flat plate vs evactubes

8/13/2019 Flat Plate vs EvacTubes

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Basic data compilation of Flat Plate vs. Evacuated Tube Solar Collectors

Besides being the most efficient collector at absorbing the suns energy, solar vacuum tubes have

other advantages. With flat panels the solar rays strike the panels at oblique angles during theearly and late parts of the day and are only perpendicular at midday, which is when they receivetheir optimal performance. Therefore the angle plays an important part in the flat panel's

collection efficiency. Whereas solar angles (the angle at which the panel meets the sun) plays a

very little role with solar vacuum tubes as the sun is always striking at an optimum angle.

Because of the vacuum, there is virtually zero heat loss. This is evident by touching a glass

vacuum tube which is cold to the touch while still generating significant heat to the glycol

solution that passes through the panel header. Almost 94% of the suns energy is directed to theinside of the tube and captured. Flat panel collectors will always loose heat through the glass,

and become very inefficient at higher temperatures.

Vacuum tubes with closed copper heat pipes have a further advantage over the solar systems in

that they can be scaled down by unplugging a couple of heat pipes. Because the internal heat

transfer fluid is isolated, removing a tube will not shut down the system. This allows for someseasonal control. In the summer months you may find that the solar system produces too much

heat, you can simply unplug a number of the tubes to scale back the output. This also means that

a broken vacuum tube will have little effect on the operation of the system.

VERSATILITY

Solar vacuum tubes can operate at a much higher temperature and as such are the preferred

choice for home heating applications, solar space heating, solar process heating, solar airconditioning and commercial solar heating applications. The growth worldwide for solar vacuum

tubes is growing exponentially and is expected to surpass flat panel collectors within the next 3-5years. Currently Asia uses 95% evacuated tubes in solar applications and Europe is quickly

adopting vacuum tubes in areas such as home heat and even solar air conditioning.

EFFICIENCY AT HIGHER TEMPERATURES

As the temperature differential between the ambient temperature (outside) and the operatingtemperature (inside the vacuum tubes) increase, vacuum tubes become the obvious choice for

hotter applications such as space heating or process heating.


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Evacuated Heat Pipe Tubes Flat-plate Solar Panels

Works with all angles of the sunsradiation

Only properly perform when the sunsradiation is directly upon the panel.

The collector is hermetically sealed

inside an evacuated glass tube,eliminating convection and

conduction heat losses and isolating

the collector from adverse ambientconditions. Therefore, no heat losses

due to convection and conduction

and no change of performanceduring the service life of the

collector due to corrosion.

The collector is put in a casing with a

glass shield to reduce heat losses. Theair gap between absorber and cover pane

allows heat losses to occur, especially

during cold and windy days. Build up ofcondensation will in due course

influence the collector greatly due to

corrosion, reducing performance anddurability.

Uses a heat-pipe for super efficientheat conduction. No water enters

into the collector.

Circulates water/htf inside insulatedareas. Prone to leakage, corrosion and

restriction of flow due to possible air

lock.

Thermal diode operation principle.

The heat pipes thermal flows one

way only; form the collector to thewater and never in the reverse.

Flat-plates can actually rob the water of

heat if the collector becomes colder than

the water temperature.

Corrosion and freeze free; there is

nothing within the evacuated tube tofreeze and the hermetic sealing of

each tube eliminates corrosion. Theinsulated manifold header with HTFcan withstand sub zero temperatures

for a long period.

Flat-plate collectors are vented and as

such the copper piping is surrounded bycolder air and the HTF/water in the

panel, unless well protected, can burstupon freezing. Corrosion can become amajor problem reducing performance!

Easy installation and little to nomaintenance. Lightweight individual

collector tubes are assembled into

the system at the point ofinstallation. Each tube is an

independently sealed unit requiring

no maintenance. If one breaks it can

be easily replaced.

Installation is difficult. Entire panelshave to be hoisted onto the roof and

installed. If one has a leak, the entire

collector has to be shut down andremoved.

Still work in overcast conditions Require direct sunlight

Relatively insensitive to placementangle, allowing architectural and

aesthetic freedom.

Requires accurate southern exposure andelevation placement.


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Evacuated Tubes vs. Flat Plate Collectors Basic System Layout

Evacuated Tube Solar Collectors are hermetically sealed within a vacuum sealed glass tube.This eliminates convection and conduction heat losses, and also isolates the collectors from

any adverse weather conditions outside.

Flat Plate collectors are built within a solid, sealed case, and covered with a tough temperedglass facing which helps to protect them

from the elements. However, the air gap within the collector can lead to conduction and

convection heat losses, especially on cold and windy days.

Heat Loss Protection

Evacuated Tube Solar Collectors use a heat pipe for super-efficient conduction and transfer.No water ever touches the solar collectors, eliminating corrosion and efficiency losses.

Flat Plate Solar Collectors circulate water or HTF (heat transfer fluid) through insulated areaswithin the collector. This makes them more prone to leakage,

corrosion, and restricted flow.

Heating System Type

The heat pipe of an evacuated tube solar collector is safeguarded from overheating by theconduction properties of the special heating fluid within it. This overheat protection keeps

your system safe from accidental heat damage.

Flat Plate Solar Collectors have no internal overheat protection methods in place, and relysolely on exterior overheat limiters. If these methods fail, damage can occur to the system.

Ease of Installation Evacuated Tube Solar Collectors are easy to install, and can be carried to the job site, and

onto the roof in pieces for a lighter, easier workload. Additionally, if one tube becomes

damaged, only that tube needs to be replaced.

Flat Plate Collectors must be installed in one solid unit, and must be brought to the jobsiteand onto the roof that way. Also, when a porti on of a coll ector fail s, the enti re collector

must be shut down and replaced.

System Design & Placement

Because of their collector design, evacuated tube solar collectors are relatively adept ataccepting most installation angles. This allows for more freedom and aesthetics in the system

design and installation.


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Flat Plate Collectors, like PV Panels, need to be placed directly at 12 degrees off south, andat roughly the same angle as the location's latitude. This means there are far fewer options for

the installation, and aestheticsmust often be sacrificed in order to protect performance.

Climate Types Evacuated Tube Solar Collectors are perfect for nearly any climate, but really shine in

northern climates with cooler temperatures, shorter days, and lower sun angles.

Flat Plate Solar Collectors can be used in most climates, but are significantly more effectivein warmer, sunnier, southern climates,

where freezing and solar angle are less likely to impact a solar water heating system.

Investment & Cost Effectiveness

Evacuated Tube Solar Collectors are often more expensive initially than Flat Plate SolarCollectors, but this investment will often be recouped quickly due to the increased

performance of these collectors in a wider variety of climates and conditions.

Flat Plate Solar Collectors typically cost less from the outset than Evacuated Tubes. However,the lower performance ratings

in many climates and increased maintenance and replacement costs mean that they are

generally less cost-effective than Evacuated Tube Collectors.

We Recommend

We recommend Evacuated Tube Solar Collectors for our clients anywhere where coldweather and possible freezing is a significant factor in the weather.

Types of solar collectors for heat

Solar collectors fall into two general categories: non-concentrating and concentrating. In the non-concentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same

as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel

absorbs the light.

Flat plate and evacuated tube solar collectors are used to collect heat for space heating ordomestic hot water.


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Flat plate collectors

Flat plate thermal system for water heating deployed on a flat roof.

Flat plate collectors, developed by Hottel and Whillier in the 1950s, are the most common type.They consist of (1) a dark flat-plate absorber of solar energy, (2) a transparent cover that allows

solar energy to pass through but reduces heat losses, (3) a heat-transport fluid (air, antifreeze or

water) to remove heat from the absorber, and (4) a heat insulating backing. The absorber consistsof a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a

matte black or selective coating is applied) often backed by a grid or coil of fluid tubing placed

in an insulated casing with a glass or polycarbonate cover. In water heat panels, fluid is usuallycirculated through tubing to transfer heat from the absorber to an insulated water tank. This may

be achieved directly or through a heat exchanger. There is a number of absorber pipingconfigurations:

harptraditional design with bottom pipe risers and top collection pipe, used in low pressurethermosyphon and pumped systems

serpentineone continuous S that maximizes temperature but not total energy yield invariable flow systems, used in compact solar domestic hot water only systems (no space heating

role)

Completely flooded absorber consisting of two sheets of metal stamped to produce a circulationzone. Because the heat exchange area is greater they may be marginally more efficient than

traditional absorbers.

As an alternative to metal collectors, new polymer flat plate collectors are now being produced in

Europe. These may be wholly polymer, or they may include metal plates in front of freeze-

tolerant water channels made of silicone rubber. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that they may be plumbed

directly into existing water tanks instead of needing to use heat exchangers which lower

efficiency. By dispensing with a heat exchanger in these flat plate panels, temperatures need not
http://en.wikipedia.org/wiki/File:Solar_panels,_Santorini.jpg


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be quite so high for the circulation system to be switched on, so such direct circulation panels,

whether polymer or otherwise, can be more efficient, particularly at low light levels.

Some early selectively coated polymer collectors suffered from overheating when insulated, as

stagnation temperatures can exceed the melting point of the polymer. For example, the melting

point of polypropylene is 160C, while the stagnation temperature of insulated thermal collectorscan exceed 180C if control strategies are not used. For this reason polypropylene is not often

used in glazed selectively coated solar collectors. Increasingly polymers such as high temperate

silicones (which melt at over 250C) are being used. Some non polypropylene polymer basedglazed solar collectors are matte black coated rather than selectively coated to reduce the

stagnation temperature to 150C or less.

In areas where freezing is a possibility, freeze-tolerance (the capability to freeze repeatedly

without cracking) can be delivered by the use of flexible polymers. Silicone rubber pipes have

been used for this purpose in UK since 1999. Conventional metal collectors are vulnerable to

damage from freezing, so if they are water filled they must be carefully plumbed so they

completely drain down using gravity before freezing is expected, so that they do not crack. Manymetal collectors are installed as part of a sealed heat exchanger system. Rather than having the

potable water flow directly through the collectors, a mixture of water and antifreeze such aspropylene glycol (which is used in the food industry) is used as a heat exchange fluid to protect

against freeze damage down to a locally determined risk temperature that depends on the

proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying

capacity marginally, while the addition of an extra heat exchanger may lower systemperformance at low light levels.

A pool or unglazed collector is a simple form of flat-plate collector without a transparent cover.Typically polypropylene or EPDM rubber or silicone rubber is used as an absorber. Used for

pool heating it can work quite well when the desired output temperature is near the ambienttemperature (that is, when it is warm outside). As the ambient temperature gets cooler, thesecollectors become less effective.

Evacuated tube collector

Most (if not all) vacuum tube collectors use heat pipes for their core instead of passing liquiddirectly through them. Evacuated heat pipe tubes (EHPT's) are composed of multiple evacuated

glass tubes each containing an absorber plate fused to a heat pipe. The heat from the hot end of

the heat pipes is transferred to the transfer fluid (water or an antifreeze mixtypically propylene

glycol) of a domestic hot water or hydronic space heating system in a heat exchanger called a"manifold". The manifold is wrapped in insulation and covered by a sheet metal or plastic case to

protect it from the elements.

The vacuum that surrounds the outside of the tube greatly reduces convection and conduction

heat loss to the outside, therefore achieving greater efficiency than flat-plate collectors,

especially in colder conditions. This advantage is largely lost in warmer climates, except in thosecases where very hot water is desirable, for example commercial process water. The high


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temperatures that can occur may require special system design to avoid or mitigate overheating

conditions.

Glass-glass evacuated tube

Some evacuated tubes (glass-metal) are made with one layer of glass that fuses to the heat pipe atthe upper end and encloses the heat pipe and absorber in the vacuum. Others (glass-glass) are

made with a double layer of glass fused together at one or both ends with a vacuum between thelayers (like a vacuum bottle or flask) with the absorber and heat pipe contained at normalatmospheric pressure. Glass-glass tubes have a highly reliable vacuum seal but the two layers of

glass reduce the light that reaches the absorber and there is some possibility that moisture will

enter the non-evacuated area of the tube and cause absorber corrosion. Glass-metal tubes allowmore light to reach the absorber and protect the absorber and heat pipe (contained in the vacuum)

from corrosion even if they are made from dissimilar materials.

The gaps between the tubes may allow for snow to fall through the collector, minimizing the loss

of production in some snowy conditions, though the lack of radiated heat from the tubes can also

prevent effective shedding of accumulated snow.

Comparisons of flat plate and evacuated tube collectors

Flat-plate collectors usually loose more heat to the environment than evacuated tubes and thisloss increases with temperature difference. So they are usually inappropriate choice of solar

collector for cold climates and/or high temperature commercial applications such as processsteam production. Evacuated tube collectors have a lower absorber plate area to gross area ratio(typically 60-80% of gross area) compared to flat plates. (In early designs the absorber area only

occupied about 50% of the collector panel. However this has changed as the technology has

advanced to maximize the absorption area.) Based on absorber plate area, most evacuated tube

systems are more efficient per square meter than equivalent flat plate systems. This makes them

suitable where roof space is limited, for example where the number of occupants of a building ishigher than the number of square meters of suitable and available roof space. In general, per

installed square meter, evacuated tubes deliver marginally more energy when the ambient

temperature is low (e.g. during winter) or when the sky is overcast for long periods. Howevereven in areas without much sunshine and solar heat, some low cost flat plate collectors can be

more cost efficient than evacuated tube collectors. Although several European companies

manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturersin the East. Several Chinese companies have long favorable track records of 1530 years.
http://en.wikipedia.org/wiki/File:Evacuated_tube_diagram.jpghttp://en.wikipedia.org/wiki/File:Evacuated_tube_diagram.jpg


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There is no unambiguous evidence that the two collector technologies (flat-plate and evacuated

tube) differ in long term reliability. However, the evacuated tube technology is younger and

(especially for newer variants with sealed heat pipes) still need to prove equivalent lifetimes ofequipment when compared to flat plates. The modularity of evacuated tubes can be advantageous

in terms of extendibility and maintenance.

For a given absorber area, evacuated tubes can therefore maintain their efficiency over a

wide range of ambient temperatures and heating requirements.Evacuated tubes are well

suited to cold ambient temperatures and work well in situations of consistently low sunshine,providing heat more consistently than flat plate collectors per square meter. On the other hand,

heating of water by a medium to low amount (i.e. Tm-Ta) is much more efficiently performed by

flat plate collectors. Domestic hot water frequently falls into this medium category. Glazed or

unglazed flat collectors are the preferred devices for heating swimming pool water. Unglazedcollectors may be suitable in tropical or subtropical environments if domestic hot water needs to

be heated by less than 20C.

Besides efficiency, there are other differences. EHPT's work as a thermal one-way valve due totheir heat pipes. This also gives them an inherent maximum operating temperature which may be

considered a safety feature. They have less aerodynamic drag, which may allow them to be laidonto the roof without being tied down. They can collect thermal radiation from the bottom in

addition to the top. Tubes can be replaced individually without shutting down the entire system.

There is no condensation or corrosion within the tubes.

F lat Plate Solar Coll ectors- Flat Plate Solar Collectors generally cost less than Evacuated Tube

Solar Collectors, and are extremely efficient and cost-effective in warm, southern, short winter

climates. However, in the north, like New England, where winters are cold and long, Flat PlateSolar Collectors can suffer from decreased efficiency, which overtime can lengthen the paybackperiod of your Solar Hot Water System and decrease the lifetime Return on your Investment.

Evacuated Tube Solar Collectors- These Solar Collectors are the most efficient and cost-effective

Solar Hot Water Collectors for Northern, cold, long winter climates like New England States. While

Evacuated Tube Solar Collectors are typically more expensive than Flat Plate Solar Collectors, their

increased efficiency in these regions allows them to completely pay for themselves as, or more,

quickly than Flat Plate Solar Collectors.

flat plate vs evactubes

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