2. Energy balance model

voltage DC power source in a TEC module, heat transfer takes place from one side to the other side. In this way, TEC module's one side is cooled and other side is heated. In a TEC module, electric current drifts from N-type element to P-type element [1]. The temperature of the cold junction gradually decreases with heat transfer mechanism from environment to cold junction at a lower temperature. This heat transfer mechanism takes place with passing of transport electrons from a low energy level inside the P-type thermocouple element to a high energy level inside the N-type thermocouple element through the cold junction. Simultaneously, transport electrons transmit absorbed heat to hot junction at a higher temperature. This extra generated heat is dissipated to heat sink, whereas transport electrons return to a lower energy level in the P-type semiconductor element, viz., the Peltier effect takes place (see Figure 1).

There is constant development and efforts made for making thermoelectric air-conditioning systems in technical competence with vapor-compression technology. The performances of thermoelectric and conventional vapor compression air-conditioners have been compared by Riffat and Qiu [2]. Results have shown that the COPs of vapor compression and thermoelectric air-conditioners are in between 2.6–3.0 and 0.38–0.45, respectively. However, thermoelectric air conditioners have several other capabilities compared to vapor-compression technology. TEC modules can be built into a planar structure on walls and false ceiling and are quiet in operation especially suitable for small offices and mini apartments. Cosnier et al. [3] have presented numerical and experimental results of a thermoelectric air-cooling and air-heating system. The maximum cooling power of 50 W per module, with a COP varying between 1.5 and 2 was reached with electrical current of 4 A and maintaining 5C temperature difference between the hot and cold sides. Cheng et al. [4] have investigated a solar-driven thermoelectric cooling module with a waste heat regeneration unit for green building applications. Their

Figure 1. Principle of thermoelectric cooling.

314 Bringing Thermoelectricity into Reality

The total energy efficiency of photovoltaic driven thermoelectric cooling devices can be increased with enhancement of photovoltaic system efficiency and with the use of thermoelectric materials with better performance. The COP of thermoelectric air conditioning devices powered through photovoltaic modules is typically not higher than 0.6 [10]. With consideration of photovoltaic system efficiency ηpv, the total energy efficiency of the system is given by the product of ηpv and COP. Mathematically it is written as:

$$E\_{\rm TEC-PV} = \eta\_{pv} \times \text{COP} \tag{1}$$

Qc ¼ N � α � I � TC � 0:5 � R � I

Qh ¼ N � α � I � TH þ 0:5 � R � I

P ¼ Qh � Qc ¼ N � α � I � ΔT þ R � I

<sup>R</sup> <sup>¼</sup> <sup>L</sup>

<sup>K</sup> <sup>¼</sup> <sup>S</sup>

COPcooling <sup>¼</sup> Qc

In order to investigate the operating energy consumption in summer, a thermoelectric coolingphotovoltaic (TEC-PV) device is simulated for building data as per Table 2, representing sunny, hot and humid outdoor air condition. Properties of TEC-PV device is provided in

The room sensible heat factor (RSHF) is defined as the ratio of sensible cooling load to total

RSHF <sup>¼</sup> Qsen

Room sensible heat factor 0.95 0.9 0.8 0.7

/h 40 m<sup>3</sup>

Peak latent cooling load 0.05 kW 0.12 kW 0.28 kW 0.48 kW

/h 90 m<sup>3</sup>

/h 120 m<sup>3</sup>

/h

Qsen þ Qlat

Table 3. Table 4 provides thermal design properties of TEC-PV device.

Outdoor air condition Sunny, hot and humid (DBT: 33–35�C, RH: 75%)

Indoor air condition Dry bulb temperature (DBT): 23�C, RH: 55%

2.1. Thermoelectric dehumidification

Floor area 9 m<sup>2</sup> Room volume 27 m3 U-value of exterior wall 0.44 W/m2 K U-value of roof 0.126 W/m2 K

Window to wall ratio 0.3 Lighting power density 0.6 W/ft<sup>2</sup> Infiltration 0.3 ACH

Ventilation rate 20 m3

Peak sensible cooling load 1 kW

Table 2. Building data.

Operation schedule 07:00 to 17:00 hours

cooling load (Eq. (9)).

<sup>2</sup> � <sup>K</sup> � <sup>Δ</sup><sup>T</sup> (3)

Building-Integrated Thermoelectric Cooling-Photovoltaic (TEC-PV) Devices

<sup>2</sup> � <sup>K</sup> � <sup>Δ</sup><sup>T</sup> (4)

<sup>2</sup> (5)

http://dx.doi.org/10.5772/intechopen.75472

<sup>S</sup> � <sup>r</sup> (6)

<sup>L</sup> � <sup>λ</sup> (7)

<sup>P</sup> (8)

(9)

317

The values of ETEC-PV are typically lower than 6%.

Commercial thermoelectric materials are alloys such as Bi2Te3, PbTe, SiGe and CoSb3 [11]. Bi2Te3 is the most commonly used thermoelectric material. The commercially available thermoelectric materials have highest ZT values around 1.0.

For a particular thermoelectric module with fixed hot/cold side temperatures, the maximum COP at optimum current is given by [11]:

$$\text{COP}\_{\text{max,cool}} = \frac{T\_c}{T\_h - T\_c} \cdot \frac{\sqrt{1 + ZT\_m} - \frac{T\_h}{T\_c}}{\sqrt{1 + ZT\_m} + 1} \tag{2}$$

where ZTm is the figure-of-merit for thermoelectric material at mean hot and cold side temperature Tm. In calculation of COP, a mean temperature between the hot and cold junction temperatures (with fixed hot side temperature of 300 K with ZTm = 1) of the thermoelectric module (TEM) is used.

A steady state energy balance model of thermoelectric cooling is used for energy performance assessment. The absorbed (Qc) heat flux and released (Qh) heat flux are obtained using Eqs. (3) and (4) respectively. The electric power (P) required to power thermoelectric module (TEM) is obtained from the difference between the absorbed and released heat fluxes (Eq. (5)). In these equations, α, R and K are the Seebeck coefficient, electrical resistance, and thermal conductance of the thermoelectric module. Whereas, I, is the electric current and N is the number of thermocouple legs in the thermoelectric module. The TEM is made up of several thermocouple legs. Thermoelectric leg characteristics are responsible for the resulting TEM performance. R and K are calculated using Eqs. (6) and (7) respectively considering the leg length (L), leg section area (S), thermal conductivity (λ) and electrical resistivity (r). The coefficient of performance (COP) of the TEM is the ratio between the absorbed heat and total electric power (including fan power) obtained from Eq. (8). In this chapter, a thermocouple leg made of bismuth telluride (Bi2Te3) is considered and its properties are shown in Table 1 [12]. The thermal resistance between the thermoelectric module and fin plate is assumed to be 0.0161 K/W [12].


Table 1. Bismuth telluride (Bi2Te3) properties.

$$Q\_c = N \cdot \left(\alpha \cdot I \cdot T\_{\bigcirc} - 0.5 \cdot R \cdot I^2 - K \cdot \Delta T\right) \tag{3}$$

$$Q\_h = N \cdot \left(\alpha \cdot I \cdot T\_H + 0.5 \cdot R \cdot I^2 - K \cdot \Delta T\right) \tag{4}$$

$$P = Q\_h - Q\_c = N \cdot \left(\alpha \cdot I \cdot \Delta T + R \cdot I^2\right) \tag{5}$$

$$R = \frac{L}{S} \cdot \rho \tag{6}$$

$$K = \frac{S}{L} \cdot \lambda \tag{7}$$

$$\text{COP}\_{\text{cooling}} = \frac{\text{Qc}}{P} \tag{8}$$

In order to investigate the operating energy consumption in summer, a thermoelectric coolingphotovoltaic (TEC-PV) device is simulated for building data as per Table 2, representing sunny, hot and humid outdoor air condition. Properties of TEC-PV device is provided in Table 3. Table 4 provides thermal design properties of TEC-PV device.

#### 2.1. Thermoelectric dehumidification

system efficiency ηpv, the total energy efficiency of the system is given by the product of ηpv and

Commercial thermoelectric materials are alloys such as Bi2Te3, PbTe, SiGe and CoSb3 [11]. Bi2Te3 is the most commonly used thermoelectric material. The commercially available ther-

For a particular thermoelectric module with fixed hot/cold side temperatures, the maximum

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZTm <sup>p</sup> � Th

> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZTm

Th � Tc �

where ZTm is the figure-of-merit for thermoelectric material at mean hot and cold side temperature Tm. In calculation of COP, a mean temperature between the hot and cold junction temperatures (with fixed hot side temperature of 300 K with ZTm = 1) of the thermoelectric

A steady state energy balance model of thermoelectric cooling is used for energy performance assessment. The absorbed (Qc) heat flux and released (Qh) heat flux are obtained using Eqs. (3) and (4) respectively. The electric power (P) required to power thermoelectric module (TEM) is obtained from the difference between the absorbed and released heat fluxes (Eq. (5)). In these equations, α, R and K are the Seebeck coefficient, electrical resistance, and thermal conductance of the thermoelectric module. Whereas, I, is the electric current and N is the number of thermocouple legs in the thermoelectric module. The TEM is made up of several thermocouple legs. Thermoelectric leg characteristics are responsible for the resulting TEM performance. R and K are calculated using Eqs. (6) and (7) respectively considering the leg length (L), leg section area (S), thermal conductivity (λ) and electrical resistivity (r). The coefficient of performance (COP) of the TEM is the ratio between the absorbed heat and total electric power (including fan power) obtained from Eq. (8). In this chapter, a thermocouple leg made of bismuth telluride (Bi2Te3) is considered and its properties are shown in Table 1 [12]. The thermal resistance between the thermoelectric module and fin plate is assumed to be

Conductivity thermal (W/m-K) <sup>λ</sup>ð Þ¼ <sup>T</sup> <sup>62605</sup> � <sup>277</sup>:<sup>7</sup> � <sup>T</sup> <sup>þ</sup> <sup>0</sup>:<sup>4131</sup> � <sup>T</sup><sup>2</sup> � �10�<sup>4</sup> Resistivity electrical (Ω�m) <sup>r</sup>ð Þ¼ <sup>T</sup> <sup>5112</sup> � <sup>163</sup>:<sup>4</sup> � <sup>T</sup> <sup>þ</sup> <sup>0</sup>:<sup>6279</sup> � <sup>T</sup><sup>2</sup> � �10�<sup>10</sup> Seebeck coefficient (Volts/K) <sup>α</sup>ð Þ¼ <sup>T</sup> <sup>2224</sup> � <sup>930</sup>:<sup>6</sup> � <sup>T</sup> <sup>þ</sup> <sup>0</sup>:<sup>9905</sup> � <sup>T</sup><sup>2</sup> � �10�<sup>9</sup>

COPmax, cool <sup>¼</sup> Tc

ETEC�PV ¼ ηpv � COP (1)

Tc

<sup>p</sup> <sup>þ</sup> <sup>1</sup> (2)

COP. Mathematically it is written as:

316 Bringing Thermoelectricity into Reality

The values of ETEC-PV are typically lower than 6%.

COP at optimum current is given by [11]:

module (TEM) is used.

0.0161 K/W [12].

Table 1. Bismuth telluride (Bi2Te3) properties.

moelectric materials have highest ZT values around 1.0.

The room sensible heat factor (RSHF) is defined as the ratio of sensible cooling load to total cooling load (Eq. (9)).


$$RSHF = \frac{Q\_{\text{sen}}}{Q\_{\text{sen}} + Q\_{\text{lat}}} \tag{9}$$

Table 2. Building data.


Table 3. Thermoelectric cooling (TEC)-photovoltaic (TEC-PV) device properties.

Relative humidity is a key control parameter for thermal comfort inside a room. The performance of a thermoelectric cooling device depends mainly on optimal positioning and layout of heat exchange & transfer surfaces.

The total heat transfer rate (Qc) of the fin heat exchanger on the cold side of the thermoelectric module (TEM) is given by [13]:

$$Q\_c = h\_c \cdot A\_c \cdot (t\_r - t\_c) + m\_w \cdot H\_c \tag{10}$$

Parameter Values Thermal resistance of cold side 0.1 K/W Thermal resistance of hot side 0.7 K/W TEM thermal conductivity 0.51 W/K TEM electrical resistance 2.236 Ω Aluminum thermal conductivity 230 W/m-K Thickness of aluminum sheet 1.6 mm Insulation thickness 40 mm Insulation thermal conductivity 0.05 W/m-K Insulation specific heat capacity 500 J/kg-K Thermal contact resistance 0.1 m<sup>2</sup>

Building-Integrated Thermoelectric Cooling-Photovoltaic (TEC-PV) Devices

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Fin thickness 1 mm Fin profile length 20 mm Fin spacing 3 mm Number of fins (cold side) per TEC module 10 Number of fins (hot side) per TEC module 10 Thermal resistance (cold side fins) 1.2 K/W Thermal resistance (hot side fins) 0.76 K/W Thermal resistance between TEM and fin plate 0.0161 K/W Height of solar PV wall mounted exhaust duct 3000 mm

Width of solar PV wall mounted exhaust duct 2 Nos.@ 1500 mm

Number of TEC modules with ceiling suspension duct (position 1) 10 covering 1.60 m<sup>2</sup> Number of TEC modules on wall (position 2) 20 covering 3.60 m<sup>2</sup>

Absorption coefficient of PV panel 0.9 Thickness of PV ventilated exhaust duct 150 mm Density of PV panel 2300 kg/m3 Specific heat capacity of PV panel 750 J/kg-K Thickness of PV panel 5 mm

Heat transfer fluid Air DC power for each fan 1.5 W Number of DC fans on supply side 2 Number of DC fans on exhaust side 2 Maximum ventilation rate per fan 60 m3 h<sup>1</sup>

Table 4. Thermal design properties of TEC-PV device.

K/W

319

where hc is the coefficient of convective heat transfer (W/m<sup>2</sup> K), Ac is the heat transfer area (m2 ), tr is the room temperature (�C), tc is average temperature of cold fins (�C) and Hc is the latent heat of condensation (J/kg-K).

The dehumidifying rate (mw, kg/s) is calculated as [13]:

$$m\_w = \frac{m\_d \cdot \left(\phi\_1 - \phi\_2\right)}{T\_{\text{sec}}} \tag{11}$$

where ma is the mass of the wet air inside the room (kg), Tsec is the dehumidifying period (sec), Φ<sup>1</sup> and Φ<sup>2</sup> are the relative humidity before and after dehumidification (%).

The convective heat transfer coefficient between adjacent fins and room air is given by [14]:

$$h\_c = 0.517 \cdot \frac{k\_{air}}{H} \cdot Ra^{0.25} \tag{12}$$

where kair is the thermal conductivity of air (W/m-K), H is the height of fin (m) and Ra is the dimensionless Rayleigh number.


Table 4. Thermal design properties of TEC-PV device.

Relative humidity is a key control parameter for thermal comfort inside a room. The performance of a thermoelectric cooling device depends mainly on optimal positioning and layout of

Placement position Inside ceiling duct Wall mounted Battery backup 10.8 kWh TEC modules 10 20 Battery @ 12 V DC 900 AH

TEC module TEC1-12710 Photovoltaic module

Operational voltage 12 V DC Total power required 1.8 kW Current max 10.5 Amp Area required 18 m<sup>2</sup> Voltage max 15.2 V Roof area 9 m2 Power max 85 W South façade area 9 m2 Nominal power 60 W Nominal power 300 W Thermocouples 127 Number of PV modules 8 Dimensions 40 � 40 � 3.5 mm On roof 4 Total number of TEC modules 30 On façade 4

The total heat transfer rate (Qc) of the fin heat exchanger on the cold side of the thermoelectric

tr is the room temperature (�C), tc is average temperature of cold fins (�C) and Hc is the latent

mw <sup>¼</sup> ma � <sup>ϕ</sup><sup>1</sup> � <sup>ϕ</sup><sup>2</sup>

where ma is the mass of the wet air inside the room (kg), Tsec is the dehumidifying period (sec),

The convective heat transfer coefficient between adjacent fins and room air is given by [14]:

kair

where kair is the thermal conductivity of air (W/m-K), H is the height of fin (m) and Ra is the

hc ¼ 0:517 �

Φ<sup>1</sup> and Φ<sup>2</sup> are the relative humidity before and after dehumidification (%).

 Tsec

Qc ¼ hc � Ac � ð Þþ tr � tc mw � Hc (10)

K), Ac is the heat transfer area (m2

<sup>H</sup> � Ra<sup>0</sup>:<sup>25</sup> (12)

),

(11)

heat exchange & transfer surfaces.

318 Bringing Thermoelectricity into Reality

module (TEM) is given by [13]:

heat of condensation (J/kg-K).

dimensionless Rayleigh number.

where hc is the coefficient of convective heat transfer (W/m<sup>2</sup>

Table 3. Thermoelectric cooling (TEC)-photovoltaic (TEC-PV) device properties.

The dehumidifying rate (mw, kg/s) is calculated as [13]:

#### 2.2. Exergy expressions

The specific exergy of fresh moist air into the duct is expressed as [15]:

$$\begin{split} \mathbf{c}\_{f,in} &= \left( \mathbf{c}\_{pu} + \mathbf{w}\_{f,in} \cdot \mathbf{c}\_{pv} \right) \left[ T\_{f,in} - T\_o - T\_o \cdot \ln \frac{T\_{f,in}}{T\_o} \right] \\ &+ R\_a \cdot T\_o \left[ \left( 1 + 1.608 w\_{f,in} \right) \cdot \ln \frac{\left( 1 + 1.608 w\_{f,in} \right)}{\left( 1 + 1.608 w\_o \right)} \right. \\ &+ 1.608 \cdot R\_a \cdot T\_o \cdot w \cdot \ln \frac{w\_{f,in}}{w\_o} \right] \end{split} \tag{13}$$

The reference temperature (To) and the humidity ratio (wo) is defined as the indoor air temperature and humidity ratio during hot and humid season. Ra is the thermal resistance of air. cpa is specific heat of moist air and cpv is specific heat inside the duct.

Thus from Eq. (13), total exergy of the fresh air flow into the duct is expressed as:

$$\mathbf{Ex}\_{f,in} = \rho \cdot M\_{\mathbf{a}} \cdot \mathbf{ex}\_{f,in} \tag{14}$$

has to be ensured from air infiltration and direct solar radiation for TEC devices fixed on windows and skylights. The mode of operation for winter can be reversed by changing the direction of current of thermoelectric modules. TEC devices can also be fixed inside air supply ventilation ducts. Buildings requiring cooling and heating with dual duct ventilation

Building-Integrated Thermoelectric Cooling-Photovoltaic (TEC-PV) Devices

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321

b. Thermoelectric module (TEM) system design: It depends on thermoelement length, number of thermocouple legs, cross sectional area, slenderness ratio. Both COP and cooling capacity of TEC devices are dependent on thermoelement length. Keeping cross sectional area constant, larger length of thermoelectric element achieves greater COP, while shorter length thermoelectric element achieves larger cooling capacity. Commercially available thermoelectric modules have thermoelement length in the range from 1 to 2.5 mm [11]. Cooling power capacity increases with decreasing the ratio of thermoelement

c. Thermoelectric cooling (TEC) system design: It depends on cooling system thermal design, heat sinks' geometry, heat transfer area, heat transfer coefficients of hot and cold side heat sinks, thermal and electrical contact resistances, fins placement and design, heat sinks integrated with thermosyphon, heat transfer fluids, phase change materials [16]. Thermal contact resistance at the interface layer of thermocouple legs is critical for its cooling capacity and COP. Because of this reason, it is not essential that increase in ZT of thermoelectric material will increase ZT of a thermocouple leg because of the presence of interface layer. The performance and efficiency of heat sinks at hot and cold side effects the cooling COP. Air cooled heat sink (forced convection with fan, example thermal resistances of 0.54–0.66 K/W [11], water cooled heat sink (thermal resistance of 0.108 K/W [11], and heat sink integrated with heat pipe (thermal resistance 0.11 W/K are most commonly used techniques. It has been found that heat pipes are not preferred as they eventually have to release heat to either air or water eventually. Heat sinks with nanofluid have potential to achieve lower thermal resistance. Hot side heat sink performance is of greater importance due to higher heat flux density in comparison to heat flux at cold side heat sink. Allocation ratios of heat transfer area with heat transfer coefficients between hot and cold sides are important for achieving maximum COP. Typical allocation ratio is around 0.36–0.47 [11]. Maximum COP with optimum cooling capacity can be achieved at given

d. Photovoltaic (PV) power system design: The most conventional way is to install PV panels on rooftop and façade of a building with thermoelectric cooling (TEC) devices. In this way, excess power can also be stored in a battery system. In case of non-availability of solar PV power, power can be fed directly from the battery backup. Active façade ventilation can be integrated with TEM and PV devices [17–19]. For heating requirements during winter season, these active façade elements can supplement with heating from TEM and

e. Performance & operational parameters optimization: It depends on electric current input, coolants, cooling methods of hot side heat sink, mass flow rate, ventilation requirements. Performance indicators are COP and energy efficiency of devices and systems.

system are good choice for using thermoelectric devices inside ducts.

length to cross sectional area.

hot and cold sides fluid temperatures.

façade integrated PV ventilated devices [20–22].

where r is density of air and Ma is airflow rate m<sup>3</sup> /h.

The exergy of the heat transferred between the fresh air and TEM is expressed as:

$$\mathbf{Ex}\_{Q\mathbf{f}} = \left[1 - \frac{T\_o}{T\_c}\right] \cdot \mathbf{Q}\_{\mathbf{f}} \tag{15}$$

where Qf is heat transfer rate (W).

The system exergy efficiency is expressed as:

$$\eta\_{\text{Ex}} = \frac{\text{Ex}\_{\text{eff}}}{\text{Ex}\_{\text{sup}}} \times 100\% = \frac{\left[\text{Ex}\_{f,out} - \text{Ex}\_{f,in}\right]}{\text{Ex}\_{\text{elec}}} \times 100\% \tag{16}$$

where Exeff is effective Exergy, Exsup is electrical exergy supplied.
