Peltier Supercooling with Isosceles Current Pulses: Cooling an Object with Internal Heat Generation

Abstract

Applying a current pulse enables a short-term transitory state where the cold junction of a Peltier couple reaches temperatures below that obtainable via maximum temperature delta steady-state current. Short-term cooling applications like on-chip hot spot and pulsed laser sensor cooling have been studied using pulsed cooling. Some studies have proposed applications that utilize consecutive repeating pulses for longer term cooling applications. These studies have found or theorized increased cooling and coefficient of performance (C O P). Considering these studies, it is desirable to have a more detailed analysis of how the additional cooling and C O P are achieved. The objective herein is to provide a detailed analysis of cooling rate and C O P during pulses using a realistically modeled system simulated in SPICE. It was found that cooling rate for long term consecutive pulse cooling applications can be increased over steady-state but C O P in most cases is reduced during current pulses. The reasons why this happens are studied in depth. From portable electronics to large scale systems and power electronics used for transportation, demand for more economical and effective heat removal means is growing. 1 Peltier devices offer a means of moving heat that has many advantages. Peltier coolers are solid-state heat pumps that convert electric current directly into a temperature difference and heat flux. The devices are reversible, imposing a temperature difference will induce a current flow. The thermoelectric effect is the operating principle utilized. As a thermal management method, using thermoelectrics has many advantages. Thermoelectrics are solid-state. Having no moving parts increases reliability and allows for no noise or vibration. Heating and cooling can be achieved from the same device at the same time. The heat flow direction can be reversed by reversing the electrical polarity to the device. The electrical current to the device can be controlled very precisely and this allows temperature control within +/− 0.01 • C. 2 With thermoelectrics, below ambient temperatures can be achieved. A coefficient of performance (C O P) greater than 1 can be achieved for heating which is not possible for resistive Joule heaters. Thermoelectrics offer high cooling power density and can be designed for very high cooling C O P. 3 In addition to high C O P, power consumption can be reduced since the devices can be used for site specific or zonal thermal management rather than cooling an entire enclosure. Thermoelectrics use no harmful refrigerants. The devices will operate in an orientation. Due to the many advantages, 4 thermoelectrics are used for a wide range of diverse applications. More recent studies and applications include using thermoelectric cooling to increase the efficiency of photovoltaic systems, 5 solving thermal management challenges with cooling integration into Silicon Photonic Systems, 6 high C O P cooling of aerospace electronics, 3 integration of thermoelectric cooling with phase change materials (PCM) for energy efficient building applications, 7 atmospheric water generation for arid climates, 8 highly efficient water distillation, 9 improved thermal comfort automotive zonal HVAC 10,11 and for hybrid and electric vehicle battery thermal management. 12-18 Some of many commercialized applications include CPU cooling, 19 kiosk cooling, 20 heated and cooled vehicle seats, 18,21 small refrigerators, 22 mattresses, seats, office chairs, 21 vehicular cup holders and mini refrigerators. 23 The cooling performance of thermoelectric devices is dependent on the semiconductor materials used, the number of thermocouples, * Electrochemical Society Member. z E-mail: alfred@thermoelectricsolutions.com the geometry of the thermoelements that make up the couples, the electrical operating condition of the device and the external heat loading and temperatures. 24 A growing area of research 12,24-55 is that of transient electrical current operation of thermoelectric devices. When a current pulse is applied to a device operating at steady-state current, a temporary lower temperature can be achieved at the cold junction of the couple or module than can be achieved with the optimum steady state current operation. This is illustrated in Figure 1. This temporary state of lower temperature is called supercooling. Conversely supercooling (transient advantage) is followed by a period of superheating (tran-sient penalty) for which the cold junction temperatures rise above the steady state value. The areas of supercooling and superheating are encompassed in shaded areas of Figure 1. It has been found that the current pulse duration and height can be optimized for maximum transient advantage or to maximize the net cooling which is the difference of transient advantage and transient penalty. 24 Outside of cooling applications , it has been shown that heat pulses can improve the thermal to electric conversion efficiency of thermoelectric generators. 56-61 Equation 1, sometimes called the "Ideal equation", 62 determines the amount of cooling that can be achieved during steady state operation. The equation holds for transient conditions with the exception that T c , T and I are time dependent. T c is the cold side junction temperature of the couple and n is the number of couples in the module. In both cases, α, R and K are temperature dependent. ˙ Q c = n αT c I − 1 2 I 2 R − K T [1] This equation is made up of a Peltier, Joule and Fourier term from left to right. α is the Seebeck coefficient of the couple α = α p − α n [V/K], T c is the temperature of the cold junction of each thermoele-ment, and I is the electric current applied to the couple. R is electrical resistance of the couple in units of ohms. K is the thermal conduc-tance of the couple in [W/K]. T is T h minus T c. R is dependent on the semiconductor material electrical resistivity ρ and geometry of the thermoelements. K , the thermal conductance is dependent on the semiconductor thermal conductivity k and geometry of the thermoele-ments. When T h > T c and α is increased while ρ and k decreased, ˙ Q c will increase. The same is true for T c > T h however ˙ Q c decreases as k decreases. The figure of merit, Z = α 2 /ρk is used to characterize thermoelectric semiconductor materials. A higher Z leads to increased ˙ Q c when T h > T c. Figure 1 shows T c vs. time when a current pulse is applied. The instant lower temperature achieved followed by a temperature rise and overshoot is due to different time constants of the Peltier cooling) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 97.70.172.17 Downloaded on 2017-12-30 to IP