Enhanced Heat Rejection for High Density Data Center Design
Higher performance compute. Higher density server racks. Higher demand for power. Fueled by an AI arms race, the data center industry has reached dizzying new heights. The one thing that must remain low amidst all the highs? Temperatures. GPUs and CPUs need to be kept cool, obviously. What may be less obvious is the fact that outdoor condensing equipment also requires sufficient circulation of relatively cool air to reject heat from the building.
There has been no shortage of discussion and innovation surrounding how to cool high density server racks. It’s the hot topic (pun intended) at most industry events, along with where to find the electricity to power it all. Direct-to-chip liquid cooling is seeing high adoption rates as a solution for data center companies providing power and space for high performance compute (HPC) applications. However, removing the heat from the data hall is only half of the challenge. This article explores how energy density creates heat rejection challenges outside of the building.
The image shows increased waste heat density correlated with increased power density in a data center
The first law of thermodynamics
Energy cannot be created or destroyed, but it can be transformed from one form to another. The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes.
With the first law of thermodynamics in mind, let’s follow the energy in a data center.
- GPUs require in the range of 8-10x the power of legacy CPUs.
- While electrical consumption has increased by orders of magnitude, the relative physical size of the data halls and data centers has essentially remained constant.
- The result is much higher energy density in terms of kW/ft2.
- The computational process transforms this electricity energy into heat energy.
- There is now 8-10x the density of heat that must be removed; first from the data hall and then from the cooling liquid and refrigerant loops.
Heat rejection and the second law of thermodynamics
The cooling transaction is not successful until the heat is fully rejected from the entire system. With few exceptions, the waste heat energy is transferred to the outside air according to the second law of thermodynamics. This is commonly accomplished by pulling ambient air over a condenser coil (as with an air-cooled chiller) or by evaporating water (as with a cooling tower). While innovations such as direct-to-chip liquid cooling have revolutionized cooling inside the data hall, heat rejection technology outside the building remains substantively unchanged.
This is where the energy density in the form of heat can really start to cause problems for a data center.
The constraints: water, power, and space
Cooling towers are energy and spatially efficient, but water and maintenance intensive. Air-cooled chillers are economical, low maintenance, don’t use water, but are spatially intensive. Air-cooled chillers must be spaced sufficiently far apart to avoid recirculating the same heat they are designed to reject from the system. In high density data center designs, there sometimes isn’t enough space for the number of air-cooled chillers that are required to reject the amount of heat generated by high performance compute.
Data center rooftops covered with rows upon rows of air-cooled chillers develop their own micro-climate. During full load operation, the air can become 30-40°F hotter than the surrounding ambient air. When this hotter air is recirculated back into the condenser coil it can lead to sharp decreases in efficiency and cooling capacity, eventually leading to compressor failure and/or units tripping offline.
The Solution
Adiabatic pre-cooling systems pre-cool the ambient air as it flows over the condenser coil. Lowering the temperature of the air across the coil increases system efficiency, protects the compressors from high head pressure, ensures cooling capacity is maintained, and mitigates the effects of hot air recirculation.
For more information on why Peak+ industry-leading controls make our adiabatic pre-cooling technology the preferred solution for sophisticated data center operators, click here.
