   According to the law of conservation of energy per unit time useful energy collector output is equal to the collector receives solar radiation energy minus the collector energy loss to the surrounding environment, i.e.:
Qu=Qs―QL (6．
QL - Qu=Qs (6

In the formula, useful energy, collector output per unit time Qu for W; 9s for the same period in the solar radiation energy, the heat receiving W; QL for the same period the collector energy loss to the surrounding environment, W.

The mathematical expressions of Qs and QL (6.24) can be obtained;
Qu=ApG(ra)e―ApUL(Tp―Ta) (6．
Qu=ApG (RA) e - ApUL (Tp - Ta) (6)

In the formula, Ap is the heat absorbing plate area, m2; G for solar irradiance, W / m2; (RA) e is a transparent cover layer and a heat absorbing plate absorption transmittance effective product than dimensionless; UL collector total heat loss coefficient, W / Tp (m2.K); the average temperature heat absorbing plate Ta, environmental temperature, K.

The collector efficiency can be defined as the useful power of the collector output and the ratio of the solar irradiance to the collector,

Namely: = Qu/GAp (6)

The instantaneous efficiency equation of the heat pipe vacuum tube collector of the solar water heater can be obtained by the formula (6.25) generation (6.26):
?=(ra)e―UL・Tp-Ta/G (6．
= (RA) e - UL (Tp-Ta/G) (6)
2．真空管集热器的总热损失系数
Total heat loss coefficient of 2 vacuum tube collector

The total heat loss coefficient of the UL heat pipe type vacuum tube heat collector, vacuum tube heat loss coefficient is U, and the thermal insulation box and the heat loss coefficient of Ub.

That is: UL=Ut+Ub (6

In the formula, Ut vacuum tube heat loss coefficient, W / (m2.K); Ub insulation box heat loss coefficient, W / (m2.K).

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