A tube solar collector refers to a successive number of glass tubes, connected to a pipe, placed parallel to each other. Air is evacuated from the individual tubes creating a vacuum aspect within them. The vacuum in the tube ensures that the surrounding temperatures are kept at a certain minimum as well as maintaining the loss of thermal heat to the least. As a result of this vacuum, these types of collectors are able to give higher temperatures than other collector alternatives like the flat plate and as such can result to extreme temperatures in the summer (Sobhansarbandi, 2017). The inner tube has a metal heat pipe running through it which is attached to a curved or flat copper or aluminum fin. A selected coating is then applied to this fin which is responsible for transferring heat to the pipes containing the running fluid. A hot bulb is then used to heat indirectly the header tanks copper manifold, this is achieved by convection; transfer heat to the hot bulb from the sealed copper heat pipes (Colombo, Bologna, & Masera, 2013). A common manifold joins all the copper pipes and is then used to heat water in a storage tank. The insulating properties of the tank allow the water to be utilized either during the day or night.
The evacuated tube collector is formed of certain components that are shown in the figure below.
(Vijayakumar, Vijayakumar, & Kumar, 2017)
Figure 1 Example of Evacuated Solar Tube Collector
Energy from the sun is absorbed by the evacuated tube. Since energy needs to be retained for longer the insulating properties of the vacuum tube come in handy. The aspect of solar absorption in this technology however dictates the design that can be best use. For instance, the twin glass tube which is a common type of evacuated tube collector technology has an outer tube that is able to withstand severe environmental hazards such as hailstones. The inner tube is however coated with an excellent solar absorber in this case (Al-N/Al). A barium getter is usually used in the design to maintain the vacuum. A pure layer of barium is formed when the evacuated tube is subjected to high temperatures during the design process. The work of the getter is to absorb the gases ousted from the tube during the operation and storage which include: hydrogen, water, nitrogen, carbon (IV) oxide and carbon (ii) oxide.
Over the years the most commonly used material used in the absorbance layer was copper but due to the price changes it made it hard to manufacture tube collectors with copper. Although it is still being used, engineers decided to use alternative like aluminum that is a readily available conductor. Collectors with aluminum reduce the cost of obtaining a collector with Cu-Cu almost by half. Another way of reducing cases of poor conductivity is by the use of heat pipes in the design. They are better than Cu and Al in terms of thermal conductivity. Between hot and cold interfaces they experience lower temperature differences. The working fluid plays an important role in the efficiency of a collector to convert heat to energy without any losses. Researchers are now insisting on using Nano fluids, this is because miniaturized technologies are coming up and clogging can of the tubes can be a problem when the conventional fluids are used.
There are two main tube configurations that can be incorporated in the design of an evacuated solar collector. These configurations are responsible for the difference in efficiency in such systems (show how the fluid will flow in the system). They include; heat pipe or direct flow, double wall tube, single wall tube (Kitcher, 2012).
This contains a copper heat pipe, attached to a reflector plate capable of absorbing heat that is usually sealed and placed in the vacuumed tube. Copper material is used to increase the efficiency of the collector during cold weather. Even though the heat pipe is evacuated, it still consists of a low pressure liquid either alcohol/water with some additives that prevent oxidation or corrosion (Laughton, 2010). When the absorber plate inside the tube is hit by direct sunlight, the liquid is forced to evaporate at a slower rate due to the vacuum inside forming a hot vapor gas that rises up and heats the top part of the pipe increasing its temperatures significantly. The tube is connected to a copper manifold that extracts the heat from the hot rising vapor and uses it to heat the glycol or water that flows through it. The hot vapor cools and condenses after losing energy and so flows back to the pipe to continue the process. To facilitate this process the evacuated tube collectors and the heat pipe need to be placed in suitable angle (preferably 30?) to allow for the condensed liquid to easily flow back (Mukhtar, 2017).
(Mike & D, 2016)
Figure 2 Heat Pipe Evacuated Solar Tube Collector
Installation of this design is easier because of the dry connection that exists between the manifold and absorber plate. Another case is that the tubes can be individually replaced in case of loss of the vacuum either by breaking or cracking making them suitable for closed loop solar designs.
These are different from the heat pipe tubes because in the centre of their tubes they have two running heat pipes. Also known as ‘U pipe collectors’ because of the shape formed when the two pipes meet at the bottom. One is the return while the other is the flow pipe (Nusz & Ramlow, 2010). The pipes are then separated by the heat absorbing reflective plate through the collector tubes. Insulation is achieved in this design option by vacuum sealing the heat transfer tube and the absorber plate.
(Shulmer & Tom, 2015)
Figure 3 Direct Flow Evacuated Tube
The overall efficiency is increased by using copper material in designing the curved or flat reflector plate and the hollow heat pipes. The only difference between this configuration and the flat plate collector is the provision of the outer tubes vacuum.
Due to the different tubes provided for in flow and out flow, this configuration is not as flexible as the heat pipes. There is a wet connection between the manifold and tube so the system will require draining if a tube is cracked because they cannot be individually replaced (Jager, 2013). This configuration is considered to be efficient than the heat pipes because it doesn’t use the concept of heat exchange between fluids. To increase the efficiency of the system, the tubes are placed one inside the other such that the flow is down to the interior of the inner tube up to the absorber tube on the outside.
These evacuated tubes do not need solar tracking because of their capability to extract both diffuse and direct radiation. But, some of the solar radiation that might have been lost can be recollected by some reflector shapes that are placed behind the tubes (Deutsche Gesellschaft für Sonnenenergie, 2005).
The issue of return on investment and cost has been discussed ever since the invention of solar thermal technologies. When the different elements of coming up with a solar system are ranked as per the prices then the control system and collector make up 1/3 of the overall costs, costs incurred during installations comes next (Cassandra & Austin, 2009). Understanding the various costs involved with an evacuated solar tube collector helps in making an informed decision. This table can be used to describe the same.
Figure 4 Costs in percentages of Production of ECT
To begin with, the cost of the whole system is determined by the variable and fixed costs incurred in its production. Some of the fixed costs include selling and distribution, factory overload (maintenance, insurance, rent, depreciation); administration expenses, selling and distribution and warranty. These usually stay constant even with change in production output. This can be described in the figure below.
Figure 5 Graph of Cost vs. Units per day
The labor costs are usually lower than the cost of materials and as a result are prone to changes in the material market. Raw materials are not the major contributor of costs in the finished product, but is dictated by the number of produced units. When the production volume reaches an optimum level then the variable costs come into play. This is when the costs of materials affect the last costs. This effect is shown in the figure below.
Figure 6 Graph showing increasing production with reduced unit cost
From the cost analysis, it is evident that the logistic expenses make up 37% of the final cost, the system instrumentation and collector account for 33%, the installation makes up 14% and finally the balance of the system makes up 5% (Norton, 2013).
When it comes to the operational costs of evacuated solar tubes, they can be compared to their flat plate counterparts. For instance, the evacuated tube collectors will cost more than the flat plate by 20%. This is however considered by comparing the cost per BTU capacity and the evacuated tube collectors are known to have a lower cost per BTU in cool climates.
They are extremely cheap when it comes to shipping costs because they can be placed vertically during shipment and thus maximize on the given space. Furthermore it only takes one person to install it. This makes it easier because the user will not incur additional labor costs for purposes of installation.
Finally, operating an evacuated tube solar collector depends on the location being used. For instance, in some regions many collectors are required to heat the same amount of water that can be heated by only a few collectors.
The following assumptions are made while giving the analysis of a solar collector tube with U-tube.
Negligible resistance to thermal effects of the outer glass tube
Conduction of gas is neglected since the vacuum between two glass tubes is perfect
With normal incidence solar radiation angle, we consider steady state conditions
Between the absorber glass tube and the aluminum fin there is a layer of air of small thickness.
The solar power absorbed S, is equivalent to optical losses times the incident solar power and can be shown as follows (Santamouris, 2014):
Where α- absorptance of the glass cover
G- Collector surface global solar irradiance
The collector’s heat balance equation for each of its parts can be outlined as follows:
The vacuum layer in between the space of the inner and outer tube eliminate the possibility of conduction of heat, this is because the coefficient of heat conduction is below 0.27 x 10-5 W/m °C and the level of vacuum is 10-4 Pa.
The equation applied at this point is:
Equation ii (Ghoneim, 2017)
Where Ta, TG, TR, are the ambient temperature, outer glass tube temperature and inner glass tube temperatures respectively.
hg-a is the coefficient of heat transfer between the ambient environment and outer glass tube.
hr-g is the coefficient of heat transfer between the outer and inner glass tubes.
Lr–g = (Lr + Lg)/2, where Lg and Lr are the outer and inner glass tube parameters.
The coefficient of the overall heat transfer between the aluminum fin and inner glass tube is represented by hr-Al. the temperature of the aluminum fin is denoted by TAl. The conduction and radiation coefficients of heat transfer between the aluminum fin and inner glass tube are given by hr-Al, cond, hr-AL,rad whereby hr-Al is given by adding the two coefficients. The outer glass tube transmission coefficient is given by τg. The inner glass tubes selective coating absorption coefficient is denoted by τr. The outer glass tube diameter is dg, the collector aperture surface receives solar radiation and is denoted by G. the loss of heat to the manifold from the tube edge is given by Qe and the perimeters of the aluminum fin and inner glass tube can be added together and the average calculated in order to obtain Lr-AL.
The thermal equation for this particular component is given by;
Where the temperature of the U copper pipe is given by TCu; The coefficient of total heart transfer between the copper pipe and aluminum fin is denoted by hAl-Cu , and can be calculated by adding the conduction and radiation transfer coefficients between the copper pipe and aluminum fin.
The usual coefficients are used for this equation with an addition of coefficient of convective heat transfer within the copper pipe. This fluctuates with speed of flow.
All these equations are modified to come up with a theoretical model that can help tell the operational effectiveness of an evacuated solar collector tube in different climatic zones.
The effective operation of these evacuated collector tubes depend on certain factors. Some of which can be influenced by surrounding climate. For instance, they can get very hot due to their design of sealed vacuum area. These temperatures can go beyond the boiling point of water especially in the hot summer months which can lead to cracking of the evacuated glass tubes (Siegenthaler, 2012). This can however be prevented by using loads that will ensure the temperatures are maintained at a constant level. For instance, the excess heat is “dumped” using large heat exchangers and by-pass valves, and also mixing of hot and cold water by mixer valves. All this is to ensure the pressure and temperature levels remain at a monitored level (preset limit) (Wang & Ge, 2016).
The heat pipe collector configuration should also not be directly exposed to sunlight when there is no fluid going through the heat exchanger. Otherwise it would be extremely hot and crack once exposed to cold water because of the sudden change in temperature. Being that evacuated tube collectors are able to heat water to high temperatures of up to 45?C, their outer tube doesn’t acquire the heat like other collectors, this is because of their intrinsic insulating properties of providing a vacuum that will prevent the outside ambient temperatures from affecting the inner tube even if it is below 0?C outside. These solar thermal technology needs to be installed in an area that is less likely to experience disasters such as landslides, or branches falling or even destruction by hail storms (Enteria & Akbarzadeh, 2013).
Otherwise, they are resilient to various weather conditions if properly mounted. These include: super strong winds, thunderstorms, excessive heat waves and cold temperatures.
In conclusion, it can be seen that the evacuated tube solar collectors is a preferable solar thermal technology. Not only will they reduce significantly the amount used in seeking for alternative means of solar thermal technology on a monthly basis. But also encourages healthy living because they solely rely on the suns energy to produce heat. All this coupled with modern technology, has given this option an upper hand because it can now be sourced easily in terms of design thus making it a profitable venture also for market providers.
References
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Colombo, E., Bologna, S., & Masera, D. (2013). Renewable Energy for Unleashing Sustainable Development. Berlin: Springer Science & Business Media.
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Norton, B. (2013). Harnessing Solar Heat. Berlin : Springer Science & Business Media.
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