1.1 Parabolic body

The stainless steel body is designed by four parabolic shaped ribs (two L-shaped and two T — shaped) which are screwed with two side plates and a trough sheet. Thus the corpus is built of seven components hold together by numerous screws. This results in a long assembly time and high expenditure of material. Since the components are individually fabricated and made of high — grade steel, the production costs are high. In addition the inaccuracy of each single component con­tributes to the total inaccuracy of the system, which in generally results in a decrease of the effi­ciency. The prototype collector at the test facility of the SIJ is shown in Fig. 2.

As a requirement for a high efficiency, the main function of the body is to guarantee the shape ac­curacy of the collector. Although the mentioned ribs were given the parabolic shape by a CNC — based bending machine, the ribs partially lost the given shaped because of material tensions. For this reason the body is supported by two tensioning and counteracting ropes as can be seen in Fig. 2. Nevertheless the shape is far of the ideal parabolic shape, especially at its borders. The DLR determined the irregularity with a photogrammetry, and figured that only about 90 % of the total collector area can be considered to be usable (see Fig. 3).

Planned optimization: The aperture area will not vary from the prototype. But in order to achieve a better shape accuracy of the body, it will be constructed of less individual parts. A deep drawing process promises good results with regard to shape accuracy and cost reduction in a series produc­tion. The body may consist of different materials like stainless steel, aluminium or even plastics. Last alternative has to be considered properly, due to the presumed linear thermal expansions. Be-

cause of the deep drawing process it is necessary to provide the side plates with a draft of 4° to en­sure the save removal of the component from the deep drawing tool. This draft also allows the stacking of the troughs for transport, thus providing the option of transporting high numbers of col­lector bodies in narrow space.

Nevertheless, the body of the first new prototypes within the optimization phase will be fabricated by rolling and welding, since the costs for a fabrication tool for deep drawing are too high for the aspired numbers of prototypes. But the rolled and welded design will be of a shape that allows deep drawing at a later series production.

New composite absorbers were developed on aluminium and cooper substrates by spray pyrolysis technique. To enhance the heat-transfer process a thin (0.5 mm) sheet of high-thermal-conductivity material (Al, Cu) was used as substrate. For increasing the adherence properties, the aluminium surface was subjected to chemical rinsing in alkaline solutions (10-15 g/L NaOH, 30-50 g/L Na2CO3, 30-50 g/L Na3PO4). Afterwards the samples were anodized in nitric acid solution for 10 minutes at 3A. The copper substrate was mechanically polished with sandpaper No 800, and then washed with deionised water before each deposition.

Due to atmosphere and/or thermal oxidation, the copper substrate surface can actually be described as Cu/CuOx, while the aluminium substrate consists of a thin alumina layer on aluminium, Al/Al2O3. The black nickel was deposited on Al/Al2O3 and Cu/CuOx (samples of 31.5 x 34 cm) in the Thin Film Laboratory from the Centre: Product Design for Sustainable Development. Thus, the absorber plates have the following structures:

(1) Cu/CuOx/NiO;

(2) Al/Al2O3/NiO.

Aqueous solution of Ni(CH3COO)24H2O (99% Across Organics) was used as precursors. Low concentration (50 ppm) of maleic anhydride copolymers (Petru Poni Institute of Macromolecular Chemistry, Romania) were used to tailor the surface morphology. In order to minimize the reflection losses a TiO2 thin layer was deposited with the same technique (SPD).The optimized deposition parameters were previously reported [7-9]. The solar absorptance (aS) and the thermal emittance (sT) of the I. R. component layers and full absorbers, determined from the reflectance spectra [10] are presented in Table 1.

The addition of the antireflexive coating has no major influence on the selective coatings, still it was used because the environment resistance is enhanced by the titania film.

In developing the layered selective coatings the reciprocal infiltration is targeted. Thus, the first (oxidized) layer has a very high roughness and the second (and third) SPD deposited layers are actually filling a mezzo-porous surface and develop a smooth, regular morphology. The surface roughness of the absorber effectively increases the thermal emittance, inducing radiation reflection and scattering compared to a smooth surface [11], Table 1. The solar absorber (2) is smoother then (1), thus better collector’s efficiency of the solar collector is expected.

Considering the results, one can conclude that an average roughness of 30 nm is enough to insure convenient emittance values, below 0.1.

7th to 10th October, Lisbon — Portugal *

N. M. Nahar

Central Arid Zone Research Institute, Jodhpur-342 003, India.

Email: nmnahar@gmail. com Fax: 91-291-2740706

Abstract

The cost of natural circulation type solar water heater has been reduced by combining both collector and storage tank in one unit and an integrated collector storage (ICS) solar water heater has been designed, developed and fabricated and its performance has also been compared with the natural circulation type solar water heater. The capacity of the heater is 100 litres of hot water per day. 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. The efficiency of the heater has been found to be 61.3% as compared to 51.9 % of natural circulation type solar water heater. 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). The cash flow of the heater with respect to different fuels has been carried out and it has been found that the cash flow is maximum with respect to fuel coal and minimum with respect to kerosene. The payback period is least, i. e. 1.42 yr, with respect to coal and maximum, i. e. 3.72 yr, with respect to kerosene (The cost of kerosene is highly subsidised). The payback periods are in increasing order with respect to fuel: coal, electricity, firewood, LPG, and kerosene. The estimated life of this solar water heater is more than 15 years. The shorter payback periods suggests that the use of ICS solar water heater is economical. The use of integrated collector storage solar water heater will conserve substantial amounts of commercial and non-commercial fuels, which are consumed for obtaining hot water.

Keywords: Solar energy, solar water heater, ICS solar water heater, energy conservation

1. Introduction

Hot water is an essential requirement in industries as well as in domestic sector. It is required for taking baths and for washing clothes, utensils and other domestic purposes both in urban as well as in rural areas. Hot water is required in large quantities in hostels, hotels, hospitals, industries such as textile, paper, food processing, dairy, edible oil etc. Water is generally heated by burning non­commercial fuels, namely firewood and cow dung cake in rural areas and by commercial fuels such as kerosene, liquid petroleum gas (LPG), coal, furnace oil and electricity in urban areas and in industries. Fortunately India is blessed with abundant solar radiation [1]. Solar radiation is available almost whole year through out India. The maximum daily average solar radiation 20.97 MJm-2 day-1 is received at Jodhpur which is known as Sun City of India while minimum 15.90 MJm-2 day-1 is received at Shillong. Solar water heaters, therefore, seem to be a viable alternative to conventional fuels for water heating.

The most commonly used solar water heater for domestic needs is natural circulation type. This type of solar water heater has been designed, developed and investigated in detail by Close [2], Yellot and Sobotaka [3], Gupta and Garg [4], Ong [5], Nahar [6], Morrison & Tran [7], Morrison and Braun [8], Vaxman and Sokolov [9], Nahar and Gupta [10], Norton etal [11], Nahar [12-17], Brinkworth [18].

Natural circulation type solar water heater has collector and storage tank in separate units, therefore, its cost is high and beyond the reach of common people. Considering this, the cost has been reduced by combining both collector and storage tank in one unit and collector-cum-storage type solar water heater has been designed, fabricated and tested. Such type of solar water heaters were studied by Tanishita [19], Richards and Chinnery [ 20], Garg [21], Nahar [22 ], and Nahar and Gupta [23-24], Fairman et al [25], Tripangnostopoulos et al [26], Smyth et al [27], Souliotis and Tripanagnostopoulos [28] and Madhlopa et al [29].These solar water heaters are simple in design, low cost, easy in operation and maintenance and easy to install. But life of this solar water heater was less than 8 years, therefore, it did not become popular in India. Considering this, integrated collector storage (ICS) solar water heater has been design, developed and fabricated.

The life of this solar water heater is more than 15 years.

2. Design

The ICS solar water heater consists of a rectangular tank 1000x1000x100 mm3, made from 3mm thick mild steel plate. The absorber area is 1.0 m2. The capacity of tank is 100 litres. The tank performs the dual function of absorbing solar radiation and storing heated water. It is encased in a galvanized steel (22 swg) tray having dimension 1240x1233x270 mm3 with about 100 mm glass wool insulation at the bottom as well as on the sides. Two glass covers have been provided over it. The front surface of the tank is painted with black board paint. Inner surface of the tank was painted with anti corrosive paint. An insulation cover is hinged over it so that the heater can be covered by it in the evening at 4 PM for getting hot water till next day morning. The heater works on push through systems. In urban areas, the inlet of the heater can be connected to water supply line through a gate valve. Hot water can be obtained by opening gate valve and collected through the outlet pipe. A funnel/bucket is provided for rural use where there is no water supply line. Hot water through outlet pipe can be obtained by putting cold water in the funnel/bucket. The heater is facing equator on a mild steel angle stand with к +15o tilt from horizontal for receiving maximum solar radiation during winter. Fig. 1 depicts actual installation of integrated collector storage solar water heater in the field.

3. Performance

The performance evaluation of the heater was carried out by filling it with cold water in the morning and recording hot water temperature at 4 PM and till next day morning when heater was covered in the evening by thermal insulating cover. The heater can provide 100 litres of hot water at an average temperature of 57.3o C that can be retained to 43.0o C till next day morning when cold water temperature was 17.0o C. The efficiencies of the solar water heaters have been obtained by the following relation:

0

Jo qu d0

П = _____________ (1)

0

A Jo Ht d0 Where,

A = Absorber area, m2 ,

HT = Solar radiation on collector plane, J m-2 hr-1,

qu = Useful heat collected by the solar water heater, j,

0 = Period of test, hr,

П = Efficiency of the solar cooker. The efficiency of the heater has been found to be 61.3%.

The impact of the slit aperture size and the flap’s sensitivity setting is shown in Fig. 5 for the measurements with set-up B on September 12 (a) and September 15 (b). On both days, the collector tilt angle was 60°. For (a) the flap was adjusted to open at approximately 80 °C and for (b) slightly over 75 °C. “Flap setting: 8 mm” in Fig. 5 means that the ventilation flap in the top of the collector frame opens when the absorber’s relative, longitudinal expansion is larger than 8 mm.

The opening of the flap can be clearly seen from the comparison of the temperature profiles of the completely closed reference collector (Tr) and the ventilated collector with flap (Ttest). The slit aperture was 15 mm in (a) and 20 mm in (b). The maximum temperature in (a) was for the reference collector 122 °C and for the test collector 110 °C with AT = (12.1 ± 1.0) K.

In (b) the maximum temperatures in the reference — / test collector were 128 °C / 111 °C with AT = (17 ± 1.0) K. In the comparison the variations of the solar irradiance for (a) between 14:30-

15:45 should be considered; still the desired effects on the temperature reduction between reference — and test collector due to increased flap sensitivity and slit aperture is obvious.

Fig. 5. Maximum temperatures in the reference — and test collector with different flap-sensitivity settings and
slit apertures for Sept. 12 (a) and Sept. 14 (b); Ig is the global solar irradiance, Ta the ambient temperature

[set-up B]

A trough collector with parabolic profile has been simulated and studied by means of ray tracing analyses. The first study examines the geometrical deformations of the parabolic profile and their effects on solar light collection. For the application to solar light exploitation, the essential quantity to be considered is the collection efficiency. It is obtained as ratio between the light focused on the absorber and the light captured by the entrance aperture of the collector. The effects of mirror deformations are expressed using collection efficiency instead of focused light or collected energy: the results are illustrated in Section 3.

The application is a solar trough collector, whose principal component is the linear parabolic mirror. The parabolic profile of this reflecting surface has been optically designed to concentrate the sunlight on a cylindrical receiver, whose centre is placed in the parabola focus.

The methodology to reproduce mirror deformations is based on the use of a mathematical representation for parabolic and deformed profiles. The mathematical approach consists in introducing conic constant and conic equation to represent the profiles of the mirror surface.

Fig. 2 presents the border profile for two examples of deformed mirrors, compared to the correct parabolic curve.

It is important to note that the length of all deformed curves must correspond to the parabola extent, since they represent

deformations of a real solar collector.

The conic equation used in this reconstruction of mirror profiles can be expressed as:

where K is the conic constant and c is the mirror curvature, defined in Eq.(2) from the curvature radius R of the parabolic mirror.

1

c =

R

The reference value for the conic constant is -1: in fact, using the conic equation Eq.(1), the parabolic curve pertains to K = -1. The values of conic constant K different from -1 correspond to deformations of the parabolic mirror, as Fig. 2 illustrates comparing the border profiles of deformed mirrors to the parabolic edge profile.

For -1 < K < 0 the profile becomes elliptic and there is a reduction on the entrance aperture of the deformed collector. The corresponding curve in Fig. 2 is the upper and internal profile.

For K < -1 the profile is hyperbolic and the deformed mirror presents a larger entrance aperture with respect to the parabolic collector. The corresponding curve in Fig. 2 is the lower and external profile (see also Table 1).

Simplicity and efficacy are the advantages of the proposed procedure to simulate the deformations of a parabolic mirror. However the most important result is that it seems to reproduce the flexibility of a real solar collector and its imperfect rigidity. In fact considering a reflecting surface, laying over a set of ribs and centrally bonded, the major deformations appear at the borders.

Thermotropic glazings change their light transmittance behavior from transparent to light diffusing upon reaching a certain threshold temperature. This autonomous shading is induced by light scattering from domains with dimensions comparable to the wavelength of the solar spectral range which exhibit an index of refraction that is different to that of the matrix above the defined transition temperature [2,3]. Thermotropic behavior may be achieved by an alteration in the structure of the liquid crystalline phase in the case of liquid crystal systems [2]. Scattering can also be induced by the formation of local differences in the refractive index by phase separation or a change in refractive index of one or more

components [2,3]. This is the case in thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains.

For industrial applications in general thermotropic materials should fulfill a set of requirements [3,4], such as:

• high reversibility and reproducibility of the switching process

• homogeneous stability and low hysteresis

• no haze and coloring in clear state

• a uniform distribution of the turbidity in the scattering state

• a high viscosity for fluid thermotropic materials filled between glass panes

• excellent long-term stability: weatherability, UV stability, non-freezing

• that the material is safely manageable, innocuously, low or non-flammable and free of organic solvents

• availability in a large area at low costs

When thermotropic layers are applied to prevent overheating of solar thermal systems, further specifications have to be fulfilled [1,5,6]:

• a solar transmittance above 85% in clear state

• a solar transmittance below 60% in opaque state (this would allow for the application of cost-efficient plastics as absorber materials)

• switching temperatures between 55 and 60°C for thermotropic glazing

• switching temperatures between 75 and 80°C for thermotropic absorbers

• a steep and rapid switching within a small temperature range

• scattering domain sizes between 200 and 400 nm in diameter

• differences in refractive index between scattering particles and surrounding matrix above 0.03

In the following the suitability of thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains to provide adequate overheating protection of a solar collector is described.

Particularly important are solar thermal air systems for solar cooling, since many countries in the world do have electricity shortages in summer. A solar cooling application uses the great offer of solar radiation in the summer period to drive a cooling application which needs heat instead of electrical power. For example an open cycle desiccant evaporative cooling (DEC) device benefits directly from hot air with high temperatures to perform best.

Especially in the south of Europe, the demand on cooling devices is great and still raising. The high amount of irradiation disqualifies every solar system that is vulnerable to stagnation in any circumstances.

1.2 Drying applications

Obviously solar air collectors are particularly suitable to drive large scale drying applications (e. g. for wood pellets, food, technical products).

But the range of suitable applications is even greater. There is a still developing field of lossless physical heat storages with sorption materials. These can be regenerated with hot air and store the energy until the reverse reaction of humidification is launched. The availability of materials improves and the processes work best with a high temperature level of the hot air.

Another field of applications might be the prevention of condensing occurrences or problems with high humidity by all means. For these a very high temperaturelevel might be not needed, but still a useful one even with bad weather conditions as low radiation or low ambient temperatures.

In any case, only a high efficient collector can meet these demands.

To achieve working temperatures around 200°C with acceptable efficiency, the absorber area has to be further reduced. Two-dimensional concentrators create a focal line, in which the absorber is placed. These concentrators have the drawback that only direct radiation can be used. One approach to build these concentrators is the development of small and cost-effective parabolic trough systems.

Extensive research on a small parabolic trough collector was done by the institute AEE INTEC (Gleisdorf, Austria) for operating temperatures of 200°C [7]. Another example is the parabolic trough collector PTC 1800 from the company SOLITEM which was developed with research support of the DLR (Germany). Demonstration plants for solar air-conditioning with this collector were already installed in Turkey.

Currently other types of linear focussing collectors are under development [8]. The linear concentrating Fresnel collector of the PSE AG (Freiburg, Germany) is already available on the market and proved its feasibility in several demonstration plants.

3. Rough Comparison

The efficiency of a solar collector can be expressed by

where K (0) is the IAM (Incident Angle Modifier) of the collector, which expresses the ratio of the optical efficiency at a solar incident angle 0 to the optical efficiency at irradiance normal to the aperture of the collector. The IAM-characteristics of a collector type for all possible directions of incidence (3D-IAM) is a very important attribute, which highly influences the energy gain of a collector. Further information about the 3D-IAM can be found in the paper of Paolo Di Lauro et. al. within the EuroSun08 proceedings.

The collector parameter n0 describes the efficiency of the light conversion into heat without thermal losses. These are expressed by the factors a1 and a2. Tav is the working temperature and Tamb is the ambient temperature. For a detailed description see [5] and [6].

4. Conclusion

Within the IEA-SHC Task 33 SHIP alternatives to standard flat-plate and evacuated tube collectors were constructed or are still under development. From the rough comparison in figure 3 it turns out that for solar air-conditioning technologies that require working temperatures below 110 °C improved flat-plate collectors or collectors of the CPC-type can be a cost-effective alternative to evacuated tube collectors. The currently developed small parabolic trough collectors are predestinated to support double effect absorption

chillers or steam ejection chillers if the fraction of direct radiation is high and the specific collector costs are comparable to those of evacuated tubes.

To select the suitable collector technology for a specific system properly, the incident angle modifier, the weather conditions and the industrial load profiles or the characteristics of the cooling machine always have to be taken into account by a detailed simulation. Simulations can only be performed properly, when the optical behaviour of a collector for all directions of the incident radiation (3D-IAM) can be well approximated.

Besides the specific costs of the collector field also aspects like the resulting collector area and building integration have an influence on the decision for a certain collector type.

References

[1] Henning, Hans-Martin: Solar-assisted air-conditioning in buildings: A handbook for planners. Wien; New York: Springer-Verlag 2004

[2] Henning, Hans-Martin: Auslegung von solaren Klimatisierungssystemen.

In: 13. Symposium Thermische Solarenergie. Tagungsband.

Bad Staffelstein, 14.-16. Mai 2003, S. 253 — 258

[3] Deutsche Gesellschaft fur Sonnenenergie: DGS-Leitfaden Solarthermische Anlagen. 7. Aufl. Berlin: Landesverband Berlin Brandenburg 2006

[4] Rommel, Matthias: Medium Temperature Collectors for Solar Process Heat up to 250°C. In: 2nd European Solar Thermal Energy Conference estec. Proceedings. Freiburg, Germany, June 21.-22., 2005,

P. 167 — 172

[5] Duffie, J. A.; Beckman, W. A.: Solar Engineering of Thermal Processes.

3rd ed. Hoboken, New Jersey: John Wiley and Sons 2006

[6] Rabl, Ari: Active Solar Collectors and their Applications.

New York: Oxford University Press, Inc. 1985

[7] Jahnig, Dagmar: Development and Optimisation of a small-scale parabolic trough collector for production of process heat. Gleisdorf, Austria: AEE INTEC 2004. Available from: djaehnig@aee. at

[8] Weifi, Werner et. al.: Process Heat Collectors. IEA Task 33/IV: Solar Heat for Industrial Processes. Gleisdorf, Austria 2008. Internet: http://www. iea-shc. org/task33/publications/index. html

[9] Hefi, Stefan: Application of Medium Temperature Collectors for Solar Air-Conditioning. In: 2nd International Conference Solar Air-Conditioning. Tarragona, Spain, 18.-19. October 2007, Proceedings S. 118 — 123.

[1] Halogen lamp

2. Luxmeter

3. Flate plate collector

4. Temperature sensor 1-inlet

5. Temperature sensor 2-outlet

6. Temperature sensor 3-water tank

7. Hot water tank

8. Hot water tank overflow connection

9. Filler valve in primary circuit

10.Regulator valve for setting the volumetric flow rate

[2] Digital display

12. Air bleed valve

Fig. 1. The experimental stand and its components

Various incidence angles, measured using as reference the horizontal plane were investigated: 0o, 10o, 20o, 30o, 40o, 50o. For each measurement the light density, the flow rate, along with the inlet and outlet temperature were recorded after 15 minutes since the experiment was started. The thermal power and the system efficiency were calculated using the equations (1) and (2).

The test procedure include the measurement of: (a) climatic parameters such as global solar irradiance on the collector plane, ambient temperature, wind direction and speed; (b) energy consumption of pumps, electronic valves and controls included in the test system and (c) operational parameters such as orientation and inclination of solar collector, initial inlet water temperature, outlet water temperature, and flow rate.

These parameters allow evaluating increase or decrease of water temperature, available energy transferred to the water during the operation period, average thermal efficiency, steady-state thermal efficiency, instantaneous efficiency, etc.

1.1.1 Average solar irradiance

In order to evaluate the average solar irradiance on the collector plane, the following equation is used

n T

.=1 n

Unglazed absorbers in polymeric materials for (outdoor) swimming pool heating have been suc­cessfully in the market for more then 20 years. The heating demand for outdoor swimming pools is such that the operational temperature in the collector lies in the range of 15 °C — 30 °С; the ambient air temperature is approximately in the same range when pool heating is necessary, hence the heat loss is small and collector glazing is not required. Normally the pool water circulates as heat carrier in the pool absorbers,

Fig. 1 (a). The solar loop is not pressurized and the operational pressure in the absorber is deter­mined by the hydraulic design. For most pool collector types the maximum operational pressure lays in the range of 1.2 — 1.5 bar. The plastics should sustain temperatures up to approximately 100 °С (during stagnation), solar irradiation and water with swimming pool chemicals. In order to avoid damages due to freezing of the water in the absorbers and pipes, most pool absorbers should be emptied during the winter season. The low temperature level and small mechanical stress allows using low-cost commodity plastics.