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M. Muttakin et al.
η = F r (τ α) e f f
η 0
−a
T f − T a
G
− b
T f − T a
2
G
(8.25)
where η 0 , a and b are constants and are usually furnished by the collector manufacturer. The mean manifold fluid temperature T f can be considered as an average of
the fluid temperatures at the inlet (T i ) and at the outlet (T 0 ), i.e.,
T f =
T i + T 0
2
and thus the efficiency can be calculated from,
η = η 0 − a
T f − T a
G
− b
T f − T a
2
G
(8.26)
It needs mentioning that the efficiency of an FPC can be expressed by the same
equation as Eq. (8.26) shown above.
8.4.2 Dynamic Modelling of an ETC
In an evacuated tube collector, there are two fluids, heat transfer fluid or refrigerant
and manifold fluid (water is considered in the current modelling). The current model
is based on the assumption that there is no refrigerant present in the ETC, rather,
water is assumed to flow directly through the heat pipes. The major assumptions are
summarized below,
• There is no presence of refrigerant. Water flows directly through the heat pipes.
• The flow of water is only in the positive x direction.
• The heat conduction in the fluid moving direction is neglected.
• The temperature dependence of thermo-physical properties of water is considered.
• Properties of glass and absorber do not depend on temperature and are assumed
to be constant.
• The infrared emissivity of the sky is unity (ε sky = 1).
Based on the stated assumptions, the heat influx to various components of an ETC
is depicted in Fig. 8.6. As shown in the figure, the model (Praene et al. 2005) consists
of 3 thermal nodes. These are, the transparent glass cover, the absorber plate and
the fluid (water) having temperatures T g , T c and T f , respectively. The heat transfer
between the sky and the glass cover is by radiation only. Again, since the absorber
plate is surrounded by the vacuum, heat transfer between the glass cover and absorber
plate, is entirely by radiation. Convective heat transfer is existent between the glass
cover and the ambient environment, and between the absorber plate and water.
M. Muttakin et al.
η = F r (τ α) e f f
η 0
−a
T f − T a
G
− b
T f − T a
2
G
(8.25)
where η 0 , a and b are constants and are usually furnished by the collector manufacturer. The mean manifold fluid temperature T f can be considered as an average of
the fluid temperatures at the inlet (T i ) and at the outlet (T 0 ), i.e.,
T f =
T i + T 0
2
and thus the efficiency can be calculated from,
η = η 0 − a
T f − T a
G
− b
T f − T a
2
G
(8.26)
It needs mentioning that the efficiency of an FPC can be expressed by the same
equation as Eq. (8.26) shown above.
8.4.2 Dynamic Modelling of an ETC
In an evacuated tube collector, there are two fluids, heat transfer fluid or refrigerant
and manifold fluid (water is considered in the current modelling). The current model
is based on the assumption that there is no refrigerant present in the ETC, rather,
water is assumed to flow directly through the heat pipes. The major assumptions are
summarized below,
• There is no presence of refrigerant. Water flows directly through the heat pipes.
• The flow of water is only in the positive x direction.
• The heat conduction in the fluid moving direction is neglected.
• The temperature dependence of thermo-physical properties of water is considered.
• Properties of glass and absorber do not depend on temperature and are assumed
to be constant.
• The infrared emissivity of the sky is unity (ε sky = 1).
Based on the stated assumptions, the heat influx to various components of an ETC
is depicted in Fig. 8.6. As shown in the figure, the model (Praene et al. 2005) consists
of 3 thermal nodes. These are, the transparent glass cover, the absorber plate and
the fluid (water) having temperatures T g , T c and T f , respectively. The heat transfer
between the sky and the glass cover is by radiation only. Again, since the absorber
plate is surrounded by the vacuum, heat transfer between the glass cover and absorber
plate, is entirely by radiation. Convective heat transfer is existent between the glass
cover and the ambient environment, and between the absorber plate and water.