Scholarly article on topic 'Performance Analysis of Earth Water Heat Exchanger for Concentrating Photovoltaic Cooling'

Performance Analysis of Earth Water Heat Exchanger for Concentrating Photovoltaic Cooling Academic research paper on "Materials engineering"

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{"Earth Water Heat Exchanger" / "TRNSYS (v17.0)" / "Parametric variation" / "Concentrating Photovoltaic."}

Abstract of research paper on Materials engineering, author of scientific article — Sanjeev Jakhar, M.S. Soni, Nikhil Gakkhar

Abstract In the present work, an earth water heat exchanger (EWHE) has been designed for Pilani, Rajasthan (India). The system is designed and simulated in transient analysis tool TRNSYS (v17.0) by varying its operating parameters which includes mass flow rate, length, pipe materials and diameter of buried pipe. The depth-wise temperature of soil has also been evaluated from the simulation and it is found that the depth of 3.5 m is sufficient for pipe burial. The results show that there is an inverse correlation between the pipe length and the EWHE outlet temperature. The comparative study between three different material shows that the performance of EWHE system hardly depends on the properties of these material. Further, the EWHE performance is found to be decreasing with an increase in the mass flow rate from 0.008kg/s to 0.05kg/s. The simulated proposed system is then compared with the existing ones in the literature for a given cooling setup of Concentrating Photovoltaic (CPV). It is observed that the proposed system gives better performance than the cooling system given in literature. To achieve the temperature drop from 48.5̊C to 25.5̊C as per the existing CPV setup in the literature, the pipe length of 60 m would be sufficient in the proposed EWHE system. Thus the coupling of EWHE with CPV plants could be economical as well as performance enhancer.

Academic research paper on topic "Performance Analysis of Earth Water Heat Exchanger for Concentrating Photovoltaic Cooling"

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Energy Procedia 90 (2016) 145 - 153

5th International Conference on Advances in Energy Research, ICAER 2015, 15-17 December

2015, Mumbai, India

Performance analysis of earth water heat exchanger for concentrating photovoltaic cooling

Sanjeev Jakhar, M.S.Soni*, Nikhil Gakkhar

Centre for Renewable Energy and Environment Development (CREED), Department of Mechanical Engineering, Birla Institute of Technology

and Science, Pilani, Rajasthan, India 333031

Abstract

In the present work, an earth water heat exchanger (EWHE) has been designed for Pilani, Rajasthan (India). The system is designed and simulated in transient analysis tool TRNSYS (v17.0) by varying its operating parameters which includes mass flow rate, length, pipe materials and diameter of buried pipe. The depth-wise temperature of soil has also been evaluated from the simulation and it is found that the depth of 3.5 m is sufficient for pipe burial. The results show that there is an inverse correlation between the pipe length and the EWHE outlet temperature. The comparative study between three different material shows that the performance of EWHE system hardly depends on the properties of these material. Further, the EWHE performance is found to be decreasing with an increase in the mass flow rate from 0.008 kg/s to 0.05 kg/s. The simulated proposed system is then compared with the existing ones in the literature for a given cooling setup of Concentrating Photovoltaic (CPV). It is observed that the proposed system gives better performance than the cooling system given in literature. To achieve the temperature drop from 48.5 °C to 25.5 °C as per the existing CPV setup in the literature, the pipe length of 60 m would be sufficient in the proposed EWHE system. Thus the coupling of EWHE with CPV plants could be economical as well as performance enhancer.

©2016 The Authors.PublishedbyElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICAER 2015

Keywords: Earth Water Heat Exchanger; TRNSYS (v17.0); Parametric variation; Concentrating Photovoltaic.

* Corresponding author. Tel.: (+91) 1596-515634; fax: (+91) 1596-244183. E-mail address: mssoni@pilani.bits-pilani.ac.in

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICAER 2015 doi:10.1016/j.egypro.2016.11.179

1. Introduction

In geothermal cooling principle, the temperature of the soil at the burial depth of about 3.5 m or more remains almost constant throughout the year, which is around the average annual ambient air temperature [1]. Specially in peak summer in dry and arid areas like Rajasthan and Gujarat, due to high solar insolation, ambient temperature reaches about 47 °C during daytime [2,3]. For such areas geothermal cooling may be considered to be a very good alternative for cooling purpose. Some researchers have used earth air pipe heat exchanger (EAPHE) for air conditioning [4-8]. Bansal et al. [9] presented a model for EAPHE and experimentally validated it. They found that the performance of EAPHE does not depend on buried pipe material. Jakhar et al. [10] evaluated the performance of the EAPHE with solar air heating duct for winter heating. They concluded that the COP of system increased up to 4.57 when coupled with solar air heating duct. However, the challenge with EAPHE is that it requires large surface area for effective heat transfer because of low thermal conductivity and low heat carrying capacity of air. The required surface area may be reduced by using water as cooling medium in EWHE because of its high heat transfer capacity. In this system, the pipes carry hot water instead of air which are buried at a certain depth below the ground, and act as a heat exchanger, where heat transfer take place between hot water to the earth thus decreases the outlet temperature of water. This outlet water from buried pipes may be used for space cooling with the help of a compact heat exchanger. Joen et al. [11] presented an analytical model for comparison the performance of EAPHE and EWHE. They calculated that the soil resistance is more dominant in case of EWHE and required small diameters tubes for effective heat transfer. Chel et al. [12] investigated the performance of an integrated system of EWHE, water air heat exchanger and air to air heat exchanger (AAHE) with the help of TRNSYS 17. They found that the integrated system together could reduce the annual heating consumption of the building by 72 %.

In the current work an attempt has been made to use EWHE for cooling of CPV cells. In such EWHE system, performance depends on various parameters which include mass flow rate of the water, depth of buried pipe, length, diameter and material of pipe, etc. By changing these parameters, the variation in outlet temperature of EWHE can be achieved. This study presents the variation in these parameters to achieve optimum EWHE outlet temperature. Further, the applicability of such proposed system is discussed by replacing the system given in literature of CPV [13, 14]. The operating temperature of the CPV system, as discussed by [15-18] is the key parameter affecting its performance, as with rise in temperature beyond certain limit results in decrease in efficiency. Hence, it is must to maintain the temperature within certain limit to achieve higher efficiency. The results obtained from the simulations shows that the proposed system performs better by giving sufficient temperature drop than the cooling system given in literature.

2. Description of the TRNSYS simulation

TRNSYS, a transient simulation system tool is used to model the renewable energy systems to estimate the transient variation [19]. Using this tool numerical simulation of the EWHE was carried out by using its inbuilt Meteonorm files for weather conditions. The model design includes inbuilt system components which take parameters and time dependent inputs of desired system and produces a time dependent outputs. Various components, which are designated as Type, can be interconnected with a flow chart. Here a given output of one component is used as an INPUT to a number of other components. In the current system the model for EWHE was used to estimate the transient output over a period of time.

The different TRNSYS component models (Types) which were used in the simulation are:

• Type 77- Simple ground temperature model

• Type 952- Earth water heat exchanger

• Type 3- Variable speed pump

• Type 15 - Weather data processor

• Type 65- Online plotter

Type 952 is important models in ground heat pump library of TRNSYS which models a horizontal heat exchanger that interacts thermally with the ground. It considers conductive heat transfer to the soil and convective heat transfer within the pipes. For the simulation, the physical and thermal parameters of the system taken are shown in Table 1.

Table 1 Physical and thermal parameters used in simulation.

Parameters

Properties

High Density Polyethylene (HDPE) pipe thermal conductivity

Galvanized Iron (GI) pipe thermal conductivity

Steel pipe thermal conductivity Fluid density Fluid specific heat Fluid thermal conductivity

0.40 W/m K

54 W/m K 1000 kg/m3 4.19 kJ/kg K 0.55 W/m K

16 W/m K

3. Methodology

In this section, the methodology of variation in design parameters for the buried pipe dimensions is discussed. The various parameters were analyzed by simulating their performance on TRNSYS. The simulation conditions were taken for Pilani Rajasthan (India) and the simulation was performed for the duration of 10 hours which is average sunshine hour during summer period. The temperature variation across the depth, over a period of a year, was calculated. The parametric simulation was performed for three pipe materials i.e. GI, HDPE and Steel pipe. For different pipe material, by keeping length, flow rate and diameter constant simulation was carried out and observed that the temperature variation between all three materials is almost similar with the temperature difference ±1.61 °C. Among three materials HDPE pipe is taken for the performance analysis because of it is cheaper as compare to other two. For HDPE pipe by keeping diameter and length of the pipe as fixed value the mass flow rate is varied and outlet temperature for each case is determined. This analysis gives the optimum flow rate for a given condition. By keeping this flow rate as constant for HDPE pipe the analysis is carried forward by changing the length of the pipe for a particular diameter. In the final analysis the effects of different HDPE pipe diameters are estimated for the fixed length and flow rate.

Further to discuss the applicability of such system, by replacing the cooling system given in literature of CPV with the proposed system, the results obtained were compared with the existing literature. For the analysis, two references have been taken and the values of outlet temperature of CPV/Thermal (CPV/T) system were taken as inlet for EWHE system. From the analysis, it has been estimated that the cost of proposed system is quite inexpensive as compared to the conventional system used in the literature.

4. Results and discussion

4.1. Performance analysis of EWHE system

The performance of EWHE system is analyzed by varying different parameters, which includes type of pipe material, length and diameter of pipe and mass flow rate of fluid within the pipe. The analysis is further carried out to estimate the effect of burial depth over a period of year, for the local conditions of Pilani, Rajasthan. Since the earth act as heat sink for the higher temperature, as the depth increases the soil temperature converges to annual average ambient temperature. Fig. 1 shows the ground temperature variation for different depths. With increase in depth from 0.5 m to 3.5 m, the average temperature over the year varies from 9.5 °C to 44.7 °C for 0.5 m depth while it remains within the range of 22.5 °C to 27.7 °C for 3.5 m depth. Considering this small variation in temperature, the depth of 3.5 m is taken for the simulations. At this depth the EWHE performance is evaluated for three different pipe materials, i.e. GI pipe, Steel pipe and HDPE pipe. The inlet temperature for each case was taken as 90 °C, which is assumed to be outlet temperature of CPV as discussed in literature [20].

/ y y / ^ ^ ^j- / y / /

% Months c-P <?

• 0.5 m A 1 m X 1.5 m

X 2 m 2.5 m I 3 m -•-3.5 m

Fig. 1. Annual ground temperature range at different depths for Pilani, Rajasthan (India).

Fig. 2 reveals the EWHE exit temperature for three different pipe materials for pipe length of 90 m, diameter of 25 mm and flow rate of fluid as 0.02 kg/s. It is concluded that the temperature difference of the fluid at outlet of EWHE for GI and Steel pipes is mere 0.2 °C while in the HDPE and Steel pipe is around 1.6 °C, for same inlet conditions. This small variation occurs because of less coefficient of friction for HDPE pipes. Although the GI and steel pipes has high thermal conductivity as compared to HDPE pipes, the coefficient of friction is quite high in these two. This causes the temperature at the outlet as marginally lower than the HDPE pipe. From this estimation, it can be concluded that in EWHE system, the properties of these materials have small impact on the performance of the system. The same also been discussed in literature for earth air heat exchanger by Bansal et al. [9]. The HDPE pipe is selected for the study as it is much cheaper as compared to GI and steel pipe.

The effect of mass flow rate on the performance of EWHE along the fixed length of 90 m and diameter of 25 mm for HDPE pipe is shown in Fig. 3 It reveals that with increase in mass flow rate the outlet temperature of EWHE increases as expected. This is evident from the fact that with increase in mass flow rate, the Reynolds number increases which in turns increases the Nusselt number and hence the convective heat transfer coefficient. But with the increase in flow rate of fluid, the contact time to which fluid is in contact with ground is reduced significantly. Thus the later effect is dominant and so the temperature drop at higher flow rate will be less as compared to lower flow rate. The maximum temperature drop observed for 0.008 kg/s but it is very low flow rate so for the analysis the mass flow rate varied from 0.02 to 0.05 kg/s.

By keeping the mass flow rate of 0.02 kg/s for HDPE pipe, with diameter as 25 mm, the effect of variation of length is estimated. The results obtained by the varying length from 50 m to 90 m are shown in Fig. 4. It reveals that with increase in length the temperature drop increases as expected. For the pipe length of 90 m, the outlet temperature is obtained as 31.9 °C for the inlet of 90 °C. The standard sizes of HDPE pipe available in market are selected for analysis ranging from 25 mm to 50 mm. The effect of variation in diameter from 25 mm to 50 mm of HDPE pipe at the flow rate of 0.02 kg/s and 90 m length is shown in Fig. 5.

10 11 12 13 14 15 16 17 18 Time (Hr)

HDPE GI -A-Steel

Fig. 2. EWHE outlet temperature vs different pipe material (pipe ^ 25 mm, Length= 90 m, Flow rate=0.02 kg/s).

m=0.008 kg/s m=0.04 kg/s

12 13 14 Time (Hr)

m=0.02 kg/s m=0.05 kg/s

15 16 17 18

m=0.03 kg/s

Fig. 3. EWHE outlet temperature vs mass flow rate (pipe ^ 25 mm, Length= 90 m, Pipe material= HDPE).

With increase the pipe diameter the outlet temperature decreases gradually over a period of time. After four hours of EWHE operations the temperature drop will be more for HDPE pipe with maximum diameter of 50 mm. The exit temperatures obtained after the 10 hours operations are 31.80 °C for 50 mm and 36.16 °C for 25 mm. Although the temperature difference is merely 4.36 °C but the 25 mm size pipe is around 44% cheaper than of 50 mm. Thus, the pipe diameter of 25 mm may be used for the practical applications.

Fig. 4. EWHE outlet temperature vs pipe length (pipe ^ 25 mm, flow rate = 0.02 kg/s, Pipe material= HDPE).

g 60 -3

£ 50 a

§ 40 H

8 9 10 11 12 13 14 15 16 17 18

Time (Hr)

mm -^D=32mm -^D=40mm *D=50mm

Fig. 5. EWHE outlet temperature vs pipe diameter (Length=90 m, flow rate= 0.02 kg/s, Pipe material= HDPE).

4.2. Applicability of EWHE system for CPV cooling

In this section, the usability of such EWHE system to replace the cooling system given in literature of CPV has been identified. For the analysis, a CPV/T system with storage tank is taken from the literature [13, 14]. Xu et al. [13] designed the CPV/T system in which solar cells were attached to straight and tree shaped cooling water channel by utilizing a thin-film thermal cladding. They used inlet temperature of 25 °C, CR of 50 suns and flow rate of 0.00045 kg/s and 0.00044 kg/s for straight and tree shaped channel to achieve CPV/T outlet temperature of 58.7°C and 55°C for respective cases. These outlets of the CPV/T system are simulated as inlet for the proposed system to

identify the appropriate length of EWHE for the same cooling effect. It is observed from the Table 2, that it would take EWHE system of 4 m and 5 m length for the respective cases to achieve the temperature drop up to inlet temperature of 25°C. The cost estimation for both cases shows that it is quite cheaper than the reference cases and hence the proposed system is much better and cheaper option.

The Table 2 also shows the comparative study with the existing system of Li et al. [14] where they used CR of 16.92 and flow rate of 0.012 kg/s for four different solar cells, i.e. Crystalline Silicon (CS), Polycrystalline Silicon (PS), Super Cell Array (SCA) and GaAs cell array. In their system they used the CPV/T system connected to storage tank heat exchanger for hot water applications. This system is modified in the present study and simulated for same inlet temperature and direct normal irradiance to achieve the same outlet temperature as shown in Fig. 6. The results obtained with proposed EWHE system are shown in Table 2, which indicates that the maximum length of 60 m would be sufficient to achieve the same cooling effect. The proposed system is cheaper as compared to reference cases. The EWHE system along with CPV may be used for the arid regions of western Rajasthan which is blessed with high solar insolation. This system will be very much helpful in summer as outside temperature is very high thus leaves negligible scope for thermal energy utilization. Also heat rejection to the ambient will be of great difficulty. Thus the proposed EWHE system will be a better solution.

Table 2. Applicability of EWHE system with existing literature.

Author

Concentration ratio (CR) (Suns)

CPV cells material

Flow rate (kg/s)

Water outlet temp. from CPV/T (EWHE Inlet) °C

Water outlet temp. from EWHE (CPV/T inlet) °C

Pipe length required for EWHE (meter)

Xu et al.

(2012)

Li et al. (2011)

50 (Straight)

50 (Tree) 16.92

16.92 16.92

Monocrystalline 0.00045 silicon

CS and PS

SCA GaAs

0.00044 0.012

0.012 0.012

55.00 48.50

47.00 42.50

24.30 25.50

25.70 26.00

1. CPV/T system 4. Pump

2. Valves 5. Cold water in

3. Storage tank / Heat exchanger 6. Hot water out

1. CPV/T system 3. EWHE

2. Valves 4. Pump

(A) (B)

Fig. 6. Schematic diagram of (A) Conventional CPV/T cooling system (B) Proposed CPV/T cooling system with EWHE.

5. Conclusion

The present work discussed the parametric study of EWHE by varying different parameters which includes burial depth, pipe material, pipe length, pipe diameter and mass flow rate. The analysis was performed for local conditions of Pilani, Rajasthan and simulations were performed using TRNSYS (v17.0). By varying burial depth, it was observed that with increase in depth from 0.5 m to 3.5 m, the soil temperature over the year varies from 9.5 °C to 44.7 °C for 0.5 m depth while it remains within the range of 22.5 °C to 27.7 °C for 3.5 m depth. Considering this small variation in temperature, the depth of 3.5 m is taken for the simulations for local condition. The comparative study between three different materials shows that the properties of these materials have lesser impact on the performance of the EWHE system so among these three materials HDPE pipe can be used for its low cost. The performance of the system increases with increase in length of the pipe and decreases with increase of mass flow rate. In case of increase in diameter, the performance of the system increases in the longer run, but the economic factor increases with increase in pipe diameter. The simulated system is then compared with cooling system of CPV as discussed by Xu et al. [13] and Li et al. [14] to estimate the effect of replacing the same with EWHE. From the analysis it was found out that the EWHE system gives better performance as compared to the reference system and hence may be used for the cooling purpose of CPV systems.

Acknowledgments

We gratefully acknowledge the support from the Center for Renewable Energy and Environment Development, BITS - Pilani Rajasthan, for this research.

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