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14 maja 2021
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Polskie prawo czy polskie prawie! Barwy Bezprawia

opublikowano: 26-10-2010

Direct heated solar collector PCT / AU 2005/001/001199 high efficency solar collector solar heating Inddividuaul panel control

REKLAMA - Direct heated solar collector

Few year ago on a sunny day in May I experienced hot water from a garden hose. Without knowing much about solar collectors I realised that the hose phenomenon could be used to produce heat. To make it efficient, the hose would need to be long or arranged in a coil or contiguous meandering lengths. Better even if the meanders were made from copper tubes, the adjacent walls of which be opened and joined together. This evolved into the solar collector presented in this article.

The principle features of the presented collector are meandering channels and heating of the fluid under the whole absorber area as shown in the subsequent figures. To understand its operation better, leat us compare it with the conventional collectors.

Fig. 1. The principle of the direct heated, meandering channel collector.

Fig. 2. Three-dimensional view of the collector proposed in this article.

Solar collectors basically come in two types: evacuated tube and flat panel. Both types of collectors absorb solar radiation through absorbers and then transfer the heat to the heat exchanging fluid.

The evacuated tube collectors minimise the heat loss by good thermal insulation achieved with vacuum inside the glass tube. Their disadvantage, however, is poor effective absorber area, typically less than 50%, caused by gaps between the absorbing tubes, thus limiting the amount of received radiation.

It is known fact, that the effectiveness of a collector, seen as a ratio of the heat transferred into the heat exchanging fluid to the total incident solar energy, is best at the lower absorber temperatures and decreases as the temperature rises.

Basic physics

In the traditional flat panel collectors the heat is transferred from the absorber plate to the fluid carrying tubes. To achieve this, a temperature gradient is required on the absorber plate. This in turn leads to the increase of the temperature away from the tubes (see Fig. 3).

Stefan Boltzmann equation shown below, states that amount of the radiated heat is proportional to the 4-th power of temperature. Therefore, the heat loss from the absorber infrared radiation increases significantly with the distance away from the tubes.

Stefan-Boltzmann equation

Where:

T is the body temperature in Kelvins.

A is the area of the body.-

is the Stefan-Boltzmann constant equal 5.67 10-8 (W/(m2K4).

e is the emissivity determined by the body’s surface, with value between 0 and 1.

Although the graphs in Figures 3 and 4 compare direct heated collector with the flat panel, the heat loss principle, in a way, applies also to the evacuated tube collectors (EVC) by virtue of the fact that the fluid carrying tubes in the EVC’s also use small fins to increase the absorption area. This in turn creates temperature gradient and unnecessary heat loss.

To minimise the heat loss through radiation, the absorber temperature has to be lowered over its whole area, which is achieved by keeping heat exchanging fluid under all of the absorber.

Fig. 3.Temperature distribution in the flat panel (tube and fins) collector as compared with the directly heated design. Non-selective coating assumed.


Fig. 4. Distribution of the absorbed power. Non-selective coating assumed.

In addition to the reduction of temperature, the presented design enforces uniform temperature distribution across the panel by preventing the rise of the fluid to the top of the collector, contrary to the thermosiphon principle. It can be shown by relatively simple mathematics, that uniform temperature distribution also improves collector efficiency over thermosiphon type, although only by a few percent.

Absorber corrugation

Another feature of the presented design is the corrugation of the absorber surface, improving the heat dissipation into the fluid (see Figures 5 and 6). It needs to be stressed that the corrugation does not increase the amount of the absorbed energy. However, by increasing the area of the absorber being in contact with the fluid, the heat transfer and thus the collector efficiency is improved.

Practical aspects

The efficiency of the direct heated collector is further improved by selective coating of the absorber. Black chrome coating is considered but this still needs to be tested.

The rest of the collector components like glass, insulation and frame, can be used directly from the existing flat panel collectors.

The suggested material to build the collector is the stainless steel, although other materials could be considered. The advantage of the stainless steel is the durability and the ease of manufacture. The collector base and absorber could

Fig. 5. Overall view of the corrugated absorber plate.

Fig. 6. Enlarged view of the absorber edge.

Fig. 7. View of the complete collector.

be pressed from single sheets of steel and then bonded together. This, after the addition of glass pane and few fittings, would form the complete collector.

The absorber should be possibly thin, to keep the cost down and minimise thermal resistance of the heat transfer into the fluid. The latter, however, will be negligible with the absorber thickness of less than 1mm. The prototype absorber was 2 mm thick and performed well, with no visible difference with respect to the copper tubes.

High volume of fluid

A distinct feature of this collector is also its high volume of fluid keeping the absorber temperature down. The exact collector capacity would depend on a specific implementation but 20 to 30 litres is a fair assumption. This is much higher than in the conventional collector types.

Prototype system

The prototype collector system has been build using flat rectangular pipes welded together to form meandering channels as shown in the Fig 1.

The control system used dedicated temperature sensors at each panel. This in turn allowed installing collectors in separate parts of the roof and compare the performance of each of the collectors.

Individual panel control and house heating

The individual panel control will be beneficial in house heating applications. The author’s experience from Australia is that during the winter, in the morning, it was nice and warm in the sunshine outside, while without the heating it was quite cold inside the house. The south-eastern oriented collectors would capture the morning sunshine energy and transfer it to the inside. The process would continue throughout the day and would benefit most the parts of the house not facing the sun at a particular time of the day.

Due to the higher volume of the heat exchanging fluid, the presented collector will take longer to “start up” comparing to the flat panel. The total amount of heat captured during the day will, however, be significantly higher.

Direct heated evacuated panel

The idea worth considering is to try to combine the evacuated tube technology with direct heating, by creating vacuum under the glass cover. This way the effective absorber area of 100% would be achieved with excellent thermal insulation.

Help needed to bring it to the commercial stage

The presented collector design has been awarded the international patent PCT/ AU2005/001199. It is the author’s view that the presented design offers efficiency close to the theoretical maximum. Due to its simplicity it should and durability it should dominated the market. Being a private individual and not the entrepreneur, the author does not have the resources to bring the design to the commercial stage and would welcome assistance from a suitable investor.

Bogdan Goczynski

bgoczynski@yahoo.com.au

zdzichu

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