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

Fig. 6 
Solar thermal collection of double-circulation water-flow window system (kWh) 
Table 3 
Monthly solar thermal collection efficiency of Cir1 and Cir2 (%) 
Solar thermal collection efficiency of Cir1 (%) 
Solar thermal collection efficiency of Cir2 (%) 
Month 
Case 2 
Case 3 
Case 4 
Case 2 
Case 3 
Case 4 
January 17.7 
18.0 
18.3 
- 
- 
- 
February 20.2 
20.6 
21.0 
- 
- 
- 
March 15.5 
15.5 
15.5 7.8 7.8 7.8 
April 15.1 
15.1 
15.1 
4.0 
4.0 
4.0 
May 14.8 
14.8 
14.8 
3.0 
3.0 
3.0 
June 15.0 
15.0 
15.0 
1.6 
1.6 
1.6 
July 13.7 
13.7 
13.7 
0.9 
0.9 
0.9 
August 15.1 
15.1 
15.1 
1.3 
1.3 
1.3 
September 14.1 
14.1 
14.1 
2.3 
2.3 
2.3 
October 15.6 
15.6 
15.6 
11.5 
11.5 
11.5 
November 16.3 
16.3 
16.3 17.2 17.2 17.2 
December 17.6 
17.8 
18.1 
- 
- 
- 
Year-round efficiency (%) 
16.2 
16.2 
16.3 
4.3 
4.3 
4.3 

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Li et al. / Building Simulation / Vol. 13, No. 3 
582 
With the proposed double-circulation water-flow window, 
around 185–187 kWh solar energy is absorbed and utilized 
for water pre-heating within Cir1, out of the total 1144.2 kWh 
incident solar radiation on every 1 m
2
of south-oriented 
window. The year-round solar thermal efficiency of Cir1 is 
as high as 16.2%–16.3%. The system efficiency is slightly 
higher in winter than in summer, as shown in Table 3.
For example, the efficiency is 20.2% in February and 14.1% 
in September. This is caused by the lower inlet water 
temperature at window cavity and higher incident solar 
radiation level in winter, both are favorable for solar energy 
absorption and utilization.
The amount of solar thermal collection of Cir2 is much 
smaller compared with Cir1. The overall water heat gain is 
49.6 kWh for Cases 2, 3 and 4. The performance of these 3 
cases are the same, since the incident solar radiation, 
indoor and ambient environment, as well as inlet water 
temperatures in both circulations from May to November 
are of the same magnitude. The comprehensive year-round 
system thermal efficiency is 4.3%. Similar to Cir1, the system 
performance is better in March and November, when
the inlet water temperature, which is the same as ambient 
temperature, is lower. 
The overall system thermal efficiency is in the range of 
20.5%–20.6% for Cases 2–4. Thermal performance of Case 4 
is slightly better compared with Cases 2 and 3. The reason 
is that the temperature of the adjacent glass pane g3 during 
heating season is slightly higher in Case 4, due to the higher 
inlet water temperature of internal window cavity f5. The 
higher temperature of g3 resulted in less heat loss, which
is favorable for solar energy utilization. For example, with
the inlet water temperature at internal window cavity of
34 °C in Case 2, 37 °C in Case 3 and 40 °C in Case 4, the 
corresponding solar collection efficiencies of Cir1 are 17.7%, 
18.0% and 18.3% respectively.