Environmental Engineering Reference
In-Depth Information
The heat balance [3]:
on air:
∂
T
∂
T
(
)
(
)
h
A
T
−
T
dxdt
+
C
GA
dxdt
+
UD
π
T
−
T
dxdt
=
−
ρ
C
Af
dxdt
(5)
v
p
p
a
p
∂
x
∂
t
on stones:
∂
2
T
∂
T
p
p
k
A
dxdt
+
h
A
(
T
−
T
)
dxdt
=
ρ
C
A
(
−
f
)
dxdt
(6)
gs
v
p
p
pg
2
∂
t
∂
x
Sizing
The collector surface
A
c
and the storage volume
V
p
are essential sizing parameters which
cannot be considered independently. We must solve simultaneously the balance equations
with the particular conditions of this installation which are in our case, the maintaining of the
substrate temperature at 40°C for all the year with a solar supply rate of 100 %.
We chose a sizing method in two stages:
•
a pre-sizing by holding account of the characteristic parameters of the installation,
•
an adjustment of the sizing according to the results obtained.
The adjustment is carried out through several simulations. The idea is to find a
compromise by gradually reducing the acceptable ranges for significant sizing parameters.
Hence, we define the parameters which have to be validated through the calculation of the
performances. The final result must lead to the evaluation of the collector surface and the
storage volume knowing the temperature and mass of the substrate.
The annual supply
E
of a commercial air collector in average operating conditions is from
450 à 500 kWh/m
2
/year. With a daily provisioning of the digester of 130 l/jour, the annual
energy need
B
is about 10 MWh/year. The collector surface A
c
= B C
s
/E = 20 m
2
.
Estimation of Biogaz Production
According to the sizing and to the digestion temperature we estimate the biogas
production and the energy equivalences (table 2). It is possible to produce 68.4 m
3
of biogas
monthly without consequent contribution of heat [4]. In Algeria, the ground temperature
ranges from 15 to 20 °C for the mayor part of the year, except in winter so the contribution of
solar and storage heat could raise the underground temperature to 40°C. For our sizing
(digester enclosure: 11,77 m3 charge: 8240 kg of digestion substrate residence time: 90 days
daily supply: 130 litters/day) the production of biogas [1] is estimated to 185,4 m
3
/month and
could be continuous during all year.
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