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.

European standards for solar thermal products in general

In 2000 and 2001 the first edition of the European standards for solar thermal systems and components was issued, and started to replace all related national standards /1/. The three standard series consisted of the following seven parts listed in Table 1.

Table 1: Titles of European solar thermal standards as published in 2000 and 2001

Number

Title “Thermal solar systems and components-…”

EN 12975-1:2000

Collectors — Part 1 — General requirements

EN 12975-2:2000

Collectors — Part 2 — Test methods

EN 12976-1:2000

Factory Made Systems — Part 1 — General requirements

EN 12976-2:2000

Factory Made Systems — Part 2 — Test methods

ENV 12977-1:2001

Custom Built Systems — Part 1 — General requirements

ENV 12977-2:2001

Custom Built Systems — Part 2 — Test methods

ENV 12977-3:2001

Custom Built Systems — Part 3 — Performance characterisation of stores for solar heating systems.

Long time durability tests of fabric inlet stratification pipes

E. Andersen* and S. Furbo

Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, DK-2800 Kgs.

Lyngby

* Corresponding Author, 5an@byg. dtu. dk

Abstract

The long time durability of seven different two layer fabric inlet stratification pipes for enhancing thermal stratification in hot water stores is investigated experimentally. Accelerated durability tests are carried out with the inlet stratification pipes both in a domestic hot water tank and in a space heating tank. Heating/cooling cycles are carried out with different operation conditions including different temperature levels and volume flow rates.

The results show that that lime contained in the domestic water is deposited in the fabric pipes in the domestic hot water tank and that this destroys the capability of building up thermal stratification for the fabric inlet stratification pipe. The results also show that although dirt, algae etc. are deposited in the fabric pipes in the space heating tank, the capability of the fabric inlet stratifiers to build up thermal stratification is unchanged for five out of seven fabric pipes within the test period.

Keywords: Hot water tanks, thermal stratification, fabric inlet stratifiers, long time durability of fabric inlet stratifiers

1. Introduction

Previous investigations have shown that thermal stratification in hot water stores can be built up in a very good way with inlet stratification pipes made of two fabric layers [1, 2, 3]. This is true even with volume flow rates through the pipes as high as 10 l/min. Many different fabric styles have been investigated including both stretchable and non-stretchable fabrics but the investigation did not result in one superior fabric being pointed out. On the contrary, several of the tested fabrics show similar good behaviour.

Based on these promising results, accelerated long time durability tests are carried out with two fabric layers inlet stratification pipes made of seven different fabric styles, see Table 2.

Table 1. Investigated fabric styles.

Fabric style #

Description

Style 361

Spun Nylon 6.6 DuPont Type 200 Woven Fabric (ISO 105/F03)

Style 769

100% Spun Dacron Type 54 Knit (Disperse Dyeable)

Style 700-3

Poly Taffetta

Style 700-12

Poly Lycra

Style 864

Spun Orlon Type 75 Acrylic Plain Weave

Style 867

Spun Acrilan 16 Acrylic Knit

The long time durability tests consisting of continuous heating/cooling periods of hot water stores with built in inlet stratification pipes have been going on for one year and the fabric pipes have been inside the tanks during this time. Hot water is pumped into the tanks through the inlet stratification pipes during all heating periods. In average each fabric pipe has been in operation in 480 hours. During this period the inlet temperatures have varied in the range 30°C — 70°C. The volume flow rates have varied in the range 2 l/min — 10 l/min. The applied operation conditions correspond to operation of a solar heating system with a solar collector area of 10 m2 with a volume flow rate in the solar collector loop of 0.2 l/min per m2 solar collector area in 2/3 year. Solar heating systems with flat plate solar collectors are typically in operation 1800 hours per year in Denmark.

Evaluation of Optical Properties

For each exposed sample a transmission spectrum was measured; in the beginning utilizing a manually operated Zeiss-Spectrometer and since 1995 with computerized Fourier Transform Spectrometers (Bruker, IFS 66), all equipped with integrating spheres. These spectra were used for the calculation of the solar transmittances based on an AM 1.5 solar spectrum (ISO 9845). In order

to approve the comparability of the different instruments, measurements performed on unexposed reference plates in the year 1985 were compared with re-measurements of the same samples in 2005. The comparison of 36 measurements resulted in a deviation of only 0.2 ± 0.5 % (relative) confirming the good comparability of these measurements. To investigate the effect of soiling half of each sample was cleaned with mild soap and a soft sponge to be measured separately. Some of the samples were additionally cleaned with ethanol to get a better differentiation of soiling and degradation effects.

2. Results

An overview of the losses in solar transmittance of the tested materials after 20 years of exposure is given in Fig. 3. The losses reported after 20 years are lower than after 10 years of exposure because of heavy rainfalls in the run-up to the sample collection in 2005, which led to non­negligible cleaning effects. Multi-skin sheets and sinuous plates are not considered in this study. For these samples additional losses were observed as a consequence of the special form and not as a material property. Detailed analysis of the weathering properties of the different materials are presented as follows.

Table 3. Comparison of the losses in transmittance in Davos and Rapperswil after 20 years of exposure. For a better comparability multi-skin-sheets and sinuous plates made of PC and PMMA are not considered.

Material

#

Davos; losses in % (rel.)

After

Total

cleaning

Gain from cleaning in % (rel.)

Rapperswil; losses in % (rel.)

After

Total

cleaning

Gain from cleaning

Low Fe glass

8

-0.2-4.5

-0.6-2.9

0.7-2.1

6.3-17.3

0.1-8.8

4.8-8.3

Float glass

8

0.7-2.7

0.0-2.0

0.0-1.1

7.7-12.1

0.7-6.7

4.2-10.3

PMMA plate

6

-0.2-1.4

-1.2-0.5

0.7-1.2

7.3-8.6

0.6-1.7

7.0-8.3

PC plate

5

3.3-7.7

0.7-10.5

-2.8-2.3

6.7-15.0

2.5-9.8

4.2-8.0

ETFE

3

15.7-18.7

14.1-18.3

-2.5-4.5

13.7-24.8

-0.3-12.9

3.3-14.1

FEP

2

9.0

0.0

9.0

9.2-15.3

-0.2-0.0

9.2-15.1

PVF

1

-1.0

-2.6

1.6

12.9

-2.0

15.0

UP

3

26.8-41.7

36.7-43.2

PET, PVC and PC films

6

destroyed

destroyed

destroyed

destroyed

destroyed

destroyed

2.1. Glass

Modern collector glazings are mainly made of low Fe glasses because of their good durability and high transmittance. The influence of a structured surface for reflex reduction (prism-like structure in the dimensions of millimetre fractions) on the soiling property of such glasses was investigated in this study. In Fig. 1 the losses in transmittance of samples with a structured surface are compared to the losses of the samples with a smooth surface. No difference according to their total transmittance losses could be identified between these two groups; the structure did not lead to increased soiling.

Eight samples of iron containing float glass were included in this test. No clear difference in terms of losses in transmittance could be identified between samples with the tin-rich float side or the tin — poor side exposed (i. e. oriented to the ambient). For both glass types losses in transmittance in the range of 10 % (relative) were observed in Rapperswil after 20 years of exposure (without cleaning). After ten years of exposure the losses were even higher due to less rainfall in the time before sample collection. At the rural site of Davos only small losses in the range of some percent were observed over the whole test period. As shown in Figure 2 on the example of float glass, the cleaning with mild soap lowered the losses to about half of the initial value. A further amelioration was achieved by the cleaning with ethanol. A slight increase in transmission that was observed around the wavelength of 1 pm was probably caused by a photo induced oxidation of Fe2+ impurities [2].

For all flat PMMA samples good weathering properties were observed; the losses in transmittance were in the range of the losses of glass or even slightly better. Especially the cleaning with ethanol had a stronger effect compared to glass; see Fig. 2 and Fig 3. For the 20 year old samples cleaned with ethanol, close to no significant losses in transmittance were measured. On the other hand a loss in transmittance in the range of two percent (relative to the initial transmittance) was observed

for the 20 year old glass samples from Rapperswil.

Rapperswil

 

Davos

 

0 Uncleand x Soap О Ethanol

 

0

 

40 days 1 3 10 20 years

 

40 days 1 3 10 20 years

 

image106

image107

Fig. 3. Soiling and cleanability of PMMA; mean value and standard deviation of the six tested flat PMMA

types.

In the unexposed state the UV-absorbing effect of some protective additives was observed for all PMMA types but with different efficiencies (the full lines in Fig.4). For four of the six types this UV-blocking property remained over the whole 20 years of exposure, similar to the sample declared as ‘strong UV-absorbing’ in Fig. 4. They proved to be suitable for the use as UV-blocking layer to protect other materials. For the other two PMMA-types a continuous gain in transmittance in the UV-range was observed over the duration of exposure. This deterioration of the UV — blocking mechanism did not cause any other visible material degradation. For these samples even a slight gain in solar transmittance was observed due to the gain in the UV-region. But it is known from artificial weathering of PMMA that some photo-degradation in the form of chain braking exists and affects the mechanical properties [3].

PMMA no declaration concerning UV-absorption

image108

 

image109

0.3 0.35 0.4 0.45 0.5

Wavelenght in micrometer

 

Fig. 4. Changes in UV-blocking for different PMMA samples.

The two tested PMMA sinuous plates with glass fibre reinforcement suffered from large losses in transmittance caused by fissures in the polymer matrix similar to the UP samples (see 4.5. UP).

2.2. PC

For all tested PC-types material degradation was observed including yellowing, surface roughening and even biological infestation for some samples exposed in Rapperswil. PC degradation is caused by photo-Fries and photo-oxidation processes caused by UV-irradiation. These reactions are influenced by additional parameters as humidity and temperature resulting in a roughened layer of degraded material of variable thickness at the sample surface [4]. In this case of open exposure a macroscopic removal of material from the surface was observed and quantified. After 20 years the

loss in plate thickness was for all PC-types and at both exposure sites in the range 0.1 mm. In Fig.

Подпись: PC, Davos Fig. 5. Erosive material removal in terms of losses in the plate thickness of a PC sample exposed in Davos.

5 the loss in plate thickness as a function of the exposure time is shown for one sample, which was exposed in Davos. This figure shows that the material removal started shortly after three years of exposure, which coincides with a lifetime estimation of three years determined by Ram et al. [5]. The exact loss in transmittance due to material degradation could not be determined as soiling can not be cleaned without irritating the weathered surface. A good estimation can be taken from the total losses in Davos, where for other materials as glass or PMMA the contribution of soiling to the total losses in solar transmittance was small. Despite of a visible yellowing the solar transmittance only decreased by 3.3-7.7 % of which a small part is still caused by soiling. For the tested PC-types the cleaning effect of heavy rain in the period before of the 20 years sample collection was more pronounced than for other materials. An elevated gain in transmittance between 10 and 20 years of exposure is caused by the fact that not only soiling but also peaces of weathered polymer were removed from the surface by rainfall. A regain in transmittance for long exposure times was also observed in artificial weathering tests by Tjandraatmadja et al. [4] who identified photo-bleaching of the yellowed layer to be the cause of this effect. The PC films (thickness 0.375 mm) were mechanically destroyed as a consequence of material degradation.

4.4. Fluoropolymers

All fluoropolymers suffered from unexpected high losses in transmittance. In the case of PVF and FEP these losses were only caused by soiling; by cleaning with ethanol the initial transmittance could be recovered even after 20 years of exposure, see Fig. 6. The losses from soiling were in the range of ten percent or more, even in Davos where comparable losses of glass or PMMA were in the range of only one to two percent. For collectors with FEP or PVF glazing a regular cleaning is important to reduce losses in efficiency caused by soiling. For ETFE the effect of cleaning was small (exception: sample No. 1 from Rapperswil). The losses of these samples are caused by persistent soiling or actual material degradation. In contrary to FEP and PVF, this polymer contains unfluorinated ethylene which could serve as point of attack for degradation mechanisms.

Fig. 6. Overview of the transmittances of all tested fluoropolymer sheets. The values of the ethanol cleaned
20 years old samples are compared to the values of the different soiled samples.

4.5. UP

All tested UP samples were reinforced with glass fibres. Similar to the reinforced PMMA plates, these samples suffered from high losses in transmittance. Fissures arise from dilatation differences of the UP matrix and the glass fibres due to high temperature changes or water accumulation [6]. Near the surface the UP matrix broke and the fibres poked out of the surface. This roughened surface lead to increased accumulation of soil. On the samples which were exposed in Rapperswil additional biological infestation was observed after 10 and 20 years of exposure.

Situation of European standards and quality assurance today

Table 1 gives an overview over the important currently valid European and International Standards which should ensure the quality of solar thermal collectors and PV-modules articulated in application area, standard and short description of their content.

Table 1. European and International standards to ensure the quality of solar thermal collectors and PV-modules.

Application Area

Standard

Short description

Solar thermal energy

EN12975-1,2:2006

European standard:

efficiency and durability test of solar

thermal collectors

AS/NZS 2712:2007

Australian standard:

Efficiency and durability test of solar thermal collectors

Photovoltaics

IEC 61215: 2005-4

International standard: Efficiency and durability test of PV-modules

ASTM E 1038-05

International standard:

Standard test method for determining resistance of photovoltaic modules to hail by impact with propelled ice balls

Tests of solar thermal collectors and PV-modules according to the valid standards and regulations by independent laboratories should guarantee the quality standard related to the state of the technology, mainly to ensure the continuous growth in order to make a contribution to the sustainable energy supply. Furthermore such tests should ensure the continuous development and should sharpen up the transparency of the European and International market for the consumer. Essential conditions to reach these aims are the general performance of the mandatory tests of all solar thermal and PV-modules in the run up to the market entrance. Furthermore useful, which means to the respective state of the technology and the environmental conditions well adjusted requirements within the different standards. The necessary permanent amendment of the standards is not always able to fulfill these requirements, because the process of the amendment always takes a long time and furthermore is subjected to totally different conflicts of interests between manufacturers, certifiers and political framing conditions.

Under a closer consideration e. g. of the development of the European standard EN12975-1.2:2006 we will see, that the reliability test to check the impact resistance is, unlikely to older versions, no longer an obligatory test, even though severe hailstorms in Europe in recent years definitely increased. Concerning this matter the amendment of the standard does not reflect the requirements resulting from real environmental conditions. Also the appliance of other EU-Standards which harmonize the building shell e. g. for roof lights, for the quality assurance of solar energy systems, is not simply possible. By the reason of different requirements concerning the functionality, formulations like “the choice of the used materials should take into account the risk of hailstorms” are not transferable. For this account it is also not possible to transfer the results from other studies performed up to now which describe the impact resistance against hailstorms of building. Additionally, today we have to think about the standardisation and adaption of testing procedures for a wider distribution of different technologies in the field of solar energy systems.

Horizontal fin ray trace analysis

Ray tracing analysis for the horizontal fin is shown

in Fig. 10. In Fig. 11 the optical efficiency plot for the horizontal fin ICPC shows an unbalanced

curve skewed to the right. The plot shows that the horizontal fin ICPC has greater energy collection efficiency in the evening than in the morning. As seen in Fig. 10, at an angle of 30 degrees, reflected ray striking the fin are both single and multiple reflections. As shown in Fig. 12, at an angle of 150 degrees, the only rays that are reflected to the fin are single reflection rays.

Fig. 13 shows efficiency effects of three different reflectivities of 1.00, 0.94, and 0.70. Efficiency is reduced for the smaller reflectivity values. The curve also skews more to the right for the smaller values of reflectivity.

Comparisons of different gaps between the absorber fin and the cover glass were analyzed by setting the reflectance to 1.00. As shown in Fig. 14, the efficiency is reduced as the gap between the absorber fin and the glass enclosure increases.

The building water supply design projects

The majority of the domestic water supply projects developed until now is based on very simply Domestic Hot Water (DHW) systems like electrically heated storage cylinder and gas or oil heating boilers. For Civil Engineers project designers the calculations were a quite simple task because they used to select the equipment without specific and demanding criteria. Nowadays, it is necessary to choose well the equipments to minimize its life-cycle cost because a minimum of three types of equipments must function together: the solar collector; the storage tank or cylinder

and the backup equipment (e. g. condensing oil or gas heating boilers). The system operation is also controlled by an automatic electronic unit. Not forgetting other system accessories, like pipes, pumps and valves [7,8]. The passive systems are commonly used on single residential buildings and they are more compact and easily mounted. But in multi residential buildings, there are several different consumers and it is necessary to think the most advantageous system installations option. Design professionals are dealing today with this dilemma because: they have little experience; they do not have much technical information to support their choices and there are not yet many multi­residential buildings with this technology in Portugal to learn with. Aware of this situation, a previous analysis for six system possible options (Table 1, Fig 1.) is presented next.

Table 1. Multi-residential buildings solar collector system options.

Solar

Collector

Water storage tank

Backup DWH System

Operation and maintenance management

System 1

Individual

Individual

Individual Boiler

All expenses in charge of each autonomous zone

System 2

Individual

Individual

storage tank internal burner

System 3 System 4

Collective

Collective

Individual

Individual

Individual boiler

water storage tank internal burner

A thermal energy meter for each autonomous zone or condominium management with a mensal rent

System 5

Collective

Collective

Collective boiler

A thermal energy meter for each autonomous zone

System 6

Collective

Collective

water storage tank internal burner

NOTE: Backup systems on electricity are not stimulated by regulations.

water storage tank

image082

Fig 1. Multi-residential building solar collector system options.

Подпись: solar collectors solar collectors solar collectors

Each of these design options has, in a preliminary perspective, positive and negative points.

Systems 1 and 2 — All equipments are individual — Advantages: self management of the system by each family (they adapt the system to their consumptions needs); minimal problems and conflicts with neighbours; the solar panels could be mounted with different tilt angles for each apartment adapted to their consumptions needs. Disadvantages: great number of water pipes and accessories, more building interior space needed and more heat losses; more home interior space to install equipments; high maintenance expenses for each family; if one home produces solar energy

in excess it is not possible to redistribute it; in seasonal or not occupied spaces there is wasteful production of energy; more complexity on the system mounting. System 3 and 4 — Centralized solar collector — Advantages: less initial investment ; better optimization of the collector area and of the solar south-oriented roof area; optimization of the captured solar energy and a rational distribution for all consumers; management and maintenance in charge of the building condominium administration; the energy of seasonal occupied or not occupied homes can be used by others; each individual consumer must adapt their consumptions needs to a collective system. Disadvantages: more home interior space to install equipments; heat exchanger in inverted system; the householders must pay a service not total adapted to their consumptions needs; some homes can take more profit from the solar collector system then others, it depends of the consumption needs schedule and habits. System 5 and 6 — All equipments are sheared by all householders — Advantages: less pipes and accessories; more reduced initial investment; no individual system maintenance; householders pays a total service of hot water supply; better optimization of the collector area and of the solar south-oriented roof area. Disadvantages: initial part of the system with inverted supply system (water supply starts from the first floors); complex management of the return water of the circuit into the water storage tank; variations on hot water temperatures can occur; lack of preparation of the condominium administrations firms to manage this service; system regulated to a constant level of water temperatures not adapted to consume households schedule with less energy efficient management; requires an adapted collective interior space for a high capacity water storage tank and backup equipments; the building structure must resist to higher loads due to the heavy equipments installed; the building architecture must be prepared for maintenance, repair and substitution of big equipments; reduced preparation of the local authorities to deal with this collective systems (taxes to pay etc); big size systems are more vulnerable to operation problems and damages.

The first two systems require some initial investment and in some cases can lead to collector areas that are not effectively used. The second group of systems seems to have more vantages due to the fact that can result in a better flexibility in the system management, made by both condominium administration and individual consumers. The last systems also have some advantages but represent a more vulnerable operating system.

Results — Performance Matrix

The result of the comparison of the failure detection methods against the criteria described in §2.4 is presented in table 4.1. The performance matrix shows the performance of the failure detection method based on the different criteria. A few things need to be clarified before interpreting the table. First of all, it is assumed that without automatic failure detection there is a possibility for checking data with a manual analysis. This can be effective but is costly (dependent on the level of the analysis). This could be (and is partially) applied for all methods. Therefore, the criterion is limited to automatic failure detection. Because a lot of information is qualitative, it is hard to determine how much a method will cost in future application when it is still in the R&D phase. Also the effectiveness of a method may increase based on practical experience. Furthermore, the (literature) publications in general do not provide a very accurate description of effectiveness and accuracy of the method.

Table 4.1 Performance matrix

Criteria

MM

FUKS

SP

IOC

ANN

GRS

KU

Automatic failure detection included?

++

++

++

++

++

Automatic failure identification included?

+

+-

+

Accuracy of failure detection

++

+-

?

+

?

+-

+

Accuracy of failure identification

++

+-

n. a.

+-

n. a.

n. a.

+-

Costs

(operational/hardware)

var

++ 100 €

+?

+

1190 €’

+?

10 k€2

+-

20-80€3

Monitored part of solar heating system (so far)4

var

sl

sl

sl, bs

sl

-aux

-aux

Qualitative scale: ++ yes/very good/c

leap via +- = reasona

ble to — no/very bad/expensive

? = unclear

1 IOC: only hardware

2 costs for measurement equipment, including one year monitoring

3 Costs per month for 20 year monitoring and at least 30 monitoring systems sold. The main costs are

expected for maintenance and improvement of software (between € 15 and 50 per month) [11].

4 var = variable, sl = solar loop, — aux = whole system besides auxiliary heating system, bs = buffer

discharging loop (optional for IOC)

The IOC is the first method which could result into the implementation of a monitoring and failure detection method into general use of larger solar thermal systems. It has been tested and is commercially available against a reasonable price, but it does not apply to the whole solar system. Manual monitoring, though more costly, is much easier adapted to extensive variation in hydraulics and systems. The method developed at Kassel University is still in development, but could also provide an automatic monitoring solution for large systems. It includes more sensors and a larger part of the system than the IOC approach, and can therefore also analyse individual components. For very small systems the approach followed in FUKS detects several failures at reasonable additional costs.

However, so far none of the above described approaches takes the auxiliary heating system into account, which is also an important source of errors.

Comparison between Steady State and Quasi-dynamic test method. according to EN 12975 — application to flat plate collectors

Jose Afonso1, Nuno Mexa1, Maria Joao Carvalho ^

1 INETI, Department of Renewable Energies, Campus do Lumiar do INETI, 1649-038 Lisbon, Portugal
* Corresponding Author, mioaoa. carvalho@ineti. pt

Abstract

Presently two test methodologies are available for characterization of the efficiency (thermal performance) of glazed collectors: i) steady state test methodology according to EN 12975­2: section 6.1 and ii) quasi-dynamic test methodology according to EN 12975-2: section 6.3.

The most commonly used methodology is steady-state according to EN 12975-2: section

6.1, considered applicable to all stationary collector, e. g., flat plate and evacuated tubular collectors.

But quasi-dynamic test methodology can have an important influence on the performance of a test laboratory since it allows for a quicker response in characterization of thermal performance of collectors as shown by Rojas, D. et al. (2008).

As preparatory work for accreditation of this test methodology at the Solar Collector Testing Laboratory, tests according to the methodology were preformed.

The necessary tools for testing, such as data acquisition programme and a MLR (Multi Linear Regression) tool were developed. First results of the test of a flat plate collector are analysed. In the case of evacuated tubular collectors, test sequences were obtained and analysed showing the need for further development of the MLR tool.

Keywords: Solar thermal collectors, Test methods, Steady-state, Quasi-dynamic

1. Introduction

Presently two test methodologies are available for characterization of the efficiency (thermal performance) of glazed collectors: i) Steady State (SS) test methodology according to EN 12975-2: section 6.1 [1] and ii) Quasi-Dynamic (QD) test methodology according to EN 12975-2: section

6.3 [1].

The most commonly used methodology is SS according to EN 12975-2: section 6.1, considered applicable to all stationary collector, e. g., flat plate and evacuated tubular collectors. But QD test methodology can have an important influence on the performance of a test laboratory since it allows for a quicker response in characterization of thermal performance of collectors as shown in reference [2].

The Solar Collector Testing Laboratory (LECS) of INETI is an accredited Laboratory for test of solar thermal collectors and the determination of thermal performance is made according to the SS test method (EN 12975-2:section 6.1 [1]). Two main reasons impose the need to develop the knowledge and the necessary practical conditions for use of the QD test methodology at LECS:

a) one is the improvement of use of time the collectors stay at the laboratory, allowing for a quicker response to the test requests [2];

b) the other is the fact that for some types of collectors, the QD test methodology allows for a better characterization of the collector behaviour [3, 4].

The SS test method requires steady conditions that significantly condition validation of the obtained data and therefore the time needed to conduct a test. The measured values for globlal irradiance, ambient temperature, fluid flow rate and fluid inlet temperature must not have significant variations (described in the standard [1]) for the set of data to be validated. When a steady state is not verified the set of obtained data is rejected.

The QD test method is much less restrictive. For example, the collector may continually be kept in the same position (facing south), regardless of the angle of incidence. There is also a greater variability of the measured values, which allows for more variable weather conditions. For this reason, use of the QD method to test solar collectors enables data to be obtained with less operator intervention. Because steady weather conditions are not required, solar collector tests may also be conducted over a greater annual period and in a more simplified manner. On the other hand, the calculation methodology, to determine the parameters that characterise the thermal performance of the collectors, is more complex.

In this work a short description of the test conditions for SS and QD test methods is presented in section 2. In this section, also the relevant equations and characteristic parameters for each test method are highlighted. Section 3. describes the different steps necessary or implementation of QD test method at the Laboratory. Section 4. presents and discusses the first test results obtained. In section 5. conclusions on the future development for implementation of QD test methodology, are presented.

Weather data

TRNSYS type 15-2 with weather data generated with Meteonorm 5.0 is used in the calculations. The weather data created with Meteonorm has been compared with reference years created based on a measurement period from 1992 to 2002. The amount of diffuse radiation is lower in the Meteonorm datafiles compared with the reference years. This is especially the case for Nuussuaq and Sisimiut. Further, the global radiation in the Meteonorm datafiles is higher than in the reference years. The direct radiation is therefore higher in the Meteonorm datafiles than the reference years.

2. Parameter variations

2.1. Collector tilt and orientation

Initial investigations are carried out with the validated models in order to determine the optimum tilt and orientation of the four solar collectors. The simulations are carried out with a constant mean solar collector fluid temperature of 60 °C during all operation hours of the year. The thermal performance of the collectors is investigated using the optimum tilt and orientation for each of the collectors. In Table 2 the values for the optimum tilt of the four solar collectors are given.

Table 2. Optimum tilt of the solar collectors. Table 3. Optimum orientation of the solar collectors.

Seido

5-8

Seido

1-8

Seido

10-20

Curved

Seido

10-20

Flat

Seido

5-8

Seido

1-8

Seido

10-20

Curved

Seido

10-20

Flat

Nuussuaq

75°

69°

76°

71°

Nuussuaq

-35°

-35°

-35°

-39°

Sisimiut

58°

58°

58°

55°

Sisimiut

12°

23°

10°

26°

Copenhagen

54°

51°

54°

52°

Copenhagen

Previous investigations have shown that the optimum tilt roughly follows the latitude. In this case that is true for Nuussuaq and Copenhagen. For Sisimiut the optimum tilt is somewhat lower than expected, this is because the amount of beam radiation is high. In Table 3 the optimum orientations of the four solar collectors are shown. In Nuussuaq the collectors are best situated turned 35° or 39° from south towards

east. In Sisimiut the collectors with the curved absorbers should be turned 10° or 12° from south towards west. The collectors with flat absorbers should be turned 23° or 26° towards west. In Copenhagen the collectors should be turned slightly from south towards west. The following analyses of the different parameters will use the optimum tilt and orientation for each of the four solar collectors according to the location simulated. In Fig 2 the thermal performance of the

 

image043

fluid temperature

 

Seido 5-8 —В— Seido 1-8

—9— Seido 10-20 with curved absorber —K— Seido 10-20 with flat absorber

 

Mean collector fluid temperature [ Cl

 

Fig 2. The thermal performance of the solar collectors in Nuussuaq using
optimum tilt and orientation.

 

image044

increasing solar collector fluid temperature faster in Nuussuaq than in Copenhagen. This is caused by

 

the cold climate in Nuussuaq. The mean solar collector fluid temperature used in the following

 

analyses is 60 °C.

 

image045

Distance between the center of the tubes [m]

 

image046image047

one glass tube to the neighbouring glass tube is reduced. Seido 10-20 with curved and flat absorber decreases more rapidly with an increase in the distance than Seido 5-8 and Seido 1-8. This is due to the larger numbers of glass tubes in the Seido 10-20 solar collectors than the Seido 5-8 and Seido 1-8. The increased centre distance will therefore result in a strongly increased length of the manifold. The

Подпись:optimum distance for each of the collectors at the different locations is shown in Table

3. The optimum tilts of the solar collectors are affected by a change in the glass tube centre distance. A change in the distance from the values given in Table 1 to the optimum distance values for the solar collectors located in Nuussuaq will result in an increase in the optimum tilt with up to 10°. For Sisimiut and Copenhagen a change in the distance from the values form Table 1 to the optimum values will only result in a small change in the optimum tilt, about 1-2°. The optimum orientation is also found for the optimum distance between the glass tubes. For the solar collectors located in Nuussuaq the optimum orientation is turned even more from south towards east, from about 35° to 65°. In Sisimiut a change in the centre distance of the glass tubes with the curved absorbers will result in a decrease in the optimum orientation from 12° and 10° to 2° and 5° from south towards west. The collectors with the flat absorbers located in Sisimiut have an increase in the optimum orientation from 23° and 26° to 37° and 36° from south towards west. In Copenhagen the optimum orientation changes only slightly with a change in the centre tube distance from about 2° to 5° from south towards west. An improvement of the centre distance between the glass tubes will only result in slight improvement of the thermal performance of the collectors with the flat fins at a mean collector fluid temperature of 60 °C. For the collectors with the curved fin the improvement of the thermal performance is in the range of 8-10 %, the largest improvement seen in Nuussuaq and the lowest seen in Copenhagen. Again a decrease in performance is seen for all the collectors at all three locations with a mean collector fluid temperature is 100 °C