INTERNATIONAL CASE STUDY BIOMES COOLING
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
Seventeen ‘supertrees’ are arranged in three clusters on the site
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.
Chilled solution 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
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 26 CIBSE Journal August 2012
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.
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, man- made ‘supertrees’.
Green giants 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 café. 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.’CJ
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