Energy-Efficient Multi-Objective Thermal Control for Liquid-Cooled 3D Stacked Architectures

3D stacked systems reduce communication delay in multiprocessor system-on-chips (MPSoCs) and enable heterogeneous integration of cores, memories, sensors, and RF devices. However, vertical integration of layers exacerbates temperatureinduced problems such as reliability degradation. Liquid cooling is a highly efficient solution to overcome the accelerated thermal problems in 3D architectures; however, it brings new challenges in modeling and run-time management for such 3D MPSoCs with multi-tier liquid cooling. This paper proposes a novel design-time/run-time thermal management strategy. The design-time phase involves a rigorous thermal impact analysis of various thermal control variables. We then utilize this analysis to design a run-time fuzzy controller for improving energy efficiency in 3D MPSoCs through liquid cooling management and dynamic voltage and frequency scaling (DVFS). The fuzzy controller adjusts the liquid flow rate dynamically to match the cooling demand of the chip for preventing over-cooling and for maintaining a stable thermal profile. The DVFS decisions increase chip-level energy savings and help balance the temperature across the system. Our controller is used in conjunction with temperatureaware load balancing and dynamic power management strategies. Experimental results on 2- and 4-tier 3D MPSoCs show that our strategy prevents the system from exceeding the given threshold temperature. At the same time, we reduce cooling energy by up to 63% and system-level energy by up to 21% in comparison to statically setting a flow rate setting to handle worst-case temperatures.

Fuzzy Control for Enforcing Energy Efficiency in High-Performance 3D Systems

Mohamed Mostafa Sabry Aly, David Atienza Alonso, Ayse Kivilcim Coskun

3D stacked circuits reduce communication delay in multicore system-on-chips (SoCs) and enable heterogeneous integration of cores, memories, sensors, and RF devices. However, vertical integration of layers exacerbates the reliability and thermal problems, and cooling is a limiting factor in multitier systems. Liquid cooling is a highly efficient solution to overcome the accelerated thermal problems in 3D architectures; however, liquid cooling brings new challenges in modeling and runtime management. This paper proposes a novel controller for improving energy efficiency and reliability in 3D systems through liquid cooling management and dynamic voltage frequency scaling (DVFS). The proposed fuzzy controller adjusts the liquid flow rate at runtime to match the cooling demand for preventing energy wastage of over-cooling and for maintaining a stable thermal profile. The DVFS decisions provide chip-level energy savings and help balancing the temperature across the system. Experimental results on 8- and 16-core multicore SoCs show that the controller prevents the system to exceed the given threshold temperature while reducing cooling energy by up to 50% and system-level energy by up to 21% in comparison to using a static worst-case flow rate setting.

IEEE Press2010

Thermal Analysis and Active Cooling Management for 3D MPSoCs

David Atienza Alonso, Ayse Kivilcim Coskun

3D stacked architectures reduce communication delay in multiprocessor system-on-chips (MPSoCs) and allowing more functionality per unit area. However, vertical integration of layers exacerbates the reliability and thermal problems, and cooling is a limiting factor in multi-tier systems. Liquid cooling is a highly efficient solution to overcome the accelerated thermal problems in 3D architectures. However, liquid cooling brings new challenges in modeling and run-time management. This paper proposes a design-time/run-time thermal management policy for 3D MPSoCs with inter-tier liquid cooling. First, we perform a design-time analysis to estimate the thermal impact of liquid cooling and dynamic voltage frequency scaling (DVFS) on 3D MPSoCs. Based on this analysis, we define a set of management rules for run-time thermal management. We utilize these rules to control and adjust the liquid flow rate in order to match the cooling demand for preventing energy wastage of overcooling, while maintaining a stable thermal profile in the 3D MPSoCs. Experimental results on multi-tier 3D MPSoCs show that proposed design-time/run-time management policy prevents the system to exceed the given threshold temperature while reducing cooling energy by 50% on average and system-level energy by 18% on average in comparison to using a static worstcase flow rate setting.

IEEE Press2011

Improved thermo-mechanical reliability of anode-supported solid oxide fuel cells

Fabio Greco

Solid oxide fuel cell (SOFC) based systems are co-generators of electricity and heat that excel for their high efficiency, low pollutant emissions and fuel flexibility. Their reliability is close to meet the requirements for market deployment. Recent research improved the reliability during both thermal cycling and polarisation, mostly by series of incremental modifications. However, the transition from small- to large-scale production imposes ever more severe costs requirements, which from the perspective of manufacturing and operation, largely depend on thermo-mechanical aspects. The end of life of SOFC stacks still often occurs because of mechanical failure, which therefore remains a major hurdle for commercialization. The purpose of this study is to advance the understanding of the mechanical failures in SOFC stacks. It has two principal scopes: (a) the characterisation of the mechanical properties of SOFC materials and (b) investigations by thermo-mechanical modelling of the failure modes. The two activities are interrelated, because the prediction of failure mode by modelling relies on the knowledge of the mechanical properties. Two setups for mechanical testing were designed and fabricated. Both are capable of testing multiple samples in one experimental stage, under the adequate temperature and atmosphere. A model-based parameters estimation approach was developed to measure the elastic and both primary and secondary creep properties and overcome the limitations of analytical solutions. The experimental and numerical developments were applied to the characterization of the NiO/Ni-YSZ electrode material and complemented with analyses using 3-D imaging and computational homogenization. The strength was further analysed by the Weibull statistics approach. An extension of thermo-elastic computational homogenisation was finally implemented to estimate the anisotropic visco-elastic properties of the gas-diffusion layers. A detailed continuum thermo-mechanical model for SOFC stack analysis was developed. Differences in modelling assumptions, selected design modifications and operation conditions were examined. Further, the effects on the stack reliability of component geometrical imperfections caused by manufacturing were for the first time modelled. Parameters for the modelling of both primary and secondary creep in Ni-YSZ were identified, which is needed for advancing stack modelling capabilities. The thermo-elastic properties were not found to vary significantly during operation. The analysis of the strength measurements highlighted the combined beneficial effects of residual stress and plasticity of the Ni phase in the heterogeneous Ni-YSZ structure. The stack simulations allowed the analysis of cell and sealing failures and distribution of contact pressure at electrically conducting interfaces. The effects of operation conditions were found to overall dominate, and worst for counter-flow with high internal reforming. Component imperfections altered the distribution of the contact pressure in the short-term, which was alleviated by creep relaxation during operation, without leading to strong history-dependence. The effect of the geometrical imperfections considered in this work remained mild, compared to that of the mechanical properties. The risk of cell failure is strongly affected by the operation conditions, whereas that of the sealants by the design and position in the stack.

EPFL2018

Improved thermo-mechanical reliability of anode-supported solid oxide fuel cells

Fabio Greco

EPFL2018