Evacuated Tube Collector
Solar Evacuated Tube Collectors for Hot Water
The evacuated tube collector (ETC) consists of a number of sealed glass tubes which have a thermally conductive copper rod or pipe inside allowing for much high thermal efficiency and working temperature compared to the flat plate solar collectors even during a freezing cold day.
In the previous tutorial we looked at Solar Flat Plate Collectors and saw that they consist of a blackened metal absorber plate and water pipes enclosed within a sealed glazed and insulated metal (or wooden) box. Pipes called riser, soldered to the absorber plate carry liquid that is heated by the sun and in a direct heating system, water is heated as it circulates through the panels to the storage tank. In indirect systems, the sun’s energy heats a glycol/water mixture that cannot freeze and which, in turn, heats the water in the tank.
While this type of solar hot water systems is cheap and easy to install, the problem with flat plate collectors is that they are “flat”. This produces one limitation to their efficiency as they can only operate at maximum efficiency when the sun is directly overhead at midday. At other times, the sun’s rays are striking the collector at varying angles bouncing off the glazing material thereby reducing their efficiency.
Solar hot water systems that use Evacuated Tube Collectors as their heat source overcome this problem because the solar collector uses individual rounded tubes which are always perpendicular to the sun’s rays for most of the day. This allows a solar hot water system using an evacuated tube collector to operate at a much high efficiency and temperature for a much longer period than a conventional single flat plate collector installed system.
Also, another advantage of solar evacuated tube technology is that the weight and roof structural problems caused by standard flat plate systems are eliminated as the solar tubes are not filled with large amounts of heavy water.
Evacuated Tube Collector
The Evacuated tube collector consists of a number of rows of parallel transparent glass tubes connected to a header pipe and which are used in place of the blackened heat absorbing plate we saw in the previous flat plate collector.
These glass tubes are cylindrical in shape. Therefore, the angle of the sunlight is always perpendicular to the heat absorbing tubes which enables these collectors to perform well even when sunlight is low such as when it is early in the morning or late in the afternoon, or when shaded by clouds. Evacuated tube collectors are particularly useful in areas with cold, cloudy wintry weathers.
So how do solar evacuated tube collectors work?. Evacuated tube collectors are made up of a single or multiple rows of parallel, transparent glass tubes supported on a frame. Each individual tube varies in diameter from between 1" (25mm) to 3" (75mm) and between 5′ (1500mm) to 8′ (2400mm) in length depending upon the manufacturer.
Each tube consists of a thick glass outer tube and a thinner glass inner tube, (called a “twin-glass tube”) or a “thermos-flask tube” which is covered with a special coating that absorbs solar energy but inhibits heat loss. The tubes are made of borosilicate or soda lime glass, which is strong, resistant to high temperatures and has a high transmittance for solar irradiation.
Unlike flat panel collectors, evacuated tube collectors do not heat the water directly within the tubes. Instead, air is removed or evacuated from the space between the two tubes, forming a vacuum (hence the name evacuated tubes).
This vacuum acts as an insulator reducing any heat loss significantly to the surrounding atmosphere either through convection or radiation making the collector much more efficient than the internal insulating that flat plate collectors have to offer. With the assistance of this vacuum, evacuated tube collectors generally produce higher fluid temperatures than they’re flat plate counterparts so may become very hot in summer.
Evacuated Tube Collector
Inside the each glass tube, a flat or curved aluminium or copper fin is attached to a metal heat pipe running through the inner tube. The fin is covered with a selective coating that transfers heat to the fluid that is circulating through the pipe. This sealed copper heat pipe transfers the solar heat via convection of its internal heat transfer fluid to a “hot bulb” that indirectly heats a copper manifold within the header tank.
These copper pipes are all connected to a common manifold which is then connected to a storage tank, thus heating the hot water during the day. The hot water can then be used at night or the next day due to the insulating properties of the tank.
The insulation properties of the vacuum are so good that while the inner tube may be as high as 150oC, the outer tube is cooler to touch. This means that evacuated tube water heaters can perform well and can heat water to fairly high temperatures even in cold weather when flat plate collectors perform poorly due to heat loss.
However, the downside of using evacuated tubes is that the panel can be a lot more expensive compared to standard flat plate collectors or solar batch collectors. Evacuated tube solar collectors are well suited to commercial and industrial hot water heating applications and can be an effective alternative to flat plate collectors for domestic space heating, especially in areas where it is often cloudy.
Evacuated tube collectors are overall more modern and more efficient compared to the standard flat plate collectors as they can extract the heat out of the air on a humid, dull overcast days and do not need direct sunlight to operate. Due to the vacuum inside the glass tube, the total efficiency in all areas is higher and there is a better performance even when the sun is not at an optimum angle.
For these types of solar hot water panels, the configuration of the vacuum tube is what’s really important. There are a few different vacuum tube configurations, single wall tube, double wall tube, direct flow or heat pipe, and these differences can determine how the fluid is circulated around the solar hot water panel.
Heat Pipe Evacuated Tube Collectors
In heat pipe evacuated tube collectors, a sealed heat pipe, usually made of copper to increase the collectors efficiency in cold temperatures, is attached to a heat absorbing reflector plate within the vacuum sealed tube. The hollow copper heat exchanger design within the tube is evacuated of air but contains a small quantity of a low pressure alcohol/water liquid plus some additional additives to prevent corrosion or oxidation.
This vacuum enables the liquid to vapourise at very lower temperatures than it would normally at atmospheric pressure. When sunlight in the form of solar radiation hits the surface of the absorber plate inside the tube, the liquid in the heat pipe quickly turns into a hot vapour type gas due to presence of the vacuum. As this gas vapour is now lighter, it rises up to the top portion of the pipe heating it up to a very high temperature.
The top part of the heat pipe, and therefore the evacuated tube is connected to a copper heat exchanger called the “manifold”. When the hot vapours still inside the sealed heat tube enters the manifold, the heat energy of the vapour is transferred to the water or glycol fluid flowing through the connecting manifold. As the hot vapour looses energy and cools, it condenses back from a gas to a liquid flowing back down the heat pipe to be reheated.
The heat pipe and therefore the evacuated tube collectors must be mounted in such a way as to have a minimum tilt angle (around 30o) in order for the internal liquid of the heat pipe to return back down to the hot absorber plate at the bottom of the tube. This process of converting a liquid into a gas and back into a liquid again continues inside the sealed heat pipe as long as the sun shines.
The main advantage of Heat Pipe Evacuated Tube Collectors is that there is a “dry” connection between the absorber plate and the manifold making installation much easier than with direct flow collectors. Also, in the event an evacuated tube cracking or breaking and the vacuum becoming lost the individual tube can be exchanged without emptying or dismantling the entire system. This flexibility makes heat pipe evacuated tube solar hot water collectors ideal for closed loop solar designs as the modular assembly allows for easy installation and ability to easily expand by adding as many tubes as you want.
Direct Flow Evacuated Tube Collector
Direct flow evacuated tube collectors also known as “U” pipe collectors, are different from the previous ones in that they have two heat pipes running through the centre of the tube. One pipe acts as the flow pipe while the other acts as the return pipe. Both pipes are connected together at the bottom of the tube with a “U-bend”, hence the name.
The heat absorbing reflective plate acts like a dividing strip which separates the flow and the return pipes through the solar collector tubes. The absorber plate and the heat transfer tube are also vacuum sealed inside a glass tube providing exceptional insulation properties.
The hollow heat pipes and the flat or curved reflector plate are made out of copper with a selective coating to increase the collectors overall efficiency. This particular evacuated tube configuration is similar in operation to the flat plate collectors, with the exception of the vacuum provided by the outer tube.
Since the heat transfer fluid flows into and out of each tube, direct flow evacuated tube collectors are not as flexible as the heat pipe types. If a tube cracks or breaks it can not be easily replaced. The system will require draining as there is a “wet” connection between the tube and manifold.
Many solar industry professionals believe that direct flow evacuated tube designs are more energy efficient than heat pipe designs, because with direct flow, there isn’t a heat exchange between fluids. Also, in an all-glass direct flow construction the two heat tubes are placed one inside the other so the fluid being heated passes down the middle of the inner tube and then back up through the outer absorber tube.
Direct flow evacuated tubes can collect both direct and diffuse radiation and do not require solar tracking. However, various parabolic trough reflector shapes placed behind the tubes are sometimes used to usefully collect some of the solar energy, which may otherwise be lost, thus providing a small amount of solar concentration.
Other Considerations when using Evacuated Tube Collectors
Due to the sealed vacuum within their design, evacuated tube collectors can get very hot, exceeding the boiling point of water during the hot summer months. These high temperatures can cause significant issues in an existing domestic solar hot water system such as overheating and cracking of the evacuated glass tubes.
To help prevent this from happening in hot summer climates, bypass valves and large heat exchangers are used to “dump” the excess heat as well as mixer valves which mix regular (cool) water with the hot water, to ensure the temperature and pressure levels never exceed a preset limit.
Also, heat pipe collectors should never be exposed to direct sunlight without a heat transfer fluid flowing through the heat exchanger. Doing so will cause the empty heat exchanger to become extremely hot and which may crack due to the sudden shock once cold water begins to flow through it.
Even though evacuated tube collectors are capable of heating water to +50 degrees Celsius in the winter, the outer glass tube of an evacuated tube does not heat up like a normal flat plate solar collectors when in use. This is due to the inherent insulation properties of the vacuum inside the tube which prevents the outer heat tube from being cooled by the outside ambient temperature which can be well below freezing.
Thus in the colder winter months, these types of collectors can not melt away the large quantity of snow that falls on them at any one time which means clearing the snow and ice from the glass tubes daily can be a problem without breaking them.
Even if it is very snowy or very cold, enough sunlight will get through to keep the tubes well above freezing and still be able to preheat the water which can then be heated further by a standard electrical immersion heater or gas burner reducing the costs of heating the water in winter.
Evacuated Tube Collectors are a very efficient way of heating much of your hot water use just using the power of the sun. They can achieve high very temperatures but are more fragile than other types of solar collectors and are much more expensive to install. They can be used in either an active open-loop (without heat exchanger) or an active closed-loop (with heat exchanger) solar hot water system but a pump is required to circulate the heat transfer fluid from collector to storage in order to stop it from overheating.
In our next tutorial about Solar Heating, we will look at another way of heating water using a type of batch collector known commonly as an Integral Collector Storage system or ICS, and see how they can be used to both generate and store the solar hot water.
Dear Sir, I would like to cite this page in my bachelor thesis. However, the author and the date of publishing are not known for this site.
Therefore, it would be very kind of you if you forward me the information that I seek via emailing or by replying the steps here. Thank you.
You can reference the website as: Alternative Energy Tutorials or the URL of the tutorial itself.
Hello
This is me Emad al anani from Egypt
I have our Poultry farm Temperature at winter 12:-5
But we have Sun we can get energy from this What I need smart solution to decrease use fuel consumption
Live in southern California where it gets quite warm in the summer. Would like to use solar tube collector to boil water for my rosemary steam extracts. Currently seems silly to boil water with electric coil when it’s 100*+ outside
Could I use a solar tube system to boil water and save on my electric costs?
Sufficient Evacuated Tube Collectors could be used to boil water at 100oC. Boiling water implies steam, so pressure safety valves and systems may be needed. Another approach could be to use Solar Concentrators that focus the sun’s rays onto a central tube.
Hi everybody, Uncle Pete in sunny RSA average 3>6kWh per sq.m. per 24h day, x 365 days per year, measured through a solar ball that always tracks and thus fully face the sun. Area facing sun, shade, dust, clouds etc. reduce the delta T collector efficiency, as do gaps in the collector area. At sunrise or sunset the sun only see the edge of the collector & atmosphere distance deflect lots.
Non sun track collector should face true pole, pitched Lat. +10.
Bubble pool blanket covers large area preventing evaporation energy loss and efficiently heats low temp water evenly flowing down thereunder (also dark plastic) or stirred down-in by returned pumped water +5d. Back insulated 4m.sq. pool solar panels also high DeltaT (Temp-available minus Water-in Temp) efficient.
In my experience re Residential Solar Water Heaters found that;
Expertly configured and installed with proper auto overheat, overpressure, hail and freeze risks mitigated, Flat Plate Direct Water Heating (or parabolic steampot ET heated) stored in a larger 400kpa cold supply fed Solar Tank that baffled feeds existing auto back-up geyser/s with max temp mixed hot water, that stir-up re-heat the water therein whilst tapping off, until Solar Tank is empty feeding cold water that auto triggers the backup Geyser/s only mid located thermostat and element. 98% Efficient.
Found indirect systems maxed out 60 degrees, thus far less efficient and far less hot water.
In RSA its a pre-condition holes must be drilled into existing backup Geyser/s to qualify for new Solar Water Heater Rebate. Compel backup thermostat and element located halfway in Solar Tank to auto kick in when half of default set 55 C hot water is tapped of. Only half volume @55 C efficient to compel electricity.
Really helpful website. I have learnt a lot.
I would like to install an evacuated solar tube system to heat a swimming pool. The water volume is 22000L. How can I calculate the size of the tube panel system and can you advise on what additional kit I may need. Do I just need the tube panels on the roof, pipe work and a pump plumbed into the existing flow and return system?
For pool heating, it really depends on what type of system (direct, indirect) you want and how much you want to spend. You could have a simple open-loop system with a panel/tubes, pump, located together on the ground with the flow and return pipes over the edge and into the water. Or you could have a larger closed-loop roof mounted system with pumps, tanks, heat exchangers, controls, filters, etc.
The relationship between solar collector area to swimming pool surface area must be sufficient to achieve the water temperatures you expect as you only want to warm the water not heat it. A roof-mounted solar heating system to heat your swimming pool will require pumps to get the water upto the panels/tubes and then gravity can bring it back down again. When the pump is not operating the water may discharge back into the pool so this needs to be considered.
Its a common fact that the heat required to raise 1kg of water by 1oC is equal to about 4.2 kilo-joules. That is 4.2kJ/kg. As the density of water is 1 kg per litre, then the mass of your 22,000 litres pool will be 22,000kg. So to raise the temperature of the water by 1oC will require 22,000 x 4200 = 92.4MJ of solar energy. Then heating the water by 1oC within one hour will require 25.7kWh of energy, as 1 Watt = 3600 J/h. The vacuum tube manufacturers datasheet will give you the thermal performance of their tubes (about 87600 BTU/hr/oC for your pool).
Also to consider. Any pipes, heat exchangers, pumps and tanks should be be swimming pool grade to avoid corrosive effects.
I have an older barn that i use as my shop and garage. (30’x16′, two story) My goal is to make it comfortable in the winter. (0-50 degrees outside temp) I have purchased a 10 tube manifold. I have a 40 gal. insulated tank and will build a forced hot air heat exchanger using baseboard heat tubes. I would like to know what type of antifreezes can be used. (I owned a camper so i have an excess of the pink stuff) and I believe a hot water expansion tank in line would be necessary. Has anyone you know of used a collector in this type of set up, or am i just full of hot air? Thanks for any suggestions.
If your installation is an indirect closed-loop system you can use a glycol antifreeze solution instead of just water as the working fluid similar to the mixtures used in cars and trucks. Glycols or methanols are commonly used as antifreeze agents in solar water systems and should ideally be non-toxic incase of leaks or contamination of water supplies. Also, the antifreeze agent used must ideally be compatible with the materials used in your system to avoid corrosion damage.
Dear sir/madam,
please, can you suggest some methods to close the open end of Evacuated tube
If the tube is open, it is not an evacuated tube and will loose efficiency
Can install in KL?
Yes you can install in KL, whatever KL is?
Very useful info here – thank you. I have a direct flow sealed system. With high temp in the tube collectors the pressure drops slowly to zero but oddly there is no fluid loss from the safety relief valve – although a tiny amount has seeped out through joints on occasions. Within 24-48 hours of lower temps the pressure then slowly returns to normal. During this zero pressure phase the system runs noisily with what sounds like air running through it. I’ve done a lot of research but still have no idea what is happening or what I can do to prevent it. Any ideas ?
Clearly at high temperatures one of your joints (or possibly relief valve) is opening as the pipework expands sucking in air (venturi effect) due to the flow of water being pumped through the pipework. As the water temperature drops the joint closes sufficiently to stop any leakage and the system re-pressurises itself again expelling the air. This is an issue as your system could potential become a vacuum collapsing the walls of any storage tanks.
Suggestions would be to firstly remove the relief valve (and/or vent lines) and seal the pipe. Refill and test. Check and seal joints, bubble test with soapy liquid application around joints one by one. Add dye to water for visual indication of any leak. Lastly, pressure test. Unfortunately, the first leak you find may not be the only leak – check the whole system.
Hi, I am In the process of installing evac tube solar Heating installation (bulb type) this will comprise 4 sets of 18 tubes. This for preheating a 3000 L underfloor heating storage tank and DHW cylinder They will be ground mounted as open access/space is not an issue. Noting the concerns about over-heating, I am wondering if the system would be more efficient connecting the manifolds in series or parallel . What are the pros and cons of each?
Clearly, a series connection is easier for the pipework, but you would effectively have one large collector comprising of 72 tubes. If you have four manifolds, No1, No2, No3, and No4. Cold water would enter No1 get heated with the warm water entering manifold No2, getting heated some more, then to No3, heated again and finally to manifold No4. You would assume the water exiting manifold No4 would be extremely hot, but not necessarily the case as the heated water flowing through manifold No4 (or even No3) maybe hotter than the heat provided by the evac tube heatpipes resulting in heat energy being dissipated back into the tubes. Then ideally, rack No4 should be positioned to be the hottest. Also there maybe a pressure drop between No1 inlet and No4 outlet due to waterflow restriction of the 72 heatpipes inside the manifolds.
For connecting manifolds in parallel branches, the pipework must be such to allow for balanced equal flow through each of the parallel connected manifold circuits. Water will always take the easiest path whichever manifold that happens to be so ensure the flow length, and subsequently the flow resistance of each manifold is the same using valves. A more uniform flow distribution of the water can be achieved by a reverse-flow connecting the four manifolds in comparison to a standard parallel connected manifold system. Parallel flow distribution can also become more uniform with a higher flow pressure. Its your choice.
Hello and thank you for a most interesting article, I have a question concerning Tubes and how to check if they are working correctly. Am I right in saying, if they are relatively cool to touch on a warm / hot day, the Vacuum is still good? or is there another way to check?
Evacuated tube solar collectors are commonly made up of vacuum glass tubes, as the absence of air inside them highly reduces the effects of any convection and conduction thermal losses. Heat extraction is accomplished using a long thin heat pipe absorber or a flow through absorber. The internal absorber and the outer glass envelope of an evacuated tube can get very hot being sat in direct sunlight, receiving solar radiation from multiple angles, due its tubular design.
Sometimes due to the lack of enough cold heat absorbing water in the tube or a connected heat exchanger, it can cause a thermal shock to the glass of the tube which may crack loosing vacuum. However, a cracked or damaged tube does not stop working but will continue to work at a lower efficiency.
Normally evacuated tubes have a getter material, usually barium, coating inside as a simple visual indicator of the tubes vacuum status. A bluey-silver tube is good, a change in colour, usually white indicates a cracked or damaged tube. Then touch alone will not identify lack of vacuum.
How do we calculae thermal efficiency η of evacuated tube solar collectors of various types? I am interested in knowing the formulas for this calculation which involve solar radiation, area of the solar collector, transmissivity, absorptivity, heat removal factor, ambient temperature and heat transfer fluid inlet temperature. I am aware about the calculations for thermal efficiency η involving Solar Flat Plate Collectors but find that the same don’t apply to Evacuated Solar Collectors. Any help would be appreciated.
The instantaneous thermal efficiency is commonly defined as being:
η = ( m.CP(TOUT – TIN) )/AC.I
Where:
m = water flow rate (kg/s),
CP = specific heat (kJ/kg-K),
TOUT = water temperature output in oC,
TIN = water temperature output in oC,
AC = collector area in m2,
I = solar radiation in W/m2
I am exploring the possibility of using evacuated tubes to heat about 2000 gallons of water in a greenhouse. My goal would be to collect heat during the day and to let the heat discharge into the building at night. All I need is to keep the space from freezing. I am thinking of building a tank which could also double as a bench for plants to grow on allowing the heat to rise up through the roots and out the sides of the tank. I would also use a thermal blanket over the crop at night. I need help in determining the feasibility of such a project and sizing of the system.
Clearly the size of your system would be determined by the available space inside your greenhouse. A bench with 2000 gallons (about 8000 liters) of water inside would weigh a lot, over 8 US tons! Then you will need a strong bench, some tubes or panel, thermostat and a pump to circulate the hot water.
What is the best way to decommission the solar tubes? Ideally I want to the them on the roof and just disconnect in the house. Thanks
Clearly empty the system of water and disconnected them
Do small direct flow tubes always have both manifolds at the same end or do some have one at each end as you would see with the much bigger Schott PTR70? Also are you familiar with small direct flow tube tolerance to heat transfer fluids in the 300degC (500F) ballpark? Working on a thermal storage DIY research project
Solar thermal evacuated tubes can be of a single ended or double ended open design with or without heat-pipe depending on the application. Parabolic trough concentrates require the absorber tubes to be exactly at the focal-point otherwise deterioration, cracking, heat transfer losses and thermal stress can result. For high temperatures evacuated tubes are made of Borosilicate glass.
Do you need a heatexchanger with the vacuum glass tubes?
The use, or not, of a heat exchanger will depend greatly on the type of Solar Thermal system. Open-loop systems generally do not require a heat exchanger, whereas closed-loop and storage tank systems would. Generally, the heat pipe of an Evacuated Tube Collector plugs directly into a copper manifold which itself acts as a heat exchanger.
Good afternoon, I am in position of 2000 plus of these tubes and I would like to give away some of them maybe for reuse or refurbishment because some of them are not in usable condition. Can you please assist?
Please state your location so that local people may reply.
on the green path of sustainability and innovation 🙂
Hello I am building a parabolic trough collector and am looking for tubes, my email is Mattwallacewebdesign@gmail.com, I’d love to hear from you.
Have you been able to give these away? I would think that the receivers would be willing to pay shipping cost.
Hi , has any one thought of using EST for heating water on a small sailboat, would they be too fragile when exposed to the sea wind conditions
Evacuated tubes are between 100cm and 120cm (about 39 and 48 inches) in length, are made using glass tubes so require a large rigid and fixed support or mounting frame. The thickness of the pyrex type glass determines the strength, longevity and efficiency, as well as cost. So if you can find suitable EV tubes and manifold to mount on a boat, then why not, but they may crack in bad weather.
Dear Sir.
According to my understanding there are 2 types of the direct flow evacuated tube collector solar water heaters.
One is an integrated system where the collector tubes are directly connected into the water storage tank and the second is a manifold system where the collector tubes are separate from the water storage tank and the heated water goes from the collector tubes into the water storage tank through a single pipe.
My question is which of the 2 systems is more efficient/effective in producing hot water with minimum hours of sun light?
2. which of the 2 systems has less over night heat loss?
3. Which system has best storage efficiency in terms of general heat loss?
4. What is the best solution when you have 3-4 hours of sun in the winter
1 system of 500 litres or 2 systems of each 250 litres connected in series?
5. Is it generally always better to have 2 smaller systems connected in series or 1 single bigger system when you have 5-6 hours of sunlight?
What is more effective in terms of producing heat quicker and with least amount of heat loss, single system or 2 smaller systems connected in series?
Very grateful for a quick answer.
Best regards Kristian
it is possible to use vacumn tubes to heat air for déshydratation of frut in solar dehydrator. d you have somme information about this systems