Experimental Performance of A Solar Collector in Solar Chimney Power Plant System

Huilan Huang College of mechanical engineering

Guangxi University Nanning, 530004,China

E-mail:huanghuilan@gxu.edu.cn

Gang Li College of electrical engineering

Guangxi University Nanning, 530004,China

E-mail:liganglyd@126.com

Hua Zhang Institute of refrigeration technology

University of Shanghai for science and technology Shanghai, 200093, China

E-mail:zhanghua3000@sohu.com

Abstract—Solar chimney power plant has been proposed as a device to economically generate electricity from solar energy in large scale in the future. There are many factors to influence on the performance of the solar collector. This paper describes details of the experimental process on the thermal storage material in solar collector. Two experimental modifications are tried on the thermal storage material in collector: (1) Pebbles and (2) introducing the water tubes with a black surface and combining pebbles and the water tubes. The former modification helped in enhancing the air temperature difference of the collector, while the latter contributed to increasing the air temperature as well as the collector efficiency. Changing the packing height of the pebbles out of the water tubes, the height of the pebbles has been tested and optimized. Experimental performance results of this model is presented.

Keywords- generate electricity; thermal storage material; pebbles; solar collector

I. INTRODUCTION Solar chimney power plant has been proposed as a device

to economically generate electricity from solar energy in large scale in the future. Solar collector is the power supply for the solar energy chimney in generating electricity system, thus it is of great significance for studying on the performance of the solar collector.Thermal storage materials in the collector play an important role in the energy conversion. The choice of the thermal storage material in the collector is a key of assuring the collector high-efficiency operation. Reference [1-4] have performed the working principle of the solar energy chimney for generating electricity. This paper describes details of the experimental performance for the collector with difference thermal storage materials.

Solar energy collector is special kind of heat exchanger that transforms solar radiation energy to internal energy of the transport medium. This is a device, which absorbs the incoming solar radiation, converts it into heat, and transfers this heat to air flowing through the collector. The ground of collector can store thermal energy after warmed by solar radiation. And overall energy of solar chimney system comes

from this energy. Therefore, the solar energy transfer can be enhanced when the material of ground inside solar collector uses high absorptivity substance near black body. The ways raising the absorptivity of ground would make ground temperature in the collector raise. This can also enhance the solar energy conversion. The temperature difference between ground and air of collector causes energy transfer. Modality of heat transfer is coupling between radiation and convection. This includes ground absorbing the solar radiation and convection transfer between flowing air and ground. The warm air is moving from the periphery of the solar collector to its center, this process is very complex. Characteristic of flowing is relevant to several factors discussed by Gannon and Von Backstrom [5], Kalogirou [6], Taumoefolau [7], Ho [8,9]. Basing on similarity principle, a modeling experiment on collector is designed and examined. There are various influence factors in the system. Some conclusions drew from experimental study are expected to disclose the correlations between characteristic of collector and these complex influencing factors.

II. EXPERIMENTAL SETUP The sketch of experimental setups for solar heat collector is

shown in figure 1. This setup consists of box of collector, solar simulative radiation device, measurement system in air flow rate, temperature and solar radiation intensity and Datataker system[10].

Figure1. Diagram of experimental setup of solar heat collector

Solar radiation energy concentrates mainly in the range of 0.2 ~ 2μm wavelength and zone of infrared ray and visible light

The scientific research foundation of Guangxi University(XBZ090789) The scientific research foundation of Guangxi University(20090033)

2010 International Conference on Electrical and Control Engineering

978-0-7695-4031-3/10 $26.00 © 2010 IEEE

DOI 10.1109/iCECE.2010.907

3718

2010 International Conference on Electrical and Control Engineering

978-0-7695-4031-3/10 $26.00 © 2010 IEEE

DOI 10.1109/iCECE.2010.907

3718

water tube wall

pebbles

a b

occupy most proportion. So we simulate the sun radiation using 21 calix lamp HW200--250 and its irradiance can be adjusted between 300~900 W/m2 by a voltage regulator. Structure dimension of collector is set at 2 m × 1 m × 0.5 m. In collector bottom, some pebbless are put into as its thermal storage material in order to enhance energy transfer in collector.

A fan is mounted in windpipe. It could be use as chimney that air would be pumped to upper layers of troposphere at outlet of solar chimney. In this condition, the fan could change air flow rates in collector outlet , so it can be simulated heat air changing in collector of which be transported to chimney under various conditions in actual solar chimney power system.

The air, which comes from environment room, is warmed in the collector. The warm air is moving from the inlet of the collector to its outlet towards the entrance of the windpipe finally vented out.

In measurement system, a TBQ-2 radiometer is used to measure the irradiance of infrared radiation lamp. There are 15 chromel-constantan thermocouples that measure the air temperatures of collector’s section, plus 2 Pt100 platinum- resistance thermometers that accuracy as A level measuring the inlet and outlet air temperature of collector. The thermocouples have been regulated in tank that accuracy is ± 0.1 zero degree. A Veturi tube in windpipe is used to measure the airflow rate.

In datataker system, there are several ADAM-4018 A/D modules and ADAM-4015 A/D modules that take signal from the thermocouples and the Platinum-resistance thermometers respectively, plus 2 ADAM-4017 A/D modules taking the signal from pressure difference transmitter and air flow rate. A host computer acquires various experiment parameters data from the Datataker.

III. TEST CONDITIONS AND METHOD Many factors influence on the performance of the solar

collector such as solar radiation, air volume flow, and compound mode of thermal storage materials. The pebbles (type ) and the combination of the pebbles and the water tubes (type ) were used for the thermal storage material. The collectors with above the two heat accumulators were studied respectively in this paper.

Under the same of radiant intensity, the temperature

the collector efficienincreasing in the different the packing height of the pebbles out of the water tubes. From then on the optimal packing height of the pebbles was gotten and the effect of the heat storage was improved. Once the water is filled in the water tubes, the water tubes will be sealed. So the water in the collector could not evaporate or leak, the principle of the heat storage has been depicted [11].

The effect of air volume flow on the performance of the collector was studied on through changing air volume flow entering the collector. Though air volume flow is a constant value and the solar collector is a huge device actually, the solar collector may be separated into many different units, moreover air volume flow of various units are different. Besides the time,

the season and the geographical location were different, even if the same as system would produce different air volume flow. So the operation of changing air volume flow is feasible in the experiment.

The flow passage of the air in the collector trunk is 0.25m in height. There were nineteen water tubes in the collector trunk. The water tubes are set with in the vertical direction of the main flow in the collector (Fig.2 and Fig.3).

Figure2. The solar collector layout

Figure3. profile chart of thermal storage material in the solar collector a the dimension of water tube for thermal storage material

b Cross-section of water tube in radial direction and the pebbles

IV. TEST RESULTS AND ANALYSIS In order to obtain the influence of air volume flow, radiant

intensity and the packing height of the pebbles on the performance of the collector, several teams of experiments were used for comparing to the temperature difference between the inlet and outlet of the collector under different working conditions. Reference[12,13] has given the formula for the collector efficiency.

A. Effect on air volume flow When the combination of the pebbles and the water tubes is

used as the heat accumulator, radiant intensity I0=715W/m2, ambient temperature t0=14.5volume flow increasing in two different conditions. (Fig.4 and Fig.5)

Fig.4 and Fig.5 show that when Qv<120m3

type are higher than that of type . When Qv 120m3

are smaller than that of type distinctly. When air volume flow increases, the air in the collector carries away more heat quantity. But the heat quantity obtained by the

reduce

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with air volume flow increasing. When air volume flow is low, is smaller than that of type (Fig. 4).

Figure4.

Figure5. Effect of Qv and type for various volume flows

This is because that the specific heat of the water is much bigger than that of the pebbles. The heat quantity stored in the collector with type becomes more, on account of less heat

of type became smaller than that of type with air volume flow increasing.

Owing to the more heat quantity is stored in the collector with type was smaller than that of type stayed in the high value for a long time. So the effect of thermal storage in the collector with type changed little because of the change of air volume flow. In this way the collector with type operated steadily.

ed effectively, when , Qv, become 80m3

conditions are lower as shown in Fig.5. The bigger the product

evidently.

B. Effect on solar radiation intensity When air volume flow is 55m3/h, the pebbles acts as the

thermal storage material and radiant intensity I0 changes from 0 to 800W/m2

lineally with radiant intensity increasing. However,gradually then reduced slightly with radiant intensity increasing.

When solar radiation intensity increased, heat exchange of the air in the collector was enhanced; the air acquired more

I0 is 300W/m2 I0 becomes much bigger, the collector acquires the heat quantity smaller, the thermal loss rises, so the bigger radiant intensity is, the higher the collector efficiency does not always become.

Figure6. The influence of I0

C. Effect on the packing height of the pebbles From the experiment above known, the collector with the

studying on the packing height of the pebbles out of the water tubes. One is water tubes on the pebbles; the other is that the packing height of the pebbles out of the water tubes is r. The

, Fig.7. And the collector efficiency of all points was calculated, Fig.8.

Figure7.

Figure8.

When I0 is 720W/m2 and the ambient temperature is 12 , nineteen water tubes were placed evenly on the pebbles in the collector. When the packing height of the pebbles was changed, the additional pebbles would not be added in the collector; otherwise the flow passage height would be changed. The flow passage height was a factor of effecting on the performance of the collector yet.

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It is show 1 higher than that of packing pebbles under the same as volume flow rate (Fig.7). This is because that the specific heat of water is further larger than that of the pebbles, packing the pebbles reduced the heat exchange area of the water tubes, this made heat exchange in the collector weakening and the heat quantity stored in the water tubes reducing. The heat quantity was carried off by the air in the collector decreasing, so in packing pebbles collector would be decreased. On account of the heat quantity stored in collector with packing pebbles was decreasingpacking pebbles with the air volume flow increasing.

of collector without packing pebbles is higher than that of collector with packing pebbles under the same as volume flow. Moreover, when air volume flow rate

of collector with packing pebbles changes little.

V. CONCLUSION There are many factors to influence on the performance of

the solar collector. It is analyzed in the effect of air volume flow rate, solar radiation intensity and the packing height of the pebbles out of the water tubes. The pebbles and the combination of the pebbles and nineteen water tubes act as the heat accumulator in solar collector. The experiment studied on the effect of volume flow rate

been analyzed.

reduces slightly with volume flow rate increasing. Moreover,flow rate is,

Solar radiation intensity effects on the performance of the collector increases gradually then decreases slightly with solar radiation intensity increasing. So the larger radiation intensity is, the higher the collector efficiency is not always following.

of collector are the larger than that of collector with

packing pebbles .

NOMENCLATURE , temperature difference between the inlet and outlet of

the collector,

I0 , solar radiant intensity, W/m2

Qv, air volume flow rate, m3/h

r , outer radius of the water tube, m

Greek symbols

, collector efficiency

Subscripts

0, packing height of the pebbles, m

r , packing height of the pebbles, m

ACKNOWLEDGMENT This authors would like to render thankfulness to project

sponsored by the scientific research foundation of Guangxi University(XBZ090789) and the scientific research foundation of Guangxi University(20090033) for their support of the research work.

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[8] K. T. K. Ho, and D. L. Loveday, “New Approach for Analyzing Solar Collectors Subjected to Unequal Front/Rear Ambient Temperatures:The Equivalent Ambient Temperature Concept, Part 1:Modeling, ” ASME Journal of Solar Energy Engineering, Vol.124, pp. 262-267, 2002.

[9] K. T. K. Ho, and D. L. Loveday, “New Approach for Analyzing Solar Collectors Subjected to Unequal Front/Rear Ambient Temperatures:The Equivalent Ambient Temperature Concept, Part 2: Validation and Implications for Design,” ASME Journal of Solar Energy Engineering, Vol.124, pp. 268-275, 2002.

[10] H. Zhang, J. M. Zhang, H. L. Huang, and F. Lu, “Experimental Device Setup for Solar Chimney Collector”, Journal of University of Shanghai for Science and Technology, Vol.29, No.4, pp.383-385.Aug. 2007(In Chinese).

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