Development and Performance Study of Solar Air Heater for Solar Drying Applications

Abstract

Open sun drying for drying of rural agro-based produces is an age-old practice for preservation as well as for the use of dried products in the future. However, open sun drying takes longer duration, and thus, it gives inconsistent quality for drying of agro-based produces. Hence, solar dryer emerges as a device to overcome the demerits of the open sun dryer and incorporates its advantage to the maximum extent. Different improved designs of solar air heater cum dryers are available to get efficient solar drying. The present investigation explores general theoretical thermodynamic performance studies of some improved solar air heater for drying applications. It follows with a detailed thermal performance studies of an improved hemispherical protruded solar air heater. The protrusion height (e) varied as 2.4, 3.0, and 3.7 mm, and the long way length (p) varied as 24, 36, and 52 mm, respectively. The effect of $\frac{e}{D}$ (0.055, 0.045, 0.035) of hemispherical protruded absorber on constant $\frac{p}{e}$ = 12 had been observed. It was seen that maximum thermal efficiency of 82% is achievable for $\kern0.15em \frac{p}{e}$ = 12 and $\frac{e}{D}$ = 0.055. Moreover, with $\frac{e}{D}$, value of 0.035 and 0.055 gives maximum and minimum effective efficiency (74 and 64%), respectively, at 12,000 Reynolds number. From economic analysis, it has been estimated that if 400 m² of black tea processing factory galvanized roof were converted by using black painting, plywood insulation, and tempered glass enclosure to convert it into a solar air heater, then average 20% of conventional thermal energy for black tea drying may be saved. The annual carbon dioxide reduction of 2189 t is achievable by using improved solar air heater. The payback period of the hybrid renewable thermal energy-based system is less than 15 months, and the benefit-cost ratio is 1:1.

Nomenclature

Cos θ :

The phase angle difference between voltage and electric current, ∼0.85

c _p :

Specific heat of drying air, kJ/kg

Ė :

Energy, kJ/s

E _fan :

Fan energy consumption, kW

Ex:

Exergy, kJ/s or kW

EUR:

Energy utilization ratio, dimensionless

g :

Gravitational acceleration, 9.8 m/s²

h :

Enthalpy, kJ/kg

I :

Electric current, ∼4 A

IP:

Exergetic improvement potential, kJ/s or kW

ṁ :

Mass flow rate of air, kg/s

m _i :

Initial weight of product, kg

m _w :

Mass of water evaporated from the product, kg/s

N :

The number of days, in January N ¼1 and in December N ¼365

n _i :

Number of holes in length or width of heat-absorbing plate

n _j :

Total number of holes in heat-absorbing plate

P :

Pitch of holes in heat-absorbing plate, m

P _sur :

Pressure, kPa (for Sabzevar City is 90.118 kPa)

P _vs :

Saturated vapor pressure of air, kPa

Q :

Heat energy, kJ/s

S :

Solar radiation, W/m²

SEC:

Specific energy consumption, kW h/kg

SMER:

Specific moisture extraction rate, kg/kW h

T :

Air temperature, °C or K

t :

Time (s)

u :

Air velocity, m/s

ū:

Average velocity, m/s

U :

Voltage, 220 V

V :

Volumetric flow rate of air, m³/s

Ẇ :

Mechanical work, kJ/s

w :

Absolute humidity, kg H₂O/kg Da

w _as :

Absolute humidity of the air entering the dryer at the point of adiabatic saturation, %

w _i :

Absolute humidity of air entering the drying chamber, %

X :

Moisture content, wb%

Z :

Height from sea level, m (for Sabzevar City is 977.6 m)

z :

Height from base level, m

1.1 Greek Letters

Δ:

Difference between two values

η :

Efficiency, %

λ :

Latent heat of water vaporization at exit air temperature, kJ/kg

φ :

Relative humidity, %

υ :

Specific volume, m³/kg

σ :

Void in the heat-absorbing plate

Σ:

Sum between several values

∞ :

Surrounding or ambient (reference)

1.2 Subscripts

a:

Air

abs:

Absolute

col:

Collector

d:

Dryer

da:

Drying air

dc:

Drying chamber

ex:

Exergy

f:

Final

I:

Initial, input, inflow

L:

Loss

o:

Output, outflow

p:

Pickup

sur:

Surrounding

sys:

System