Chilled beam air conditioning systems were a rarity in Australia until around 2005 when there was heightened awareness about energy efficiency, air‐quality and greenhouse gas emission reduction.
There are a few examples of Passive Chilled Beam (PCB) systems with 100 per cent once through outside air cycles, however, the majority of newer projects adopt an Active Chilled Beam (ACB) configuration with recirculated air.
The ARBS Education and Research Foundation commissioned research paper (from page 6)
provides a detailed study of Active Chilled Beam (ACB) systems, and compares the performance of two chilled water plant configurations.
The first is Standard using a single Chilled Water Plant (CHWP) of multiple chiller configuration, which uses a Heat eXchanger (HX) to deliver High Temperature Chilled Water (HTCHW) to Active Chilled Beams (ACBs) at the zone level.
The second is a High Temperature Chiller using a dedicated chiller to generate and distribute HTCHW to the ACBs at zone level.
This chiller runs at a much higher COP for this duty and allows the overall HVAC system to function more efficiently.
Both systems use a central Air Handling Unit (AHU), that has a function similar to that of a Dedicated Outside Air System (DOAS), which delivers de‐humidified and conditioned air to the ACBs at zone level.
In keeping with recent design trends for such systems in Australia, the AHU is set to recirculate air and includes an economy cycle that is carefully controlled.
Since the first evaluations, carried out two years ago, heat outputs from lighting and IT equipment, mainly computers, has decreased significantly, as outlined in the National Construction Code (NCC) 2019.
Construction systems, window systems, internal loads, thermostat settings and infiltration rates have been changed to reflect the requirements of the National Construction Code 2019.
The new requirements for the Verification Methods using energy simulation require a test of thermal comfort measured using PMV, to be maintained between ±1 for 95 per cent of the conditioned floor area for 98 per cent of operating hours, as per code definitions.
These requirements required the building enveloped to be improved beyond the minimum Deemed‐To‐Satisfy requirements.
Energy efficiency
The predicted energy efficiency outcomes for each of the two system configurations, the Standard (STD) and High Temperature Chiller (HTCH) variants are provided in greater detail in the report.
But the results indicate a 16 per cent predicted reduction in system energy consumption between the Standard (STD) and High Temperature Chiller (HTCH) variations of the modelled Active Chilled Beam (ACB) HVAC system.
The predicted results are logical. There are energy savings in the chiller energy consumption for the system with the Dedicated High Temperature Chiller (HTCH). Some of these savings are offset by additional pumping energy required to move larger quantities of high temperature chilled water around the system.
The predicted performance for both configurations of the ACB systems modelled above is estimated to be within the operational requirements of a 5 to 5.5 star Base Building NABERS rating for office buildings. A striking result outcome of this study is the fact that the NCC2019 simulation parameters specified have resulted in the predicted hourly thermal demand being similar in quantum to that of the predicted hourly cooling loads.
Plant modelling
The peak thermal load for the building was estimated to be around 1,000kW.
The Standard (STD) chilled water plant configuration was modelled with three equal sized chillers plumbed in parallel. Each chiller was sized to provide 350 kWr of refrigeration.
Design COP was set to a conservative 5.5 value (modern chillers can achieve more than 6 at design conditions).
The reference values for leaving chilled water and leaving condenser water are set to 6.67C and 35C (AHRI conditions). The chillers are not allowed to unload below 20 per cent.
Supply of High Temperature Chilled Water (HTCHW) for the STD configuration has been modelled using a secondary chilled water loop that incorporates the Heat eXchanger (HX supplying 14C water to the ACBs on demand via a variable speed secondary chilled water pump.
The second HTCH (High Temperature Chiller) chilled water plant configuration is modelled with one dedicated 400 kW chiller supplying the ACBs directly with 14/17 C water.
The two other 350 kW chillers, plumbed in parallel, serve the DOAS AHU cooling coil with 5/13 C chilled water.
The heat rejection system for the HVAC plant has been modelled to be a single cooling tower, a simple single speed fan and cycling control operation.
The cooling tower has its own dedicated constant volume pump designed to meet a 200 kPa head. Cooling tower sizing is based on a 29C/34.5C loop split.
The sump water temperature is controlled to follow ambient wet bulb temperature down to 20C with an approach of 3C.
The heating hot water loop has been modelled to be a single natural gas fired boiler, running a 80C loop design temperature at 80% efficiency, with a 20C temperature differential.
A variable speed pump circulates the hot water across the system and it is designed to meet a head of 200 kPa.
The single Air Handling Unit (AHU), modelled for both HVAC system configurations, has a function similar to that of a Dedicated Outside Air System (DOAS).
It delivers de‐humidified and conditioned air to the ACBs at zone level. In keeping with recent design trends for such systems in Australia, the AHU is set to recirculate air and includes a carefully controlled economy cycle.
The supply air condition leaving the DOAS type AHU has two control conditions imposed on it. The first condition is imposed by the cooling coil which imposes a de‐humidification priority cooling algorithm that maintains a zone maximum absolute humidity of 8 gm of water vapour per kg of dry air.
The 2nd control conditioned is imposed on the supply air condition downstream of the supply fan and is based on an outside air reset condition. When the outside air ambient dry bulb temperature is 15C or less, the AHU supplies air at 18C to the Active Chilled Beams (ACBs). When the outside air ambient dry bulb is 18C or higher, the AHU supplies air at 12C to the ACBs.
The zone level Active Chilled Beams (ACBs) have been represented by four pipe induction units, incorporating a heating coil and a cooling coil.
The unit receives treated air from the DOAS type AHU and induces room air past the coils with a three fold induction ratio.
In practise, modern chilled beams are a single coil (2‐pipe component) which can accept either heating hot water or high temperature chilled water via a multi‐port control valve. Therefore two sets of neighbouring beams can have opposite duties, with one working in cooling mode and the other in a heating mode. This allows for a high degree of tenant fitout flexibility.
System insights
There were a number of system level insights from the modelling exercise. For example,
the dedicated chiller supplying High Temperature Chilled Water (HTCHW) to the ACBs is very efficient in delivering the cooling duty; however, selection of this machine must be carried out carefully to ensure stable operation across all load ranges.
Chilled Beam systems will call for cooling all through the year in Sydney and similar climates. However, Chilled Beam systems are less ‘forgiving’ and control strategies need to be commissioned carefully and monitored continuously.
Chilled Beam systems work best with an efficient façade; their response time is slower than that of all air systems.
Since chilled water (and hot water) needs to be pumped around the building and in around occupied spaces, there is always the risk of leaks or hose connector failure with the tenant space. This must be managed by ensuring high quality components are specified and installed correctly.