To model the collector array as simply as possible, a very small network is used. Using a unique non linear neuron on the hidden layer and a linear output neuron leads to good results when considering modeling the array when the fluid is flowing during two consecutive hours; which means that the system is in a quasi steady state and that it is not necessary to use complex models as presented in [9]. This is also similar to the conclusion given in [10] stating that parameter estimation of a solar collector array is reliable when high temperatures are obtained within the collectors. The inputs are the inlet temperature T4, the global radiation, the ambient temperature. The output is the outlet temperature T1. Figure 3 shows the differential (in %) between estimated values and actual values when the system is stable all year long. Due to the fact that the system is more likely to run two consecutive hours in summer, the error is lower during these months. Nevertheless, the maximum error is less than 1%. The equation representing the estimated values

For the connecting pipes a similar network is used, the inputs are the inlet temperature of the pipe, the ambient temperature, and the global radiation. The output is the outlet temperature of the pipe. In this case the regression R value are closer to unity, the difference is -2.6 10-6, and the equation is

Test = 0.9997 Tact — 0.0174

It can be seen from Figure 4 that the thermal performance of the all glass ETC 5 is always larger than that of the reference collector, ETC 4, simply due to the fact that ETC 5 has a larger transparent area. Despite of small fluctuations, it is clearly shown that the performance ratio increases from winter to summer and decreases from summer to winter meaning that the all-glass ETC 5 performs relatively better in summer compared to the winter. The reason could be the difference of the two collectors in

tube orientation and the distance between the tubes. The all glass ETC 5 has east-west oriented horizontal tubes while ETC 4 has south-north oriented tubes with a tilt of 67°. Since the solar azimuth variation is much larger than the solar altitude variation, especially in the summer, the shadowed tube area, caused by shadows from neighbouring tubes, is much larger for ETC 4 than for ETC 5 in large parts of the day. Furthermore the ratio of tube diameter to tube centre distance is 63-65% for ETC 5 and 75-81% for ETC 4. Therefore there is a relatively larger tube distance and thus less shadow from neighbouring tubes of the ETC 5 compared to ETC 4. The performance ratio of ETC 5 to ETC 4 is insignificantly influenced by the mean collector fluid temperature.

The energy output of ETC 5 in the three phases is summarized and presented in Figures 5, 6 and 7. It can be concluded that ETC 5 has the smallest thermal performance per m2 gross area while it has the second largest thermal performance m2 transparent area because ETC 5 has the smallest ratio of tube diameter to tube centre distance and thus has the largest distance between the tubes.

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.

References

[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

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

Abstract

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.

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 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)

pa

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).

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.

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.

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.

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.

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.

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.

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.

References

[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.

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),

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

i

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.