Evacuated tube collectors

The high vacuum inside the closed glass tube of the evacuated tube collector is easier to maintain over a long period of time than that in an evacuated flat-plate collector. Glass tubes can resist the ambient air pressure due to their shape so that no supports are necessary between the back and front sides.

A metal absorber sheet with a heat pipe in the middle is embedded inside a closed glass tube with a diameter of a few centimetres. A temperature sensitive working medium such as methanol is used inside the heat pipe. The sun heats up and vaporizes this heat pipe fluid. The vapour rises to the condenser and heat exchanger at the end of the heat pipe. There, the vapour condenses and transfers the heat to the heat carrier of the solar cycle. The condensed heat pipe fluid flows back to the bottom of the heat pipe where the sun starts heating it again. To work properly, the tubes must have a minimum angle of

Table 3.6 Absorptance a, Transmittance t and Reflectance p for Different

Absorber Materials

Table 3.6 Absorptance a, Transmittance t and Reflectance p for Different

Absorber Materials

Visible

Infrared

Material

a = e

t

p

a = e

t

p

Non-selective absorber

G.97

G

G.G3

G.97

G

G.G3

Black chrome

G.87

G

G.13

G.G9

G

G.91

Black nickel

G.88

G

G.12

G.G7

G

G.93

TiNOX (TiN + TiO +

TiO2)

G.95

G

G.G5

G.G5

G

G.95

inclination to allow the vapour to rise and the fluid to flow back. A cross-section illustrating the operating principle of the evacuated tube collector is shown in Figure 3.12. The photo in Figure 3.13 shows a detail of an evacuated tube collector.

There are also evacuated tube collectors with a heat pipe that passes through the end of the glass tube. The heat transfer medium of the solar cycle can flow directly through the heat pipe of these collectors. Then, a heat exchanger is no longer needed and the collector need not be mounted with a minimum angle of inclination.

View from top

Glass tube with high vacuum

Condenser, heat exchanger -,

Glass tube with high vacuum

Condenser, heat exchanger -,

Selectively coasted absorber sheet

Solar cycle heat carrier

Selectively coasted absorber sheet

Solar cycle heat carrier

. Solar radiation vaporizes the heat pipe fluid. The vapour rises along the heat pipe

2. Heat pipe fluid transfers heat to solar cycle and condenses

View from side

. Solar radiation vaporizes the heat pipe fluid. The vapour rises along the heat pipe

2. Heat pipe fluid transfers heat to solar cycle and condenses

View from side

3. Heat pipe fluid flows down and Is heated again by the sun

Figure 3.12 Assembly and Function of the Evacuated Tube Collector with

Heat Pipe

3. Heat pipe fluid flows down and Is heated again by the sun

Figure 3.12 Assembly and Function of the Evacuated Tube Collector with

Heat Pipe

Figure 3.13 Photo of the Connections of the Evacuated Tubes to the

Solar Cycle

Figure 3.13 Photo of the Connections of the Evacuated Tubes to the

Solar Cycle

The entrance of atmospheric hydrogen into the vacuum cannot be avoided even with nearly perfectly closed glass tubes because hydrogen atoms are extremely small. This destroys the vacuum over a long period of operation. Therefore, so-called getters, which can absorb hydrogen over a long period of time, are installed inside the evacuated glass tube.

Evacuated tube collectors can obtain a significantly higher energy gain especially in the cooler months of the year. Thus, a solar thermal system with evacuated tube collectors needs a smaller collector area compared to standard flat-plate collectors. The specific collector prices of evacuated tube collectors are higher than that of flat-plate collectors. Furthermore it is not possible to integrate tube collectors directly into a roof; they must always be installed on top of the roof. This reduces the architectonical possibilities for collector integration.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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