Category Archives: The Experimental Analyze Of The Solar Energy Collector

The second stage of the experiment

A carton paper overhanging part is added to the bottom of the selective coated collector. An artificial suction effect is tried to be constituted by using two fans. In this case, air pressure in the chambers is increased but isn’t enough to provide air flow by convection. As seen in Table 1, the surface temperatures of the collector are variable. The surface temperature values are rather high in the middle part of the collector. So increase of the temperature and velocity of the incoming air from bottom of the collector are unproportional. Consequently air exit from the top of the collector is unrealized by reason of air flow losses.

New Concepts For Solar Collectors In 2030

Marco Bakker1, Jochem Nijs2, Wim van Helden1, Angele Reinders2
1 ECN, Energy Research Centre of the Netherlands, P. O. Box 1, 1755 ZG Petten, The Netherlands,
telephone: +31 224 56 8079, fax: +31 224 56 8966, email: m. bakker@ecn. nl
2 University of Twente, Faculty of CTW, Department of Design, Production and Management,
P. O.Box 21, 7500 AE Enschede, The Netherlands

Introduction

In 2030, the energy demand of newly built houses will be fully covered with solar energy, according to the vision of the European Solar Thermal Technology platform. Naturally, this should not interfere with the increasing demand for indoor comfort and the need for affordable accommodation. This places a lot of demands on future solar thermal collectors.

With the materials and production technologies that are currently being used, it is not possible to make the transition to a next generation of solar collectors with a strongly improved price/performance ratio. New collector concepts are required, based on new materials, new production technologies, and on increased intelligence of the collector system. In this paper we will illustrate how new concepts for solar collectors have been developed with a focus on implementation in 2030.

Backcasting

Within WAELS, a Dutch national long-term research project, a vision on the solar thermal collector of the future was developed. To start as unbiased as possible, a backcasting method was used. This method starts with a future vision, and describes the developments between then and now that are needed to reach that future.

A series of three workshops was organised to get an overview of the developments in the various sectors related to solar thermal collectors. Experts from a. o. construction, installation, materials science, and information technology were invited to discuss the developments in their fields of expertise. These developments were extrapolated and combined in order to define a set of opportunities (e. g. new materials, sensors, construction technologies, etc.) and boundary conditions (e. g. evolved energy demand, competing technologies, etc.) for solar thermal collectors in 2030.

The main trends that were found as a result of the workshops—aspects that will become increasingly important for solar collectors—were:

•f lexibility: both building and installation can easily adapt to newly added components or to changes in user demands;

• integration: building and installation components are fully integrated and interwoven;

• intelligence: the building can independently adapt its behaviour to the ambient conditions and the user’s demands and behaviour;

• modularity: building and installation components are easily exchangeable;

• independence: the building can provide in its own energy needs.

Unglazed absorbers

High market penetrations have pool absorbers of (a) rigid, extruded sheets of polypropylene (PP) with intrinsic channels, Fig. 6 (a). Common are also flexible absorber mats of ethylene propylene diene monomer (EPDM) rubber pipes, here in red in order to match the colour of roof tiles (b).

Further examples of pool absorbers with pipe structure are plain polyethylene (PE) pipes (d) and ripped PP pipes (e). Fig. 6 (c) and (d) illustrate two examples of rigid, blow-moulded absorbers in high-density polyethylene (HD-PE): (c) flat panels of 200 x 110 x 1.5 cm and (f) pool absorber cassettes with honeycomb structure of 30 x 30 x 3 cm. Both absorbers can be interconnected in a matrix, parallel and in series. Commonly, pool absorbers are placed on the top of flat or slightly tilted roofs, various sophisticated solutions for fixing and preventing the collectors from displace­ment by wind, rain and snow exist.

image117

Fig. 6. Various designs of polymeric absorbers for swimming pool heating

[Source: FAFCO (US), MAZDA Solar (D), Roth-Werke (D), PIPELIFE Austria (AT), Solar Ripp (D), VK TEAM (CZ)]

2.1 Glazing

The glazing has to sustain the temperature gradient between the collector inside and the ambient temperature, solar irradiation, load of weather impacts due to wind, snow, hail and rain. Compre­hensive work on the durability of polymeric glazing has been performed, e. g. [8, 9, 10]. UV-resistant, thermotropic and anti-soiling coatings for polymeric surfaces are e. g. studied in Subtask C of IEA-SHC Task 39.

The aesthetics of the collector glazing is essential from an architectural point of view. Especially when large parts of a building’s facade or roof are covered by solar collectors, the integrated de­sign, size, shape, surface structure, colour and reflectance will contribute to the building’ s appear­ance. Rather common and known from the building and construction sector are twin-wall sheets of polycarbonate (PC) as collector cover (Fig. 7 (a)). Fig. 7 (b) shows a solar air collector with a PC twin-wall sheet as cover. Different collector cover designs are dome structures of PC (c), corru­gated roof coverings with 6 mm twin-wall structure of PC (d), and acrylic glazing of an integrated storage collector (e). The polymeric glazing in the examples (c) and (d) cover conventional, selec­tively coated metal absorbers with pressurised solar loop and water-glycol mixture as heat carrier.

image118

Fig. 7. Polymeric glazing for solar thermal collectors: PC twin-wall sheets, air collector with flat PC twin — wall sheet, dome-structured glazing, corrugated roof covers, acrylic glazing of integrated storage collector;

(Img. source: Sabic IP (NL), SolarVenti Ltd (DK), Imagination Solar Ltd (UK), Eternit-Werke (AT), Solarpower GmbH (D))

Background and theory

The present study has three objectives, i) to investigate the temperature reduction in polymeric collectors by passive ventilation, ii) to demonstrate a built-in, temperature triggered mechanism to initiate the ventilation process iii) to present a simplified method for determining the temperature dependent heat loss coefficient and the heat capacity of glazed polymeric solar collectors from stagnation measurements. All measurements are performed under stagnation conditions (no heat carrier in the absorber). Central issues for the investigations in the present work are:

• Non-selective absorbers of polymeric materials (low temperature performance plastics);

• Building integrated installations;

• Solar heating system design: non-pressurised, water as heat carrier, drain-back function;

• Simple control system design, self-protective overheating control;

2.1. Polymeric collector

For the design of a polymeric collector it is important to have a high system performance, which is competitive with conventional collector systems. Due to the overall system design, the present system is not operative for absorber temperatures typically above ~90 °С, the heat carrier drains back to a drain-back reservoir and the absorber is filled with air. The present absorber material is a modified PPE/PS blend [12] and studied e. g. in [13, 14].

Here, the maximum temperature reduction due to ventilation is studied, however not whether the reduced stagnation temperature is suitable for a long service life of the polymeric absorber. Still it can be indicated that the stagnation temperature in the present collector should be significantly below 130 °С; the system design should be such that this temperature is only reached during short periods of time.

Loads and Loading Cases

The tracker is equipped with a PV panel with a surface 1.48×0.67 m2, approx. 1 m2. The maximum wind speed for the Bra§ov region is 30 m/s resulting a maximum wind force on the panel Wmax = 580 N for a maximum wind pressure pm = 580 N/m2 [3]. The wind direction can be considered towards the front of the panel (fig. 2, a.. .c) or towards the back of the panel (fig. 2, d.. .f). Different assumptions can be considered on the distribution of the wind pressure on the panels: uniform pressure [4], resulting bigger wind force (fig. 2, a, d); trapezoidal distribution, approximating the distribution presented in [5], reversed for opposite wind direction (fig. 2, b, e); triangular distribution [6], reversed for opposite wind direction (fig. 2, c, f). For all six wind load cases presented in fig. 2, the load can be reduced to a single wind force W and a moment M, placed on the axis of the panel, with values according to Table 1.

Подпись: pm pm pm pm pm/2 a b c d e f image018pm pm

image019

Wind

Fig. 2. Wind load (a — wind case 1; b — wind case 2; c — wind case 3; d — wind case 4; e — wind case 5; f —

wind case 6).

Table 1. Wind loads for wind pressure distribution cases.

Wind direction towards front of panel

Wind direction towards back of panel

M

W

Wind case 1

Wind

case 2

Wind case 3

Wind case 4

Wind case 5

Wind

case 6

W

W

max

Iw

2 max

3W

4 max

Wmax

— IW

2 max

—W

4 max

M

0

—W l

12 max

—W l

24 max

0

—W l

12 max

—W l

24 max

The only weight that is considered for preliminary design is G = 250N, the weight of the panel together with all the parts (frame) directly attached to it.

Table 2 presents the loading cases given by the extreme positions of the tracking system.

Table 2. Loading cases.

1 — Winter solstice sunrise

2 — Winter solstice sunset

3 — Winter solstice noon

4 — Summer solstice sunrise

5 — Summer solstice sunset

6 — Summer solstice noon

Y *= 55°; P*= +48°

Y*= 55°; P*= -48°

Y*= 67°; P*= 0°

Y*= 0°; P*= +65°

Y*= 0°; P*=-65°

Y*= 23°;

P*= 0°

Movements and mechanical stresses in gas-filled flat plate solar collectors

J. Vestlund1*, M. Ronnelid1 and J. O. Dalenback2

1 Solar Energy Research Center, Hogskolan Dalama, Borlange, Sweden
2 Chalmers University of Technology, Goteborg, Sweden
* Corresponding Author, ive@du. se
Abstract

Sealed gas filled flat plate solar collectors will have stresses in the material since volume and pressure varies in the gas when the temperature changes. Several geometries were analyzed and it could be seen that it is possible reducing the stresses and improve the safety factor of the weakest point in the construction by using larger area and/or reducing the distance between glass and absorber and/or change width and height relationship so the tubes are getting longer. Further it could be shown that the safety factor won’t always get improved with reinforcements. It is so because when an already strong part of the collector gets reinforced it will expose weaker parts for higher stresses. The finite element method was used for finding out the stresses.

Keywords: Solar collectors, modelling, mechanical stresses, Inert gases

1. Introduction

One way of getting more energy efficient solar collectors is changing the air inside the collector with a more suitable gas with respect to the unwanted heat losses. Though, a construction with an enclosed gas will cause new challenges. Since the temperature in a solar collector can vary in a range of about 240 to 500 K there will be a volume and/or a pressure variation in the gas. These variations has to be considered, otherwise they can imperil the expected life length of the construction. This study has examined a collector box consisting of an ordinary glass and an ordinary absorber in all respects expect that it is formed as a tub fixed to the glass so a cavity is formed that can host a gas.

2. Method

. Adaptation of a glass batch coating plant for the selective coating of solar absorbers

Roll-bond panels are produced by screen printing the channel pattern on an aluminium sheet with a separating ink, rolling it together with another aluminium sheet at high pressure and high temperature and finally inflating the channels with pressurized air. Due to this process it is not possible to use pre­coated aluminium sheets, because the selective coating would suffer damages during the rolling process. Therefore it is necessary to apply the selective coating after the roll-bond absorbers have been built, which is only possible with a batch coating plant. Batch coating plants which can apply sputtered coatings on glass panes already exist. One aim of the BIONICOL project is to adapt such a plant of an industry partner in order to be able to coat the roll-bond absorbers.

2.2. Development and testing of appropriate heat transfer fluids

Roll-bond solar absorber were already built in the seventies and eighties, but then vanished from the market. One reason for this was the fact that corrosion occurred in some of the absorbers. Therefore one of the most important tasks within the project is the development and comparative examination of a tailor-made inhibited glycol based heat transfer fluid. The intended experimental work comprises ASTM D 1384 related corrosion tests as well as the investigation of solar fluids operated under varying field test conditions. Moreover, it is also planned to carry out corrosion tests of fluids in a closed cycle with roll-bond panels in an early stage of the project.

Intellectual property protection

This design is the subject of a European patent application number EP 05255415 in the joint names of Macgregor Solar and Solartwin Ltd.

10. Special features

The collector is freeze-tolerant and so does not need any anti-freeze or heat exchanger. It can be directly connected to an existing heating system using fresh water. It could also be used to heat corrosive water e. g sea-water since the rubber used is resistant to corrosion.

Reference

1. Bartelsen et al , Elastomer — Metal Absorber, Development and Application, Proc ISES World Solar Energy Congress, Israel, 1999

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).