PERGAMON Renewable Energy I 5 (I 998) 20 I 204

PRODUCT DEVELOPMENT IN SOLAR WATER HEATING

H.A. de Beijer

De Beijer RTB B.V. Meijnerswijk 5, 6841 HA Arnhem, Netherland

The development of Solar Water heaters is dominated by Technology Push instead of Demand Pull. xc 1998 Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION

The current concepts of Solar Water Heaters are thirty years ago designed.

The Solution@ concept is a result of questions from the market. The program of demands was: - no storage tank in the house - simple construction - low price and installationcosts - no mainenance - no electrical energy required for circulation pump or control unit

The Solution Solar Water heater is a water heater in which the solar energy absorber and the hot water storage tank have been merged into one unit. Normaly solar collectors are installed on a roof or a tilted structure and are, for the northern hemisphere, oriented to the South, with a storage tank in the house. To increase the possibilities of installation a Solution system is also developed for east-west orientati- on.

0960-148 1/98/$~- see front matter 0 1998 Published by Elsevier Science Ltd. All rights reserved PII: SO960-148l(98)00156-6

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Fig. 1 The solar hot water heater is compact and simple

WORKING PRINCIPLE

The principle of the system is two tubes whereby one tube is assembled into the other as shown in figure 2. The inner tube serves as storage tank for the hot water (1). The outer tube (3) has been provided with a special solar selective layer by which the absorbed sun rays are efficiently transferred into heat. The space between the inner and the outer tube is vacumated, as a result of which the storage tank works as a thermos flask. Some water (5) is placed in this space (4) in order to transport heat from the outer to the inner tube. When the outer tube is heated, this water evaporates and consequently condenses on the colder side of the inner tube. In this way the solar heat is efficiently transported to the storage tank. In order to improve the evaporation efficiency, the inside of the outer tube has been equipped with a capillary structure (6) so that a large part of the outer tube is wet.

The system is initialy designed to be installed in the traditional way. To make it possible to use the system for other situations as mentioned in the introduction, the idea was developped to use a cylindrical cover. By this it should be possible to use the solar system for non traditional situations. In figure 2 an idea is given to use the solar collector instead of the ridge tiles of a roof.

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Fig. 2 Solar hot water system installed in the top of a roof

RESEARCH PROGRAM

The first part of the study are investigations to get an idea of the possibilities of the system. Three main points have been investigated: - The changes in heat losses. - The influence of the cylindrical cover. - The reactions in the market from architects e.g.

Heat losses

Because the cylindrical cover has a bigger surface it is obvious that the heat losses will increase. To get an idea of the influence a test model is built for laboratory testing. The measured heat losses are converted to heat loss per mz aperture surface. These values makes it possible to compare the result with the values of the Solution collector with a flat cover. The test results show values slightly higher than the values of the Solution collector with a flat cover. The conclusion can be made that the heat losses did not realy increase for the new cilindrical construction. This can be explained because the R-value of the gap between inner and outer tube is high and therefore has a major influence on the total R-value.

Cvlindrical cover

Calculations are made for the transmitted solar energy through the cover. For the calcula- tionss a, in The Netherlands often used, reference year is used. Hourly values for beam and diffuse radiation and solar altitude and azimuth are given.

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A computer program was made to calculate for a plane with any orientation the hourly value of the angle of incidence for solar radiation. Herewith the solar transmittance for the cover is calculated. With this coefficient and the hourly values of solar energy, for every plane the transmitted solar energy can be determined. Counting up all hourly values gives the yearly total. To get the result for the cylindrical cover the cover is devided in small plane parts. Table 1 gives the result for Dutch circumstances.

Season

winter

spring

ummer

Difuse Beam Total

69 14 83

261 101 362

355 175 530

autumn 148 47 195

337 I 1.170

Table 1. Solar Energy in kWh, transmitted through a transparant half part of a cilinder. Diameter 1 m, length 1 m, material PMMA.

Reactions in the market

The conversations about the design are hold with several architects, building companies and manufacturers of roofs. Most of them seems to be quite happy with the other appro- ach of installing solar collectors, even for higher costs.

Because of the result of these preliminary investigations, the aim for 1998 is to make a full size prototype that will be installed on a house. The installation will be monitored.