Saving energy with cooling systems, such as chillers, often sounds easy: just increase the setpoint, and the compressor has to work less hard. While this is true, there’s more to it than that. In this discussion, we will explore how this works and why you should be cautious with expectations regarding savings.
How?
When you increase the water-evaporation temperature of a chiller (by raising the setpoint), the suction pressure of the compressor increases. This reduces the pressure difference between the suction and condensation pressures, allowing the compressor to work less hard. While saving energy, it also affects other system parts.
In a typical cooling system, cold water is directed through pipes and valves to heat exchangers, extracting heat from air or other medium. This often occurs using a variable-flow system with two-way valves. When the water’s evaporation temperature rises, these valves open further to maintain the same cooling capacity. This results in a lower pressure in the chilled water network.
Now, the chilledwaterpump steps in. The pump speeds up to maintain the water flow and compensate for the lower pressure. The problem? This increased pump energy usage can offset part of the energy savings gained from the compressor.
How Does That Work?
It’s also important to understand that the amount of energy a compressor moves depends on the enthalpy of the refrigerant. When the suction pressure increases, the compressor can operate more efficiently because it pumps more refrigerant without using significantly more energy. However, you should always consider the entire system, not just the compressor.
Additionally, the condensation pressure can change. If the condenser doesn’t remove heat quickly enough, the pressure can rise, forcing the compressor to work harder again. This reduces the savings you were hoping to achieve.
In the maritime world, there is sometimes a belief that raising the setpoint is the solution, as it can be challenging to verify energy consumption, unlike in a building. In a building, it’s easy to see what the consumption is, but on a ship, that is often not the case. Therefore, it’s essential to base decisions on calculations that consider a combination of efficiency and energy consumption. This provides a better understanding of the actual performance of the system.
Replacing with More Energy-Efficient Installations
Many companies install energy-efficient chillers in existing systems and expect significant savings, but without adjusting the rest of the system, the results often fall short. Real savings only occur when you monitor and optimize the entire system: from the chiller (compressor, condenser, evaporator, controls) to the air handling units (water distribution, airflow, controls). Often, a quick win is pursued or only one of the components is examined, leading to a narrow focus that misses the bigger picture.
Finally
In summary, saving energy in cooling systems is not just about raising the setpoint. It requires a comprehensive understanding of all components of the system. Only by thoroughly monitoring and adjusting everything can you achieve real results.
