PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS
Prog. Photovolt: Res. Appl. 2004; 12:441–448 (DOI: 10.1002/pip.550)
Special Issue
Operational Performance of
Grid-connected PV Systems
on Buildings in Germany
Ulrike Jahn1*,y and Wolfgang Nasse2
1
Institut für Solarenergieforschung GmbH Hameln/Emmerthal (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany
Solar Engineering Decker & Mack GmbH Vahrenwalder Straße 7, D-30165 Hannover, Germany
2
This paper presents operational performance results of grid-connected PV systems
in Germany, as collected and elaborated for the Photovoltaic Power Systems
Programme (PVPS) of the International Energy Agency (IEA). Performance ratios
obtained from 235 PV installations in Germany and from 133 PV plants in other
countries are compared and discussed. For Germany, a significant rise in PV system
performance and reliability was observed for new PV installations due to higher component efficiencies (e.g., inverter) and increased availabilities.
There is a lack of long-term experience in performance and reliability of PV
systems, owing to the absence of monitoring programmes. As an outcome of IEA
PVPS collaborative work, Task 2 provides reliable and worldwide monitoring performance data and results (www.task2.org). Technical and operational data is available
for system planning and comparison, for teaching and training purposes as well as for
future developments of financing schemes (e.g., feed-in-tariffs) in order to stimulate
the PV market. Copyright # 2004 John Wiley & Sons, Ltd.
key words: operational performance analysis; trends in long-term performance; reliability of
grid-connected PV systems; low-yield analysis; system availability; PV performance database
TECHNICAL PERFORMANCE RESULTS
he analysis of PV system performance is based on the European Guidelines, Document B,1 and the IEC
Standard 61724.2 Key parameters for evaluation of PV system performance are energy yield, efficiency
and performance ratio. The most appropriate performance indicators of grid-connected PV systems are
final PV system yield Yf, reference yield Yr and performance ratio (PR).1–4 The performance ratio PR is the ratio
of PV energy actually used to the energy theoretically available (i.e., Yf/Yr). It is (nearly) independent of location
and system size and indicates the overall effect of losses on the array’s nominal power due to module temperature, incomplete utilization of irradiance and system component inefficiencies or failures.
From the performance analysis of 334 grid-connected PV systems supplied by the Performance Database5 of
the International Energy Agency, it was learnt that the average annual yield Yf fluctuates only slightly from one
T
* Correspondence to: Ulrike Jahn, Institut für Solarenergieforschung GmbH Hameln/Emmerthal (ISFH), Am Ohrberg 1, D-31860 Emmerthal,
Germany.
y
E-mail: ujahn@easynet.de
Contract/grant sponsor: German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU); contract/grant number:
0329640B.
Copyright # 2004 John Wiley & Sons, Ltd.
Received 14 February 2004
442
U. JAHN AND W. NASSE
Table I. Annual performance ratio (PR) of 368 PV systems operating in different countries in Europe for comparison
Country
Austria
Germany
Italy
Netherlands
Switzerland
Systems analysed
22
235
29
20
62
Monitoring (yrs)
44
787
81
52
301
Monitoring period
Performance ratio
Begin
End
Minimum
Maximum
Mean
1994
1991
1992
1992
1990
2002
2002
2002
2000
2002
0425
0378
0448
0588
0497
0798
0883
0826
0796
0854
0639
0671
0673
0694
0694
year to another, and has typical average annual values for one country (700 kWh/kWp for the Netherlands,
730 kWh/kWp for Germany, 790 kWh/kWp for Switzerland and up to Yf ¼ 1840 kWh/kWp for Israel). However,
there is a considerable scatter around these average values for individual systems, ranging from 400 to
1030 kWh/kWp in Germany and from 450 to 1400 kWh/kWp for plants in Switzerland.6
Table I shows the annual performance ratio (PR) in terms of minimum, maximum and average values for 368
grid-connected PV systems in five different countries. The broad range of mean annual PR values by a factor of
2–3 (e.g., in Germany PRmin ¼ 0378 and PRmax ¼ 0883) is quite significant and has system-specific reasons.
The results, experiences and lessons learnt are gained from the analysis of grid-connected PV systems, which
were supported and monitored under different PV programmes such as the German 1000-Roofs Photovoltaic
Project, Sun-at-School Programme and others. The PV systems investigated have an installed capacity between
1 kWp and 1 MWp adapted to different applications and located all over Germany.
Figure 1 shows the distribution of annual performance ratios calculated from 787 annual datasets of 235 gridconnected PV systems operating during 1991–2002, which are grouped into two installation periods. Early
installed PV systems (1991–1994) have their maximum in the PR range of 065–07 and an average mean
PR of 065 for 609 annual performance data. The newer installations (1996–2002) have their maximum in
the range of 075 to 08 with an average value of PR ¼ 074 for 178 annual datasets. This is a significant rise
in PV system performance and reliability, which new systems have achieved in Germany.
Figure 1. Distribution of 787 annual performance ratios of 235 grid-connected PV systems in Germany for two installation periods
Copyright # 2004 John Wiley & Sons, Ltd.
Prog. Photovolt: Res. Appl. 2004; 12:441–448
PERFORMANCE OF GRID-CONNECTED PV SYSTEMS ON BUILDINGS
443
Former investigations on annual PR have shown that for well-operating grid-connected PV systems during
1990–1999, PR values between 06 and 08 can be expected.6 It was found that well-maintained PV systems
show PR values of typically 075 at an availability of higher 98%. Taking into account realistic efficiency values
of the improved inverter types of today, optimum values of annual PR between 081 and 084 may be achieved.7
A tendency to increasing annual PR values during the past five years has been observed.
TRENDS FROM GERMAN 1000-ROOFS PV PROGRAMME
Within the German 1000-Roofs Photovoltaic Programme more than 2250 grid-connected PV residential
systems of 1–5 kWp were installed as PV rooftops during 1991–1994. This programme was accompanied by
an analytical monitoring and evaluation programme, which delivered remarkable results and experiences.8–11
The 78 PV systems under investigation were part of the 1000-Roofs PV project in Northern and Eastern
Germany and were installed between 1991 and 1994. These residential rooftop systems have an average
installed capacity of 25 kWp and have operated for more than ten years. Global monitoring data are available
from the standard monitoring and evaluation programme (SMAP) for 1993–2000. All PV rooftops are equipped
with a calibrated silicon sensor to measure the in-plane irradiation at the site of the PV array. Additionally 99 PV
systems spread all over Germany were monitored as part of the intensive monitoring programme (IMAP) and
high-resolution monitoring data are available for 1994–1997.
Figure 2 shows the annual performance ratios of 78 SMAP and 99 IMAP systems operating since the early
1990s. The annual PR differs significantly from plant to plant, and shows a broad range for all operating years.
The average annual PR of 067 in 1993 drops to an average value of 060 in 2000. The average performance
values clearly show a negative tendency. Energy yields and PR have been reduced by 11% between 1993
and 2000.
The registered failures at 21 PV systems during ten years of operation show that defects and problems
decreased at first (1992–1995), but have started to increase in recent years (1999–2000). Considering a total
number of 47 failures in 210 operational years, a statistical failure will occur every 45 yr per plant. Inverters
contribute 63%, PV modules 15% and other system components 23% to the total failures.
Figure 2. Trend of annual performance ratios of 177 residential PV rooftop systems in Germany installed between 1991
and 1994
Copyright # 2004 John Wiley & Sons, Ltd.
Prog. Photovolt: Res. Appl. 2004; 12:441–448
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U. JAHN AND W. NASSE
RESULTS FROM LOW-YIELD ANALYSIS
Low-yield analysis was carried out at 17 selected PV systems to find quantified reasons for reduced yields, which
cannot be explained by system losses and tolerances due to system components. The selection criteria for the
systems under investigation were the performance ratio (annual value lower than 060) and the final energy yield
(75% less than the average value in that region) in addition to practical considerations, which allow technical
inspections and 4–6 week monitoring campaigns at the selected sites. The detailed investigations of this project
resulted in four additional reasons for very low yields and performances of grid-connected PV systems.12
Deviation from manufacturer’s module specifications
A specific deviation of the PV module nominal power from the quoted rating was found for 17 selected systems12 ranging from 5% to 26%.
Shading due to trees, buildings or walls
Shading of PV systems leads to a significant reduction of the system’s energy yield. For ten systems, low- and
high-degree losses were identified:
*
*
*
partial shading of five of the six strings during mornings by a tall (18 m) broadleaf tree resulted in a reduced
yield of 4%;
partial shading of a PV array by the roof of neighbouring building as well as by surrounding trees led to a
reduced yield by up to 10%;
significant shading of the array by several trees located close to the house and extending above the PV
rooftop resulted in annual energy losses of higher than 20%.
Defects of DC installations
For the 17 PV systems under investigation, different components of the DC system caused various defects and
failures of one and more strings12: defect clamp of a switch; defective screwed contacts inside the PV junction
box; defective string diodes; cracked PV modules due to bubbles inside the laminate; defect connectors for PV
roof tiles; defective monitoring equipment and defective string fuses. It is advisable to check the string currents
and open-circuit voltage on a regular basis (e.g., yearly maintenance).
Inverter problems
Partial or complete failure of the inverter, non-optimum maximum power point (MPP) tracking, bad power regulation and fixed voltage adaptation losses were observed for the 17 systems under investigation. Except for
complete inverter failures, the energy losses due to non-optimum operating inverters were found12 to be
below 10%.
TRENDS FROM NEW PV INSTALLATIONS IN GERMANY
The 58 new installations built between 1996 and 2002, are of different sizes (1 kWp to 1 MWp), and differ in
their application (residential system, power plant, school programme) and mounting (rooftop, freestanding,
BIPV). In Figure 3 the trends of the annual PR are compared for early and later installations. For the new
German installations significantly higher values are obtained between 1998 and 2002. Mean annual PR values
between 073 and 076 are found, resulting in an average mean annual PR of 074 for all 58 new installations and
178 operational years. Furthermore, the range of annual PR values has decreased during 1998–2002 as a result
of improved quality of the new systems.
It can be concluded that a negative tendency in terms of performance and yields was observed for these early
installations (1991–1994) from the rooftop programme during eight years of operation. Learning experience
Copyright # 2004 John Wiley & Sons, Ltd.
Prog. Photovolt: Res. Appl. 2004; 12:441–448
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445
Figure 3. Trends of annual performance ratios of 177 residential PV systems installed between 1991 and 1994 compared
with 58 new PV systems installed after 1996 in Germany
was gained for early inverter developments that had led to frequent inverter failures, which resulted in significant reductions of the annual energy yields for some PV plants. New PV installations (after 1996) in Germany
revealed that they reach realistic PV module ratings, higher component efficiencies (e.g., inverter) and higher
performance ratios ( > 080). The high level of average annual PR (074) and no change of average PR in five
operational years (1998–2002) indicate that the quality of the late systems in Germany has clearly increased.
The frequency distribution of the 177 early and the 58 new German installations are shown in Figure 1. For
early systems the PR spread is between < 04 and 085 and with an average PR of 065. The PR spread of new
systems lies between 050 and 085 with an average PR of 074. Here, a significant rise in mean annual PR of
13% was found.
COMPARISON OF DESIGN DATA AND REAL PERFORMANCE RESULTS
FROM BIPV PROJECT IN A PUBLIC SCHOOL
A grid-connected building integrated PV system of 504 kWp was installed as rooftop at a public high school in
Northern Germany and has been monitored since 2002. The well-planned system shows good performance
values and annual PR ¼ 074 in 2002. The interesting features of this PV system are:
*
*
*
The 336 PV modules were specified each with a rated power of 150 Wp, resulting in nominal array power
504 kWp. Outdoor I–V measurements of 10 test samples of these 336 PV modules resulted in an average
measured power of 1431 Wp, leading to an average plant power of 481 kWp. The reduced power of 5% is a
typical problem that often remains undiscovered, if manufacturer-independent module measurements are not
available. For the calculations of energy yield and PR, the real nominal power (1431 Wp) is used.
Energy reductions due to partial shading were observed for one of seven strings (of the order of 10% of that
string) although shading problems were minimised by using dummies.
During the planning phase expected energy yields and performances were calculated using the simulation
tool PV*SOL 22 and the manufacturer’s nominal power of 150 Wp. The comparison of the design values and
the real performance results shows rather good agreements on an annual basis, taking the corrected real
nominal power into account. Monthly energy yields and irradiation data are compared in Figure 4, resulting
in annual Yf ¼ 820 kWh/kWp (simulation) and Yf ¼ 793 kWh/kWp (measured) and annual PR ¼ 765%
Copyright # 2004 John Wiley & Sons, Ltd.
Prog. Photovolt: Res. Appl. 2004; 12:441–448
446
U. JAHN AND W. NASSE
Figure 4. Design and monitoring values of monthly energy yields and irradiation for comparison during 2002 for a 50 kWp
building-integrated PV system at public high school in northern Germany
(simulation) und PR ¼ 736% (measured). Thus a difference of 3–4% was found for the comparison between
design and real performance results. The critical point of design values is the used basis of irradiation data,
which may lead to significant monthly discrepancies of irradiation values—here for the months June and
July, as shown in Fig. 4.
LONG-TERM PERFORMANCE AND RELIABILITY OF
GRID-CONNECTED PV SYSTEMS
How reliable are PV systems and their components? To investigate the trend of system availabilities, 116 gridconnected PV systems in Germany, Switzerland and Italy were analysed.
Figure 5 shows the distribution of annual system availability for 116 systems grouped into two different
installation periods. For the 82 systems installed before 1995, the average annual availability is 946% compared with 956% for the 34 new installations. While this modest increase of 1–2% in system availability
was found for the 116 systems during the two periods, the annual mean PR increased by 4% for the same
systems and periods. On the other hand, the inverter efficiencies of early and later PV systems were compared,
and it was found that the mean annual inverter efficiency has increased by 1% from inv ¼ 891% for the early
systems to inv ¼ 900% for new systems.
Thus the rise in PV system performance (PR ¼ 4%) of the PV systems investigated is due to improvements
of inverter efficiencies (delta inv ¼ 1%) and to the increased system availability (1–2%) as well as to improvements of realistic PV module ratings, and due to avoiding shading, mismatch and module temperature losses. It
was further observed that the system availability is generally higher for systems which are intensively monitored.
CONCLUSIONS
While many PV systems perform according to plan, a common performance experience of the early PV systems
was that, in terms of final energy yield, the system did not meet expectations. Dominating performance constraints were the poor reliability of inverters, long repair times and shading problems.
Copyright # 2004 John Wiley & Sons, Ltd.
Prog. Photovolt: Res. Appl. 2004; 12:441–448
PERFORMANCE OF GRID-CONNECTED PV SYSTEMS ON BUILDINGS
447
Figure 5. Frequency distribution of annual system availability of 116 grid-connected PV systems from Germany, Italy and
Switzerland (488 datasets) operating from 1992 to 2001 for two different installation periods
From the performance analysis of 235 grid-connected PV systems in Germany, a clear tendency to improved
performance was found for new PV installations. Realistic PV module ratings, better inverter efficiencies and
higher system availabilities are identified as reasons for this. At the same time, the broad spread of annual PR,
which is due to system and component failures, shading effects, MPPT mismatch, badly oriented PV arrays and
high module temperatures, has decreased, and indicates improved quality of PV system performance.
Despite good results obtained with many of the new grid-connected systems, the investigation of the operational behaviour of the PV systems has identified further potential for optimization. Average annual PR values
higher than 075 are achievable for well-planned PV systems.
For the new systems of lower performance, the same problems as for the earlier installations were found and
the same recommendations have to be given. For BIPV, it is a challenge to combine building requirements,
architectural design criteria and highest technical performance.
There is a lack of long-term experience in performance and reliability of PV systems, owing to the absence of
monitoring programmes. For a wider dissemination of PV in future, monitoring activities are essential to
improve quality management and reliability of PV systems.
Acknowledgements
The authors would like to thank all their colleagues in IEA PVPS Task 2 who contributed their PV system data
to the performance database. The PV promotion programme in Lower Saxony ‘Solaroffensive Niedersachsen’
provided additional data for PV system analysis. This work is supported by the German Federal ministry for the
Environment, Nature Conservation and Nuclear Safety (BMU) under Contract 0329640B.
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Copyright # 2004 John Wiley & Sons, Ltd.
Prog. Photovolt: Res. Appl. 2004; 12:441–448