iisrt z abhijit devaraj

International Journal of Mechanical Civil and Control Engineering

Vol. 1, Issue. 3, June 2015 ISSN (Online): 2394-8868

27

Evaluation of efficiency and collector time constant of a

solar flat plate collector at various intensities of light

and constant wind speed by using forced mode

circulation of water Abhijit Devaraj

1 Abhishek Hiremath

2 Akshay R Patil

3 Krushik B N

4

Department of Mechanical Engineering, BMS College of Engineering, Bangalore, INDIA

Abstract- The present attempt of the work is to calibrate the

efficiency and collector time constant of a flat plate collector

which is used to heat water flowing through the pipes by forced

circulation at varying intensity of heat flux, when wind is blowing at a constant speed. It was observed that these factors

affect the flat plate collector in a profound way. This work

helps us in giving an insight on practical scenarios where solar

collectors are usually placed at high elevations to receive heat

as high altitudes involve flow of wind across the collector.

Keywords: Flat plate collector, solar water heater, intensity of

sunlight, wind flow

I - INTRODUCTION

Solar Energy is one of the major alternative sources of

energy being used in the current world scenario. Processes

of industrialization and economic development require

important energy inputs. Reserves of fossil fuel are limited

and their large scale use is associated with environmental

deterioration.[2]

Solar energy is considered one of the main

promising alternative sources of energy to replace the

dependency on other fossil fuel resources[3] [4]

There are

adverse environmental effects caused by greenhouse gas

emissions from fossil fuel combustion.[5]

Solar energy is an

ecologically clean source of energy and freely available to

everyone over long time periods at all parts of the earth.[6]

Incoming solar radiation is converted into thermal energy

using black bodies which trap the excess heat emitted from

the sun in the form of infrared radiations Availability of

solar energy depends on day and night cycles and weather

conditions hence collectors are used to trap solar energy

radiated from the sun. Solar Water Heating (SWH) is the

conversion of sunlight into renewable energy for water

heating using a solar thermal collector. The heat collector

used here is a Flat-plate solar collector which is used to

collect heat for various applications such as space heating,

domestic hot water or cooling with an absorption chiller.

There are two types of solar water heating systems namely

passive and active. Flat plate collectors can be either glazed

or unglazed and either air or liquids can be us ed as heat

transporting fluids. [1]

This experiment involves an active

water heating system where a pump is used to circulate

water which allows us to have the collector tank above the

collector and also use drain back tanks.

The advantages of solar flat plate collector are that we

receive hot water throughout the year, it decreases our daily

fuel consumption and reduces our energy bills and also

reduces carbon emissions.

II – IMPLEMENTATION

FIGURE 1- Block Diagram of the experimental setup

Cold Water

Tank

Flat Plate Collector

Hot Water

Tank

Valve 5

Valve 1

Pump

Valve 7

Valve 3



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FIGURE 2 – Experimental Setup of ECOSENSE water

heating system based on solar flat plate collector.

FIGURE 3- Panel used to display input and output

parameters.

The setup consists of the following components:

Radiation meter: To measure the radiation level that is

received by the collector a radiation meter is supplied with

the system. It is a sensing based device. It can measure the

radiation level in the range of 0 to 200 W/m2

.

Thermometer: Four thermometers are connected to the

system. The sensors are RTD based platinum probe and

work on the principle of variation of resistance with

temperature. The probes are class A RTD and can measure

the temperature in the range of 200°C to 650°C. Pressure

Gauge: Two pressure gauges are there in the setup. They

work on the principle of generation of electric signal by

semi-conductor device due to exertion of pressure. Pressure

gauges can measure the pressure in the range of 101.3 to

650 KPa. Water flow meter: To measure the water flow

rate a panel mount flow meter with a mini turbine flow

sensor is connected near the collector inlet. It is a

programmable meter. It can measure the flow rate in the

range of 0.5 to 25 liters/minute. A temperature limit of

meter is up to 80°C. Pump: We are using an AC pump to

fill up the collector tank as well as to circulate the water

through the collector at some regulated speed. A continuous

regulator is there to maintain the flow rate. Anemometer:

An anemometer is supplied with the system. This can be

used to measure the air velocity and ambient air

temperature. The air flow sensor is conventional angled

vane arms with low friction ball bearing while the

temperature sensor is a precision thermistor. The

Anemometer can measure the wind velocity in the range of

0.5 to 45 m/s while the temperature range is 10 to 60°C.

Fan: One AC fan is integrated with the system to generate

artificial wind speed. To set the wind speed as per

requirement a regulator is there in the control unit. Valve:

Different valves are there to direct the water flow as per

requirement. [7]

A - Specifications

The specifications of the equipment are as follows:

Tank capacity: 50 litres

Collector area: 0.716m²

Tungsten halogen fixture’s area: 0.72m²

Halogen system Power: 150 watt each

Radiation meter range: 0 to 1999 w/m²

Water pump power: 0.12hp

Water flow range: 0.5 to 25 LPM.

Water flow maximum pressure: 17.5 bar

Thermometer sensor: class A sensor

Thermometer range: 200 to 650˚C

Anemometer range: 0.4 to 45 m/s

Fan range: 0 to 5 m/s [7]



29

B - Assumptions made in the setup

1. The collector is in steady state condition.

2. Headers cover a small area of the collector and can be

neglected.

3. Headers provide uniform flow to riser tubes.

4. Flow through the back insulation is one dimensional.

5. Temperature Gradients around the tube are neglected.

6. Properties of materials are independent of temperature.

7. No energy is absorbed by the cover.

8. Heat flow through the cover is one dimensional.

9. The covers are opaque to infra red radiation.

10. Same ambient temperature exists at both front and back

of the collector.

11. Dust effects on the cover are negligible.

12. There is no shadowing of the absorber plate.

13. Temperature drop across glass tube is uniform.

14. Solar radiation transmitted through glass cover is

reflected not absorbed. [7]

III – RESULTS AND DISCUSSIONS

A – Formulae

Calculations were performed using the following

formulae’s:

Heat Supplied= specific heat flux *area of collector

Water flow rate = 2.35 Lpm = 2.35/60

= 0.03916 Kg/s

Heat Radiated = Qrad = Q = σA∆T4 = σA(T1

4-T∞

4)

[8]

Collector Time Constant = R

R = [T3- T3(0)] / [ T4- T3(0)] [8]

Collector Plate Efficiency = ɳ = (Qrad/ Qsup)*100 [8]

C – Methodology

The cold water tank 1 was filled with water at atmospheric

temperature. Valve 1 and valve 7 were opened which allows

flow from the cold water tank 1 to the Flat Plate Collector

inlet. The pump was switched on and the regulator was set

at the minimum power at which the pump can work. A

suitable flow rate was set whose value can be observed on

the flow meter screen. Valve 3 was opened which allows

flow from the Flat plate collector outlet to the hot water

tank. After waiting for some time to get a stable reading the

fan regulator was adjusted to get the desired wind speed

which in this case is 5 m/s. The wind speed was measured

using an anemometer. Once the flow rate and the wind

speed were set the initial readings of collector plate

temperature, water inlet temperature, water outlet

temperature and hot water temperature were noted down at

time= 0 sec. The Halogen system was then switched on and

the radiation was set to desired level which in the first case

is 100 W/m2. The cold water was allowed to flow through

the Flat plate Collector which absorbed the heat and was

then allowed to flow into the hot water tank. The

temperature readings as mentioned above were noted down

for every one minute for a total duration of 10 minutes.

After the experiment was completed the pump was switched

off and the valve 1 was closed and valve 5 was opened

which allows the water to drain from the hot water tank to

the cold water tank. The water was allowed to cool for some

time. The experiment was repeated two more times by

following the exact same procedure but the flux rates were

set at 130 and 160 W/m2 for the next two trials respectively

and the readings were tabulated. The heat supplied was

obtained by multiplying the flux supplied by the collector

area. The collector time constant and radiative efficiency of

the collector were calculated using suitable formulae.

Graphs were plotted for efficiency vs. time and collector

time constant vs. time for various specific flux rates.



30

B - Tables

Table 1: Readings for a specific Heat flux of 100 W/m2

Table 3: Readings for a specific heat flux of 160 W/m2

Heat

Supplie

d (Qin)

in watts

Wind

Velocity

(V) in

m/s

Time

in sec

Flow

Rate (ṁ)

in Kg/s

Plate

Temp (T1)

in °C

Inlet

Water

Temp

(T2) in

°C

Outlet

Water

Temp

(T3) in

°C

Hot

Water

Temp

(T4) in

°C

Heat

Radiated

(Qrad) in

watts

Collector

Time

Constant

(R)

Efficiency

of plate

ɳ (in % )

71.6 5

0 0.03916 38.8 30 31.8 32.6 32.39 0 45.23

60 0.0383 37.7 29.2 32.5 32.8 27 0.7 37.71

120 0.037 36.6 29.2 32.2 33 21.6 0.33 30.16

180 0.0386 36.3 29.1 32.1 33.1 20.23 0.2307 28.25

240 0.0408 36 29.1 31.9 33.2 18.79 0.0714 26.24

300 0.0408 35.9 29.1 31.9 33.3 18.31 0.0667 25.57

360 0.04 35.7 29.1 31.9 33.3 17.36 0.0667 24.24

420 0.0391 35.7 29.1 31.9 33.3 17.36 0.0667 24.24

480 0.0383 35.7 29.1 31.9 33.3 17.36 0.0667 24.24

Heat

Supplied

(Qin) in

watts

Wind

Velocity

(V) in

m/s

Time

in sec

Flow

Rate

(ṁ) in

Kg/s

Plate

Temp (T1)

in °C

Inlet

Water

Temp

(T2) in

°C

Outlet

Water

Temp

(T3) in

°C

Hot

Water

Temp

(T4) in

°C

Heat

Radiated

(Qrad) in

watts

Collector

Time

Constant

(R)

Efficiency

of plate

ɳ (in % )

93.08 5

0 0 44.7 29.3 32.6 33 62.27 0 66.9

60 0.0167 42.6 29.4 33.5 33.2 51.44 1.5 55.26

120 0.022 39.6 29.2 33 33.2 36.34 0.667 39.04

180 0.0195 38.4 29.2 32.9 33.3 30.42 0.428 32.68

240 0.0225 37.9 29.2 32.7 33.3 27.98 0.1413 30.06

300 0.0204 37.7 29.2 32.8 33.4 27 0.25 29

360 0.02 37.6 29.3 32.7 33.5 26.52 0.111 28.49

420 0.0175 37.6 29.3 32.8 33.6 26.52 0.2 28.49

480 0.017 37.6 29.3 32.8 33.6 26.52 0.2 28.49

Heat

Supplied

(Qin) in

watts

Wind

Velocity

(V) in m/s

Time

in sec

Flow

Rate

(ṁ) in

Kg/s

Plate

Temp

(T1) in °C

Inlet

Water

Temp

(T2) in

°C

Outlet

Water

Temp

(T3) in

°C

Hot

Water

Temp

(T4) in

°C

Heat

Radiated

(Qrad) in

watts

Collector

Time

Constant

(R)

Efficiency

of plate

ɳ (in % )

114.56 5

0 0.03 42.4 33 33.1 33.5 50.42 0 44.01

60 0.0104 42 30.5 36.9 33.6 48.39 7.6 42.24

120 0.0175 41.7 30.2 36.3 33.9 46.87 4 40.91

180 0.0175 40.8 30 35.5 34.1 42.33 2.4 36.95

240 0.0216 40 29.9 34.2 34.2 38.33 1 33.46

300 0.0212 40 29.7 34 34.4 38.33 0.692 33.46

360 0.0179 39.9 29.6 33.6 34.5 37.83 0.357 33.03

420 0.0191 39.9 29.6 33.7 34.6 37.83 0.4 33.03

480 0.02 39.8 29.6 33.7 34.6 37.34 0.4 32.59

Table 2: Readings for a specific heat flux of 130 W/m2



31

C – Efficiency of Collector

FIGURE 4 – Plot of Efficiency vs. time for a flux of 100

W/m2


W/m2


W/m2

The plots of efficiency v/s time for each specific flux rate

(figure 4, figure 5, and figure 6) showed that the efficiency

decreased as time increased. This was due to wind blowing

constantly over the Flat plate Collector which reduced the

collector plate temperature resulting in reduced heat

radiation. This resulted in decreased efficiency. The graph

after a certain time interval becomes almos t linear. This was

because after some amount of cooling of the Flat plate

Collector had taken place, the plate attained an almost

steady temperature which gave steady heat radiation and

almost constant efficiency.

D – Collector time Constant

FIGURE 7 – Plot of Collector time constant vs. time for a

flux of 130 W/m2

0

10

20

30

40

50

0 60 120 180 240 300 360 420 480

Eff

icie

ncy i

n %

Time (seconds)

Efficiency v/s time plot for 100 W/m2 flux

Efficiency

0

10

20

30

40

50

60

70

80

0 60 120 180 240 300 360 420 480

Eff

icie

ncy i

n %

Time (seconds)


efficiency

0

5

10

15

20

25

30

35

40

45

50

0 60 120 180 240 300 360 420 480

Eff

icie

ncy i

n %

Time (seconds)


efficiency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

60 120 180 240 300 360 420 480

R

Time (seconds)

R v/s time for flux of 100 W/m2



32

FIGURE 8 – Plot of Collector time constant vs. time for a

flux of 130 W/m2

FIGURE 9 – Plot of Collector time constant vs. t ime for a flux of 160 W/m2

Collector time constant is required to evaluate the transient

behavior of a collector. It can be calculated from the curve

between R and time as shown above. The plots of Collector

time constant R v/s time for each specific flux rate showed

that R decreased as time increased. The graph for a flux rate

of 100 W/m2 becomes almost constant or linear between the

time interval of 240 and 480 seconds and hence R = 0.75 for

figure-7. The graph for a flux rate of 130 W/m2 becomes

almost constant or linear between the time interval of 420

and 480 seconds and hence R = 2 for figure-8. The graph for

a flux rate of 160 W/m2 becomes almost constant or linear

between the time interval of 360 and 480 seconds and hence

R = 0.35 for figure-9.

IV - CONCLUSIONS

In the present study on Flat plate Collector’s the potential

barriers to using them in practical scenarios at high

elevations involving wind flow was determined. From the

readings obtained and the graphs plotted it was inferred that

the Collector Time Constant R decreased as time increased.

Also as time increased the temperature of Flat Plate

Collector decreased due to which the heat radiated

decreased. This resulted in a decrease in efficiency. The

temperature drop was due to cooling of the Flat Plate

Collector due to the constant wind blowing over it. Also the

efficiency decreased as heat flux incident normally on the

collector plate decreased. Hence in practical scenarios

maximum efficiency is obtained at noon when maximum

normal heat flux is incident on the Flat plate Collector.

This particular study helped us understand the influence

of day night cycles and wind flow velocity on flat plate

collectors. It gave us estimation that for solar collectors to

heat water to higher temperatures and generate more

efficiency, the collectors should be kept at a high altitude to

receive more sunlight and also at a location where the wind

is blowing at minimum or negligible speed to avoid cooling

and temperature drops.

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

60 120 180 240 300 360 420 480

R

Time (seconds)

R v/s time for a flux of 130 W/m2

0

1

2

3

4

5

6

7

8

60 120 180 240 300 360 420 480

R

Time (seconds)

R v/s time for a flux of 160 W/m2



33

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[7] – Insight Solar Manual by ECOSENSE

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