IMD

Thermal Energy Storage (TES) employed at Massmart’s new seven-story Sunninghill building is part of a progressive approach to Green Building design with respect to its innovative heating, ventilation, and air conditioning.

Massmart is the third-largest distributor of consumer goods in Africa and achieved annual sales of R43.1 billion in 2009. Its progressive approach to Green Building design includes the building envelope and services which have been designed and constructed to reduce the impact on the environment.

Gaining with glass
From initiation the structure, orientation and construction of this new building were considered to ensure minimum heat gain. By calculating the sun angles the northern and southern facades, that contain large glazed areas, were orientated to minimise incoming solar radiation. Additional sun-shading was provided by purposefully designing the balconies to provide maximum shading. The eastern and western facades are provided with minimal glazing to reduce the amount of sun entering the building during the mornings and afternoons. Performance glazing was used on all the glass facades to reduce heat gain due to direct solar radiation, while still providing the maximum visible light into the building.

Breaking TES boundaries
The air conditioning system contains a TES system. Traditional HVAC systems rarely operate at full load capacity. The systems normally operate during the day to cater for the building`s demand and remain idle at night. The cooling capacity is designed to satisfy the maximum instantaneous cooling demand that might occur only a few days of the year. TES operates by storing thermal energy at night and using that energy during the day to provide cooling to the building. This results in a higher efficiency by only requiring half the peak electrical energy of that compared to an equivalent non-TES chilled water system.

Thermal energy storage is particularly well adapted to air-conditioning systems by regulating the production of cooling energy and optimising the use of electrical resources. It enables a significant reduction in installed chiller capacity and enables the building owner to utilize low tariff electricity to create significant running cost savings. Thermal storage solutions are commonly used for peak lopping but are also a useful technology for providing a back-up air-conditioning solution during power failures and load shedding.

During a power failure or scheduled load shedding stored cooling is available and only the pumps, controls and fans in the air- conditioning unit needs to operate under emergency power to effectively cool the building. The size of the standby generators can be reduced and subsequently the emergency power usage cost will decrease. Cristopia is a 'closed loop' TES system, designed to overcome the past problems of conventional ice storage systems. The system consists of a storage tank, filled with glycol/ water in which is suspended plastic spherical nodules that are filled with a Phase Change Material (PCM).

Overnight the chiller is used to store cooling energy in the thermal storage tank. The glycol/water heat transfer fluid is cooled by the chiller to temperatures below the phase change temperature of the PCM. Circulating the fluid through the tank causes crystallisation of the PCM contained in the nodules. The energy is stored at constant temperature as latent heat. The thermal storage tank can be used for back-up applications, when the chiller is off. The heat transfer fluid enters the thermal storage tank at a temperature higher than the PCM fusion temperature and is cooled by the nodules.

When the system demand is greater than the installed chiller capacity the cooling energy is provided by the chiller and thermal storage tank combined. The chiller works at full capacity with the thermal storage tank providing the shortfall.  During the night, the chiller charges the thermal storage tank and supplies the cooling demand of the building, should there be any as a result of personnel working after hours, limited to 10-15 percent of chiller capacity.

Two central air cooled chillers are positioned in a freely ventilated plant area on the roof of the building. The chillers are designed to accommodate 50 percent of the building`s cooling capacity and two thermal storage tanks filled with Cristopia PCM nodules absorb the remaining 50 percent of the load. Chilled water is reticulated to in-ceiling fan coil units by means of a reverse-return variable speed pump.

The same amount of kilowatt hours of cooling are required as with a standard chilled water system, but a considerable portion of the energy consumption by a TES can be ‘shifted’ to after hours i.e non-peak periods. The theory is that the chiller, sized to cater for 50 percent of the building’s total cooling load, will charge the thermal storage tank sized to store the other 50 percent of the required load. During summer both the chiller and the thermal storage system will operate to produce the required 100 percent load. The 100 percent cooling load would only be required for three to four months in the year. For the remainder of the time only 50 percent of the cooling load would be required, and all the cooling would be provided by only using the thermal energy stored overnight in the tanks.

The electrical load required for a direct-expansion split type system is generally between 60 W/m² to 70 W/m². The electrical load for a well designed chilled water system is in the region of 50 W/m² to 55 W/m². The TES only requires 30 W/m² of electrical power. During a power failure scenario only the pumps and fan/coil units require generator power. The BMS system keeps the heating elements out under power failure conditions, ensuring that only 10 W/m² of generator power is required to provide 100% percent of the full peak cooling load for up to four hours.

The air is conditioned by in-ceiling fan coil units. The conditioned air is introduced into occupied zones by means of constant volume ceiling diffusers. Each fan coil unit conditions a temperature zone varying in size between 25 m² and 80 m². Dedicated units are allocated to particular sensitive areas i.e. executive offices, boardroom and meeting rooms.

Leveraging Technology
The motion sensors that were installed to control the lights are incorporated into the operation of the air conditioning system. When an area is unoccupied the air- conditioning units switch to an economizer mode. The unit’s temperature set-point is then adjusted to 26°C. This allows reduction of the energy demand and simultaneously enables reduction of the pull-down time to bring the office’s temperature down to the set-point temperature. The BMS Fan Coil Manager allows for complete control flexibility. The HVAC system can either be controlled centrally by the facilities manager or individually from any desk-top computer or laptop.

Complete Package
Through the use of the appropriate glass technology, thermal energy storage mechanisms, sensor controllers and building management systems, C3 Engineering has demonstrated how a South African building can fully utilize available technologies to cut costs and be environmentally conscious.

Accreditation:
Owner:                                                 `                    Massmart
Consulting Engineer:                                             C3 Engineering
HVAC Contractor and Installer:                            HVAC Installations cc.
Chiller Supplier:                                                      Ciat (Skyshot)
TES supplier:                                                           Christopia (Skyshot)
Fan Coil Unit Supplier:                                           Hammer (Skyshot)
BMS Supplier:                                                         Skyshot