Category Archives: EuroSun2008-5

AN EXPERIMENTAL ANALYZE OF A SOLAR COLLECTOR

G. Zorer Gedik1* and A. Koyun2

1Yildiz Technical University, Architecture Faculty Istanbul /Turkiye
2Yildiz Technical University, Mechanical Engineering Fac., Istanbul /Turkiye,
Corresponding Author, gzorer@hotmail. com

Abstract

In this study the experimental results of a solar energy collector which is installed to a south classroom window of a school in Istanbul are presented. The collector unit had been tested experimentally and numerical to determine its thermal performance before its integration into the south window. In this paper, the experimental analyze will be given in detail. The collector is tested using infrared radiation lamps in the laboratory. The collector reaction to change in the value of heat transfer is measured. Air temperatures and velocities are measured at the bottom (air entry) and top (air exit) of the collector. Moreover, performance of the collector geometry is analyzed using Computing Fluid Dynamics (CFD). The results of the measurements and theoretical analysis are compared. The back face of the collector is insulated to generate effective convective air flow on the basis of the test results. Then thermal efficiency measurements were executed in the classroom and the efficiency curves displayed and evaluated. A computer system with a software was designed to obtain air temperatures, velocities and solar radiation data in the classroom.

Keywords: Solar heating, solar collector, experimental, collector geometry.

1. Introduction

A research project supported by TUBITAK (The Scientific and Technical Research Council of TUrkiye) is designed to improve the thermal efficiency of classroom design through the use of solar energy. [1] This paper presents a part of the results of the project. A solar energy collector was attached to a south classroom window of a school in Istanbul which has existing large classroom windows. The main components of the solar air collector are the glazing, the air space between the glazing and the collector plate, the aluminum collector profiles with air chambers and the insulated backing of the collector. (Figure 1) The function of the glazing is to admit as much solar radiation as possible and to restrict the transmission of heat radiation back through the glazing so that the optimum greenhouse effect results. Hence, the existing single glazed in front of the collector in the classroom changed the special glazing has ninety-two percent transmittance of light. [2]

The wavelength selective coating is applied to the front face of the collector profiles that have a low emmisivity of energy in the infrared wavelengths. Since infrared energy flow makes up a large part of the total energy loss of the system through the glazing, selective surfaces is quite effective in improving performance. [3]

image001
image002

Figure 1:The detail of solar collector. Figure 2. The profiles of solar collector.

Fault diagnostic method for a water heating system based on. continuous model assessment and adaptation

Sylvain Lalot[1]*, Soteris Kalogirou[2], Bernard Desmet1, and Georgios Florides2

1LME, Universite de Valenciennes et du Hainaut Cambresis, Le Mont Houy,

59313 Valenciennes Cedex 9, France

2Cyprus University of Technology, P. O. Box 50329, 3603 Lemesos, Cyprus
* Corresponding author, sylvain. lalot@univ-valenciennes. fr
Abstract

The objective of this work is the development of an automatic solar water heater (SWH) fault diagnostic system (FDS). The latter consists of a modelling module and a diagnosis module. A data acquisition system measures the temperatures at four locations of the SWH system (outlet of the water tank; inlet of the collector array; outlet of the collector array; inlet of the water tank). In the modelling module a number of artificial neural networks (ANN) are used, trained with the very first values when the system is fault free. Then, the neural networks are able to predict the fault-free temperatures and compare them to actual values. When the differences are low, the corresponding networks are unchanged. On the contrary the networks are retrained. Then the diagnosis module analyses the difference between the current connection weights and the initial weights. When a persistent significant modification occurs, a flag is set to signify that a default is present in the SWH.

The system can predict three types of faults: collector faults and faults in insulation of the pipes connecting the collector with the storage tank (to and from the tank) and these are indicated with suitable labels. It is shown that all faults can be detected well before the end of the drifts, without any false alarm, when the networks and thresholds are well tuned and that the observation window has the right size. It is shown that this does not depend on the draw off profile.

Keywords: fault diagnostic, model adaptation, neural network, water heating system

type. As it has been shown that continuous drifts can be analysed by neural networks in heat exchangers [3-5], ANN has been chosen here to test such tools. In particular, a method based on neural models is presented according to the study detailed in [6] which shows that a continuous assessment of a model and its adaptation is efficient. In a first part, the solar system is presented along with the drifts that are taken into account. The drift detection tool is detailed in the second part, and results are given in the third part.

A Long Term Test of Differently Designed Evacuated Tubular Collectors

J. Fan*, J. Dragsted, S. Furbo

Department of Civil Engineering, Technical University of Denmark,

Brovej 118, DK 2800, Kgs. Lyngby, Denmark
Corresponding Author, iif@byg. dtu. dk
Abstract

During three years seven differently designed evacuated tubular collectors (ETCs) utilizing solar radiation from all directions have been investigated experimentally. The evacuated tubular solar collectors investigated include one SLL all-glass ETC from Tshinghua Solar Co. Ltd, four heat pipe ETCs and one direct flow ETC from Sunda Technolgoy Co. Ltd and one all-glass ETC with heat pipe from Exoheat AB. The collectors have been investigated side-by-side in an outdoor test facility for a long period. During the measurements, the operating conditions — such as weather conditions and temperature of the inlet fluid to the collectors have been the same for all collectors. The volume flow rate through each of the collectors is adjusted so that the mean solar collector fluid temperature has been the same for all collectors. Thus a direct performance comparison is possible. The side-by-side tests were carried out with different mean solar collector fluid temperatures and in different seasons of the year. The results of the measurements are presented in this paper. The influence of the mean solar collector fluid temperature on the thermal performance of the different collector designs will be discussed. Further, the collector performances are compared for different times of the year and it is illustrated how the performance of the different collector types depends on weather conditions.

Keywords: Evacuated tubular solar collector, collector design, thermal performance, test.

1. Introduction

In recent years the evacuated tubular collectors have gained an increasing share of the market. On the world’s largest solar thermal market, China, evacuated tubular collectors have increased the market share from 30% in 1998 up to 94% in 2007 [1]. In Europe, evacuated tubular collectors of up to 240,000 m2 were installed in 2007 [2].

On the market there is a large number of collector manufactures providing evacuated tubular collectors with a variety of types such as all-glass, heat pipe, all-glass with heat pipe, direct flow, with and without reflectors. As far as the heat pipe evacuated tubular collector is concerned, there are designs with different tube diameters and different shapes of the absorber. It is therefore important to know how the different designed evacuated tubular collectors perform. He et al. [3] made a comparison of optical performance of evacuated collector tubes with flat and semi-cylindrical absorbers. The collector tubes are utilizing solar energy from the front side. The absorbed energy of the absorber is used in the comparison. They found that the semi-cylindrical absorber outperforms the flat absorber by 15.9% annually if it is located at latitude 40° N. Fan et al. [4] carried out side-by-side outdoor tests of four heat pipe evacuated tubular collectors with a flat fin or a semi-cylindrical fin. The collectors

utilize solar radiation from all directions. The measurements show that at latitude 57° the ETC with a flat fin performs better than the ETC with a semi-cylindrical fin for a tube diameter of 70 mm and a collector tilt of 67°. The ETC with flat fin tends to perform better than the ETC with the curved fin in winter and at high collector fluid temperatures.

Evacuated tubular collectors have a substantially lower heat loss coefficient than standard flat plate solar collectors. This makes ETCs very suitable for high latitude regions like the Arctic. The advantages of evacuated tubular collectors at high latitudes are not only their low heat loss and high efficiency, but also the ability to utilize solar radiation from all directions due to the large variation of the solar azimuth. The aim of this paper is to present the result of a long term outdoor test of differently designed evacuated tubular collectors utilizing solar radiation from all directions. Side-by­side tests of seven differently designed evacuated solar collectors were carried out in a period from February 2006 to August 2008. The thermal performances of the differently designed evacuated tubular collectors are compared. Based on the observations from the measurement, it will be elucidated how the collector performance is influenced by the solar collector designs, the weather and operation conditions.

2. Experiments

Seven differently designed ETCs utilizing solar radiation from all directions have been investigated experimentally. Detailed data sheet of the investigated ETCs is given in Table 1.

Side-by-side tests were carried out in an outdoor test facility at the Technical University of Denmark, latitude 56°N, see Figure 1. On the test platform, five collectors can be tested under the same conditions at a time. The collectors are directly facing south and have a tilt angle of 67° which is suitable for typical operation conditions in the Arctic. The collectors can utilize solar radiation from all directions. A glycol/water mixture of 41% by weight is used as the solar collector fluid. The fluid flow rate through each of the collectors is measured by a flow meter type Brunata HGQ1-R0. The inlet and outlet temperatures of the collector are measured by copper/constantan thermal couples, type TT. The difference between the outlet and inlet temperature is measured by a thermopile. The five collectors are parallel connected to a temperature control unit so that the inlet temperatures to the collectors are the same. A pump is used to circulate the solar collector fluid during all hours so that the inlet temperature of the fluid to the collectors is kept constant. The flow rates through the collectors are adjusted in such a way that the average temperatures of the collector fluids in all the collectors are approximately the same during the test. The accuracy of the absolute temperature measurement and temperature difference measurement is 0.5 K and 0.1K, respectively. The accuracy of the flow rate measurement is estimated to be 1.5%. The measurement data are monitored and logged every two minutes by LabView.

The weather data are measured in a climate station located on the roof of a building close to the test platform. The total and diffuse solar irradiance on horizontal surface and the ambient air temperature are measured.

The thermal performance of the ETCs were measured in the period from February 2006 to August 2008. The experiment is divided into three phases:

Phase 1: February 2006 — June 2006, collectors tested: ETC 1, ETC 2, ETC 3, ETC 4 and ETC 5.

Phase 2: July 2006 — May 2007, collectors tested: ETC 2, ETC 4, ETC 5 and ETC 6.

image062

Phase 3: May 2007 — August 2008, collectors tested: ETC 2, ETC 4, ETC 5, ETC 6 and ETC 7. During the test period, four mean collector fluid temperature levels are used: 26°C, 43-47°C, 63-68°C and 75-78°C.

Fig. 1. The side-by-side test facility.

Table 1. Data of the tested evacuated tubular collectors.

ETC no.

1

2

3

4

5

6

7

Collector type

Seido 5-8

Seido 1-8

Seido 10-20 with curved fin

Seido 10-20 with flat fin

SLL 1500

VA1858

Seido 2-16

Note

Vertical tubes, heat pipe

Vertical tubes, heat pipe

Vertical tubes, heat pipe

Vertical tubes, heat pipe

Horizontal

tubes

Vertical tubes, heat pipe

Vertical tubes, direct flow

Manufacturers

Sunda Technology Co. Ltd

Sunda Technology Co. Ltd

Sunda Technology Co. Ltd

Sunda Technology Co. Ltd

T singhua Solar Co. Ltd

ExoHeat AB

Sunda Technology Co. Ltd

Number of tubes

8

8

20

20

50

24

16

Tube diameter

100 mm

100 mm

70 mm

70 mm

47 mm

58 mm

70 mm

Tube length

2000 mm

2000 mm

1750 mm

1750 mm

1500 mm

1800 mm

1700 mm

Tube centre distance

111-120 mm

111-120 mm

86-93 mm

86-93 mm

72-75 mm

79-84 mm

89-91 mm

Tube diameter / tube centre distance

0.83-0.90

0.83-0.90

0.75-0.81

0.75-0.81

0.63-0.65

0.69-0.73

0.77-0.79

Transparent area

1.54 m2

1.54 m2

2.36 m2

2.36 m2

3.30 m2

2.45 m2

1.87 m2

Collector height

2.16 m

2.16 m

1.90 m

1.90 m

2.00 m

1.97 m

1.90 m

Collector width

0.96 m

0.96 m

1.86 m

1.86 m

3.20 m

1.99 m

1.82 m

Gross area

2.07 m2

2.07 m2

3.53 m2

3.53 m2

6.40 m2

3.92 m2

3.46 m2

Absorber area

3.66 m2

2.80 m2

6.60 m2

4.00 m2

8.71 m2

6.17 m2

3.20 m2

Absorber

material

Aluminum

Aluminum

Aluminum

Aluminum

Glass

Glass

Copper-

Aluminum

Absorber

thickness

0.47 mm

0.47 mm

0.6 mm

0.6 mm

0.6

Selective coating

Aluminum Ni

Aluminum Ni

Aluminum Ni

Aluminum Ni

Aluminum Ni

Aluminum Ni

Aluminum Ni

Absorptance

0.92

0.92

0.92

0.92

0.90

0.92

0.92

Emittance

0.08

0.08

0.08

0.08

0.08

0.08

0.08

Glass thickness

2.5 mm

2.5 mm

1.7 mm

1.7 mm

1.6 mm

1.6 mm

1.7 mm

Transmittance at incidence angle 0°

0.91

0.91

0.91

0.91

0.91

0.91

0.91

Manifold

diameter

28 mm

28 mm

38 mm

38 mm

45 mm

38 mm

38 mm

3. Results and Discussion

Set up of the test facility

One aim of the newly developed all-in-one test facility is to combine all three test methods in a single test facility. This test facility must be able to fulfil all requirements and qualifications result­ing from the above mentioned standards.

The housing of the test facility is a conventional 20 foot office container. In this container the hy­draulics of the temperature unit as well as the measuring equipment and the data logging instru­ments are located. In order to operate the facility independent from a cooling network, a chiller combined with a 600 litre cold water store is installed. With the exception of the chiller, all com-

Подпись: Fig. 1: Arrangement of major components (bird's view)

ponents are located inside the container. Figure 1 shows the schematic layout of the major compo­nents.

The facility is designed in such a way that it allows for parallel testing of

• 4 collectors (according to EN 12975 / ISO 9806) or

• 4 systems according to ISO 9495-2 (CSTG-method) or

• 2 systems according to ISO 9495-5 (DST-method)

Подпись: Fig. 2: Circuit diagram of the complete hydraulic arrangement

To realise these different test configurations, the hydraulic arrangement consists of one main loop that can be divided into six smaller loops by using several valves. These six loops are required for testing two solar thermal systems according to the DST-method at the same time. The hydraulics for testing according to ISO 9459-2 and ISO 9806 / EN 12975 consist of only one hydraulic loop, which can be used to test four systems (CSTG-method) or four collectors simultaneously. In figure 2, the layout of the complete hydraulic arrangement is shown.

On the left side of the diagram the four connections for testing the systems (ISO 9459-2, CSTG — method) or for the collectors (EN 12975) are located (1-4). For the DST-method (ISO 9459-5), connections for two collectors (1-2) including the two necessary connections to the solar collector loop (DST-solar 1-2) are depicted. The middle part of the hydraulic circuit is solely related to the DST-method, since there are the connections for the thermal auxiliary heating loops (DST-aux 1-2) and the storage tanks (DST-tap 1-2) of the two systems to be tested. The first heat exchanger (HX1) in flow direction offers the possibility to connect an optional external cooling net. If this is not available, the heat will be removed via the second heat exchanger (HX2), which is on the sec­ondary side connected to the cold water store of the test facility. The cooling circuit also supplies the air conditioning unit inside the container test facility with cold water. On the right hand side of the vertical line, which symbolizes the wall of the container, the chiller is located.

Подпись: Fig. 3: Chiller connected to the container of the test facility

This chiller is positioned outside the container because this allows a better supply of fresh air to the refrigerant condenser and reduces the noise level inside the container test facility. Figure 3 shows the chiller connected to the container of the test facility.

Подпись: Fig. 4: Hydraulic connections to systems and collectors

All hydraulic connections to the equipment being tested are located outside the container and are realized with conventional 1 inch screw fittings, countersunk into the container walls. In figure 4 the connections to the collectors / systems being tested are shown.

Options of HVAC and DHW equipments

Since the energy label in Portugal is based on an evaluation of primary energy, equipments for ambient heating, ambient cooling and production of domestic hot water have, besides the envelope and the climate, a critical influence in the result. In order to analyse the influence of the equipments, several scenarios were considered, as listed in Table 4.

Table 2 : Main characteristics of case-studies (as in the base-cases)

Apartment T1

Dwelling

Floor area (m2)

53.5

150.4

External wall area (m2)

30.4

132.7

Shape factor

0.70

0.75

Main wall U-value (W/m2.°C)

0.57

0.68

Planar thermal bridge U-value (W/m2.°C)

1.12

0.86

Roof area

80.1

Main roof U-value (W/m2.°C)

0.42

Window area (m2)

12.2

28.6

Main window U-value (W/m2.°C)

3.1

2.9

Main window orientation

NE

NW and SW

Main window shading device

External roller blinds, except 1 glazed door with internal curtains.

External roller blinds

Nominal air change rate (infiltration)

0.94 ach’1

1.1 ach-1

Thermal inertia

high

high

Other characteristics

— Garage under the floor.

— Shading from nearby buildings

— 2 floors, incl/ garage

— No relevant obstructions

Table 3: Main characteristics of the selected climates

Climatic Zone

solar radiation [kWh/m2]

Location

Heating

Heating

Winter, on

Summer, on

Winter

Summer

degree-days

months

South surface

Horizontal surface

per month

June to September

Braganja

I3

V2 N

2850

8

90

790

Evora

I1

V3 S

1390

5.7

108

820

Faro

I1

V2 S

1060

4.3

108

820

Guarda

I3

V1 N

2956

8

90

730

Lisboa

I1

V2 S

1190

5.3

108

820

Penhas Douradas

I3

V1 N

3400

8

90

730

Viana do Castelo

I2

V1 N

1760

6.3

92

730

Table 4: Heating, cooling and DWH equipment scenarios considered

Scenario

Ambient heating

Ambient cooling

Domestic hot water

Eqpt. 1

Gas boiler

Heat pump

Gas boiler

Eqpt. 2

Gas boiler

Heat Pump

Solar collector + gas boiler

image153

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

Eqpt. 3

Heat pump

Heat pump

Electrical resistance

Eqpt. 4

Heat pump

Heat pump

Gas boiler

Eqpt. 5

Heat pump

Heat pump

Solar collector + electrical resistance

Eqpt. 6

Heat pump

Heat pump

Solar collector + gas boiler

Current status of Solar Keymark Certification

At present (summer 2008) approximately 430 different types of solar collectors and nearly 40 factory made systems are Solar Keymark certified. It is expected that two third of all solar thermal collectors sold in Europe are already qualified with a Solar Keymark /6/. A large number of the tests required for awarding the Solar Keymark were carried out at the Test and Research Centre for Solar Thermal Systems (TZS) located at the Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart.

6.1 Future development of Solar Keymark Certification Solar collectors

An assessment of the thermal performance of solar collectors directly on the basis of the efficiency parameters is not appropriate. Hence it is common practice to use the annual collector gain as a performance criterion, e. g. with regard to the qualification for subsidy programs. At present several national methods exist for the determination of the collector performance. This results in additional effort for manufacturers active in different European countries.

In order to overcome this problem, a method for the determination of the annual collector gains will be incorporated into the revised version of the Solar Keymark scheme rules.

Factory made systems

According to the current Solar Keymark scheme rules every factory made system has to be tested even if the system is part of a so-called system product line (or family respectively) and only differs in the collector area and/or the store volume. The huge effort resulting from this requirement is also the reason why the number of Solar Keymark certified collectors is approximately ten times higher than the number of certified systems. In order to overcome this problem a procedure was developed that allows the determination of the system performance of systems which are part of a system family by a mathematical procedure based on the test of only one system out of the product line /7/.

At present it is not finally decided if this procedure or an-other one /8/ based on the standard series EN 15316 (Heating systems in buildings — Method for calculation of system energy requirements and system efficiencies) will be included in the final revised version of the Solar Keymark regulations.

Custom built systems

Custom built systems are standardised in the European standard series CEN/TS 12977. Since the major share of systems sold in central and northern Europe are small custom built systems, it is favourable to extend the Solar Keymark also to that category of systems. Due to the fact that Solar Keymark certification must, in principle, be based on EN standards this is quite difficult since a CEN/TS document is formally not considered as an EN standard. At present several options to overcome this problem are under discussion but, even in the optimal case, a solution can not be expected before 2009.

3. Conclusions

The current status and the latest developments related to standards for solar thermal products and to Solar Keymark certification were described. The revised version of the European standard series

EN 12975, EN 12976 and CEN/TS 12977 contains appropriate requirements and test procedures for the products on today’s solar thermal market

The large number of already certified products clearly shows the success of the Solar Keymark. The Solar Keymark is on the best way to be established as THE quality and qualification label for solar thermal products all over Europe.

The combination of the European solar thermal standards and Solar Keymark certification provides an excellent basis for the further development of as sustainable solar thermal market in Europe. In the long-term all involved players — customers, manufacturers as well as test laboratories and certification bodies — will benefit.

References:

[1] Harald Druck, W. Heidemann, H. Muller-Steinhangen, A. Veenstra: European standards for thermal solar systems — the finals, Proceedings of ISES 2001 Solar World Congress, Pages 589 — 595, Adelaide, Australia, ISBN 0-9586192-7-1

[2] H. Druck, S. Fischer, H. Muller-Steinhagen: Solar Keymark Testing of Solar Thermal Products, Proceedings of ISES 2007 Solar World Congress, September 18 to 21, 2007, Beijing, China, ISBN 978-7­302-16146-2, Tsinghua University Press, Beijing and Springer-Verlag GmbH Berlin Heidelberg, CD: ISBN 978-7-89486-623-3

[3] H. Druck, E. Hahne: European test standard for the performance characterisation of stores for solar heating systems, Proceedings of Terrastock 2000, Pages 285 — 290, Stuttgart 2000, ISBN 3-9805274-1-7

[4] M. Peter, H. Druck, Testing of controllers for thermal solar systems, Solar Energy 82 (2008), pp. 676-685

[5] CEN: Specific CEN Keymark Scheme Rules for Solar Thermal Products, Final version 8.00 — January 2003 (ed. J. E. Nielsen). Available via

http://www. estif. org/solarkeymark/Links/Internal_links/solar_keymark_scheme_8.00.pdf

[6] www. solarkeymark. org

[7] H. Kerskes, B. Mette, H. Druck, H. Muller-Steinhagen: The Solar-Keymark Testing for Factory Made Systems by Means of an Extrapolation Procedure, Proceedings of Eurosun 2008, October 7 to 10, 2008, Lisbon, Portugal, to be published

[8] EN 15316-4-3: Heating systems in buildings — Method for calculation of system energy requirements and system efficiencies — Part 4-3: Heat generation systems, thermal solar systems

More information about the Solar Keymark is available at: www. solarkeymark. org.

The European Standards mentioned above are available from: www. beuth. de

Optical Concentration by Primary Mirror Field and Secondary Concentrator

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.

image079

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.

image080

Figure 7: Influence of optical quality on electricity costs for a fictitious solar power plant using the Linear

Fresnel Concept

Overview of Monitoring and Failure Detection Approaches for Solar Thermal Systems

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

Flexibility of the new testing facility for air-collectors

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.

image165

Figure 4: Some of the components of the solar air collector testing loop: one of two gas turbine volume flow meters, one of two ventilators, electric control cabinet and data acquisition, water-to-air heat exchanger. The water side is connected to a thermostat by which the inlet temperature of the collector to be tested is controlled. All components are well insulated. They are covered with weatherable protection housings which

are not seen in the pictures.

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.)

image166

Figure 5:Air-collector testing loop installed for outdoor tests using the solar tracker at Fraunhofer ISE.

image167

Figure 6: Components of the air-collector testing loop used in a field test and not in one of our laboratories.

image168

Figure 8 shows an example of an efficiency curve of a flat-plate air-collector with a selective absorber, measured under the solar simulator at Fraunhofer ISE. The air flows under the absorber plate which has downward-oriented fins to increase the heat exchanging surface from the absorber to the fluid. In order to give the complete information from the measurements we always use all three possibilities to present the collector efficiency: In the term (Txy-Tamb)/G on the x-axis, Txy can be choosen to be the mean fluid temperature Tm (defined as in EN12975 as the arithmetic mean value of Tin and Tout) or the collector inlet temperature Tin or the collector outlet temperature Tout. A

*

§

cn

ІП

Oi

5

0 0.01 002 0.03 0.04 0.05 0 06 0 07 0.08 0.09 0.1

(Txy-Tamb)/G in (K m2)/W

Подпись: Figure 7: Components of the air-collector testing loop used in the indoor testing laboratory with solar simulator at Fraunhofer ISE.

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.

Hardware Structure

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.

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Fig.3. Physical structure of hardware