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

Keywords
 
double-circulation water-flow window, 
building energy saving, 
room heating terminal, 
solar collection, 
static payback period 
Article History
 
Received: 25 August 2019 
Revised: 06 November 2019 
Accepted: 18 November 2019 
© Tsinghua University Press and 
Springer-Verlag GmbH Germany, 
part of Springer Nature 2020 
1 Introduction 
Energy conservation in buildings is gaining more and more 
attention worldwide for the purpose of fossil fuel consumption 
reduction and relevant environment pollution prevention 
(Qian et al. 2019). To reduce heating/cooling load, energy- 
efficient envelope structures were widely investigated and 
applied to both commercial and domestic buildings in the 
past decades (Cha et al. 2014; Lenin et al. 2019). Renewable 
energy systems and high-performance devices further cut 
the building energy consumptions (Liu et al. 2018b; Wu 
and Skye 2018). And currently, it is a trend to collaborate 
different energy-saving measures in indoor environment 
regulation (Lyu et al. 2020; Pandya et al. 2019).
Renewable energy, including solar energy, geo-thermal 
energy and wind power are recommended in building energy 
conservation (Qu et al. 2019; Saaly et al. 2019). Solar energy 
has been used for room heating for a long period. Passive 
heating with solar radiation does not consume additional 
power and causes no pollution. Elaborately designed solar 
house can take full advantage of incident solar radiation 
(Liu et al. 2018a). Active solar heating utilizes water or air 
as heating media to provide thermal energy in cooling 
seasons (Lyu et al. 2017). In recent years, solar water heating 
system has been applied extensively in China (Su et al. 2018; 
Wang et al. 2015). Solar collectors are normally located on 
the rooftop or attached to building envelope, and the most 
popular collector forms include flat-plate, heat pipe and 
B
UILD 
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(2020) 13: 575–584
https://doi.org/10.1007/s12273-019-0600-y 

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Li et al. / Building Simulation / Vol. 13, No. 3 
576 
evacuated tubular (Guo et al. 2018). Building-integrated 
solar collectors are more attracting for the advantages of 
space saving and better aesthetic outlook, which leads to 
extensive research in this field and the initial proposal of 
solar-absorbing water-flow window in 2011 (Chow et al. 
2011; Maurer et al. 2017).
Water-flow window is a novel building-integrated solar 
thermal system, it uses two parallel glass panes as absorption 
plates of solar thermal collector with water flowing in the 
middle. Positioned in external wall, the window collector is 
capable of reducing indoor cooling load substantially (Li 
2012; Li et al. 2019). It is a combination of both passive and 
active solar heating/cooling, which is favorable for maintaining 
suitable indoor thermal environment at low energy cost. As 
a multi-functional transparent envelop, water-flow window 
was proven to be effective for both solar collection and 
indoor cooling load reduction. Experiments were carried out 
to verify its performance (Chow and Li 2013). During the 
experimental period in August, the daily thermal efficiency 
was found ranged from 5.9% to 12.4% with natural water 
circulation in the heat exchanger and window cavity.
This means that for 1000 J incident solar radiation on the 
glazing surface, around 59 J to 124 J of the incoming solar 
thermal energy can be stored in the form of hot water for 
building use.
Numerical simulation was proven to be effective in the 
investigation of various building energy saving techniques, 
and the same approach was applied in the research of 
water-flow window. Validated code was utilized to optimize 
system configurations for higher efficiency (Chow and Lyu 
2017b). Comprehensive simulation carried out in ESP-r 
showed a year-round AC system load reduction of 7.8% if 
the air-gapped double glazing was replaced by water-flow 
window under subtropical climate (Chow et al. 2012). 
PCMs-filled heat exchanger was proposed to control energy 
storage to better cope with different usage pattern (Chow 
and Lyu 2017a) and antifreeze was added in experimental 
research to cope with freezing prevention requirement, 
which extends the scope of water-flow window application 
to various climate regions (Lyu et al. 2018). Energy-saving 
performance of water-flow window in Madrid, Spain was 
also studied (Gil-Lopez and Gimenez-Molina 2013). Results 
showed that the energy consumed for space heating and 
cooling could be reduced by as large as 18.26% for room 
equipped with water-flow window. The research proved 
great potential of extensive application of water-flow window 
from both environmental and economic aspects.
The possibility of utilizing water-flow window as room 
heating terminal with warm water flowing within window 
cavity was proposed, which made it more effective and 
controllable in indoor thermal environment regulation (del 
Ama Gonzalo et al. 2017). Low-grade geo-thermal energy 
was recommended as an economic and environmental- 
friendly energy source for warm water generation. Previously, 
radiant heating through floor, ceiling and wall were considered 
as energy-efficient and thermal-comfort way of indoor 
environment regulation (Lin et al. 2016; Yang, et al. 2019). 
It is even more attractive to use water-flow window as means 
of room heating. The main advantages include better 
utilization of solar energy, less initial investment and more 
convenient to management and maintenance. Numerical 
simulation was carried out to confirm that indoor radiant 
heating with water-flow window could improve the overall 
thermal comfort of indoor environment. The investigation 
focused on indoor heat loss reduction during heating season 
through supplying of warm water at different temperatures 
(Lyu et al. 2019).
In summary, the effectiveness and economic potential 
of water-flow window in solar energy utilization, as high- 
efficiency room heating terminal and energy-efficient building 
fenestration were proved through experimental and numerical 
investigations, whilst there are still some realistic problems 
need to be solved. For instance, the possibility of heat loss 
from warm flowing water to cold environment through glass 
pane exists and may cause energy waste. Meanwhile, it is 
necessary to carry out energy and economic analysis based 
on year-round performance from the aspect of practical 
application. Under such circumstances, a systematic and 
convertible numerical model for improved configuration 
would favor future study and project application at large 
scale. 
Accordingly, the object of the present investigation is a 
compact double-circulation water-flow window composed 
of 2 water flowing circuits, with a sealed air layer in the 
middle of the window for better thermal insulation. 
Numerical model of this innovative building component is 
built up and used to predict the annual thermal and economic 
performance. The model may later be inserted into various 
building simulation tools as a universal module. Following 
the present instruction, the system configuration and operation 
mechanism is presented in Section 2, with characteristics
of the proposed system described in Section 3. Numerical 
model setup is introduced in Section 4 and system 
performance is presented in Section 5 from the aspects of 
solar collection and indoor heating/cooling load reduction. 
System energy saving potential and pay-back period prediction 
is in Section 6 and conclusions are pointed out in Section 7.