Category Archives: The Experimental Analyze Of The Solar Energy Collector

Comparative performance with natural circulation type solar water heater

Performance of ICS solar water heater was compared with natural circulation type solar water heater. Natural circulation type solar water heater consist of a flat-plate collector made of copper fin having absorber area 1.9 m2 and doubled walled storage tank of 100 litres capacity. The detailed design has been described by Nahar [16]. Both the solar water heaters were filled in the morning and hot water temperatures were recorded regularly. The average hot water temperature was 57.3o C and 62.0oC at 1600 hours same day that was retained to 43.0o C and 50.4o C till next day 0800 hours when tap water temperature was 17.0o C in ICS and natural circulation type solar water heater respectively. This suggest that performance of ICS solar water heater is as good as of natural circulation type solar water heater while cost of ICS solar water heater is Rs 8000.00 where as cost of natural circulation type solar water heater is Rs. 12000.00 ( 1.0 Euro = Rs 60.00).

Heat loss coefficient. UL determined from thermal stagnation measurements (set-up A)

The heat loss coefficient UL is determined according to Section 2.3 for the measurements with the reference collector (not ventilated) and a collector tilt angle of 45° (data of June 10, 2006). The slope U2 and the interception with the y-axis U are found by linear regression, see Fig. 6 (a, b). In the case of (a) and (b) we find: UL= 3.8 + 0.030 AT (red line).

image156 image157

The collector’s heat capacity is not accounted for in (a), hence the derived UL-values of the measurements in the morning and afternoon do not coincide. By choosing the heat capacity Ce=9 kJ/(m2 K) in (b) the AT-dependent heat loss coefficient from the morning and afternoon match. The chosen heat capacity Ce does not have a significant influence on the UL-values as long as the measurements before and after noon cover about the same AT interval. The U-values were determined for different tilt angles and the resulting collector efficiencies have been compared with earlier efficiency tests of the same collector. The presentation and discussion of these results would exceed the frame of the present work; it is referred to [19].

Fig. 6. U-value UL as a function of the temperature difference AT between the mean absorber temperature
and the ambient temperature if the collector’s heat capacity Ce is set to zero (a) and to 9 kJ/(m2 K). The line
represents the least square fit; results based on raw data of measurements performed at set-up A.

Effect of mirror deformations

image036After having introduced the procedure for mirror deformation reproduction, the first study examines the consequences produced by the deformations of a solar trough collector. The study is based on the use of ray tracing analyses that allow controlling all the optical parameters. Considering the application of sunlight exploitation, the most important features probably are collection efficiency, angular aperture and acceptance angle. The collection efficiency E is the ratio between the light focused on the absorber and the light received by the collector entrance aperture. The angular aperture of a collector represents the total aperture angle receiving the sunlight rays and it is usually expressed as Field Of View (FOV). While the acceptance angle is the limit aperture for which the collection efficiency keeps its maximum value. The angular analyses of the solar trough collector are discussed in Sections 4-5.

The collection efficiency of solar trough has been monitored in order to evidence how much it is affected by the geometrical deformation of mirror profile.

The configuration considered in this study is characterised by the following parameters: f=780mm, D=50mm,

G=70mm, T=2mm. The absorber centre is located in the focal point of the parabolic mirror.

The simulations are carried out for various values of the conic constant K,

corresponding to elliptic and hyperbolic Fig. 3. Deformation effects versus wrtiral

edge displacement.

deformations. The results are graphically reported in Fig. 3, whose calculation parameters are detailed in Table 1. The sampling for the conic constant K (Column 2) is not linear because we preferred to choose as reference parameter the deformation at the mirror extreme. The reference quantity chosen to represent the mirror deformation is the vertical displacement of mirror extreme Delta Z, reported in Column 6 of Table 1.

Column 1 indicates the deformation type (specifying the curvature direction of the deformation with reference to Fig. 2): elliptic (internal/up) or hyperbolic (external/down). Then Column 2 reports the value of each corresponding conic constant K.

The successive three columns of the table refer to the geometrical parameters of every deformed profile. Referring to Fig. 2, the point of mirror extreme has as coordinate (Ymax; Zmax), respectively reported in Column 3 and Column 5. Still referring to the semi-profile, half of the length of each deformed profile is in Column 4, confirming that the rigid deformation is calculated keeping almost unchanged the total length of the mirror. The calculation is approximated considering only integer values of mirror semi-aperture Ymax.

Table 1 — Effect of mirror deformations.

Deform.

Conic

Mirror

Mirror

Edge

Vertical edge

Collection

type

constant K

semi-aperture

semi-length

coordinate

displacement

Efficiency

(curvature

direction)

Ymax (mm)

(mm)

Zmax (mm)

Delta Z (mm)

E (%)

Elliptic

(int./up)

-0.5713564

896

947.2

267.1

7.50

63.71

Elliptic

-0.725855

898

948.0

264.6

5.00

74.60

Elliptic

-0.7903212

898

947.3

263.1

3.50

82.34

Elliptic

-0.8381976

899

947.1

262.6

3.00

84.67

Elliptic

-0.8602855

899

947.8

262.1

2.50

86.25

Elliptic

-0.954064

900

948.1

260.6

1.00

86.10

Elliptic

-0.9769435

900

947.9

260.1

0.50

86.15

None

-1

900

947.7

259.6

0.00

86.14

Hyperbolic

(est./down)

-1.0232346

900

947.5

259.1

-0.50

86.17

Hyperbolic

-1.0735775

901

948.2

258.6

-1.00

86.16

Hyperbolic

-1.1452315

901

947.6

257.1

-2.50

86.16

Hyperbolic

-1.1694875

901

947.4

256.6

-3.00

86.15

Hyperbolic

-1.221419

902

948.1

256.1

-3.50

82.32

Hyperbolic

-1.2962255

902

947.5

254.6

-5.00

74.33

Hyperbolic

-1.4532411

903

947.4

252.1

-7.50

64.16

Finally the effect of mirror deformations is reported in Column 7 as collection efficiency values; the calculation has been carried out with linear sampling in Delta Z. Figure 3 visualises the variations of collection efficiency as a function of the mirror deformation parameter Delta Z.

The sign of the Delta Z values reported in Table 1 is in agreement with Fig. 2: the vertical edge displacement is positive in the elliptic case (int./up in Fig. 2) and is negative in the hyperbolic case (ext./down in Fig. 2).

In conclusion for the examined trough collector the effect of elliptic deformations is significant only for vertical edge displacement Delta Z > 2.5 mm.

While in the hyperbolic deformation case the effect of solar trough deformations is considerable only for absolute values of Delta Z > 3 mm.

Thermotropic hydrogels

Thermotropic hydrogels are chemically or physically cross-linked polymer networks which are poured with appropriate water content. At low temperatures the aqueous solution is dissolved homogeneously on the molecular level, so that a clear state is achieved. Above the switching temperature scattering domains are formed by an aggregation of the polymers and/or by separation of free water from the polymer network [2,3,7,8].

Numerous thermosensitive hydrogels exhibiting a cloud point are discussed in the literature. Examples are aqueous solutions of polyvinylethers [9], polyvinylalcohol [10], poly(N-substituted acrylamide) [11], poly(N-vinyl alkylamide) [12], ethyleneoxide-polypropyleneoxide [13], copolymers of N-vinyl — 2-pyrrolidone and hydroxyethylmethacrylate [14], poly(methyl-2-acetamidoacrylate) [15],

poly(methyl-2-acetamidoacrylate-co-methyl acrylate) [16], polyvinylacetone [17], poly(N-oxazolines) [18] or cellulose derivatives [4].

As to performance properties thermotropic hydrogels possess a high potential for solar collector applications. The materials are characterized by a high transparency in the clear state (>82%) along with low haze and a change in solar transmittance by 77% at temperatures adjustable between 5 and 100°C. The materials exhibit an excellent switching performance with a steep switching gradient, a high reversibility, and low hysteresis [4,19,20,21]. But as the transition is based on physical interaction between the components the materials have several problems with long-term stability and ageing. Furthermore the materials have to be UV protected [4]. The water makes the thermotropic hydrogel susceptible to freezing and limits the operation temperature range [21]. Thermotropic hydrogels filled into the intervening space of a double glazing, place high demands on sealing of the glazing. If not sealed properly the layer will dry out [2]. If synthetic materials are used thermotropic hydrogels are quite expensive. If biopolymers are used costs decrease significantly. However, the use of biopolymers requires a thorough protection against microorganisms [22].

A thermotropic hydrogel glazing, a sandwich of two glass panes and the encapsulated hydrogel, is announced by Affinity Ltd. (Japan) [2]. However, no thermotropic hydrogel is described currently that exhibits the working temperature range and switching temperatures required to provide adequate overheating protection for solar collectors. Further developments should focus on the adjustment of adequate switching temperatures (55-80°C) and the improvement of the long-term stability.

Introduction of a new solar air collector

Advantages of a solar air collector system in general

• Air is for free.

• Air cannot freeze.

• No deterioration of the heat-transfer-fluid.

• No high pressure thrusts during a stagnation situation.

• No costs for secure systems, e. g. safety valves, membrane expansion vessels.

• No environmental or security problems in case of system leakages.

• Collector is intrinsically safe to itself and to the system.

• Solar air collectors enable easy and cost-effective highly scaleable solar thermal collector fields.

Disadvantages of a solar air collector in general

• Air has a lower heat capacity than water.

• Movement of air can cause aerodynamic noises.

• To achieve the same mass flow of air then of water, you need more electrical energy for the fan than for a water pump.

First prototype in Freiburg and demonstration plant for solar cooling in Bergamo

Our first prototype collector was installed in Freiburg, Germany in Dec 2005 [1]. The evaluation of measurements that were performed during summer 2006 confirmed the theoretically derived performance parameters [3].

In summer 2006 the second linear Fresnel process heat collector with 132 m2 aperture area (66 kWp, th) was installed on the roof of a building of the company Robur S. p.A. in Bergamo, Italy, to power a H2O/NH3 absorption chiller with 17 kWth nominal cooling power (5TR). Since September 2006 the solar cooling system is continuously operated and monitored.

[4] Present address: Institut fur Solarenergieforschung GmbH, Am Ohrberg 1, 31860 Emmerthal, Germany.

Total solar energy

Equation (2) allows evaluating the total solar energy on the collector scope.

n

G = ^ Idt (2)

.=1

1.1.2 The actual useful energy

Equation (3) allows calculating the actual useful energy extracted to the solar energy.

Q = mCfAT = mcf ((out — T. n ) (3)

1.1.3 The instantaneous efficiency

Equation (4) allows calculating the instantaneous efficiency defined as the ratio of the actual useful energy extracted to the solar energy intercepted by the collector.

Подпись:Q

AgG

1.1.4 Reduced temperature difference

When the temperature at the collector inlet is employed, the reduced temperature difference is calculated as:

1.1.5 Graphical presentation of instantaneous efficiency

Graphical presentation shall be made by statistical curve fitting, using the least squares method, to obtain an instantaneous efficiency curve of the form

Подпись: (6)Подпись: (7)

image062
Подпись: (5)

П = П — a1T* — a2G(T*)

or

П = П — UT

Integrated storage collectors and thermosiphon systems

Integrated storage collectors and thermosiphon systems are typically installed in climates without freezing during the winter season. The storage is the collector (ICS), Fig. 1 (c), or is closely con­nected to the collector (thermosiphon system), Fig. 1 (b). These systems have compact designs, are normally on-roof mounted and a relatively easy supplement or replacement of a domestic hot water (DHW) boiler. Here the introduction of polymeric materials contributes to considerable reduction of weight and opens for innovative, functional designs and shapes (see examples, Fig. 9).

2.1 Flat plate collectors and favourable system design

Europe has a sophisticated market for different solar thermal applications, as systems for DHW preparation or combined solar heating systems for DHW preparation and space heating (combisys- tems) in single — and multi-family houses, hotels and large-scale plants for district heating [3]. Due to the heat loss such collector systems require a collector cover (glazing).

Already during 1977-1985 substantial R&D was performed in the US on polymeric solar thermal collectors, first of all for solar DHW systems [6, 7]. Presently there exist few commercial glazed collectors with polymeric absorbers. These are mostly designed for low-pressure systems, which are open vented and have pure water without antifreeze additives as heat carrier. Depending on the

application polymeric absorbers have different design criteria to the solar collector system than conventional, metal-based absorbers; some designs have a built-in overheat/freezing protection mechanisms for the solar collectors, e. g. drain-back technology, ventilation or other designs to avoid thermal stagnation or freezing of the heat carrier in the solar loop.

For polymeric absorbers of commoditive or engineering plastics the operative system temperature should be as low as possible in order to minimize thermal load for the polymer, favour long ser­vice-life and high overall system efficiency. Glazed absorbers of high temperature performance plastics with spectrally selective coatings are not in the market yet, but might/will contribute to serve the retrofit market of small solar DHW systems.

image112
image113

Examples of hydraulic system designs, which are favourable for the application of glazed poly­meric collectors, are — among others — solar heating systems with large DHW demand (sport centres, hospitals, nursing homes, etc., see Fig. 2 (a)); further solar combisystem with large heat store (~100 l/m2 collector area), low temperature heating system (floor/wall heating), avoiding intermit­tent heat exchangers between solar loop, storage volume and heat emission system (Fig. 2 (b)).

(a) (b)

Fig. 2. Examples of hydraulic system designs, which are favourable for the application of polymeric collec-
tors; (a) solar DHW system covering large DHW demand and (b) solar combisystem.

Comparing Measurements with Expectations

With collector parameters and the measurement data for irradiation, ambient temperature, inlet and outlet temperatures of the collector, it is possible to calculate a theoretically expected collector power. The comparison of expected and measured collector power is shown in Figure 4.

Additionally as a result of Raytracing calculations the theoretical optical efficiency was plotted over time in the same graph. The oscillation of the efficiency is produced by the shadow of the receiver, which moves from mirror row to mirror row. At 14:45 the shadow moves out of the mirror field, so that the optical efficiency theoretically increases up to 51% at 15:37 on that day.

Table 3. simulation parameters

Specular reflectivity of primary and secondary mirrors

R1=0.92, R2=0.77

Transmissivity of glass tube

0.96

Absorptivity

0.94

Circum solar radiation CSR

17%

Total mirror error

atot=10 mrad

image143

Trestle and bearings

The design of the trestle aimed at the possibility of rotating the parabolic trough about a single axis automatically (east-west or north-south) and about the second axis manually. The possibility of the manual tracking allows the reduction of the IAM (incident angle modifier) by positioning the para­bolic trough with the ideal angle to the sun, especially during the experimental phase.

As identifiable in Fig. 2, the trestle is built up of a frame base of square steel bars with the dimen­sions of 2 m x 2 m, which gives the construction the necessary stability. In the middle of the frame base, a vertical square bar is fixed. With a bearing at the top, the vertical square bar holds the car­rier for the collector body. The whole construction is mounted on concrete blocks to stabilize it and to avoid slipping. Its weight is about 60 kg without the concrete blocks. The main disadvantages of the trestle are the vast dimensions, the high mounting expenditure, the high material usage and the restricted possibility to build several parabolic troughs in a row.

As mentioned above, the collectors’ rotation axis is equal to its caustic line, which means that the trough rotates about the absorber system. For this reason the absorber always remains in a fix posi­tion, so that the inlet and outlet pipes can be assembled inflexible.

The bearing of the trough comprises of a combination of a fixed bearing at the bottom and a loose one at the top. They are mounted with the carrier, so that the trough rotates between the carriers’

blades. The absorber system is inserted into the trough from the outside by sliding it through the fixed bearing, which is designed with an appropriate opening. The absorber system is positioned between the bearings on a cushion to protect the sensitive glass pipe.

If several parabolic troughs are arranged in a row it is not possible to change the absorber system of one trough in the middle of the row in the case of damage. That is a big disadvantage in view to the handling since it would be necessary to remove several parabolic troughs to change one ab­sorber.

Planned optimization: It must be possible to arrange several collectors in a row. For this reason the possibility of a manual tracking about a second axis will be relinquished, so that automatic tracking will be feasible only about one axis (east-west or north-south). In a north-south arrange­ment angles of rotation of 160°, respectively in an east-west arrangement of 80°, will be necessary.

Between two collectors in the row, there will be only one post, at which both collectors are fixed from each side. To reduce costs, the 1 m high post will be designed of standardized steel I-section, which is zinc coated.

The bearing concept will be assembled of seven components which will meet the following de­mands: bearing of the collector (static and dynamic load), support of the absorber system, transfer­ence of torsion moments from one collector to the next collector in a row and compensation of lin­ear thermal expansions to avoid restraints. In addition, the bearing must balance the draft of the collector body (see above) to make a rotation about its axis possible. Mainly because of the last mentioned reason, there is no product on the market which meets the mentioned demands. Because of this it is necessary to develop a new concept.

To compensate the above mentioned side plates’ draft of 4°, all components are screwed on ac­cording slant mounting plates, which are also screwed on both side plates of the collector. The bearing itself consist of a bearing-seat, in which a sliding bearing is forced. Due to the open inner diameter of 65 mm, the connecting pipes of the absorber system fit through this opening. The rota­tion of the collector will be around a hollow shaft, which sticks in the sliding bearing and will si­multaneously be used as a support for the absorber system. The hollow shaft is designed with a flange, so that it remains in its fixed position by screwing it at the post. To transfer the torsion moment, the slant mounting plates are connected by stiffening components between the collectors.