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

7 Conclusions 
Thermal, energy and economic performances of double- 
circulation water-flow window under the climate of Shenzhen 
are analyzed numerically in the current study. The energy 
savings from both solar energy utilization for water heating 
and indoor heat gain increment/reduction in heating/cooling 
seasons are taken into consideration. By subtracting the extra 
energy consumption caused by the pumps and heating device 
within the system, the annual electricity charge saving and 
the static payback period are predicted. The results show 
promising application potential of water- flow window in 
buildings with strong incident solar radiation and stable hot 
water demand. The main conclusions are: 
(1) The direct solar transmission is largely reduced with 
double-circulation water-flow window, compared with 
common curtain wall. The comprehensive solar transmissivity 
is reduced from 30.6% to 16.0% during cooling season. 
(2) Solar energy can be absorbed by the flowing water 
and contributes to energy saving of water heating device. 
The thermal collection efficiency is about 16.2% for the 
external water circulation (Cir1) and 4.3% for the internal 
water circulation (Cir2). 
(3) With the assumption of extra investment of 1500 CNY 
for this 9 m
2
of water-flow window compared with common 
curtain wall, the static payback period is predicted to be 
around 7 years. The payback period is influenced by the 
water temperature at the inlet of window cavity. 
In conclusion, the double-circulation water-flow window 
under investigation is able to reduce building energy con-
sumption compared with common curtain wall. The related 
extra investment can be earned back with electricity charge 
saving. After around 7 years, the system can realize over 
20 CNY/m
2
saving annually. Considering the continuous 
payback and convenient maintenance of the system, there 
is a great potential for its extensive application. 
In the present investigation, the lower supply water 
temperature at the inlet of window cavity promotes larger 
amount of electricity saving, which provides possibility of 
utilizing low-grade heat source in buildings, such as solar 
thermal collector and ground source heat pump (GSHP). 
Meanwhile, the indoor heat gain through the window can be 
regulated according to occupants’ demand by adjusting the 
inlet temperature and flow rate of water within the window 
cavity.