performance enhancement of flat plate collectors using distilled water … · 2018-07-15 ·...

PERFORMANCE ENHANCEMENT OF FLAT PLATE

COLLECTORS USING DISTILLED WATER

A.Senthilkumar1,Jaicharan umakanth2 ,Gautham reddy

3,Hussain kani

4

1Assistant Professor Grade-II , Deaprtment of Mechanical Engineering,Aurupadai Veedu

Institute of Technology,Vinayaka Missions University,Chennai. 2,3,4

Students of Mechanical Enginnering, Aurupadai Veedu Institute of

Technology,Vinayaka Missions University,Chennai.

ABSTRACT

Solar energy is one of the widely used renewable energy that can be

harnessed either by directly deriving energy from sunlight or indirectly. Solar

water heating system, on the other hand, is one of the applications of solar energy

that has drawn great attention among researchers in this field. Solar collectors,

storage tanks and heat transfer fluids are the three core components in solar water

heat applications. In the present work, an attempt has been made to enhance the

heat transfer in solar water heater by using distilled water. Considerable

improvement in the solar collector efficiency is envisaged with increasing of

distilled water and different pitch in the internal grooved tube. The outlet water

temperature is expected to increase with increase of distilled water concentration

and mass flow rate in the turbulent region.

INTRODUCTION

Solar energy is one of the most widely used that can be harnessed

either by directly delivering energy from sunlight or indirectly. Solar water heating

system, on the other hand is one of the application of solar energy but has drawn

great attention amongst researchers in the field. Solar collectors, solar tanks and

heat transfer fluids are the three core components in solar water heater

applications. A typical flat plate collector is a metal box with glass or plastic cover

(glazing) on top and dark colored absorber plate on the bottom. The sides and

International Journal of Pure and Applied MathematicsVolume 119 No. 16 2018, 1813-1834ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

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bottom of the collector are usually insulated to minimize the heat losses. To find

out the applicability of internal grooved tubes for solar applications replacing the

plain ones. To estimate the performance variation by replacing plain tube with

grooved tube having different pitch. To study the performance of system for

different working fluids such as distilled water and aqueous glycol mixtures.

The essential parts required to construct the conventional liquid flat plate

collector are a transparent cover, selectively coated absorber plate, tubes, container

and thermal insulation material. The transparent cover prevents wind and breezes

from carrying the collected heat away from the absorber plate (convection).

Together with the frame and the cover protects the absorber from adverse weather

conditions. The main part of a flat-plate solar collector is the absorber plate. It

covers the whole aperture area of the collector and hold out three functions: absorb

the maximum possible quantity of solar irradiance, accomplish this heat into the

working fluid, and lose a minimum amount of heat back to the collector

surroundings. Solar irradiance passing through the glazing is absorbed directly on

the absorber plate without intermediate reflection as in concentrating collectors.

Absorber surface coatings have a high absorptance of short-wave length. Usually,

these coatings appear "dull" or "flat" indicating that it absorbs radiation coming

from all directions, and the resulting of the black surface absorb over 95% of the

incident solar radiation.

The internal grooving inside the absorber tube is used for optimized of better

heat transfer rate.The benefits of inner grooving are high surface quality, easy to

form and bend and high recycling value. Due to a micro grooving contact area

inside the absorber tube is increased. Inner grooving increases the ratio between

surface area and volume of the tube material. The heat transfer rate of the inner

grooving tube depends on the number of fins, total thickness, bottom wall

thickness, groove depth, apex or top angle, helix angle, and base material.

To study the flow characteristics in a circular pipe having internal grooved

and finding the variation of plain tube pitch to internal grooved. Heat transfer

International Journal of Pure and Applied Mathematics Special Issue

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characteristics – Nusselt number in a plain tube and internal grooved fin with water

and nano fluids. Calculation of efficiency and comparison of the performance with

and without grooves on collector tubes. Compare net performance improvement

due to distilled water and aqueous glycol mixtures.

METHODOLOGY AND EXPERIMENTAL VALUES

FABRICATION OF THE FLAT PLATE COLLECTOR

The essential parts required to construct the liquid flat plate collector are

absorber plate, tubes, transparent cover, collector box and thermal insulation

material.Riser tubes are placed parallel to each other and tube center to center

distance is equal. Brass is used to join the riser tube with lower header and upper

header tube. To get a good circulation of working fluid, inlet and outlet should

opposite side of the collector. Riser tubes are integrally fitted with absorber plate

as an aluminum sheet. Due to this more contact area between the absorber plate

and riser tube. The absorber plate is painted with the dull black plate and which act

as a black body. Low iron content glass is provided for the transparent cover of the

collector to reduce the top loss. This cover prevents wind to take the heat away

from the collector. Glass wool is provided the back and side of the absorber plate

of the collector to reduce the thermal loss. The parts of the collector enclosed and

hold up by the wooden frame.

TECHNICAL SPECIFICATIONS

Component Specification

Collector material

Length

Width

Wood

2000 mm

420 mm

Absorber plate material

Length

Width

Aluminium

1850 mm

360 mm


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Thickness

Thermal conductivity of plate

0.6 mm

k= 205 W/m-K

Plate absorpitivity of solar radiation

Plate emissivity for re-radiation

0.94

0.14

Plate to cover spacing 30 mm

Riser tube material:

Outer diameter:

Length of Tube:

Copper

9.52 mm

1900 mm

Riser tube center to center distance

Number of tubes:

120 mm

3

Top and Bottom Header:

Outer diameter:

Inner diameter:

Length of Tube:

Copper tube

15.87 mm

14.07 mm

450 mm

Number of glass cover:

Glass Cover emissivity/ absorpitivity

Thickness of glass cover

Refractive index of glass relative to air

One

0.88

5 mm

1.526

Back insulation material

Thickness

Thermal conductivity of Insulation

Glass wool

50 mm

k=0.04 W/m-K

Table 1: Design of the collector

DIMENSIONS OF TUBES

Component Plain tube 1st Inner grooved tube(G1) 2

nd Inner grooved tube(G2)

Outer diameter (mm) 9.52 9.52 9.52

Inner diameter (mm) 8.52 8.52 8.56

Bottom wall thickness (mm) 0.5 0.3 0.28

Fin groove depth (mm) - 0.2 0.15

Total wall thickness (mm) 0.5 0.5 0.43

Apex or top angle - 53 53

Helix angle - 18 18


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Number of teeth - 60 60

Table 2: Dimensions of the tube

DESIGN APPROACH DETAILS

Heat exchanger

The heat exchanger is a thermal device that transfers the heat from higher to

lower temperature medium. The use of spiral tube heat exchanger is to reduce the

pressure drop and to increase the thermal efficiency. The spiral copper channel is

kept inside the insulated container made of steel. The spiral tube has the hot

working fluid from the collector and the container filled with cooled to carry the

heat from the tube.

Solar radiation measurement

The flux intensity of the global component of solar radiation is measured

using Pyranometer is also called as a direct response. It absorbers all flat spectral

sensitivity of the electromagnetic spectrum. It should be fixed on the surface

parallel to the solar collector surface in order to avoid the shadow on the absorber

plate. The direct beam of solar radiation is measured using the pyrheliometer

apparatus. Thermopile inside the pyrheliometer converts heat from the sunlight to

an electrical signal. The voltage signal recorded in the multimeter is converted into

watt per meter square using numerical formula.


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Figure 1: Photographic view of Pyranometer and Pyrheliometer

Pyranometer Pyrheliometer

Make LP Pyra Make EPlab

Instruments

Spectral

Range

305nm/

2800nm

Voltage 0-10mv

Table 3: Specification of Pyranometer and Pyrheliometer

Temperature measurements

Thermocouples are used to measure all temperatures such as inlet

temperature, outlet temperature, ambient temperature, plate and cover temperature.

The thermocouple works on the principle of Seebeck effect which means that

generation of electromotive force wherever the temperature difference between the

dissimilar metals. The K-Type thermocouple is made with the combination of

chromium and aluminum junction. The sensitivity of K-Type thermocouple is

approximately about 41μV/oC.

Figure 2: Photographic view of Temperature indicator with Thermocouple


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Thermocouples with Temperature indicator

Type K- Type

Make Scientific Instruments, India

Measurement Range -50oC to 1300

oC

Operating Range 0oC to 50

oC

Table 4: Specification of Thermocouples with Temperature indicator

Wind velocity measurement

The anemometer is to measure the wind speed at the top of the collector. It

can be classified into 2 types, namely a measure of wind pressure and a measure of

wind speed. The measured values are integrated to get the average wind velocity

for each 15 minutes of testing time.

Experimental Setup

The schematic diagram of the newly developed solar flat plate collector is

shown below.

Fig.3. Schematic diagram of experimental test setup


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The experimental setup is a closed loop consisting of liquid flat plate solar

collector, heat exchanger, storage tank and liquid pump respectively. The solar flat

plate collector is facing south with a tilt angle of 22oC and absorbs the heat from

the solar and transfers to the working fluid. The heat exchanger helps to control

and adjusts the inlet fluid temperature as required value to the solar collector. The

purpose of the heat exchanger is transfer heat from the outlet fluid of the collector

to cold water around the coil. The cooled outlet water from the heat exchanger is

finally stored in the insulated tank with thermacol. The quarter HP pump is used to

circulate the liquid across over the tube of the collector with the help of an

alternative current motor. The mass flow rate can be adjusted to the desired value

with the aid of bypass just about the pump.

NUMERICAL EQUATIONS FOR FLAT PLATE COLLECTOR

Convective heat transfer coefficient( hi )

The properties of the fluid are taken from the heat and mass transfer data

book. The properties are to be evaluated at fluid average temperature. The type of

flow is identified from the value of the Reynolds number.

Reynolds Number (Re)

Nusselt number

Laminar flow: Fully developed thermal layer

L>>D

The coefficient of heat transfer (hi) is calculated from the Nusselt Number.

Collector heat removal factor and overall loss coefficient


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Bottom loss coefficient

Side loss coefficient

Where, L1*L2 - dimension of the absorber plate in meter

L3 - height of the casing in meter

Top loss coefficient

Empirical equation for the top heat loss coefficient is given by

Where,

Collector efficiency factor

The tubes are fabricated integral with the absorber plate.

Heat removal factor

Useful heat gain

The above equation is a Hottel Whillier Bliss Equation

Heat loss from the collector


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Instantaneous efficiency

For theoretical

For experimental

Where, ∆t = To - Tfi

RESULTS AND DISCUSSIONS

It is seen that the values of the useful heat gain and efficiency increase

sharply from 10 AM to around noon and then drop sharply to 3 PM. The

instantaneous efficiency and solar radiation are valid for every 30 minutes on the

side of the instant considered. The variation obtained is typical for a solar flat plate

collector and indicates that the strong dependence of these factors on the amount of

radiation incident on the collector is shown in fig4.


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Figure 4: Peak temperature variations in day

Experimental efficiency of solar collector in plain tube(m=0.01lph)

Figure 5. Variation of theinstantaneous efficiency of a plain tube collector over a

day (10AM to 3PM) at different lph.

Experimental efficiency of solar collector in grooved tube I(m=0.01lph)

Figure.6: Variation of the instantaneous efficiency of a grooved tube I collector over a

day (10AM to 3PM)


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Experimental efficiency of solar collector in grooved tube II (m=0.01lph)

Figure.7. Variation of theinstantaneous efficiency of a grooved tube II collector over a

day (10AM to 3PM)

Where R = (water flow inlet – Tamb)/Ig

From the above graphs the efficiency obtained from grooved tube is higher

than that of compared to the plain tube. Now the same apparatus is said to be

run by another mass flow rate of m=0.02 lph.


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Experimental efficiency of solar collector in plain tube(m=0.02lph)

Figure.9. Variation of theinstantaneous efficiency of a plain tube collector over a day

(10AM to 3PM)

Experimental efficiency of solar collector in grooved tube I(m=0.02lph)

Figure.10. Variation of the instantaneous efficiency of a grooved tube I collector over a

day (10AM to 3PM)


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Experimental efficiency of solar collector in grooved tubeII(m=0.02lph)

Figure.11. Variation of theinstantaneous efficiency of a grooved tube II collector over a

day (10AM to 3PM)

Experimental efficiency curves in plain tube m=0.01lph (time vs solar

radiation , efficiency)


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Figure.12.Variation in efficiency and solar radiation across time

Experimental efficiency curves in plain tube m=0.02lph(time vs solar




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Experimental efficiency curves in grooved tube I m=0.0lph(time vs solar




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Experimental efficiency curves in grooved tube I m=0.02ph(time vs solar



Experimental efficiency curves in grooved tube II m=0.01ph(time vs solar




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Experimental efficiency curves in grooved tube II m=0.02ph(time vs solar



CONCLUSIONS

Based on the analysis of circular plain tube and inner groove tube as absorber

tube of the solar collector, the following conclusion can be made that efficiency

obtained from internal grooved tubes is much better than that of compared to plain

tube.

The result shows that the efficiency of the liquid flat plate collector is

significantly increased with the use of helical inner grooved absorber tube

having a higher pitch.


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The heat transfer rate and pressure drop in the smooth and groove tube

areincreases, but friction factorsincreases with the increasein Reynolds

number.

The heat carrying capacity of inner grooved solar flat plate collector is quite

higher than the plain tube collector.

The size of the collector is reduced and had an advantage ofreduction of

absorber material cost.

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performance enhancement of flat plate collectors using distilled water … · 2018-07-15 ·...

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