By: Ilana Koegelenberg – assistant editor*
With sea water heat exchange and an underfloor displacement ventilation (UDV) system, the No 1 Silo building is on course to make history with a prestigious 6-star rating and an innovation credit from the Green Building Council.
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| The Silo No.1 building at the V&A Waterfront in Cape Town. |
The No 1 Silo building at the Victoria & Albert waterfront in Cape Town is specifically designed as the corporate head office of Allan Gray. But a long-term tenant that values staff wellness, put a high priority on whole-of-life costs and environmental responsibility – and gave the design of the HVAC system an importance beyond what is normally found in an A-grade office.
Design specifications
At the beginning of the project the V&A, Allan Gray and the core design team visited Australia to learn first hand from the experiences of building owners, occupants and facilities managers owning, living and working in green buildings. Australia has its own operational energy benchmarking tool: the National Australian Built Environment Rating System (NABERS), which is widely adopted and was recently legislated as mandatory for all office buildings being sold or leased. This has put special emphasis on energy efficiency and specifically energy-efficient HVAC design for the base buildings in Australia. The two systems that achieved high NABERS ratings were passive chilled beams and underfloor displacement ventilation (UDV) systems.
Returning to South Africa the team investigated the merits of these two systems, in conjunction with a more conventional variable air volume (VAV) system. The client and the design team decided on an UDV system mainly due to the ease and flexibility of electrical distribution the raised floor system offers. The cost of the raised floor was almost exclusively offset against the reduced cost and environmental impact of churn throughout the life of the building. The increased health benefits of a UDV system was a close second, whereas the energy consumption is similar to that of a chilled beam system and about 10% more efficient than a traditional VAV system.
In this UDV system air is supplied to a floor void from air-handling units on the roof. Air is let into the space at low velocity via grilles placed in the floor. The temperature of the air is a few degrees below room temperature. As it picks up heat from occupants and equipment it rises, taking pollutants with it to a high level where it is extracted back to the air-handling unit on the roof.
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| The sea water plant room. |
An UDV system not only controls pollutant levels, but also reduces the risk of cross-contamination over a conventional mixing system by providing low-velocity vertical laminar flow from bottom to top, thus not mixing air from one work space to another.
In the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA)’s published book, Displacement ventilation in non-industrial premises it states that ‘the contamination concentration is always better in the occupied zone in a displacement-ventilation room than in a room ventilated by mixing ventilation’. According to REHVA the air change effectiveness in a displacement ventilation system is mostly higher (60%-70%) than a mixing ventilation system (50%).
Another innovative feature of the building is the sea water heat exchange replacing the need for cooling towers. This system comprises of three fully independent titanium plate heat exchangers (each with dedicated supply and return sea water pipework, sea water pumps and filters) operating in a duty/duty/ standby configuration to provide full N+1 redundancy. Condensed water from the main building chillers pass through these plate heat exchangers, at which point it gets cooled by heat transfer between condenser water and sea water. All the sea water pipework are installed in medium or high-density polyethylene (MDPE and HDPE) and no insulation is required.
The nature and magnitude of the environmental benefits are:
• Chillers last 5-10 years longer due to full control over the condenser water quality
• The use of domestic water (in cooling towers) is avoided
• Increased chiller coefficient of performance (COP) from five (air-cooled chillers) to eight, therefore the chiller electrical consumption is almost halved due to a decrease in the condenser water temperatures
• The elimination of the risk of legionella.
With sea water temperatures in Cape Town harbour ranging from 12°C-16°C it is expected that the chiller will regularly work at a COP of eight.
The V&A already have two sea water systems in operation which enabled feedback on the operations and maintenance regime for a feasibility study. Arup also undertook a full environmental study to prove to the necessary authorities that the design would bear no negative impact on the harbour and its marine life.
Challenges
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| The dust house. |
The development also incorporates the tenant’s data centre for which the requirement was to achieve a Tier 3 rating. However, to meet the client’s and tenant’s aspirations on energy reduction, it was impossible to use dual independent air-cooled chillers to achieve the rating. The design thus provides chilled water to the data centre from the highly efficient base building chilled-water system for normal day-to-day operation, with a single air-cooled chiller, dedicated to the data centre only, providing the necessary backup. Integrating the two systems proved to be a challenge due to their differing operating characteristics.
Budget
The capital cost of the system is higher than a conventional system specifically around the sea water heat exchange. The team went through extensive value engineering exercises during design development and detailed design to reduce the capital cost. They took a balanced approach to ensure that they still meet the client brief, don’t compromise any whole-of-life benefits and provide a value for money system.
Product choices
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| The Silo No.1 building is still under construction. |
In the original scheme the building was served by independent chillers and heat pumps (for respectively cooling and heating). As part of the value engineering process the team finally incorporated the use of fully reversible heat pumps to meet both heating and cooling requirements. The heat pumps were also selected on their capability to operate at significantly reduced condenser water temperatures (from traditional cooling towers) to make best use of the sea water heat exchange system. Three units are used, any two of which meet the building cooling load, with the third unit doing double duty: meeting the building heating load in day-to-day operation, and providing back-up to the two units in chilling mode in the event of a failure.
The challenge incorporating the reversible heat pumps was that the change-over had to be accommodated within the chilled water, heating water, and sea water heat exchange pipework connections to the heat pump: in heating mode the evaporator bundle is connected to the sea water, and in chilling mode the condenser bundle is connected to the sea water. This means that all three systems are interlinked (via the change-over valves) and, therefore, need to operate at the same pressures and with the same chemical dosing regimes.
The offices are provided with air conditioned air via a pressurised floor void. This allows for stratification of the air within the occupied zone and enhanced ventilation effectiveness. An additional benefit is that the supply air temperature is elevated (above the industry standard) at about 18°C, meaning that the periods when the AHU economiser cycle may be used is greatly extended with significant energy cost savings.
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| The air-handling plant room. |
With very few exceptions all pumps and fans are on variable speed drives, with the unit speed adjusting to meet the imposed load. This provides significant electrical energy savings. The only situation where constant speed pumps are used, is where the pump is matched to a plant item with a constant flow requirement – for example the chiller primary pumps. Constant speed fans are provided where code requirements stipulate air flow rates.
The reversible heat pumps each have dual independent refrigerant circuits (with dual power supplies) to provide additional redundancy, but this also allows the unit to turn-down significantly to meet the imposed load without the unit having to cycle on and off.
The use of evaporative cooling was reviewed, but, with the benefits of the sea water heat exchange system and the elevated supply air temperature, was not viable.
Impact on electrical use
As part of the design process and Green Star certification, a full energy model was carried out. Various complexities of models were used from the conceptual stage to test alternatives and make informed design decisions. From the double skin façade performance through to chiller selection was tested. The final model shows that the building has the potential to consume 60% less energy than the base case SANS 10400(XA) compliant building. The HVAC system will contribute about 45% of the building’s final energy consumption. It will be tuned by the contractor and the relevant design team members through the first year of operation with re-commissioning after a year.
A pleasant surprise is the effectiveness of the CO monitoring system controlling the basement ventilation system. The design team was aware of the positive energy data collected from an Arup-designed Green Star building in Sydney, but it wasn’t until they undertook a detailed investigation that they realised the significance.
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| The floor was raised to accommodate the under- floor displacement ventilation system. |
The system consists of two separate types of fans. The first is the main extract fans that exhaust the air from the basement. The second is the impulse fans that move air from the interior of the car park to the exhaust grilles on the perimeter. The main extract fans are fitted with VSD drives and the schedule, as per the CIR, is used to turn down the fan speed to achieve the required air flow. The impulse fans are also controlled by CO censors, but they switch on or off based on the censors. This system will results in energy consumption of less than 10% compared to a conventional fix airflow basement extracts system.
Green innovation
Both the UDV system and the sea water heat exchange system received an innovation credit from the Green Building Council for being one of the first in South Africa – and this contributed to the building achieving a 6-star Green Star Office Design rating.
Compliance
Due to the building’s double skin façade and efficient HVAC system the building well exceeds the new SANS10400(XA) energy-efficiency codes. The fresh air rate for the building also exceeds the latest codes by 75%. The Environmental Impact Assessment that was done for the sea water system did, however, impose strict design criteria: limiting the overall heat transfer; the sea-water discharge temperature rise; imposing strict inlet and discharge velocities; dictating the physical location of the inlets and discharges; and prohibiting materials (for example uPVC) and dosing chemicals (for the control of marine growth).
*With inputs by Nic Smith, associate mechanical services and Jaco Kemp, associate and sustainable building specialist at Arup on behalf of the V&A Waterfront.
