Cooling load and noise characterization modeling for photovoltaic driven building integrated thermoelectric cooling devices

. Photovoltaic driven thermoelectric cooling devices are investigated for installation in a modular outdoor test-room. Because of Peltier effect in a thermoelectric cooling (TEC), heating and cooling is achieved by applying a voltage difference across the thermoelectric module. Theoretical design modeling of cooling load and noise characterization of building integrated Thermoelectric (TEC) Devices is analyzed. System design of photovoltaic driven TEC devices is investigated with varying fresh outdoor ventilation rates. Building integrated design of TEC devices inside ceiling suspended duct along with TEC devices mounted on wall driven by rooftop and active façade photovoltaic devices is considered in the analysis. In this way, two-stage dehumidification is achieved by two different sets of TEC devices. The investigation is conducted for effect of voltage, air flow rate and height of fin heat transfer surface. Expressions along with results for noise characterization in photovoltaic driven building integrated TEC devices are also provided.


Introduction
Thermoelectric module is a solid-state energy conversion device made up of thermocouples, which are wired in series electrical circuit and parallel thermal junctions. A thermocouple consists of N-type and P-type semiconductor elements, to generate thermoelectric cooling (viz., Peltier -Seebeck effect) when a voltage difference in appropriate direction is applied through the connected circuit. 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 Ptype 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). The design of thermoelectric cooling system is based on temperature difference across the hot and cold sides of the TEC module and the required cooling capacity. In this paper energy balance model and noise characterization is presented for evaluating system design of a prototype thermoelectric coolingphotovoltaic (TEC-PV) device. The prototype consists of an integrated design with ceiling suspended, wall mounted, rooftop and active façade TEC-PV devices [1].

Energy Balance Model
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 [2]. 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: The values of ETEC-PV are typically lower than 6%.
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. ( In order to investigate the operating energy consumption in summer, a thermoelectric cooling-photovoltaic (TEC-PV) device is simulated for building data as per Table 1, representing sunny, hot and humid outdoor air condition. Properties of TEC-PV device is provided in Table 2.

Thermoelectric Dehumidification
The room sensible heat factor (RSHF) is defined as the ratio of sensible cooling load to total cooling load (Equation 4).
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 [4]: Where, hc is the coefficient of convective heat transfer (W/m 2 K), Ac is the heat transfer area (m 2 ), 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 [5]: ( 6) Where, ma is the mass of the wet air inside the room (kg), Tsec is the dehumidifying period (sec), Φ1 and Φ2 are the relative humidity before and after dehumidification (%). The convective heat transfer coefficient between adjacent fins and room air is [6]:

Design Considerations for thermoelectric Cooling -Photovoltaic (TEC-PV) Devices
Building integration parameters: Thermoelectric cooling (TEC) devices can be fixed in a building on wall and ceiling as radiant cooling panels. Due consideration should be given for placing thermoelectric modules with or without heat sinks. Heat sinks can be placed towards building interior zone and towards exterior zone. The thermoelectric modules can be placed on a cut section of a wall, with provision of cooling the hot side heat sink. The thermoelectric cooling devices can also be fixed on max, a window or a skylight. Proper protection 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 system are good choice for using thermoelectric devices inside ducts. 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 mm to 2.5 mm. Cooling power capacity increases with decreasing the ratio of thermoelement length to cross sectional area. 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 [7]. Thermal contact resistance at the interface layer of thermocouple legs are 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, water cooled heat sink (thermal resistance of 0.108 K/W, and heat sink integrated with heat pipe (thermal resistance 0.11 W/K are most commonly used techniques. 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 [8]. For heating requirements during winter season, these active façade elements can supplement with heating from TEM and façade integrated PV ventilated devices.

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 Design of Thermoelectric Cooling-Photovoltaic (TEC-PV) Device
The system design consists of: i) outdoor fresh air ventilation; ii) thermoelectric cooling (TEC); iii) building integration; iv) photovoltaic power generation; and v) exhaust air ventilation. Operation: The outdoor fresh air is cooled down and dehumidified as it flows over a heat sink/exchanger attached to thermoelectric cooling (TEC) module. The cool air enters the indoor environment which is to be maintained at 23 °C and 55% RH. The stale air is taken out through ducted exhaust air ventilation system. The exhaust air also cools down the heat sink/exchanger attached to hot side of thermoelectric module (TEM). The outdoor fresh air is introduced into the single zone building air volume at varying rates as mentioned in Depending on dew point of the air, cooling dehumidification and iso-thermal dehumidification can take place on fins inside cooling duct and on wall with TEC modules. The schematic of a building zone with two stage cooling through TEC modules by means of supply duct and wall mounted TEC modules with solar PV façade exhaust duct is illustrated in Figure 2 a. The performance characteristics with voltage variation of analysed TEC1-12710 modules in TEC calculator is provided in Figure 2 b. The variation in theoretical values of COP (cooling) and temperature (cold) for ZTm=1 is provided in Figure 3 a. The variation in theoretical values of cooling capacity with temperature difference is provided in Figure 3 b. The variation of theoretical heat transfer coefficient with height of heat transfer surface (fins) is provided in Figure 4 a. The theoretical variation of cooling capacity load served inside room with height of heat transfer surface (fins) is provided in Figure 4 b. All the results are based on theoretical values irrespective of actual performance values of the prototype TEC-PV device.

Noise Characterization
A unified theory for stresses and oscillations is proposed by the author [9]. The following standard measurement equations are derived and adopted from the standard definitions for sources of noise interference [10,11,12,13,14,15]. Noise of Sol: For a pack of solar energy wave, the multiplication of solar power storage and the velocity of light gives solar power intensity I. On taking logarithm of two intensities of solar power, I1 and I2, provides intensity difference. It is mathematically expressed as: (8) Whereas logarithmic unit ratio for noise of sol is expressed as Sol. The oncisol (oS) is more convenient for solar power systems. The mathematical expression by the following equality gives an oncisol (oS), which is 1/11 th unit of a Sol: (9) Noise of Therm: For a pack of heat energy wave, the multiplication of total power storage and the velocity of light gives heat power intensity I. The pack of solar energy wave and heat energy wave (for same intensity I), have same energy areas, therefore their units of noise are same as Sol. Noise of Scattering: For a pack of fluid energy wave, the multiplication of total power storage and the velocity of fluid gives fluid power intensity I. On taking logarithm of two intensities of fluid power, I1 and I2, provides intensity difference. It is mathematically expressed as: Whereas, logarithmic unit ratio for noise of scattering is Sip. The oncisip (oS) is more convenient for fluid power systems. The mathematical expression by the following equality gives an oncisip (oS), which is 1/11 th unit of a Sip: (11) For energy area determination for a fluid wave, the water with a specific gravity of 1.0, is the standard fluid considered with power of ±1 Wm -2 for a reference intensity I2. Noise of Elasticity: For a pack of sound energy wave, the product of total power storage and the velocity of sound gives sound power intensity I. On taking logarithm of two intensities of sound power, I1 and I2, provides intensity difference. It is mathematically expressed as: (12) Whereas, logarithmic unit ratio for noise of elasticity is Bel. The oncibel (oB) is more convenient for sound power systems. The mathematical expression by the following equality gives an oncibel (oB), which is 1/11 th unit of a Bel: (13) There are following elaborative points on choosing an onci as 1/11 th unit of noise [15,16]: i) Reference value used for I2 is -1 W m -2 on positive scale of noise and 1 W m -2 on negative scale of noise. In a power cycle, all types of wave form one positive power cycle and one negative power cycle [9]. Positive scale of noise has 10 positive units and one negative unit. Whereas, negative scale of noise has 1 positive unit and 10 negative units; ii) Each unit of sol, sip and bel is divided into 11 parts, 1 part is 1/11 th unit of noise; iii) The base of logarithm used in noise measurement equations is 11; iv) Reference value of I2 is -1 W m -2 with I1 on positive scale of noise, should be taken with negative noise measurement expression (see Eqs 9, 11 and 13), therefore it gives positive values of noise; v) Reference value of I2 is 1 W m -2 with I1 on negative scale of noise, should be taken with positive noise measurement expression (see Eqs 9, 11 and 13), therefore it gives negative values of noise. The choosing of onci in noise units is done so as to have separate market product & system of noise scales and their units distinguished from prevailing decibel units (which has its limitations) in the International System of Units. More discussions on energy conversion, noise characterization theory and choice of noise scales and its units are presented in many papers by the author [15,16]. Tables 3, 4, 5 and 6 have presented sensitivity analysis and noise characterization values for the exterior duct based on mass flow rate, solar irradiation and size of duct. Appendix has provided noise calculation charts.  Table 6 Noise of elasticity with air particle velocity (Impedance Z0 = 413 N·s·m -3 at 20°C)

CONCLUSIONS
Thermoelectric cooling (TEC) is one of the specialized areas in "Thermoelectrics". This paper has presented the summary of energy modeling parameters representing various cooling load performance and noise characteristics of building integrated thermoelectric cooling-photovoltaic (TEC-PV) devices. There is significant growing interest level in thermoelectric cooling (TEC) because of their useful control aspects. This is because TEC modules are readily operated at partial load by changing the electric current. Moreover, there is increase in cooling COP with reduction of cooling power. Air-conditioning of fresh outdoor air for direct indoor use through proper system design of supply air ventilation system and exhaust air ventilation system is another key benefit of thermoelectric cooling (TEC). In addition, photovoltaic (PV) roof-top power generation and photovoltaic (PV) ventilated façade are integrated into the system design, thus making it further sustainably sound in terms of input electricity requirements through green power and active ventilation system for supply and exhaust air. Thermoelectric modules (TEM) offer airconditioning solutions with flexible electrical loads in contemporary context of smart energy systems for buildings. Finally, the noise interference and characterization equations as per speed of a composite 1   Example: To find oSol corresponding to a pressure ratio of 363 Ratio of 363 = 11X33 In oSol = +22+32 oSol = +54 oSol