Performance of double-circulation water-flow window system as solar collector and indoor heating terminal Chunying LI 1

part can be calculated with convective/radiation heat transfer 
coefficient and temperature difference between the inner 
glass pane (g6 in Fig. 2) and indoor air. 
5 Results 
and 
discussions 
Thermal performance and energy saving potential of 
double-circulation water-flow window systems (Cases 2–4) 
are analyzed and compared with common double-pane 
curtain wall (Case 1). Indoor heat gain through the window, 
solar thermal collection and system thermal efficiency are 
evaluated.
5.1 Indoor heat gain through the window 
The proposed double-circulation water-flow window 
regulates indoor thermal environment by adjusting the 
indoor heat gain through the window. As mentioned above, 
the comprehensive indoor heat gain through the window 
is composed of direct solar transmission and convective/ 
radiative heat gains. Year-round thermal performance of 
the 4 comparative cases is shown in Figs. 4 and 5. 
Thermal performance of the advanced water-flow 
window during heating season is presented in Fig. 4. The 
accumulated incident solar radiation intensity on the 
south- facing vertical curtain wall during heating season is 
365.9 kWh/m
2
, and 125.7 kWh/m
2
is transmitted inward to 
indoor space directly in Case 1. The overall transmissivity 
is 34.4%. Whereas the direct solar transmission is only 
72.5 kWh/m
2
for water-flow window (Cases 2–4), which
is 42.3% lower compared with Case 1. This is caused by 
thermal insulation of the additional glass panes and the 
thermal extraction of the flowing water layers. The inlet 
water temperate is preset to be 34 °C, 37 °C and 40 °C in 
Cases 2, 3 and 4, so there is heat release from water stream 
to indoor air through the internal glass pane. Considering 
this part of heat transfer, the comprehensive indoor heat