Mechanical engineer and NABERS accredited assessor, Ahmad Fraij, presents a range of options to improve chiller system efficiency during the design stage.
To select the best system, a holistic approach is key to make budget and achieve optimal efficiency. When it comes to efficiency my personal preference is a variable speed chiller with magnetic bearing compressors. Air cooled chillers should have EC fans for the condenser and should be equipped with an adiabatic system to achieve even greater efficiencies.
Generally, chiller efficiency depends on the compressor lift, which is the difference between the suction and discharge pressures.
The smaller the difference in pressures the higher the efficiency of the chiller. The pressure is proportional to the temperature and therefore, the compressor lift.
Chiller efficiency depends on the leaving chilled water temperature and the leaving condenser water temperatures for the water cooled chillers or the leaving air temperature for the air cooled chillers.
Based on this, we can see that increasing the leaving chilled water temperature from the chiller will improve its efficiency. As a rule of thumb, for every 1°C increase in leaving chilled water temperature, there is a three per cent reduction in chiller energy consumption. Therefore, always design your system with high leaving chilled water temperature. Note that increasing the chilled water temperature leads to bigger coils in the AHU’s and FCU’s so it is important for the designer to carry out a life cycle cost analysis to select the best system.
The designer should also consider selecting chilled beams or displacement diffusers for the chilled water system because these types of systems require higher chilled water supply temperature that can reach 16°C. This high chilled water temperature allows you to select smaller chillers while also improving chiller efficiency.
Most engineers design the chiller system at 5°C delta T on both the chilled water side and also on the condenser side if the chiller is water cooled. This is the old way to design a system but due to the need for energy efficiency, we need to increase delta T to 8 – 10°C to reduce the flow rate so we can select smaller pumps to reduce energy use.
Further, reducing the condenser water flow rate for the water cooled chillers, will lead to a smaller cooling tower and smaller tower fan, which also reduces energy consumption. Don’t forget reducing water flow rates means smaller pipes, reducing the capital cost of the project.
With most projects the connection is parallel for water cooled chillers but there is a more efficient option, especially for the large capacity chillers.
Series counter-flow chillers configuration (as shown in the figure below) reduces the chillers lift and improves their efficiency. You can see from the figure below that the chiller that has lower leaving chiller water temperature has also the lower leaving condenser water temperature and vis versa.
This configuration makes both chillers see approximately the same smaller lift (26°C) than seeing the bigger lift (31°C) if they are both connected in parallel. Chiller manufacturers claim that this configuration can improve chiller efficiency by up to 13 per cent.
Variable Speed Drives (VSDs)
Motor energy consumption is proportional to the cube of the flow rate so reducing the water or air flow rate will reduce the energy consumption of pumps and fans drastically. This should be specified with VSD’s including:
- Chilled Water Pumps with emphasis on variable primary flow configuration, which saves on the initial and running costs.
- Condenser Water Pumps
- Cooling Tower Fans
- Air Handling Units Fans
Using a Pressure Independent Control Valve (PICV) for the air terminal units such as FCU’s and AHU’s is much better than using the traditional combination of the 2-way control valve and balancing valve. The 2-way control valve can change the flow rate supplied to the terminal unit based 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 return chilled water temperature to the chiller and causes low delta T syndrome.
This reduces the chiller efficiency and leads to more chillers to run, which increases energy consumption and reduces system efficiency. Further, 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 difference between the two ports. Therefore, it eliminates the overflow to happen and makes the system more efficient.
Thermal storage tanks
Thermal storage tanks whether it is chilled water, ice or phase change material (PCM) can be used to shift the electricity demand from the peak time to other 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 because of its large volume, it is not always practical to include it.
The ice conductivity is low and requires low operating temperatures, which reduces the chiller efficiency and therefore, can be used for demand shifting.
But for energy efficiency, a more detailed analysis should be done to make sure that operating 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 and this material has 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 include in a building.
The building management system (BMS) should be designed and specified well and should include 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 variable speed chilled water pumping strategy and therefore, it is more suitable for constant flow systems and for variable primary flow systems the 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
Finally, the designer should specify a Chiller Plant Optimization Controller, which is a controller dedicated only for the chiller plant (chillers, cooling towers, pumps and valves) and has algorithms to optimize water temperatures and flows to make sure the plant equipment operate at their maximum efficiency.