Peltier Testing

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Solid state physics II

Testing a Peltier Element

Abstract

The purpose of the experiment was to investigate the cooling properties of a Peltier element and specifically the relation between the electric current through the element and the temperature in a refrigerator cooled by the element. The “refrigerator” consisted of a cavity in a block of polystyrene foam with the Peltier element covering the opening of the cavity. A fan on top of it cooled the hot side of the element. The temperature inside the cavity was measured with a thermocouple.

The measurements showed that the temperature depends on the current as a second order function and that and that there exist a current for which the temperature reaches a minimum value. For the investigated setup, the lowest measured temperature was 11.1°C (approximately 12°C below the surrounding temperature). It was achieved at a current of 1.3A, corresponding to a power consumption of 9.7W.

Sammanfattning

Peltier-effekten upptäcktes 1834 av fransmannen Jean C. A. Peltier och beskriver värmeflödet som uppstår då en elektrisk ström leds genom gränsskiktet mellan två olika ledande material. Denna effekt utnyttjas idag i kommersiella applikationer för kylning.

I denna artikel presenteras resultaten av en undersökning av en sådan produkt; ett Peltier-element, med avseende på dess förmåga att kyla ned ett litet hålrum i ett block av polystyrenskum. I synnerhet undersöktes sambandet mellan den elektriska ström som leddes genom Peltier-elementet och den temperatur som uppnåddes i det isolerade utrymmet.

Vid mätningen konstaterades att den lägsta uppmätta temperaturen inte uppnåddes vid den högsta strömmen genom elementet, utan vid en lägre ström. Temperaturen varierar med strömmen enligt ett kvadratiskt samband, med ett markant extremvärde. Kyleffekten tilltar alltså inte alltid med ökande ström, utan antar ett maximalt värde vid en viss ström, som kan antas vara olika för olika experimentella förhållanden (bortledning av värme från den varma sidan av elementet, isolering på den kalla sidan etc.). Vid det aktuella försöket uppmättes en lägsta temperatur av 11,1°C (cirka 12°C under omgivande temperatur) vid strömmen 1,3A, motsvarande effekten 9,7W. Introduction

The Peltier effect was first observed in 1834 by the Frenchman Jean C.A. Peltier. It describes the heat current that arises as a result of an electric current through the interface of two different conductors.

The purpose of this project is to measure the maximum cooling of a commercial Peltier element, in terms of the lowest achievable temperature inside a small refrigerator built using the element. The project also includes investigating the limiting factors of Peltier cooling and presenting the relevant theory behind it.

Theory

The heat current, [pic], at an interface between two conductors (A and B) can be expressed as a function of the electrical current, I, through the interface, and the Peltier coefficients of the two materials, ΠA and ΠB. The heat current at the interface is:

[pic](1)

Heat is generated if ΠA> ΠB and the electrical current flows from A to B. If the direction of the electrical current is reversed, so is the direction of the heat current.

By combining two interfaces, A to B and B to A, a hot and a cold junction can be created. In this case, heat is generated at one of the interfaces and the same amount is absorbed at the other.

In most commercial Peltier elements, three different materials are used (a P-doped semiconductor (P), a metal (M) and an N-doped semiconductor (N)). The Peltier coefficients of the three materials are different, ΠN> ΠM> ΠP. If the materials are arranged according to figure 1, a hot and a cold side are created, using several interfaces between the materials (and thus maximizing the cooling) with simple serial electrical connections. The arrangement functions due to the fact...
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