Characterization of a thermoelectric cooler based thermal management system under different operating conditions
Highlights
► A model was developed for thermal management systems using thermoelectric coolers. ► Model predictions were in good agreement with experimental results. ► An operating envelope was developed for peak and off peak conditions. ► The effect of the number of thermoelectric coolers on the system was determined.
Introduction
Refrigeration cooling systems using thermoelectric coolers (TEC) have been considered for a number of electronic packaging systems to keep the chip temperature below its design point. Being solid state devices, TECs can provide high reliability and control in chip cooling [1]. Their relatively small form factor and weight, noise free operation and absence of moving parts provide advantages over many other cooling methods. Much of previous research on TECs has focused on characterizing the maximum capacity (e.g., minimum chip temperature or maximum heat dissipation from the chip) under standard ambient conditions [2], [3], [4], [5]. TECs are also advantageous in thermal management applications at elevated ambient temperatures due to the reduced driving temperature potential for heat transfer under these conditions. The maximum output capacity at these conditions can be predicted using models for the TECs [3], [4], [5].
The performance of TEC based cooling systems has been evaluated through both models and physical experiments [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. This includes investigations to optimize the geometry of thermo-elements in a TEC [2], [7], [8] and thermo-element materials [9], [10]. The performance of TEC based thermal management systems in practical cooling applications have been considered for both forced air [6], [13] and liquid cooling [5]. The results show that TEC based systems can dissipate larger heat fluxes than systems without TECs under a range of operating conditions [2], [5], [13]. The prediction of performance of a TEC based system was found to be in reasonable agreement with experiments [4], [5]. The analysis of TEC based thermal management systems show that the hot side thermal resistance (i.e. the thermal resistance from the hot side of the TEC device to the ambient) often has a more significant effect on the performance than the cold side thermal resistance between the chip and the TEC [12], [13]. This is mainly because the electric power input to the TEC module(s) must be transferred through the hot side thermal resistance in addition to the heat load from the chip. An increase in the hot side thermal resistance also reduced the range of operating current where the TEC was effective [12].
The performance of a TEC based thermal management system is typically considered at optimal operating current [2], [3], [4], [5], [13] that yields maximum heat flux [3], [5] or minimum chip temperature [2], [4], [5], [13] for a given operating condition. This is used to assess whether the thermal management system can perform adequately at the most extreme conditions for the system. In many cases, the thermal management system for a chip is operated at the most extreme condition for only a fraction of the time unless it is operating at a fixed heat load and a fixed ambient condition. Thus, the performance of the thermal management system at off peak conditions is also important in selecting a design. The objective of this investigation was to characterize how a given TEC based thermal management system performs under a range of ambient temperatures at different heat fluxes. An operating envelope is developed to characterize the system performance for peak and off peak thermal conditions. The maximum coefficient of performance of the system at different conditions within this operating envelope is of particular interest. The analysis was performed using standard thermo-physical equations of the TEC and a thermal resistance network model for the remainder of the system. The results of the model predictions were validated by experiment. The effects of different design parameters were then considered. The model is presented in the next section followed by a description of the experimental facility. The results of the model predictions and experiments are then discussed. The results show that the design parameters have a significant impact on the performance of the device at off peak conditions.
Section snippets
Modeling
The thermal management system was modelled as a TEC device with a cold side thermal resistance Rc between the cold side of the TEC and the chip and a hot side thermal resistance Rh between the hot side of the TEC and the ambient as shown in Fig. 1. Following [2], [3], [5], [12], [13] the TEC module(s) is modelled assuming that (i) the interconnect material between the n and p-type thermo-elements are ideal, (ii) the material properties of the n and p-type thermo-elements are identical [14] with
Experimental facility
The experiments to characterize the TEC based thermal management system were performed for a system that approximates one used in a sealed computer where the heat is transferred from the chip to the base plate of the heat sink through heat pipes and then dissipated to the ambient by natural convection. The experimental set up, shown schematically in Fig. 2, consists of a flexible heater to simulate the heat dissipation from the chip and is attached to an aluminium heat spreader, on which the
Results and discussion
For the experiments performed here, the cold side ceramic substrate of the TEC modules was the cold side thermal resistance and the hot side thermal resistance consists of the hot side ceramic substrate, the heat pipe unit and the heat sink. The variation in the heat source (or chip) temperature and coefficient of performance of the system with applied current to the TEC modules for an ambient temperature of 38.5 °C is shown in Fig. 3. The heat source temperature decreases with the applied
Conclusions
A thermal resistance network model for a thermal management system incorporating TEC modules was used to examine the performance of TEC based thermal management systems at off peak conditions. The case of a thermal management system for a sealed computer operating at a range of heat loads and ambient temperatures or (Tdesign − T∞) was considered. Experiments were performed to determine the typical properties of the components and to validate the model predictions. The model predictions were in
Acknowledgements
The support from SmallPC and Ontario Centres of Excellence (OCE) is gratefully acknowledged.
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