EXPERIMENTAL INVESTIGATIONS OF A SOLAR PARABOLIC TROUGH COLLECTOR FOR CIRCULAR AND ELLIPTICAL ABSORBER

Solar parabolic trough collector is one of the most efficient and an effective technology to deal with environmental pollution and it has gained much attention due to the recent energy demand. The solar parabolic trough collector is one of the most promising techniques for absorbing the heat from the sun. This heat is utilized for electricity generation and other industrial heating applications. This paper describes the theoretical and experimental assessment of performance of the circular and elliptical absorbers used in solar parabolic trough collector. The absorber tube of parabolic trough collector is used to transfer the heat to the working fluid. The working fluid considered over here is water which is the best operating medium in direct steam generation. The mass flow rate of water in absorber tube is analyzed in 3 stages as 0.016, 0.024 and 0.030 kg/s respectively. The experimental test is done in Chennai-Tamilnadu, Southern part of India which experiences a superior temperature throughout the year. The experiment is conducted for the period of one year from June 2015 to May 2016. The performance improvement focuses on collector efficiency, useful heat transfer rate, outlet temperature of working fluid, temperature gradient, overall heat transfer rate and the thermal losses.


INTRODUCTION
In the recent decades solar energy with the advancement in technology became the best energy resource for reducing the power demand from the non-renewable sources like coal, petroleum products etc. Currently the energy crisis has made a steep decrease in industrial development and commercial life of increasing populations. Most of the countries which extract power from solar energy uses Parabolic Trough Collector (PTC) device. The parabolic trough collector lies on the concentrated type device with a range of thermal energy absorption of about 400 o C, hence used for electricity generation by steam power cycle [1]. The parabolic trough collector has a receiver where heat transferring fluids like water or oil or some organic fluid circulates. The receiver is generally placed at the focus of the parabola. Since India being a tropical country it receives high solar insolation. Many works has been carried by many researchers on PTC for effective utilization of energy by optimization, thermal analysis, numerical analysis etc.
Experimental investigation has been done on the receiver, with black epoxy coated receiver and the work carried with and without glass cover on the receiver.The concluded result shows that the instantaneous collector efficiency has been increased [2]. Direct steam generation method in the absorber tube is also an important process used in electricity generation, but the superheated steam causes overheating on the absorber tube and result in non-homogeneous heat flux so the absorber tube performance should be pondered [3].
The thermal analysis on the modified receiver with porous disc receiver in parabolic collector when tested for different collector orientation angle also by varying height and width of disc has made some changes by exhibiting different heat flux. The use of porous disc receiver shows better performance [4]. Obviously the receiver is also provided with internal helically finned tubes for reducing the thermal losses also to minimize thermo mechanical stress and thermal fatigue, considerably the parabolic trough solar plant efficiency has also increased by 3% resulting in escalation of 20.6MW electricity production [5]. Solar parabolic trough collector is also studied with the heat transferring fluid as the gasphase nano fluids (CuO and Ni nano particles), resulting an increase in the heat exchange surface which leads to the improvement in thermal efficiency by 62.5% and the temperature of the nano fluid outlet as 650 o C [6].
Michael Geyer and Eckhard Lupfert have proposed and experimentally constructed Euro Trough (ET) parabolic collector ET100 and ET150 for high performance with the standard receiver and reflecting mirror panel. They also added a finite element analysis validation with their experimental work and concluded that their proposed system has reduced weight and cost reduction [7]. The Solar Parabolic Trough Collector (SPTC) is also used as small scale field in hotels for hot water generation by adding some modification as fiber reinforced plastic parabolic trough collector which has less weight and have the capacity to keep the food stuffs in hot condition, they have also done a cost analysis over the outmoded electricity method and clinched as SPTC as economical [8].
Thermal losses on the absorber tube are also a major problem and much research work has been done with the SPTC power plant. Yaghoubi etal. have done a numerical simulation using the experimentally measured data from 250 kW Shiraz (Iran) solar power plant. Authors have done a study with three types of absorber tubes, i) vacuum ii) lost vacuum (air) iii) broken glass tube (bare). They finalized that the use of absorber tube with vacuum reduces the thermal losses compared to other types and also stated that the poor isolation leads to thermal efficiency losses in solar thermal power plant [9]. Many studies have been done and are still on progress about the numerical simulation of absorber tube I S S N 2 3 2 1 -8 0 7 X V o l u m e 1 3 N u m b e r 7 J o u r n a l o f A d v a n c e s i n c h e m i s t r y 6349 | P a g e F e b r u a r y 2 0 1 7 w w w . c i r w o r l d . c o m using finite element method and finite volume method. Changfu You et al. have done a heat transfer model of absorber considering flow study using finite element method for direct steam generation plant with a conclusion that the solar radiation fluctuation drastically affects the operation of SPCT for direct steam generation [10]. Finite Volume Method (FVM) is also employed in direct steam generation thermal plant, for improving the heat transfer rate in absorber computational fluid dynamics study has been done and the results are evaluated with the experimental value [11]. Hachicha et al. have also studied about the effect of solar flux around the SPTC using FVM and solar ray trace technique by considering the size of sun [12]. The wind load in SPTC is also taken into account by many studies, where the wind is one of the major problems causing parameter. The wind velocity can damage the solar tracking method and change the focus point of SPTC which results in the poor heat transfer by the absorber. Naeeni et al. have studied the effect of wind load for scrutinizing the performance of solar collector and absorber by varying the orientation angle from 90 o ,60 o ,30 o ,0 o ,-30 o ,-60 o and -90 o of parabolic collector, specifically they also considered the effect of wind along the absorber [13]. Hence the various studies have been done on the performance improvement on absorber of SPTC for various purposes like heating applications and mainly for power generation. In this present study elliptical absorber was designed, constructed and investigated theoretically and experimentally under different water mass flow rate and inlet water temperature.

Experimental Specifications
The set up consists of solar parabolic trough collector with a storage tank of 70 liter capacity, non-return valve for maintaining the flow direction and control valve for regulating the water flow rate. To obtain the measurement data's the following measuring instruments are attached namely, pyranometer, anemometer, flow meter and thermocouple with digital display. The pyranometer is used for measuring the solar radiation in the apparatus, digital anemometer for measuring the wind velocity, flow meter for measuring the mass flow rate of water and the thermocouple with digital display for temperature measurement. The basic parameters of parabolic trough collector and the designed elliptical absorber dimensions are shown in table 1.

EXPERIMENTAL SETTING
The experiment work has been done for a period of one year (from June 2015 to May 2016) and on every month the test was conducted for three sunny days with the normal circular absorber and another three days with the elliptically designed and constructed absorber. The test was recorded on hourly basis. In this experiment the cold water from the storage tank is passed through the absorber tube in solar parabolic trough collector via gravity method, because the storage tank is kept above the level of collector, and the heated water again enters the storage tank through the nonreturn valve. The flow of hot water takes place by thermo siphon (natural circulation) and hence the hot water is replaced by the cold water from the bottom of the storage tank. The measurement data's such as the total solar radiation on the collector, wind velocity, water mass flow rate and the temperatures at inlet, outlet and the ambient temperatures are noted for every one hour. The temperature on the outer cover of the glass tube was also noted. Similarly the experiment is repeated with the elliptical absorber tube and all the readings were recorded. In this experimental set up the sun tracking mechanism has been attached hence the set up tilts from east to west automatically. The mass flow rate of water maintained for the test was 0.016,0.024 and 0.030 kg/s. The layout of the experimental setup is shown in Fig.1.    The table 2 and 3 illustrates the average variation of temperature and solar radiations for the mass flow rate of 0.014 kg/s, which are measured directly using thermocouples and pyranometer. It shows a successive improvement in final temperature which leads to a better absorption of solar radiation. Similarly the test was conducted using a different mass flow rate of about 0.024, and 0.030 kg/s results in improved heat absorption by the elliptical absorber

THEORETICAL METHODOLOGY
The thermal performance of the solar parabolic trough collector is based on the heat transfer rate of the absorber. The useful energy obtained for the collector is calculated using the following equations, [14,16] Qu = mCp(Tout -Tin) Qu= mCp{ + amp -Tin}{1-exp[- The useful heat transfer energy gained per unit of the collector length (Qu`) can be expressed in terms of the local receiver temperature (Tm) also known as absorber mean surface temperature, [14,16] Qu`= Q u (4) Where, F` is the collector efficiency factor and can be defined from the equation (6) Here hf = heat transfer coefficient on inside surface of tube (W/m 2 o C)

FR= Collector heat removal factor
The collector heat removal factor is given by the following equation, Hence the collector efficiency can be obtained by dividing useful heat transfer energy by the product of collector aperture area and Ibr, and the instantaneous collection efficiency (ηin) can be calculated by the following equation (9) [14,16] ηin = (9) (i) Overall loss coefficient and heat co-relations: The overall heat loss coefficient in the solar parabolic trough collector is based on the convection and radiation, and the heat loss rate per unit length is given by the following equation (10 &11 Hence the rate of heat transfer coefficient for the enclosed space between elliptical absorber and glass cover is given as (iii) Heat transfer coefficient on the inside surface of the elliptical absorber tube : The heat transfer coefficient hf through the convective mode on the inside surface of the elliptical absorber tube is given by the following equation, Also for the Reynolds number greater than 2000, the flow is turbulent and heat transfer coefficient may be given as

Time of Day Vs Average Temperature
The experimentally found result as shown in fig.2 reveals that the proposed system with the elliptical absorber has high heat transfer rate when compared to the ordinary circular absorber. The solar radiation is utilized maximum in the elliptical absorber resulting in better outlet temperature and comparatively high temperature gradient. The vacuum condition made between the absorber and the glass cover has reduced the thermal losses considerably, because the elliptical absorber has more contact area and the major amount of heat are carried away by the mass of water passing through the absorber.

Fig. 3 Average Solar Radiation absorbed to Time of Day
The solar radiation absorbed with respect to time of day is shown in fig.3.The solar flux absorbed by the absorber in the form of sensible heat causes the working fluid to absorb more heat. The solar flux absorption is based on the surface exposed to the solar radiation, here in the elliptical absorber have more surface ie, the major axis of the ellipse is large when compared to the normal absorber.

Fig 4. Average Overall heat loss coefficient to Months of Year
Since the enclosed annular area between the absorber tube and the glass tube is maintained in vacuum condition, the thermal losses are reduced to a maximum. Also in the elliptical absorber more vacuum condition is maintained. The overall heat loss is calculated as the average of every month from June 2015 to May 2016 as shown in fig.4.

Fig 5 Average Useful heat transfer to the Time of Day
The time of day to the average useful heat transfer is shown in fig.5.Useful heat gained by the collector is based on its heat absorbing surfaces, hence while comparing to the normal absorber the elliptical absorber shows more useful heat transfer rate.

Fig. 6 Average Instantaneous efficiency to the Months of Year
The instantaneous collector efficiency is based on the performance of the collector including heat transfer rate, thermal losses, solar flux absorption capacity ie, the collector aperture area. In this paper the mass flow rate of water is made from 0.016, 0.024 & 0.030 kg/s. It is also found that the outlet temperature has not been affected much beyond the mass flow rate of about 0.030 kg/s. Hence from fig.6, the instantaneous collector efficiency for the designed and constructed elliptical absorber tube shows better performance in solar parabolic trough collector.

CONCLUSION
From this study, the solar parabolic trough collector can be employed in industries for heating applications and of course this technique can also be employed for power generation. Since, Chennai in Tamilnadu, India receives proper annual sunny days per year and the solar energy can be harvested for electricity generation. In this paper, the experimental work is carried for a period of 11 months using the normal absorber and newly constructed elliptical absorber. Finally it shows the following results.
1) The instantaneous efficiency has been increased with an average of 9% for the period of June 2015 to May 2016 with the use of newly designed elliptical absorber tube when compared to normal PTC of circular absorber tube.
2) The outlet temperature of the water is increased, where as the temperature gradient increased considerably.
3) The useful rate of heat transfer received by the collector has been increased to 82.86% because of the more surface for heat absorption in the elliptical absorber. The receiver temperature is also increased to about 12% for elliptical absorber when compared to the normal PTC. 4) Comparatively, the heat losses have been reduced because of perfect vacuum condition and proper contact surface of radiation absorption. The thermal losses reduced to about 89.05% for elliptical absorber tube when compared to the circular absorber tube. 5) The average solar radiation is found to be high for elliptical absorber and recorded as 1002 W/m 2 which is 34 W/m 2 higher than circular absorber at 1 pm.