Selasa, 28 Maret 2017

An 8-Bit, 40-Instructions-Per-Second Organic
Microprocessor on Plastic Foil
Kris Myny, Student Member, IEEE, Erik van Veenendaal, Gerwin H. Gelinck, Jan Genoe, Member, IEEE,
Wim Dehaene, Senior Member, IEEE, and Paul Heremans
Abstract—Forty years after the first silicon microprocessors, we Plastic electronics refers to the technology to make transistors demonstrate an 8-bit microprocessor made from plastic electronic and circuits with thin-film organic or plastic semiconductors technology directly ontion is as low as 100today limited to 40 instructions per second. The pW. The ALU-foil operates at a supply voltageflexible plastic foil. The operation speed isower consump- on arbitrary substrates, including not only rigas glass, but also flexible plastic foils. A variid substrates suchety of organic of 10 V and back-gate voltage of 50 V. The microprocessor can molecules and polymers have been developed as semiconducexecute user-defined programs: we demonstrate the execution of tors, and the best ones [1]–[4] today feature a charge carrier
the multiplication of two 4-bit numbers and the calculation of themoving average of a string of incoming 6-bit numbers. To executesuch dedicated tasks on the microprocessor, we create small plastic mobility on the order of 1–10 cmlower than that of silicon. When integrated/Vs, some 100into circuits, theto 1000 times circuits that generate the sequences of appropriate instructions. realistic mobility values are somewhat lower but nevertheless The near transparency, mechanical flexibility, and low power con- sufficient for applications such as backplanes for flexible sumption of the processor are attractive features for integration on active-matrix displays, in particular for flexible electronic everyday objects, where it could be programmed as, amongst otheritems, a calculator, timer, or game controller. papers [5]. The first dedicated circuit applications of organic thin-film transistors have also appeared in recent years, such cessor,organic processor, organic transiIndex Terms—flexible processor, organic circuits, organic microprocessor,Dual-gate, flexible circuits,stor, plastic circuits, plasticflexible micropro- as recently demonstrated by the idriver for an organic active matrix OLED display [6]. Such cir-ntegration of an organic line microprocessor, plastic processor. cuits can be made directly on thin and ultra-flexible plastic foils, which allows them to be very simply laminated on everyday objects, and furthermore provides appealing characteristics in
                                         I. INTRODUCTION                                                  terms of bending radius and robustness: we no longer talk of
LECTRONICS pervades everyday life and is undeniably flexible electronics but of truly crinkable electronics [7].
making its way from computing to telephony and to as- Here, we investigate the possibility to use this technology to sisting us in everyday tasks through products such as electronic realize microprocessors on plastic foil. As the cost of an elecpaper to read and write, electronic noses to sense gases, smart tronic chip decreases with production volume, ultralow-cost milighting with electronics to save energy, and so on. The key en- croprocessors on easy-to-integrate flexible foils will be an enabler of these pervasive electronics applications is the fact that abler for ambient intelligence: one and the same type of chip integration of ever more transistors with ever smaller dimen- can be integrated on vastly different types of objects to perform sions has resulted in the cost of a single semiconductor tran- customized functions, such as identification, simple computing, sistor, or switch, to dwindle to the level of ten nano-dollars per and controlling.
transistor. Nevertheless, if the cost of a transistor in a chip is The organic microprocessor has been implemented as two negligible and decreasing, the cost of placing and routing elec- different foils: an arithmetic and logic unit (ALU) foil and an tronics on daily objects is not necessarily proportionally low. instruction foil. The ALU-foil is a general-purpose foil which can execute a multitude of functions. On the other hand, the in-
Manuscript received May 07, 2011; revised July 17, 2011; accepted struction foil is a dedicated chip that generates the sequence of September 05, 2011. Date of publication November 04, 2011; date of current instructions to obtain a specific function. It sends this sequence version December 23, 2011. This paper was approved by Guest Editor Satoshi of instructions to the ALU-foil such that the combination of both

Shigematsu. This work was supported in part by the EU-Projects COSMIC(ISTIP-247681) and ORICLA (FP7-ICT-2009-4 247798).        foils results in the execution of a specific algorithm. The first

K. Myny is with imec, 3001 Leuven, Belgium, the Katholieke Universiteit prototype of the organic microprocessor [8] had only one inLeuven, 3001 Leuven, Belgium, and also with the Katholieke Hogeschool Lim- struction foil available and could operate up to six operations burg, 3590 Diepenbeek, Belgium (e-mail: van Veenendaal is with Polymer Vision, 5656 AE Eindhoven, The Nether- per second (OPS). In this paper, we report an improved organic lands. microprocessor that can run 40 OPS and can operate with two
J. Genoe is with imec, 3001 Leuven, Belgium, and also with the Katholieke           different instruction foils. We first discuss the technology and
Hogeschool Limburg, 3590 Diepenbeek, Belgium.G. H. Gelinck is with the Holst Centre/TNO, 5605 KN Eindhoven, The choice of logic family used for the microprocessor foil. Sub-
Netherlands.                                                                                                        sequently, we report on the design and measurement data of
W. Dehaene, and P. Heremans are with imec, 3001 Leuven, Belgium, and the ALU-foil. Next, a complete integrated microprocessor is also with the Katholieke Universiteit Leuven, 3001 Leuven, Belgium.Color versions of one or more of the figures in this paper are available online demonstrated by combining the ALU-foil with the instruction at foil. Finally, we conclude by comparing the organic micropro-
Digital Object Identifier 10.1109/JSSC.2011.2170635                                         cessor to the silicon Intel 4004 early-days processor.

0018-9200/$26.00 © 2011 IEEE

back-gate as  -control gate (right). (Figures from [11].)


In our organic thin-film transistor (OTFT) technology, all layers to make the circuits are processed directly on a 25-m-thick PEN (polyethylene naphthalate) foil and consist of polymers or organic molecules, with the exception of metals (Au) for gates, sources, drains, and interconnect lines between the transistors [9]. The OTFT technology is a unipolar p-type, single- technology, using pentacene as semiconductor. The basic transistor has a channel length of 5 m.
The yield of larger integrated circuits in such a single-, p-type-only technology is intrinsically limited, as a result of the parameter variability [10]. Myny et al. have demonstrated an increased circuitrobustnessby the additionofan extra gate to each OTFT, leading to the availability of multiple ’s in a unipolar p-type technology [11]. The organic microprocessor has been designed in this technology. A cross section is shown in Fig. 1. As depicted, each TFT comprises two gates, a front gate and a back gate. The front gate controls the channel current while the back gate, which is weakly coupled to the semiconductor channel, is used to shift the transistor’s threshold voltage. This is depicted in Fig. 1. As a consequence, the  of each single transistor can be independently tuned.
The key factor when determining the choice of logic family for the basic circuit gates is the circuit robustness parameterized by the noise margin. Fig. 2 shows the noise margin (at 20 V) of typical zero- inverters when no back-gate is used, compared with the noise margin achievable with an optimized dual-gate zero- topology. In this optimized topology, the back gatesoftheload transistorsareconnectedtothefrontgates, while all back gates of the drive transistors are connected to a common rail, to which a back-gate voltage is applied externally [11].
The typical spread on threshold voltage in organic TFT technology is 0.2 to 0.5 V, which is large compared with the noise margin achievable with single-gate technology. As a result, it is common practice in the field of organic electronics to use a transistor-level approach to design (simple) circuits. Indeed, it is usually necessary to simulate the schematic entry with an analog circuit-level simulator (such as Spectre or Spice) and use
Monte Carlo simulations to predict yield. However, such analog circuit-level simulators are not adapted to deal with the needed level of complexity to design and simulate an organic microprocessor due to the large number of (parallel) switching gates, large amount of input, control, and output signals. In contrast, in our optimized dual-gate, the much improved noise margin allows to make use of common digital design practices. Starting from the basic characteristics of inverters and other logic gates, we designed a robust library of basic digital logic gates (inverters, NANDs, buffers). This standard cell library was used to
design the organic microprocessor by means of a gate-level design approach. Therefore, after modeling, simulating, and measuring the basic building blocks, we used a gate-level simulator (Modelsim) with our standard cell library to design and simulate the organic microprocessor. The ratio between drive and load transistor for the logic gates in the library was a 1:1 ratio beneficial for area, with a minimal of 140/5 m/ m.
Figs. 3 and 11 show photographs of some microprocessors on foil. The 25-m-thick foil is highly flexible. Furthermore, the complete circuit is nearly transparent, as only the metal electrodes of gates, sources, drains and interconnect lines are reflective.


Characteristic to a microprocessor is that its hardware is not dedicated to a single function or operation, but is designed such that the operations performed on (digital) inputs can be programmed and defined after manufacture of the processor. The challenge, therefore, is to manage the plurality of possible critical data paths in the microprocessor, for all different instruction codes and inevitable variations due to nature of organic technology on foil. Our microprocessor has been constructed around an 8-bit arithmetic and logic unit (ALU) which comprises three blocks as schematically represented in Fig. 4. The first block adds or subtracts the incoming numbers (arithmetic unit), the second block performs logic operations on the incoming data (logic unit) and the third block shifts the incoming bits (bit shift unit). The arithmetic unit is designed as a ripple carry adder/subtractor. Detailed control over each of these three blocks and the actual selection of the output of the ALU unit is determined by the microprocessor’s instruction set, also called
Fig. 4. Symbol (top-left), instruction set (bottom-left), and architecture (right) of the main building block of the microprocessor, namely the 8-bit ALU. Three operational code bits, opcode(2:0), are used to select among the different instructions.
operational codes or “opcodes” (Fig. 4). As the architecture depicts, the ALU executes every clock cycle instructions on each of the three units in parallel. Subsequently, a multiplexer selects the desired instruction to be executed in that clock cycle.
Fig. 5 outlines the complete architecture of the microprocessor foil. Around this ALU, a minimal set of 8-bit registers has been placed, for storing the working data (accumulator A, working registers (C0, C1 and C2) and an output register). The storing and loading of the data in these registers is also controlled by the instruction set. The registers select bits (RR in the table of Fig. 5) correspond to bits 7 and 8 of the opcode and are used to select between the four working registers, C0 to C3. Working register C3 is implemented as a hard-coded decimal 1 in order to ease the implementation of the increment and decrement instructions.
Fig. 5. (A) Architecture of the microprocessor core, comprising the Arithmetic and Logical Unit (ALU), accumulator register “A” and output register “OUT” at the top and the input multiplexer and storage registers “C” at the bottom. (B) Implemented instruction set: RR refers to the binary representation of the selected
We have tested all of the individual instructions of the microprocessor foil extensively for different bias conditions. Fig. 6(A) shows that the microprocessor can operate at up to 40 OPS, when powered at a 20-V supply voltage and an appropriate back-gate voltage. This maximum frequency is determined by the 25-ms critical path delay in the design. Fig. 6(B) shows that the microprocessor can operate at voltages down to 10 V. The critical path is defined by the subtract operation, where the carry bits need to ripple subsequently through each of the bits. As shown in Fig. 7, the contribution of these logic gates was measured separately on different kinds of ring oscillators, as a function of the capacitive load of the gates. An inverter driving a single subsequent stage has a minimum capacitive load, and in that case its gate delay is 83 s. Similarly, the minimum gate delay of a two-input NAND is 126 s, while one input is connected to . However, when a logic gate has to drive multiple subsequent stages in parallel, it is slowed down: we show in Fig. 7 that a gate driving nine identical inverter gates in parallel is slowed down to 1 ms. This gate delay, combined with the length of the critical path, explains why with our current design and topology, the processor frequency is 40 OPS. Moreover, because it was the first time a circuit of this complexity was realized in organic technology, we preferred conservative design choices. For instance, we utilized only gates with a fan-in of 2 and our signal buffering strategy was very conservative. By alleviating these restrictions and by optimizing the design, we estimate that the frequency can improve towards the hundreds of OPS range. Another reason for the current limitation to the tens of OPS range is related to the choice of logic family, where we have chosen for robustness. Other unipolar logic types (dual-gate, diode-connected) are more advantageous in terms of speed [11]. As Figs. 6(A) and 7 also depict, the frequency of the ALU and the ring oscillator decreases when the back-gate voltage
gate delay of the individual inverters of the chain. The stage delay of inverters driving five and nine gates have corresponding labels. The triangles show the measured stage delay of two-input NAND gates driving one, five, and nine subsequent gates.
of the drive transistors increases. This can be explained by a negative -shift of the drive TFT yielding less drive current
Fig. 8. Demonstration of user-programmability of the microprocessor. (A) Time evolution of the 4-bit input signals and the 8-bit output signal when the multiplier instruction set (see Fig. 9) is implemented in the organic microprocessor foil. The top axis shows the cycle number of the program loop. (B) The 6-bit input signal (left axis) and the 7-bit output signal (right axis, which has one significant binary digit more than the input) when the running averager program (see Fig. 10) is executed. The top axis shows the cycle number of the program loop.
Fig. 9. Architecture and operation of instruction sequence generating circuits on foil. (A) Schematic of instruction generating foils: n is 5 in case of the multiplier foil and 4 for the running averager. (B) Program listing of the dedicated instruction set of the multiplier, the execution of which can be followed by the example shown in (C): the instruction lines of the code (B) have been colored using the same color code as the outcome that they produce in the multiplication of 1010 (10)

and 0111 (7) in (C). (D) shows the program instruction for the moving averager.
Furthermore, we show that our microprocessor is truly a general-purpose machine that can be programmed for multiple uses by executing instruction codes for different applications. To this end, a test board was developed that programs the microprocessor. In a first example, shown in Fig. 8(A), the microprocessor is programmed to execute a multiplication of two numbers. The solid line shows a sequence of cycles, in each of which two 4-bit numbers are multiplied to give an 8-bit output value, shown as dotted line. The input values are shown on the left scale (from 0 to 15), while the output is shown on the right scale (0 to 255). The first cycle shows the multiplication of the binary numbers 0111 (decimal 7) and 1010 (decimal 10) to 01000110 (decimal 70). This value remains at the output while two new numbers are fed in at the input (0000 and 1111) and multiplied. In the third cycle, the output of this multiplication (00000000) appears at the output while a new set is provided and executed, and so on. The processor clock can be verified to run at 40 Hz during this execution, as explained above.
Fig. 10. Demonstration of operation of the plastic microprocessor commanded by an instruction foil. (A) Decimal representation of the measured seven least significant bits of the instruction generated by the running averager instruction foil running at 70 Hz. The clock is shown at the right-hand axis. The data is valid on the rising edge of the clock. (B) Measured output of the microprocessor foil connected to the running averager instruction foil. As the input bit stream switches from 000000 to 000111, the output gradually increases to the same level over three loop cycles, but with seven significant binary digits.
In a different example, chosen from the application domain of digital signal processing, the microprocessor executes the weighed time-averaging of a stream of incoming digital inputs to reduce random noise. This algorithm is known to clean up the output signal of a sensor after digitization by an analog-to-digital converter (ADC). By virtue of its applicability to large area
Fig. 11.  Photograph of the 8-bit ALU-foil.
substrates, plastic electronics is suited to develop large-area sensors [12], and the first plastic ADC converters were shown recently [13], [14]. We implemented the algorithm of a moving averager, i.e., an averaging algorithm in which the weight of the past data decreases exponentially, and demonstrate the execution of the algorithm in Fig. 8(B). The 6-bit input provided to the microprocessor is shown as the red line: 001111 (15) during the first four loop cycles, then 111101 (61) during the next 10 cycles, then 000110 (6). The running averager calculates the weighed average as a 7-bit number, which can be seen to tend to the steady input values after they have been provided for some cycles. Here again, the clock speed of the processor is 40 Hz.


Until now, only the ALU-foil of the microprocessor core was discussed. In the above demonstrations, the instructions for the microprocessor were generated by external test equipment. To come to a complete plastic solution, we also designed a plastic control unit, shown Fig. 9(A). This control unit has as task to take instructions from a memory in the appropriate order controlled by a program counter. These instructions are sent as opcodes to the microprocessor core. Opcodes for the program counter are also generated to enable branching in the program. In the ideal case, the program would be stored in programmable, nonvolatile memory on the foil. However, programmable nonvolatile memory compatible with plastic thin-film transistors on foil is still subject of research today and is as such not available for our experiments. Therefore, like in the early days of silicon technology, we used true read-only memory (ROM): the


instructions are hardcoded on the foil. A different foil is designed for every program. For the low-cost, low-complexity but high-volume applications that are envisaged here, this procedure could even be a realistic commercial scenario. The instruction sets generated by the multiplier instruction foil and the moving averager foil are shown in Fig. 9(B) and (D), respectively.
The operation of the running averager instruction foil by itself is shown in Fig. 10(A). This circuit does not contain a ripple carry adder, and therefore it has a shorter critical path compared with the microprocessor. Stand-alone, the instruction circuit can run at a clock speed of 70 Hz.
Finally,wedemonstrate thecombined operation of aninstruction foil with the microprocessor. We conducted this experiment with the running averager. The correct operation of this combination is shown in Fig. 10(B). This demonstration shows that the microprocessor can indeed accept its instruction set from a dedicated plastic circuit and is not limited to instruction sets from a test board.


In Table I, we summarize the circuits fabricated and demonstrated in plastic technology. With less than 100 W, the power consumption of the flexible chips is already quite low and could further be reduced by voltage scaling in the future [15], [16]. Such very low power consumption is very important for widespread mobile applications on everyday objects.
To conclude, we compare in Table II the characteristics of the first plastic microprocessor with the early silicon processors made in p-type-only silicon technology some four decades ago.[1] Significant correspondence can be seen regarding parameters such as gate length, supply voltage and transistor count, but some marked differences are also clear. The instruction rate of the plastic technology is about three orders of magnitude slower than the early silicon processor, as a direct consequence of the three-orders-of-magnitude lower carrier mobility in organic semiconductors. However, on the positive side, the power consumption is also four orders of magnitude smaller. In future implementations, semiconductors such as amorphous oxides [17] could boost the performance to an intermediate speed, with still very attractive power consumption for low-cost, lowperformance, and mobile applications.


This work was performed in a collaboration between imec and TNO in the frame of the HOLST Centre.


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Kris Myny (S’08) was born in Hasselt, Belgium, on July 26, 1980. He received the M.S. degree from the Katholieke Hogeschool Limburg, Diepenbeek, Belgium in, 2002. He is currently working toward the Ph.D. degree on the design of organic circuits at Katholieke Universiteit Leuven, Leuven, Belgium. He joined imec, Leuven, Belegium, in 2004 as a Member of the Polymer and Molecular Electronics group. His main research interests are the design, fabrication and optimization of digital organic circuits for amongst others organic RFID tags and AMOLED-backplanes.
Mr. Myny was the recipient of the imec 2010 Scientific Excellence Award.
Erik van Veenendaal received the Ph.D. degree in physical chemistry from the University of Nijmegen, Nijmegen, The Netherlands, in 2001.
In the same year, he joined Philips Research Eindhoven, Eindhoven, The Netherlands,to work on characterization and modeling of organic electronics. In 2003, with the launch of Polymer Vision as an internal Philips company, his work focused on the characterization of organic electronics enabled displays and setting up the quality and reliability program for rollable displays. Currently, his main responsibilities
as principal scientist at Polymer Vision BV, Eindhoven, include R&D into future generations of rollable displays and management of subsidy R&D programs.
Gerwin H. Gelinck received the Ph.D. degree from the Technical University of Delft, Delft, The Netherlands, in 1998.
That same year, he joined Philips Research as a Senior Scientist, where he began working on polymer and organic transistors and their use in integrated circuits, displays and memories. In 2002 he was co-founder of Polymer Vision BV, Eindhoven, The Netherlands. From 2002 to 2006, he was Chief Scientist of Polymer Vision. Since 2007 he is
Program Manager of “Organic and Oxide Circuitry”
at the Holst Centre, which is a joint research initiative of TNO and imec.
Jan Genoe (S’87–M’02) was born in Leuven, Belgium, on May 19, 1965. He received the M.S. degree in electrical engineering and Ph.D. degree from the Katholieke Universiteit Leuven, in 1988 and 1994, respectively.
Afterward, he joined the Grenoble High Magnetic Field Laboratory as a Human Capital and Mobility Fellow of the European Community. In 1997, he became a Lecturer with the Katholieke Hogeschool Limburg (KHLim), Diepenbeek, Belgium. Since
2003, he has been both Professor with KHLim and
head of the Polymer and Molecular Electronics (PME) group of imec. His current research interests are organic and oxide transistors and circuits as well as organic photovoltaics. He is the author and coauthor of approximately 90 papers in refereed journals.
Wim Dehaene (S’89–M’97–SM’04) was born in Nijmegen, The Netherlands, in 1967. He received the M.Sc. degree in electrical and mechanical engineering and Ph.D. degree from the Katholieke Universiteit Leuven, Leuven, Belgium, in 1991 and 1996, respectively. His dissertation is entitled “CMOS integrated circuits for analog signal processing in hard disk systems.”
After receiving the M.Sc. degree, he was a Research Assistant with the ESAT-MICAS Laboratory of the Katholieke Universiteit Leuven. His research
involved the design of novel CMOS building blocks for hard disk systems. The research was first sponsored by the IWONL (Belgian Institute for Science and Research in Industry and agriculture) and later by the IWT (the Flemish institute for Scientific Research in the Industry). In November 1996, he joined Alcatel Microelectronics, Belgium. There he was a Senior Project Leader for the feasibility, design, and development of mixed-mode systems-on-chip. The application domains were telephony, xDSL and high speed wireless LAN. In July 2002, he joined the staff of the ESAT-MICAS laboratory of the Katholieke Universiteit Leuven, where he is now a Full Professor. His research domain is circuit level design of digital circuits. The current focus is on ultra low power signal processing and memories in advanced CMOS technologies. Part of this research is performed in cooperation with imec, Leuven, Belgium, where he is also a part-time Principal Scientist. He is currently teaching several classes on electrical engineering and digital circuit and system design.
Paul Heremans received the Ph.D. degree in electrical engineering from the University of Leuven, Leuven, Belgium, in 1990, on hot-carrier degradation of MOS transistors.
He then joined the Opto-electronics Group of imec, Leuven, Belgium, where he worked on optical interchip interconnects, and on high-efficiency III-V thin-film surface-textured light-emitting diodes. His current research interest is oxide and organic electronics, including circuits, backplanes and memories, as well as organic photovoltaics. He is an imec Fellow, Director of imec’s Large Area Electronics department and part-time Professor at the Electrical Engineering Department of the University of Leuven and editor of Organic Electronics.

[1] Historic data are collected on the Intel Museum. [Online]. Available: The specifications can be found at datashts/intel-4004.pdf

Cooling of microprocessors with micro-evaporation: A novel two-phase cooling cycle
Jackson Braz Marcinichen a,*, John Richard Thome a, Bruno Michel b
aLaboratory of Heat and Mass Transfer (LTCM), Faculty of Engineering (STI), E´cole Polytechnique Fe´de´rale de Lausanne (EPFL), Station 9, CH-1015 Lausanne, Switzerland
b IBM Research GmbH, Zurich Research Laboratory, Sa¨umerstrasse 4, CH-8803 Ru¨schlikon, Switzerland Dedicated to Professor Dr.-Ing. Dr.h.c.mult. Karl Stephan on the occasion of his 80th birthday.

a r t i c l e i n f o                                                   a b s t r a c t

Article history:
Received 4 March 2010
Received in revised form 3 June 2010
Accepted 5 June 2010 Available online 12 June 2010
Cooling circuit
Heat recovery
Three micro-evaporator cooling cycles, one with a pump, one with a compressor and a hybrid of the two together, are proposed for cooling a computer blade server. The hybrid cycle is characterized by the interchangeability between the first two cycles, where the decision on the cycle to operate is based on the season (necessity or economical benefit for heat recovery) or the maintenance of cycle’s driver. The main characteristics of each cycle are presented as well as the details of the micro-evaporator cooler for the blade’s CPU. Analysis of the cycle overall efficiency and the potential for heat recovery shows that the best cycle to use depends mainly on the end application of the heat recovered. Four refrigerants were evaluated as the possible working fluids for cooling the microprocessors.
HFC134a and HFC245fa were found to be the best choices for the desired application. ª 2010 Elsevier Ltd and IIR. All rights reserved.
Refroidissement des microprocesseurs a` l’aide de la microe´vaporation : un cycle de refroidissement diphasique et innovant
Mots cle´s : Refroidissement ; Composant ; E´lectronique ; Microprocesseur-conception ; Comparaison ; Circuit frigorifique ; COP ; Re´cupe´ration de chaleur
* Corresponding author. Tel.: þ41 21 693 5894; fax: þ41 21 693 5960. E-mail address: (J.B. Marcinichen).
0140-7007/$ e see front matter ª 2010 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2010.06.008

1.             Introduction

Cooling of data centers causes estimated annual electricity bills of 1.4 billion dollars in the United States and of 3.6 billion dollars world wide (Koomey, 2007). Currently, the most widely used cooling technology is refrigerated air cooling of the data centers’ numerous servers. According to recent articles published at ASHRAE Winter Annual Meeting at Dallas (January, 2007) typically 40% or more of the refrigerated air flow bypasses the server racks in data centers. Furthermore, servers that are turned off or on standby are cooled as if they were operating, wasting a significant amount of the energy for the unnecessary flow. This poor energetic performance in one of industries leading technological sectors is quite startling and motivates the search for a green thermal solution for future generations of higher performance servers that consume much less energy to operate and cool while they also provide the possibility to recover a large quantity of waste heat. This is the topic of research addressed here.

Current chip cooling technology consists of conducting the microprocessor’sJouleheatingawaythroughthesiliconchipdie itself,thenacross a thermal interface material (TIM) toa copper or aluminum heat spreader/finned cooling element and finally byconvectiontorefrigeratedairenteringat10e15C.Lookingat thisonamaterialbasis,themicroprocessorcircuitryhasamass of about 5 mg, the silicon die about 5 g and the metallic cooling element about 0.5 kg, representing about five orders of magnitudeintheratioofthematerialsinvolvedandthuspointsoutthe huge opportunity to improve this whole process.

mw ODP
critical heat flux [W cm2] coefficient of performance [-] global warming potential [-] refrigerant mass flow rate [kgh1] water mass flow rate [kg h1] ozone depleting potential [-] suction pressure [bar] discharge pressure [bar] cooling capacity or power generated by electronic components [W] subcooling [K] discharge temperature [C] inlet evaporating temperature [C]
outlet evaporating temperature [C]
inlet water temperature [C] outlet water temperature [C]
compressor power [W] pumping power of water in the condenser [W] liquid pump power [W]
xoutlet Greek r
Dhcomp DP
DPME DhME hcycle hcycle_LP hcycle_VC
comp Cond
disc evap ME
r_pump pumping power of water in the
subcondenser [W] volumic refrigerating effect [kJ m3]
outlet vapor quality [%]
specific mass [kg m3] compressor enthalpy difference [kJ kg1] pressureincreaseprovided by the liquid pump[Pa] micro-evaporator pressure drop [bar] specific cooling capacity [kJ kg1] cycle overall efficiency [-]
liquid pump cycle overall efficiency [-]
vapor compression cycle overall efficiency [-]
compressor condenser discharge evaporating micro-evaporator suction water

Thermal designers of data centers and server manufacturers now agree about the long term need to improve the cooling process by implementing liquid or two-phase cooling directly in the server itself, eliminating the poor thermal performing air as a coolant all together (Greenberg et al., 2006; Hannemann and Chu, 2007; Samadiani et al., 2008). That said, there is a clear need for a detailed design and evaluation of these new cooling strategies in order to arrive at an improved solution. They should provide more efficient heat transfer from the chips, memories, etc. using water-cooled or boilingcooled elements, eliminating air as a means of heat transfer, while also reducing energy consumption for driving the cooling system by a significant amount. Some examples of design and evaluation of these new cooling strategies can be found in Scott (1974), Bash (2001), Peeples (2001), Heydari (2002), Schmidt and Notohardjono (2002), Maveety et al. (2002), Phelan and Swanson (2004) and Suman et al. (2004). Additionally, since data centers often dissipate on the order of

5e15 MW of heat, this makes heat recovery an important

Data center with 64 Blades (Refrigerated air cooling vs. Twophase on-chip cooling system)

Fig. 1 e Comparative power supply for a data center containing 64 blades for air cooled and two-phase on-chip cooling with and without heat recovery.

energetic and environmental issue to consider and will greatly reduce the CO2 footprint of the data center.

Fig. 1 shows the comparative energy consumption required by a data center with 64 blades (325 W per blade) when using traditional air cooling, two-phase on-chip cooling and twophase on-chip cooling when the energy is recovered, using a vapor compression cycle. For air cooling, it is assumed that the power required to cool the data center is the same as that required to run the information technology equipment (Koomey, 2007; Ishimine et al., 2009). This is plotted as a function of the compressor overall efficiency. It is seen that, if no heatwasrecovered,thecostofcoolingthedatacenterwouldbe approximately 59% that of traditional air cooling when operatingatacompressorwithanoverallefficiencyof60%,whichis typical of light commercial systems. However, if the heat was toberecoveredandconsidering60%ofrecoveryefficiency,this value drops down to about 24% that of traditional air cooling. These results show that the cost of cooling could be drastically decreased when using on-chip cooling, representing a huge potential for the next generation of data centers cooling systems.

Recent publications show the development of primarily four competing technologies for cooling chips (all with their own pros and cons): microchannel single-phase flow, porous media flow, jet impingement cooling and microchannel twophase flow (Agostini et al., 2007). Single-phase liquid cooling is now fairly well known and can be used to remove high heat fluxes. Leonard and Phillips (2005) showed that the use of such new technology for cooling of chips could produce savings in energy consumption of over 60%. Despite the potential of this technology,itsapplicationseemslimitedsofarduetotheneed of a high pumping power to keep the temperature gradient in the fluid from inlet to outlet within acceptable limits. Moreover,theworkingfluidmodeledinmoststudiesisusuallypure water, which presents a problem with its high freezing point, and hence the even higher pressure drop/pumping power of watereglycol mixtures, the real liquid working fluid, has to be dealt with for realistic evaluations. Furthermore, manufacturers are reluctant to use water-based fluids directly in their servers and mainframes due to reliability questions.

The use of a porous media with single-phase and twophase flow has as the main advantage of the large surface area for heat transfer. Nevertheless, a high pumping power remains as a limitation. Jet impingement is another promising cooling solution that can reach low thermal resistances without any thermal interface and yield nearly uniform surface temperatures with multiple jets requiring potentially high pumping power. However, possible problems related to surface erosion as a consequence of the continuous impingement of the high velocity jets needs to be further investigated.

Finally, two-phase flow in microchannels, i.e. evaporation of dielectric refrigerants, is a promising medium to long term solution, despite the higher complexity involved. This solution consumes a low pumping power (only 1/10 as much as water cooling according to Hannemann et al., 2004), has good temperature uniformity (Agostini et al., 2008), very high heat transfer coefficients (as high as 270 000 W m[1] K1 according to Madhour et al., in press), and provides high heat flux dissipation. Studies demonstrate that the thermal resistance decreases with increasing heat flux and with decreasing hydraulic diameter. Possible problems with flow instabilities have been resolved using micro-orifices at the channel inlets (Agostini et al., 2007) while the prediction methods of local heat transfer coefficients (Consolini and Thome, 2010), the critical heat flux (Mauro et al., 2010), and pressure loss (Cioncolini et al., 2009) in the two-phase region are still improving. On the other hand, published tests using water evaporating in microchannels to cool computer chips are not a viable solution (too low absolute pressure and vapor density at 60 C relative to the ensuing pressure drop and speed of sound) and hence this fluid is not considered here.

Marcinichen and Thome (2010) showed results of a simulationcodeforsingle-phaseliquidwaterandtwo-phaseHFC134a cooling cycle, both with a liquid pump as the driver. The liquid water cooling cycle presented a pumping power consumption 16.5 times that obtained for the two-phase HFC134a cooling cycle, considering a design of components and piping so that thetotalpressuredropinthecyclewasabout1bar.Theresults permitted them to conclude that the two-phase HFC134a coolingcyclecanoperateatamuchlowerenergyconsumption compared with a single-phase liquid water cooling cycle. This result can be considered a differential when compared with demonstration projects, such as that for the new supercomputer called AQUASAR (Ganapati, 2009), which considers the implementation of a liquid water cooling cycle on a rack cabinet with power consumption of around 10 kW.

In this context, the objective of the present study is to proposeandanalyzepotentialtwo-phasecoolingcyclesableto maintain the temperature of the chip below its upper operating limit and to recover energy from the cycle’s condenser forexternalapplications,suchasheatingabuilding,residence, hospital,preheatingofboilerfeedwater,etc.Themainfocusis to work with two-phase flow of dielectric refrigerants, using a liquid pump or a vapor compressor to drive the fluid, which can reduce the demand for cooling energy by an impressive amount compared to the large refrigeration chillers currently used to cool air in data centers. A specific analysis of the potential working fluids for this application and the results of critical heat flux obtained with mathematical models developed for micro-evaporators are presented. Also, qualitative and quantitative comparisons of the cooling cycles proposed are made here, using the cycle overall efficiency and the potential for heat recovery as the factors of merit.

had a mass flow rate, a pumping power and a condenser size that were 4.6, 10 and 2 times smaller than the water-cooled system. The coolant temperature rise was 10 C for the water but negligible for HFC134a. In their study, a demonstration radar cooling unit was also designed and built for a 6.4 kW heat load (sixteen 400 W cold plates with convoluted fins). For 25 C ambient air temperature, the working fluid saturation temperature was maintained at 32 C with a total volumetric liquid flow rate of 376 L h1 and a cold plate outlet vapor quality of 30%, providing a safety factor for dry-out. The system was stable, easily controllable and provided essentially isothermal conditions for all the cold plates. They emphasized the significant benefits from efficiency, size and weight that were provided with the PLMC solution.

Mongia et al. (2006) designed and built a small-scale refrigeration system applicable for a notebook computer. The system basically included a minicompressor, a microchannel condenser, a microchannel evaporator and a capillary tube as the throttling device and is considered to be the first refrigeration system developed that can fit within the tight confines of a notebook and operate with high refrigeration efficiencies. HC600a (isobutane) was the working fluid, chosen from an evaluation of 40 candidate refrigerants. According to them, HC600a presented the best efficiency at a low pressure ratio and was readily available, although flammable, but the system required only a very small fluid charge (a few milliliters). Two evaporators were used, the first one a microchannel evaporator to cool the high heat flux component (chip) and the second one a superheater (conventional finned evaporator) to cool lower heat flux components, such as memories, which also guaranteed that superheated vapor was delivered to the minicompressor inlet. Two thermal test vehicles were used to simulate the chip and the power components. For a baseline operating condition, when the evaporator and condenser temperatures and the heat load were 50 C, 90 C and 50 W, the coefficient of performance (COP) obtained was 2.25. The COP reached 3.70 when the evaporator and condenser temperatures increased and decreased by 10 C from the baseline conditions and the heat load was reduced to 44 W. The smallscale refrigeration system achieved 25e30% of the Carnot efficiency (ideal COP for a Carnot cycle), values comparable with those obtained in today’s household refrigerators.

Trutassanawinetal.(2006)designed,builtandevaluatedthe performance of a miniature-scale refrigeration system (MSRS) suitableforelectronicscoolingapplications.TheirMSRShadthe following components: a commercial small-scale compressor, a microchannel condenser, a manual needle valve as the expansion device, a cold plate microchannel evaporator, a heat spreader and two compressor cooling fans. A suction accumulatortoavoidliquidflowtothecompressor,anoilfiltertoreturn oil to the compressor and guarantee good lubrication, and heat sources to simulate the chips were also installed. HFC134a was the working fluid. System performance measurements were conducted at evaporator temperatures from 10 C to 20 C and condenser temperatures from 40 C to 60 C. The cooling capacityofthesystemvariedfrom121Wto268WwithaCOPof 1.9e3.2 at pressure ratios of 1.9e3.2. Their MSRS was able to dissipate CPU heat fluxes of approximately 40e75 W cm2 and keep the junction temperature below 85 C for a chip size of

1.9 cm2. It was concluded that a new compressor design for electronics cooling applications was needed to achieve better performanceofthesystem(themostsignificantlossesoccurred in the compressor, which was not designed for the operating conditions of electronics cooling). It was also recommended to study the development of an automatic expansion device and a suitable control strategy for the MSRS.

Trutassanawin et al. (2006) also mentioned some alternative cooling approaches such as heat pipes, liquid immersion, jet impingement and sprays, thermoelectrics and refrigeration. For refrigeration, the following possible advantages were cited: (i) one of the only methods which can work at a high ambient temperature, (ii) chip to fluid thermal resistances are considerably lower, resulting in lower junction temperatures, which could lead to higher heat fluxes being dissipated, and (iii) lower junction temperatures can also increase the microprocessor’s performance and increase the chip’s reliability. Possible “disadvantages” were characterized to be: (i) an increase in the complexity and cost, (ii) possible increase in the cooling system volume and (iii) uncertainties in the system reliability (moving parts in the compressor).

Thome et al. (2007) surveyed the advances in thermal modeling for flow boiling of low-pressure refrigerants in multimicrochannel evaporators for cooling of microprocessors. According to them, multi-microchannel evaporators hold promise to replace the actual air-cooling systems and can compete with water cooling to remove high heat fluxes, higher than 300 W cm2, while maintaining the chip safely below its maximum working temperature, providing a nearly uniform chip base temperature (Agostini et al., 2008) and minimizing energyconsumption.Variablessuchascriticalheatfluxes,flow boilingheattransfercoefficientsandtwo-phasefrictionfactors were evaluated and characterized as important design parameters to the micro-evaporator for high heat flux applications.

Thome and Bruch (2008) simulated two-phase cooling elements for microprocessors with micro-evaporation. Heat fluxesof50Wcm2 and150Wcm2 inamicro-evaporator with channels 75 mm wide, 680 mm high and 6 mm long with 100 mm thick fins were simulated for flow boiling. The size of the chip was assumed to be 12 mm by 18 mm and the micro-evaporator was considered with the fluid inlet at the centerline of the chip and outlets at both sides, i.e. a split flow design to reduce the pressure drop but increase the critical heat flux. Results of pumping power, critical heat flux, and junction and fluid temperaturesweregeneratedforHFC134aataninletsaturation temperature of 55 C (chosen to allow for heat recovery). The followingconclusionswerereached:i)theinfluenceofmassflux on the fluid, chip and wall temperatures was small, ii) for the heatflux of 150 W cm2, the chip temperature was 70C orless, i.e.wellbelowitsoperationallimitof85C,iii)fortheheatfluxof 150 W cm2, the junction-to-fluid temperature difference was only 15 K, which is lower than that with liquid cooling systems, iv)thefluidtemperaturecouldstillberaisedby10Ktoajunction temperature of 80 C while rejecting heat at 65 C for reuse, and v) the critical heat flux increased with the mass flux and the lower limit was about 150 W cm2 for 250 kg m2 s1. The channel width had a significant effect on the wall and junction temperatures, and there was a turning point at about 100 mm when considering 1000 kg m2 s1 of mass flux and 150 W cm2 of base heat flux, at which these temperatures reached a minimum. For the same mass flux and base heat flux, the reduction of channel width also reduced the energy consumption to drive the flow (pumping power).

From a system viewpoint, Thome and Bruch (2008) showed an approximate comparison of performances of liquid water cooling versus two-phase cooling. For the same pumping power consumption to drive the fluids, two-phase cooling allowed the chip to operate about 13 K cooler than water cooling or it could operate at the same junction temperature but consume less pumping power using a lower refrigerant flow rate. The two-phase cooling system appeared to be more energy-efficient than classical air-cooling or direct liquid cooling systems while also exhausting the heat at higher reusable temperatures. Regarding the choice between a pump and a compressor as the driver for a micro-evaporation heat sink system, they emphasized that the choice depends on the economic value of the re-used energy. The system with a compressor is ideal for energy reuse because of the higher heat rejection temperature; however the additional energy consumed by the compressor compared to the pump has to be justified by the reuse application.

Mauro et al. (2010) evaluated the performance of a multimicrochannel copper heat sink with respect to critical heat flux (CHF ) and two-phase pressure drop. A heat sink with 29 parallel channels (199 mm wide and 756 mm deep) was tested experimentally with a split flow system with one central inlet at the middle of the channels and two outlets at either end. Three working fluids were tested (HFC134a, HFC236fa and HFC245fa) and also the parametric effects of mass velocity, saturation temperature and inlet temperature. The analysis of their results showed that a significantly higher CHF was obtainable with the split flow system compared to the single inlet-single outlet system (Park and Thome, 2010), providing also a much lower pressure drop. For the same mass velocity, the increase in CHF exceeded 80% for all working fluids evaluateddueto theshorterheatedlengthof asplitsystemdesign. For the same total refrigerant mass flow rate, an increase of 24% for HFC134a and 43% for HFC236fa were obtained (no comparabledatawereavailableforHFC245fa).Theyconcluded that the split flow system had the benefit of much larger CHF values with reduced pressuredrops and further developments in the design of split flow system could yield an interesting energetic solution for cooling of computer chips. Fig. 2 shows the details of the two configurations of multi-microchannel copper heat sink regarding the inlet and outlet flow system.

Fig. 2 e Schematic of micro-evaporator: a) one inlet/one outlet and b) one inlet/two outlets.

Itisworthnotingthatthefocusoftheabovestudieswasthe development of multi-microchannels evaporators able to

remove “in loco” the heat load generated by the microprocessors and also the development of two-phase cooling systems able to: i) control the operating conditions in the micro-evaporator, ii) maintain the microprocessor temperature at acceptable levels, iii) recover the heat for a secondary process and iv) operate at a much lower pump energy consumption compared with a single-phase liquid water cooling system.

3.              Present work

Three micro-evaporator cooling cycles are proposed here:

1.   one with a liquid pump as the driver of the working fluid,

2.   one with a vapor compressor as the driver of the workingfluid, and

3.   one hybrid cycle that is a combination of the first twocycles.

The main characteristics of each cycle are presented below with a focus on their advantages and the functions of the components. Additionally some simulations are presented showing the following: (i) performance of the vapor compression cooling cycle for four refrigerants as the possible working fluids for cooling the microprocessors, (ii) operational limits for one specific geometry of a micro-evaporator (critical heat flux, outlet vapor quality, pressure drop, etc) to demonstrate its suitability for this type of application, and (iii) potential for heat recovery and the cycle overall efficiency for the first two cycles proposed.

3.1.            Two-phase micro-evaporator cooling cycle

Figs. 3e5 depict the cycles with a liquid pump, a vapor compressor and hybrid of these two, respectively. The goal is to control the chip temperature to a pre-established level by controlling the inlet conditions of the micro-evaporator (pressure, subcooling and mass flow rate). It is imperative to keep the micro-evaporator (ME) outlet vapor quality below that of the critical vapor quality, which is associated with the critical heat flux. Due to this limitation, additional latent heat is available, which can be used by other heat generating components. The critical heat flux and outlet vapor quality are

Fig. 3 e Schematic of the liquid pump cooling cycle.

Fig. 4 e Schematic of the vapor compression cooling cycle.

predicted using methods developed by Revellin and Thome (2008), which are a function of micro-evaporator inlet conditions and microchannel dimensions.

Another parameter that must be controlled is the condensing pressure (condensing temperature). The aim is, during the winter, to recover the energy dissipated by the refrigerant in the condenser to heat buildings, residences, district heating, etc. In order to accomplish this, the idea is to use a variable speed compressor and an electric expansion valve, as will be discussed below.

Fig. 3 depicts the two-phase cooling cycle where the flow rate is controlled by a liquid pump. The P-h diagram (Fig. 6), which was drawn for low pressure refrigerant HFC245fa, shows the thermodynamic conditions for specific points along the cooling cycle, considering 9.9 K and 60 C for the subcooling and evaporating temperature at the ME inlet, respectively. The pressure drops in the micro-evaporator and microchannel cooling plate for the memory chips (MPM) were simulated to be on the order of 0.5 bar and 0.0 bar (it is negligible), respectively, based on preliminary calculations. These values are representative and were defined only for cycle interpretation. The components considered and their main functions are presented below:

a)    Variable speed liquid pump: controls the mass flow ratecirculating in the system.

b)   Stepper motor valve: controls the liquid flow rate tocontrol the outlet vapor quality in each micro-evaporator (0%e100%).

c)    Micro-evaporator (ME): transfers the heat generated by themicroprocessor to the refrigerant.

d)   Microchannel cold plate for memories (MPM): additional component used to cool the memories using the remaining latent heat, which is available due to the limitations enforced on the micro-evaporator.

e)    Pressure control valve (PCV): controls the condensingpressure.

Fig. 5 e Schematic of the hybrid cooling cycle highlighting the possibility of interchangeability between liquid pump and vapor compression cooling cycle.

f)    Condenser: counter-flow tube-in-tube exchanger or a micro-condenser.

g)   Liquid accumulator: guarantees that there is only satu-rated liquid at the subcooler inlet, independent of changes in thermal load.

h)   Temperature control valve (TCV): controls the subcoolingat the inlet of liquid pump.

This cycle is characterized in having low initial costs, a low vapor quality at the ME outlet, a high overall efficiency, low maintenance costs and a low condensing temperature. This is a good operating option when the energy dissipated in the condenser is not recovered, typically during the summer season. However, the heat can still be recovered if there is an appropriate demand for low quality heat (low exergy).

Fig. 6 e HFC245fa P-h diagram showing the thermodynamic conditions for specific points of the liquid pump cooling cycle.

Fig. 4 shows a two-phase cooling cycle where a vapor compressor is the driver of the working fluid. The P-h diagram (Fig. 7), which was also drawn for low pressure refrigerant HFC245fa, shows the thermodynamic conditions for specific points along the cooling cycle, considering 0.69 K and 60 C for the subcooling and evaporating temperature at the ME inlet, respectively. The pressure drops in the ME and MPM were considered to be the same as for the liquid pump cycle above. The components considered and their main functions are:

a)    Variable speed compressor: controls the ME inlet pressureand consequently the level of inlet subcooling.

b)    Pressure control valve (PCV): controls the condensingpressure.

c)    Condenser:       counter-flow             tube-in-tube              exchanger or

a micro-condenser.

Fig. 7 e HFC245fa P-h diagram showing the thermodynamic conditions for specific points of the vapor compression cooling cycle.

Fig. 8 e Effect of superheating at the inlet of the VSC on the isentropic COP.

d)    Liquid accumulator: guarantees that there is only satu-rated liquid at the internal heat exchanger (iHEx1) inlet.

e)    Internal heat exchanger liquid line/suction line (iHEx1):increases the performance of the cooling system. Fig. 8 shows the ratio of the isentropic COP with superheating at the inlet of the VSC relative to the saturation COPsat (as defined by Gosney, 1982). Condensing and evaporating temperatures of 60 C and 90 C were considered, respectively. It is worth noting that for the four potential working fluids analyzed, the ratio increases with superheating, although some fluids, such as ammonia, shows decreasing performance (Gosney, 1982).

f)     Electric expansion valve (EEV): controls the low-pressurereceiver level.

g)    Lowpressurereceiver (LPR):this componentcan be seen asa second internal heat exchanger liquid line/suction line, which increases the EEV inlet subcooling and allows an overfeed to the ME since the ME outlet returns to this receiver. The same analysis considered for the iHEx1 can be considered here, i.e. the LPR increases the performance of the cooling system as it, together with the iHEx1, generates the superheating and the subcooling at the inlet of the VSC and EEV, respectively.

h)    Stepper motor valve: controls the liquid flow rate tocontrol the outlet vapor quality in each micro-evaporator (0%e100%).

i)     Micro-evaporator (ME): transfers the heat away from the microprocessor.

j)     Microchannel cold plate for memories (MPM): cools thememories.

This cycle is characterized by a high condensing temperature (high heat recovery potential), a high range of controllabilityof theME inletsubcooling (characteristic ofsystemswith VSC and EEV), a medium overall efficiency when compared with the liquid pumping cooling cycle (uncertain, evaluate potential for heat recovery in the condenser). This is a good operating option when the energy dissipated in the condenser is recovered for other use, typically during the winter season when considering a district heating application (high exergy).

Fig. 5 considers a hybrid two-phase cooling cycle, i.e. this multi-purpose cooling cycle makes it possible to interchange the cycles driven by the liquid pump and the vapor compressor. The change of cycle would be accomplished through the shut off valves 1e7 (SOV). The decision on the cooling cycle to operate would depend on demand for the heat recovered, or allow for cycle maintenance (repair of the compressor or pump). The microprocessors cannot operate without cooling and thus the interchangeability of the cycles represents a safety mechanism in case of failure of the pump or compressor. The “cons” of the hybrid cycle would be mainly the higher initial cost but certainly the advantages (system online reliability, controllability, cycle interchangeability and flexibility in heat recovery) may prove to justify the higher initial cost. Furthermore, this hybrid cycle represents a plugand-play option where any one of the three cycles can be installed based on the particular application, minimizing engineering costs.

Fig. 9 e Typical blade with two microprocessors and a heat generation capacity higher than 300 W.

It is worth mentioning that the applicability of these cooling cycles is not restricted to only one microprocessor but can be applied to blade servers and clusters, which may have up to 64 blades per rack cabinet. Each blade, such as that shown in Fig. 9, can have two (or more) microprocessors with a heat generation capacity higher than 150 W. If the auxiliary electronics (memories, etc.) on the blade are included, the total heat generation per blade can be higher than 300 W. Thus, the microchannel cold plate (MPM) described in the cooling cycles has the function to cool the auxiliary electronics that can represent about 60% of the total heat load on the blade, but have a larger surface area compared to the CPU and thus a lower heat flux.

Finally, when considering an entire rack, a very sizable heat loadis generated,which representsa good opportunity to recover the heat rejected. In this case, reuse of the heat removed from the blades for a secondary application will greatly reduce the CO2 “footprint” of the system. For example, if we consider a data center with 50 vertical racks, where each rack has 64 blades and each blade dissipates 300 W, the total potential amount of heat to be recovered will be 0.96 MW. Such a heat recovery system requires a secondary heat transfer fluid to pass through all the condensers (either water or a refrigerant) and then transport the heat to its destination. 3.2. Working fluids

Thepresenceofoil inthe coolingcycleswouldadverselyaffect the performance of the heat exchangers and also possibly lead toproblemsofcloggingofsmallcomponentsandgenerationof contaminants (Marcinichen, 2006). So, for this reason, these cyclesshoulduse drivercomponents that do not requireoil for lubrication purposes (that is, an oil free liquid pump and/or an oil free vapor compressor should be used).

Table 1 shows a comparison of four refrigerants in relation to their environmental parameters (BNCR35, 2008), where GWP is the global warming potential (ratio of the warming caused by the substance to the warming caused by a similar mass of carbon dioxide, GWP ¼ 1 for CO2) and ODP is the ozone depleting potential (ratio of the impact on ozone of a chemical compared to the impactof a similarmassof CFC11, ODP¼ 1 for CFC11). It is worth noting that the refrigerants considered have an ODP of zero, but still have rather high values of GWP.

The four working fluids (HFC236fa, HFC245fa, HFC134a and HC600a-isobutane) were evaluated with regard to COP and the volumic refrigerating effect for the vapor compression cooling cycle proposed (Fig. 4). The cycle considers two microchannel cooling components, ME and MPM, the first to cool the microprocessor (outlet vapor quality set to 30%) and the second to cool the auxiliary electronics (memories, DC/DC, etc) on the blade microprocessor (outlet vapor quality set to 90%, which is the estimated value that considers the blade manufacturer’s information that the auxiliary components

Table1 eEnvironmental parameters GWPandODP for the four potential working fluids.
Refrigerant                             GWP (100 year)                         ODP
HFC236fa 6300 0 HFC245fa 950 0
HFC134a                                                               1300                                                           0
HC600a                                                                         3                                                           0
Table 2 e Boundary conditions for the working fluids analysis on the vapor compression cooling cycle.
1)  Condenser
> condensing temperature ¼ 90 C, outlet vapor quality ¼ 0%
2)  Micro-evaporator on chip (ME)
> inlet saturation temperature ¼ 60 C,
>outlet vapor quality ¼ 30%
3)  Microchannel cold plate on memories (MPM)
> outlet vapor quality ¼ 90%
4)  Effectiveness of iHEx1 ¼ 90%
5)  Input data
> fluids: HFC245fa, HFC236fa, HFC134a and HC600a
> total pressure drop in the two evaporators
(ME and MPM) ¼ 0.5 bar
6)  Outlet data
> discharge temperature (isentropic compression)
> enthalpy difference in the two evaporators and in the compressor
> volumic refrigerating effect (qualitative idea of compressor size)

can represent up to 65% of the total heat load). It was also considered that iHEx1 has an effectiveness of 90% and the two microchannel cooling components have a total pressure drop of 0.5 bar.

The volumic refrigerating effect (wv) is determined by calculating the ratio between the sum of the ME and MPM enthalpy differences and the specific volume in the compressor suction (Gosney, 1982). This parameter indicates comparatively the size of compressor for the different working fluids, i.e. a higher volumetric refrigerating effect means that a smaller swept volume rate is required for a particular cooling capacity.

Table 3 shows the results considering the conditions in Table 2. For this cycle, the COP was determined by dividing the sum of the ME and MPM enthalpy differences (DhME) by the compressor enthalpy difference (Dhcomp). It can be observed that HFC245fa has the lowest suction and discharge pressures

(Psuc and Pdisc), which is advantageous for the compressor and cooling system (allows a less robust construction that enables material cost savings). However, it also has a lower volumic refrigerating effect, meaning that a larger compressor will be necessary. The best working fluid, when looking at the volumic refrigerating effect, is HFC134a since its value is more than 2 times higher than that of HFC245fa, but requires operation at a higher Psuc and Pdisc. It is worth noting that HC600a (isobutane) has the highest specific cooling capacity (DhME), as shown in Fig. 10, implying lower mass flow rates for

the same cooling capacity.

Relatively small differences in COP are observed in Table 3 for the four fluids, showing no significant effect on the choice of the working fluid. The same can be said about the compressor discharge temperature (Tdisc). The high values of COP observed are justified by the fact that the thermodynamic analysis does not consider the irreversibilities inherent in the cycle. However, due to the high evaporating temperatures considered here (for attaining a high performance green



Fig. 10 e HC600a P-h diagram highlighting the large specific cooling capacity.

computing solution), the COP values are higher than those found in household refrigerators and light commercial systems (actual COP about 2 or 3). Finally, it can be observed that HC600a and HFC134a present the lowest pressure ratios, which is an advantage because they represent compressors with high volumetric efficiencies.

Fig. 11 shows the effects of iHEx1 effectiveness and condensing temperature on the cycle COP. The same conditions described in Table 2 were considered and HFC134a was used as working fluid. It can be observed that the COP increases when the iHEX1 effectiveness increases. However, the condensing temperature has a greater effect on the COP, decreasing with an increase of condensing temperature. It is worth mentioning that there might be an optimal condensing temperature to obtain the maximum economical value of recovered heat for the penalty paid in compressor power consumption.

ME must be able to maintain the microprocessor’s operating temperature from 70 C to 75 C (83 C is the nominal maximum operating temperature).

Basedontheaforementionedinformation,Table4showsthe results obtained by the methods developed to evaluate the performance of the ME’s. The three-zone model (Thome et al., 2004) was used for two-phase heat transfer since it was shown to predict many fluids and geometries with good accuracy (Dupont et al., 2004), the numerically based model of Revellin and Thome (2008) was used for critical heat flux calculations and the homogeneous model was used for two-phase pressure dropssinceitwasfoundtopredictmicrochannelpressuredrops withrelativelygoodaccuracy(Ribatskietal.,2006).Themethods werealsousedtoestimatethemassflowrateintheME.Theheat load was considered to vary between 90% and 100%, i.e. from 146.25W to162.50 W. The ME inlet subcooling, Sub, wasfixed at

5 K and two inlet evaporating temperatures, Tevap_inlet, were considered, 50 C and 60 C. The dimensions of the ME were 170mmoffinwidthandchannelwidthand1700mmoffinheight, with a heated “footprint” of 18.5 mm by 13.5 mm. The working fluid selected for the present analysis was HFC134a.

The results show that the mass flow rate, mr, to guarantee the cooling capacity must be from 10.82 kg h1 to 11.90 kg h1 when the outlet vapor quality, xoutlet, is 30% and the inlet evaporating temperature is 60 C. For this case the lowest critical heat flux, CHF, was 141.2 W cm2, a value well above the actual value of 65 W cm2 (safety factor of 2.2). However, when the outlet vapor quality was set to 50%, the smallest CHF was then only 83.1 W cm2, a value judgedto be too nearto the actual value for the standard blade (65 W cm2) since the accuracy in predicting CHF is about 20%. Thus, it is best to consider an outlet vapor quality of 30%. While not done here, it is also possible to use the one-dimensional numerical method of Revellin and Thome (2008) to analyze the CPU die’s power dissipation map to verify the local safety factors in CHF with respect to the local hot spots.

3.4. Analysis of the cycle overall efficiency and potential for energy recovery

Table 3 e Results of simulations on the vapor compression cooling cycle/potential working fluids.

COP         Tdisc (C)           Pdisc (bar)          Psuc (bar)          Pdisc/Psuc                 DhME (kJ kg1)            wv (kJ m[2])
Dhcomp (kJ kg1)

The overall efficiencies (
hcycle) of the proposed cycles were evaluated considering the potential for energy recovery. This is determined by the ratio of the recovered energy in the condenser and subcooler to the energy consumed to drive the working fluid. Some additional terms were also considered to take into account the pumping power of the secondary fluid in the condenser and subcooler. Thus, considering the possible heat recovery in the heat exchangers, hcycle will be influenced by the type of heat recovery application, since different types

Effects of iHEx1 and condenser on the cycle COP

Fig. 11 e Effects of iHEx1 effectiveness and condensing temperature on the cycle COP.

of condensers, subcoolers and condensing temperatures could be chosen to maximize hcycle for the particular situation.

For an ideal case, the power dissipated by the microprocessor and memories, QMEþMPM, and the power consumed by the compressor, Wcompressor, or the liquid pump, Wpump, are fully recovered in the condenser and subcooler. This also holds for the power consumed by the pumps associated with the secondary fluid in the condenser, WCond_pump, and subcooler, WSubcooler_pump. The cycle overall efficiency for the liquid pump and vapor compression cycle can, therefore, be written as:

a)   Liquid pump cyclehcycle LP ¼ QMEþMPM þ Wþpump þ WCond pump þ WSubcooler pump

                          Wpump                WCond pump þ WSubcooler pump

               ¼ þ              þ          QMEþMPM                                                                                                                                  (1)


                        Wpump                WCond pump þ WSubcooler pump

b)  Vapor compression cycle

QMEþMPM þ WCompressor þ WCond pump

Presently, we are not concerned with the performance of the secondary system heat exchanger, which will be a function of its unknown (for now) mass flow rate and fluid properties. As noted in Eqs. (1) and (2), the hcycle will depend mainly on the pumping power of the secondary fluid, which in itself is a function of the type of application of the secondary system (heat exchanger size, type and properties of the secondary fluid). It is worth noting that the difference in cycle overall efficiency for the two cycles is in the denominator. In general, the compressor power is higher than the liquid pump power, due to the work needed to obtain a differential pressure associated with the compressor. This could lead to the conclusion that the hcycle for the liquid pump cycle is always higher than for the vapor compression cycle. However, the pumping power of the secondary fluid through the condenser

is higher for the liquid pump cycle than for the other cycle because of the lower condensing temperature, with the possibility of the opposite to be true. Furthermore, the hcycle will depend on the efficiency of each component and on the end use of the energy recovered in the condenser and subcooler.

The results of a simplified analysis evaluating the potential of heat recovery for the cycles with the liquid pump and the vapor compressor are depicted in Table 5. To develop this analysis, the results in the second line of Table 4 were taken into consideration as well as the following assumptions:

a)   water was considered as the secondary fluid,

b)   the condenser and subcooler water pumping powers werenot considered,

c)   the HFC134a liquid pumping power was determinedthrough Eq. (3) for 100% liquid pump overall efficiency. The liquid pump inlet subcooling was considered 10 K and the inlet pressure was considered that at the ME outlet,

d)   the compressor suction pressure was considered to be thesame pressure as at the ME outlet and with 10 K of


e)   the vapor compression was considered isentropic and100% compressor overall efficiency, vapor compression cycle,

Table 4 e Operational limits for one micro-evaporator. HFC134a as working fluid.
Boundary conditions in the micro-evaporator (working fluid: HFC134a)
Tevap_inlet (C)                Sub (K)            xoutlet (%)              Qw                      mr (Kg h1)             DPME (bar)              Tevap_outlet (C)                CHF (W cm2)
50                                                     5                                 30                         162.50                         10.82                               0.0092                                    50.0                                          145.7
60                                                     5                                 30                         162.50                         11.90                               0.0096                                    59.9                                          148.9
50                                                     5                                 30                         146.25                         10.28                               0.0082                                    50.0                                          141.2
60                                                     5                                 30                         146.25                         10.82                               0.0082                                    59.9                                          141.2
50                                                     5                                 50                         162.50                            7.03                               0.005                                       50.0                                             91.9
60                                                     5                                 50                         162.50                            7.57                               0.005                                       59.9                                             91.2
50                                                     5                                 50                         146.25                            6.28                               0.0045                                    50.0                                             83.3
60                                                     5                                 50                         146.25                            6.76                               0.0043                                    59.9                                             83.1

Table 5 e Comparative analysis for the liquid pump and vapor compression cooling cycle regarding heat recovery.
Cycle                               Energy recovery (W)             Tw_inlet (C)                            Condenser                                           Subcooler
                                                                                                              Tw_outlet (C)             mw (Kg h1)             Tw_outlet (C)               mw (Kg h1)
Liquid pump                                                    162.5                                              30                                    49.9                                    4.85                                    49.9                                    2.18
Vapor compressor                                          206.7                                              30                                    80.0                                    3.56                                       e                                          e

h) the condenser and subcooler outlet water temperature was assumed to be 10 K less than the condensing


The refrigerant pumping power is thus calculated as:


Wpump ¼DP           (3) r

where m is the mass flow rate, r is the specific mass and DP is the pressure increase provided by the pump.

In Table 5, it can be observed that there is an increase of 27.2% of heat recovery for the vapor compression cycle (this additional heat is associated with that imparted by the compressor) while there is an increase of 98% of total water mass flow rate, mw, for the liquid pump cycle that is associated with a lower water temperature difference in the condenser and subcooler. The final result shows that the liquid pump cycle will require a larger pump to circulate water in the condenser and subcooler, i.e. a higher pumping power and possibly a larger heat exchanger (condenser þ subcooler). Finally, it is important to remember that the results presented above are only for a simple example case and that a more detailed analysis considering all components and their thermal efficiencies needs to be done to better understand the behaviorand performance of eachcycle and its particular heat recovery application.

4.             Conclusion

1.   Three two-phase cooling cycles for cooling of data centerservers have been proposed for more energy-efficient cooling of blade server microprocessors and their memories. The cycles use two-phase boiling in microchannels for removing the heat from the microprocessors and memories and the heat can be dissipated either to the ambient or, better, it will be recovered for heating of buildings, preheating of boiler feed water, etc. This second solution has the potential to be a key step in the realization of a new generation of green, high performance data centers.

2.   To integrate operating flexibility and higher system operating reliability into one cooling cycle, include the possibility to recover heat or not, and to facilitate maintenance while still operating the server’s cooling system, a hybrid cooling cycle was proposed with interchangeability between the liquid pump and vapor compression driven cooling cycles. As the cooling of the servers should have a very high online availability, the interchangeability will

also guarantee uninterrupted operation in case of forced maintenance of the compressor or the pump.

3.   The vapor compression cooling cycle proposed was considered to determine the best working fluid for cooling applications of microprocessors and memories. The analysis took into consideration the following variables: suction and discharge pressures, volumic refrigerating effect, pressure ratio and COP. Of the four refrigerants considered, HFC134a and HFC245fa appear to be the best choices.

4.   Methods taken from the literature to evaluate the thermalperformance of the ME’s were used here to estimate the CHF of the ME and compare to the total heat flux of a specific blade. For an evaporating temperature, subcooling and outlet vapor quality of 60 C, 5 K and 30%, respectively, the predicted CHF was about 2.2 times the actual maximum heat flux of the blade server using fins that were 1700 mm high, 170 mm thick and channels 170 mm wide. This safety factor was considered sufficient since the accuracy in predicting CHF is about 20%. For an outlet vapor quality of 50% the factor decrease to only 1.3 times, a value judged to be too low to guarantee problem free operation.

5.   The micro-evaporator cooling cycles proposed were analyzedinrelationtothecycleoverallefficiency(hcycle)and the potential for energy recovery, after the aforementioned constraint of critical heat flux was taken into account. The qualitative comparison showed that the best cycle, i.e. that with the highest hcycle, will depend mainly on the end application of the energy recovered in the condenser and subcooler, which will influence the design of the cooling cycle and the thermodynamic conditions. A quantitative comparison showed that the vapor compression cycle is capable of recovering more energy for a lower water mass flow rate. Also, it was shown that a higher water temperatureisachievedwiththevaporcompressioncycleduetothe higher condensing temperature.


The Commission for Technology and Innovation (CTI) contract number 6862.2 DCS-NM entitled “Micro-Evaporation Cooling System for High Performance Micro-Processors: Development of Prototype Units and Performance Testing” directed by the LTCM laboratory sponsored this work along with the project’s industrial partners: IBM Zu¨rich Research Laboratory (Switzerland) and Embraco (Brazil). J.B. Marcinichen wishes to thank CAPES (“Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de N´ıvel Superior”) for a one year fellowship to work at the LTCM laboratory.

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[1] 2.                                                                                                Literature review
Hannemann et al. (2004) have proposed a pumped liquid multiphase cooling system (PLMC) to cool microprocessors and microcontrollers of high-end devices such as computers, telecommunications switches, high-energy laser arrays and high-power radars. According to them, their system could handle applications with 100 W heat loads (single computer chip) as well as applications with short time periods of kW heat loads (radar). Their PLMC consisted basically of a liquid pump, a high performance cold plate (evaporator) and a condenser with a low acoustic noise air mover to dissipate the heat in the ambient air. A comparison between a singlephase liquid loop (water) and the system proposed with HFC134a wasmadefor a 200 Wheat load. The HFC134asystem
[2] .3.        Microprocessor heat load and micro-evaporator model
To evaluate the performance of the cooling cycles proposed, the first step was to define a standard blade model where we need to control the microprocessor and memory temperatures. A photograph of the standard blade is shown in Fig. 9. According to the manufacturer, when we consider only one microprocessor and the auxiliary electronics associated with it, the maximum heat flux will be about 65 W cm2 and the area for heat transfer is about 2.5 cm2 (this is the worst case, i.e. the entire heat load assumed to be concentrated on the small area of the chip and its ME). Thus the maximum heat transfer rate will be 162.5 W per ME. The cooling capacity per