dome, mixing the air and preventing it from stratifying over the height of the mountain.
Above the mountain, stratification is allowed and this is where the extract air intake has been located. The removed air can either be cooled and recirculated or used to regenerate the desiccant.
Evaporative misters, mounted on the underside of suspended walkways, will add a fine spray of water droplets to the dome's air. This will increase humidity within the space; the droplets will also absorb heat as they evaporate, reducing the cooling load.
The 8,000 kW of cooling needed to air condition both domes is supplied from plant hidden from view in the adjacent five-storey plant room, which is located beneath a hill south of the domes.
In keeping with the garden theme the primary source of energy for the cooling system is waste wood. The client, the National Parks Board, is responsible for about three million trees, which generate about 5,000 tonnes of hardwood waste a month. Instead of being dumped in landfill, the forestry residue is chipped and mixed with dry wood from waste shipping containers from the nearby port. It is then burnt in a 30 m long, 16 m high 7.2 MW biomass boiler, situated in the site's energy centre, to create superheated steam. This is used to drive a turbine generating 1.2 MW of electricity to power the four centrifugal chillers that cool the domes' supply air, and to meet part of the site's power requirements.
"The domes are effectively carbon neutral for cooling, if you ignore the carbon transporting the materials to site," says Bellew. The resulting ash is split into two steams: fine ash, which is high in nitrates, is mixed with the park's vegetation waste to make fertiliser; while the larger ash particles are taken off-site for use in concrete manufacture.
The chillers are able to draw electricity from the grid when the generator is not working. As a secondary backup, the domes are also connected to the Singapore district cooling network. In addition to driving the electric chillers, the high temperature hot water downstream of the steam turbine also drives two absorption chillers.
"Because the biomass boiler and steam turbine are not easy to modulate, the absorption chillers serve as a heat dump to help stabilise the system, in addition to providing the base cooling load," says Bellew.
The combined outputs of the chillers meet the domes' sensible cooling requirements. The chillers are connected to a variable temperature chilled water circuit, which supplies the air cooling coils and floor cooling loads. The system's elevated evaporator temperatures ensure significant energy savings are achieved by increasing the chillers' efficiency. Cooling towers situated on the upper level of the energy centre reject surplus heat from the circuit.
In addition to the absorption chillers, heat from the biomass boiler is also used to regenerate a liquid desiccant circuit. The desiccant removes moisture from the fresh air supply to the domes, which means it requires less energy to cool it. The fresh air passes through the desiccant and is mixed with the return air before passing over the cooling coils to lower its temperature before it is supplied to the domes. Water removed in regenerating the desiccant is exhausted to atmosphere through a flue concealed in the trunk of one of the scheme's giant, manmade "supertrees".
The supertrees, which are up to 50 m high, are a feature of the landscape. There are 17 of them located in three clusters on the site. Their trunks are actually formed from steel filigree surrounding a hollow concrete core; the metal lattice acts as a supporting frame for vegetation to climb up. The trees are topped by steel branches, which, in seven of the trees, support photovoltaic panels to generate additional power for the site. In addition to concealing the exhaust from the desiccant regeneration, the trunk of another tree conceals the main boiler flue. Two supertrees even contain lifts to carry visitors up to an aerial walkway, while another houses a treetop cafe.
Amazingly, given that this is a scheme to air condition two giant greenhouses, it is aiming for Platinum accreditation under the Singapore Building and Construction Authority's Green Mark scheme, the country's equivalent to LEED and BREEAM.
Can constructing two domes on the equator, and then modifying the climate within, ever be described as a sustainable proposition? "Perhaps not," says Patrick Bellew, "but given that the scheme was going to be built in any case, Atelier Ten has succeeded in developing a solution with a positive outcome."
He concludes: "We've stretched every sinew to make the most of the resources at our disposal from the local climate and environment, and endeavoured to identify virtuous cycles where the project can be beneficial to the local environment."
How the liquid desiccant works
More than 10,000 litres of liquid desiccant are used to dehumidify the air for the giant biomes at Bay South. Liquid desiccant, rather than solid desiccant, was used on this project because it allowed the supply and exhaust air ducts to be located in different parts of the biome.
A highly concentrated solution of lithium chloride dissolved in water is sprayed into the stream of fresh air. "Desiccant helps strip the moisture down to 30% relative humidity," says Atelier's Patrick Bellew.
As the air passes through the liquid curtain, the desiccant removes moisture without altering the air's enthalpy. As it dries the air, the concentrated desiccant solution absorbs moisture, diluting it and increasing its volume so that more of the solution leaves the airstream than is sprayed into it.
The weak solution is regenerated by boiling off the excess moisture using waste heat before it is returned to the system.
The drying process slightly increases the temperature of the airstream, which then has to be cooled to the supply condition by passing it over a conventional cooling coil. According to Bellew, this is a much more energy efficient solution than the conventional one of removing moisture from the air by passing it over a cooling coil - which has to cool it more than is necessary to remove the moisture - and then reheating the air to supply condition.
Because desiccants store energy in the form of latent heat of vaporisation of water, rather than the specific heat of water, the concentrated desiccant solution in the system's giant buffer tank stores about 10 times more energy than in the equivalent volume of chilled water. The stored desiccant will help balance supply and demand loads.
Client: National Parks Board of Singapore
Landscape architect: Grant Associates
Sustainability and building services: Atelier Ten
Local M&E engineer: CPG Corporation
Structural engineer: Atelier One
Architect: Wilkinson Eyre