Category Archives: The Experimental Analyze Of The Solar Energy Collector

The Ray-Tracer software

image050 image051 Подпись: R r image053 Подпись: (20)

The software uses the equations (1) to build the concentrator and the equations (2) — (19) to calculate the radiant flow density on the input aperture and the optical concentration factor [16, 17, 18, 19]. We consider that the radiation that is incident on the photovoltaic cell is fully absorbed by it. The radiant flow density on the output aperture, Brec, is calculated with the equation:

The meanings of the measures in the (20) equation are: Bconc — the radiant flow density on the

input aperture, R — the input aperture radius; r — the output aperture radius; Ninitial — the initial numbers of rays; Nabs — the number of rays on the output aperture considered to be absorbed of photovoltaic cell. The measures given by the user are: the input aperture radius, R (mm); the output aperture radius r(mm) considered to be equal to the receptive cell radius; the position of the receptive cell, H0; the initial moment of the measures, t0; the final moment of the the measurings, t1; the incrementation of the time At; the inclination angle of the roof, s; the month of the year, l; the day of the month, %.

The calculated and shown measures are: the intensity of the solar, Bn; the angle of incidence of the radiation on the input aperture, в; the density of the radiant flow in the plane that contains the input aperture, Bconc; the density of the radiant flow in the plane that contains the photovoltaic cell Brec; the optical concentration factor, Coptic; the quantity of energy that passes the input aperture during the measurements, Qconc; the quantity of energy received by the cell, Qrec ; the concentrator’s efficiency q.

The concentration is efficient if the optical concentration factor is bigger than 1. With the help of the tables or of the graphics we can determine the maximum angle of incidence 6max and the time period for which 6<Bmax.

Modeling Software

Theoretical modeling was done applying a software generated by AEE INTEC (Gleisdorf, AUT), which provides a comprehensive theoretical mathematical description of flat-plate solar collector performance [4]. It allows for the evaluation of collector conversion factors and for the determination of efficiency graphs. For the present investigation specific functional elements, such as thermotropic layers have been implemented additionally. In real mode of operation the maximum absorber temperatures are a function of climatic conditions, incident irradiation and angle of incidence. To evaluate the frequency distribution of the maximum absorber temperatures in the course of the year a statistic module was implemented. Basically this software considers

• direct, diffuse and scattered solar irradiation reaching the absorber,

• transmission, absorption and reflection on multi-layer collector glazing including the thermotropic layer,

• solar absorption and reflection as well as thermal emission of absorber coatings,

• heat transport from absorber to fluid

• heat losses by convection, thermal conduction and radiation due to collector glazing and casing and

• climate data for Graz (for the statistic module).

Oxide titanium thin films with organic pigment

Titanium oxide films were deposited on glass and metallic substrates with dc magnetron sputtering and also with pulsed dc magnetron sputtering at room temperature from 7.5mm titanium target with 99.995% purity, in reactive atmosphere with mix of argon and oxygen gas. The sputtering was carried out using a 5kW power supply, with possibility to change pulsed frequency from 5 to 350 kHz, and reverse time from 0.4 to 5ps, and reverse voltage 10% of operation voltage.

The gas argon was fed to the chamber independent of the reactive oxygen. Flows for both, argon and oxygen were regulated using Bronkorst mass flow controller, operated by Bronkorst control unit. Pressure monitoring in the sputtering chamber was made by Balzers penning and pirani with TPG 300 monitor unit.

All experiments were performed with target to substrate distance of 6cm, and depositions were started after pumping the chamber to a base pressure of 1×10-6mbar. Reflexion data were taken on a Perkin Elmer Lambda 9 NIR/UV/VIS Spectrophotometer and solar absorption was evaluated to be in account solar spectrum (AM1, 5) partition in 20 wavelength ranges of equal energy. Thermal emissivity was measured with an AE emissometer of Sevices & Services at an equilibrium temperature of 82°C and after adequate calibration. Optical properties values were evaluated with an uncertainty not exceeding ± 1%.

First experiments had in consideration the already known dependence of film morphology relatively to deposition parameters, and allowed to narrow the possible range of values variation for deposition parameters, with the final objective to reach highest as possible solar absorber selectivity [3]. Some coatings were prepared with constant oxygen flow rate and others with oxygen flow rate increasing gradually from the beginning to the end of deposition. Pulsed dc frequency and reverse time were also parameters taken in consideration.

The third stage of the experiment

The surface temperature of the collector should have a proportional increase to get adequate airflow exit. Thus optimum geometry should be designed for the collector. For this purpose, the front and back faces of the collector are coated with foil paper both to constitute new air chambers and to stabilize the surface temperatures. (Figure 9) In which case, the selective surface isn’t muddy. It is metallic color. Once more the system is run and measurements are taken.

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Figure 9: Measurement points of the collector coated with foil paper Resultant, both air entry and air exit are seen at the collector. The collector coated with foil paper doesn’t absorb heat as much as muddy selective surface from infrared radiation lamps. The system keeps less heat therefore entry velocities decrease significantly and pressure difference comes to normal values and air exit is seen. Thereby measurements could be taken from all chambers and holes. (Table 2)

Consequently, the back and the front face of the aluminum profile is enclosed with it’s standard cover. In this case, new chambers are formed for air entry and exit. (Figure 10) The wavelength selective coating is applied to the front face of the collector profiles that have a low emissivity of energy in the infrared wavelengths. Moreover the back face of the collector is insulated to generate effective convective air flow on the basis of the test results. The front and back face of the collector are framed by aluminum framing. (Figure 11) The insulated backing of the solar collector serves the dual function of blocking heat transfer between the classroom and the back of the profiles (and air chambers) and insulating the window area to reduce heat transfer from the classroom to the outdoors. [4]

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Figure 10. The aluminum profile enclosed with it’s standard cover.

 

Figure 11. View of top-bottom of the collector

 

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Figure 12. F*yranometer on the collector.

Table 2. The measurements of the collector coated with foil paper.

Entry velocity (m/s)

Exit velocity (m/s)

Entry temperatures (°С)

Exit temperatures (°С)

0,19

0,15

24,0

36,5

2

0,12

0,09

24,1

36,5

3

0,20

0,14

24,0

36,6

4

0,21

0,16

24,0

36,6

5

0,13

0,10

24,1

36,6

6

0,21

0,15

24,0

36,6

7

0,22

0,17

24,0

36,6

8

0,13

0,10

24,1

36,6

9

0,22

0,15

24,0

36,7

10

0,23

0,17

24,0

36,7

11

0,12

0,11

24,1

36,7

12

0,25

0,16

24,0

36,7

13

0,26

0,17

24,0

36,7

14

0,14

0,10

24,1

36,6

15

0,25

0,17

24,0

36,7

16

0,26

0,16

24,0

36,6

17

0,14

0,10

24,1

36,6

18

0,24

0,16

24,0

36,6

19

0,24

0,16

24,0

36,6

20

0,12

0,10

24,1

36,6

21

0,24

0,15

24,0

36,6

22

0,20

0,12

24,0

36,9

23

0,18

0,11

24,0

36,6

24

0,21

0,13

24,0

36,9

25

0,19

0,12

24,0

36,6

26

0,22

0,14

24,0

36,9

27

0,20

0,13

24,0

36,7

28

0,22

0,10

24,0

37,0

29

0,20

0,11

24,0

36,7

30

0,21

0,14

24,0

37,0

31

0,19

0,15

24,0

36,7

32

0,23

0,12

24,0

37,0

33

0,20

0,13

24,0

36,6

34

0,21

0,14

24,0

36,9

35

0,19

0,13

24,0

36,6

mance of the collector geometry is analyzed using Computing Fluid Dynamics (CFD). Measured temperature and velocity values at the exit of the collector are compared with CFD analyze results and they are coherent. [5]

Generation and selection of concepts

The trends and ideas resulting from the series of workshops were analysed and combined into twelve sketches of the solar thermal collector of the future. The purpose of these sketches was to visualise the outcomes from the workshop, and to get an impression of the consequences and feasibility of the realisation of these trends. In a multi-criteria analysis, the sketches were analysed and ranked. The highest-scoring two sketches were then worked out into two primary concepts, one for the longer term, aimed at 2030, and one for the shorter term, aimed at 2015.

Two concepts

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The first concept, aimed at 2015, is based on a passive house, which roof is covered with a roof — integrated side-by-side solar system with both PV panels and vacuum tube collectors. The heat generated in the collectors is stored in a thermochemical seasonal heat storage. The energy flows, both heat and power, are regulated by an intelligent energy management system. This system is able to fully provide the domestic energy use (assumed to be 6.5 GJ for space heating, 12 GJ for DHW, and 3,500 kWh for electricity), covering the energy demand of both the user and the building itself.

The second concept, aimed at 2030, is based on durable, modular construction elements, in which energy production, energy storage, building insulation, and an indoor climate system can all be integrated. By choosing the appropriate construction elements, the building can be completely tweaked to the user’s particular demands. Although the energy production of this concept is of course very dependent on the chosen configuration, many energy-producing configurations are possible, in which the total energy production exceeds the total domestic energy use.

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The WAELS project is a cooperation between ECN, TNO, and the Eindhoven University of Technology, and partly funded by SenterNovem, an agency of the Dutch Ministry of Economic Affairs. The work described in this abstract was partly carried out by an Industrial Design student of the University of Twente.

Reference

image076Journal of Heat and Mass Transfer 48 (1) (2005) 53-66.

Absorber

Example (a) in Fig. 8 shows a thermosiphon collector with blow-moulded absorber of PE. (b) and (c) are flat-plate collectors with 10 mm PC twin-wall sheets as collector cover. In example (b) is

the absorber made of silicone rubber tubing partly compressed between metal plates. A drain-back collector with stiff, extruded absorber plate of polyphenylene ether/polystyrene (PPE/PS) blend is shown in (c). Example (d) is a solar air collector of PC where the extruded structure has a transpar­ent surface as collector cover and a black rear side as absorber. A transparent, hollow roof tile of polymethyl methacrylate (PMMA) was developed as solar collector with a black liquid as absorber and heat carrier (e). Example (f) represents a hybrid absorber of EPDM pipes pressed into a metal, modular roof system with omega-shape profile; it is available with/without glazing and as a hybrid PV/solar thermal collector. The examples (c), (d) and (f) in Fig. 8 are modular systems of poly­meric or hybrid-polymeric collectors, which are available in various lengths and designed for re­placing conventional roof — or facade covers.

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Fig. 8. Glazed collectors with polymeric components

(Img. source: Solco (AUS), Solartwin (UK), Solarnor (N), PUREN Gmbh (D), Geasol (SI), MAAS Profile Gmbh & Co (D))

Ventilation flap

If ventilation should be applied as an overheating mechanism in polymeric collectors, it is necessary to limit heat losses when the collector is operative. Hence a flap at the top of the collector frame was introduced, which should be opened when a critical temperature in the collector/of the absorber is reached (Fig. 1). Due to the high thermal expansion coefficient of polymeric materials (~10-4 K-1), the temperature of the absorber itself and its longitudinal

expansion can be used to trigger the opening of the flap. This is a simple, self-controlled mechanism, which will also work during power failure.

Product description

Подпись: Fig. 3. Tracker’s components. The equatorial tracking system with linear actuators developed on the platform of Transilvania University of Bra§ov is presented in fig. 3. Panel 1 is assembled on a frame made of L profiles 2 mounted along the panel welded together by another two L profiles numbered 3. At the middle of the L profiles 3, bearings 4 and 5 together with the U profile central beam 6 create the daily rotational axis of the panel. The daily rotational movement (with angle P) is performed with the screw linear actuator 10, attached by the rotational link 11 on the rectangular beam 9 mounted with screws on the central beam. The end of the actuator’s screw is attached with rotational link on the bolt of the brida 12 mounted on the L profile 3.

The altitudinal (season) movement is performed around the rotational axis made of bearings 7 linking the middle of the central beam 6 with the pillar 8. The seasonal rotational movement (with angle y) is performed with the screw linear actuator 14, attached on the pillar 8 with the rotational link 15. The end of the actuator’s screw is attached with rotational link on the lever 13 mounted on the central beam 6.

Подпись: Fig. 4. Daily rotational axis.
Подпись: Fig. 5. Seasonal rotational axis.

With the purpose of diminishing the friction, double sealed ball bearings are used for the main rotational axes of the tracking system. Figure 4 presents a longitudinal section through the bearing 4 of the daily rotational axis. The journal part 16, on which the inner ring of the bearing is mounted, is assembled on part 3. The cap 17, on which the outer ring of the bearing is mounted, is assembled on the part 18 belonging to the central beam 6.

Figure 5 presents a longitudinal section through the bearings 7 of the seasonal rotational axis. The journal part 19, on which the inner ring of the bearing is mounted, is assembled on the central beam 6. The cap 20, on which the outer ring of the bearing is mounted, is assembled on the pillar 8.

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Figure 6 presents extreme positions of the panel rotating around the daily axis (a, b) and around the seasonal axis (c, d).

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Fig. 6. Specific positions.

Conditions, dimensions and material

All calculations started out from common used design, dimensions and material of existing collectors. Sizes of 1 to 3 m2 were examined and different relations between length, width and height were investigated. The used directions of x, y and z can be seen in figure 1. The collector model consisted of a sheet of glass and an absorber formed as a tub where the edge of the tub was fixed on the glass sheet. The connection between glass and absorber in the model was the simplest possible where the absorber mesh was connected to the glass mesh as a "T" in an imagined cross section. The cavity was the volume between glass and absorber. All folds in absorber had a radius of 5 mm. The glass was sticking out 40 mm outside the absorber tub. 12 mm tubes with 0.85 mm material thickness were supposed to be placed on the backside of the absorber with a centre distance of 120 mm.

Подпись: Fig. 1. A quarter of a solar collector seen from the back.

Material properties comes from soda glass, copper and aluminium. A complete list of used conditions can be seen in table 1.

Table. Physical properties and geometries used in the mathematical model.

Properties of materials in solar collector

Glass

Copper

Aluminium

Young’s modulus (GPa)

69

118

70

Poisson’s ratio, (-)

0.23

0.3

0.3

Thickness absorber tp (mm)

0.25

0.5

Geometry properties

Tube spacing, dtt (mm)

120

Tube outer diameter, 0t (mm)

12

Tube thickness tt (mm)

0.85

Radius of curvatures of absorber tub (mm)

5

Simulating conditions

Ambient Pressure (kPa)

100

Minimum mean temperature inside collector (K)

240

Mean temperature inside stressless collector (K)

300

Maximum mean temperature inside collector (K)

500

2.1. Calculations

The Solar collector box had known dimensions, and it was possible calculate the volume enclosed by a list of faces (Venclosed) at different pressures. We supposed that when T=300K, the pressure inside

and outside of the box is 100kPa and all stresses in the material is 0. Then we wanted to find a P when T=500K. We used the ideal gas law for finding the pressure and volume at the new temperature.

Подпись: Haigh diagram era Fig. 2. Haigh digram

Estimations of fatigue were made for having an understanding of if the stresses will imperil the function of the expected lifetime of the construction. A Haigh diagram, which can be seen in figure 2, is a help when doing valuations of fatigue.

Подпись: -+ - Подпись: = 1 Подпись: (1)

The stresses are divided in to a static and a dynamic part. The static stress is put on the x-axis and dynamic stress on the y-axis. We use Goodman’s rule as the limit between the safe and the unsafe area[1]. Goodmans rule can be seen in (1).

Vtjts

The Goodman’s rule will give an line between (oUTs,0) and (0,oe) in the Haigh diagram. Plastic deformation is also unwanted; therefore a line representing that is added as an additional limit. The smallest value from Goodman’s rule and the plastic deformation line is then used as the safe area. [2]. The formula of the plastic deformation line can be seen in (2).

Подпись:^ a, p0.2,allow ^ m, p0.2,allow ^p0.2

The plastic deformation limit will be a line between (op0.2, 0) and (0, op0.2). This means that the limit is op0.2 no matter the proportions between dynamic and static stress which gives us the allowable stress expressed as in (3).

Подпись: (3)^T 500, allow ^T 500,p0.2 ^p0.2

With table values of Copper and Aluminium, the maximum allowable stresses will become as in table 2 [2].

Development and field testing of solar collectors with the new absorbers

The small FracTherm® absorber had been mounted in a collector casing and tested as a stand-alone device, which means that interconnection between several collectors arranged in parallel had not been relevant. Therefore one important future task is to develop concepts for header channels and appropriate adaptors. It is important that the cross section of the headers is large enough in order to keep the pressure drop as low as possible and to ensure a uniform flow distribution in all collectors.

There are basically the following different possibilities to build header channels:

• Additional header tubes connected to the roll-bond absorber

• Wide, flat header channels integrated into roll-bond absorber (same height as absorber channels)

• Narrower, higher header channels integrated into roll-bond absorber (much higher than absorber channels)

• Other concepts generating channels by forming parts of the roll-bond absorber

These different concepts have to be investigated in detail especially with respect to feasibility, production tolerances, costs, pressure resistance, corrosion, coating, flexibility.

Подпись: Fig. 4. Collectors with non-rectangular shapes (left) and example of how a non-rectangular FracTherm® absorber could look like (right)

The FracTherm® algorithm allows generating channel structures on given areas which need not to be rectangular. One of the industry partners of the BIONICOL project already now offers custom-made non-rectangular collectors which fit the shape of the roof (Fig. 4, left). However, these are individually made meander absorbers, resulting in high production costs and high pressure drops in operation. With FracTherm® it is possible to generate channel structures which fit the given boundary form and lead to a uniform flow distribution as well as a low pressure drop (Fig. 4, right). Since roll-bonding is a series production process, it will not be economical to produce individual absorbers, but it would be possible to offer some standard non-rectangular absorbers which might be adapted to customers’ specifications to a certain degree.

The collectors developed in the BIONICOL project will be tested at Fraunhofer ISE. The collector performance tests will be carried out with aluminium roll-bond prototype collectors with different absorber designs (meander absorber, harp absorber, FracTherm® absorber) as well as with state-of-the — art collectors for comparison. The tests will be carried out according to the European standard EN 12975 (collector efficiency curve, exposition test, rain penetration test, internal and external thermal shock test, pressure drop). Moreover, the emptying behaviour of the collectors will be tested

and optimized which is important with respect to stagnation behaviour and a potential application in drain-back systems. Stagnation tests can be carried out using the indoor solar simulator of Fraunhofer ISE (determination of steam penetration depth, “steam producing power”, volume of fluid remaining in the absorber).

Field test collectors will be produced in a small-scale series production and installed in different test sites in Europe (maritime, urban, rural and industrial atmosphere). The field test collectors will be operated for one year. It is planned to publish the results of the final evaluation in several workshops.