Category Archives: EuroSun2008-5

Conclusions and further perspectives

The development and set-up of a mobile, stand-alone test facility for solar thermal collectors and systems based on a 20 foot office container as well as the test facilities required for durability and reliability testing were described.

For the future it is intended to extend the mobile, stand-alone solar thermal test facility in such a way that it additionally allows for testing of hot water stores according to ENV 12977-3. Furthermore, it is intended to deliver turn-key test facilities to several other test institutes, manufacturers and universities.


[1] The European Standards (EN and ENV) and the International Standards (ISO) mentioned above are available from: www. beuth. de, or www. cen. eu/cenorm/standards_drafts/index. asp

Quantification of the impact resistance of solar thermal collectors and. photovoltaic-modules against severe hailstorms

Stefan Mehnert*, Matthias Rommel, Stefan Brachmann, Joseph Steinhart, Thorsten Siems,
Anne-Marie Behringer, Georg Mulhofer, Korbinian Kramer, Johannes Scherer

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrafle 2, D-79110 Freiburg, Germany
Corresponding Author, stefan. mehnert@ise. fraunhofer. de


Damages caused by severe thunderstorms are dramatically increasing in recent years and it can be assumed that this trend will continue within the climatic change. A survey of the most common thunderstorms in Europe shows that associated climatologically secondary phenomena like hail have likely the same or actually in some cases even a higher potential for damage than their primary wind phenomena. Current studies report an increasing of severe hailstorms in the European Union (EU). It must be assumed that severe hailstorms will become one of the most pressing problems posed to society by severe thunderstorms, mainly in the summer season. The enormous financial losses caused by hailstorms each year in moderate climate regions will further increase. This fact results in the basic necessity to perform some representative studies to quantify the impact resistance of solar thermal collectors and photovoltaic-modules against severe hailstorms. The benefit of such studies will be the mitigation of the threat of physical conditions posed by hailstorms, a better basis of information for the insurance industry for the assessment and the coverage of severe hailstorms, and last but not least to back up essential guidelines given from the EU to ensure the sustainable growth.

Keywords: quality assurance, impact resistance, severe Hailstorms

1. Introduction

The Fraunhofer ISE developed a new testing facility to simulate hail impacts with ice balls with the objective to perform impact resistance tests of solar thermal collectors and PV-modules according to the valid standards. This testing facility has recently been set-up and gives us the possibility to perform experimental research as well as the possibility to perform tests commissioned by the industry. This paper describes the background of the relevance of impact resistance test procedures for the quality assurance of solar thermal collectors and PV-modules. Furthermore, this paper compares and assesses the different requirements of the testing procedures within the currently valid European and international standards.


Fig. 1. Launcher of the impact resistance test facility, Fraunhofer ISE

. Array Layout and Absorber Orientation

The new ICPC evacuated tubes were fabricated with two absorber orientations, one with a vertical absorber fin and one with a horizontal fin. A cross-section of the collector tube illustrating the two orientations is shown in Fig. 3.

The collector array is made up of three banks. The north bank consists of all horizontal fin tubes, the middle bank consists of all vertical fin evacuated tubes and the south bank includes an even mixture of the two types. The two differently oriented fined collectors gave essentially identical

Fig. 4: 60 Degree Nominal Angle Ray Tracing
for the Vertical fin ICPC.

performance. The flow pattern through the 112 evacuated tubes in each bank is parallel and the three banks are plumbed in parallel.

The Portuguese Building Thermal Regulations on solar collectors

The new Thermal Regulations are applied to the all new residential and to some office buildings. Some cases of important retrofit work are also enclosed. The regulations are applied separately to each autonomous zone of a building (each zone with an electricity energy meter). It establishes the obligatory use of solar collectors for hot water supply if the building has a sufficient solar exposure [2]. To have sufficient solar exposure means the building roof must not be shadowed during the period that begins two hours after sunshine and ends two hours before sunset, oriented between SE and SW. They also establish the obligatory use of a minimal of 1 m2 of solar collector area (SCA) (aperture area) per each household of an autonomous zone (conventional nr of households = nr of bedrooms +1). This measure, recognised as a government policy effort to increase solar energy use in Portugal and meet strategic goals, immediately create some discussion in project designers about its cost-effectiveness. The law does not provide anything about the system efficiency and the project climate zone. There is only a regulation exception that allows breaking this minimal area. The value can be reduced to 50% of the total south-oriented roof area. Recently, the Portuguese Energy Agency (ADENE), that rules the Portuguese System for Energetic Certification of Buildings (SCE), clarified this situation and has allowed the use of a smaller collector area. Applying the regulations minimal area, the thermal project designer must proof that with less area can produces at least the same energy than a standard system (optical efficiency n0=69% and loss coefficients a1=7,500 W/(m2.K) and a2=0,014 W/(m2.K2)). The thermal regulations methodology uses the Esolar to quantify the energy produced by the solar collector system. The same methodology allows also using other renewable energy systems (Eren). The total energy for heating water using conventional energy (Nac) is calculated by the following equation (1).

Nac = (Q — Esolar — Eren) / Ap (kWh/m2.year) (1)


The Qa value represents the total energy needs during a year for hot water heating and the pa represents the equipment efficiency. The minimum value of pa adopt by these regulations is 65%. The Nac value is later comparable with a maximum value defined by regulations (Na) and contributes also to the final value of total primary energy (Ntc) in kg of petroleum equivalent (2).

Nic Nvc

Ntc = 0Л(—)Fpui + 0,1(—————— )Fpuv + NacFpua (kgpe/m .year) (2)

pi pv

Better values of Esolar, led to more reduced values of conventional energy consumptions therefore conducting to a more easily satisfaction of the requisites imposed by the thermal regulations. One important aspect can be noticed from this analysis, the equipments efficiency (pi, pv and pa), for space heating, cooling and DHW became fundamental to achieve a good building thermal performance. Is not only important to have a strong envelope insulation to meet energy conservation, as the project designers were used to with the previous regulations, but now the efficient use of energy is a crucial aspect. Therefore, choosing the equipments efficiency became one of the mainly concerns that thermal project designers must be prepared for. Also, the selection of the conventional energy is fundamental, being electricity much more penalized than oil or gas (Fp oil/gas =0,086 Kgpe/kWh and Fp electricity = 0,290 Kgpe/kWh).

Function Control without Heat Measurements (FUKS)

In the second half of the 90’s the FUKS-approach (function control without heat measurements) was developed in Germany as a cheap function control (<100€) [3]. This approach applies algorithms to measurement data and notices whether components function correctly. An overview of the developed algorithms is provided in table 3.2. The values in the algorithms could be adapted for other systems.

Several other algorithms were developed which can only be applied with additional sensors, e. g. resistance sensors for defect or inaccurate temperature sensors for the collector output or in the storage tank.

Table 3.2 Failure algorithms developed in the FUKS-approach [4]

Failure description

Result of failure


1. Collector circuit points interchanged

2. Collector T-sensor falsely positioned

Pump clocks

Pump running time < 10 s.

3. Leakage of heat exchanger

4. Power outage

System pressure too high

(Tkol = 20 °C) AND

(psystem psystem soll + 2 bar)

5. Incorrect controller software

6. False volume flow setting

7. Air in hydraulic circuit

8. dT setting is inappropriate

9. Defect input or output to the controller

dT too high

Pump on AND dT = dTsoll + 15 K

10. Gravity brake is open

11. Fouling of gravity brake

12. Time switch is programmed wrongly

Pump on at night

Pump on AND

time between 22:00 and 6:00

The method was for test purposes partially implemented in controllers of Esaa and Wagner. Several failures were recognized, but there were also false positives, detection of a failure while the system was functioning correctly. The approach stays cheap by using mainly sensors that are used for the control, however, with analysis and pressure measurements, the price may be slightly higher than mentioned. Although the method succeeds at detecting several failures in the lab, location of failures is limited, as can be seen in Table 3.2. Furthermore, there is no yield measurement, which could mean that large energy losses are not detected.

Two-Axis Tracker

PSE has been developing uniaxial and biaxial trackers for several years, for use in linear concentrating Fresnel collectors [2], heliostats, measurement of solar thermal collectors, etc. The mechanical design of this testing tracker is especially rugged; it has a maximum load allowance of 700 kg on the 5 x 2.5 m test surface. The guaranteed tracking precision is ±1°; however, the average precision during normal operations is much better. The tracking precision was verified using a “Heliosensor” [3], PSE’s sun position sensor.

Подпись: Figure 2: Data measurements taken with a heliosensor during calibration

The heliosensor is used to calibrate both the rotary encoder, which records the azimuth angle, and the inclination sensor, which records the zenith angle, during installation. After the sensors have been calibrated, the orientation of the testing surface is updated using an astronomical algorithm.

So as not to compromise the tracker’s precision, the track-laying and mounting of the two-axis tracker onto the moveable carriage must be carried out with especially high precision.

The tracker has three different operating modes: manual, automatic tracking, and user-defined tracking.


Figure 3: Left: Two-axis tracker with a testing surface of 5 m x 2.5 m; Right: Screenshot of the corresponding software.

The tracker is operated using software installed on a local network PC, which communicates with the tracker’s controls, an embedded Linux system, via the network connection.

The testing surface is completed with four cross-flow fans, which ensure that the collector is ventilated according to current norms during measurements.

Determination of the Degree of Mixing

Before the start of test, mix the water in the storage tank by using a small pump to circulate the water from the top to the bottom of the store. The water in the store is assumed to be at a uniform temperature when the temperature of the water at the outlet of the store varies by less than 1 K for a period of 15 min. Draw off water from the store at a constant flow rate of 600 lit/hr. The cold water

entering the store shall be at a constant temperature of less than 30 °С, shall not fluctuate by more than

±0.25 K and shall not drift by more than ±0.2 K during the draw-off period. Measure the temperature

at least every 15 s and record an average value at least every time a tenth of the tank volume is drawn off. Fig. 8 shows that a mixing draw-off temperature and water flow rate trend and relationship change with draw-off time.


Fig.8 Mixing raw-off temperature and flow rate graph 4.3. Determination of storage tank heat losses

Before the start of the test, mix the preconditioned water in the store by using a pump to circulate the water from the top to the bottom of the store. The water in the store is assumed to be at a uniform temperature when the temperature of the water at the outlet of the store varies by less than 1 K for a

period of 15 min. The average temperature over these 15 min is to be taken as the initial temperature of the tank. After cooling the tank for a period of between 12 h and 24 h, the water should be recirculated in the storage tank so that it reaches a uniform temperature. The temperature is assumed to be uniform when the temperature at the outlet of the tank varies by less than 1 K for a period of 15 min. The average temperature during this 15 min period shall be taken as the final temperature of the tank.

3. Conclusion

In this paper, automatic control system for domestic water heating test system has been studied and developed. Test procedure according to ISO 9459-2 is a series of complicated work and the whole process is a high cost test for water heating system. By the use of Labview a set of automatic control system can operate the test system, collect and record the corresponding data, and manipulate all sorts of equipments. In this case, it is easier and lower cost than has ever been before.


[1] ISO9459-1(1993). Solar heating—Domestic water heating systems—Part 1: Performance rating procedure using indoor test methods.

[2] ISO9459-2(1995). Solar heating—Domestic water heating systems—Part 2: Outdoor test methods performance characterization and yearly performance prediction of solar-only systems.

[3] ISO9459-3(1997). Solar heating—Domestic water heating systems—Part 3: Performance test for solar plus supplementary systems.

[4] ISO9459-4. Solar heating—Domestic water heating systems—Part 4: System performance characterization by means of components tests and computer simulation.

[5] ISO9459-5. Solar heating—Domestic water heating systems—Part 5: System performance characterization by means of whole system tests and computer simulation.

Selection of the best fitting surface

To be able to calculate the thermal performance of one specific thermosiphon DHW system, it is necessary to identify the surface that describes the thermal behaviour of the system in the best way. For the selection of the best fitting surface, the result of at least one DST test per product line is needed. In Table 2, an example for a product line of six different DHW systems is given. The solar fractions of system 1, 2 and 6 have been determined based on measurements according to the DST method and on a long term performance prediction. The solar fractions of system 3, 4 and 5 are not known a priori and have to be calculated by the extrapolation procedure. The best fitting surfaces is defined as the surface where the discrepancies between the values offsol obtained by the DST method and obtained with equation (2) are minimum. These values are compared with the fSol-values that have been derived directly for the DST measurements. The differences Afsol, between measured and predicted values is calculated with equation (3),

Подпись: (3)Подпись: (4)sol, i, j sol, measured sol, calc, i, j

with i = 1…3: system number, j = 1….12: surface number.

The total difference AFsol is defined as the summation over the differences Afsoi? i

AF, o =Z|A«,u


Table 2. Example for one typical product line

System no.

Collector aperture area [m2]

Storage tank volume [m3]

fsol [-]

























It is assumed that the surface where the value of AFsol is minimal describes the system behaviour in the best way. This function will then be used to compute the solar fraction for the remaining systems 3, 4 and 5. Table 3 lists the results determined by the mathematical extrapolation procedure according to equation (3) for the solar fractions of systems 3, 4 and 5.

Table 3. Results determined by the extrapolation procedure for system 3, 4 and 5

System no.

Collector aperture

Storage tank

fsol [-]

area [m2]

volume [m3]

1st International Congress on Heating, Cooling, and Buildings^ 7th to 10th October, Lisbon — Portugal /

























Development of new system testing facilities

Different SDHW systems (thermosiphon systems, integrated collector-storage systems and forced — circulation systems) have been tested at Fraunhofer ISE since 1997 The testing facilities for pre­assembled systems have now been expanded to satisfy the growing demand of these tests according to the European testing standard EN 12976. From now on it is possible to perform up to four tests of SDHW systems at the same time. Furthermore, it will be possible to carry out tests of heat storage tanks.

Elements of system testing

SDHW systems are tested for the Solar Keymark certification according EN 12976-1,2. This includes an efficiency test on the system, functional tests on the system and functional tests on the collector applied in the system. The tests will be briefly described in the following.

Efficiency test

The parameters of the systems are experimentally determined by an outdoor test with a number of testing days. The boundary conditions like daily irradiation, ambient temperature and cold water inlet temperature for a testing day to be valid are defined in the testing standard.

A simulation is carried out using the parameters which were determined in the efficiency test. The performance prediction simulations are carried out for different reference locations (Wurzburg, Stockholm, Davos and Athens are the standard locations but the calculations can be carried out also for other locations.).



Fig. 2.The parameters determined in the outdoor test are used for a prediction of the system output for reference locations in Europe.

Functional tests on the system

• Freeze resistance

The aim of the freeze resistance test is to check whether the necessary precautions have been taken to protect the system from temperatures below zero.

• Pressure resistance

Within the pressure resistance tests all circuits are checked on their ability to hold and withstand pressure.

Ability of the solar-plus-supplementary-system to cover the load

The intention of this test is to assess the ability of the system to provide the maximum daily load on days without contribution out of the solar loop.

• Over temperature protection

The test is done to make sure that the system is able to withstand a period of high solar radiation without hot water drawn from the tank. The intention is also to make sure that no dangerous situations occur for the user.


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Подпись: 0 00

Fig. 3. Measurements of an over temperature test. The graph shows the irradiance, the power consumption of
the pump in the collector loop and the outlet temperature of the collector array (as mean values averaged
over five minutes). By means of the controller of the system the pump is operated intermittingly in order to
prevent the collector from overheating. The storage tank is cooled by the collector (it is a selective flat plate
collector in this case) until a certain threshold temperature is reached. The test showed that the risk of a
stagnation phase with evaporation of the fluid in the collector loop is reduced sufficiently..

Functional tests on the collector

Additional functional tests are performed on the collector of the system unless a complete test according EN 12975 has been performed already. The tests include:

• Exposure test

• High-temperature resistance test

• External thermal shock tests (twice)

• Internal thermal shock tests (twice)

• Rain penetration test

• Stagnation temperature

• Mechanical load test

• Final inspection

The FDS procedure

The kernel of the system is the neural networks. So, it is necessary to consider a first step to train the networks. This period should be as short as possible because during this period the system has

to be inspected regularly to be sure that nothing has happened (e. g. it is necessary to clean the collectors). It has been found that getting 30 values with a sampling period of 1 hour is a good compromise between sensitivity of the procedure and initial length.

At the end of the initialization period, three networks are available. They are used to estimate the temperatures. The latter are compared to the acquired temperatures through the computation of the RMSE. If the error is very small, the initial connection weights are stored in a matrix. If the error is higher than a threshold (0.05 for the collector array, 0.015 for the connecting pipes), the networks are re-trained. The new connection weights are stored in the connection weights matrix. If a given number of consecutive connection weights are different from the initial weights, an alarm is fired. It has been found that 5 consecutive values are sufficient, and do not lead to long delays between the fault and its detection.

2. Results

Подпись: Fig. 4 Detection of the F' drift for two different draw off profiles

To generate the data, the typical meteorological year (TMY) files of Nicosia have been considered. Figure 4 shows the detection time for the F’ drift considering the two draw off profiles.

Although it seems that the detection is quite late, a plot of the temperatures about the detection time shows that it would be very difficult to detect the drift by the simple analysis of the temperatures (Fig. 5).


Fig. 5 Differential between temperatures when the system is stable all year long

and when F’ evolves

Figure 6 shows the detection time for the UL drift. On the one hand, it can be noted that a combined drift leads to an earlier detection. On the other hand, it has been checked that the defaults on the connecting pipes do not lead to any detection by the analysis of the collector array data.

Подпись: Figure 6: Detection of the UL drift

3. Conclusions

An on-line fault diagnostic system has been presented. The main advantage of this system is that it does not need a long training period. It has been shown that the drifts are detected well before the increase of the auxiliary electrical power is higher than 7.5%. This means that the FDS is sensitive. As the neural networks are very simple, this should not be a problem to implement the FDS in real world applications.


The financial support of the French Ministry of Foreign Affairs (under contract EGIDE ZENON 11999SD) and of the Cyprus Research Foundation (under contract KY-rA/0305/02) is greatly acknowledged.


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[2] S. Kalogirou, S. Lalot, G. Florides, B. Desmet, Development of a Neural Network-Based Fault Diagnostic System for Solar Thermal Applications, Solar Energy, Vol. 82, No. 2, pp. 164-172, 2007.

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[10] M. Bosanac, S. Jensen, In-situ solar air collector array test, Solar Energy laboratory, Danish Technological Institute, December 1997