Category Archives: The Experimental Analyze Of The Solar Energy Collector

Parabolic trough collector

image031During the last decade we developed solar collectors from optical design, deeply analysed using ray tracing simulations, to realisation and test. Several optical layouts have been considered: linear concentrators, as well as, lens configurations with increased complexity from simple parabolic collectors to lens mirror combinations [1-7]. Our solar concentrators are designed to be applied in the Photo Voltaic sector or in the thermal field.

The studies presented in this paper analyse a trough collector with parabolic profile, whose characteristics and performance have been examined by ray tracing simulations. The major components of the solar trough are linear mirror and linear absorber in Fig. 1. The linear parabolic mirror
concentrates the sunlight over the absorber represented by a metal pipe, surrounded by a glass tube. The configuration parameters considered in this paper are:

■ Linear parabolic mirror

— Focal length: f = 780 mm.

— Dimensions of collector aperture:

— Width W = 1.8 m;

— Length L = 5 m.

■ Linear Absorber

— Dimensions of metal pipe:

— External diameter D = 50 mm;

— Length L = 5 m.

— Dimensions of glass tube:

— External diameter G = 70 mm;

— Thickness T = 2 mm;

— Length L = 5 m.

The first study describes an original methodology to reproduce rigid deformations of the linear parabolic mirror. Successively it analyses how much these mirror deformations affect the energy collected by the solar trough. The second study examines the interactions between mirror deformations, misalignment and tracking errors. The optical characteristics considered in this paper are: mirror edge deformation, collection efficiency, misalignment angle and acceptance angle.

Thermotropic materials for overheating protection of solar collectors

K. Resch1* and G. M. Wallner2

1 Polymer Competence Center Leoben GmbH, RoseggerstraBe 12, A-8700 Leoben, Austria
2 Institute of Materials Science and Testing of Plastics, University of Leoben, A-8700 Leoben, Austria

Corresponding Author, resch@pccl. at

Abstract

Within this paper thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains are reviewed with respect to their capability to prevent overheating of solar collectors. As to their switching ranges (77% for thermotropic hydrogels, 52% for thermotropic polymer blends and 25% for systems with fixed domains) the functional layers are well suited for solar collector applications. For an all polymeric flat-plate collector with twin — wall sheet glazing and black absorber thermotropic hydrogels, thermotropic polymer blends and TSFD would limit maximum absorber temperatures to 75, 90 and 125°C, respectively. This would allow for the use of cost-efficient plastics as absorber materials. However, thermotropic materials have not yet been developed systematically for the application in solar thermal systems with corresponding switching temperatures and properties to resist the demanding environmental conditions especially for prolonged periods.

Keywords: overheating protection, thermotropic hydrogels, thermotropic polymer blends, thermotropic systems with fixed domains

1. Introduction

The application of cost-efficient plastics as absorber materials for all-polymeric solar collectors requires appropriate overheating protection. A feasible way to limit stagnation temperatures is the reduction in optical gain especially by the use of thermotropic layers [1]. Thermotropic glazings are actively switchable layers which permit the light and energy flux to be adapted dynamically to temperature conditions within the collector. In the past various thermotropic systems for active daylight control in transparent facades have been developed and investigated mainly [2,3]. Within this paper existing thermotropic systems are discussed with respect to their capability to prevent overheating of solar collectors.

The need for a new collector

Kollektorfabrik started in June 2006 to incorporate the must-haves of a highly efficient collector and the don’ts of a stagnation vulnerable solar thermal system. In this process we questioned the common water based solar heat transfer medium and put our attention on air.

1.1 Solar fraction

To raise the total share of solar thermal energy, there are different possibilities:

• New fields of solar thermal usages could be implemented. Basically, every process that is in need for heat up to 130 °C can be satisfied or supported by a solar thermal application. Often processes need a huge amount of thermal energy, which leads — if supported by solar thermal systems to huge solarfields with all their necessities. Except for uncovered absorbers for low temperature applications there is not really a wide range of products which can be used for large scaled collectorfields. A field of air collectors is not limited in size, since an inconvenient stagnation behaviour cannot occur.

• The solar fraction of every single application could be raised. The chart in figure 1 is well known as the demand and offer of a solar thermal system for a private household in the run of a year. A big collectorfield enhances the solar fraction but exceeds the heat demand for domestic hot water in summer. This can cause trouble if, for instance due to lack of knowledge, no measures are taken to deal with the energy overrun.

In fact the size of the collector area alone has a minor impact on the system costs. Even systems that support space heating, often cut the size of the collector area due to the maximum content of the installed storage tank and not only to the maximum of the installable collector area.

The solar fraction of an application therefor is often not a question of the affordable collector area, but how to deal with the risks of energy overrun in the summer period and the associated cost of implemented measures.

image130

Fig 1. Demand, offer and overrun of energy for a household with a solar system which supports space­heating.

Stationary, Low Concentrating and Non-Imaging Collectors

Another approach is to increase the ratio between aperture and absorber area. This ratio is called geometrical concentration ratio. CPCs (Compound Parabolic Concentrators) are applied, because these optical concentrators have no focal point. Thus tracking can be avoided in combination with concentration ratios lower than two.

Figure 2 shows a stationary, non-evacuated CPC-type collector without a vacuum and with a concentration ratio of 1.5. It is made up of riser tubes centred in symmetrical CPC valleys with two asymmetrical CPC valleys for the headers. The absorber is V-shaped (inverted V). The collector has only two inlet/outlet connections and has a minimum back insulation of 3 cm. There

image166

is enough distance between the glass and the top of the absorber for the collector to be provided with anti-convective barriers such as a Teflon® foil.

The research institute INETI (Lisbon, Portugal) was involved in the development. The collector is available on the market and has an operating temperature level of 110°C, e. g. suitable to operate an absorption cooling machine.

Test Method. Characteristics

The p. roposed test method is similar to the way according to ISO 9806-1 (1994), and basically, the main difference is that except the steady-state measurement there are some integral tests included to determine the overall thermal performance. It is a strict test condition to satisfy all of parameters for steady-state measurement, when solar irradiance would be influenced by cloud. So the whole process for measurement would be a long period and expensive. In the case of the actual utilization, the surrounding condition always is exposed in a dynamic mode. In order to simplify this in this proposal and broaden the test scope, during the day test continuous measurements should be operated whatever the fluctuation of solar radiance and a more widened range for temperature and flow rate. Through this kind of measurement a set of comprehensive data that would access to actual needs to some extents could be provided successfully.

The test determines steady-state efficiency of all-glass evacuated tube solar collectors in order to evaluate the thermal performance and consequently comparison with other kinds of solar collectors. The average global thermal efficiency, the relationship between the increments of outlet temperature and total solar energy on the collector plane, and the relationship between the increments of instantaneous efficiency and total energy on the collector plane are also determined. Moreover, manufacturers can optimize their products or design solar water heating system depending on the actual test results.

1.1. Scope

This test is a systematic and comparatively comprehensive test for determining the thermal performance of water heating solar collectors. The thermal behaviour is characterized by means of
whole collector tests using a “black box” approach. In the whole test process it contains that the experiment of determining the steady-state thermal performance of solar collectors, the experiment of determining the instantaneous efficiency of solar collectors, and the experiment of determining the thermal performance during the whole day operation.

Polymeric Solar Collectors — State Of The Art

M. Meir1*, J. Buchinger2, S. Kahlen3, M. Kohl4, P. Papillon5, J. Rekstad1, G. Wallner6

1 University of Oslo, Department of Physics, P. O. Box 1048, N-0316, Oslo, Norway

2) Arsenal Research, Giefinggasse 2, A-1210 Vienna, Austria

3) Polymer Competence Center Leoben, Roseggerstrasse 12, A-8700 Leoben, Austria

4) Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, D-79110 Freiburg, Germany

5) CEA-INES, BP 332, 50, avenue du Lac Leman, F-73377 Le Bourget du Lac, France

6) University of Leoben, Institute of Materials Science and Testing of Plastics, Franz-Josef Str. 18, A-8700

Leoben, Austria

* Corresponding Author: mmeir@fys. uio. no
Abstract

The state of the art of polymeric materials in solar thermal applications is reviewed in IEA — SHC Task 39. The present status with regard to solar thermal collectors is summarized. Ex­amples of favourable solar heating system — and solar collector designs of polymeric materi­als are illustrated. Overheating protection opens for the application of commodity plastics in glazed collectors. Various absorber types are compared from a polymer-engineering point of view.

Keywords: Polymeric materials, solar collectors, IEA-SHC Task 39, plastics;

1 Introduction

Approximately 40% of the final energy demand in the EU25 member states is used for low tem­perature heating and cooling — a share, which is in principle easily accessible with solar thermal technology [1]. However, the fraction of energy use covered by solar thermal is nearly negligible and still below 0.05% in the European countries [2]. In the Sustainability Report 2006 by Bank Sarasin [3] an annual growth rate of 25-30% of newly installed global collector capacity is ex­pected up to 2010. Conventional solar collector systems are based on materials (e. g. copper) with limited availability. The material supplies will not be large enough to cover up for the expected growth in solar thermal installations. These issues demand the introduction of new materials, of which polymers seem to have a strong preference in all respects. Polymers reveal a large cost — reduction potential due to mass production, reduction in weight, freedom in structural and func­tional design and the potential to lead to a breakthrough for solar thermal energy production.

Polymeric collectors had a market share of 19% of the worldwide solar heating capacity in opera­tion in 2006 [4], which are almost exclusively unglazed absorbers for swimming pool heating. The US represents the largest market for polymeric pool absorbers with a power production of

19.2 GWth in operation at the end of 2006. Pool absorbers are applicable in the low temperature range. In order to meet the requirements from the market for heating applications in the medium and high temperature range, the introduction of new polymeric materials and technology is essen­tial. New materials can only be applied if the service-life is comparable to those in conventional products. Task 39 is a collaborative effort in the International Energy Agency’s Solar Heating and Cooling Programme, which brings together solar thermal — and polymer experts from research insti­tutions and industry working on these challenges. IEA-Task 39 is divided in three Subtasks,

A: Information, B: Collectors and C: Materials. An on-going effort is the preparation and update of a database on existing applications, prototypes and patents with regard to polymers in solar thermal applications. The present work gives a brief overview limited to solar thermal collectors and inte­grated storage collectors.

2 System design

The PSE linear Fresnel process heat collector

The collector design is modular in steps of 4 m length and consists of 11 primary mirror rows with a width of 0.5 m each. The primary mirrors are made of tempered white glass with silver backside coating. They are mechanically bent to concentrate the sunlight on a tubular vacuum receiver (SCHOTT PTR® 70) with secondary CPC concentrator 4 m above the mirror field. The secondary is an aluminium mirror in CPC shape (Alcan Singen / Alanod). Each mirror row is moved by an individual drive connected to an embedded Linux unit with astronomical control software.

Table 1. Geometric parameters of the collector

Length

Modular in steps of 4 m

Width of the mirror field

7.5 m

Aperture width

5.5 m

Distance between mirrors

0.2 m

Height of the receiver

4 m above mirror field

Number of mirrors

11

Width of mirrors

0.5 m

Optical Performance parameters were derived out of Raytracing calculations, thermal performance parameters could be deducted from reported measurements [2]. For practical use we defined a site independent number of thumb for the peak capacity.

Table 2. Performance parameters of the collector

Optical efficiency

0,62

Thermal loss coefficient

4.3 10-4 W/(m[3]K2)

Peak capacity

500 W/m2

image140

Figure 1. PSE linear Fresnel Process Heat Collector in Bergamo, Italy.

Robur has installed two of their NH3/H2O absorption chillers in a modified version with a heat exchanger for pressurized water. These can be seen in Figure 1 on the right hand side of the picture. The collector is aligned with the building, which is not exactly north-south oriented, but turned 7.6° towards SE-NW.

Features and optimization potential

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.

image011

Fig. 2. Prototype collector at the SIJ test facility

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).

image012

Fig. 3. Accumulative lead error of the prototype collector [DLR]

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.

Absorbers’ synthesis

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 *

Absorber

Sample

as

St

Surface roughness, [nm]

Al/Al2O3/NiO/TiO2

(1)

А1/АІ20з

0.94

0.12

330.4

А1/АЬ0з/№0

0.92

0.09

28.5

Al/Al2O3/NiO/TiO2

0.92

0.08

3.6

Cu/CuOx/NiO/TiO2

(2)

Cu/CuOx

0.94

0.08

303.5

Cu/CuOx/NiO

0.96

0.06

8.8

Cu/CuOx/NiO/TiO2

0.95

0.05

1.9

Table 1 Solar absorptance (aS) and thermal emittance (st) of absorbers deposited on Al and Cu substrates

DEVELOPMENT OF A LOW COST INTEGRATED. COLLECTOR STORAGE (ICS) SOLAR WATER HEATER

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.

image123

Fig. 1 Integrated collector storage solar water heater

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%.