2. Thermoelectric coolers

relatively low COPs. Various studies on thermoelectricity have examined its operation with power directly supplied by photovoltaic to produce the required electricity to run the cooling systems [3, 4]. The electrical current supplied by photovoltaic which is consumed by TE

Using TE modules, several researchers reported cooling small volumes such as submarines [5]. TE modules have been proposed to be used in building applications using active building envelopes [6, 7]. Such studies underlined the promising future of the TE modules in cooling

General comparison between these three types of coolers for air conditioners is shown in

Recent studies provide two possible directions that can lead to considerable progress in TE

2. improving thermal design and optimization of the current available TE cooling modules. Introducing efficient heat sinks at both the hot and cold side of TE coolers greatly influences the cooling COP. Air cooled heat sink forced convection with fan [9, 10], water cooled heat sink [11] and heat sink integrated with heat pipe [12, 13] are frequently employed techniques. This review will focus on the development of TE cooling with great concerns on advances in

devices, is a direct current so that no DC/AC inverter is required.

1. improving intrinsic efficiencies of TE materials, and

materials, modeling and optimization approaches.

Table 1. Comparison between the three types of coolers for air conditioners.

applications.

248 Bringing Thermoelectricity into Reality

Table 1 [8].

cooling [3]:

When two different metals or semiconductors are connected together and the two connections held at different temperatures, there are many irreversible phenomena that can take place at the same time [14]. These are the Joule effect, Fourier effect, Thomson effect, Seebeck effect and Peltier effect. The Peltier effect is the most interesting among them for TE cooling. If a circuit contains two connections between different conductors or semiconductors, applying a DC volt will cause heat to transfer from one junction to the other. For producing the Peltier effect, semiconductor alloy materials, such as Bi2Te3 and SiGe, are better than metals [15]. The principle of TE coolers utilizing semiconductor Peltier effects is shown in Figure 1. The heat is

Figure 1. Principle of thermoelectric coolers utilizing semiconductor Peltier effects.

Figure 2. A conventional thermoelectric module with multiple thermoelements.

transferred from the cooled space to the hot-side heat sink through n-type and p-type semiconductor thermoelements which rejects the heat to the environment. The heat flow direction through the semiconductor materials will be reversed if the electric current direction is reversed.

system is not easy to install [28, 29]. Thermoelectric cooling has also been applied in other occasions, such as generating fresh water [30–33] and active building envelope system [7, 34]. TE systems can be directly connected to a PV panel. Since TE devices are low voltage driven devices, they can accept a power supplied by PV panel without conversion. This advantage makes TE devices attractive for building air-conditioning applications [27, 30]. This solar cooling technique can reduce the energy consumption and the environmental impact issues raised by conventional refrigeration and air-conditioning systems. Batteries can also be used to store DC voltages when sunlight is available while supplying DC electrical energy in a discharging mode in the absence of daylight. A battery charge regulator is needed to protect the battery form overcharging. Solar thermoelectric can be used in cooled ceiling applications to remove of a large fraction of sensible cooling load. In this case, the TE modules are connected in series and

sandwiched between aluminum radiant panels and heat pipe sinks in ceilings [35].

COPc <sup>¼</sup> Qc

COPC,id <sup>¼</sup> Qc

COPh,id <sup>¼</sup> Qh

The basic unit of the TE cooler is the n-type and p-type thermoelements. A bottom-up modeling approach is to construct the model at element level with the assumption that both types of thermoelements are exactly the same but opposite direction of the Peltier-Seebeck effect.

In the cooling mode, the cooling capacity Qc = (mcp)c (Tcout - Tcin), the heat dissipated in the hotside heat sink Qh = (mcp)h (Thout - Thin), the electric input power W=Qh - Qc, and the cooling

> <sup>W</sup> <sup>¼</sup> <sup>1</sup> Thout�Thin

where Tcin is the temperature of the inlet fluid in the cold side of the TE system, Tcout is the temperature of the outlet fluid in the cold side of the TE system, Thin is the temperature of the inlet fluid in the hot side of the TE system, Thout is the temperature of the outlet fluid in the hot side of the TE system, (mcp)c is the thermal conductance of cold side of the TE system, (mcp)h is the thermal conductance of hot side of the TE system, m is the mass rate of the fluid, cp is the

> ð Þ mcp <sup>h</sup> ð Þ mcp <sup>c</sup>

If some of the parameters for TE elements are available, the ideal COPc (COPC,id) and COPh

<sup>W</sup> <sup>¼</sup> <sup>α</sup>pnTc � KRΔ<sup>T</sup>

<sup>W</sup> <sup>¼</sup> <sup>α</sup>pnTh � KRΔ<sup>T</sup>

<sup>V</sup> � <sup>1</sup> 2V

VR <sup>þ</sup> <sup>1</sup> 2V

Tcout�Tcin Cr � <sup>1</sup> (1)

Thermoelectric Cooling

251

http://dx.doi.org/10.5772/intechopen.75791

is the heat capacity ratio. In the heating mode,

<sup>V</sup> <sup>þ</sup> <sup>α</sup>pnΔ<sup>T</sup> (2)

<sup>V</sup> <sup>þ</sup> <sup>α</sup>pnΔ<sup>T</sup> (3)

4. Analysis of thermoelectric elements

COPc can be expressed by:

COPh<sup>¼</sup> Qh

specific heat capacity of the fluid and C<sup>r</sup> =

<sup>W</sup> ¼ 1 + COPc.

(COPh,id) can be expressed as:

A typical TE module usually consists of a large number of n-type and p-type bulk semiconductor thermoelements that are connected electrically in series and thermally in parallel and sandwiched between two ceramic plates, as illustrated in Figure 2.
