EXPERIMENTAL DETERMINATION OF THE ...

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ABSTRACT: The use of concentrator solar cells in photovoltaic plants ... the aim of reproducing the thermal load conditions on a concentrator photovoltaic cell.
20th European Photovoltaic Solar Energy Conference, 6 –10 June 2005, Barcelona, Spain

EXPERIMENTAL DETERMINATION OF THE DISSIPATION SYSTEM THERMAL BEHAVIOUR FOR CONCENTRATOR PHOTOVOLTAIC CELLS R. Fucci, G. Leanza, F. Pascarella, A. Romano, F. Roca ENEA Research Center Portici – Località Granatello 80055 – Naples – Italy, email:[email protected] ABSTRACT: The use of concentrator solar cells in photovoltaic plants significantly reduces the number and area of expensive cells and contributes to reduce the total system cost. Unfortunately the remarkable power concentration weighing upon limited areas is only partially turned into electric energy; therefore its dissipation towards the outside becomes necessary. The characterization of the thermal dissipation structures used turns out to be necessary, in order to highlight its effectiveness in real working conditions. As the theoretical analysis of the heat transfer is a very complicated task, it is usual to obtain the total thermal resistance Rth (difference between the cell and the ambient temperature to the available irradiance on the cell) from experimental measurements. It has been realized an experimental apparatus with the aim of reproducing the thermal load conditions on a concentrator photovoltaic cell. By studying and comparing the thermal behaviours of various solutions, it is possible to come to the comprehension of the thermal phenomenon, so allowing the choice and the configuration of the best structure from the point of view of the performances/cost ratio. Keywords: Concentrator Cells, Thermal Performance, Experimental Methods 1

INTRODUCTION

The difficulties met in the last few years for the development of thin film materials alternative to the crystalline silicon and the hypothesis, very plausible, that in the next future very high efficiency cells can be available (>30%) even if at high cost for surface unity, have created the preambles for a new and renewed interest toward the photovoltaic (PV) concentration. In this technology the active area covers only a little part of the PV module while the light is concentrated on the sensitive element by lenses or mirrors realized with materials which are widely common and not very expensive, like plastic or glass materials. At present the PV Concentrators technology (PV-C technology) is considered by the international scientific community to be an attractive and promising application for its capacity to speed up the reduction of the photovoltaic technology cost. In such context, the PhoCUS [1] (Photovoltaic Concentrators to Utility Scales) project has been started at ENEA. The basic unit of such plant is constituted by a 5 KWp generator; each generator panel is constituted by a set of PV-C modules for the attainment of the total power of the basic unit. The concentration of the luminous energy on the cells takes place through lenses with a concentration factor of 200 X. In the planning stage of these modules an important aspect to be faced is the remarkable power concentration weighing upon limited areas (~ 1 cm2), which produces power density even in the order of 20 W/cm2 for concentration of 200X. Such solar power density is only partially turned into electric energy; therefore its dissipation toward the outside is necessary. The effect of possible excessive raising of the temperatures is dual; on the one hand, the electric efficiency of the cell turns out to be damaged [2], on the other the presence of materials with different Coefficient Thermal Expansion (CTE) involves mechanical problems. An experimental apparatus has been realized for the thermal behaviour determination of the cell when some operating conditions change; the aim is to reproduce the thermal load conditions present on a working PV-C cell. It’s possible to rebuild the dissipation structure which 2364

we want to use (cell, cell package, heat-spreader, heatsink) and characterize it in all the various working conditions: environment temperature, heat to be dissipated, wind influence. It’s also possible to determine the response of the system in connection with the time, with the possibility to obtain a model. This knowledge allows to obtain the cell working temperature in any condition, also in case of variable regime, which can happen in case of cloudy days or at the initial and final part of the day. In these situations the direct radiation changes very quickly (the hypothesis of stationary regime is not true). The most important advantage of such structure consists in the possibility of a control of the heat to be dissipated, free from outside direct radiation conditions, and in a quick evaluation of the system performances. 2

EXPERIMENTAL APPARATUS

The study is reduced to the analysis of a sub-module constituted by a single cell mounted on a heat dissipation structure; within the limits of the approximations made, such an apparatus allows the use of various constructive solutions. The fundamental elements at the base of the experimental apparatus are the simulator of the thermal load, the mock-up which in some way reproduces the conditions present inside the concentration module, and the outside circuit of measurement which allows the application of the thermal power to be dissipated and the measurement of the temperatures and the electrical variables. 2.1 Thermal load simulator It allows to dissipate all the necessary heat from the structure (Fig. 1); the reproduction of the thermal load for a single cell and its related heat-sink is realized depositing on a structure exactly equal to our cell a contactable heating resistance. The heat dissipation is caused by Joule effect. Therefore, on a silicon substrate of (12*12) mm2 dimension (area of the cell), was deposited an electrically insulating layer between the substrate (silicon) and the heating resistance; for the first cells we have deposited

20th European Photovoltaic Solar Energy Conference, 6 –10 June 2005, Barcelona, Spain

silicon nitride (0.5 µm) by PECVD (Plasma Enhanced Chemical Vapour Deposition ), while the heating structure is realized in silver deposited by electron beam evaporation.

temperature measurements are realized thermocouples in close contact with the cell.

Heat-sink

by

K-

Lens

Figure 1: Thermal load simulator Finally, the thermal load simulator is completed with the heat dissipation structure under investigation (Fig. 2).

Cell

Substrate

Figure 3: Sketch of the mock-up

DC Power Supply

Shunt Resistance

Heat-sink

Cell

Figure 2: Thermal load simulator with heat dissipation structure 2.2 Mock-up indoors The structure so obtained, which has the aim to reproduce the sub-module to be characterized, is completed by a housing in plastic material (PMMA) (Fig.3); the walls are 21 cm in height (focal distance expected by the lenses used in the PhoCUS module). With an opportune thermal insulation of the lateral walls, it is possible to suppose that the system works in adiabatic conditions: it can represent a good approximation of the behaviour of an internal cell of the module in stationary working conditions. The frontal side of the mock-up isn’t thermally insulated and reproduces, from the point of view of heat transmission, the behaviour of the lens. The realized mock-up is mounted on a support structure which allows the rotation, with the chance of measuring at variable elevation (Fig. 3). 2.3 Measurement circuit The measurement circuit is constituted by a continuous current source which provides the energy to be dissipated on the heating resistance. The voltage and the current on the cell are acquired by a data-logger;

Datalogger Figure 4: Measurement circuit 3

EXPERIMENTAL MEASUREMENTS

The experimental activity realized is referred to temperature measurements on the pseudo-cell in connection with time applying various kinds of inputs and using different assemblies. 3.1 Output of the system to different inputs On a typical structure constituted by a cell and the related heat-sink, a series of measurements has been taken, changing the input: applying a step input of opportune amplitude, it is possible to determine the regime temperature of the cell in reply to the input. Opportunely changing the amplitude of the input, it is possible to determine the overall output of the cell (Fig.5)

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20th European Photovoltaic Solar Energy Conference, 6 –10 June 2005, Barcelona, Spain

100

90

90

80

80

60

T [°C]

60

T [°C]

70

1W 5W 10 W 15 W 20 W

70

50

50 40

40

30

30 20

20

10

10

0

0 0

500

1000

1500

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2500

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3500

0

5

time [sec]

Figure 5: Typical output of the heat dissipation structure for PV-C applications in case of step inputs.

10

15

Power [W]

20

25

Figure 6: Increase of cell temperature vs. heat to be dissipated in different heat dissipation structures. In table I there are the related equivalent thermal resistances:

Observing the time evolution of the temperature on the cell (Tcell) during the heating, it is possible to interpolate the system output through the sum of two exponentials having different time constants. A first evaluation of these two terms consists in supposing that the first exponential (the one with the shorter time constant) depends on the load and unload phenomena on the cell-substrate structure which, because of its reduced dimensions and its structure, reaches the regime temperature very quickly; the second exponential, that is the transient one with longer time constant, depends on the load and unload phenomena of the heat-sink, which presents a very important heat dispersion phenomenon, as its lateral dimensions are bigger and the heat source concentrated. T = T + Tth1* [1 - exp(-t / τ )] + Tth2 * [1 - exp(-t / τ )] C

C0

c1

c2

The regime temperature on the cell will be: T

reg

= T + Tth1+ Tth2 C0

-Aluminium sheet 5mm thickness, black oxidized -Aluminium sheet, 2mm thickness black oxidized -Aluminium sheet, 2mm thickness white oxidized -Steel sheet, 2mm thickness black oxidized -Heat-sink PhoCUS

2,0623 2,5697 4,061 2,0177

Table I: Equivalent thermal resistances 3.3 Measurements at variable elevation The heat-sink used in the PhoCUS module has been completely characterized changing the elevation angle. In this configuration, between the cell and the heat-sink is inserted an electrically insulated substrate of silicon nitride. The obtained results allow a thorough knowledge of the thermal resistance of the dissipation structure (Fig.7;Table II).

where TC0 is the starting temperature of the PV cell The behaviour of the system in function of the power is linear (Fig. 6; Fig. 7) ; so it’s possible to define a useful equivalent thermal resistance:

60 55 50 45

Rth = (Treg -Tamb) / Pc

80°

T [°C]

where Pc is the heat to be dissipated.

Rth [°C / W] 2,1655

40 60°

35

30°

30

3.2 A comparison among different dissipation structures On the base of these first measurements, it is possible to realize a comparison among various dissipation structures by comparing the maximum regime temperatures on the cell, changing the power to be dissipated (Fig. 6; Table I); the comparison has been realized at an elevation angle of 0°. In particular we have compared some flat structures of aluminium and steel of different thickness and emissivity (in case we want to utilise a metallic housing for the PV-C module, which works as heat dissipation structure, excluding the use of a further heat-sink) and an aluminium heat-sink used in the first implementation of the PhoCUS modules (which use a plastic material housing)

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25 20 15 0

5

10 15 Power [W]

20

25

Figure 7: Increase of cell temperature vs. heat to be dissipated at variable elevation. Elevation angle Rth [°C / W] 80° 2,6983 60° 2,6823 30° 2,5863 0° 2,5569 Table II: Equivalent thermal resistances

20th European Photovoltaic Solar Energy Conference, 6 –10 June 2005, Barcelona, Spain

Comparing the results from Table I and Table II related to the same heat-sink and elevation (0°), we can calculate the contribute to the whole thermal resistance of the substrate of silicon nitride mentioned above (Rth=0,54[°C/W]). 3.4 Daily thermal load The value of maximum temperature which the system reaches in the space of a day can be obtained by a complete day lasting measurement on such apparatus, applying thermal power steps obtainable from the trends of the available direct radiation. In a similar way, the value of the equivalent thermal resistance can be useful to determine some information about the maximum temperature reached by the PV cell. For the heat-sink used in the first PhoCUS modules, we have obtained these results (Table III):

-Tamb=40°C DNI=1000 W/m2 C=200X Cell Area=1,21 cm2 Lens optical efficiency=85% Cell electrical efficiency=0%

Cell Temperature [°C] 95,5

5

PERFORMANCE / COST RATING

To obtain a performance rating of our dissipation structure, it may be useful the use of a model that is equivalent to the lens – PV cell – heat-sink system (Fig.9), which allows an aimed design of the dissipation structure in function of the ambient parameters and the structures used for the concentration (lens - PV cell).

Plight

Pelectric Optical efficiency

Electrical efficiency

Thermal resistance

Figure 9: Equivalent model of the lens – PV cell – heat dissipation system.

Table III: Maximum temperature on the cell

It’s possible to model the heat-sink by an equivalent R-C circuit, or by introducing the equivalent thermal resistance value (hypothesis of almost-stationary input), as seen above; the PV-cell can be modelled by an equivalent circuit in function of the cell temperature and the direct radiation [3]. In both cases, the performance is represented by the annual energy production, which such a structure involves, combined with the cost which the choice of a dissipation structure involves. Clearly the choice depends strongly on the meteorological characteristics of the site in which the PV-C module will be used.

4

6

-Tamb=40 °C DNI=1000 W/m2 C=200X Cell Area=1,21 cm2 Lens optical efficiency=85% Cell electrical efficiency=20%

84,4

EQUIVALENT ELECTRICAL CIRCUIT

The previously seen interpolation with two exponentials of the system output allows to model the structure from the thermal point of view by means of an equivalent R-C circuit with two time constants (Fig. 8):

Pcell

∆T1

∆T2

Figure 8: Equivalent electrical circuit From experimental measurements, we can prove that there’s not any difference, from the point of view of the regime cell temperature, between applying a single step of heat to be dissipated and arriving at that power by degrees; the system is therefore linear and time-invariant. It is also possible to show the linearity of the system referred to the ambient temperature.

CONCLUSIONS

By the experimental apparatus realized, we are able to measure quickly and easily the performances of the heat dissipation structures used in PV concentrator modules. In relation to the characteristics of the lenses and the PV cells, we can calculate the performance of our system referring it to the meteorological characteristics of the site in which the PV-C module will be used. REFERENCES [1] A.Sarno et al. ,”The PhoCUS Project” Proceeding of the PV in Europe Conference Rome October 2002. [2] A. Luque:”Solar Cells and Optics for Photovoltaic Concentration”.THE ADAM HIGLER SERIES ON OPTICS AND OPTOELECTRONICS (1989). [3] B. Marion “A Method for Modeling the CurrentVoltage Curve of a PV Module for Outdoor Conditions" Prog. Photovolt.: Res. Appl. 2002: 205214.

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