Category Archives: EuroSun2008-5

Power calculations based on available test methodologies

Подпись: n image124 image125 Подпись: (1)

Collector instantaneous efficiency n is defined as a ratio between the useful heat Q delivered and the hemispherical irradiance Gcol on the collector aperture Aa, according to (Rabl, 1985):

The hemispherical radiation Gcol reaching the collector aperture plane, to which the collector instantaneous efficiency is referred to, is calculated by the summation of the different components of radiation, for a given beam radiation incident angle в, and the plane tilt angle P, according to (Rabl, 1985):

Gcol =Icos6 + D(1 + cose)/2 + Rg (1 — cosy?)/2 (2)

where the ground reflected component — Rg = pgG — depends both on the global radiation G reaching the horizontal (ground) plane and on the ground reflectivity (albedo) pg.

As known, in the steady-state efficiency test (EN 12975-2; section 6.1) the collector efficiency curve is described by four parameters (considering a glazed collector): the optical efficiency n0, a global heat loss coefficient ai and (in the second order approach) a temperature dependent coefficient for the

2

global heat loss coefficient a2. The test includes also the measurement of incidence angle modifiers K(O) based on hemispherical irradiance, to be used in instantaneous power calculations.

In the quasi-dynamic efficiency test (EN 12975-2; section 6.3), the collector efficiency curve is described by five parameters (considering glazed collectors) and the incidence angle modifier values based on beam radiation. The five parameters are: the optical efficiency for beam radiation ц0Ь, the incidence angle modifier for diffuse radiation Kd, a global heat loss coefficient ci, a temperature dependent coefficient for the global heat loss coefficient c2 and a dynamic response coefficient c5 representing the effective heat capacity of the collector.

Besides the treatment of the dynamic response of the solar collector to temperature changes included in the quasi-dynamic test methodology, the major difference to the steady-state methodology lies in the decoupling of the radiation components, allowing the separation of effects affecting differently each of those components (e. g. optical effects, as referred by NEGST (2006) and Horta et al. (2008)).

According to EN 12975-2; section 6.1 the calculation of instantaneous collector power from steady — state efficiency curve parameters follows equation 3:

Подпись: (3)Qss = nAO PcoA — a, (f — Ta ) — a2 (Tf — Ta ) Aa

whereas the same calculation using dynamic test efficiency curve parameters follows equation 4 [EN 12975-2; section 6.3]:

image127

(4)

 

Horta et al. (2008) suggested a power correction methodology, applicable to power values determined after Eq.(3), accounting for the collector optical effects, affecting differently the radiation components which reach the absorber surface. According to this methodology, the power value is corrected using the following equation:

Подпись: (5)ss_______

1 — f (1 — KdfJ

where:

D

 

f

 

(6)

 

image129

image130

is a diffuse radiation fraction to be suggested by the efficiency test laboratory after reference irradiation conditions for the collector test (Horta et al., 2008), and:

J Jні»,.в, )cos (O )sin (в )ів{ dOl

_п — л/

K = /2 /2_______________________

dif. h

J J cos(O )sin(O ‘)dOtdOl — П2 — П2

is a weighted average hemispherical incidence angle modifier (Carvalho et al, 2007), calculated after the longitudinal and transversal incident angle modifier (IAM) values measured in the steady state efficiency test. Since this correction applies to test results performed according to steady-state test method, the incidence angle modifier is based on hemispherical radiation.

Implementation of the QD test method

3.1. Necessary steps

In order to implement the QD test methodology at LECS, three main aspects had to be dealt with:

a) Install an operational test circuit

The available test circuit was initially used for SS tests. It allows for the control of the collector inlet temperature at a constant value. Calibrated sensors were installed and a fixed stand for installation of the collectors was used. For measurement of irradiance on the collector aperture, two pyranometers were used, one with diffuse band for measurement of diffuse irradiance. Beam irradiance was calculated based on measurement of global and diffuse irradiance.

b) Develop a data acquisition programme

The data acquisition system was composed by a DMM equipment allowing reading of several sensors with voltage and resistance signals. A data acquisition program was developed using Visual Basic 6 language. The program’s purpose is to gather data from the measurement sensors, according to the methodology set out in the standard [1]. The measured data is recorded on a file with the collector name, test date and ORI extension. Files with ORI extension contain sensors readings in volts and ohms. The information on sensor readings is supplemented by information such as the date and time (hour, minute, second) and reading number.

The acquisition program also calculates physical values using the calibration parameters of each sensor and records them on a file with the collector name, test date and TPO extension.

The time step for data acquisition is 5 s, which is in agreement with the suggestions given in the standard.

c) Develop a tool for parameter identification

The software chosen for the development of the tool for parameter identification is Mathlab. With this software a pre-processing of the data collected with the data acquisition programme is also performed. This pre-processing generates files with mean values of relevant data for five minutes intervals. Generation of graphic for analyses of the data collected according to the recommendations of the standard is also performed. Example of these graphs can be seen in section

4., Fig. 1 of this work. This tool also includes a multilinear analyses of the data collected, in order to determine the characteristic parameters according to equation (4).

3.2. Multilinear regression

The following (conventional) multiple linear regression model describes a relationship between the k independent variables, xj, and the dependent variable Y

Y = Д) + Ax1 + Ax24 + Pa + є-> (6)

This model was applied to equation (4), heat balance equation for QD test method, for determination of the collector characteristic parameters (5). Since the parameter IAM for beam radiation, Keb(0), is dependent on the incidence angle, 0, an assumption for its functional form is needed. In a first approach this dependence was considered to be given by;

Подпись: (7)Keb(0)= 1 — b0(— -1 I cos 0

image178 Подпись: (8)

and equation (4) could be re-written as:

Characteristic parameters: Pi= F(xa)en ;p2= F(xa)enb0; p3= F(xa)enK0d ;p4=d; p5=c2; p6=c5

A programme was developed for determination of the characteristic parameters based on the measured data (5 minute average values).

The same programme also produces graphs for analyses of data variability, according to the EN 12975-2; section 6.3. These graphs are:

• Difference between mean fluid temperature and ambient temperature versus global irradiance;

• Beam versus global irradiance;

• Beam irradiance versus incidence angle;

Based on the calculated parameters and measured values, it calculates the power delivered by the collector using the collector heat balance model of equation (8) and represents it graphically for comparison with the measured power delivered by the collector during the test sequences.

Tube radius

Calculations with different glass tube radius are carried out. Other parameters are here changed as well: For the collectors with the curved absorbers, the glass tube centre distance and the radius of the absorber fins are changed as well. For the collectors with the flat fins the glass tube centre distance and the width of the fins are changed as well. For Seido 5-8 and 1-8 the radius of the glass tubes is varied from 0.035 m to 0.070 m. For Seido 10-20 with curved and flat absorbers the glass radius is varied from 0.02 m to 0.05 m. The thermal performance is shown as a function of the glass tube radius for

Подпись: Nuussuaq in Fig 6. The results for Sisimiut and Copenhagen are similar. The higher the radius the higher the thermal performance, especially for Seido 10-20, which is expected since they consists of more tubes, resulting in a larger increase in the area. The effects on the optimum tilt are by changing the glass tube radius again most seen in Nuussuaq. The solar collectors should be mounted more vertically the higher the tube radius is. In Sisimiut and
Подпись: Performance of the solar collectors as a function of tube radius
image054
Подпись: —A— Seido 5-8 -В- Seido 1-8 —©— Seido 10-20 with curved absorber —X— Seido 10-20 with flat absorber

image056image057Copenhagen the glass tube radius almost does not influence the optimum collector tilt. The change in the optimum orientation follows the same tendency as seen with a change in the other parameters. The collectors in Nuussuaq should be turned more towards east. In Sisimiut the collectors with the curved absorbers should be turned slightly from south towards west, and the collectors with flat absorbers should be turned 35° from south towards west. In Copenhagen the change in the orientation is only about 2° more from south towards west. In Fig 7 the thermal performance of the collectors in

image058 Подпись: Here a large increase in the thermal performance is seen for all the collectors, though highest for Seido 10-20 with curved and flat absorbers. If the result are viewed taking into account the increasing collector area the improvements are minimal for all the collectors in all the locations.

Nuussuaq is shown with an increased tube radius for all the collectors.

Mean collector fluid temperature [ Cl

Fig 7. Improvement of the thermal performance as a result of an increase
in tube radius in Nuussuaq.

5. Conclusion

The last simulations were done with the models using the improved centre distance, larger strip angle and width and larger transmittance-absorptance product. The thermal performance of the improved

collectors in Nuussuaq in seen in Fig 8. The overall improvement of the collectors results in an increase in thermal performance of up to 9 % with a mean collector fluid temperature of 60 ° for the collectors in Nuussuaq. For Sisimiut the improvement of the collectors is about 5 % for a mean collector fluid temperature of 60 °C. In Copenhagen the least overall improvement is detected. The highest improvement is in Copenhagen seen for Seido 10-20 with flat absorbers.

image060Подпись:References

[1] L. J. Shah, S. Furbo, (2005). Theoretical investigations of differently designed heatpie evacuated tubular collectors, Denmark.

[2] L. J. Shah, S. Furbo (2005). Utilization of solar radiation at high latitudes with evacuated tubular collectors, Denmark.

[3] J. Fan, J. Dragsted, Rikke Jorgensen, S. Furbo (2006). B^redygtigt arktisk byggeri i det 21. arhundrede, Denmark.

[4] J. Fan, J. Dragsted, S. Furbo (2007). Validation of simulation models for differently designed heat-pipe evacuated tubular collectors, Denmark.

Collector tests according to ISO 9806 / EN 12975

In order to determine the efficiency parameters of solar thermal collectors according to ISO 9806 or EN 12975, two different procedures can be used: the steady state test method and the quasi dy­namic test method.

During the steady state test, all boundary conditions such as solar radiance, ambient temperature and collector inlet temperature must be constant. After recording data points over a representative range of operating conditions, the collector efficiency curve can be determined by means of multi­linear regression using the least square method.

During the quasi dynamic test the boundary conditions must vary. Based on a series of measure­ments, specific collector parameters are determined, as well. With the quasi dynamic test method, additional parameters such as the heat capacity of the collector and the incident angle modifier co­efficient can be determined in addition to the efficiency curve.

Case — studies

1.1. Case-study buildings

Two case study buildings were considered. Case-study 1 is a T1 apartment (with one bedroom) located in the ground floor of a 6-floor apartment building [4]. The second case-study is a T3 detached dwelling (three bedrooms). Table 2 presents for each the main characteristics influencing their thermal behaviour.

1.2. Climatic regions / locations analysed

As stated in the introduction, one of the aims of this study is to understand the influence of the climate in the resultant energy label, and what levels of envelope and equipment sophistication are needed to cope with the climate (first to fulfil the regulation and then to reach class A+) in each of the locations. To this purpose, a selection of locations was made in an attempt to represent the

diversity of climate of mainland Portugal. The locations selected were: Lisboa, Faro, Guarda, Penhas Douradas, Evora, Viana do Castelo and Bragan? a.

Table 3 shows the main characteristics of each zone in order to understand the difference between them.

Solar Keymark Certification

Подпись: Fig. 2: Keymark Label In order to remove trade barriers inside Europe and to support the establishment of a uniform European market the European standardisation bodies CEN and CENELEC[4] created a uniform methodology for the marking of products on the basis of European standards. The mark resulting from this approach is the so-called Keymark which aims to be accepted all over Europe (see Figure 2). For solar thermal products the corresponding label is named Solar Keymark.

The Solar Keymark is a voluntary third-party certification mark. By obtaining the Solar Keymark, the solar product qualifies for nearly all the different European member state regulatory and financial incentive schemes.

The elaboration of the specific Solar Keymark scheme rules /5/ which provides the basis for Solar Keymark certification started at the beginning of 2000 within the framework of a European project initiated by the European Solar Thermal Industry Federation, ESTIF and co-financed by the European Commission. At the beginning of 2003 the final version of the Solar Keymark scheme rules were published. During the years 2006 and 2007 these scheme rules were revised within an other European project. In both projects representatives from the solar thermal industry, the leading solar thermal test institutions and certifications bodies co-operated.

The Solar Keymark for solar collectors requires that the products fulfil the requirements stated in the European Standard EN 12975-1 and are tested according to EN 12975-2.

In order to certify factory made solar thermal systems according to the Solar Keymark scheme rules they must be in line with the requirements of EN 12976-1 and must be tested according to EN 12976-2.

In order to obtain the Solar Keymark the following three major steps are required:

• Picking of random test samples from the factory production or the manufacturer’s stock by an accepted inspector.

• Successful type testing of the solar collectors or systems respectively by an accredited test lab

• Inspection of the manufacturer’s quality management system

If all the requirements mentioned above are fulfilled the Solar Keymark certification can be issued by certification bodies empowered by CEN Certification Board.

Stability of Absorber and Mirror Coatings

Modern selective absorber coatings usually are thin layer systems on metal substrates which selectively absorb short wavelength photons — a process which is enhanced by multiple scattering at small metal particles in a dielectric matrix (so-called Cermet layers). Long wavelengths average over the particle mix and do not resolve the particles of typical size (order of magnitude is 10nm). They experience an effective medium formed by the metal-dieelectric mixture. Interference effects are being used to maximize reflectance in the infrared range. Therefore the exact thickness, the volume fraction of the metal content of layers and the size of particles determines the optical and thermal performance — solar absorptivity and thermal emissivity. The high-temperatures as reached in concentrating collectors during operation on the other hand tend to increase the mobility of atoms in the layers. Therefore interdiffusion might deteriorate simple layer systems. Additional barrier layers therefore are used to restrict mobility. In some cases adhesive layers can improve the bonding stability of the system.

image077

time in h

Figure 2: Change of absorptivity and emissivity of prototype coating

Within the project Fresnel-II Fraunhofer ISE developed a sputtered air-stable absorber coating on stainless steel that resists temperatures up to 450°C. When heated at 500°C after a relaxation time the coating did not change the properties even after weeks of heating. The obtained solar absorptivity is 94% (AM1.5 global) with an emissivity of 18% in respect of a black radiator at 450°C. The application of such a coating is the single-tube absorber of a Linear Fresnel Collector. Of course, other applications are conceivable.

Other thin layer systems can be used to produce mirror coatings for secondary concentrators. These mirrors have to withstand elevated temperatures due to the proximity to the absorber tube. The solar reflectance shall be high, therefore first surface mirrors were the aim of the project. On the basis of highly reflecting silver an optical layer system has been developed and improved. Again barrier and adhesion properties have to be taken into account. Also mirror coatings change their properties slightly upon heating. It could be shown that the reflectivity changes ceased after a few hours when being heated at 150°C and 250°C (which seems to be sufficient for a secondary reflector in a linear Fresnel collector), but not at 350°C. Therefore the application e. g. for tower receivers seems to be problematic. Further work is needed to develop mirror coatings which are stable at even higher temperature loads.

image078

Figure 3: Change of solar reflectance (AM1.5 global) for first surface mirror due to heating

at different temperatures

The durability and resistance against several environmental factors, not only temperature but also UV, humidity, salt and air pollution, is a main concern with regard to the cost effectiveness of solar technologies. Large investments are needed in order to harvest the sun at nearly no cost. Longevity, performance and cost have to be considered in parallel. Innovative complex and possibly expensive new components cannot to be cost effective if their performance degrades after short time. However, cheap products might be replaced when disassembly and replacement can be standardized.

In climatic chambers and similarly in outdoor exposition components can be checked and qualified. Certainly unsuitable materials can be excluded. Relative comparisons between various development options are possible. However it is not trivial to translate results from accelerated indoor testing (using higher loads than experienced in reality) to a quantitative lifetime estimation. Comparisons with actual outdoor weathering need very long time and are not available for new products. Thus risk can only be minimized by understanding the material degradation processes and the real load factors during application. Combining this material and process specific knowledge with carefully designed accelerated indoor tests at least can minimize the risk of failure.

Application of the different test methods

A CPC collector having an aperture area of 1.87 m2 was analysed. The collector uses a circular absorber tube with an outer diameter of 19 mm. With an aperture width of 103 mm this results in a concentration ratio of C = 1.73.

Table 1 shows the results determined with the tests under quasi-dynamic and steady state conditions. The mean diffuse fraction during the test under steady state conditions was D = 0.3.

Figure 3 shows the power curves calculated using the collector parameters determined under quasi­dynamic conditions for diffuse fractions of 0.1, 0.3 and 0.5 together with the power curve calculated with the collector parameters determined under steady state conditions.

Table 1. Collector parameters determined

П0

[-]

Kdfu

[-]

a1

[W/(m2K)]

a2

[W/(m2K2)]

ceff

[kJ/(m2K)]

quasi-dynamic

0.798

0.725

3.483

0.009

13.65

steady state

0.725

3.599

0.007

Figure 3 shows the significant dependency of the collector output on the diffuse fraction D. For a diffuse fraction of D = 0.5 the maximum collector output is reduced by 160 W/m2 and 11 % respectively compared to the collector output at a diffuse fraction of D = 0.1.

image122

— •-D = 0.1 ………. D = 0.3 ——— D = 0.5 steady state

Fig. 3. Power curves (G = 1000 W/m2) for different diffuse fractions and under steady state conditions

The power curve determined using the test method under steady state conditions shows a similar appearance as the power curve determined for a diffuse fraction of D = 0.3. This attributes to the fact that the mean diffuse fraction during the test under steady state conditions has been D = 0.3.

From the presented investigation two main conclusions can be drawn:

1. The collector parameters gained from the test under quasi-dynamic conditions are very well suited to calculate the collector performance for different diffuse fractions. 2

3. Conclusion

The test method under quasi-dynamic conditions is, contrary to the test method under steady state conditions, very well suited to determine the thermal performance of CPC collectors having a concentration ratio larger than 1. Especially the differentiation between diffuse and beam irradiance permits a reliable modelling of the thermal performance under arbitrarily diffuse fractions. The level of detail provides a more exact estimation of the yearly energy gain and thus a better planning reliability during the dimensioning of solar thermal systems using CPC collectors.

Due to the poor reproduction of the incident irradiance the test method under steady state conditions is not suited for CPC collectors. The inaccuracy of the test method even grows with rising concentration factors.

The increasing efforts in the fields of solar thermal process heat and solar cooling have led to a rising number of concentrating collectors on the European market. Against this background it is appropriate to nominate the test method under quasi-dynamic conditions as the sole test method to be used for concentrating collectors within the next revision of the European standard EN 12975.

Nomenclature

a1

[W/(m2K)]

Heat loss coefficient

a2

[W/(m2K2)]

Temperature dependent heat loss coefficient

A

[m2]

Aperture area

C

[-]

Concentration ratio

ceff

[J/(m2K)]

Effective heat capacity of the collector

D

[-]

Diffuse fraction

G

[W/m2]

Hemispherical irradiance

Gdfu

[W/m2]

Diffuse irradiance

Gdir

[W/m2]

Beam irradiance

Gnet

[W/m2]

Useful irradiance

K(0)

[-]

Incidence angle modifier for hemispherical irradiance

Kbeam(0)

[-]

Incidence angle modifier for beam irradiance

Kdfu

[-]

Incidence angle modifier for diffuse irradiance

N

[-]

Fraction of useful irradiance

Q

[W]

Collector output

П0

[-]

Conversion factor

0

[-]

Angle of incidence

0a

[°C]

Ambient temperature

0fl, m

[°C]

Mean fluid temperature

t

[s]

Time

References

[1] DIN EN 12975-2:2006, Thermal solar systems and components — Solar collectors — Part 2: Test methods -, 2006.

Improved new testing possibilities for air-collectors

image159 image160 image161

As explained in the abstract, Fraunhofer ISE has already operated a test facility for solar air collectors for some years. We have now started to extent and improve our testing possibilities. Our concept was to have a mobile testing facility. That means that either it may be used within the existing indoor test stand with the solar simulator. But additionally, we also made it possible to use the components of the air-collector testing loop at the outdoor testing facility with sun tracker. In both cases the testing facilities allow measurements with very high accuracy and reproducibility. Figure 1 shows the set-up of the air-collector testing loop. Exactly the same rules as described in EN12975 for the testing of water-collectors are used for the air-collector tests.

Fig. 1. Set-up of the air-collector testing loop for efficiency measurements

Information on the components of the loop are summarised here, and will be commented after the list:

• Two ventilator units, each with a flow rate performance between 0 — 500 m3/h, 400 Pa

• Two flow meters (MR) with a measurement range of 0 — 1000m3/h

• Two humidity sensors(FR) to measure the relative humidity of the air in the test loop

• four temperature sensor (TR) to measure the inlet — and outlet-temperature of the collectors, and temperature measurement points necessary to determine the density and the heat capacity of the air at the flow sensors

• water-to — air heat exchanger to condition the air in the test loop

• pressure difference sensor (PR)

• solar radiation (RR)

• Ambient air temperature (TR)

Compared to a water-collector test loop, we added some additional features due to our testing experience with air-collectors:

1. Not one, but two air ventilators are used. Although in principle solar air-collectors should be tight and do not have leakages in the absorber, many of them (especially at the beginning of a development process) are not air tight. This is disturbing the efficiency measurements. It is therefore very helpful, if two ventilators are installed in the test loop. Because they can be operated in such a way (one is pushing air, the other is sucking air) that with the proper adjustment as a result there is only a small difference between the absolute pressure of the air in the collector and the pressure of the ambient air. Thus, air leakage does only have a small influence on the efficiency measurement and development measurements can be carried out in a reproducible was. Of course, the air leakage rate is nevertheless important for the performance of a collector and should be measured and reported in separate measurements. And as mentioned already, the collector should be air tight.

2. Water is an incompressible fluid, but air is not. Therefore the influence of the absolute pressure of the fluid has to be taken into account. Figure 2 shows the dependence of air density on temperature and pressure. The variation within the typical measurement boundaries can be seen from Figure 2. In any case, the influence can easily be included in the measurement and evaluation procedures.

t

 

£

tfl

c

&

 

70 90 110

Temperatur in °C

 

-30

 

-10 10

 

30

 

50

 

130

 

150

 

170

 

190

 

image162

image163

image164

Figure 2: Dependence of air density on pressure and temperature.

Figure 3: Dependence of specific heat capacity of air on temperature and humidity (xw20 denotes an

absolute humidity of 20 g/m[16]).

Automatic Control System

The whole automation system is mainly divided into two parts: one part is the hardware equipments consisted of all kinds of devices used in the testing system; the other part is the software program based on Labview language. I/O device is used to connect hardware with software. Through the automation system the test system need to be operated normally and accurate measurement is also acquired. Because of the strict requirements under the test conditions according to ISO 9459-2, the system needs to accurately identify solar time and determine the test process whether it is in a exactly true position in sequence. At the same time data acquisition system will be operated to collect and record data with real-time curve of all sorts of signals. After the whole test process is finished, all of data will be used to produce the test report. Therefore, the main purpose of automatic control system is to release the tedious labour work of the operator and develop a set of efficient and reliable system in order to measure and control the test system automatically. The system should also be reliable and stable in different conditions to guarantee the normal operation of the whole test system.