Within group of solar collectors without concentration are the vacuum tubes collectors.

Currently they are the most used.

As was seen in flat collector’s analysis, conversion of radiant energy from the sun to thermal energy leads to radiation, conduction and convection losses that progressively decrease the yield as temperature difference between collector and environment increases.

Improvement provided by vacuum tube collectors is to avoid conduction and convection losses.

If less heat is lost, we will in most cases obtain more yield for same amount of Sun energy.

We will see that this is not always the case and depends on temperature of use.

Vacuum collectors find their main application in intermediate temperature systems (heating, air conditioning, industrial processes, etc.) and in cold places with high differences between collector temperature and the environment.

The vacuum technique used by fluorescent tubes manufacturers has been developed and is the one used by vacuum tube manifolds manufacturers.

Vacuum tube collector systems are based on evacuated tubes.

These are formed by two concentric tubes between which the air has been sucked up producing a vacuum. At one end, both tubes are joined by sealing the vacuum. Inside both tubes are located the different types of absorbers that determine the different systems.

Single evacuated tubes are evacuated tubes, assembled directly with accumulation tank or independently, which may contain only water or water plus antifreeze.

A dark colored layer of absorbent material is located on evacuated tube inner wall.

When solar radiation strikes the absorbent material layer it is transformed into heat and raises the temperature of the fluid that is in contact with it.

The fluid is heated by convection and begins to rise through the tube being replaced by cold fluid which in turn heats up and restarts the process.

This type of vacuum tube offers the advantage of having the aforementioned few heat losses and the disadvantages of being very sensitive to pressure.

U-Pipe vacuum manifolds are used both in individual collectors and in compact solar systems with integrated tank.

Absorber can be placed on tube wall as in evacuated tube case or on an absorbent material sheet.

In any case, absorber is run on its surface by a pipe (preferably copper) through which the fluid raises its temperature in contact with it flows.

U-Pipe tube manifolds have the advantage of being able to adopt both horizontal and vertical position without impairing their performance since tube can rotate on its axis by tilting absorber in the most appropriate way in case the absorber has sheet shape.

Finally, in vacuum tube technology we find heat pipe manifolds.

They employ a mechanism consisting of a closed tube into which a vaporizing fluid (alcohol mixture) of specific properties is introduced.

When sun hits absorber attached to the tube, fluid evaporates and absorbs heat (latent heat). As gas rises above liquid to top of the tube where the cold spot is located. There it liquefies (condenses) and yields its latent heat to the fluid we are interested in heating by falling back to tube bottom by capillarity or gravity.

This process (evaporation – condensation) is repeated for sun’s radiation duration or until collector has reached a very high temperature (around 130 ° C or more).

They have the advantage that each tube is independent being able to change in full system operation. It is highly frost resistant.

Since tubes can also rotate on their axis, it is possible to adopt vertical and horizontal positions as in the case of U-Pipe systems, although in this case generally a minimum tube inclination (between 15º and 20º according to manufacturer) to allow fluid, once liquefied, to fall by gravity.

There are 3 qualities of these collectors:

– Dry union: heat exchange occurs without direct contact between heat transfer fluid and tube, which makes them very suitable in areas with unfavorable water qualities.

– Diode function: heat transfer is always carried out only in one direction, from absorber to heat transfer fluid, and never other way round.

– Temperature limitation: evaporation – condensation cycle takes place as long as vaporizing fluid critical temperature is not reached, thus avoiding uncontrolled temperature rise inside the tubes risks.

This content was extracted from the Solar Thermal Energy Technical-Commercial Manual and is part of Solar e-learning.

All you need is Sun. All you need is Sopelia.

Within the solar collectors without concentration we find the flat plate.

They were the most used, but have lost ground in favor of vacuum tube.

In flat collectors, the collector is located in a rectangular box (housing), whose usual dimensions are between 80 and 120 cm wide, 150 and 200 cm high, and 5 and 10 cm thick (although there are larger models).

The face exposed to the sun is covered by a very fine glass, while the remaining five faces are opaque and are thermally insulated.

Inside the box, on the face that is exposed to the sun, is placed a metal plate (absorber).

This plate is attached or welded to a series of conduits through which a heat transfer agent (usually water, glycol, or a mixture of both) flows.

A selective surface treatment is applied to mentioned plate or is simply black painted, to increase its heat absorption.

Flat solar collectors work taking advantage of greenhouse effect (the same principle that can be experienced when entering a car parked in the sun in summer).

After passing through the glass (transparent for wavelengths between 0.3 μm and 3 μm) radiation reaches absorber surface, which is heated and emits radiation with a wavelength between 4.5 μm 7.2 μm, for which the glass is opaque.

Approximately half of this last radiation diffuses to the outside, being lost; but the other half returns inward and thus contributes to absorber surface further heating.

As it passes through the box, the heat transfer fluid heats up and increases its temperature at absorber expense, which temperature will decrease.

The heat transfer fluid then transports that heat energy to where it is desired.

The flat solar collector is formed by 4 main elements:

1) Transparent cover: it must possess the necessary qualities (suitable transmission and thermal conductivity coefficients) to provoke the greenhouse effect and to losses reduce; ensuring manifold water and air sealing, in conjunction with housing and joints; do not keep outer surface dirt adhering so that rain easily slips.

2) Absorber: receives the solar radiation and converts it into heat that is transmitted to the heat transfer fluid.

Shapes are diverse: metal plates separated by a few millimeters, a metal plate with welded or embedded tubes or two metal plates with a circuit inside.

Face exposed to sun must capture the largest radiation amount so it is usually black painted or endowed with a selective surface (very absorbent to radiation and with low emissivity).

Paints are cheaper than selective surfaces and have a better overall thermal behavior at near-ambient temperatures, but are marred by ultraviolet radiation continued action and temperature variations between day and night.

Selective surfaces generally have a better behavior and are obtained by several layers superposition (metal and metal compounds) or special surface treatments.

The most modern manufacturing technique is laser welding.

3) Insulation: it is used to reduce thermal losses in absorber rear part that must be of low thermal conductivity. Materials can be glass wool, rock wool, cork, polyethylene or polyurethane.

4) Housing: generally made of aluminum or stainless steel, it protects and supports manifold elements, also allowing manifold anchoring and holding to assembly structure. It must withstand temperature changes (dilatations) without tightness losing and must resist corrosion.

Solar energy wherever you are with Sopelia.

Solar Layout is the App for collectors and solar modules on site positioning.

This is the most intuitive Solar App of the market.

To use it on field is not necessary to have an Internet connection because it works from place latitude, obtained by GPS.

Today we will see solar thermal energy part.

To begin press left command shown in initial screen with the house, the solar collector and the user taking a hot shower.

If our Smartphone GPS is not enabled, the App will ask us to activate it to locate our position.

Intermittent earth planet image immediately appear with the legend “Localizing”.

When our device GPS have located our position, the following screen appears to confirm it.

By confirming our location the Solar Equipment Use Menu displays.

There are 3 applications in the Menu:

1 Hot water: represented by a shower image
2- Heating represented by a radiator image
3- Outdoor pool conditioning: represented by a pool ladder image

By selecting one of the 3 applications, Options Menu will display.

There are 3 variables in the Menu:

1- Inclination: represented by collector and angle image
2- Orientation: represented by collector and cardinal points image
3- Distance: represented by 3 collectors rows image

By pressing the Inclination option, we get recommended inclination value for location and solar application selected, accompanied by some Tips considering collector type used.

Pressing Orientation option, we obtain procedure to fix collectors orientation description and access to recommended compass App discharge, if we don´t have it.

Pressing Separation option, the Kind of Surface Menu is displayed for us to select the appropriate option (Horizontal / Non horizontal). If the surface on which the collectors will be placed is horizontal, we only must enter Collector Height in cm data.

If the surface on which the collectors will be placed is non horizontal, in addition to Collector Height in cm data, we must enter Surface Inclination Angle data. We will enter a positive value if it matches the collector inclination direction and a negative value if it is different.

In this way we obtain the Separation (distance) between collector’s rows in meters.

Pressing i button Tips related to shadows and singular locations (snow, desert and rain areas) are deployed.

Download Solar Layout and placed solar thermal collectors on site in the most intuitive way with Sopelia.