understanding temperature effects on pv system performance

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ABSTRACT. Within the framework of Task 2 of the Photovoltaic. Power System Programme (PVPS) of the International En- ergy Agency (IEA) performance ...

UNDERSTANDING TEMPERATURE EFFECTS ON PV SYSTEM PERFORMANCE Thomas Nordmann, Luzi Clavadetscher, [email protected], [email protected] TNC Consulting AG, Seestrasse 141, CH-8703 Erlenbach, Switzerland.

ABSTRACT Within the framework of Task 2 of the Photovoltaic Power System Programme (PVPS) of the International Energy Agency (IEA) performance results of grid-connected and stand-alone PV systems in different countries have been compared. In this paper the effect of the elevated cell-temperature on the annual performance of 18 selected grid-connected PV systems of different mounting (freestanding, roofmounted and integrated PV facades) from different geographic location in 5 countries are analysed. Annual datasets of hourly data have been used for this in depth analysis. 1. INTRODUCTION

Freestanding

Flat roof

System Losses

Stadtmühle R

Laus

Marzili Muttenz

Bolzano

Zulehner

0 Portugaller

0 Domat

20

Nagoya City 4

1

Wildkogelbahn

40

Buchinger

2

Hiroshima-City

60

Becker

3

Bologna

80

Aachen

4

Klammt AG Stadtmühle F

100 Temperature [°C], PR [%]

Sloped roof

5

Stadelhofen

kwh / kWp * d

• • • •

Facade (integrated) Sloped roof Freestanding Flat roof

Final Yield

The following figures (2 to 6) show the rise of the module temperature (Tm - Tam) in respect to the irradiation for the different types of mounting. The figures represent hourly data during the operation of the plant over one year. For each type of mounting two samples are shown, usually one with high and one with a low module temperature. A wide spread in cluster of points would indicate influences other than the irradiation, such as wind or airflow. 3.1 Sloped roof, highly integrated The plant Stadelhofen in Switzerland (figure 2) shows a high module temperature. The cells are mounted on the inside of a compound insulation glass of a slightly sloped roof. It seems that there is little air circulation inside the building in the roof area. The maximum measured module temperature was 85°C and the mean rise in temperature from ambient is about 55 K at 1000 W/m2. This plant showed the highest module temperature of the plants compared, this results in an annual loss of 11.3 %. The integrated compound modules are part of the architectural design and provide daylight in corridor of a school building. 60

Delta T, T_m - T_am [K]

2. PLANTS INVESTIGATED

Conversion Losses

The types of mounting are grouped in 4 main types:

3. MODULE TEMPERATURE

The IEA PVPS Database now contains monitored monthly datasets from 370 PV systems of different technologies, located in 15 countries [1] [2]. About 48 % of these are mounted on a sloped roof, 45 % are freestanding, mounted on a flat roof or on a soundbarrier and 6 % are on a facade. The degree of building integration varies from non to highly integrated. Some of the datasets in the IEA PVPS Database contain the measured mean module temperature. By using annual datasets of hourly monitored data it was possible to determine the rise in module temperature with the irradiation and also the quantify the energy losses due to elevated module temperature for specific type of mounting.

Facade

The 18 grid connected PV plants investigated are located in Austria, Germany, Italy, Japan and Switzerland. The monitored annual datasets were supplied by the Task 2 members of each country. All the 18 plants are grid-connected systems and the modules are made from crystalline silicone cells.

50 40 30 20 10 Stadelhofen 0

Performance

Availability

Modultemperature

0

0.2 0.4 0.6 0.8

1

1.2

Irradiation, G_i [kW/m^2]

Figure 1, shows the annual values of the normalised losses and yields, the performance, the availability of the plant and the module temperature of the 18 plants analysed.

Figure 2, rise in the module temperature from ambient, plant Stadelhofen Switzerland.

3.2 Facade, integrated

The facade of the plant Klammt AG consists of opaque, insulated PV-cladding elements with an air-gap between the module and the insulation for cooling the air-gap seems to narrow for proper cooling, resulting in a higher cell temperature. The typical rise in module temperature from ambient for the 3 integrated facade system is between 46 and 52 K and the annual temperature losses are between 5 and 7 %.

50

30 20 10 Aachen 0

40 30

30 20 10

0.2 0.4 0.6 0.8

1

20 10 Bolzano

0

0 0

0.2 0.4 0.6 0.8

1

1.2

0

0.2 0.4 0.6 0.8

4. TEMPERATURE LOSSES For the purpose of comparison only the data during the full operation of the plant were used for the calculation of the values represented in figure 6. Any large differences in the conversion losses (LC) are due to shading, partial disconnection of strings or disconnection single inverters in multi inverter systems.

Facade

Klammt AG 0

0.2 0.4 0.6 0.8

1

Sloped roof

Freestanding

Flat roof

100%

50

80%

40

60%

30

40%

20

20%

10

1.2

Irradiation, G_i [kW/m^2]

3.3 Sloped Roof

0%

Delta T, T_m - T_am [K]

20 10

40

Modultemperature

Muttenz

Stadtmühle R

Laus

Marzili

Bolzano

Zulehner

Portugaller

Domat

Nagoya City 4

Wildkogelbahn

Buchinger

Hiroshima-City

Becker

Bologna

Stadtmühle F

Aachen

Klammt AG

Final Yield Ambient Temperature

20

Losses 10

Temperature

16%

60

12%

40

8%

20

4%

0

0%

-20

Becker 0 1.2

Irradiation, G_i [kW/m^2]

0

0.2 0.4 0.6 0.8

1

1.2

Irradiation, G_i [kW/m^2]

Figure 4, rise in the module temperature from ambient, plants Buchinger and Becker in Austria.

Temperature Loss

0 1

System Losses

Conversion Losses

30

Buchinger 0.2 0.4 0.6 0.8

Temperature Losses

Figure 6, yield and losses as percentages, mean daytime ambient temperature and mean module temperature for 18 the plants investigated.

50

30

0

Stadelhofen

Figure 4, is an example of a well cooled and not so well cooled sloped roof system. The range of the rise of temperature from ambient in the systems investigated is between 20 and 34 K and the temperature losses range from 1.7 to 7 %. The cooling of a PV array mounted in a sloped roof depends on the level of integration or on the size of the air-gap between the roof and the modules.

40

1.2

Figure 5, rise in the module temperature from ambient, plants Zulehner in Austria and the plant Bolzano in Italy.

10

1.2

50

1

Irradiation, G_i [kW/m^2]

Irradiation, G_i [kW/m^2]

Figure 3, rise in the module temperature from ambient, plants Achen and Klammt AG in Germany.

Delta T, T_m - T_am [K]

30

20

Irradiation, G_i [kW/m^2]

0

40

Zulehner

0 0

Delta T, T_m - T_am [K]

40

Temperature [°C]

40

50

-40

-4%

3.4 Freestanding and Flat Roof Freestanding and flat roof systems usually allow a free airflow around the modules and have therefore lower temperature losses. Of the freestanding and flat roof systems analysed the temperature rise is between 20 and 28 K and the temperature losses between 1.7 and 5 %

Rise in Temperature [K]

Delta T, T_m - T_am [K]

Delta T, T_m - T_am [K]

50

Delta T, T_m - T_am [K]

50

Figure 3 shows the module temperature of two facade integrated systems in Germany. The facade elements of the plant Aachen are partly transparent compound glass insulation modules like on the plant Stadelhofen. There seems to be a free airflow on the inside of the building, resulting in a lower cell temperature.

Sloped roof, highly integrated

Facade

Sloped roof Freestanding

Flat roof

Figure 7, is an overview of the results of all the 18 PV systems, showing the temperature losses and the rise in module temperature from ambient (K at 1000 W/m2) grouped by the type of mounting.

Annual Mean Temperatures Module Temperature

Plants analysed Tilt Mounting

Name

Power

Country [° ]

Sloped roof highly integrated Stadelhofen

CHE

5

Façade

Irradiation Ambient Ambient Module Maximum Daytime

Rise

Temperature Loss

P0

Hi

T am

T am, d

Tm

T m, max

T m, Hi

L

[kW]

[kWh / m2]

[°C]

[°C]

[°C]

[°C]

[K/kWh/m2]

[%]

[%]

[kWh/kWp]

896

13

17

51

85

55

11.3

80

720

9.4

PR

t

Y

f, a

integrated

Aachen

DEU

90

4.0

785

17

40

67

46

6.4

62

486

integrated

Klammt AG

DEU

90

20.1

852

15

41

64

52

7.2

59

506

integrated

Stadtmühle F

CHE

90

16.4

731

11

16

37

65

46

5.3

82

599

Becker

AUT

30

3.2

1359

10

14

37

64

34

5.2

63

852

Bologna

ITA

24

2.3

1138

22

38

61

23

5.5

83

946

Buchinger

AUT

26

1.8

1250

10

16

29

57

20

1.7

68

849

Sloped roof

Hiroshima City

JPN

19

2.9

1450

19

23

41

70

31

7.0

74

1073

Wildkogelbahn

AUT

26

4.7

1407

1

6

20

48

22

-2.1

74

1047

Soundbarrier

Domat

CHE

45

104.0

1522

10

14

31

54

24

2.8

82

1241

Freestanding

Nagoya City 4

JPN

10

3.6

1338

17

21

36

65

26

5.0

82

1098

Freestanding

Portugaller

AUT

1.3

1300

9

15

33

57

25

3.7

52

670

Other

Zulehner

AUT

50

2.0

1157

9

15

29

59

20

1.7

70

813 1004

Flat roof

Bolzano

ITA

40

1.6

1344

16

34

58

27

4.0

75

Laus

AUT

39

2.4

1151

11

17

35

64

25

4.2

78

893

Marzili

CHE

35

22.7

1290

10

16

34

55

25

3.8

74

956

Muttenz

CHE

45

21.2

1164

8

15

31

58

25

2.8

78

911

Stadtmühle R

CHE

25

15.0

1092

11

15

31

61

28

2.5

79

859

Table 1, key performance and temperature data of the systems analysed. Of some systems only daytime data was available and therefore the annual mean ambient temperature is missing.

Figure 7 shows clearly that the type of mounting and the manner of integration can have a significant influence on the rise in module temperature and the temperature losses. 5. MONTHLY DATA

Temperature Losses

System Losses

Conversion Losses

Final Yield

Modultemperature

December

12%

8%

4%

0%

-4%

Temperature [°C]

0

October

50%

November

10

September

60%

July

20

August

70%

May

30

June

80%

April

40

March

90%

February

50

January

100%

Figure 8 is typical example of the monthly variation of the module temperature and the temperature losses. This plant in Japan is located in an area with high daytime temperatures in the summer months. The temperature losses range from 11 % in July to a gain of 3.6 % in January (figure 9). Figure 10 shows the annual values for the final yield, system losses, conversion losses and the temperature losses.

Temperature Losses

Of the 18 systems analysed 17 showed an annual temperature losses ranging from 1.7 % to 11.3 %. One alpine system, Wildkogelbahn in Austria, due to low ambient temperature, has a temperature gain of 2 %. A well cooled PV array can have a temperature rise of about 20 K and a temperature loss of less than 3 %.

10

20 30 40 50 Modultemperature

60

Figure 9, mean module temperature vs. temperature losses, plant Nagoya City 4 in Japan. Temperature Losses (5%)

Conversion Losses (7%) System Losses (5%)

Ambient Temperature Final Yield (83%)

Figure 8, yield and losses as percentages, mean daytime ambient temperature and mean module temperature, monthly values of a freestanding plant, Nagoya City 4 in Japan.

Figure 10, annual yield and losses, plant Nagoya City 4 in Japan.

6. CONCLUSIONS The analyses of these 18 grid-connected PV systems showed the importance of the optimal mounting and in the case of building integration a well designed layout to achieve an efficient cooling of the PV-modules. Freestanding and flat roof mounted systems show the lowest rise in temperature. Sloped roof systems need a free airflow between the roof and the modules. Integrated facade systems require a high degree of sophistication to get sufficient cooling of the modules. Analysing PV systems data from various sources, it became clear that optimal placement of the temperature sensor for the ambient and module temperature is important. It also seems that the DC-power measurement is the most difficult to get reliable results.

The authors wish to thank their colleagues from IEA PVPS Task 2 for supplying the data for this study.

7. REFERENCES [1] IEA PVPS Task 2 Report, Analysis of Photovoltaic Systems, Report IEA-PVPS T2-01: 2000, April 2000. [2] IEA PVPS Task 2 Report, Operational Performance, Reliability and Promotion of PV Systems, June 2002. [3] IEA PVPS Task 2, Performance Database, May 2003, www.task2.org. [3] Guidelines for the Assessment of Photovoltaic Plants, Document A and Document B, June 1993, JRC, Ispra Italy. [4] International Electrotechnical Commission (IEC): Standard IEC 61724, Photovoltaic System Performance Monitoring - Guidelines for Measurement, Data Exchange and Analysis. [5] H. Häberlin and Ch. Beutler, Normalized Representation of Energy and Power for Analysis of Performance and On-line Error Detection in PV-Systems. Proc. 13th EU PV Conference, Nice, 1995.

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