When looking for energy savings in an existing chiller plant or during plant renovation, look beyond the chiller. Despite having the highest peak load of any component, the chiller may not be the largest contributor to total annual energy consumption. It’s more important to consider how all of the components work together throughout the year to maximise savings. Paying attention to part-load efficiency is critical for maximising savings.
There are also simple and low- or no-cost operational changes that can be investigated to significantly reduce an existing system’s energy use without jeopardising plant operation or occupant comfort. Here are 3 things to think about.
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Consider variable speed retrofits
The majority of components in the Hitachi chiller cooling system and Hitachi chiller air conditioning system can get the benefit of VFDs that have variable speeds. In actuality, all modern energy codes require VFDs for these parts in new systems as well as major retrofits. VFD prices have also decreased drastically over the last few years.
As can be seen in the chiller variable and constant speed performance charts, which are on the page 38 there’s an immense benefit of using VFDs in chillers, however, only if condenser water temp relief can be used.
Cooling tower fans offer another way to reduce energy consumption by using VFDs. With the loading and outdoor wet-bulb temperature drop Variable speed fan motors don’t just conserve energy for fans because of fan law benefits (a fan operating at 50 percent speeds draws 12.5 percentage of power used by the fan running at 100%) However, they also offer better temperature control.
Variable speed pumping could provide energy savings opportunities however it requires a careful examination of other components that comprise the entire system. For chilled water, switching variable flow retrofit could require extensive and expensive modifications to the control valves as well as control sequences. Additionally, the capabilities of variable flow of existing chillers must be re-examined. Limits on flow of chillers could hinder the viability of variable chilled water pumps. On the condenser side, the variable flow control can be limited by the flow limitations of chillers and cooling tower freeze/fouling issues. But, if the pumps are overloaded with a constant circulation system, the balancing of the pump flow by lowering the speed or the flow limitation with a balancing factor could be a viable option and without the addition of variable flow throughout the system.
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More means less. Running several parallel devices can maximize savings.
The plant equipment of the chiller generally runs more efficiently with a load of part. Chillers for instance can operate at their maximum efficiency between 40 – 60% of their capacity. Fans and systems pumps for cooling connected in parallel could also benefit from a system of control which operates more pieces machinery at slower speeds opposed to a staging system that allows the operating equipment to expand up to maximum capacity before settling for the next unit. For chillers and cooling towers the more equipment you run, the greater the area of heat transfer across all points of operation that improves the efficiency of the equipment and also reduces pressure drop. Pumps, making the most of savings from the law of pumps and operating at optimal pump efficiency points is essential. (The pump law works comparable with the law of fan: if the speed of the pump decreases the energy consumption will be reduced by the cubic root of the decrease in speed.) But, any control system modification must take into account chiller and cooling tower minimum flow limits.
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Increase the temperature of the supply
The majority of commercial systems are constructed with the temperature of chilled water that ranges from 40 F to 45 F. This usually allows the proper dehumidification process and an acceptable temperature of supply air for the occupants of spaces in peak hours. But, the peak conditions of weather and load are not often observed.
The implementation of a reset for the temperature of supply air control can reduce energy consumption in many ways. First, when cold supply air temperatures are not required (acceptable humidity levels and no zones at peak load), raising supply temperatures can help prevent over-dehumidification of spaces and unneeded latent cooling. In addition setting higher air supply setpoints will permit chilled water supply temperature to increase significantly, increasing the efficiency of chillers. Overall, chiller efficiency increase about 2 percent per degree chilled water supply temperatures is increased.
Other features of design and control are also worth a look for the best energy efficiency. As mentioned in the past, VFDs on all or specific components must be considered. Also, taking a review of temperature resets, and other control sequences that are simple to change are typically efficient and require the lowest expense.
However, these strategies do not suit all systems. Based on your budget and financial requirements, climate, anticipated load profile, and equipment limitations, your ideal solution will differ and may need an engineering analysis and annual calculations of energy consumption.
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