Efficiency in Solar Thermal Panels and their testing

Genersys solar panels have been tested in many places for literally dozens of different countries. The testing for the European Union covers certification for each member of the European Union. In addition, some nations within the European Union, like France and Spain, have additional tests to which Genersys panels have been subjected. The latest country to test Genersys panels was South Korea. In every case Genersys panels have “passed” these certification tests.

Unfortunately both for the consumer and the solar thermal professional the information obtained in the tests can be misleading. At first sight it may be thought that the more heat that the solar panel creates the better, but this is not in fact the case. Solar thermal works on the basis of being able to store the energy collected by the sun. A larger water store with a properly designed and sized heat exchanger will provide more useful heat than a very hot panel.

Indeed, there are great disadvantages in making the panels too hot. In very hot countries – parts of the United States, Australia  and South Africa panels designed to be very hot will either need to be covered in summer and/or have the generated heat dumped. In other places the panel will simply overheat the system in very hot weather.

A panel designed on heat performance will over heat regularly. Overheating causes heat damage to the panels’ absorbers, to the system seals and valves. Over heated panels and solar systems are over stressed and they do not last long and they will have heavy regular maintenance charges.

At Genersys we aim to design our panels so that they are not damaged by great heat stresses and so that they also provide useful energy in low light conditions. These features often mean that on simple test comparisons it may appear to the untrained and unqualified eye that other panels perform better. This is not the case. Genersys panels perform as well as any other panels but by their design withstand heat stresses better, last longer and do not over heat or cause the system to overheat.

Michaela Wheeler is our Chief Engineer in the United States, (where solar thermal technology is relatively under developed) and she has has occasionally had to answer questions about panel performance. She has now put together her scientific comparison of panel testing. She concnetrates her figures around the Genersys 1000-10 panel and its US competitors.

You will note that different testing stations (even in Europe) often provide different test results! Michaela’s explanation creates a proper comparison of different solar collectors.  Here conclusion is “After converting the EN 12975 report into the OG-100 format, the Genersys 1000-10 has a peak efficiency that is comparable with its competitors and that all major panels operate at more or less the same peak efficiency as to make no difference.” and I am told that her mathematics proves it.

1000-10 Panel Thermal Test Results

Test

Laboratory

Year Peak Efficiency Flow Rate (ml/s)

Test Fluid

EN-12975

Fraunhofer

2003

0.814

34

Distilled Water

OG-100

Bodycote

2005

0.588

13

40% Propylene Glycol

EN-12975

Arsenal Research

2007

0.776

36

Distilled Water

    Source: Genersys USA

As shown in the chart above, the 1000-10 panel was tested using the EN-12975 and the OG-100 test.  

The thermal performance test procedure for the EN-12975 is the EN-12975-2 and the thermal performance test procedure for the OG-100 is the ISO 9801-1.   The test procedure for the EN 12975-2 was adopted from the ISO 9806-1.[1]   Given the same testing conditions both test procedures should arrive at the same data.  

The difference between the EN-12975 and the OG-100 is how they calculate instantaneous efficiency and collector area.   The EN-12975 calculates instantaneous efficiency using the arithmetic average of the fluid inlet and outlet temperatures.[2]  The OG-100 calculates instantaneous efficiency using the inlet temperature of the fluid.[2]   The EN-12975 uses aperture area whereas OG-100 uses gross area for its collector area.[2]   Instantaneous efficiency can be calculated in both formats using either EN-12975-2 or ISO 9801-1 testing procedures.

 Comparing OG-100 Results with Competitors

 

 

Model

Test

Year Peak Efficiency Flow Rate (ml/s)

Test Fluid

Genersys 1000-10 OG-100

2005

0.588

13

Propylene Glycol & Water

Stiebel Eltron Sol 25 Plus OG-100

2006

0.649

54.9

Water

Viessmann Vitosol 200F OG-100

2006

0.7203

49.8

Propylene Glycol & Water

Schuco USA L.P. Premium OG-100

2005

0.718

50

Water

Bosch Thermo… Buderus OG-100

2007

0.715

48

Water

Marathon Int. Baxi OG-100

2007

0.696

38.2

Water

     Source: SRCC: http://www.solar-rating.org

                 Comparing the Genersys 1000-10 panel with its competitors reveals that the peak efficiency is significantly lower.   However, comparing the test conditions between the panels reveal that the flow rates and test fluids are significantly different.   This difference prevents us from making a proper comparison between the Genersys product and its competitors.  

 In order to provide fair comparison between panels, the OG-100 has an equation for determining flow rates for each panel.   The OG-100 application form states, “For glazed flat-plate liquid-type solar collectors the ASHRAE standard flow rate per unit area (transparent frontal or aperture) is 0.02 kg/(s m2) [14.7 lb/(hr ft2)]. “[3]   Given this equation, the Genersys 1000-10 panel should have been tested at 35.6 ml/s.   Unfortunately, the 1000-10 panel was tested at 13 ml/s.

 How to Create a Proper Comparison

In order to create a proper comparison between the Genersys 1000-10 and its competitors, the Genersys 1000-10 panel would need to be tested at the proper flow rate of 35.6 ml/s and with a test fluid of water.   Thankfully this test has already been performed in the EN 12975 certifications shown on the first chart.   Since the testing procedures are the same for the EN 12975-2 and the ISO 9806-1, we can use the data from the EN 12975 report to calculate the efficiency in the OG-100 format.   The data for the 2003 EN 12975 report done by Fraunhofer Institut Solare Energiesysteme is shown in the table below.

 

m

tin

tout

tout – tin

tm

ta

tm -ta

(tm-ta)/G

Ŋ

kg/hr

°C

°C

K

°C

°C

K

Km2/W

121.32

23.04

32.94

9.9

27.99

29.58

-1.59

-0.0016

0.822

121.4

23.05

32.95

9.9

28

29.64

-1.64

-0.0017

0.822

121.34

23.05

32.96

9.91

28

29.72

-1.72

-0.0018

0.823

124.87

46.16

54.38

8.22

50.27

29.72

20.55

0.0211

0.702

124.76

46.18

54.42

8.24

50.3

29.97

20.33

0.0208

0.703

124.74

46.2

54.45

8.25

50.32

29.98

20.34

0.0209

0.704

126.02

69.29

75.85

6.56

72.57

30.75

41.81

0.0429

0.567

125.97

69.3

75.87

6.56

72.58

30.73

41.85

0.0429

0.567

126.04

69.32

75.88

6.56

72.6

30.71

41.89

0.0429

0.567

123.59

92.14

96.99

4.85

94.56

30.94

63.62

0.0652

0.413

123.65

92.16

97.01

4.86

94.59

31.1

63.49

0.0651

0.413

123.68

92.18

97.04

4.86

94.61

31.12

63.49

0.0651

0.414

Source: Genersys PLC: 2003 EN 12975 Fraunhofer Institut  Solare Energiesysteme

 

                m = mass flow rate

                tin= temperature in

                tout= temperature out

                tm= average temperature between in and out

                ta= ambient temperature

                G = Irradiance (975 W/m2)

                Ŋ = Efficiency

 

The equation for determining efficiency using the OG-100 format is shown below.[2]

 

12ÅŠ = mCpTout-TinAcGt’>

                                                           

                                                            Ŋ = efficiency

                                                            12m’> = mass flow rate

                                                                                Cp = specific heat of water  (4.186 J/g °C)

                                                                                Tout = temperature out

                                                                                Tin = temperature in

                                                                                Ac = gross area of collector (2.028 m2)

                                                                                Gt = Irradiance on collector (975 W/m2)

 

Peak efficiency is obtained when the difference between Tout and Tin is greatest.   Looking at the test data chart above, this occurs when Tout – Tin is 9.91 K.   This line on the chart is shown below.

 

m

tin

tout

tout – tin

tm

ta

tm -ta

(tm-ta)/G

Ŋ

kg/hr

°C

°C

K

°C

°C

K

Km2/W

121.34

23.05

32.96

9.91

28

29.72

-1.72

-0.0018

0.823

Source: Genersys PLC: 2003 EN 12975 Fraunhofer Institut  Solare Energiesysteme

 

Entering the data on this line into our equation results in the following efficiency:

 

120.707 = 121.344.1869.912.028*975 (10003600)’>

 

 

Comparing Genersys 1000-10 Panel with Competitors using Proper Comparison

 

The chart below shows the efficiency of the Genersys 1000-10 panel with its competitors using the EN 12975 test results converted into OG-100 format.

 

  Model

Test

Year Peak Efficiency Flow Rate (ml/s)

Test Fluid

Genersys 1000-10 Converted

2005

0.707

34

Water

Stiebel Eltron Sol 25 Plus OG-100

2006

0.649

54.9

Water

Viessmann Vitosol 200F OG-100

2006

0.7203

49.8

Propylene Glycol & Water

Schuco USA L.P. Premium OG-100

2005

0.718

50

Water

Bosch Thermo… Buderus OG-100

2007

0.715

48

Water

Marathon Int. Baxi OG-100

2007

0.696

38.2

Water

    Source: SRCC: http://www.solar-rating.org

 

Conclusion

 After converting the EN 12975 report into the OG-100 format, the Genersys 1000-10 has a peak efficiency that is comparable with its competitors and that all major panels operate at more or less the same peak efficiency as to make no difference.

 References

 

[1] Werner W. Weiss. Solar Heating Systems for Houses.  Earthscan Publications Ltd.; 2004: 280

[2] John A. Duffie, William A. Beckman. Solar Engineering of Thermal Processes.  New York: Wiley-Interscience Publication, 1991: 304-305

[3] SRCC. “SRCC” Solar Ratings Certification Corporation. http://www.solar-rating.org/ratings/OG100_ratings_info.htm