Dual-Subsystem Air Handling Unit for Extended Free Cooling Range
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Solution Overview
Problem
Data center air handling units often rely on mechanical cooling due to insufficient temperature gradients for free cooling, leading to inefficient use of free cooling, especially when ambient air temperatures are between 14°C and 18°C, where free cooling with hydronic loops is frequently replaced by mechanical cooling.
Innovation Solution
An air handling unit with dual cooling subsystems, each comprising a refrigeration apparatus and a water circuit with heat exchangers, allowing for selective operation in free cooling or mechanical cooling modes based on outside air temperature, enabling the use of free cooling in a broader temperature range by adjusting the water connections and control mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If free cooling with hydronic loops is used, then energy efficiency is improved, but it requires ambient air temperature to be 14°C or lower, limiting its applicability
Solution Approach 1:
The cooling system is divided into two independent cooling circuits: a first cooling circuit for pre-cooling return air using outside air when temperature conditions permit, and a second cooling circuit for final cooling using the refrigeration apparatus. This segmentation allows each circuit to operate optimally within its own temperature range, enabling free cooling to be utilized when outside temperature is between 14-18°C rather than requiring temperatures below 14°C.
Solution Approach 2:
Return air from the conditioned space serves as an intermediary medium between the outside air and the refrigeration apparatus. The first cooling circuit uses outside air to cool the return air stream, which then becomes the input to the second cooling circuit. This intermediary approach allows the refrigeration apparatus to operate at higher temperatures, improving its efficiency when free cooling conditions are partially met.
2Reliability
If mechanical cooling is used, then cooling performance is maintained, but the Coefficient of Performance (COP) is only about 4 to 5, reducing energy efficiency
Solution Approach 1:
The mechanical cooling system is segmented into two stages: the first cooling circuit handles the bulk of the cooling load when outside air conditions are favorable, and the second cooling circuit handles the remaining cooling requirement. This segmentation reduces the temperature lift required by the refrigeration apparatus, thereby improving its COP from the typical 4-5 to higher values during part-load operation.
Solution Approach 2:
The first cooling circuit performs preliminary cooling of the return air stream using outside air before the air enters the refrigeration apparatus. This preliminary action reduces the temperature of the air entering the mechanical cooling system, allowing the refrigeration apparatus to operate more efficiently and achieve higher COP values.
3Device complexity
If a single cooling system is used, then device complexity is reduced, but the ability to maximize free cooling across different temperature conditions is limited
Solution Approach 1:
The air handling unit is designed with multi-functionality: it can operate in pure free cooling mode when outside temperature is below 14°C, in hybrid mode (first cooling circuit + refrigeration apparatus) when outside temperature is between 14-18°C, and in pure mechanical cooling mode when outside temperature is above 18°C. This universal design allows the system to adapt to various temperature conditions while maintaining a relatively compact structure that integrates both cooling circuits within a single unit.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration maximizes the use of free cooling, achieving a high annualized Coefficient of Performance (COP) by allowing primary air cooling through free cooling in frequent temperature intervals, reducing the reliance on mechanical cooling and enhancing overall efficiency.
Implementation Method 1
a first water circuit connected to the condenser and comprising at least one outside heat exchanger exposed to outside air, in which the water releases heat to the outside air under action of a fan
Implementation Method 2
a second water circuit connected to the evaporator and comprising at least one indoor heat exchanger exposed to the indoor airflow, in which the water draws heat from the indoor air flow
Implementation Method 3
a refrigeration apparatus comprising an evaporator and a condenser
Data Source
Figure 1
Figure 2
Figure 3
AI summary
An air handling unit (1) for cooling down an indoor airflow (A1) comprises at least one fan (3) circulating the indoor airflow inside the air handling unit (1) and a first and a second cooling subsystems (5, 15) comprising each: - a refrigeration apparatus (50, 150) comprising an evaporator (500, 1500) and a condenser (504, 1504), - a first water circuit (52, 152) connected to the condenser and comprising at least one outside heat exchanger (520, 1520) exposed to outside air (A5, A15), - a second water circuit (56, 156) connected to the evaporator and comprising at least one indoor heat exchanger (560, 1560) exposed to the indoor airflow, - water connection means (62, 64, 162, 164) for selectively connecting, depending on a temperature of the outside air: - the first water circuit to the condenser and the second water circuit to the evaporator in a mechanical cooling mode where the refrigeration apparatus is operating, or - the first water circuit to the second water circuit and disabling water circulation in the evaporator and the condenser, in a free cooling mode where the refrigeration apparatus is stopped.