Milkwood Office Park in Umhlanga Ridge boasts a flexible design that offers excellent air quality and temperature control, while still keeping energy consumption to a minimum. The contract commenced on 31 March 2015, and the first tenants were operational by December 2015.
The design brief was for a green building that would deliver a high-quality environment, both in terms of air quality and temperature control. Energy consumption was to be minimised without compromising internal conditions, particularly regarding humidity control during the tropical KwaZulu-Natal east coast summers.
The design was to accommodate the potential division of the four buildings in tenancies ranging from 200m2 to a full building of 4 000m2. This demanded a degree of flexibility to maintain control and proper air distribution regardless of tenant layouts. Our design priorities for the system were as follows:
1. High fresh air quantities
While this is always desirable, the humid Durban climate brings an added penalty in the form of latent heat gains. With the target of 10ℓ/s per person, this gave an overall cooling load of some 250kW to bring the fresh air to room condition.
The solution was the use of four dedicated energy recovery units, housing desiccant dehumidifying energy wheels. There is one per building, which provides all the fresh air requirements to the plantrooms while using the conditioned exhaust air as pre-cooling.
Wheel efficiencies are 75% and this provides a free cooling effect of some 174kW at summer design conditions.
2. Excellent temperature control
The buildings were designed prior to tenants being engaged and it was, therefore, important that the base system design would be flexible enough to allow any combination of use. From open plan to cellular offices, boardrooms to call centres, the system needed to be able to accommodate the varying requirements within the tenant fit-out phase without compromising the base system design.
The system chosen was variable air volume (VAV) air handling units (AHUs), incorporating variable speed drives (VSDs), chilled water metering, and control valves in a single valve body. Diffusion is done with Rickard VAV diffusers with the final choice of master/slave arrangements forming part of the tenant fit-out. This gives full control of water and air-side volumes to allow accurate control of space temperature. The chilled water control and metering is done with Belimo equipment.
3. Low energy usage
It goes without saying that this is a priority on any project and in this case, we saw an opportunity to drive this in tandem with reducing capital cost. We had been involved with the same client on resolving a problem they had experienced on another development. The chillers installed exceeded the required capacity to such a great extent that they could not run consistently at even the 25% minimum that the screw compressors required.
The design cooling load of the Milkwood Office Park is 850kW, while the available chiller from the other site had a 500kW capacity. We were already planning to use ice storage on the project, and the potential to re-use what was essentially redundant capital equipment made both economic and ecological sense. The system design was then completed based on utilising the 500kW chiller in an assisted ice storage scheme.
4. Low running costs
The only disadvantage of running an ice storage plant is the higher energy usage due to the reduced coefficient of performance (COP) when running glycol in the chiller. From a financial perspective, no penalty exists, since the time of use tariff saving while running the chiller in ice-making mode at night is substantial, giving very good payback periods on the investment.
Article written by HVAC&R Online