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

6 System energy saving potential and pay-back period 
Compared with common double-layer curtain wall, water- 
flow window can achieve energy saving through solar energy 
utilization and indoor heating/cooling load reduction. For 
economic analysis purpose, the corresponding electricity 
charge saving is calculated with energy saving by water-flow 
window and the corresponding electricity consumption by 
pumps and heat source device, as listed in Table 4. 
In Table 4, the annual solar thermal collections of this
9 m
2
water-flow window are over 2100 kWh for Cases 2–4. 
Taking electricity-to-heat energy conversion efficiency of 
water heating device as 450%, the corresponding electricity 
savings (shortened as ES1) of water heating device are 
468.8 kWh, 470.9 kWh and 473.0 kWh for Cases 2, 3 and
4, respectively. Meanwhile, the electricity saving of air- 
conditioning system caused by indoor heat gain increment 
during heating season (ES2) and indoor heat gain reduction 
during cooling season (ES3) is calculated. With Case 1 as 
benchmark, the corresponding electricity savings from 
air-conditioning load reduction (ES2+ES3) are 132.6 kWh, 
162.4 kWh and 192.2 kWh for Cases 2, 3 and 4, with COP 
of air-conditioning system to be 3.5 for cooling and 4.5 for 
heating. Overall speaking, the double-circulation water-flow 
window (Cases 2–4) help to reduce electricity consumption 
by 601.4 kWh/year, 633.4 kWh/year and 665.3 kWh/year, 
respectively.
It should be noted that the electricity saving is achieved 
at the cost of extra electricity consumption of the circulation 
pumps in both circulations, and the heating device in Cir2.
Table 4
Energy-saving potential of double-circulation water-flow window cases (9 m
2
) 
Case No. 
Case 2 
Case 3 
Case 4 
Total solar thermal collection (Cir1+Cir2, kWh) 
2109.8 
2119.3 
2128.7 
Electricity saving of water heating device (ES1, kWh) 
468.8 
470.9 
473.0 
Indoor heat gain increment (heating season, compared with Case 1, kWh) 
178.7 
313.2 
447.2 
Indoor heat gain reduction (cooling season, compared with Case 1, kWh) 
325.0 
325.0 
325.0 
Electricity consumption reduction of AC system(ES2, heating season and compared with Case 1, kWh) 
39.7 
69.6 
99.4 
Electricity saving of AC system(ES3, cooling season and compared with Case 1, kWh) 
92.8 
92.8 
92.8 
Comprehensive electricity saving by water-flow window (ES=ES1+ES2+ES3, kWh) 
601.4 
633.4 
665.3 
Total water flow amount within Cir1 (m
3
) 236.5 
236.5 
236.5 
Total water flow amount within Cir2 (m
3
) 236.5 
236.5 
236.5 
Total electricity consumption of circulation pump (EC1, kWh) 
49.6 
49.6 
49.6 
Heat release of water within Cir2 (heating season, kWh) 
512.4 
667.5 
834.4 
Corresponding heat source electricity consumption (EC2, kWh) 
113.9 
148.3 
185.4 
Comprehensive electricity consumption by water-flow window (EC=EC1+EC2, kWh) 
163.5 
197.9 
235.0 
Net electricity saving by water-flow window (NES=ES−EC, kWh) 
305.4 
273.0 
238.0 

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Li et al. / Building Simulation / Vol. 13, No. 3 
583
The corresponding electricity consumptions (shortened as 
EC) are 163.5 kWh, 197.9 kWh and 235.0 kWh for Cases 2, 
3 and 4. The net electricity savings by double-circulation 
water-flow window are thus 305.4 kWh, 273.0 kWh and 
238.0 kWh for Cases 2, 3 and 4. The net electricity saving 
of Case 4 is the lowest, given the highest inlet water tem-
perature at internal window cavity (f5) in heating season. 
The performance is adversely affected since higher water 
temperature means more heat loss to ambient. This enhanced 
water heat gain within the external window cavity (f2). This 
explanation is confirmed by the higher system thermal 
collection of Cir1 in Case 4 in heating season, as shown
in Fig. 6. 
With electricity price of 0.78 CNY/kWh for commercial 
buildings in Shenzhen, the annual electricity charge savings 
are 238.2 CNY, 213.0 CNY and 185.7 CNY for Cases 2, 3 and 
4, respectively. As for the extra investment, the glass panes, 
pumps, water tank, heating device and pipework should all 
be taken into consideration. Considering the Low-e glass is 
much more expensive than common clear glass, the overall 
cost of the 4 layers of clear glass panes within double- 
circulation water-flow window is considered to be close to 
the curtain wall in Case 1, which is composed of 1 layer of 
clear glass and 1 layer of Low-e glass. Therefore, the extra 
first investment is predicted to be 1500 CNY for this 9 m
2
double-circulation water-flow window. It is intriguing that 
the extra cost is not from the window itself, but from the 
attached pipework, pumps and so on. In the real application, 
this part of investment can be quite flexible. The building 
scale, window size and hot water demand can all influence 
the system configuration and its investment. Based on 
these assumptions, the static payback period of water-flow 
window is 6.3 years for Case 2, 7.0 years for Case 3 and 8.1 
years for Case 4.