VODAFONE – 116 m² EVACUATED TUBE | Solar Cooling System – REACH Renewable
SHORT DESCRIPTION OF THE CARBON NEUTRAL BUILDING SYSTEM
On July24, 2011, Vodacom unveiled its Vodafone Site Solutions Innovation Centre, the first-ever building to be awarded 6-stars by the Green Building Council of South Africa. Located at Vodacom’s Midrand head office, Vodafone Site Solutions Innovation Centre (SSIC).
This is a carbon neutral building that will house a team of 12 experts who will look at technological ways to reduce the company’s footprint across the globe and reduce the cost of building and maintaining cellular networks.
The centre aims to speed up the development of the Vodafone Group’s sustainability goals: to reduce global CO2 emissions by 50% by 2020and to achieve a 20% carbon intensity reduction target for emerging markets by March 2015.
Vodafone chose to create this Innovation Centre in South Africa because it is, in many respects, both an emerging and developed economy. This particular location would help the group achieve its targets in a more consolidated and focused manner.
Solar Thermal design elements making the SSIC so green include a highly efficient solar-powered air conditioning, heating and ventilation system that delivers 2 500 l/s of fresh air to the office during operation, and 1250 l/s of heated fresh air in heating mode.
PRINCIPLE AND SYSTEM CHARACTERISTICS
The chiller uses lithium bromide as the refrigerant, which has both zero ozone depleting potential (ODP) and zero greenhouse warming potential. All thermal insulations used also have no ODP.
The integrated HVAC system, developed by WSP in partnership with Voltas Technologies, uses solar power in combination with a Yazaki absorption chiller as the sole source of both cooling and heating for the entire Vodafone carbon neutral building. “Titanium-coated evacuated tube solar collectors transfer the energy of sunlight into hot water, which we store in a stratification tank around 95 ‘C..In summer this hot water is used as a primary source of energy to power the absorption chiller, while in winter, the hot water is used directly for space heating.
The energy required to ‘fire’ the 35 kW chiller at Vodaworld’s SSIC is collected by 58 vacuum tube U-pipe collectors mounted flat on the building’s roof. U-pipe collectors do not need to be mounted at an angle. The liquid being heated is pumped through each tube, so it does not rely on convection to transfer the heat. The result is a reduction of material, weight and wind load onto the roof and therefore structural savings that further benefit the green star rating.
Off all the different solar collection options, this technology also represents the highest efficiency for applications where high temperature water is required. Absorption chillers from Yazaki operate of hot water at temperature of around 88’C and the U-pipe technology perfectly matches this requirement. TO prevent the water from boiling, though, the whole system operates under pressure.
Titanium coated vacuum tubes were chosen over conventional linings. This maintains the efficiency of the vacuum tube and significantly increases the useful life of the collector. With collector temperatures frequently exceeding 110 ‘C during normal operation, copper coatings tend to flake off more quickly due to the stresses induced by the day-time/night-time temperature variations.
Because the system is not backed up in any way by a conventional powered electrical system, storage is critical. A specially designed and insulated 7,000 Litre hot water storage tank, locally manufactured by Tactical Fluids, allows for one to two days of energy storage when direct sunlight is limited. The tank is separated into two chambers, with insulation between the two. In summer the top chamber is used as a buffer for the cooling process. Only during times of excess solar power is the bottom half enabled. During winter time the whole tank is utilised for storage to increase the amount of available energy.
In cooling mode the Yazaki absorption chiller is the heart of the system. These chillers can be efficiently energised by hot water heated by any industrial process; a co-generation system, solar collectors or any other heat source. A solution of Lithium bromide and water is used under vacuum as the working fluid. A diluted solution of lithium bromide is first boiled to separate it into water vapour (the refrigerant) and concentrated lithium bromide. The water vapour is then cooled, condensed and passed through an orifice into a deep vacuum chamber where it evaporates over the chilled water coils, removing the heat.
The deep vacuum is maintained by the affinity of the concentrated lithium bromide solution for the refrigerant water vapour. The evaporated water is absorbed by the concentrated lithium bromide solution flowing across the surface of a cooling water coil in an absorber. Heat of condensation and dilution are removed by the cooling water and rejected to an external cooling system. The resulting dilute lithium bromide solution is then preheated into a heat exchanger before returning to the generator where the cycle is repeated. Electrical power is only needed for the small pump that circulates the diluted lithium bromide back to the generator.
Another true innovation of this system lies under the buildings foundations. A rock store under the carbon neutral building is used to pre-cool air. This air is then used as the cooling source for the absorption chiller’s external dry cooling system and the pre-cool the supply air entering the air handling unit.
This means that the chiller plant at Vodafone SSIC does not require a cooling tower, which would have been a net water consumer. So as well as being energy efficient, no water is consumed by the HVAC system, which is critical for achieving high green credentials.
A Rehau thermally activated under floor slab provides the base of the buildings heating and cooling system. Water passes through un underfloor piping system to maintain the temperature of 21 ‘C throughout the year, the water temperature being adjusted depending on the seasonal changes.
For fine temperature adjustments and fresh air supply in a heating unit and diffusers are used to adjust the inside air temperature. Underfloor systems react quite slowly so the system is used only to establish the buildings base temperature. The fully automated system is controlled by a number of temperature and humidity sensors installed in and outside the carbon neutral building to maintain a comfortable climate.
Adding to the comfort levels inside the carbon neutral building, water is also pumped through a series of chilled water fins. These help to cool down and dehumidify the surrounding air. By creating a temperature difference between the air and the fins, moisture from the air condenses on the chilled surfaces and it’s simply channel to provide water for plants below.
In order to achieve the highest levels of energy efficiency for electric driven pumps and fans, all motors exceeding 100 W have been equipped with variable speed drives. Each motor is driven at the exact speed required to meet the cooling and heating demand, therefore minimizing electrical energy consumption.
- The success of this project is a result of Voltas own investment in the solar powered HVAC systems at the Moot hospital, the first ever such system to be installed in South Africa.
- The merits of such systems are being noted and adopted. But relying completely on a buildings solar power for the HVAC needs is not usually cost effective. More integrated systems are necessary to make the best use of solar HVAC installations.
- We need to be looking at both heating and cooling and the use of terminal devices with chilled and hot water coils to replace conventional units with electrical resistance heating elements.
- There is great potential in hybrid systems: the combination of a solar system, taking care of the peak cooling demand at the hottest time of the day, and a conventional high COP ammonium based chiller to take care of the base cooling load. The beauty of this approach lies in the fact that the cooling demand curve tracks the sun, so solar energy is available when you need it most.
- By making use of all the integration opportunities much more energy saving potential can be harnessed leading to significantly better long term returns.