Design and thermal performance evaluation of an air heater with low cost thermal energy storage

Journal Pre-proofs Design and thermal performance evaluation of an air heater with low cost thermal energy storage Abhishek Saxena, Prashant Verma, Ghanshyam Srivastava, Nandkishore Yadav PII: DOI: Reference:

S1359-4311(19)36431-2 https://doi.org/10.1016/j.applthermaleng.2019.114768 ATE 114768

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Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

16 September 2019 28 November 2019 3 December 2019

Please cite this article as: A. Saxena, P. Verma, G. Srivastava, N. Yadav, Design and thermal performance evaluation of an air heater with low cost thermal energy storage, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114768

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Design and thermal performance evaluation of an air heater with low cost thermal energy storage Abhishek Saxena1*, Prashant Verma2, Ghanshyam Srivastava3, Nandkishore Yadav4 1Mechanical

Engineering Department, Moradabad Institute of Technology, Moradabad, India 2Mechanical Engineering Department, GBPUAT, Pantnagar, India 3Mechanical Engineering Department, Eshan College of Engineering, Mathura, India 4Mechanical Engineering Department, Moradabad Institute of Technology, Moradabad, India Corresponding author email: - [email protected], Fax- +915912452207

Abstract- In the present experimental study a modified solar air heater (SAH) has been tested which integrated with cylindrical copper tube carrying low cost thermal energy storage (TES) material. The blackened copper tube is flexible to integrate with any flat plate type thermal system for performance enhancement especially for providing the hot air for long deliberating hours in low ambient conditions. Total, three low cost TES materials namely; granular carbon powder (GCP), paraffin wax and a unique combination of these two has been tested to evaluate its significance on the performance of SAH in geographical location of Moradabad city, India. Total three different models of air heater (SAH-B, SAH-C and SAH-D) have been developed and experimentally tested on TES encapsulated flexible copper tubes for performance enhancement. Flexibility of the tube is that it can be easily placed on the solar absorber of SAH whenever required by filling any desired TES material. Results show that all the new developed air heaters have been found better over the conventional air heater (SAH-A) but the model SAH-D has been found the best among all models in terms of heat transfer (485.31 W/m2K), thermal efficiency (78.31%) and exhaust temperature (50oC). The overall cost of the modified system is only $67.75 which is much economical to the other studied models. Keywords- air heater, energy storage, low cost, heat transfer, performance enhancement NomenclatureSAH- Solar air heater LHS- Latent heat storage SHS- Sensible heat storage PCM- Phase change material SAC- Solar air collector SAHS- Solar air heating systems GCP- Granular carbon powder TES - Thermal energy storage material NTU – Number of transfer units Tmelt - Melting temperature of PCM (oC) Tamb - Ambient temperature (oC) h - Heat transfer coefficient (W/m2oC) hrd - Radiative heat transfer rate (W) hcv - Convection heat transfer rate (W) hcd - Conduction heat transfer rate (W) Uover - Overall heat loss transfer coefficient (W/m2oC) 1

Symbol A- Area (m2) M - Mass (kg) D- Hydraulic diameter Va - Velocity of the air (m/s) t - Time (sec) gl - Glass p - Projection for tubes out - Outside the system in – at the entrance of the system abs - Absorber ht - Heat transfer ts- Tube surface air - Inside hot air α - Absorptivity τ - Transmissivity ρ – Density ƞ- Efficiency (%) v - Kinematics viscosity (m2/s) k - Thermal conductivity C - Specific heat (J/kgoC) H - Latent heat of fusion (J/kg) R - Radius of the tube (m) L - Length of the tube (m) I - solar irradiance (W/m2) stl - Solid to liquid phase change comp- Composite TES material

1.0 Introduction Energy crisis is increasing everywhere and the people around the world are seeking for an optimal solution to overcome this problem. Although renewable energy sources are playing a major role in this crisis situation but among all these resources solar energy is really a powerful tool to minimize this energy demand up to some extent. There are different applications of solar energy in which heating, cooling and power generation is common. In solar heating applications solar air heaters (SAHs) are continued to use for energy saving purposely for those systems which require low or moderate temperature to be operated. SAHs are mainly used for space heating, agriculture or industrial drying, marine products, textile etc [1]. In comparison of other solar collectors, SAHs has some diverse advantages such as; the heat transfer from a collector (solar absorber) to the employed fluid is a major difference because the air is directly used as the working fluid in a quite simple design. Although the poor heat transfer from absorber tray to the employed fluid is a major problem in conventional design of a SAH but it can further be improved through various efficient techniques available as a simple solution to increase heat transfer and the overall efficiency [2]. These competent techniques deals with extended surfaces over a flat plate collector (FPC) with geometries of different patterns (like round, conical, etc.) or incorporation of a quality thermal heat 2

storage (TES) material to the system [3-4]. Previous reviews on the performance of SAHs with the use of extended surfaces [5] show the successful rate of experiments comparatively in terms of better output over conventional design. In these different reviews [5-8], the outcome is almost similar that extended surface enhanced the heat transfer between system’s absorber and flowing fluid for a higher rate inside the system as well as minimized the thermal losses to the environments. Similarly the implementation of TES materials in solar applications give quite impressive results (either it is a SHS or LHS) to enhance the thermal performance for example a simple solar collector [11], a solar cooker [12-13], or solar distillation unit [14] etc. The concept of extended surfaces and applying the TES materials to the solar energy systems has been noticed from previous available data and scientific reports for an improved performance with enhanced heat transfer. Table 1 shows the scientific research on SAHs in early phase. References Lof and Nevens [15]

Design of the system A simple air collector has been developed to study thermal performance of efficient class of the designed collector under cold climatic conditions.

Later, Mathur [16]

Authors has been emphasized on the SHS materials like rock bed etc., especially for room heating and cooling for long term storage as well as to increase thermal efficiency of a solar system. The simulation of packed bed air heating and cooling of solar systems has been carried out. The system has been chosen to investigate three different modes of performance. The authors has been applied the Schumann model in which performance evaluation of the said system has been carried out with the help of few assumptions.

Huges et al., [17]

Morrison and Khalik [18]

A transient response has been presented of a TES material inside the air and liquid based SAHs. The effects of the TES storage potential on the heat transfer rate and design of SAH has been investigated through paraffin wax and sodium sulfate decabydrate.

Theunissen and Buchlin [19]

The transient simulation (code- TSUNS) of a SHS has been carried out through both the SHS (rock bed storage) and LHS (CaCI26H2O) for a comparison at geographical location of Belgium and USA.

3

Results A relationship between the net heat recovery, solar irradiance intensity, ambient temperature and glazing patterns has been developed to obtain an optimum design. The heat recovery efficiency has been estimated to be 50%. The modified system has been tested for an improved heat storage efficiency and found the tested system better over other designs In mode 1, the sun energy is available to be stored for a planetary heating or cooling load. In mode 2, solar energy is accessible for storage at the time when heating is required. In Mode 3, no solar heat can be stored but the load for space heating is required. The NTUc has been found adequate for gravel beds and obtained value of the NTUc is greater than ten. Optimum range of TES storage volume has been recommended on the basis of thermal performance enhancement of the system. Little gain in fraction of load supplied by solar source has been realized to be increased per unit collector area by 25 kg/m3. Results indicated that TES is an important component for performance improvement in SAHs. On the basis of NTU and available solar fraction LHS is found better over SHS for the designed SAH. The value of NTU has been obtained between 5 to 10

Bhargava et al., [20]

Enibe [21]

Prasad et al., [22]

Saxena et al., [23]

Saxena et al., [24]

Karthikeyan et al., [25]

Ali et al., [26]

A model of a single pass double glazed SAH has been investigated experimentally and theoretically. The system performed on rock bed storage. The model has been found quite well for experimental analysis especially for the effects of heat transfer coefficients (conductive, convective and radiative) and number of glazing. A passive type PCM integrated SAHS has been designed for performance evaluation in the climatic condition of Nigeria. The PCM containers have been equispaced across solar absorber tray to be acted like as an air heating channel. A packed bed air heater integrated with a wire mesh has been investigated (which has used as packing material). Obtained experimental data has been used for developing correlations between Colburn (Jh) factor, friction factor (ff) and the Reynolds number (Re). Granular carbon has been used as a quality TES material inside a SAH to increase heat transfer rate and to provide the long term heating. The use of halogen lamps inside the ducts made the heating possible in low ambient environments or in night. The dessert sand has been tested inside a SAH and compared the results to a same deigned SAH operated simultaneously with modified system under the climatic condition of Moradabad city, India. Parametric studies have been carried out of a SAHS integrated with PCM infused packed bed storage which is found appropriate for low temperature heating.

A double pass SAH has been investigated on 04 different configurations viz; (i) a simple SAH performed on only absorber plate (ii) SAH performed on copper tubes carrying TES (iii) SAH performed on aluminum rods inserted in the center of each tube and (iv) SAH performed on SAH performed on copper rods inserted in the center of each tube. Results showed that 03 and 04

4

The average ƞtherm for single glazing SAH has been obtained around 27.10% while for double glazing SAH it has been observed to be 32.6%.

The system has been found the best configured at a flow rate around 0.057 kg/s with 22% thermal efficiency (ƞtherm). Heat transfer data and friction factor characteristics have been analyzed for diverse mass flow rates (0.0159 to 0.0347 kg/s) for 08 different groups of matrices. It has been observed that an enhancement up to 89.5% is possible to obtain in thermal efficiency. The new designed system has been found better over a conventional SAH of the same design with a thermal efficiency improvement of 20.77% (on natural convection) and 55.64% (on forced convection). Thermal efficiency (ƞtherm) has been found to be improved as 18.74% on natural convection while 69.06% on forced convection Results of experimental study shown that size of PCM container, mass flow rate and the inside air temperature influenced the coefficient of convective heat transfer and other physical parameters. The surface heat transfer coefficient has been calculated for the three different mass flow rates and the obtained values have been noticed to be 43.10, 37.20 and 28.9 W m−2 K-1, respectively. For the first configuration, ƞmax has been estimated to be 88% at 5:30 hrs. The second configuration has been provided the useful heat till 7:00 hrs with ƞmax of 91% at 7:00 hrs. The next configurations 3 and 4, has been given the extended performance up to 7:30 hrs by reaching to their ƞmax values of 93% and 96%, respectively, at 7:30 hrs.

Saxena et al., [27]

Mahmood et al., [28]

Edalatpuor et al., [29]

Zhu et al., [30]

Poongavanam et al., [31]

configured SAH models have better performance over 01 and 02 configurations. A mixture of two SHS mediums has been prepared and tested inside a modified SAH. The mixture has been prepared by mixing the desert sand and granular carbon into ration of 4:6 and tested under a new designed air heater. The single and double pass air heaters with diagonal blackened fins placed on equal spaced sections has been constructed and tested. An alternative absorber tray has been developed by using 16 wire mesh layers. A double glazed SAH has been designed and tested in which paraffin wax has been used as PCM. Both the exergy and energy analysis has been carried out in climatic conditions of Mashhad, Iran. The thermal response of vacuum tube SAC has experimentally studied which has been established on an array of micro heat pipes. The studied analysis has been typically based on heat losses from the system. Some investigations has been carried out on a modified SAH to examine the heat transfer characteristics as well as friction characteristics for laminar stream conditions. The duct surface with corrugated absorber tray has been roughened through shot blasting. Results have been obtained for higher value of Nusselt number (Nu) for corrugated plate.

Bezbaruah et al., [32]

The thermo-hydraulic characteristics of a modified SAH duct have been investigated by developing conical vortex generators.

Abuska et al., [33]

The three model of SAH (with same specifications) has been compared for performance characteristics. The effect of PCM has been investigated with the effect of honeycomb as internal fin in PCM on six different mas flow rates (0.008 kg/s to 0.048 kg/s). The significance of forced convection operation has been studied on the performance of modified SAH with crossmatrix absorber which dealing with hollow metallic square shaped tube. Total four different geometries has been developed and investigated for measuring the efficiency, pressure drop and heat transfer characteristics on diverse fluid flow rates.

Razak et al., [34]

5

The efficiency improvement has been observed around 20.78% on natural convection while on forced convection it is 80.05%. The efficiency improvement has been observed around 62.50% and 55% for double and single pass air heater, respectively. After conducting the experiments it has been concluding that the daily ƞenergy has been varied from 58.33% to 68.77% while daily ƞexergy varied from 14.45% to 26.34%. It has been concluded that the daily ƞenergy has been varied from 50% to 70%. The value of ƞmean for conventional and modified shot blasted absorber trays have been varied from; i) 14.87% to 19.7% at flow rate of  0.01 kg/s. ii) 26.04% to 34.16% at flow rate of 0.015 kg/s. iii) 33.4% to 56.01% at flow rate of 0.02 kg/s, respectively. Re has been found to be varied from 3000 to 15, 000 for different angle of attacks. ANSYS FLUENT has been used for CFD analysis. The value of Nu has been found to be improved by 7%. Daily ƞtherm has been found to be varied from 8.4% to 9%. The modified systems have been found better over the conventional SAH.

The first configuration has been found the best for obtaining high value of efficiency (76%), temperature rise (15.3oC), pressure drop (1.33 Pa) and the Re (50, 794).

Sivakumara et al., [35]

Mzad et al., [36]

Sahu et al., [37]

Thermal analysis of pin-fin SAH has been carried out on forced convection under the climatic conditions of Coimbatore, India. Various performance parameters like, ƞenergy, ƞexergy, ƞthermohydraulic, and temperatures of the system and ambient has been studied carefully. The design of a SAH has been optimized by considering all the factors of design like, glazing material, quality of energy storage, solar collector and type of insulation. The unit has been tested on four different azimuth angles and tilt angles. The use of extended surfaces like fins and their significance on the thermal response of SAH has been investigated. Geometry of the fin, system type and its operating parameters has been compiled and presented.

The exhaust temperature of the system if found higher around 17oC than a conventional SAH while the ƞenergy has been increased by 12% in comparison of reference model.

It has been concluded that optimized system has an improve heat transfer rate over a conventional SAH and being this optimization the cost of the SAH has also been found reduced. The review study on the heat transfer analysis and correlations between heat transfer parameters for SAHs has been presented well for efficiency and heat transfer enhancement especially by using extended surfaces. Table 1- Previous literature review on SAH with energy storage materials

Above literature shows the significance of TES materials and extended surfaces on the efficiency and heat transfer augmentation of SHS. Both techniques are practically good to the solar thermal systems for performance enhancement in terms of higher thermal efficiency, improved heat transfer, minimized heat loss and improved life cycle. Therefore, a combination of both the techniques has been developed by keeping the economic factor in the mind for a long term aspect and applied in SAH for efficient space heating or drying process. 2.0 Materials and Methods 2.1. Fabrication There are two models of SAH which has been developed for performance testing i.e., model SAHA, which is a reference model and the model SAH- B which is a model that deals with cylindrical copper tubes encapsulated with granular carbon as SHS. It is notable that the model SAH-B has been again modified into a new model SAH-C that deals with copper tubes encapsulated with paraffin wax as LHS. It is also remarkable that the model SAH-C is again re-modified into a new model SAH-D which integrated with flexible copper tubes encapsulated with a specific mixture of granular carbon powder (GCP) and paraffin wax as a quality TES material. In that way total three models of air heater (SAH-B, SAH-C and SAH-D) has been developed along with a reference model SAH-A for the same specifications. Because of restricted heat capacity of solar absorber or collector, conventional type SAH is able to provide only the hot air during peak solar hours. But by introducing a TES material capability would add no complexity to a conventional type SAH design. Thus, by filling the tubes with an appropriate low cost TES material, the storage capability of the system will be increased as well as thermal losses will be reduced.

6

For the fabrication of the SAH plywood of approximately 01 cm thickness has been used. The absorber tray has been fabricated by laminating a 0.05 mm thick aluminum sheet on the plywood having a specific area of 151 x 54 cm2 and painted black for maximum energy storage. To minimize the heat losses to the environment from the system glass-wool is used as an insulator. Insulator thickness is 2 cm and it inserted between solar collector plate and wooden cabinet from the bottom and side walls. A single transparent glass (high temperature resistant- 350oC, k = 0.8 W/mK, refractive index n=1.52, transmissivity = 0.97) with a thickness of 0.3 cm has been used as glazing [23]. Properties of paraffin wax Melting temperature Heat storage capacity Specific heat capacity Density solid (at 25oC) Density liquid (at 80oC) Heat conductivity (both phases) Volume expansion Flash point Max. operation temperature

Value 41-44oC 250 kJ/kg 2 kJ/kg K 0.8 kg/l 0.7 kg/l 0.2 W/m.K 12.5% >180oC 70oC

Properties of carbon powder Density Thermal conductivity Specific heat capacity Thermal diffusivity Emissivity Absorptivity Particle size range Pore volume Molecular weight

Table 2 Properties of paraffin wax

Value 460 kg/m3 0.11W/mk 0.93 kJ/kg K 10.2 m2/s/106 0.91 0.97 0.31-0.84 1.04 cm3/g 12 g/mol

Table 3 Properties of granular carbon

A cylindrical copper tube of 0.5 sheet thickness with diameter of 02 cm and approximated 53.8 cm in length is designed for storing the TES material. The tube is closed from its bottom end while the top end is designed for a threaded enclosed arrangement (fig 3a). The benefit of this flexible leakage free tube is that TES material can be easily replaced whenever required. Another benefit is that there is no need to fix it permanently over the solar collector for a typical SAH design. It could better be placed through thermal conductive glue on the solar collector and removed easily when not required. This tube is also painted black for a better solar heat capturing and a set of tube is located at equidistance by 2 cm distance on the absorber. It is notable that at first the tubes (14 tubes) has been vertically positioned for performance enhancement but convective heat transfer is obtained low with a high frictional value due laminar viscous sub-layer. Therefore the said tubes again modified in to 35 same specifications tubes to place horizontal inside the system on equidistance for creating high turbulence. Glazing of the system and absorber has a distance of 10 cm to each other and side walls of the heater is exposed to the sun by 115o angle to receive the fair amount of irradiance [12]. For the air supply to the inlet duct a CPU cooler fan (San Ace MC™109x9812H0016) has been fixed to the duct through a speed controlled knob for variations and controlling of air velocity. For forced convective operations it has been set to a velocity 6 m/s. Two ducts of aluminium made and of similar design have been used for supply of the air and for exhausted air. Both the ducts of heaters are blackened. The stand of the SAH has been fabricated by galvanized iron rods which hold the system at 43o from horizontal plane [38]. Two different low cost TES materials granular carbon powder (SHS) and paraffin wax (LHS) has been considered to a prepare quality mixture which acts a high thermal heat storage and can provide a long term heating. Carbon powder has an excellent energy storage capacity and the use of it diminishes the problem of low thermal conductivity while utilizing in solar energy applications [39-40]. Thermal properties of both the tested TES materials have been given in table 7

2 and 3. The properties of carbon have been taken from the reference [51] and the properties of paraffin wax have been taken from the datasheet provided by the supplier [52]. References [41-42] show that paraffin wax is a modest and monetary method for energy storage in solar thermal applications. Paraffin wax is an appropriate LHS for solar energy systems in which sun energy or heat is stored by means of phase change in the storing medium. Therefore both the materials have been mixed up well to prepare a quality TES material. It is low cost ideal solution to increase the efficiency of a SAH for space heating and industrial drying. Before conducting the experiments on SAH-A (figure 3b) and SAH-D models(figure 2), both the TES material has been filled in the different tubes in the form of a TES mixture in different ratio such as; 20:80, 30:70, 40:60, 50:50, 60:40 and 70:30. The testing of TES infused tubes has been carried out with the help of reference [27]. The tube with a ratio of 30:70 for carbon to paraffin has been found to be melting of composite mixture first and got the solidification in late hours. Therefore, all the tubes followed by the same ratio of tested mixture in SAH-D All the copper tubes and absorber are painted dull black for maximum heat gain. Horizontal positioned tubes

Vertical positioned tubes SAH without tubes

Figure 1 (a) SAH integrated with flexible tubes

(b) Solar air heater of conventional design

The temperature variations at different points of conventional and modified SAHs along with Tamb have been measured through a 12 wire K-type copper constantan thermocouple meter which has an accuracy of ±1oC. Solar radiation on the horizontal plane has been directly monitored through a common used solarimeter (CEL™-201, with accuracy is +1 W/m2). The wind velocity outside and inside the system has been measured through an anemometer with + 1% accuracy. The air 8

temperature has been measured by a mercury-in-glass thermometer. Readings have been taken for the temperature at different points and ambient, insolation and the wind velocity for every 30 minutes and discussed for hourly basis for easy understanding.

2 3 4

1 1. 2. 3. 4.

Inlet duct Outlet duct PCM filled tubes Absorber tray

Figure 2 3D model of SAH integrated with flexible

Granular carbon

Paraffin wax

Figure 3 (a) Copper tubes to be filled by TES material

(b) Granular carbon and paraffin wax

The experiments have been carried out on both the natural and forced convection operations for a set mass flow rate of 0.034 kg/s and 0.48 kg/s, respectively for each configuration. The experiments have been conducted from 10:00 hours to 18:00 hours on consecutive days of the June 9

2019 in Moradabad Institute of Technology, India. It is notable that both the systems have been taken 10-12 minutes for installation at the place of experiment conduction and in this duration reading could not be taken. 2.2 Thermal Analysis Present design of the modified system carries a set of cylindrical copper tubes carrying low cost TES material and installed over the absorber tray of the modified SAH. At a certain point along the air blown direction the captivated solar energy rises the plate temperature to Tp. Then heat is transported from the absorber to the surrounding air at Tamb over the bottom loss coefficient Ub, to the flowing fluid at Tf from the coefficient of convective heat transfer as well as to the base of the glazing over the linearized radiation coefficient of heat transfer. Finally, the heat is lost to the surrounding air by collective convective and radiative heat transfer coefficient Ut [38]. The heat transfer process for the modified system has been presented in figure 4. Schematic diagram (figure 3) shows different elements of modified air heater. The equations of energy balance show the energy exchange between the absorber (a set of PCM infused tubes on absorber tray), single transparent glazing, an air-flow heater, and the surroundings (environment). 2.3 Energy balance equations Now, energy balance for the modified design of SAH can be expressed as [46-48]; For the straight tubes set (solar absorber);

 dT M abs C p.abs  abs  dt

   ( I . Ap ) gl . abs  hrd(abs gl)  hcd(abs air)  hTES (t) 

(1)

For glazing (single glass cover);

 dT M gl C p. gl  gl  dt

   I . gl  hrd(abs gl)  hc v ( gl  amb )  hcv ( gl  air )  hrd (gl sky) 

(2)

For inside fluid (air) flow;

heffect  m air C p.air Tout Tin air  hc v ( abs  air )  hcv ( gl  air )  hbase

(3)

The terms used in equation (1) to (3) for heat transfer has been expressed in appendix 1. Effective heat transfer rate through the surface of collector plate to the bulk temperature of inside air can also be estimated from;

heffect  hAHT Tabs Tair 

(4)

10

The efficiency of air heater increases by increasing the heat transfer rate as well as the surface contact area of heat transfer and also by decreasing the heat losses to the surroundings. The efficiency can be obtained by;



heffect I . Ap



 I .A

p

 Qloss 

(5)

I . Ap

For thermal energy storage material; (a) For granular carbon (SHS);

 d .T  hgl (t)  M gl C p.TES  TES   dt 

(6)

(b) For paraffin wax (LHS);

 d .R(t)  hgl (t)  TES .(2 RL).H stl    k gl .(2 RL).(Tabs  Tmelt ) /  R  dt 

(7)

In case of composite TES material, the value of Ccomp, kcomp and ρcomp will be;

C p.comp  C p. gc M gc  C p. pw M pw kcomp  k gc vgc  k pwv pw

comp   gc vgc   pwv pw Where, v pw  V pw / V and m pw  M pw / M c are the volume fraction and mass fraction of paraffin wax while vgl  Vgl / V and mgl  M gl / M c are the volume fraction and mass fraction of granular carbon 2.4 Modelling for discharge process of PCM/Composite TES Heat balance for each tube filled with TES material

 p Tin  Tout air  2 RLh(Tin  Tts ) mC

(8)

For a small value of Biot number (Bi) Tts ≈ critical temperature (Tcr), the steady state temperature of the fluid after cylinder j can be estimated as [49]; T j 1   Tcr  T j (1   ) (9) And the freezing time for each encapsulated tube can be estimated as;

t j   1  ( j  1) 

(10) 11

Where,  

2 RLh R 2 H   1 ks  Rh and Biot number = Bi    ,    p ) air (mC 2ks T  2 hR  k

The coefficient of heat transfer can be estimated through [50]; h

k b2 Re n 2R

(11)

2.5 Data reduction In the present experimental study the efforts have been put to improve the system efficiency through improving the heat transfer rate as well as by decreasing the heat losses by storing the maximum amount of solar energy inside the system. Therefore it is necessary to evaluate the thermal response of the system due to modifications. For this, the value of certain parameters has been obtained through experimental data such as; The net useful heat gain for circulating air in test section of SAH can be computed as;

 p  air (Tout  Tin ) Quse  mC

(12)

Heat transfer coefficient for the SAH test section can be obtained through;

h  Quse / Ap (Tts  Tair )

(13)

Where, the bulk temperature of the air (Tair) can be estimated from;

Tair  (Tin  Tout ) / 2

(14)

The Reynolds number (Re) and heat loss through the base of solar absorber can be written as;

Re  Va D / v

(15)

Qb  U b . Ap . Tair  Tamb 

(16)

Where, Ub is bottom heat loss and can be obtained by system’s insulation thickness and its thermal conductivity value [38].

12

Warm/Hot air out

Absorber tray Glazing Outlet Duct

Solar radiation Inlet duct Insulation

Hi-speed fan

PCM infused tubes Air in at ambient conditions Support frame of G.I Fig 4 Schematic view of experimental set-up of new SAH

Solar radiation

Tout

Ut Tg

Glazing

hw

hTES

I

Tp Ub

Tin

PCM infused tubes

Tb

Glass wool

43o

Air in at Tamb Fig 5 Heat transfer mechanism of new air heater (SAH-D)

3.0 Result & Discussion In the present work, a low cost TES material has been prepared and tested experimentally for efficiency enhancement of a SAH. Here, two different TES materials have been investigated to observe the significance on the thermal response of modified air heater. Experiments have been conducted under the climatic conditions of Moradabad city, India. As discussed in section 2.1 two same specifications SAHs (SAH-A and SAH-B) has been designed and fabricated in which SAHA is the reference system and system SAH-B has been carried the GCP and after examining the 13

thermal behavior of SAH-B, it has been re-modified into the new system SAH-C which carried paraffin wax (LHS). Lastly the system SAH-B has been re-modified into SAH-D which has been carried a specific composition of GCP and paraffin wax. The experimental study has been carried out from 10:00 hrs to 18:00 hrs in the month of June 2019. It is notable that the copper tubes has been positioned parallel to the direction of fluid flow inside the SAH to conduct the experiments on 04.06.2019 and got the thermal efficiency around 49.7% on forced convection. Therefore, on the next day, the same model has been re-arranged for a perpendicular structure of copper tubes to the fluid flow and this time the efficiency enhancement become possible due to turbulence occurrence. Mass flow rate has been set for a value of 0.034 kg/s for operating the system on natural convection operation while 0.48 kg/s for forced convection operation for all the configurations. In the present experimental study, the main parameters that have been studied for the output results are the thermal efficiency and heat transfer coefficients for heat gain and heat loss. Investigation of the experiments also showed the significance of TES material over the performance of the tested model under low ambient conditions. All the three models of SAHs (SAH-B, SAH-C and SAHD) have been tested on consecutive days of June 2019 (as given below) along with a reference model SAH-A. 1. 2. 3. 4. 5. 6.

SAH-A and SAH-B has been operated on 06.06.2019 on natural convection SAH-A and SAH-B has been operated on 07.06.2019 on forced convection SAH-A and SAH-C has been operated on 08.06.2019 on natural convection SAH-A and SAH-C has been operated on 09.06.2019 on forced convection SAH-A and SAH-D has been operated on 10.06.2019 on natural convection SAH-A and SAH-D has been operated on 11.06.2019 on forced convection

All the measuring instruments are properly checked each time for any reading error before starting of experiments. All the experiments have been carried out on natural convection and forced convection. The experimental procedure has been shown in figure 6 with the help of a flow chart.

START

Filling of TES (carbon: paraffin) in tubes in the ratio of 14 20:80, 30:70, 40:60, 50:50, 60:40, 70:30

Figure 6 Flow chart of the process followed for results of experiments

3.1 Copper tube with sensible heat storage inside SAH-B

15

The new solar heater contains thirty five same specifications copper tubes which have been placed in horizontal position inside the system at an equidistance for better heat transfer in between solar absorber and the inside air. In first this configuration copper tubes have filled with GCP. The tested GPC has been sieved to a 20x50 (US sieve) mesh and resilient a range from 0.31 to 0.86 mm for particle size. The complete surface extent is around 1051 m2/g and the total pore volume is closed to 1.04 cm3/g [23]. The experimental set-up has been checked well for any leakage in the tube or system and error in measuring devices before conducting the experiments. On 06.06.19, the experiments have been started at 10:00 hrs and finished at low ambient conditions in the evening around 18:00 hrs. Both the systems have been tested simultaneously under the same environment. On the day 1 the testing of SAH-A and SAH-B has been operated on free convection. The Tamb has been observed around 30.9oC with 590 W/m2 of insolation and the wind velocity has been observed around 0.35 m/s. The Tp of the SAH-A and SAH-B has been observed around 64.7oC and 67.2oC, respectively at 10:00 hrs. Maximum value of the Tp obtained by the system SAH-B is to be 77.1oC at 14:00 hrs when the Tamb is around 36.2oC. The Tp of the SAH-A has been found to be 70.8oC at the same time. The SAH-B has been found to be a fast thermal response solar thermal system due to the incorporation of SHS. The exhaust air temperature of system SAHB (Texh-B) has been observed to be 47oC and the exhaust air velocity has been notified to be 0.38 m/s and for the system SAH-A (Texh-A) it has been monitored as to be 43.9oC with exhaust velocity around 0.37 m/s. The temperature variation in the system SAH-B is due to the SHS medium. It absorbs the much quantity of solar heat due to better heat storage capacity in comparison of SAH-A and also have a greater heat transfer area in comparison of reference system. Figure 7 shows that the modified system SAH-B has been found to be capable for providing the hot air around 40.1oC in the late evening times due to stored heat energy while the reference system provide less temperature due to absence thermal energy storage.

Figure 7 Temperature variations in thermal performance of model A and B on natural convection

Thermal response of the SAH-B has been found better over the reference system SAH-A due to GCP infused tubes. The maximum thermal efficiency of the system SAH-B (ƞB-therm) has been 16

found around 11.9% while for the reference system it has been observed to be 10.51%. The heat transfer rate has also been found to be improved for SAH-B. The maximum value of h has been found to be 98.05 W/m2.K for reference system SAH-A while for the modified system SAH-B it has been observed as to be 137.61 W/m2.K. The Qu has been found for a range of 136.68 W to 375.81 W for SAH-B and 135.61 W to 273.36 W for the reference system. After successful completion of the experiments on natural convection both the systems has been experimentally studied for thermal performance enhancement in the same manner on forced convection on 07.06.19. The Tamb has been found around 31.9oC and solar radiation has estimated to be 585 W/m2 around 10:00 hrs on the second day of experiments. The mass flow rate has been fixed for a value of 0.48 kg/s for forced convection. The Tp of the SAH-A and SAH-B has been observed around 59.8oC and 63.1oC, respectively at 10:00 hrs. The GCP infused tubes has successfully attained a surface temperature around 75oC on forced convection and provide better heat transfer from solar absorber to the inside air. Maximum value of Tp has been obtained by the system SAH-B to be 75.2oC around 14:05 hrs when the Tamb has been observed around 38.8oC and Tp of the SAH-A has been observed as to be 71.9oC. Exhaust air temperature of air heater SAH-B (Texh-B) has been observed to be 43.3oC and the exhaust air velocity has been notified to be 1.1 m/s and for the system SAH-A (Texh-A) it has monitored as to be 41.8oC with exhaust velocity around 1 m/s. This configuration has been found better over previous configuration for both the heaters due to forced convection.

Figure 8 Temperature variations in thermal performance of model A and B on forced convection

The maximum thermal efficiency of the system SAH-B (ƞB-therm) has been found around 56.01% while for the reference system it has been observed to be 39.22% on forced convection. For both the models heat transfer rate has been improved much on forced convection. The maximum value of h has been found to be 192.88 W/m2.K for the SAH-A while for the modified system SAH-B it has been computed as to be 370.61 W/m2.K. The Qu has been found for a range of 481.05 W to 17

1929.60 W for modified air heater SAH-B and 482.40 W to 1447.20 W for the reference system. Figure 8 shows that the modified system SAH-B has been found to be capable for providing the hot air around 37.5oC in the late evening times on forced convection. 3.2 Copper tube with latent heat storage inside SAH-C After completion of the testing of GCP infused copper tubes in SAH-B the same system have been modified into new model SAH-C. In this GCP has been replaced with paraffin wax for performance valuation while there is no change in reference system. The PCM has filled only 90% in the tubes due to thermal expansion and has been placed in the same fashion as configured in SAH-B. On 08.06.19, performance investigation has been carried out from 10:00 hrs to 18:00 hrs on natural convection for both the systems simultaneously under the same conditions. The Tamb has observed around 30.8oC with 580 W/m2 of insolation and the wind velocity (Vw) has been noticed around 0.32 m/s. The Tp of the SAH-A and SAH-C has been observed around 63.9oC and 63.2oC, respectively at 10:00 hrs. The maximum value of the Tp has been obtained by SAH-C to be 79.3oC around 14:05 hrs when the Tamb has been found around 36.9oC and Tp of the SAH-A has been noticed to be 72.9oC. The exhaust air temperature of system SAH-C (Texh-C) has been observed to be 49.2oC and the exhaust air velocity has been notified to be 0.36 m/s and for the system SAH-A (Texh-A) it has been monitored as to be 44.8oC with exhaust velocity around 0.35 m/s. The temperature variation in the system SAH-C is due to the LHS medium. It absorbs the much quantity of solar heat due to large capacity for heat storage and greater heat transfer area in comparison of reference system.

Figure 9 Temperature variations in thermal performance of model A and C on natural convection

Figure 9 shows that the modified system SAH-C has been found to be capable for providing the hot air around 39.2oC in the late evening times due to stored heat energy while the reference system 18

provide less temperature due to absence TES material. It has been notified that when the PCM is discharging the surrounding air pick up the heat from the tube surface and transport it to the exhaust and provide the hot air in evening hours in the absence of solar energy. After making the calculations through the experimental readings the maximum thermal efficiency of the system SAH-C (ƞC-therm) has been found around 12.71% while for the reference system it has been observed to be 10.8%. The maximum value of h has been estimated to be 108.2 W/m2.K for the SAH-A while for the modified system SAH-C it has been observed as to be 141.4 W/m2.K. The Qu has been found for a range of 138.1 W to 410.21 W for modified system SAH-C and from 131.22 W to 307.53 W for the reference system SAH-A. The new model SAH-C has been found better over the reference model SAH-A on natural convection testing. Temperature variations of tested PCM in different tubes with respect to time can be shown in figure 8 and 9, for a fixed mass flow rate. It can be observed that average temperature of tested PCM steadily increased up to 11:30 hrs and afterward it has been increased rapidly almost across all the tubes. Average value of temperature of PCM has been found to be closed to its melting point and sometimes it has been crossed its melting point. Therefore, it can be said that heat has been stored in the form of latent heat as well. After successful completion of the experiments both the models SAH-A and SAH-C have been tested on forced convection on 09.06.19. At staring of the experiments the Tamb has been found around 29.7oC and solar radiation has noticed to be 595 W/m2 around 10:00 hrs on the fourth day of experiments. The mass flow rate has been fixed for forced convection as in previous forced convection configuration. The PCM infused tubes attained the temperature around 77.1oC on forced convection due to high energy storage density and provide better heat transfer between solar absorber to the inside air. The Tp of air heater SAH-A and SAH-C has been observed around 60.8oC and 62.2oC, respectively at 10:00 hrs. The maximum value of the Tp has been obtained by the system SAH-C to be 77.1oC around 14:50 hrs when the Tamb has been found around 37.6oC and Tp of the SAH-A has been observed as to be 73oC. Exhaust air temperature of system SAH-C (Texh-C) has been observed to be 46.2oC and the exhaust air velocity has been notified to be 1.1 m/s and for the system SAH-A (Texh-A) it has monitored as to be 43.7oC with exhaust velocity around 1 m/s. This configuration has been found better over the previous configuration for the modified SAH due to forced convection. The maximum thermal efficiency of the modified system SAH-C (ƞc-therm) has been found to be 69.61% while for the reference system it has been observed to be 42.94% on forced convection. For both the air heaters heat transfer rate has been found to be much improved on forced convection. The maximum value of h has been found to be 231.71 W/m2.K for the SAH-A while for the modified system SAH-C it has been computed as to be 446.35 W/m2.K. The Qu has been found for a range of 497.5 W to 1520.22 W for reference air heater SAH-A and from 516.47 W to 2412.19 W for the modified system. Figure 10 shows that the modified system SAH-C has been found to be capable for providing the hot air around 38.3oC in the late evening times on forced convection

19

Figure 10 Temperature variations in thermal performance of model A and C on forced convection

3.3 Copper tube with combined sensible and latent heat storage inside SAH-D After completion of the testing of PCM infused copper tubes in SAH-C the same system have been re-modified into new model SAH-D. The common paraffin is an assortment of pure alkanes and has relatively an extensive range of the temperature for phase change and also has low heat conductivity which can be improved by mixing some high thermal conductivity materials in optimum ration such as; carbon or graphite [43-44]. Therefore in the present case a mixture of the GCP and PCM has been prepared. In this a mixture GCP and PCM is mixed in 30:70 which shows the rapid temperature increase than the pure GCP and pure PCM. The properties of the paraffin wax have been found vastly improved (by well mixing GCP) such as; thermal conductivity, absorptivity, energy storage etc., and it work as a direct solar absorber. The previous model of solar heater SAH-C is re-modified in which PCM infused tubes of SAH-C has been replaced by the new composite TES material and the new system configured as SAH-D. The PCM is filled only 90% in the tubes due to thermal expansion and has been placed in the same fashion as configured in SAH-B and SAH-C while there is no change in SAH-A. On 10.06.19, the performance investigation has been carried out from 10:00 hrs to 18:00 hrs on natural convection for both the systems simultaneously under the same ambient conditions. At this time the Tamb has observed around 30.7oC with 585 W/m2 of insolation and the wind velocity (Vw) has noticed around 0.36 m/s. In the initial hours the modified system SAH-D has been achieved a higher surface temperature in comparison of all previous configurations. The Tp of the SAH-A and SAH-D has been observed around 62.9oC and 65.2oC, respectively at 10:00 hrs. The maximum value of the Tp has been obtained by SAH-D found to be 87.1oC around 13:55 hrs when the Tamb is around 37.8oC and Tp of the SAH-A has been noticed to be 76.8oC (figure 11). Exhaust air temperature of system SAH-D (Texh-D) has been observed to be 50.1oC and the exhaust air velocity 20

has been notified to be 0.34 m/s and for the system SAH-A (Texh-A) it has monitored as to be around 46.5oC with exhaust velocity around 0.33 m/s.

Figure 11 Temperature variations in thermal performance of model A and D on natural convection

The experimental results on natural convection indicated that the maximum thermal efficiency of modified air heater SAH-D (ƞD-therm) has been found around 21.19% while for the reference system it has been observed to be 11.5%. The maximum value of h has been estimated to be 109.71 W/m2.K for the SAH-A while for the modified system SAH-D it has been observed to be 157.1 W/m2.K. The Qu has been found for a range of 273.4 W to 471.11 W for re-modified system SAHD and from 131.22 W to 318.2 W for the reference model SAH-A. The new model SAH-D has been found better over the reference model SAH-A on natural convection testing. After completion of the experiments on natural convection operation the models SAH-A and SAHD have been experimentally studied for performance enhancement on forced convection on 11.06.19. At staring of the experiments the Tamb has been found around 31.8oC and solar radiation has been noticed to be 590 W/m2 around 10:00 hrs on the last day of experiments. The mass flow rate has been fixed for forced convection same as in previous forced convection operations for other models. The last configuration has been observed to attain the higher temperatures in reduced timings for the model SAH-D. The Tp of the SAH-A and SAH-D has been observed around 61.9oC and 65.2oC, respectively at 10:00 hrs. The maximum value of the Tp has been obtained by the system SAH-D to be 83.9oC around 14:05 hrs under the peak solar hours when the Tamb is around 37.8oC and Tp of the SAH-A has been observed as to be 73.7oC. At this time, the exhaust air temperature of system SAH-D (Texh-D) has been observed to be 52.8oC and the exhaust air velocity has been notified to be 1.1 m/s and for the system SAH-A (Texh-A) it has monitored as to be 44.8oC with exhaust velocity around 1 m/s. This configuration has been found superior over the previous tested models on the forced convection operation. The maximum thermal efficiency of the modified system SAH-D (ƞd-therm) 21

has been estimated to be 78.31% while for the reference system it has been observed to be 46.61% on forced convection. The maximum value of h has been found to be 231.71 W/m2.K for the SAHA while for the modified system SAH-D it has been computed as to be 446.35 W/m2.K. The Qu has been found for a range of 504.1 W to 1685.4 W for the reference heater SAH-A and from 681.2 W to 3105.7 W for the modified system SAH-D. Figure 12 shows that the modified system SAH-D has been found to be capable for providing the hot air around 50oC in the late evening times on forced convection.

Figure 12 Temperature variations in thermal performance of model A and D on forced convection

The modified system SAH-D has got the high surface temperature with a fast response and provides the long term air heating for space heating and drying purpose. Figure 12 shows that the modified system SAH-D has been found better over than all other tested models. The new model is capable for providing the hot air around 50oC in the late evening times due to stored heat energy. Parameters Tp ƞtherm h

Qu

SAH-B The average value of Tp is 68.33 while the maximum value is 75.2oC The average value of ƞtherm is 36.51% while the maximum value is 56.01% The average value of h is 301.16 W/m2K while the maximum value is 370.61 W/m2K Ranges from 482.40 W to 1447.40 W

SAH-C The average value of Tp is 69.71 while the maximum value is 77.1oC The average value of ƞtherm is 52.01% while the maximum value is 69.61% The average value of h is 399.16 W/m2K while the maximum value is 446.35 W/m2K Ranges from 516.47 to 2412.19 W

SAH-D The average value of Tp is 74.51 while the maximum value is 83.9oC The average value of ƞtherm is 67.92% while the maximum value is 78.31% The average value of h is 411 W/m2K while the maximum value is 485.31 W/m2K Ranges from 681.2 to 3105.7 W

Table 4 Average and maximum value of different parameters of all three models on forced convection operation

It has been notified that when the PCM is discharging the surrounding air pick up the heat from the tube surface at a high temperature and then transport it to the exhaust and provide hot air in 22

evening hours during the off sunshine hours. The thermal performance of tested air heater has been found to be improved on each new configuration. The comprehensive analysis of transitory temperature distribution of diverse TES materials inside the tested models of SAHs (figure 7 to 12) designated that the model SAH-D is better over than SAH-C and SAH-B, while SAH-C is better over than SAH-B which is better over SAH-A (table 4). The temperature variation in the system SAH-D is due to the composite TES material, fluid turbulence near tubes hot surface and due to larger contact surface of area of heat transfer. The TES material absorbs the much quantity of solar heat due to large capacity for heat storage as well as much better thermal properties of composite TES in comparison of pure PCM and GCP. The introduction of the TES infused flexible copper tubes with high energy storage potential to a conventional SAH has been remarked with reducing of heat losses.

Figure 13 Temperature variations of model A and D on forced convection (off sunshine hour’s case)

Besides this, the experiments have been repeated on 21.10.19. For this, the system has been placed under the full sunshine hours and started the experiments on a sunny day of October 2019. The main objective of this experiment is to observe the duration of the hot air supply in non-sunshine conditions from the modified system (after once the system has attained its peak output). The experiment has been stared at 10:00 hrs and around 14:00 hrs, it has been observed that exhaust temperature reached to its peak value about 48.11oC. Therefore the system SAH-D has been fully covered at the same time from a thick cloth by ensuring that the sunlight is completely absent on the collector plate of SAH-D. Figure 13 shows that the modified system is much better than a conventional system (SAH-A). The value of Texh-D has been observed 48oC and the Texh-A has been found to be 42.6oC at 14:00 hrs. Afterwards it has been noticed that Texh-D fallen slightly towards the Tamb while the Texh-A has been fallen much quickly than Texh-D. The output air temperature of SAH-D has been found around 34.8oC around 16:21 hrs when the Tamb has noticed to be 34.31oC and the Texh-A has been noticed to be equivalent to the Tamb at the same time. The experimental 23

study has been shown the capacity of the modified system for supplying the hot air in non-sunshine hours which is better over the conventional system. The modified system has the potential to supply hot air continuous for 2-3 hours (in the absence of sun shine) once the TES material inside the tube is gotten fully charged. The role of the flexible PCM infused copper tubes to enhance the rate of heat transfer has been successful. Blackened surface of the tubes supports to capture the maximum energy from the sun. Due to the purity of both the TES materials and a specific composition the new PCM shows a remarkable heat storing capacity in range of narrow temperature. In addition, the new composite is chemically safe. After completion of the life cycle of TES material in PCM infused tube it can be easily replaced due to flexible attachment of the tube over the absorber or easily replaced for testing another TES material due to threaded cap sealing provided at one end. Flexibility of the tubes allows arranging the order of the tubes in any new pattern for evaluation of thermal performance. The PCM infused tubes can be used in any flat plate base solar thermal system for high thermal energy storage such as; box cookers (under study) or solar still etc. This heat storage element is an ideal extension to conventional thermal systems for heat transportation solution. The main parameters of the experimental study; heat transfer rate, heat transfer area, mass flow rate, efficiency, storage heat capacity and cost of the system summarizes the model SAH-D as an economic and optimum design. This design is appropriate for space heating and drying operations [53] for different geographical operations. 4.0 Economic Analysis The modified system has been found to be economic to its users. Table 5 shows the expenditure of the modified SAH in which mostly items have been purchased from local market at reasonable prices. The overall cost of the fabrication is approximately 4800 ₨ (Indian rupees) or equivalent to $67.75 which much economic then other studied models [54-57]. The modified system has also been found economic in comparison of some designs developed in other countries such as; Wazed et al., [54] has been design a cost effective SAH which has been fabricated the local available materials. The system has been tested in climatic conditions of Bangladesh. The cost of the fabrication has been assumed around 70 USD. Edalatpour et al., [55] has been designed and fabricated a double glazed SAH and tested under the climatic conditions of Masshad, Iran. The performance evaluation has been carried out to estimate the daily energy efficiency, exergy efficiency and the total fabrication cost which has been obtained as 68.77%, 26.34% and $113.3, respectively. Poole et al., [56] has been developed a plastic made transpired solar air collector and tested under the climatic conditions of Turkey. The system has been designed for a specific size room heating and the cost estimated for $12/m2. Besides this, IEA [57] has been reported by collecting the data (on the cost effectiveness of a SAH) from various countries such as; Germany, France, Italy, Japan, U.K., USA, etc., that en efficient air heating system (for a specific size space) has a range to its fabrication cost from $10/m2 - $14/m2. Now, by comparing the other design [15-37 and 54-57] of air heater the present design is not only economical but a low cost SAH which provides the long term heating in low ambient conditions also. Items Aluminum sheet Plywood

Cost (Indian currency) 350 ₨ 1000 ₨ 24

Glass wool Transparent glass Copper tubes PCM Granular carbon powder Fan Adhesive Iron rod (for stand) Miscellaneous (Paint, labor, nail, transportation, brushes, wires etc.) Total Table 5 Cost of different items for fabrication of SAH

300 ₨ 200 ₨ 900 ₨ 1100 ₨ 200 ₨ 100 ₨ 150 ₨ 400 ₨ 200 ₨ 4800 ₨

The total annual cost of the energy demand can be estimated by using the methodology explained in references [58-59] as; Annual cost (AC) = ACC (annual capital cost) +AMC (annual maintenance cost) + running fuel cost (FR) – annual salvage value (ASV) Where annual capital cost is given by; ACC = capital recovery factor (CRF) x initial capital investment (CI) CRF can be estimated as; 𝐼(𝐼 + 1)𝐿

(17)

𝐶𝑅𝐹 = (𝐼 + 1)𝐿 ― 1 Where interest rate, I and life time of the solar collector, L can be assumed suitably The annual salvage value of the collector can be found by; ASV = salvage fund factor (SFF) x salvage value (SV) Where SFF is obtained as; 𝐼(𝐼 + 1)𝐿

(18)

𝑆𝐹𝐹 = (𝐼 + 1)𝐿 ― 1 Cost of useful energy can be evaluated by; Annual cost

Cost of useful energy = Annual delivered useful energy

(19)

Annual delivered useful energy can be calculated as; Annual useful energy = solar insolation (W/m2) x net collector area (m2) x collector efficiency x approximate number of sunny days x number of sunshine hrs/day (20) Annual useful energy = 580* (W/m2) x 0.8154 (m2) x 0.78 x 250 x 8 W h/yr = 737.77 kWh/yr 25

*solar insolation measured experimentally is averaged for the year Assuming life (L) of the SAH to be 10 years and interest rate (I) of 5 % the Annual cost of useful energy can be estimated as given in Table 6 S. No 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Parameters Gross collector area CI (Rs) S (10 % of CI) (Rs) CRF SFF ACC (Rs) ASV (Rs) AMC (Rs) (10 % of ACC) AC (Rs) Annual cost of 1 kWh of useful energy (Rs)

Numerical value 0.8154 ( m2) $68.06 $6.81 0.1295 0.0795 $8.81 $0.54 $0.88 $10.24 $0.014

Table 6 Cost analysis of the new design of SAH

If this system would have been run on electricity then the rate of per unit consumption in India varies from $ 0.69 per unit which is higher than the rate of $0.014 as evaluated in table above making saving in the range of 50-80 % hence it is more economical. Regarding the assessment of payback period (PBP) than it can be estimated by using the following equation; 𝑇𝑜𝑡𝑎𝑙 𝑖𝑛𝑣𝑒𝑠𝑡𝑚𝑒𝑛𝑡

(21)

𝑃𝐵𝑃 = 𝐻𝑒𝑎𝑡 𝑝𝑟𝑖𝑐𝑒 ∗ 𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 = 4 𝑦𝑒𝑎𝑟𝑠 5.0 Uncertainty Analysis

Results from the instruments used for measuring the temperature variations, solar irradiance and mass flow rate have a definite degree of uncertainty and it has been computed by referring and applying the experimental theory of Coleman and Steele [45] and is shown in table 7. The tests have been repetitive for total two repetitions (N). Methods for enumerating the particular set of investigational data are shown in equation (22) and (23) and therefore experimental uncertainty has been estimated by equation (24); For the mean value (x) N

x

 (x ) i

i 1

(22)

N

For the standard deviation ()



1 N ( xi  x ) 2  N  1 i 1

(23) 26

Now, experimental uncertainty in the present investigation can be computed by dividing equation (17) by equation (18) as; 

x

(24)

 S. No. 1 2 3 4 5 6 7 8 9 10

Parameters (oC)

Tamb Solar radiation (W/m2) TP of SAH-B (oC) TP of SAH-C (oC) TP of SAH-D (oC)

ƞtherm

Texhust of SAH-B (oC) Texhust of SAH-C (oC) Texhust of SAH-D (oC) Wv

Uncertainty ± 1.14% ± 2.01% ± 1.81% ± 1.62% ± 1.22% ± 2.51% ± 1.79% ± 1.70% ± 1.29% ± 2.11%

Table 7 Uncertainty of some major parameters

6.0 Conclusion In the present work, a cylindrical copper tube has been deigned which carries a low cost thermal heat storage. A set of the said tubes has been placed and tested inside a simple designed SAH for efficiency enhancement and length of duration of supplying the hot air for space heating and drying purposes. Total three different configurations has been developed and tested under the same climatic conditions along with a same specification reference model. Results showed that model SAH-D with 78.31% thermal efficiency is the best configured among all modified systems. The modified system SAH-D has also been found to be capable for providing the hot air around 50oC in the late evening times on forced convection. The main parameters of the experimental study; heat transfer rate, thermal efficiency, plate surface temperature, duration of hot air supply, storage heat capacity and cost of the system summarizes the model SAH-D as an economic and optimum design for space heating and for drying operations. The PCM infused copper tube has been remarked for extent heat storing capacity in range of narrow temperatures and the new composite PCM inside the tube has been found completely chemically safe. Therefore the PCM infused tubes has a good possibility for adoption as a PCM packet or container for thermal systems for better heat storage and heat transfer. Elimination of a blower/pump makes the design economical and simple. The modified system’s deals with a low cost fan which consume less electricity which can be produced by a PV panel and system will operate completely on solar power and can be referred as a pure solar energy system. Acknowledgement- The valuable support of Er. Amit Khatter (CAM Solutions, Ludhiana) is appreciated.

References [1] A. Saxena, Varun, M. Karakilcik, Solar food processing and cooking methodologies, Applications of Solar Energy, Springer-Nature Series, (2018) 251-294 [2] A. Saxena, N. Agarwal, Review of recent patents of solar air heaters, Recent Patents in Engineering, 9 (2015) 2-20 27

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Appendix-1 30

The terms given in equations from (1) to (5) can be stated as; The intensity of irradiance (I);

 t  I  I max .sin    tmax 

(25)

The ambient temperature (Tamb);

Tamb  Tamb.min  (Tamb.max  Tamb.min ).  I / I max 

(26)

Convective heat transfer by glazing to the surroundings and to the fluid flow can be expressed respectively as equations (20) and (21);

hcv (gl amb)  hgl amb . Ap (Tgl  Tamb ) hcv (gl amb)  (5.7  3.8Vw ). Ap (Tgl  Tamb )

(27)

hcv (gl  air )  hgl  air . Ap (Tgl  Tair )

 k hcv (gl  air )   air  L

 0.33 0.5   0.664 Pr Re  Ap (Tgl  Tair )  

(28)

Radiation heat transfer by glazing to the sky can be obtained by;



hrd (gl  sky )   . g . Ap Tgl  273  (0.0551(Tamb  273)1.5  273)  273 4

4



(29)

Convective heat transfer through SAH absorber to the air

hcv (abs_air)  habs _ air . Aht . Tabs  Tair   k hcv (abs_air)   air  L

 0.8   0.356 Re  Aht . Tabs  Tair   

(30)

HIGHLIGHTS

 

The tested low cost heat storage has large capacity of solar energy storage Cylindrical copper tube has the flexibility of easy replacement of storage material 31

 

The modified heater provide exhaust air temperature around 52.8oC with exhaust velocity of 1.1 m/s The cost of the modified system is only 67.45$.

32

To,

Editor in chief International Journal of Applied Thermal Engineering

Sir,

There is no financial or other relationship with other people or organizations that may in appropriately influence the author’s work and similarly not by side of Institution.

Thanks

33

Dr. Abhishek Saxena (Editor- MIT Transactions) Associate Professor, [Member of ISES (Florida), ATINER (Athens), SESI (India)] Department of Mechanical Engineering Moradabad Institute of Technology, U.P., India Contact- +91-9837623458

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