The present study is composed, in order that a laboratory experiment on an evacuated solar thermic aggregator can be analytically presented and discussed. A solar thermic aggregator consisted of 10 evacuated tubings, has been tested in footings of its efficiency, in laboratory conditions. The intent of this study is the presentation of the obtained efficiency curve and the probe of its characteristics. Effort has been posed to the rating of the public presentation of the tried solar thermic aggregator in relation to internal and external factors of the system. The degree of cogency of the experiment and any deductions of the consequences have been besides discussed.
It has been concluded that… The consequences showed that…
Today, domestic hot H2O every bit good as infinite warming can be expeditiously provided, working solar radiation by agencies of evacuated tubing solar thermic aggregators engineering. Despite their high cost and their complicated design, they are more preferred, compared to other solar thermic types, because of their high efficiencies, when installed in the appropriate environment.
A solar thermic aggregator is a type of solar energy system for medium to high temperature aggregation. Solar radiation is used as a beginning of energy. The operation of a solar thermic aggregator is based on the soaking up of solar heat, and its circulation through the system, in order that a working fluid ( H2O for case ) can be heated.
The system consists of a figure of tubings, made of dual bed glass, which lie in sequence. An enclosed vacuity system operates when solar radiation reaches the tubings, insulating the collected heat. The heat absorbed by the absorber home bases of the tubing is so transferred to the circulated H2O by agencies of heat pipes.
Picture 1: Operation manner of an evacuated tubing solar thermic aggregator
The efficiency of a solar aggregator can be defined as the part of solar energy, which is converted into thermic energy. The entire efficiency is the amount of the instantaneous efficiencies, which are experienced by the solar thermic aggregator. Analyzing these distinct efficiencies, decisions can be made, sing the behavior of the system, in footings of its entire efficiency and its influential factors. The expression for the instantaneous efficiency of an evacuated tubing solar thermic aggregator, is presented below:
I: the solar radiation ( W/m2 )
A: the country of the aggregator ( M2 )
: the mass of go arounding H2O through the system in 1 2nd
cp: the specific capacity of the H2O ( J/kgK )
?? : the difference between the recess and outlet temperature of the flow ( K )
As it can be seen from the equation above, the efficiency of the solar aggregator is influenced by the given features of the system every bit good as its environing environment. These conditions define the system ‘s ability to change over the available solar radiation to utile heat.
Hence, the entire efficiency of the system can be estimated if its heat additions and losingss are evaluated. Convection and radiation are the chief grounds for losingss. However, conductivity, convection and radiation losingss can be besides experienced as a consequence of the features of the different constituents of the aggregator.
Picture 2: Heat additions and losingss, taking topographic point during the operationof the system
Carefully analyzing the solar energy balance in the solar aggregator ( equation 2 ) , Hottel-Whillier expression operation for foretelling the system ‘s entire efficiency can be easier understood.
n0: the initial efficiency
Uracil: the heat transportation coefficient
Tamb: the ambient temperature
Titanium: the recess temperature of the H2O into the system
Stagnation temperature is a important temperature of a solar aggregator, when its efficiency reaches a nothing value. When the stagnancy point is reached, the flow rate of H2O is zero, which means that the system has lost its ability to change over solar energy into utile heat.
The aim of the present experiment is to analyze the behaviour of an evacuated tubing solar thermic aggregator. A sequence of temperature records helps in specifying the system ‘s efficiency curve, which is used for the rating of the system ‘s operation manner. It is besides of import to be stated that the experiment takes topographic point in laboratory unreal conditions, which means that a big discrepancy in values from the world is expected. This discrepancy is logically expected as the research lab visible radiations can non exactly imitate the Sun and the experimental setup is a scaly non-optimized theoretical account.
The experimental process was implemented in consecutive stairss so that it can be completed successfully, pull outing accurate and dependable consequences.
First, the pump was turned on, so that H2O was able to be circulated through the system. Its selected volume flow rate was of 0.3 litre per minute. Next, a computing machine connected by detectors with the experimental set up, was assigned to roll up information on temperatures. After exchanging on the visible radiations, the temperature of the armored combat vehicle, the recess and mercantile establishment temperatures of the circulating H2O every bit good as the ambient temperature were recorded for every minute of operation. The process was completed after one hr, when the computing machine had already selected 60 values of temperatures. Finally, the recorded values were saved in an excel file for farther analysis, while the system was turned off.
Six chief constituents contributed to the procedure of the experiment. These equipments are presented below:
Beginning of visible radiation: solar radiation was simulated by 8 tungsten visible radiations. The fake irradiance was 1000 W/m2.
Solar energy aggregators: 10 evacuated tubings were responsible for the aggregation of irradiance from the tungsten visible radiations. Their entire country was of 1m2.
Heat pipe: heat is transferred from the solar aggregator to the heat pipe bulb.
Manifold: an equipment through which H2O was go arounding in order to absorb the gathered heat. Good insularity is a important parametric quantity which enhance the efficiency of the system.
Pump: an indispensable constituent for the circulation of H2O. Water was pumped to keep its rate of flow to 0.3 litres per second.
Flow metre: a constituent used so that the volume flow rate can be checked.
Water armored combat vehicle: an equipment where the circulating H2O was temporarily stored.
Detector: the detector was connected to three thermometers by agencies of three twosomes of overseas telegrams and it was responsible for the sensing of ambient, recess and mercantile establishment temperatures.
Computer: a device which was responsible for the temperature record. It was having information from the detector patchboard and was entering them by agencies of an appropriate package.
Using the known parametric quantities of the system into the appropriate equations, which govern the behavior of the system, utile consequences can be extracted in footings of the system ‘s efficiency every bit good as the temperature trends during the experimental process. The known parametric quantities are the undermentioned:
Volume flow rate = 0.3 liter/min
Density of H2O = 1 Kg/liter
Irradiance = 1000 W/m2
Area of the aggregator = 1 M2
Specific heat capacity of H2O = 4200 J/KgK
Some unit transitions were necessary in order that all the equations could be sold, utilizing a incorporate unit system ( SI ) .
Volume flow rate = 0.3 liter/min = & A ; gt ; 0,3Kg/60sec = 0.005 Kg/sec
The recorded recess and mercantile establishment temperatures of the flow, every bit good as the recorded ambient temperatures are presented in the diagram below ( diagram 1 ) :
Diagram 1: Inlet, mercantile establishment and ambient temperatures during a period of 1 hr ( 60 proceedingss )
The instantaneous efficiencies of the evacuated tubing solar aggregator during the one hr period ( 60 proceedingss ) of the experiment, are presented in the diagram below ( diagram 2 ) :
Diagram 2: Instantaneous efficiencies during a period of 1 hr ( 60 proceedingss )
Diagram 3 is a typical graph for an evacuated tubing solar aggregator. It illustrates its efficiency against ( Ti-Tamb ) /I values. A additive graph was applied to come close the initial graph in order that the unknown features of the system can be easy determined.
Diagram 3: Efficiency against ( Ti-Tamb ) /I
The heat transportation coefficient ( U ) every bit good as the initial efficiency of the solar aggregator can be calculated by agencies of the undermentioned equation:
The heat transportation coefficient ( U ) can be represented by the incline of the approximative line graph. Hence, the heat loss rate between the system and its environment can be easy defined as 1.4786. ( U= 1.4786 )
In add-on, the initial efficiency which means the efficiency of the system when its recess temperature is of the same value as that of its milieus, can be straight extracted from the above equation as 0.3072. ( n0=0.3072 )
Finally, the stagnancy temperature of the solar aggregator can be besides determined by agencies of the above expression. Harmonizing to its definition, the stagnancy temperature of the system is the temperature which is achieved when the efficiency is zero. Using the definition to the above equation, the undermentioned expression is extracted:
ten = = & A ; gt ; ( Ti-Tamb ) /I = = & A ; gt ; ( Ti-Tamb ) =
Assuming an mean ambient temperature of 28.4 & A ; deg ; C,
Ti= 236.16 & A ; deg ; C
Discussion and Decisions
Analyzing the recorded recess, mercantile establishment and ambient temperatures which are illustrated in graph 1, of import decisions, in footings of the system ‘s adaptability of the new conditions at the beginning of the experiment, can be extracted. The warm up stage of the system can be easy observed as the recess temperatures of the flow at the beginning of the experiment are lower than the ambient temperatures. These low recess temperatures are a consequence of a faulty operation of the heat money changer which can non adequately heat the circulating H2O. Hence, these values have to be discarded in order that accurate consequences can be obtained.
By and large, larger values of mercantile establishment temperatures can be observed compared to the recess temperatures of the H2O flow. This is likely to go on because of the direct heat exchange between the heat money changer and the mercantile establishment flow. Equally good as this, a changeless addition of the recess temperatures can be noticed. Perversely, there is a rapid growing of the mercantile establishment temperatures, particularly during the warm-up period. It can be besides seen that after 29 proceedingss of operation and when the warm up period has already ended, a changeless difference between the recess and mercantile establishment temperatures is achieved.
Analyzing the instantaneous efficiencies of the evacuated tubing solar thermic aggregator from graph 2, it can be seen that a maximal efficiency of around 37.5 % is achieved. The instantaneous efficiencies of the aggregator addition quickly during the first 16 proceedingss of operation, making their maximal value and so they experience a little bead until the 29th minute. The instantaneous efficiencies remain about changeless at a value of about 29 % from that minute on.
The 3rd graph is representative of the behaviour of the solar aggregator. It can be clearly observed that the efficiency was non changeless during the experimental process. The smaller the difference between its recess temperature and the ambient temperature, the larger the efficiency that can be achieved. This is because big differences between these temperatures lead to greater heat losingss from the system in an attempt that a sensible operational temperature is maintained.
The intersection point of the efficiency line and the x axis is of great significance as this is the point where stagnancy of the system occurs. When the line reaches this point, it means that the system ‘s efficiency is equal to zero. In this instance there is no transition of solar energy into utile heat as its entire sum is converted straight to heat losingss. The stagnancy temperature of the system constitute a important parametric quantity for a solar aggregator design.
Mistakes observed during the experimental process
Taking into consideration that experiments are semisynthetic systems, tested under unreal conditions, great disagreements are expected when comparing obtained with real-life consequences. The experimental process is merely an approximative representation of the world, which sometimes introduces imprecise consequences. This impreciseness demand to be highlighted, in order that more dependable decisions can be stated.
First, ambient temperature is a factor that was non suitably counted. The recorded values were temperatures of the interior infinite of the research lab, where the experiment took topographic point. In a existent state of affairs, the recorded ambient temperature would be the temperature of the out-of-door air. Equally good as this, the place of the thermometer, near to the light beginning, it might take to an overestimate of the recorded temperatures.
Additionally, the lower degree of insularity applied to the manifold and the H2O armored combat vehicle, comparing to a existent system ‘s insularity, could be a factor of inaccurate efficiency appraisals ; the overestimate of heat losingss perchance lead to an underestimate of the system ‘s efficiency.
Equally good as this, solar radiation was inexactly represented by a changeless beginning of visible radiation, as hourly fluctuations expected by real-scale solar radiation, could non be counted.