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Fig.6.11 Variation of HTF outlet temperature and dT versus time during charging and discharging process. 157 Fig.7.3 Photograph of the helical coil heat exchanger 157 Fig.7.4 Variation of solar insolation and outlet water temperature.

Preface

Harvesting Solar Energy

Flat Plate Collector

The absorber plate material can be thermally stable polymers, steel, copper or aluminum which can be coated and painted black to increase the absorption capacity. Several types of absorbent tube arrangements exist for FPC, viz. serpentine, tube formed in sheet metal, parallel tube and drop type corrugated plate.

Application of solar thermal technology

Thermal Energy Storage Systems

Among the four, sensible and latent heat storage are very common in thermal energy storage systems. With sensible heat storage (SHS), the energy is stored, whereby the temperature of the solid or liquid medium is raised.

Fig. 1.5 Mechanisms of thermal energy storage
Fig. 1.5 Mechanisms of thermal energy storage

Thermal Management of Biogas Digester

However, this injection of hot water increases the water content of the sludge and should only be used if such dilution is necessary. The indirect heating system is done with the help of heat exchangers located inside or outside the biogas digester.

Motivation

This type of heating system depends on the shape of the digester, the nature of operating mode and types of substrate (feed material) used. The current work is aimed at developing a solar heating system integrated with heat storage together with control devices to maintain a constant temperature of the feed material of a biogas digester.

Objectives of the Thesis

The application of the integrated solar thermal system (solar collector, latent heat storage and control device) for biogas production process is experimentally tested and the results are reported. To investigate the efficiency of the integrated solar thermal system in biogas production process in terms of temperature variation with and without control conditions.

Outline of the Thesis

Chapter - 6: Presents the result and discussions of the experimental and numerical study of the thermal storage system (latent heat storage). Chapter - 7: The experimental setup and results of controlled and uncontrolled experiments for the integrated solar thermal system along with its application in biogas production have been presented and discussed.

Introduction

Solar Energy

Fig.5.8 The results of straight tube solar water heating system in no-loop 5.4.6 Exergy efficiency for straight tube collector in closed loop system. Since the inlet of the LHS is the outlet of the solar collector (Fig.4.5 and 4.6), the temperature of the latent heat storage at the inlet was taken as 343 K, which was the maximum temperature of the solar collector.

Figure 2.1 presents the growth rate of bacteria versus temperature.
Figure 2.1 presents the growth rate of bacteria versus temperature.

Flat Plate Collector

Factors Affecting the Performance of Flat Plate Collector

Effect of tube spacing and geometry

A comparative study of the thermal performance of FPCs with 11 cm and 16 cm tube spacing was performed by Fatigun et al. The results show no significant changes in the thermal efficiency of the solar collector by changing the tube geometry.

Effect of air gap between plate and glass

2014) performed numerical study with circular, triangular, hexagonal and square tubes on the thermal performance of FPC and observed higher thermal performance for circular tube geometry compared to others. The thermal efficiency with circular, triangular, square and hexagonal tubes was reported and 6.6%, respectively. 2012) assessed the thermal performance of FPC with different absorber plate geometric configurations.

Effect of glazing material

Comparative study between single, double and triple glass collectors with a cover of 5 mm thickness on thermal efficiency was carried out by Mustafa and Ismail (2013). The study reveals higher thermal efficiency for triple glazing, while single glazing showed the lowest thermal efficiency.

Effect of absorber plate coating

The result indicated that increasing the thickness of the glass cover reduces the heat losses.

Effect of collector tilt angle

Effect of fluid flow rate

Analytical study on solar collector thermal efficiency by Duffie and Beckman (1991) revealed higher collector efficiency factor with higher flow rate. The investigation by Badach et al. 2012) found that fluid mass flow rate had higher influence on the thermal efficiency of FPC compared to solar radiation and/or inlet tube diameter.

Effect of inlet water temperature

Experimental studies on the effect of varying flow rate on outlet water temperature by Ismail (2005) and Ismail (2007) indicate high outlet water temperature at low flow rate. Parametric study by He et al. 2016) indicates a drop in outlet water temperature resulting in an increase in solar collector thermal efficiency from 60.8% to 70.0%.

Effect of ambient temperature

Effect of solar insolation

Effect of wind speed

Effect of insulation

The validation was performed by taking the average temperature of the four thermocouples T1 to T4. The experimental procedures for both controlled and uncontrolled experiments with the heat exchanger are briefly presented in Chapter 7. Head loss due to friction of fluid was determined using Eq. . 3.31). The variation of pressure drop versus mass flow rate is depicted in Fig.5.23.

Review on Exergy Analysis of Flat Plate Collectors

Numerical Study on Flat Plate Collectors

Thermal Energy StorageSystem

Latent heat energy storage system

The effect of fins on the performance of the LHS system was investigated by Darzi et al. The study by Jesumathy et al. 2014) found that the inlet temperature had a greater influence on the melting rate compared to the mass flow rate.

Melting Enhancement of Phase Change Material

Ho and Gao (2009) conducted experimental investigation on the thermal properties of paraffin wax by dispersing 5 wt. of nonionic surfactant and 10 wt. CuO nanoparticles on the melting and solidification rate of paraffin wax and found higher thermal performance for Al2O3 compared to CuO nanoparticles.

Thermal Management for Anaerobic Digestion

Better quality and higher biogas yield were achieved for thermophilic digestion compared to mesophilic digestion. 1995) conducted experimental investigation into the effect of temperature on biogas production for lignocellulosic biomass. 2001) conducted experimental studies on the effect of temperature on biogas production for digestion of pig manure and found higher biogas yield for thermophilic followed by mesophilic digestion.

Temperature Controlling Methods inside Biogas Digester

The result revealed that in the winter season (i) a constant slurry temperature of 29 ºC could be maintained for the conventional biogas plant with windows and movable insulation, (ii) a constant slurry temperature of 26 ºC could be achieved in the conventional plant with windows. and (iii) a slurry temperature of 21 ºC could be maintained in the conventional biogas plant without glazing and insulation. The figure indicated higher slurry temperature for movable insulation and lowest slurry temperature for conventional biogas plant.

Summary

Furthermore, using the same for heating raw material for a biogas digester is a challenging task. The uncontrolled conditions help to develop the relationship between shell and biogas digester in terms of temperature.

Introduction

Theoretical Analysis of Flat Plate Collector

  • Designing of flat plate collector
  • Analysis of optical property of glass
  • Energy analysis
  • Energy analysis for cascaded system
  • Exergy analysis

The useful energy gain with respect to the inlet (Ti) and outlet (Til) temperature of the fluid was determined using the expression The temperature distribution on the surface of the solar collector along the direction of the fluid flow was determined by the expression (Duffie and Beckman 2013).

Table - 3.1 Materials and properties used for the designing of flat plate collector
Table - 3.1 Materials and properties used for the designing of flat plate collector

Pumping Power

Theoretical Analysis of Thermal Energy Storage System

Design of latent heat storage system

Performance Parameters of Latent Heat Storage System

  • Charging time
  • Discharging time
  • Melt fraction
  • Energy stored
  • Energy recovered

The total heat stored in the LHS system (ET,C) consists of sensible heat (ES,C) and latent heat (EL,C). The total heat removed from the LHS system (ET,D) consists of sensible heat (ES,D) and latent heat (EL,D).

Modified Thermo-Physical Properties of nano-PCM

Numerical Modelling for the Flat Plate Collector

Model geometry and meshing for straight tube collector

Model geometry and meshing for bent tube collector

Governing Equations for Flat Plate Collector

The governing equations used to simulate the FPC system are performed based on assumptions. First order upwind was used to discretize pressure and second order upwind for the momentum and energy equations.

Numerical Modelling for Thermal Energy Storage System

Model geometry for the latent heat storage system

Meshing for latent heat storage system

Governing Equations for Thermal Energy Storage System

Governing equations for latent heat storage system

The mushy region constant is defined as the velocity transition in the mushy region. In this study, a value of 104 for the mushy region constant was determined based on a report (Niyas et al. 2017).

Governing equations for nanofluid based latent heat

Boundary Conditions Used for the Thermal Energy Storage

Summary

Introduction

The uncertainty in the independent variable such as Reynold's number, thermal efficiency and energy efficiency is shown in Table 5.3. The formula used to determine the uncertainty scores is given in Appendix-D. There was difficulty in achieving convergence for the model developed with less than 237,458 mesh elements during the loading process. The average temperature difference between 392,382 elements and 306,464 elements was less than ± 0.5%.

Figure 5.3 depicts the variation of temperature across the outlet tube diameter. The  temperature at the tube surface is 18 K higher compared to the centre
Figure 5.3 depicts the variation of temperature across the outlet tube diameter. The temperature at the tube surface is 18 K higher compared to the centre

Details Experimental Representation

Experimental Setup

Details of the open loop system

Cold water from the storage tank is pumped into the FPC through the lower part of the collector and passes through the top of the collector pipe. The surface of the collector was cleaned to remove dust and to increase the absorption capacity before starting the experiments.

Details of the closed loop system

In the closed-loop test, water is pumped from the reservoir to the collectors, allowing heat absorption by FPC. The heat exchanger transfers heat from the collector loop fluid to the fluid held in the storage tank.

Details of the cascaded collector system

Water from the FPC is circulated through the heat exchanger into the storage tank and back to the reservoir. The surface of the collector was cleaned to remove dust and to increase the absorbency before the start of the experiments.

Experimental Procedure of the Flat plate Collector

The open loop test was performed only for straight tube collectors, while the closed loop test was performed for straight, bent as well as for the cascade collectors. Similar methodologies were followed for the experiments with the latent heat storage system in closed loop for the cascade collectors.

Experimental Descriptions and Procedure of the Latent

Finally, the integrated thermal system was tested with and without control conditions in the closed-loop arrangement.

Summary

Introduction

Experiments

Measured Input Parameters Using Straight Tube Collector

The figure indicated that for the above two cases, the solar radiation showed a parabolic path with the peak value reached at mid-day, which subsequently decreased. Correspondingly, the ambient temperature and the temperature of the inlet water rose from the morning and reached the peak value respectively at 2 p.m. and 4 p.m.

Model Validation Using Straight Tube Collector

  • Temperature distribution of the straight tube collector
  • Comparison between experimental and predicted
  • Comparison between experimental and predicted
  • Thermal efficiency of the straight tube collector in open
  • Thermal efficiency of the straight tube collector in
  • Exergy efficiency of the straight tube collector in

The outlet water temperature slowly increased from 8:00 a.m. to 10:00 a.m. due to lower solar insolation and the morning ambient temperature. Measured input parameters, namely ambient temperature, inlet water temperature, solar insolation and outlet water temperature.

Figure 5.4 illustrates the variation of  absorber plate temperature at the top  surface
Figure 5.4 illustrates the variation of absorber plate temperature at the top surface

Measured Input Parameters Using Bent Tube Collector

The input data used for the exergy efficiency of the bent tube collector is shown in Fig 5.16. The average temperature variation of the paraffin wax during the charge/discharge process is illustrated in Fig.6.9.

Model Validation Using Straight Tube Collector

Temperature distribution of the bent tube collector

The water temperature changes along the length of the bent pipe at 12 o'clock shown in Fig.5.11 show higher outlet water temperatures at the top of the pipe compared to the bottom. Because of this, it takes longer for the heat to reach the center of the convection tube.

Figure 5.12 presents  the variation of temperature  across the outlet  tube diameter at  12:00 h
Figure 5.12 presents the variation of temperature across the outlet tube diameter at 12:00 h

Comparison between experimental and predicted outlet

The temperature variation along the length of the absorber plate shows a difference of 38 K between the inlet and outlet sides of the plate. Since the riser is in contact with the absorber at mid-width, there is a variation of about 6 K across the width of the plate.

Comparison between experimental and predicted absorber

For all investigated flow conditions, the experimental values ​​of the plate temperature were higher than the simulated values ​​with a maximum error of deviation ≤ 4%. This is mainly due to the assumption that perfect contact was maintained between the plate and the tube leading to the higher rate of heat transfer from the plate to the fluid compared to the experimental conditions.

Thermal efficiency of the bent tube collector in closed loop

The temperature reaches the maximum value at 12:00, after which it starts to decrease for all flow rates studied. Furthermore, both inlet and outlet water temperature increased almost linearly and followed the same trend.

Exergy efficiency of the bent tube collector in closed loop

Comparison of Straight and Bent Tube Solar Collector

  • Measured input parameters
  • Comparison study using outlet water temperature
  • Comparison study using absorber plate temperature
  • Comparison study using thermal efficiency in closed loop
  • Comparison study using exergy efficiency

Therefore, the absorber plate temperatures for the straight pipe header were found to be higher than the bent pipe header. The maximum experimental absorber plate temperature of 346 K for the straight tube collector and 342 K for the bent tube collector was observed.

Table - 5.1 Result of experimental values used for simulation
Table - 5.1 Result of experimental values used for simulation

Pressure Drop and Pumping Power in Straight and bent tube

Variation in pressure drop

Variation of pumping power

The bent tube collector required higher pumping power to circulate the working fluid compared to the straight tube collector. At the lowest mass flow rate, the pumping power required for straight and bent tube headers was almost the same.

Cascaded Collector System

Variation in solar insolation and water temperature using

The higher inlet water temperature of collector-2 resulted in a higher temperature of the absorber plate of the solar collector. Due to this, the heat losses from the absorber plate of the collector increased as a result of re-radiation and convection (Sukhatme and Nayak 2008).

Thermal efficiency of cascaded collector

Since the outlet water temperature of solar collector-1 at lower flow rate was higher, the heat absorption rate of solar collector-2 is lower. Further, it is also evident that the thermal efficiency of the cascaded collector was higher for higher flow rate compared to lower flow rate.

Effect of Operating Parameters on Thermal Efficiency

  • Effect of ambient temperature on thermal efficiency
  • Effect of inlet water temperature on thermal efficiency
  • Effect of solar insolation on thermal efficiency
  • Effect of flow rate on thermal efficiency
  • Effect of plate material on thermal efficiency
  • Effect of transmissivity coefficient on thermal
  • Effect of wind speed on thermal efficiency
  • Effect of mass flow rate and collector loss factor on

Figure 5.29 Thermal efficiency and water temperature at the outlet according to solar insolation 5.10.4 Effect of flow on thermal efficiency. Figure 5.30 Thermal efficiency and temperature of outlet water relative to inlet water 5.10.5 Effect of plate material on thermal efficiency.

Figure  5.30  shows  the  plot  of  collector  efficiency  and  outlet  water  temperature  versus  inlet  water  velocity  at  303  K  inlet  water  temperature,  298  K  ambient  temperature and 700 W.m -2 solar insolation
Figure 5.30 shows the plot of collector efficiency and outlet water temperature versus inlet water velocity at 303 K inlet water temperature, 298 K ambient temperature and 700 W.m -2 solar insolation

Effect of Operating Parameters on Exergy Efficiency

  • Effect of solar insolation on exergy efficiency
  • Effect of ambient temperature on exergy efficiency
  • Effect of optical efficiency on exergy efficiency
  • Effect of mass flow rate and collector loss factor on
  • Effect of insulation thickness on exergy and thermal

Fig.5.36 Variation of exergy efficiency versus ambient temperature 5.11.3 Effect of optical efficiency on exergy efficiency. Fig.5.38 Variation of exergy efficiency versus mass flow rate and collector loss parameter (Ti -Ta)/I.

Figure 5.36 shows the plots of exergy efficiency versus ambient temperature at 303  K  inlet  water  temperature,  700  W.m -2 solar  insolation  and  0.0167  kg.s -1  mass  flow  rates
Figure 5.36 shows the plots of exergy efficiency versus ambient temperature at 303 K inlet water temperature, 700 W.m -2 solar insolation and 0.0167 kg.s -1 mass flow rates

Uncertainty Analysis

Fig.5.40 Variation of exergy and thermal efficiency versus collector loss parameter (Ti -Ta)/I using water and ethylene glycol. Uncertainty in measuring pipe diameter (mm) ±0.006 Uncertainty in measuring temperature (ºC) ±0.2 Uncertainty in measuring solar insulation (W/m2) ±5.0 Uncertainty in measuring water mass flow rate (kg/s) ±0.001 Uncertainty in Reynolds number measurement (%) ±2.5 Uncertainty in thermal efficiency of solar water heater (%) ±3.7 Uncertainty in energy efficiency of solar water heater (%) ±3.9.

Summary

Introduction

Experiments

It is observed from Fig.6.9 (a) and (b) that the variation of average temperature curve at the beginning of the process indicates a sharp increase/decrease of temperature during charging/discharging process. Two thermocouples are placed at the top (Ta t) and bottom (Ta b) of the shell tank.

Preliminary Numerical Model for Paraffin Wax Filled Latent Heat

Optimization of number of charging/discharging tubes…

Increasing the number of charge/discharge tubes of the HTF, using fins, inserting metallic material into PCM, and mixing nanoparticles with PCM were among several techniques used by various researchers to increase the heat transfer rate. Further increases in the number of charging tubes resulted in only a marginal reduction in the charging time of the LHS system.

Effect of tube arrangement

It is shown from the figure that the discharge rate of 4 tubes in the middle and 12 tubes on the outside [Fig.6.3 (b)] was faster than 8 tubes in the middle and 8 tubes on the outside or with 6 tubes at the middle and 10 tubes on the outside. Since the requirement of latent heat storage is to have a faster discharge rate, it was found that 4 tubes in the middle with 12 tubes on the outside [Fig.6.3 (b)] is the best arrangement for the present investigation.

Effect of cylinder orientations

The arrangement of 8 tubes in the middle with 8 tubes in the outer [Fig.6.3 (a)], 4 tubes in the middle with 12 tubes in the outer [Fig.6.3 (b)] and 6 tubes in the middle with 10 tubes at the outer [Fig.6.3 (c)] was considered for this study to select the best arrangement. Therefore, the melting rate in the horizontal system is much higher than the vertical unit.

Grid Independent Test

Furthermore, it is also noticeable that the time it took to get a complete melt was almost 52 minutes for the horizontal orientation, while it was 70 minutes for the vertical orientation. The HTF inlet flow rate of 0.0083 kg/s and 343 K inlet temperature were measured during the charging process for the grid independent test.

Model Validation and Performance Study

  • Plot of average melt fraction
  • Validation of top surface temperature during charging/
  • Average melt fraction of paraffin wax
  • Average temperature of the paraffin wax
  • Energy stored /discharged
  • HTF outlet temperature variation

A comparison between the experimental and numerical average temperature of the paraffin wax on the upper surface of the storage tank during the filling/discharging process is shown in the figure. The temperature of the paraffin wax reaches approximately 343 K / 304 K in 90 minutes / 120 minutes during the filling and discharging process.

Temperature Variation of Paraffin Wax during Charging

The variation of sensible heat stored/released for nanofluid and pure paraffin wax filled LHS is depicted in Fig.6.18. The temperature variation at the inlet and outlet side of the heat exchanger versus time for the uncontrolled test is plotted in Fig.7.7.

Figure 6.13 shows a plot of temperature versus time for the paraffin wax during the  charging process
Figure 6.13 shows a plot of temperature versus time for the paraffin wax during the charging process

Melting Enhancement and Performance Evaluation of LHS

Variation in thermo-physical properties

The change in thermodynamic properties, namely density, dynamic viscosity, thermal conductivity, latent heat of fusion and specific heat with respect to temperature and volume fraction of nanoparticles was determined based on the correlations given in Appendix –C. Fig.6.14. Variation in (a) density (b) dynamic viscosity (c) thermal conductivity (d) specific heat and (e) latent heat versus temperature.

Plot of average melt fraction

The total heat stored depends on the latent heat of fusion and the specific heat of the PCM. Heat is further transferred to the jacket tank through a heat exchanger to raise the temperature of the liquid in the jacket tank.

Figure  6.16  plots  the  variation  in  average  melt  fraction  with  time  during  charging  and discharging process
Figure 6.16 plots the variation in average melt fraction with time during charging and discharging process

Effect of nanoparticle addition on melt fraction

Effect of nanoparticle addition on average temperature

From the simulation data (Figure 6.17 (a)), it can be seen that the time required to reach a fully charged state for pure paraffin and nanofluid is 90 minutes and 40 minutes, respectively, i.e. 1.8 times faster for nanofluid. In general, the dispersion of Al2O3 nanoparticles in paraffin wax is more effective for the charge/discharge process compared to pure paraffin.

Energy Stored /Released

Sensible heat stored/discharged

6.18 (a)) that the maximum sensible heat stored was 1.37 MJ for the nanofluid at 40 min, while it was 1.8 MJ for the pure paraffin wax filled LHS at 90 min. This shows that the sensible heat discharged using pure paraffin-based storage was 1.45 times higher compared to nanofluid storage.

Latent heat stored/discharged

6.18 (b)), the amount of sensible heat released from the nanofluid in the storage was 1.1 MJ after 68 minutes, whereas this value was 1.6 MJ for pure paraffin-filled storage after 120 minutes. Similarly, the amount of latent heat discharged from the nanofluid (Fig. 6.19 (b)) was 2.2 MJ at 68 min and 3.2 MJ for pure paraffin-filled storage at 120 min during the discharge process.

Fig.  6.19  Variation  of  latent  heat  stored/released  during  (a)  charging  and  (b)  discharging  process
Fig. 6.19 Variation of latent heat stored/released during (a) charging and (b) discharging process

Total heat stored/discharged

From the equations it can be seen that the latent heat of PCM fusion affects the latent heat storage capacity. Since the dispersion of nanoparticles lowers the latent heat and specific heat of PCM, the total energy stored in LHS is expected to be lower than pure paraffin-based LHS.

Summary

From Figure 6.20 (a), it can be seen that during the charging process, the total stored heat was 3.57 MJ for the nanofluid after 40 min, while it was 5.24 MJ for the pure paraffin wax-based LHS after 90 min. Similarly, during the discharge process, the total heat released from the nanofluid-based reservoir was 3.3 MJ at 68 minutes and 4.8 MJ for the pure paraffin-based LHS at 120 minutes (Figure 6.20 (b)).

Introduction

Uncontrolled Conditions

Experimental setup and procedure of uncontrolled

Results and Discussion

Temperature variation in biogas digester and shell tank…

It is clear from Fig.7.5 that during the period 8:00 h – 10:20 h, the temperature inside the biogas digester was 5 K lower than the ideal value. The figure showed that the temperature of the biogas digester varied in the range of 303.1 K to 314 K, while the variation inside the shell tank was in the range of 310 K – 318 .1 K.

Heat exchanger temperature variation

Controlled Conditions

Experimental setup and procedure of controlled

This algorithm was to divert the water flow away from the shell tank when the shell temperature exceeds 315 K. At the end of the experiment (night period), when the temperature drops below the desired limit, the controlled electric heater is placed in the shell tank. heats the biogas solvent.

Results and Discussion

Summary

Conclusions

Result of flat plate solar collector

Result of thermal energy storage

Result of the integrated solar thermal system

Scope for Future Work

Figure

Fig. 1.5 Mechanisms of thermal energy storage
Figure 2.1 presents the growth rate of bacteria versus temperature.
Figure  2.4  depicts  the  variation  of  time  dependent  slurry  temperature  for  a  (i)  conventional biogas plant (ii) water heater, (iii) water heater with solar canopy and  (iv)  water  heater  with  solar  canopy  and  movable  insulation
Table - 3.1 Materials and properties used for the designing of flat plate collector
+7

References

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