Presented By

G. Kalaiarasi[2011189008]

PRESENTATION OVERVIEW

IntroductionLiterature Review Objective of the present workExperimental InvestigationResults & DiscussionConclusion

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REVIEW PAPERSTitle :Review of Efficiency improvement of flat plate air

heaters

Author :Ho ming yeh and tonstshein lin

Journal :Energy 1996;21(6) :435-43

Inference: The Paper presents the following To increase heat transfer area of the collector. To study the effect of parallel barriers on collector efficiency Applications used by drying and space heating

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Cont.Title :Absorption of thermal radiation in a v-groove cavity

Author :E.M.Sparrow,S.H.Lin

Journal :Int J Heat mass transfer (1962;5:1111-5)

Inference: The Paper presents the following Energy absorption(solar radiation) of v-groove cavity Types of Incoming radiation are studied. Specular cavity more efficient absorber under diffuse radiation.

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IntroductionWe are going to look into one of the most important

potential applications of solar energy, solar drying.Solar drying of agricultural crops helps in reducing the post

harvest losses in developing coutries which is at present account for 30 to 40%.

In the present work, we are going to study and compare the efficiency of three most commonly used solar collectors.Flat Plate solar collectorFinned Plate solar collectorV-Corrugated solar collector

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Literature ReviewChoosing the right design of all the available designs of

solar air collector decides the economy of solar drying. In the present design of solar collectors , major losses occur

at the front cover since it has been exposed to ambient temperature whereas the back and sides of the collector can be insulated adequately.

As per the present design air may be allowed flow above, below or both the sides of absorber plate in solar collector.

Double pass concept of solar collectors was introduced by Satcunanathan and Deonaraine to reduce the cover losses, whereas Wijeysundera confirmed their concept experimentally over a wide range of operating conditions

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Cont.The air type solar collector has 2 main disadvantages lead to wastage of

solar energy in absorber plate Low thermal capacity of air Low absorber to air heat transfer coefficient.

To improve the heat transfer coefficient between the absorber plate and air, different types of absorbers were suggested. Some of them are plates with fins attached, V- corrugated, and matrix type

Literature reports higher efficiency of v-groove and finned collectors without significantly increasing pressure drop but for matrix type the pressure drop is high.

The literature lacks the study on all three collectors using the same flow rate, test procedure and set up, construction material and climatic conditions. Double pass operation of finned and v-corrugated collectors has not been experimentally investigated.

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OBJECTIVE OF THE PRESENT WORK

To develop an air type solar collector with high thermal performance suitable for solar drying.

To study different types of air collectors with various flow configurations and to improve our ability to predict the thermal performance of three types of air collectors with the help of reliable literature and extensive experiments.

To study the factors influencing the performance in both single pass and double pass air collector in identical meteorological conditions and test facility .

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EXPERIMENTAL INVESTIGATION.The thermal efficiency of a collector is defined as the ratio of

the useful thermal energy to the total incident solar radiation averaged over the same time interval.

The steady state thermal performance of a solar collector is expressed by the Hottel-Whillier-Bliss equation

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Cont.

Cont. The temperature rise equation (Eqn. 5) is helpful to design an air collector for

given meteorological condition. A series of experiments were conducted based on ASHRAE standard under

Singapore meteorological condition ( 01º22´N, 103º55´E). A common collector framework with interchangeable absorber plate was

designed to avoid differences in thermal performance due to construction. Collector Framework design:

The flat plate and finned collectors had 25 mm flow duct height and the v-corrugated collector had 50 mm v-height, leading to a flow passage area of 0.0175 m2 for all three collectors.

Window glass was used as the glazing for all the collectors. The bottom of the v-groove and finned absorbers rested on the aluminum plate of

the collector, whereas the flat plate collector was suspended from the c-channel. Supports were provided to prevent bending of the flat plate absorber.

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Cont.

For single pass operation, air will be flow along single surface and drawn from the rear end of the collector through the opening in the back plenum.

For double pass air entered at the top left opening will be turned 180º at the back plenum and drawn at the bottom right opening.

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Cont. Perforated sheets were used in the air flow channel to

distribute the air uniformly throughout the channel.

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Cont.Initial conditioning before starting the measurementsThe collectors were warmed up and run at least 1 h before tests were

conducted. Airflow rate and inlet temperature were kept constant with the help of

controllers.The collector slope was adjusted to 10º, considered to be suitable for

Singapore geographical location.To limit the solar irradiation fluctuation to 50 W/m2, the solar irradiation

must be above 630 W/m2, and wind speed should be less than 4.5m/sTests were conducted between 2 h before and 2 h after solar noon. As all the

data were taken in near normal incident angle, the effect of incident angle on the thermal performance should be negligible.

All the sensor data were monitored and entered in 20 channel data acquisition system for every 20 seconds.

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RESULTS & DISCUSSIONSSingle Pass Mode:

With the recorded information, the efficiency of the collector was calculated using Eqn. 1 for all the three collectors.

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Cont.From graph it can be seen that efficiency is strongly depend upon

air flow rate. Efficiency increase strongly up to 0.05Kg/ m2

It shows that the finned collector is more efficient than the flat plate, and the v-corrugated collector is more efficient than the finned and flat plate collectors due to increased heat transfer area.

This study shows that the v-groove collector is 7–12% more efficient than flat plate collectors and 5–9% more efficient than finned collectors.

Double Pass Mode: In this case, the curves were drawn for the air flow at both sides of

absorber plate

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Cont.

Like single pass, in double pass as well the improvement in efficiency is insensitive after 0.056Kg/ m2.

From the study, the efficiency of all the collectors increased for double pass. The v-groove collector is 2–5% more efficient than the finned collector and

5–11% more efficient than the flat plate collector.

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Cont.ASHRAE recommends drawing the efficiency curve at two

flow rates, one is 0.01 m3/ m2s and another is at a higher flow rate to investigate performance at various flow rates.

The efficiency curve, i.e. efficiency η against (Ti-Ta)/IT, was plotted using more than 16 data points.

As per Eq. (2), this results in a straight line with slope FRUL

and y- intercept equal to FR (τα).

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Cont.

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Cont.

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Cont.

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Cont.From the curves, it can be seen that the loss coefficient (FRUL) is

higher in the flat plate collector, followed by the finned collector and least in the v-groove collector for the same flow rate

Collector outlet temperature is another important parameter for drying applications. So we need to come up with optimum flow rate that would satisfy both the efficiency and suitable temperature for most of the agricultural products.

Combined curves were drawn across Efficiency & Outlet temperature Vs Flow rate.

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Cont.

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Cont.From these curves, the optimum flow rate is identified and

tabulated below.

Information on temperature rise of the air with incident radiation should also be considered for collector design .

If the inlet temperature Ti is close to Ta, the temperature rise varies almost linearly with incident radiation.

The typical plots of the three collectors are shown in figure below along with the equation for temperature rise and the threshold value, Ith, for incident radiation.

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Cont.

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CONCLUSION

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A flat plate collector with single pass is least efficient, and the v-corrugated collector is found to be most efficient.

Double pass operation further increases the efficiency of the collector. The increase is most significant in the flat plate collector and least in the v-groove collector in double pass mode.

The efficiency of all three air collectors is a strong function of air flow rate and it saturates after the flow rate 0.056 kg/ m2s. The optimum flow rate is 0.035Kg/ m2S

Since the v-corrugated collector shows better efficiency in both single and double pass operation and also is structurally stable, this collector is considered predominantly useful for drying applications.

Reference

M.A.Karim and M.N.A.Hawlader “Development of solar air collectors for drying applications” Journal of Energy conversion and Management

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