Page 31 - Climate Control News Magazine July 2021

Troubleshooting
on the pressure difference between its ports. This means that many times this 2-way valve will allow more water flow rate to the terminal unit than required. This overflow reduces the re- turn chilled water temperature to the chiller and
causes low delta T syndrome.
This reduces the chiller efficiency and leads to
more chillers running, which increases energy consumption and reduces system efficiency. Fur- ther, this overflow makes the pumps run at higher speed to meet the required overflow, which also increases the energy consumption of the pumps.
PICV maintains the required flow rate to the terminal units regardless of the pressure differ- ence between the two ports. Therefore, it stops the overflow happening and makes the system more efficient.
THERMAL STORAGE TANKS
Thermal storage tanks whether chilled water, ice or phase change material (PCM) can be used to shift the electricity demand from peak time to oth- er times to reduce peak demand charges. However, these tanks occupy big areas of a building and it is not always possible to accommodate them. The largest tank is the chilled water storage tank and the ice tank which requires around 20 per cent of
the volume of the chilled water tank for the same thermal capacity, while the phase change material tank requires 50 per cent of the volume.
The chilled water thermal storage tank is the easiest option to include in the design but due to its large volume, its inclusion is not always practical.
The ice conductivity is low and requires low operating temperatures, which reduces the chill- er efficiency and therefore, can be used for de- mand shifting.
But for energy efficiency, a more detailed analysis should be done to make sure that oper- ating the chillers at low ambient temperature during the night, for example, will outperform the reduction in efficiency due to the low chilled water temperatures.
The most common material used for PCM tanks is Eutictics with a freezing point of around 8.3°C, which make it the best option to choose for chilled water systems to save energy. However, the size of the tanks are still a challenge to in- clude in a building.
CONTROLS
The building management system (BMS) should be designed and specified well, including energy efficiency control strategies.
Below are examples of energy efficiency control strategies that should be included in the BMS:
• Chilled Water Temperature Setpoint Reset.
Note that this strategy conflicts with the vari- able speed chilled water pumping strategy. As such, it is more suitable for constant flow sys- tems and for variable primary flow systems. Precedence should be for the variable speed pumping and after the flow is at minimum, then the chilled water temperature setpoint reset should apply.
• Condenser Water Temperature Setpoint Reset. • Optimum Start/Stop
• Demand Limiting
Ahmad Fraij is a mechanical engineer and NABERS Accredited Assessor with more than 20 years experience inn sustainability, energy saving and building services working here and overseas.
ABOUT THE AUTHOR
CLIMATE CONTROL NEWS JULY 2021
31