Page 176 - Low Temperature Energy Systems with Applications of Renewable Energy

166 Low-Temperature Energy Systems with Applications of Renewable Energy

overall CO 2 emissions differ depending on the type of fuel that is used to generate the
dryer’s electricity.
Based on the data shown in the figure, one can conclude that by deploying heat
pump technology for drying wood compared to solid fuel plants, the emission of
3
greenhouse gases can be reduced by about 125e160 kg of CO 2 /m of the drying cham-
ber per year. As for plants using natural gas as primary fuel, this indicator ranges from
3
10 to 50 kg of CO 2 /m of the drying chamber per year. When this environmental
benefit is combined with superior thermodynamic energy performance, it is clear
that HPD demonstrate important advantages over traditional dryers.
According to the data from Ref. [21], the worldwide production of heat pump wood
dryers for the last few years is about 800e1000 units per year with a growth trend indi-
cated. This may indicate an increase in the recognition of heat pump technologies for
drying wood.

4.5 Summary

Eight typical drying systems are presented. These include two basic open air systems
with supplementary heaters and recirculation of air, the drying agent. Six arrangements
include a heat pump: two systems are open and may use a supplementary heater; the
other four are closed loop and may include recirculation or bypass. Thus a wide assort-
ment of dryers is available for a variety of drying applications. A survey is included of
worldwide research being carried out in drying with heat pumps, along with a hierar-
chy that classifies the many uses of heat pumps in this industry.
The systems are described, analyzed, and the results presented with the aid of the
psychrometric chart, in tables and in graphs. Optimizations are performed. Compari-
sons are drawn among the systems to show which are preferred in certain situations.
Care is taken to define the alternative fuel/energy use and the mode of drive for the
heat pump compressor in making comparisons, since many possible arrangements
exist in practice. When proper accounting is done to include the energy required for
alternatives to heat pumps in drying grains and wood, heat pumps are the most effec-
tive on the basis of overall energy consumption per unit of dried product.

Nomenclature

B bypass factor
COP coefficient of performance
E cooling cooling coefficient
h enthalpy
K recirculation ratio
m flow rate
P pressure
Q capacity
q heat load
t temperature
V air flow