performance analysis of solar parabolic trough collector used

Performance analysis of solar parabolic trough

collector used for absorption chillers

Wael Nesim

Department of Mechanical Power Engineering

Faculty of Engineering, Ain Shams University, Cairo, Egypt

Email- [email protected]

Mahmoud Abo Elnasr




Hany Saad




Abstract- In this paper, A parabolic trough collector (PTC) has been adopted for a solar air conditioning. The system is

required for a factory of 240TR cooling load. An experimental and analytical analysis using Ansys CFD model are carried

out in addition to the performance of the solar air conditioning system with the PTC is investigated. The results showed

that the solar air conditioning system with able to supply the factory by the required cooling load through the day. The

average parabolic trough collector efficiency is 98 %. Also, it was found that the error between CFD model results and

exp results varying from 0.3 % to 4.3 %. As a result, this model can be used in generating a PTC model geometry and

material to meet the load of an absorption chiller requirements. In addition to, using PTC in absorption chillers

decreasing the running cost because it reduces the consumption of fuel used in absorption chillers.

Keywords – PTC, Parabolic trough collector, absorption chiller, Ansys CFD, Solar energy

Nomenclature

Aa Collector Aperture area (m2)

Ar Receiver area (m2)

L Collector length (m)

W Collector width (m)

Do Outer diameter of the receiver (m)

C Concentration ratio

Keff Convection coefficient through the receiver annulus (m)

High Technology Letters

Volume 27, Issue 6, 2021

ISSN NO : 1006-6748

http://www.gjstx-e.cn/34

Dci Cover inner diameter (m)

Dco Cover outer diameter (m)

Dr Receiver diameter (m)

Tr Receiver mean temperature (℃)

Tf Fluid temperature (℃)

Tci Cover inner diameter temperature (℃)

σ Stephan Boltzman constant (5.67 x 10-8 W/m2 K4)

εr Receiver emittance

εc Cover emittance

Kc Conduction coefficient of the glass cover

hw Wind heat transfer coefficient

Tsky Sky temperature

Ta Ambient temperature

Nu Nusselt number

Re Reynolds number

Density (kg/m3)

Dynamic viscosity of air (kg/m3)

Velocity (m/s)

UL Heat loss coefficient based on unshaded collector area

Uo Overall heat transfer coefficient

hfi Heat transfer coefficient inside the tube “absorber”

K Thermal conductivity for the tube “absorber”

Collector efficiency

Collector flow factor

HTF mass flow rate (kg/s)

Cp specific heat at constant pressure (kJ/kg K)

Qu Useful heat gain (kW/m2)

S Absorbed radiation per unit area of unshaded aperture area

Temperature

Average temperature drop from the outside of the receiver to the fluid.

Heat transfer fluid

HCE Heating Collector Element

COP Coefficient of performance

Parabolic trough collector

Ton Refrigerant

Et Total solar radiation (kW/m2)

Greek symbols

Specular reflectance of the concentrator

Intercept factor

Cover transmittance

Absorber absorbance

Incidence angle

I. INTRODUCTION

The installation of the Absorption chillers consumes high energy because of fuel consumption, in addition to the

effect of burning fuel on Ozone layer. As a result, using the soler energy technology to reduce fuel consumption and

its environmental effect in absorption chillers is a challenge for many researchers, [1].



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Mat S et al. [2] investigate in detail the energy consumption distribution for commercial buildings and residential

one. They found that all over the world the new energy technologies that reduce energy consumption and

environment pollution are attracted for many researchers. As an evidence of high energy consumption, it is found

that residential and commercial energy consumption increased between 20 % and 40 % in developed countries, [3].

The more one applied in the cooling and heating for building is the solar energy [4].

The solar energy technology in air conditioning come into being, which have many advantages. The building

cooling load required usually large when the solar radiation is strong, i.e., in the cooling system, the refrigeration

load increases with the increase in the intensity of the solar radiation, which match with the changing of the building

cooling demand. Also, the equipment higher utilization rate, i.e., the solar system can provide the hot water for the

building in winter and cooling in summer [5]. In addition to it promote the environmental protection, saving energy.

The absorption refrigeration cycle is used for the solar air conditioning which use (LiBr/H2O) in different solar

collectors temperature from low to high [3]. Recent researcher focused to increase the medium temperature solar air

conditioning to be high temperature. In Italy, a 25 KW double effect absorption chiller with medium temperature

using parabolic trough collectors was installed [4]. The COP of the systems was very high compared with low

temperature solar system using absorption refrigeration system with single effect [4]. Mathematical modeling for

solar absorption chiller paid attention for many researchers. A new mathematical model was established by Shirazi

A et al. [6] for an absorption chiller. He studied the performance of the chiller at several external temperatures, for

example, the temperatures of cooling water and the heat source. Also three types of solar air conditioning systems

are compared by [7], A vacuum tube collector, vacuum flat plate collector and parabolic trough collector. They used

TRNSYS17 and MATLAB software to find the relationship between the solar refrigeration system performance and

the direct radiation ratio. They concluded that the double effect combination with solar collector could reduce

energy consumption and CO2 emissions when compared with compression refrigeration cycle.

[8] Concluded that, the preferred technology to be used among other technologies such as Linear Fresnel collectors,

parabolic dish is (PTC) parabolic trough collector technology, because of it is light structure system, in addition to it

is efficiency is high.

Drosou V et al. [9] established a numerical simulation for a PTC, solar parabolic trough collector, with a double

effect absorption chiller for a solar refrigeration system for an office building. The results showed that this system

was more efficient than the traditional flat plate collector and required less collector area.

In general, the PTC can provide driving temperature from 140 to 200 °C to drive the double-effect LiBr/H2O

absorption chillers and the COP ranged from 1.1 to 1.5.

Also, high COP generated from PTC is a reason to use PTC technology, as the driving heat source temperature for

single-effect chillers is about 80-100 ℃, while their COP is limited to 0.7 in range. Double and triple effect chillers

on the other hand, require driving temperatures of around 180-210 ℃, and can reach COPs of up to 1.4 and 1.8

respectively [4].

The solar radiation system is unstable when used to power solar refrigeration systems or for HVAC systems as it

need area with high intensity of the solar radiation. To solve this problem, there are two common ideas [10,11]. The

first one is using auxiliary heat source to supply the system or using a backup conventional refrigeration equipment.

The second one is using the storage system of solar energy. Many energy storage methods include the latent, the

sensible and the thermochemical energy storage are used in solar air conditioning systems [12, 13, 14, 15].

One of the factors that have a great effect on the performance of the PTC, are the size of receiver tube which

increase the performance as well as the heat losses decreases with the small size of receiver tube. This is due to the

increase in the concentration ratio with small tubes area [5].

The tracking system of PTC is also an important parameter to increase the PTC efficiency. The PTC tracking system

can be east-west or north-south direction [14].

In general, it is required to develop low cost, and small PTC. More than 30 small models PTC with aperture width

less than 4 m developed by many companies in the last 10 years which used in air conditioning chillers [25].

For example, an independent-developed PTC collector was developed by Y. Bi et. al. [24] for solar air conditioning

system. They developed an experimental system to measure the PTC performance.

Yuehong Bi et. al. [24] studied the operation strategy of an independent-developed solar parabolic trough collector

(PTC) using absorption refrigeration chiller and three-phase accumulator. They concluded that the system with a

three-phase accumulator enable to supply cooling for the building continuously and steadily. The system also

improves PTC efficiency as well as energy storage efficiency.

In this paper a mathematical model was established using ANSYS to design a PTC collector to supply energy to

absorption chiller LG 240 TR for a plant in Cairo, Egypt. The model is validated using experimental data of

Yuehong Bi et al. [24]. The system performance was measured and compared to provide the theoretical guidance

for the practical application of the system.



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II. AIR CONDITIONING AND PTC SOLAR SYSTEM DESCRIPTION

An air conditioning solar system is shown in Fig.1.

Fig.1-A

Fig.1-B

The system consists of parabolic trough collectors (PTC), absorption chiller, cooling tower, circulating pumps, auxiliary natural gas source, etc. The PTC and the NG source provide thermal energy for absorption chiller during day and night. During the day, the solar energy is direct applied to the absorption chiller through the PTC for cooling to meet the required cooling load, and any excess solar energy is lost as the system is not equipped with storage system. In case of low solar energy, the NG system is operated to compensate the load difference. The application used need the cooling load from 8 am to 5 pm only as it is a food factory. Therefore, no need to storage system.



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The PTC system used consists of the (HCE) and a bending linear parabolic reflector, the (HCE) consists of an absorber tube surrounded by a glass tube with a vacuum layer in between to reduce convection heat losses Fig.2 adapted from [26].

Fig.2 adapted from [26]

The materials of absorber tube used is stainless steel with a selective absorber surface which provides the required optical and radiative properties with a high solar radiation absorbance and low emittance for the temperature range in which the surface emits radiation [8].

To have a complete evacuation between steel and glass tubes, the vacuum pressure is about 0.013 Pa. Also, there is no hydrogen required in vacuum envelope so chemical getters are used to absorb hydrogen [9].

The (HCE) is located in the focal line of the parabolic trough. The result of the area ratio between (PTC) and (HCE), a large amount of energy absorbed by steel tube that heat the (HTF), this area ratio is the concentration ratio and equal 24 [24].

During the daylight, the reflector of the PTC reflects the solar radiation to the vacuum absorber tube which is located at the focal line. The solar energy reflected heating the thermal oil the absorber tube. A circulating pump is used to circulate the heating thermal oil into the absorption chiller, in this case the absorption chiller will operate.

III. ANSYS CFD MODEL VALIDATION

A numerical solution is commissioned to reveal the heat transfer and heat losses of PTC with the same dimensions used in experiments [24]. In order to validate the Ansys CFD model to be used to define PTC geometry needed in a specific condition. The model is validated and compared with experimental data of the PTC geometry and properties which are listed in (table 1) [24].



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Name Value (unit)

Parabolic trough collector length

Aperture width

Reflectance of concentrating mirror

Concentrating ratio

Optical efficiency

4 m

2.4 m

≥94 %

24

74.5 %

Heat absorbr tube

Inner/outer diameter

Absorbance/emittance (at temp)

Thermal conductivity

Steel

0.0272 m/0.0320 m

0.95/0.1 (300 ℃)

54 w/(m.k)

Outside glass tube

Inner/outer diameter

Transmittance

Thermal conductivity

Vacuum degree

High borosilicate 3.3

0.066 m/0.070 m

≥95.5 % (AR film)

1.2 w/(m.k)

≤0.001 mbar

Table 1 (adapted from [24])

There are many experiments that are compared with Ansys CFD model, and the experiments are listed in (table 2) [24].

Parameter name Unit

Intensity of solar radiation W/m2 610 506 630 959 940 725 139.7 237

Ambient wind speed m/s 1.36 1.36 1.51 4.2 2.75 4.47 2.7 3.4

Mass flow rate kg/s 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4

Ambinet temperature ℃ 18.8 17.2 17.3 15.7 14.5 14.1 12.3 16.8

Inlet temperature ℃ 176.7 206.4 194 174.8 159.7 150.3 134.4 155.2

Experimental Measured outlet temperature ℃ 183.4 211.8 199.2 181.8 167.1 157.6 142 158.2

Ansys model outlet temperature ℃ 184 212.5 201.6 186.4 171 159 136 158.5

The relative error % 0.327 0.331 1.205 2.53 2.334 0.888 -4.23 0.19

Parameter Value

Table 2 (adapted from [24])

The significant difference analysis and the relative error between the model and the experimental results are determined to verify the accuracy of the Ansys model.

IV.GOVERNING EQUATION

In order to calculate the absorbed solar beam radiation, heat losses should be calculated. Fig.3 adapted from [26]



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Fig.3 adapted from [26]

represent the heat transfer circuits for the absorber tube. The required equations are as follow:

Parabolic trough collector aperture area:

(1)

Parabolic trough collector area concentration ratio [15]:

(2)

(3)

Heat transfer from the absorber to the HTF [16]:

(4)

Heat transfer from the receiver” absorber” to the inside of the cover “glass pipe” [16]:

(5)

Previous equation can be written as following in case of Keff = Zero [17]



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(6)

Heat transfer from inner cover “glass pipe” to outer cover [18]:

(7)

Heat transfer from outer cover “glass pipe” to surroundings at Ta [19]:

(8)

Sky temperature [20]:

(9)

Wind heat transfer coefficient [21]:

(10)

Nusselt number [18]:

in case of 1000 < Re < 50000 (11) in case of 0.1 < Re < 1000 (12)

Reynolds number [18]:

(13)

Loss coefficient [22]:

(14)

Overall heat transfer coefficient “based on the outside receiver tube diameter” between the surroundings and the

fluid [22]:

(15)

Collector efficiency [23]:

(16)

Collector flow factor [4]:

(17)

Relation between collector efficiency and collector flow factor [4]:

(18)

Useful heat gain [6]:



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(19)

Fluid temperature rise [6]:

(20)

Average temperature drop from the outside of the receiver to the fluid [23]:

(21)

Absorbed solar radiation to be calculated from the following equation and it is necessary to get PTC heat losses.

Absorbed radiation per unit area of unshaded area [23]:

(22)

V.RESULTS AND DISCUSSION

V.I. MODEL VLIDATION

The analysis of the Ansys model results of the PTC is compared with the experimental results of Table 2 [24]. The significant difference analysis and the relative error between the model and the experimental results are determined to verify the accuracy of the Ansys model, as shown in Table 2 [24]. It is clear from Table 2 [24], that the error between the model and the experimental results is about 0.19 to 4.23 %. which means that the model results are close to experimental results of Table 2 [24]. As a result, the calculation from the Ansys model of PTC reflect the actual operating data which can be used to analyze the PTC performance. For example, when applying experiments in table 2 in the Ansys CFD model, the output results are as shown in Fig.4.

Fig.4



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Fig.5 shows that the main reason in increasing HTF outlet temperature is the sun intensity although increasing or decreasing the value of ambient temperature and wind speed.

Fig.5

In case of sun intensity equal to 139.7 w/m2 , wind speed equal to 2.7 m/s, ambient temperature equal to 12.3 ℃ & inlet HTF temperature equal to 134.4 ℃, it was found that outlet HTF temperature from ansys model equal was 136 ℃, as a result the temperature difference between inlet and outlet HTF temperature equal to 1.6 ℃ as shown in Fig.5. this means that the temperature difference is very low when using PTC in absorption chiller cycle. In the other hand if the alternative method used in absorption chiller as direct fired it will consume a lot of NG fuel.



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The low temperature difference is because of low of useful heat gain from sun as shown in Fig.6, as it is equal to 898 Watt, this value is low compared with other values in other conditions.

Fig.6

As a result the PTC is not recommended in areas with low in sun intensity.

On the other hand and in case of sun intensity equal to 506 w/m2 , wind speed equal to 1.36 m/s, ambient temperature equal to 17.2 ℃ & inlet HTF temperature equal to 206.4 ℃, it is found that outlet HTF temperature from Ansys model equal to 212.5 ℃. As a result, the temperature difference between inlet and outlet HTF temperature equal to 6.1 ℃ as shown in Fig.5. The temperature difference is higher than value in the first case because of high sun intensity.

When the sun intensity equal to 610 w/m2, wind speed 1.36 m/s, ambient temperature is 18.8 ℃ & inlet HTF temperature equal to 176.7 ℃, The outlet HTF temperature from Ansys model was found equal to 184 ℃. In this case the temperature difference between inlet and outlet HTF temperature equal to 7.3 ℃ as shown in Fig.5. The temperature difference in this case is higher than in the first two cases as the sun intensity is higher.

The same result as when increasing the sun intensity to 630 w/m2, with wind speed equal to 1.51 m/s, ambient temperature 17.3 ℃ & inlet HTF temperature equal to 194 ℃. It is found that outlet HTF temperature from Ansys model equal to 201.6 ℃. The temperature difference between inlet and outlet HTF temperature equal to 7.6 ℃ as shown in Fig.5.

Increasing the sun intensity to 725 w/m2 and to 940 w/m2 respectively at the same wind speed 4.47 and 2.75 m/s, ambient temperature equal to 14.1 ℃ and 14.5 oC respectively & inlet HTF temperature equal to 150.3 ℃ and 159.7oC respectively, it is found that outlet HTF temperature from Ansys model are 159 ℃ 171 ℃ respectively. As a result, the temperature difference between inlet and outlet HTF temperature equal to 8.7 ℃ and 11.3 oC respectively as shown in Fig.5. These cases mean that increasing the sun intensity increasing the temperature difference between inlet and outlet HTF which increase the heat gain from the sun.



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V.II. PTC PERFORMANCE

V.II.I. FACTORY COOLING LOAD

The factory typical cooling load during the day is calculated by HAP cooling load software. The result from HAP is shown in Fig.7. From this figure the maximum cooling load of the Factory is 240 TR (844 kW). The large cooling load is at 3 P.M, and the cooling load is almost zero at night. In this regard, the absorption air conditioning system are designed to meet the maximum cooling load which is 240 TR (844 kW).

0

100

200

300

400

500

600

700

800

Data for August

Lo

ad

( k

W )

Hour of Day

0001

0203

0405

0607

0809

1011

1213

1415

1617

1819

2021

2223

Total Cooling Total Heating

Fig.7

V.II.II. APPLYING ANSYS MODEL ON A PROJECT

From obvious experiments and results, Ansys model is verified, and it can be used in generating results in other practices. And it will be used in a project that have absorption chiller parameters as per table 3 adapted from [25].



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Parameter Name Unit Parameter Value

Cooling Capacity TR 240

COP Unitless 1.51

Chilled Water Flow Rate m3/hr 145.2

Entering Chilled Water Temperature ℃ 12

Leaving Chilled Water Temperature ℃ 7

Cooling Water Flow Rate m3/hr 240

Entering Chilled Water Temperature ℃ 32

Leaving Chilled Water Temperature ℃ 37

Entering Hot Water Temperature ℃ 95

Leaving Hot Water Temperature ℃ 72

Hot Water Flow Rate kg/s 9.83

Chilled Water Data

Cooling Water Data

Hot Water Data

Table 3 – absorption chiller parameters. (adapted from [25])

By applying Ansys model to get the PTC dimensions used to apply parameters in table 3, the calculations were made on 21st August and the average solar radiation intensity in this day = 534.5 Watt/m2 as shown in Fig.8. from this figure, the length of that PTC is 50 m, the PTC aperture width is 21 m. This result in case of solar intensity is less than 534.5 Watt/m2, an auxiliary heating source is needed to heat the water from 72 ℃ to 95 ℃.

Fig.8



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V.II.III. CHARACTERISTICS OF THE COLLECTOR

The performance of the PTC is shown from figures 9 to 20. Fig.9 to Fig.11 show that from January to March and Fig.14 to Fig.20 from June to December. It is clear from these figures that the auxiliary NG is needed before 9 A.M and after 3 P.M, but from 9 A.M to 3 P.M, no auxiliary NG is needed. But on contrary, the solar radiation is more than needed, therefore, a storage heat will be used.

As per Fig.12 to Fig.13 it is shown that thru April and May, auxiliary NG needed is only before 10 A.M and after 2 P.M, but within 10 A.M to 2 P.M, no auxiliary NG needed. In this case also, the solar radiation is more than needed and this lead to use a storage heat.

Fig.9



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Fig.10

Fig.11

Fig.12



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Fig.13

Fig.14



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Fig.15

Fig.16



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Fig.17

Fig.18



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Fig.19

Fig.20



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VI. CONCLUSION

Solar air conditioning system is used to meet the cooling load of a certain factory. First, a mathematical model was developed using ANSYS to be used to design a PTC collector to supply energy to absorption chiller with LG 240 TR capacity. The model is validated using experimental data of Yuehong Bi et al. [24]. The system performance was measured and compared to provide the theoretical guidance for the practical application of the system. An experimental and analytical analysis using Ansys CFD model are carried out In addition to the performance of the solar air conditioning system with the PTC is investigated. The results showed that the solar air conditioning system was able to supply the factory by the required cooling load through the day. The average parabolic trough collector efficiency is 98 %. Also, it was found that the error between CFD model results and exp results varying from 0.3 % to 4.3 %. As a result, this model can be used in generating a PTC model geometry and material to meet the load of an absorption chiller requirements. In addition to, using PTC in absorption chillers decreasing the running cost because it reduces the consumption of fuel used in absorption chillers.

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[20] Igarashi T. Heat transfer from a square prism to an air stream. Int J Heat Mass Trans 1985;28(1):175–81.

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[26] Ricardo Vasquez Padilla et al. Heat transfer analysis of parabolic trough solar receiver.



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performance analysis of solar parabolic trough collector used

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