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Tube Solar Collector

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Introduction

The solar collector generally consists of Evacuated tubes that are responsible for absorbing heat in solar water heaters. This technology has been there for many years and it gained popularity in countries such as Canada, China, UK and Germany among other countries that pioneered     industrialization. These tubes collect solar energy and convert it into heat that is used in water heating. It is worth noting that there are different types of evacuated tubes that are used in solar industry in different countries. For instance, Apricus tube solar collectors use the most common twin glass tube due to its reliability, low manufacturing cost and high performance. All –glass vacuum tube collectors and commonly used in China since it is cheap, economical and has the ability to withstand cold seasons without freezing. For example in the year 2005, China only produced over 15 million all-glass vacuum tube collectors and majority of these have been used in domestic water heaters (Zhiyong, Chen,  Luo, Zhang & Xue 2010).

Types of tube collectors and their functioning

There are two major categories of solar collectors. There are those that are for heat generation and those that are for electricity generation. Solar collectors that are for heat generation are further divided into flat plate collectors and evacuated tube collectors and all- glass vacuum tube collectors. On the other hand, solar collectors for electricity generation are further divided into parabolic trough, power tower and parabolic dish among others. For the purpose of this study, the concentration is on solar tube collectors which are normally for heat generation in water heaters with more emphasize on the all- glass vacuum tube collectors that are used in China (American Society of Mechanical Engineers 2011).

The fact that the Tube solar collector has a vacuum between its inner and outer walls enables it to lose less heat by convention. The vacuum cover around the absorber surface prevents excessive loss of heat to the environment making tube solar collectors more suitable in terms of performance than flat plate collectors. Though evacuated tuber solar collectors are efficient especially were high temperatures are required, they are also very expensive when compared to less effective solar collectors such as batch solar collectors, integrated tank collectors and flat plate collectors which are cheap to purchase and operate. However, tube solar collectors are the most appropriate when it comes to commercial use and industrial heating especially in areas where there is little sunlight.

Evacuated -tube collectors use heat pipes for their instead of passing liquid directly through them. Generally, these pipes are made of several evacuated glass tubes and each of them has an absorber plate connected to the heat pipe. Heat from the hotter parts of the heat pipes is transferred to the transfer fluid of a domestic hot water or hydronic space heating system in a heat exchanger called known a manifold. The transfer fluid is normally made up of water or an antifreeze mix which is mostly propylene glycol (Garg2000). The manifold has insulation and is wrapped by a sheet of metal or plastic case that protects it from its elements. It is the vacuum that surrounds the outer part of the tube that prevents loss of heat through convection and conduction making them more efficient than flat plate collectors especially in areas where temperatures are normally low. However, this also becomes a problem in warmer places unless in situations where very hot water is required such as in commercial and industrial use. Therefore, the collector needs a special system in warm climates to prevent overheating conditions that could be very dangerous to both the collector and the user. This is the reason why flat plate collectors are still used in some parts of the world regardless of the fact that more efficient solar collectors have been invented and are readily available.

1. Direct flow evacuated tube collectors

This form of a tube solar collector is made up of two pipes that run down and back inside the tube. One of the tubes brings cold water into the heater and the other one carries the heated water out of the heater. Due to this continuous flow of water in and out of the heater, the tubes are not easily replaced. Therefore, in case of a breakage of the tube, all the fluid is pumped out of the heater if it uses a closed loop or water is allowed to flow out freely if the system uses an open loop. (American Society of Mechanical Engineers 2011)

2. Heat pipe evacuated tube collectors

This is another type of a tube solar collector that is domestically used in many parts of the world. The collector is made up of a heat pipe that is made up of copper and is attached in the absorber plate inside the vacuum sealed solar tube. This copper pipe is hollow and its inner parts are also evacuated. Inside the heat pipe there is a small quantity of liquid which normally purified water or alcohol with special addictives. The vacuum that is between the inner and the outer pipes allows the water to boil at lower temperatures than it could have happened in normal conditions. Basically, when sunlight falls on the surface of the absorber, the liquid in the heat tube is immediately converted into vapor and rises to the top of the pipe making water or glycol to flow through a manifold where it picks heat.

Consequently, the fluid in the heat pipe condenses and flows back down the tube. This process remains continuous as long as there is sunshine. Heat pipe evacuated tube collectors are more effective than direct flow collectors due to presence of a dry connection between the absorber and the header make installation much easier. More so, individual tubes can also be exchanged without empting the entire system of its fluid hence breakage in one of the tube has insignificant effect to the whole system. However, heat pipe collectors have to be placed in a tilt position of about 23° to ensure that the internal fluid of the heat pipe returns to the hot absorber (Garg2000).

Considering its functioning, heat pipe evacuated tube collectors has solar heat collecting panels that are well designed making them appropriate for increasing requirements of building aesthetics, high efficiency and working with system pressure. This makes them widely used in individual residence and in large hot water project such as central heating, industrial heating process and in swimming pool heating. The collector is also designed to be used up to 8 bar/116psi. This means that the collector is able to overcome low pressure and most mains pressure domestic hot water systems.  In closed loop an expansion vessel is fitted to the tube to prevent pressure buildup as water expands during hot seasons.  Heat pipe evacuated tube collector does not have a built in tank but only the manifold of about 510ml/1pint of water in a 20 tube solar collector. A circulation pump is required to circulate the water through the manifold and back to the solar storage tank. Generally a controller is used to control the pump. A flow rate of only 2L/min is required for most domestic installations, and therefore a low wattage pump is sufficient. Larger pumps are only necessary when several solar collectors are connected in series, or when the pump is required to overcome head pressure (American Society of Mechanical Engineers 2011).

Heat pipe evacuated tube solar collectors are also manufactured in such as way that they suit aesthetic requirement of most of the users. This is because research has shown that many people are more concerned with how solar collector will appear in the roofs of their houses before stalling them. Therefore, the tubes are black in color and they match nicely with colors used in most roofs. In additional to that, this collector uses pure copper piping and silver braze for the header to avoid corrosion especially in situations where the environment is warm and water is highly chlorinated. It should be noted that copper is less corrosive and is used in making domestic plumbing hence making heat pipe evacuated tube solar collector one of the most appropriate water heaters for domestic usage (Garg2000).

3. All glass vacuum tube solar collector

This type of a solar tube collector is the one mostly used for domestic heating of water in China. All glass vacuum tube collectors are made up of vacuum tubes and a manifold header which interconnects the tubes and circulates heat transfer fluid.  Basically, the tubes are made up of three parts which are two concentric glass tubes sealed at one end, annular vacuum space between the tubes and the selective coating on the outside wall of the inner glass tube. The water in the tube is heated by solar radiation, and then rises along the top of the tube to the manifold header mixed with water in it, after mixing they flow into the next tube together driven by the pump. At the same time, the mixture water from the pre-section of manifold enters into the bottom of this tube and replaces the heated water.

Generally, all glass vacuum tube solar collectors contain a double layer of glass fused together at lone or both ends with a vacuum between the layers which allow the absorber and the heat pipe to remain at a normal atmospheric pressure. However, the glass- glass tubes have a weakness in that though the device tubes have highly reliable vacuum seal, its two layers of glass reduces the light that reaches the absorber and this increases the likelihood of moisture entering the non evacuated area of the tube causing corrosion. This is a challenge that is not experienced in glass- metallic tubes which allow enough light to the absorber and prevent it and the heat pipe from corrosion. However, the good thing is that gaps between the tubes allow snow to fall through the collector hence reducing loss of heat production during snowy conditions.

Challenges of tube solar collector

Though tube solar collectors have proven to be very efficient in water heating in both commercial and domestic use, the technology is also faced with several challenges. The major challenge is the fact evacuated tube collectors can get very hot above the boiling point of water and this is very dangerous in an existing domestic solar water system (Garg2000).This forces the users to ensure that all the hot water is used on daily basis to avoid overheating of water in the tanks. This is the only way to ensure that mixers are easily installed just after the last hot water tank and mix regular water supply with the hot water to ensure the temperature do not go above the set limit.  A single day without using water from tube solar collectors raises the temperature of water above the boiling point resulting to high pressure in the tank. This pressure blows hot water onto the lawn killing the grass and also creating safety hazard (Window 1999).

Another challenge arises when the panel as to be mounted on the ground. Mostly one is required to cover the unit before leaving the house for a long time. It is therefore worthy noting that their tubes are normally made of annealed glass which is more delicate than tempered glass. This means that one has to be very careful while covering them. More so one has to take care of overhangs i.e. slide ice and snow loads and falling branches while selecting location for the panels. This makes it difficult to operate tube solar collectors in areas densely populated or in areas where there is cases of snow falling. It is also a challenge to use evacuated tube collectors in winter unlike with flat plate collectors since they are not able to melt large quantities of snow that fall on them making them less efficient. There is a big likelihood of breaking the tubes while clearing the snow off the glass tubes (Window 1999).  

Comparison between tube collectors and flat plate solar collectors

There has been differing opinions from researchers and energy scientists about the reliability of these two technologies on the basis of which one is more efficient and reliable. Unlike the flat plate solar collectors, tube collectors have a discontinuous absorbance area. It also has open space between collector tubes is vacuum filled to minimize excessive loss of heat through conduction and radiation processes (Garg2000). Therefore, the collector gross area is larger in tube collectors than it is in flat plate solar collectors since the square meter of roof area covered with evacuated tubes is larger than the actual absorbers in flat plate solar collectors. On the other hand, evacuated tubes occupy more space on the roof than flat solar collectors making them less suitable to some users. However, it should be noted that specification of the ISO 9806 standard on how solar thermal collectors efficiency should be measured are ambiguous making it even difficult to rank the two collectors on this basis. Consequently this makes it difficult for users to make informed decisions on which collector to purchase.

In terms of heat loss, evacuated tube solar collectors loss little heat to the environment that it is with flat pate collectors. According a report released by solar energy researchers in China show that show that flat plate collectors lose twice as much heat that is lost by evacuated tube collectors making then inappropriate choice of solar collector for high temperature commercial applications in processes such as steam production. In terms of absorber plates, evacuated tube collectors have a lower absorber plate area when compared to flat plates. However, based on the absorber area efficiency, evacuated tube collectors are more effective per square meter than equivalent flat plate systems. Consequently evacuated tube solar collectors are more suitable where there is limited roofing especially in areas where the population is very high and occupants of a building exceeds the roof space for mounting the collector.

Another difference between the two collectors is that evacuated heat pipe tube (EHPTs) serve as a thermal one way valve due to their heat pipe which is not evident in flat plate collectors. Consequently, tube solar collectors have an inbuilt maximum operating temperature making them safer than flat plate collectors although there are arguments that glass tube collectors are likely to shutter after a long period of exposure to sunshine without being filled with cold water due to their in ability to resist internal thermal shock .

Conclusion

It is evident that tube solar collector is a newer technology when compared with flat plate solar collector which have been in use for long time in the history of solar energy. Tube solar collector has several advantages and disadvantages but considering its efficiency and solar collector demands in the modern world, the technology is the most appropriate for both domestic and commercial water heating. However, tube solar collector technology need to addresses few things such the installation and maintenance cost, durability of the system, and cases of vacuum leakage after some time. This will make tube solar collectors more popular in other parts of the world other than Asian countries, where the technology is highly embraced currently both by domestic and commercial users.

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