Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Hotspot-adapted Cold Plates to Maximize System Efficiency
Thomas Brunschwiler, Hugo Rothuizen, Stephan Paredes, and B. Michel
IBM Research GmbH, Zurich Research Laboratory, 8803 Rüschlikon, Switzerland
tbr@zurich.ibm.com, +41 44 724 86 81
Evan Colgan
IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA
Pepe Bezama
IBM East Fishkill, 2070 Route 52, Hopewell Junction, NY 12533, USA
This modeling study is focused on the potential and the
limitations of hotspot-adapted liquid heat removal to improve
on system pumping power and on the re-usability of output
heat, for various packaging schemes at the component level.
This is in particular important to improve the power efficiency
of datacenters with the consequence to reduce total cost of
ownership and their impact on the environment. Inefficient air
cooling is responsible for up to 40% of their total power
consumption. High-performance liquid cooling has the
potential to reduce this number substantially and makes the
direct re-use of produced heat in neighborhood-heating
networks viable. The application of normal-flow and crossflow
cold plate architectures is discussed.
Custom-tailored normal-flow cold plates can be produced with
high spatial contrast in heat transfer with a granularity of
1 mm 2 . For conventional processor chip packages this results
in a flow rate reduction and fluid temperature differential
(T fout -T fin ) increase of 28%. This also translates into a net
pumping power decrease of 43% for a server rack with
multiple heat sources.
Heat flux tailoring with cross-flow heat exchangers is subject
to the additional constraint of a fixed volume flow over the
length of the channels, which calls for modulation of the heat
transfer geometry along the channel in order to address hot
spots. In this study the fluid flows through a layered-mesh
network, in which the number of mesh layers is modulated.
For standard packages employing thermal grease interfaces,
we find that for a given flow rate, there is little benefit in terms
of maximal junctions temperature at the expense of a
significant increase in pressure drop. The parameter
improving is the on-chip temperature variation.
We conclude the study with recommendations on how to
design hotspot-adapted cold plates.
I. INTRODUCTION
Worldwide, the number, size, and power consumption of
datacenters doubled between 2000 and 2005 due to
information technology (IT) consolidation trends and
increased demand from Web2.0 applications, such as video
streaming. Large datacenters consume up to 200 MW of
electricity, of which 40% is used only to run the cooling
infrastructure and is representing a significant portion to the
operating cost [1]. This is the result of the use of inefficient
air-cooling technology, which relies on computer-room airconditioners.
The power usage effectiveness (PUE), defined
as the ratio of total datacenter power divided by the power
consumption of the IT equipment, can be improved by
implementing advanced liquid-cooling technology [2]. For
low thermal gradients from junction to the fluid of ≤ 20 K
and a junction temperature limit (T jmax ) of 85°C, the ITequipment
can be cooled without chillers all year long.
Moreover, in cold and moderate climates, the ≥ 65°C
“waste” heat from liquid cooling can be sold to district
heating networks [3]. The value of this available heat
depends strongly on the its quality, namely temperature
level.
In this study we report on the potential benefits of hotspotadapted
cold plates on the component level. Cooling the
spatially non-uniform power dissipation of processors [4] by
means of uniform heat transfer is not a efficient solution [5].
Too much pumping power is spent on chip areas with low
heat flux. Furthermore exergy is reduced as the cold and hot
fluids mix at the outlet manifold of the cold plate, reducing
the value of the available heat. Moreover, this type of
cooling also results in high temperature gradients on the die,
causing thermo-mechanical-stress-induced aging.
Tailored, steady-state normal-flow and cross-flow heat
transfer concepts on the component level are presented and
benchmarked against uniform heat removal. Their benefit in
different packages is discussed. Finally the cold-plate
efficiency in a complete server rack with multiple heatgenerating
devices is reported.
II. SPATIALLY RESOLVED HEAT REMOVAL
Non-uniform heat transfer for a given unit-cell heat-transfer
geometry can be realized in normal flow (Fig. 1a) and jet
array cold plates [10] by local flow throttling. The parallel
fluid feed allows each unit cell to be addressed
independently, by changing the fluid delivery diameter of
each unit cell. The modulation in cross-flow heat
exchangers [9] is constrained. The volume flow rate can
only be changed for zones transversal to the flow direction,
but is constant for subsequent unit cells. Therefore the heattransfer
geometry (such as the channel hydraulic diameter or
cavity porosity) is modified to modulate all unit cells
individually (Fig. 1b).
©EDA Publishing/THERMINIC 2009 150
ISBN: 978-2-35500-010-2