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D. Ivanova et al. / Energy Conversion and Management 44 (2003) 763–769
3.2. Combined scheme for energy supply
For heating the water in the serpentine tubing, a conventional energy source, electricity, is used
at night. The construction enables an additional heating of the dryer air during flow (shown by
arrows) in the incoming air duct 2, Fig. 1, at night. Thus, the heat losses to the environment are
reduced without expensive thermal insulation. The dependence of the efficiency on the mass flow
rate of the working fluid has been determined by means of many dry experiments, without hu-
midity in the drying chamber. The efficiency has been calculated as a ratio between the heat flow
at the exit of the drying chamber and the heat flow released from the serpentine tubing. This
released heat flow is calculated by means of measuring the temperature differences of the hot
water at the entrance and at the exit of the serpentine tubing
mqcpðTout ꢀ TaÞ
gw ¼
ð2Þ
gwcwðtw;i ꢀ tw;out
Þ
If the mass flow rate of the working air increases, the efficiency grows in a nonlinear way from 0.3
for m ¼ 0:045 m3/s to 0.7 for m ¼ 0:33 m3/s if the temperature of the circulating water in the
serpentine tubing is 90 ꢁC. In this case, it is advisable to work with a mass flow rate of the working
fluid between 0.25 and 0.33 m3/s, as it will assure the highest efficiency.
In order to determine what part of the heat flow returns through the surrounding walls and
enters the drying chamber with the incoming air, laboratory experiments have been made. The
average night air temperature in the laboratory has been maintained steady. For the months from
June till September, the average night temperatures are as follows:
TJune ¼ 19:1 ꢁC; TJuly ¼ 21:3 ꢁC; TAugust ¼ 21 ꢁC; TSeptember ¼ 16:9 ꢁC:
The additional heating energy, ensured by the conventional source, to maintain the steady tem-
perature of the drying agent above the serpentine tubing at T1 ¼ 60 ꢁC, under the condition that
the installation works 12h, Dt ¼ 12h in the nights, in such regimen is
Q ¼ mqcpðT1 ꢀ TaÞDt
ð3Þ
This means the monthly amount of heat is as follows: QJune ¼ 4858:9 kW h; QJuly ¼ 4750:8 kW h;
QAugust ¼ 4787:6 kW h; QSeptember ¼ 5120:3 kW h or the annual thermal load in the nights is
19 517.6 kW h.
From the air movement in the incoming air duct, the temperature rise reaches 4.8 ꢁC. The
utilized heating energy is
Qut ¼ mqcpðTf4 ꢀ TaÞDt
The monthly values are
ð4Þ
QJune ¼ 534:6 kWh; QJuly ¼ 515:6 kWh; QAugust ¼ 515:7 kWh;
QSeptember ¼ 570:3 kWh:
In the period of exploitation June–September, the saved energy by utilization of the heat under
this regimen is 2136 kW h, which means 11% of the annual thermal load. The money saved,
depending on the energy sources, is between 43.5 $ with coals and 148.7 $ with electricity.