ENERGY CONTROL STRATEGY CHILLED WATER SYSTEM
RTE’s site mast
The issue of matching system response to AHU loads required rigorous testing to ensure comfort conditions were always met and this was achieved through a considerable amount of analysis facilitated by the BMS system
AHUs. An additional goal of the energy audit was to reduce the AHUs’ operating schedule, providing not only a reduction in both operational and maintenance costs, but also providing for a significant reduction in the operating hours of the CHWS due to this reduced demand. The findings of the audit allowed for a daily reduction in the operating schedule of three hours (16%), which is achieved without adversely affecting the thermal conditions and health of occupants.
Chilled water system energy audit The initial findings from the air conditioning system audit also highlighted the inadequacy in the initial control strategy to deliver an efficient mode of operation for the CHWS. The process of developing a new control strategy incorporated the monitoring of each AHU’s cooling coil requirement, and using this information to control the CHWS based on a demand-led strategy. The following outlines the strategy:
•Each of the Television Centre’s 22 AHUs that require cooling energy were separated
•High priority are transmission critical AHUs, while low prioritisation covers
•Each prioritisation group has a ‘threshold on/threshold off’ facility for the cooling
non-technical areas
•This tuning facility will allow for the strategy to cope with both fluctuations
32 CIBSE Journal December 2012
coil percentage valve openings, allowing for a ‘delay on’ facility for that event
into two groups that formed ‘priority high’ and ‘low’ formats
•The respective priority group set-points can be user-adjusted at the BMS to suit
in load – due to the on/off status of the different AHU time schedules – ensuring that the strategy provides protection to the chillers’ compressors, by reducing the potential for unnecessary stop/starts, which would decrease the life expectancy of the compressor
the seasonal operational requirements of the CHWS
Modulating control strategy It is through the modulating control strategy that the CHWS achieves its load matching capabilities. The BMS was initially limited in its ability to control the chilled water process, and only allowed for a system on/ off function by way of a time schedule. The CHWS audit led to a reduction in the actual system ‘on’ times. However, more savings were available with the introduction of modulating control to the control strategy. The principle is that the demand for the
CHWS is reduced by weighting its response to the AHUs according to their criticality. Put simply, the CHWS responds to demands from more critical AHUs quicker than less critical AHUs. When the CHWS is eventually enabled,
it ramps up in a phased manner, firstly by slowly increasing CHW flow rates; then, if demand continues, it reduces supply water temperature until the load is met. Compared to the previous method of turning the CHWS onto 100% capacity for a set number
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