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M. Muttakin et al.
8.3.2 Efficiency of an FPC
An efficient solar collector should possess high absorptance for radiation in the solar
energy spectrum. At the same time, in order to minimize the losses, it must have low
emittance for long wave radiation. The efficiency (η) of an FPC is defined as the ratio
of useful gain (Q u ) to the incident solar radiation power (Q T ),
η =
Q u
Q T
=
Q u
G A c
(8.3)
where G represents the solar irradiance in W/m
2 , and A c represents an aperture area
of the collector. With G s being the absorbed energy, the useful energy gain can be
defined by,
Q u = A c [G S − U L (T c − T a )]
(8.4)
where U L is the overall heat transfer coefficient, T c and T a are the mean absorber
plate temperature and ambient air temperature, respectively. Hence, U L (T c – T a )
is the thermal energy lost from the collector to the ambient through conduction,
convection and infrared radiation.
In order to determine the efficiency of the collector, all the parameters of Eq. (8.4)
must be known. The parameters G, A c , T c and T a can be measured through experiments. The absorbed energy G s can be calculated from,
G S = G(τ α) e f f
(8.5)
where (τ α) eff is called the effective transmittance-absorptance coefficient. It can be
approximated from the known value of incidence angle modifier of a collector, as
defined by,
K τ α =
(τ α) e f f
(τ α) n
where (τ α) n is transmittance-absorptance normal to the collector surface. For FPC
with flat covers, the incidence angle modifier depends on the angle of incidence θ
following the below equation (Souka and Safwat 1966),
K τ α = 1 + b 0
1
cos θ
− 1
where b 0 is the incidence angle modifier constant, and it has a positive value.
The thermal network of an FPC with two covers is shown in Fig. 8.3. The absorbed
energy G s is converted to useful energy gain Q u after losing a portion to the ambient
environment through the top and bottom of the collector. At some typical location, let
T p be the absorber plate temperature. From the top of the collector, heat loss is due to
M. Muttakin et al.
8.3.2 Efficiency of an FPC
An efficient solar collector should possess high absorptance for radiation in the solar
energy spectrum. At the same time, in order to minimize the losses, it must have low
emittance for long wave radiation. The efficiency (η) of an FPC is defined as the ratio
of useful gain (Q u ) to the incident solar radiation power (Q T ),
η =
Q u
Q T
=
Q u
G A c
(8.3)
where G represents the solar irradiance in W/m
2 , and A c represents an aperture area
of the collector. With G s being the absorbed energy, the useful energy gain can be
defined by,
Q u = A c [G S − U L (T c − T a )]
(8.4)
where U L is the overall heat transfer coefficient, T c and T a are the mean absorber
plate temperature and ambient air temperature, respectively. Hence, U L (T c – T a )
is the thermal energy lost from the collector to the ambient through conduction,
convection and infrared radiation.
In order to determine the efficiency of the collector, all the parameters of Eq. (8.4)
must be known. The parameters G, A c , T c and T a can be measured through experiments. The absorbed energy G s can be calculated from,
G S = G(τ α) e f f
(8.5)
where (τ α) eff is called the effective transmittance-absorptance coefficient. It can be
approximated from the known value of incidence angle modifier of a collector, as
defined by,
K τ α =
(τ α) e f f
(τ α) n
where (τ α) n is transmittance-absorptance normal to the collector surface. For FPC
with flat covers, the incidence angle modifier depends on the angle of incidence θ
following the below equation (Souka and Safwat 1966),
K τ α = 1 + b 0
1
cos θ
− 1
where b 0 is the incidence angle modifier constant, and it has a positive value.
The thermal network of an FPC with two covers is shown in Fig. 8.3. The absorbed
energy G s is converted to useful energy gain Q u after losing a portion to the ambient
environment through the top and bottom of the collector. At some typical location, let
T p be the absorber plate temperature. From the top of the collector, heat loss is due to