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Free cooling isn’t a new concept, but it’s often underutilised, especially in sectors where cooling demands are ever-growing, such as industrial and datacentre cooling. The UK’s cooler average temperatures provide an excellent opportunity to implement free cooling systems, which can significantly reduce energy consumption and operational costs. However, there are both advantages and potential pitfalls to consider when integrating these systems into your cooling strategy.
What is Free Cooling?
Free cooling refers to the process of using naturally cool outdoor air to cool water for systems that require chilled water throughout the year. In the UK, the average annual temperature hovers around 9°C, often dipping much lower during the colder months. When the outdoor air temperature falls below the return temperature of the chilled water, this cold air can be used to cool the system without relying on mechanical chillers. This not only reduces energy consumption but can also enhance the efficiency and lifespan of your cooling equipment.
Traditional Free Cooling Systems
Historically, free cooling systems have relied on a combination of chillers and stand-alone dry coolers. These systems use valves and costly controls to incorporate free cooling into the cooling loop. While these systems offer flexibility and reliability, they often come with several challenges:
· Large Footprint: The need for multiple independent machines, such as separate chillers and dry coolers, requires significant space.
· High Installation Costs: The interconnecting pipework, pumps and other components increase both the complexity and cost of installation.
· Operational Complexity: Managing the coordination between the chillers and free-cooling components often requires third-party control solutions.
Despite these challenges, this approach has been widely used due to its flexibility and effectiveness in providing free cooling throughout the year.
The Rise of Integrated Free-Cooling Chillers
In recent years however, a more efficient solution has emerged: integrated free-cooling chillers. These systems combine both mechanical and free-cooling capabilities into a single, compact unit. Using futureproofed low-GWP refrigerants, these machines can autonomously manage cooling requirements, optimising water production with onboard control strategies. The benefits are significant:
· Smaller Footprint: The need for separate cooling systems is eliminated, reducing both physical space requirements and installation costs.
· Simplified Installation: Since the necessary hydraulics are pre-assembled and the control logic is integrated, the installation process is streamlined and less complex.
· Higher Efficiency: With free cooling included, these chillers can achieve an Energy Efficiency Ratio (EER) around 5-6 times greater than when in mechanical cooling mode.
BIG Efficiency Gains
The greatest advantage of free cooling is the substantial increase in overall system efficiency, leading to lower operational costs.
When cooling, chillers typically achieve an EER of 3-5, meaning for every 1kW of energy input, the system produces 3-5kW of cooling output. But with free-cooling, that ratio skyrockets to 25-30 EER, offering up to 30 times more energy output than energy input. This dramatic improvement in energy efficiency makes free cooling a no-brainer for systems that require year-round cooling. These efficiencies can also be boosted by increasing the delivered water temperature setpoint.
When cooling, chillers are operating compressors, condenser fans and other internal components to maintain the temperature setpoint whilst balancing the needs of the vapour compression cycle, making mechanical cooling one of the biggest energy costs and carbon contributors to the industrial market. In free-cooling mode, the EER dramatically increases because the refrigerant circuit and its compressors are switched off, only using a set of fans to manage the water temperature.
Additionally, the use of free cooling reduces reliance on mechanical cooling, reduces wear and tear on the chiller components, extends lifespan of the system and lowers the cost of maintenance. After all, it’s far cheaper and easier to replace a fan than a compressor.
Operational Modes: Maximising Flexibility
Modern integrated free-cooling chillers can manage key operational conditions, offering flexibility to maximize efficiency based on the ambient conditions:
· Chiller-Only Mode: When the ambient temperature exceeds the chilled water return temperature, the system operates purely as a chiller. The free-cooling coil and FC fans remain inactive, and the system operates like a traditional chiller. An integrated 3-way valve bypasses the free-cooling coil to prevent unnecessary head losses, and fan speed modulation controls condensation.
· Mixed Mode: When the ambient temperature drops below the chilled water return temperature, the system switches to mixed operation. The free-cooling coil is activated, and the water leaving the coil is pre-cooled by the outside air before entering the evaporator. The chiller then operates at reduced capacity, only producing the amount of cooling necessary to reach the required water temperature.
· Free Cooling Mode: When the outdoor air temperature is low enough to fully meet the cooling demand, the system operates exclusively in free-cooling mode. An integrated 3-way valve bypasses the chiller evaporator to prevent unnecessary head losses, and fan speed modulation controls water production. The compressors are turned off, and the system relies entirely on outside air to provide cooling.
The Risk of Freezing and How to Manage It
One of the key risks associated with free-cooling systems is the potential for freezing. Since the system uses outdoor air to cool the water, the temperature of the water can drop to levels where freezing is a concern, especially during colder months. This risk is typically mitigated by adding an antifreeze solution, such as glycol, to the water.
However, in traditional free-cooling systems, this can be costly due to the large volume of water that needs to be treated. The good news is that integrated free-cooling chillers can offer a "no glycol" version, where an onboard heat exchanger separates the free-cooling coil from the main chilled water loop. This allows glycol to be added to the free-cooling section only, reducing the overall costs and complexity of antifreeze management where applicable.
Modular Designs and Maintenance Considerations
Free-cooling chillers can be designed with a modular approach, where the refrigerant air coil and free-cooling water coil are placed in separate sections. This design increases the overall length of the unit, but it improves flexibility, makes it easier to
maintain the heat exchanger surfaces and improves the free cooling potential by separating the mechanical cooling components. A modular design also reduces the air pressure drop across the system, which in turn reduces the power required by the fans.
In contrast, some systems combine the refrigerant and free-cooling coils into a single overlapped compact chiller unit. While this reduces the footprint of the system, it can make maintenance more challenging as the coil faces are positioned closely together. This configuration can often impact performance, increase fan power requirements, and limit machine capabilities due to the “shared air” restrictions imposed.
When considering mixed operation as the predominant mode with the overlapping coil configuration, the free cooling potential may also be reduced. In fact, if you run the fans at full speed to maximize free cooling capacity, then you risk lowering the condensing temperature of the refrigeration circuit leading to reduced efficiencies and operational envelope issues. The mitigation for this type of chiller would be to compromise free cooling operation in favor of mechanical cooling.
The modular design avoids this conflict, allowing independent fan control that balances condensing temperature with maximum free-cooling output and optimises machine operation based on actual outdoor conditions.
Conclusion: Free Cooling as the Future of Cooling Efficiency
In conclusion, free cooling offers a compelling opportunity for businesses in the UK to reduce energy consumption, lower operational costs, and increase system longevity using sustainable technologies.
While traditional free-cooling systems require a large footprint, complex installation, and third-party controls, integrated free-cooling chillers offer a more streamlined, efficient and cost-effective solution. By leveraging the UK’s cool climate, businesses can take advantage of substantial energy savings and operational benefits, especially for applications with large year-round cooling needs.
With the right low-GWP system design and operational strategy, free cooling can deliver long-term financial and sustainability benefits, making it a smart investment for today and the future.
