Thermodynamic Performance Evaluation of a Geothermal Drying System

Abstract

Renewable energy sources such as geothermal energy can be used in drying processes as a heat source due to the high energy costs of fossil fuels.

In this study, geothermal cabinet type dryer was constructed and situated in Balcova-Narlidere Geothermal Field, Turkey where the clean city water of district heating system is used as an energy source for the dryer. The dryer was tested on site for drying of olive leaves and energy and exergy analyses of the drying process conducted under two cases: Case 1. Exhaust air was rejected to the environment. Case 2. A portion of exhaust air was re-circulated. Energy Utilization Ratio (EUR) was determined as 7.96 for Case 1 and 50.36 for Case 2. The highest rate of exergy destruction occurred in the fan, followed by heat exchanger and the dryer, accounting for 0.2913, 0.05663 and 0.0115 kW, respectively. Exergetic efficiency of the drying chamber was calculated as 89.66 %. Re-circulating the exhaust air decreased the exergy value at the outlet of the dryer from 0.1013 to 0.08104 kW, indicating that re-using the air increases the performance of the dyer.

Nomenclature

A:

Area (m²)

C_p :

Specific heat (kJ/kg K)

EU:

Energy utilization (kW)

EUR:

Energy utilization ratio (%)

${E}^{\prime }x$ :

Exergy rate (kW)

h:

Specific enthalpy (kJ/kg)

$\dot{m}$ :

Mass flow rate (kg/s)

P:

Pressure (kPa)

$\dot{Q}$ :

Heat rate (kW)

RH:

Relative humidity (%)

r:

Re-circulation ratio (%)

s:

Specific entropy (kJ/kg K)

T:

Temperature (°C)

V:

Velocity (m/s)

$\dot{W}$ :

Work rate (kW)

ρ:

Density (kg/m³)

ψ:

Specific exergy (kJ/kg)

ε:

Exergetic efficiency (%)

ω :

Specific humidity ratio (kg water/kg air)

da:

Drying air

dc:

Drying chamber

des:

Destroyed

env:

Environment

He:

Heat exchanger

in:

Inlet, inflow

out:

Outlet, outflow

re:

Re-circulated

sat:

Saturated

0:

Dead (reference) state