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Analysis of thermal performance on evacuated tube solar collector without and with reflector
1I.George, 2R.Kalaivanan
1,2Department of Mechanical Engineering, Annamalai University, Annamalai Nagar, India Email: [email protected]; [email protected]
[Received:1st Sept.2017; Accepted: 30th Sept.2917]
Abstract— Solar collector is an important component of solar hot water system. The purpose of this research is to design, fabricate and test thermal efficiency of the evacuated tube solar collector (ETSC) with flat reflector.
The thermal performance of an ETSC is experimentally investigated at different mass flow rates. Water is used as a working fluid in experimental setup and tested in Indian climatic conditions. The ETSC consists of fifteen evacuated tubes and manifold channel. The manifold channel consists of a square hollow manifold through which water flows.
The temperature difference and efficiency are studied with different mass flow rates, arrangement of collector tubes and reflectors. The effect of shadow due to adjacent tubes is taken into account by changing the centre distance between centres of the tubes. The reflectors are used to enhance the performance of ETSC. It is observed that in case of reflector ETSC gives higher temperature difference and has better thermal efficiency as compared to the case of without reflector.
Key words: Efficiency, Evacuated tube collector, Reflector.
I. INTRODUCTION
Nowadays the demand for the energy is increasing day by day. Majority of the energy production is from conventional energy sources like fossil fuels. As these sources are not sufficient in quantity to meet the growing demand, the stepping to the renewable sources of energy is very essential. The renewable sources can be utilized for many applications. One of them is the utilization of solar energy to heat the water. The solar thermal collectors absorb the incident solar energy to heat up the running water through the tubes. Different types of solar thermal collectors are available based on the requirements, amount of heating required etc. and researches are going on to find out the ways to increase the efficiency further. In the case of solar thermal collectors, the two main types are flat plate collector and evacuated tube collectors. In both types, water is used as the transferring medium for heat. The evacuated tubular collector consists of glass vacuum sealed tubes. The presence of vacuum medium reduces the conductive and convective losses when compared to flat plate collector.
A review of the literature suggests that considerable efforts have been made in the research and development
of the evacuated tube solar collectors. E.Zambolin and D.Del Col 1 investigated experimentally on flat plate and evacuated tube solar collector and compare the result in steady state and quasi dynamic method. In the result the evacuated tube collector had better performance.
Budihardjo and Morrison 2 evaluated the performance of water-in-glass evacuated tube solar collector systems with flat plate solar collectors in a range of locations.
The performance of a typical 30-tube evacuated tube array was found to be lower than a typical 2-panel flat plate array for domestic water heating in Sydney. Ma et al. Shah and Furbo 3 performed experiments on vertical evacuated tubular collectors. The collector had a tubular absorber and could utilize solar radiation coming from all directions. Results from calculations with the model are compared with measured results and it was found that there occurs a good degree of similarity between the measured and calculated results. And further the thermal performance of the evacuated solar collector was compared to the thermal performance of the flat plate solar collector with an optimum tilt and orientation.
Hayek et al. 4 investigated experimentally the overall performance of solar collectors. Two types of evacuated tube solar collector namely, water-in-glass tube and heat pipe designs are used . It was found that the heat pipe based collector is better than the water-in-glass tube and their efficiency is 15 to 20% higher. Yadav and Bajpai5 investigated experimentally the thermal performance of one-ended evacuated tube solar air collector at different flowrate. Both parallel and counter flows are experimentally examined by changing the position of the blower. It is found that the setup operates more efficiently at a higher flow rate and counter flow.
Avadhesh Yadav and V.K.Bajbai 6 conducted experimental study for up flow and down flow at various flow rates of air at different solar intensity on evacuated tube solar collector. Up flow is more efficient and achieve 60∙C, when water is used in downward flow.
G. L. Morrison and M. Behnia 7 developed a number of heat extraction methods from all glass evacuated tubes and found the water in glass concept to be the most successful due to its simplicity and low manufacturing cost. Shah and Furbo8 investigated heat transfer and flow structures inside all glass evacuated tubular
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collectors for different operating conditions by means of computational fluid dynamics. The investigations were based on a collector design with horizontal tubes connected to a vertical manifold channel. Generally, the results showed only small variations in the efficiencies.
This indicates that the collector design is well working for most operating conditions. Morrison et al. 9 evaluated the characteristics of water-in-glass evacuated tube solar water heaters including assessment of the circulation rate through single-ended tubes. A numerical model of the heat transfer and fluid flow inside a single-ended evacuated tube had been developed assuming no interaction between adjacent tubes in the collector array.
Kim and Seo 10 investigated experimentally and numerically the thermal performance of evacuated tube solar collector. Four different shapes of absorber are considered to find the best shape of absorber tube for the solar collector. L. Ma and Z. Lu11 investigated the thermal performance of the individual glass evacuated tube solar collector and analysed the heat loss coefficient, heat efficiency factor by using one- dimensional analytical method and also studied the influence of air layer between the absorber tube and the copper fin on the heat efficiency. The results showed that the function relation of the heat loss coefficient of the glass evacuated tube solar collector with temperature difference between the absorbing coating surface and the ambient air was nonlinear. Paul Thomas and Praveen Ajith 12 carried out an experimental study on evacuated tube solar collector and compared the performance using flat and concave reflector. Runsheng Tang and Wenfeng Gao 13 Analyse effect of center distance between glass tube and optimal tilt angle of south facing evacuated tube collector.
Nomenclature
ETSC: Evacuated tube solar collector
o: Outlet temperature of water ∘C
i: Inlet temperature of water, ∘C
pw: Specific heat of air, J/kg K
ċ: Mass flow rate of air, kg/hr.
D: Diameter of evacuated glass tube, m LE: Length of evacuated glass tube, m
c: Area of evacuated tube solar air collector, m2 : Solar radiation intensity, W/m2
Q: Thermal power transmitted to liquid : Evacuated tube solar collector efficiency.
II. EXPERIMENTAL
The objective of this experiment is to study the thermal performance of ETSC and to produce hot water. The photograph of the experimental setup is shown in Fig. 1.
The setup consists of fifteen glass evacuated tubes . The length and diameter of outer glass tube and absorber tube are 1.5 m, 0.047m, and 0.0343.m, respectively. The
surface area of evacuated solar air collector is 1.66m2. The open ends of evacuated tubes are connected to the manifold channel and closed end is supported by frame.
Manifold channel is 1.16m in length and it consists of square pipe at the center of manifold channel. Regulator is used to control the water in to the evacuated tube and to vary the mass flow rate.
Fig: 1 Experimental set up
The experiment is carried out with three sets of ETSC of various center distances like 115mm, 90mm, and 65mm at same instant. The reflector is used to enhance the performance of ETSC.
The experimental setup consists of the following parts:
A. Evacuated tubes, B. Manifold channel, C. Reflector.
Fig. 2. Schematic diagram of (a) complete header, (b) cross-sectional view of header
A. Evacuated tubes
The evacuated tubes used in this system are shown in Figures 2(a) and 2(b). Each evacuated tube consists of two concentric glass tubes which are made from borosilicate glass and between two tubes there is a vacuum. The outer tube is transparent and the inner tube is coated with coating (Al-N/Al) for better absorption of solar radiation.
B. Manifold channel
The manifold channel used in this system is shown in Figures 3(a) and 3(b).
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Fig. 3. Schematic diagram of (a) complete header, (b) cross-sectional view of header
It consists of two square pipes. One is outer square pipe (stainless steel) and the other is inner square pipe (mild steel). For the surface of outer square pipe, a polyurethane insulation is used to prevent the heat transfer from manifold channel to atmosphere. The inner square pipe is centrally passed through outer square pipe with one end closed. The fifteen holes are made on the surface of the outer square pipe in which the open ends of the evacuated tubes are attached and closed ends are supported by the frame.
C. Reflector
Reflector is used under the evacuated tubes to reflect the sunlight on to the evacuated tubes. The size of reflector is 1.55 × 1.20 m2. It is a GI sheet (galvanized iron) made of mild steel and coated with zinc which has good reflectivity. It can easily reflect the incident solar radiation to the tubes due to which the outlet temperature is increased.
MEASURING DEVICE AND INSTRUMENTS:
Different parameters are measured in these experiments:
(i) Inlet and outlet water temperatures (ii) Solar intensity
(iii)Water flow rate.
These parameters are measured by the following devices:
T – Type thermocouple is used to measure the temperatures at different points. It is connected with a digital temperature indicator (model: TPLR-96-16U, supplied by Tempsen devices, India) that shows the temperature with a resolution of 0.1∘C.
The solar radiation intensity is measured during the day using a Eppley radiometer (model PSP), supplied by The Eppley laboratory, USA. The mass flow rate of water is measured at the beginning of experimental work. A rota meter is attached to the inlet pipe to measure the mass flow rate with an accuracy of 1 lit/sec. A separate probe
Anemometer of model AM-4201 is used to measure the wind velocity with an accuracy {± (2% + 1d)} and resolution 0.1m/s.
SYSTEM OPERATION:
Initially, the water is supplied to the ETSC and then exposed to the sun. This water circulates through manifold header to evacuated tube as shown in Fig.4
Fig. 4: Experimental setup of evacuated tube solar air collector
Evacuated tube absorbed solar energy converting it into heat for use in water heating. A reflector of polished steel is placed underneath the vacuum glass tubes separated by 2 cm from the tubes lowest line. The reflector surface is shaped to form parallel line to work for reflecting solar radiation to the back and sides of each tube. A T- Thermocouple (Temperature Detector) that measures the inlet and outlet temperatures of the water and controls the flow of water in the system by regulating valve. The piping network is insulated by a layer of polyurethane foam in order to minimize heat losses. A rotameter is included in the circuit for the purpose of measuring the flow rate at inlet and outlet of the system. Finally, solar radiation was measured by a pyranometer, connected to a data logger. The observation was recorded from 10:00am to 4:00pm at an interval of 15 min. without and with polished reflector.
PERFORMANCE ANALYSIS:
Thermal performance of the ETSC can be estimated by the solar collector efficiency, which is defined as the ratio of output to the input. Output in this case is the heat gained by water flowing through the manifold channel and input is the energy of the solar radiation falling on evacuated tubes:
Solar collector efficiency = (1)
Where,
= w pw ( out − in)
= c (2) Area of the evacuated tube solar collector is given by ( c)
= Number of tubes × 2DLE.
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III. RESULTS AND DISCUSSION
In this experimental setup, the main concern is the heating of water at different mass flow rates. The experiments were carried out by changing the center distance between the tubes and with and without reflector. The experiments were carried out from 10:00am to 4:00pm at an interval of 15 min. In this work, influence of center distance between the glass tubes on the ETSC was observed and shown in the form of three different cases as following.
Performance of Evacuated tube collector without and with reflector at 15 kg/hr. flow rate:
The ETSCs with three different center distance were exposed to solar radiation, the reading were taken. At a low mass flow rate of water without reflector, it can be observed that the temperature of outlet water from the evacuated tube solar collector steadily increases.
Fig.5 Variation of temperature difference of water with time in evacuated tube collector without reflector at 15 kg/hr. flow rate
Fig. 6 Variation of temperature difference of water with time in evacuated tube collector with reflector at 15 kg/hr. flow rate.
The fig. 5 shows the effect of center distance between glass tubes on the rising temperature of evacuated tube collectors with different solar intensities. The maximum solar intensity was 1607W/m2 at12:45pm, after that it started decreasing with time. The outlet temperature and temperature difference of water in ETSC depend upon solar intensity and mass flow rate. The temperature of outlet water kept on increasing at higher rate than the ambient temperature due to high absorption rate of solar
radiation in evacuated tubes and minimal reflection properties. The outlet temperature and temperature difference are found to be maximum for the center distance of 90mm and lower for other center distances.
When the solar intensity started decreasing, the inlet, outlet temperature and temperature difference of water also started decreasing. The outlet temperature and maximum temperature difference of water are 80.4 and 38.7o C at 2.30 pm for 90mm center distance.
Performance of evacuated tube collectors at 15 kg/hr.
mass flow rate of water with reflector according to the collector center distance is shown in fig.6. It can be observed that the difference of temperature of water from the evacuated tube solar collectors are increased up to noon and almost steady afternoon. Among the three collectors the outlet temperature and temperature difference is maximum of 90.8oC and 46.5oC at 2.30 pm for 90 mm center distance collector.
Fig. 7 Variation of efficiency with time in evacuated tube collector without reflector at 15 kg/hr. flow rate.
Fig. 8 Variation of efficiency with time in evacuated tube collector with reflector at 15 kg/hr. flow rate.
Figs. 7-8 show the thermal performance of the solar collectors according to the collector tube center distance.
The efficiency as shown in figs are inversely proportional to solar intensity and directly proportional to the temperature difference achieved. Thus, at peak value of solar intensity, the efficiency was nearer to its lowest value. It increased sharply between 04:00 and 05:00pm as solar intensity was decreasing. For the rest of the time, the graph is nearly as flat as before noon; the increase in temperature difference is countered by the increase in solar intensity. The maximum efficiency was found to be 0.702 and 0.048 at its lowest solar intensity for the center distance of 90mm ETSC without and with
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reflector. The other center distance collectors were of lower efficiency for all time on the day.
Performance of Evacuated tube collector without and with reflector at 30 kg/hr. flow rate:
The evacuated tube collectors with three different center distances were analysed at mass flow rate of 30kg/hr.
without and with reflector. At a medium water flow rate without reflector, it can be observed that the temperature of outlet water from the tube solar collector slightly decreased. Fig.9 shows the variation of temperature difference of water, and solar intensity with time in evacuated tube collectors without reflector.
Fig. 9 Variation of temperature difference of water with time in evacuated tube collector without reflector at 30
kg/hr. flow rate.
Fig.10 Variation of temperature difference of water with time in evacuated tube collector with reflector at 30
kg/hr. flow rate.
The maximum solar intensity was achieved in the afternoon, after that it started decreasing with time. The maximum solar intensity was 2132W/m2 at 12:00pm.
The outlet temperature and temperature difference of water depend upon solar intensity and water flow rates.
The outlet temperature and temperature difference achieved at high flow rate were lower than the case of low water flow rate in ETSC. This is due to the reduction in residence timing of water for heat transfer inside the ETSC. The outlet temperature and temperature difference of water achieved in this case were 65.2oC and 23.4oC at 02:30pm for 90mm center distance collector.
Performance of evacuated tube collectors at 30 kg/hr.
mass flow rate of water with reflector is shown in fig.10.
It can be observed that the difference of temperature of
water from the evacuated tube collectors increased up to noon and steadily decreased at afternoon. Among the three collectors the outlet temperature and temperature difference is maximum of 80.9oC and 37.8oC at 2.00 pm for 90 mm center distance collector.
Fig.11 Variation of efficiency with time in evacuated tube collector without reflector at 30 kg/hr. flow rate.
Fig.12 Variation of efficiency with time in evacuated tube collector with reflector at 30 kg/hr. flow rate.
Figs. 11-12 show the variation of efficiency with time in evacuated tube collectors without and with reflector.
The efficiency is inversely proportional to solar intensity and directly proportional to the temperature difference achieved. Thus, at peak value of solar intensity, the efficiency was nearer its lowest value. It increased sharply between 03:00 and 04:00pm as solar intensity was decreasing. At high water flow rate the efficiency achieved was higher than the case of low mass flow rate in collector because the flow rate of water increased very fast, but the change of temperature difference was very small. The maximum efficiency was found to be maximum viz. 0.80 and 0.52 at its lowest solar intensity for 90mm center distance collector without and with reflector. The other center distance collectors were of lower efficiency for all time on the day.
Performance of Evacuated tube collector without and with reflector at 45 kg/hr. flow rate:
At a high flow rate without reflector, it can be observed that the temperature of outlet water from the ETSC steadily increased up to noon due to higher intensity because of an increase in the residence time of water at low flow rate.
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Fig.13 Variation of temperature difference of water with time in evacuated tube collector without reflector at 45
kg/hr. flow rate.
Fig.14 Variation of temperature difference of water with time in evacuated tube collector with reflector at 45
kg/hr. flow rate.
Fig. 13 shows the variation of temperature difference of water with time in evacuated tube collectors. The maximum solar intensity was 2033W/m2 at12:30pm.
The outlet temperature and temperature difference of water in collectors depend upon solar intensity and water flow rate. The outlet temperature was found to be maximum at noon. When the solar intensity started decreasing then outlet, inlet temperature and temperature difference of water also started decreasing. The outlet temperature and maximum temperature difference of water achieved were 67.5oC and 21.7oC at 2:00 pm 90 mm center distance collector.
Performance of evacuated tube collectors at 45 kg/hr.
mass flow rate of water with reflector is shown in fig.14.
It can be observed that the difference of temperature of water from the evacuated tube solar collectors were increased up to noon and almost steady afternoon. In this case, with reflector, the outlet temperature and temperature difference achieved were higher than that without reflector. Among the three collectors, the outlet temperature and temperature difference were maximum of 66.7oC and 26.7oC at 2.30 pm for 90 mm center distance collector.
Fig.15 Variation of efficiency with time in evacuated tube collector without reflector at 45 kg/hr. flow rate.
Fig.16 Variation of efficiency with time in evacuated tube collector with reflector at 45 kg/hr. flow rate.
Figs. 15 - 16 show the variation of efficiency with time in solar collectors at low mass flow rate without and with reflector. The efficiency is inversely proportional to solar intensity and directly proportional to the temperature difference achieved. The efficiency was found to be increasing as the solar intensity decreased after 02:00pm due to inverse ratio of temperature difference and intensity at a constant mass flow rate.
The maximum efficiency was found to be 0.842 at 2.30pm and for 90mm center distance collector without reflector. The other center distance collectors are of lower efficiency for all time on the day.
IV. CONCLUSION
It can be concluded that the performance of evacuated tube collector (ETSC) can be significantly increased with the use of reflector. Further, efficiency, outlet temperature and temperature difference also increase with the use of reflector for 90mm center distance evacuated tube solar collector.
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