The concentrator optics of the LFC consists of the mirror field of elastically bent primary mirrors having a focal length — depending on the actual collector — of several meter, and a secondary concentrator. Figure 4 shows schematically the configuration of a single tube collector. The primary mirrors may have different focal lengths and therefore different curvature radii. Radiometric flux density measurements at the aperture of the secondary concentrator may reveal the optical efficiency of the mirror field for a certain sun position. An alternative method we used for characterizing the incident flux is the photometric evaluation of the focal line on a calibrated white target at the receiver aperture. However, it is quite time consuming to evaluate sufficient daily and seasonally variing sun positions. Moreover, a measurement does not easily give the reason for inadequate focusing.

Therefore we adapted the so-called Fringe reflection Technique (FRT) already used for smaller specularly reflecting objects like lenses and optical glasses to large mirrors. With FRT one may determine the exact shape of a primary mirror as well as local slope and local curvature deviations. Several variations of this methodology have been investigated. One is working with passive (printed) patterns, another with dynamically generated active patterns using a computer LCD display or a beamer plus projection area. The principle can be used indoors in the laboratory or production, and outdoors with more variable light conditions and is described in detail by Heimsath [5]. After having qualified a statistically significant number of mirrors one may simulate by raytracing the optical efficiency of the mirror field for every sun position.

In order to characterize the secondary receiver we developed a method yielding the acceptance for radiation coming from each different primary mirror. Due to the reversibility of light paths one may take pictures of the absorber images from the primary mirror position and use these for the evaluation of acceptance as a function of incidence angle. For unambiguous evaluation we developed a colour — coded cover for the absorber tube. Weighting the reflected images by the reflectivity of the secondary mirror material we arrived at an angular acceptance plot which we compared with raytracing.

A series of optical tests were described which can be used to evaluate the optical performance of a Linear Fresnel Collector. An example calculation of the levelised electricity costs for a fictitious solar power plant using the LFC was performed using different parameters for optical concentration quality. An average statistical error including mirror shape, scattering, tracking and torsion was taken to show the influence of optical quality.

A. C. de Keizer, K. Vajen and U. Jordan

Kassel University, Institute of Thermal Engineering, 34125 Kassel, Germany, www. solar. uni-kassel. de

Corresponding Author, solar@uni-kassel. de

Abstract

Continuous monitoring and failure detection during the life time of a solar thermal system is important to detect occurring failures as quick as possible. Therefore, several methods have been developed during the last decades. However, so far application is mainly limited to research and demonstration projects. In this paper several failure detection methods are described and compared with a partial multi-criteria analysis.

Up to now monitoring approaches have primarily been applied with data analysis by an expert, but without an automatic analysis of the data through the method. There are some methods that include automatic failure detection; which is based on a static function control or on a simulation based comparison. Up to now none of the systems include the auxiliary heating. Keywords: monitoring, failure detection, solar thermal systems

1. Introduction

The solar thermal energy market is growing. Solar thermal systems are designed to function for at least 25 years, but failures and malfunctions of parts of the system are likely to occur at a certain time. This can cause energy and economic losses. These can be minimized or largely avoided with the right monitoring approaches. System failures are not easily noticed without performance checks, since the auxiliary heating system always backups the hot water supply. Furthermore, changing weather circumstances and hot water demand make a prediction of the energy yield difficult. During the last decades several methods for monitoring have been developed [2-14], however, so far they have mainly been applied in research and demonstration projects.

Several terms will be distinguished here. Monitoring is defined as data logging of a variable amount of measurement data, however this data is not automatically analysed, so it does not automatically lead to a failure declaration. In a failure detection method, measurement data is automatically analysed and in case of a malfunction a failure indication follows. Failure identification or localisation goes further in that it requires the identification of the type of failure. This will make reparation much easier.

In this paper a partial Multi-Criteria Analysis (MCA) will be used to describe the performance of several monitoring methods for solar thermal systems. The MCA procedure will be described in chapter 2, consecutively the monitoring methods will be described in section 3. The results of the MCA will be given in section 4, conclusion and discussion are described in section 5.

2. Method

The components and measurement devices of an air-collector testing loop are more voluminous than for water-collectors. This simple fact causes additional difficulties when a solar collector testing loop is installed in a indoor laboratory and also for the conditions of outdoor measurements. In order to achieve the desired flexibility for testing indoor, outdoor and on systems installed in the field we followed the concept to install each component of the testing loop on its own carriage with wheels.

Figure 5 shows the test loop installed for outdoor air collector tests using the solar tracker. Figure 6 shows the situation, when the same components are used for field tests, i. e. when the collector is installed already and tests have to be made in situ. In these measurements the water-thermostat was not used. Therefore it is more difficult to control the inlet temperature of the collector. It can be achieved to a certain extent by mixing ambient air with the heated air from the collector outlet. But of course, the accuracy of the measurements decreases a bit and the range of inlet temperatures available also depends on the power of the collector. Another challenge of field measurements of

collectors installed in a fixed position is to correctly take care of the influence of the Incidence Angle Modifier IAM. (But this is not different from corresponding water-collector performance measurements and not special for air-collectors.)

Figure 8: Measured efficiency curves of a flat-plate air-collector: The collector reference temperature Txy is
Tin, Tm, Tout respectively for the curves from bottom to top.

3. Conclusions

We have started to improve our testing facilities to be able to carry out very exact air collector performance measurements. We will further improve them in the near future. Air collectors may be characterized exactly based on the same principles as described in EN12975. This concerns not only the performance measurements but also all functional and quality tests defined in EN12975.

There are obvious advantages that the instrument based on PXI technology possesses a high speed, a small size and easy expansion, so the hardware structure will use PXI bus technology and relevant products based on the PXI bus. The system concludes PXI-1042 chassis, PXI-8186 Controller, PXI — 6221 multifunction data acquisition card, PXI-4351 high-precision temperature and voltage logger, PXI-6513 digital I/O card, and SC-2345 signal conditioning modules.

Fig. 3 shows the physical structure. PXI-1042 as chassis has 8 slots to comply with PXI and CompactPCI specifications with universal AC power supply. PXI-8186 as embedded controller can greatly improve the system scalability and integration. PXI-6221 multifunction data acquisition card can support two 16-bit analog outputs and has 24 digital I/O, 32-bit counters and digital triggering. PXI-4351 as high-precision temperature and voltage logger includes 8 digital I/O lines (TTL), 16 voltage or 14 thermocouple inputs and autozero and cold-junction compensation and its accuracy can reach 0.12°C for RTDs. PXI-6513 digital I/O card possess optical isolation in banks of 8 channels, 64

sink outputs. This card is a low-cost solution with advanced features for industrial control and manufacturing test applications and it is also a high-reliability industrial feature set. SC-2345 series have connector block and configurable connectors.

H. Kerskes*, B. Mette, H. Drtick, H. Mtiller-Steinhagen

Institute for Thermodynamics and Thermal Engineering (ITW),

University of Stuttgart, Pfaffenwaldring 6, 70550 Stuttgart, Germany
Tel.:++49-711-685-63534

Corresponding Author: kerskes@itw. uni-stuttgart. de
Abstract

System tests according to the EN 12976 test procedures are a basis for factory made Solar Keymark system certification. To obtain the Solar Keymark certificate, each system configuration of a solar domestic hot water system (DHW) has to be tested by an accredited testing laboratory. As companies often offer a product line of their solar DHW systems, it is desirable to have a calculation tool that is able to predict the thermal performance of the whole product line without the need to test each of the system configurations. This paper presents the main elements of the Solar Keymark certification program as well as a mathematical procedure developed at the ITW with which it is possible to extrapolate performance test results of one tested system to systems of the same product line.

Keywords: Solar Keymark certification, solar domestic hot water systems, DST

1. Introduction

With the implementation of Solar Keymark certification in the year 2003, a pan-European quality label for solar thermal products was established for the first time /1/.

The aim of the Solar Keymark label is to break the existing trade barriers within the European market. A solar thermal system or solar collector certified according to the Solar Keymark scheme rules fulfils the relevant European Standards and is hence qualified for acceptance by the national regulatory and financial authority, e. g. with regard to financial incentives.

Thereby it is avoided that a system already tested by an accredited testing laboratory has to be tested again by national laboratories. For manufacturers of solar thermal systems the Solar Keymark is an instrument to save time and money and to demonstrate the quality of their products. On the other hand customers benefit from the guarantee to buy an approved product.

In 2004 the first system tests according to the EN 12976 test procedures /2/ as basis for factory made Solar Keymark system certification were carried out at ITW. In the meantime about 35 factory made solar thermal systems are Solar Keymark certified by several European certification bodies and the number of requested system tests is growing rapidly.

The collector was tested according to both steady state test (EN 12975-2;section 6.1) and dynamic test (EN 12975-2; section 6.3) methods. The corresponding characteristic parameters are presented in Table 1.

Incidence angle modifier (IAM) values obtained, after both test methodologies, for longitudinal and transversal incidences are presented in Table 2.

Longitudinal and transversal IAM functions were constructed after 2-points based and linear approximations, respectively (Carvalho et al, 2007). The composed IAM, illustrated in figure 2, follows the McIntire (1983) approximation:

K(9) = K(9l,9t )* K(9l,0)K(0,9t) (8)

The weighted average hemispherical IAM, calculated according to Eq.(7), yields Kdfh =0.529.

From the test sequences obtained for QD test method, test periods according to SS test method conditions were selected and the parameters of efficiency curve determined (equation (3)). The values determined are listed in Table 2.

Comparison of results between the QD and SS methods is presented in the form of an efficiency curve with respect to Tm* = (tm-ta)/G*, where G* is the global irradiance incident on the collector. The following values were considered:

G* = 1000 W/ m2; Gd = 150 W/ m2; 0: = 15°

according to the standard’s recommendations for the presentation of results.

The above figure shows good agreement between the two test methods for the flat plate collector tested.

An evacuated tubular collector was also tested. It was a double glazed tubular collector and had as heat transfer device a heat pipe.

Test sequences were obtained but the identification of characteristic parameters was not successful since further development of the MLR tool was needed due to the fact that IAM can not be characterized by equation (7) for this type of collectors.

Also a strange behaviour of the collector was detected in the test sequences, showing that, after a certain time period the collector performance was enhanced, i. e, the outlet collector temperature raised in almost step wise way after a couple of hours, showing a different behaviour between morning and afternoon. The reason for this behaviour could not be determined.

5. Conclusion

The QD test methodology was implemented at LECS. For this implementation data acquisition programme and MRL tool were developed. First results were obtained for a flat plate collector and showed good results, although a better choice of test sequences is needed in order to incorporate a better weather variability, i. e., sequences with cloudy conditions.

For test of collectors with higher optical complexity, adaptation of the MLR tool is needed. This is the case of CPC type and evacuated tubular collectors.

References

[1] EN 12975 (2006), Thermal solar systems and components — Solar collectors — Part 2: Test Methods, Section 6.1. and Section 6.3, European Standard.

[2] Rojas, D. et al., Thermal performance testing of flat-plate collectors, Sol. Energy (2008), doi:10.1016/j. solener.2008.02.001

[3] Carvalho, M. J., P. Kovacs, Fischer, S., Project NEGST — New Generation of Solar Thermal Systems, "WP4-D2.1.k — Resource document — Definitions and test procedure related to the incidence angle modifier”, 2006, in http://www. swt-technologie. de/WP4 D2.1 ges. pdf

[4] Horta, P., M. J. Carvalho, S. Fisher (2008), Solar thermal collector yield — experimental validation of calculations based on steady-state and quasi-dynamic test methodologies, Submitted for presentation at Eurosun2008, Lisboa.

[1] Introduction

Solar water heating (SWH) systems are not usually equipped with any fault diagnostic system (FDS). Any faults are usually identified either by regular inspection by servicing personnel or when the system is not producing appropriate quantities of hot water, which is the most frequent. Usually people forget the existence of the solar system and this is inspected only after hot water is not available, indicating some problems. This results in problematic operation of the systems for long periods of time, which reduce the effectiveness and viability of the systems. Primarily works present the possibility of on-site determination of faults [1-2]. But the drifts were of a step-by-step

[3] Du to changes in the official CEN nomenclature the previously used abbreviation ENV for the indication of a preliminary standard is now replaced by the abbreviation CEN/TS (CEN: Comite Europeen de Normalisation — European Committee for Standardisation ; TS: Technical Specification).

[4] European Committee for Electrotechnical Standardization — Comite Europeen de Normalisation Electrotechnique

[5] Duff, W. S., Winston, R., O’Gallagher, J., Henkel T. and Bergquam, J., “Five Year Novel ICPC Solar Collector Performance”, 2003 American Solar Energy Society Solar Energy Conference, Austin TX, June 2003

[6] Duff, William, Roland Winston, Joseph

[7] The CSTG — method was originally developed within an European Project by the “Complete System

Testing Group” (CSTG). Today this method is standardised in ISO 9459-2

[9] DST: Dynamic System Test. The DST-method is standardised in ISO 9459-5.

[10] The collector performance calculated from the collector parameters gained under steady state conditions is only valid for the diffuse fraction prevailing during the measurements. The usage of these results leads to an under-estimation of the collector output for diffuse fractions smaller and to an over-estimation for diffuse fractions larger, than the values predominant during the steady state measurement.

[11] ++ well developed / can be applied directly, +- further research and development, — early R&D

[12] — not automated, +- partly automated, ++ fully automated

The solar system considered in this work is a large hot water production system suitable for a small hotel, blocks of flats, offices or similar applications. Although the FDS system developed can be applied to small systems as well it is thought that the expenditure required would not balance the extra benefits incurred in such cases and in domestic applications the users are usually more sensitive to the maintenance of their own system in comparison with the maintenance staff of a hotel for example or the tenants of a multi building installation where everybody but really nobody is responsible. The system schematic is shown in Fig. 1. The system consists of 40 m2 of collectors, a differential thermostat (not shown in Fig. 1) and a 2000 L storage tank. The system is also equipped with a data acquisition system which measures the temperatures at four locations of the SWH system; the collector array outlet (T1), the storage tank inlet (T2), the storage tank outlet (T3) and the collector array inlet (T4). The global solar radiation, the ambient temperature, and the pump state (on/off) are also recorded.

In fact, a TRNSYS model is used to simulate the system. Two draw off profiles have been used. The first one is repeated day after day, the second one is computed using the free generator developed at the Univeristat Marburg [7] and used in [8]. Being totally different, these profiles will show that the results do no depend on them. In a real application, the temperature readings would be affected by noise. So, it has been decided to add random noise to each computed temperature. It has to be noted that the time step is one hour.

Four drifts or defaults are taken into account in this study. For the collector, two parameters are considered: F’ (linked to the fin efficiency), and UL (linked to the thermal insulation); for details see TRNSYS manual. For F’, a progressive decrease is computed so that the value decreases from 0.7 to 0.6. For UL, a sudden increase is considered (from 3 to 4 W/m2.K) followed by a progressive increase (from 4 to 5 W/m2.K). For the connecting pipes, the variations of the U value are similar to the variations of the UL value. These drifts will be shown in the last section when presenting the results. It has to be noted that it has been necessary to write our own components for TRNSYS to be able to read the values from files, which allow continuous variations of the parameters (one value corresponding to one hour).

The drifts have been both considered separately and combined. In the latter case, which leads to the lowest performance of the system, the yearly increase of the auxiliary electrical power needed to deliver the hot water is less than 7.5%. It can be concluded that if the FDS is able to detect the faults before the end of the drifts, it is sufficiently efficient.

The power of the collectors during the day is studied to investigate the transient thermal performance of the ETCs. Figure 2 shows the collector power in an autumn day. In the morning, the direct flow ETC 7 starts up first, followed by the heat pipe ETC 4, ETC 2 and the double glass ETC with heat pipe. There is a sharp increase of the power of the heat pipe ETCs which is most likely caused by the late start-up of evaporation in the heat pipe causing “overheated” absorber temperatures. A possible explanation is that the upper part of the collector is heated up first, but the heat pipe will not be able to work until the bottom part of the heat pipe is heated up to the evaporation temperature. After 10:00 and before 15:00, the power of the ETC 6 levels out while the power of the other collectors increases in the morning and decreases in the afternoon. That can be explained by the cylindrical shape of the absorber of ETC 6. When there is almost no shadow on the tubes between 10:00 and 15:00, the irradiated surface area of the cylindrical absorber does not change significantly, therefore there is insignificant change of the collector power. The heat pipe ETC 2 and 4 and the direct flow ETC have a flat absorber, so the collector power will increase in the morning due to a decrease of incidence angle and an increase of irradiated surface area. In the afternoon the power will decrease due to increased incidence angle and reduced irradiated surface area.

The direct flow ETC 7 performs almost the same as ETC 4 but in the early morning and the late afternoon ETC 7 performs a bit better than ETC 4. The all-glass ETC 5 on the other hand starts up slowly and stops almost 1 hour later than the other collectors. This is due to its large thermal capacity since a large quantity of collector fluid is stored in the glass tubes.

Figure 3 shows power of the collectors in a summer day. In the morning ETC 4, 6 and 7 almost start up at the same time. The power of the collectors increases gradually and smoothly. The power of ETC 6 is higher than ETC 4 and 7 in the morning and in the afternoon. That is because the cylindrical absorber of ETC 6 has a larger irradiated surface than a flat absorber in the morning and in the afternoon.

3.1. Long term thermal performance

The thermal performances of the seven ETCs are compared. Figure 4 shows relative thermal performances of the differently designed ETCs. The performance ratio is defined as the ratio between the weekly thermal performance of the collector in question and the weekly thermal performance of the reference collector shown in the figure. The mean solar collector fluid temperature during operation is given at the bottom of the figure.

In addition to the equipment required for thermal performance testing, the test facility is delivered with the complete equipment required for durability and reliability testing of solar thermal collectors according to EN 12975-2. This comprises test facilities for outdoor exposure, external and internal thermal shock, rain penetration, mechanical load test and internal pressure test.

Like for thermal performance testing, it is also possible to combine test facilities required for several durability and reliability tests into a single test facility. Figure 5 shows the test facility for the rain penetration which can also be used for performing external thermal shock tests.

For carrying out exposure tests, high-temperature resistance tests and internal thermal shock tests, the same test facility can be used since only the frame with the spray nozzles has to be moved sidewards.

Furthermore, the collector test procedures related to the positive and negative mechanical load test, the internal pressure test, and the impact resistance test can be performed using only one single test facility. For this purpose, the collector is mounted on the mechanical load test facility; see figure 6.

Obviously, the mechanical load tests (negative and positive pressure) may be performed subsequently.

After connecting the equipment for the internal pressure test of the collector, the corresponding test can be performed on the mounted collector without any changes on the test facility.

According to EN 12975-2 two methods for performing the impact resistance test are specified. One method is using a steel ball and the other one ice balls.

For performing the impact resistance test by means of a steel ball, the collector does not need to be remounted since the collector is mounted stiff enough on the test facility. The supporting frame of the mechanical load test facility was designed by IAO as a push loading drawer. Due to this it is possible to pull out the support frame including the mounted collector underneath of the frame simulating the mechanical load; see figure 7.

For performing the impact resistance test with ice balls, it becomes necessary to utilize a complete different test facility. Since the ice balls must have a certain speed at the time of the impact on the collector cover the application of a certain kind of acceleration mechanism (“ice ball gun”) is essential. Such an acceleration mechanism is shown in figure 8.

The performing of an impact resistance test according to this method makes it necessary to mount the collector vertically.

1.1 Patent

The mobile, stand-alone solar thermal test facility has been registered by SWT for patenting under the number AZ 102007018251.3 at the German Patent Office. Major issues of this patent are the mobile and stand-alone characteristics of the test facility combined with the possibility to perform tests according to three different standards with one single test facility.