Evaluation on year-round performance of double-circulation water-flow window
Graphical abstract
Double-circulation water-flow window combines the functions of solar thermal collector, indoor cooling/heating terminal and sun shading, and realizes the utilization of both solar and geothermal energy within buildings.
Introduction
Windows are attracting worldwide attention for its influence on not only indoor thermal/light environment but also the related energy consumption. In the past decades, extensive studies were carried out relating to high-performance windows [1]. In the field of passive solar design, i.e. to block incident solar radiation and to reduce indoor/outdoor heat transfer through the window, there are multiple types of smart glazing, such as thermochromic [2], electrochromic [3], liquid crystals [4] as well as suspended particles device switchable glass [5]. These high-tech products provide shading and privacy conveniently, whereas the high price still restricts the extensive application. Unconventional window designs such as ventilated window [6], PCM filled window [7], silica aerogel filled window [8] are proven efficient to reduce the air-conditioning load. Yet, the incident solar energy is absorbed and released to the surrounding environment later on without proper utilization.
On the other hand, there are active solar design windows which utilize the incident solar energy in the form of heat or electricity. Semi-transparent PV window converts incident solar energy into electricity, which is a well-known bonus point in green building assessment. However, the expensive PV window usually results in a payback period as long as over 20 years depending on the radiation intensity, and sometimes causes complaints about the obstruction to the window vision. Pipe-embedded window [9] provides cooling/heating through pipes within window cavity, which utilizes low-grade energy source and reduces the energy consumption of air-conditioning system tremendously. There is one drawback which might discourage the building owners from adopting this energy-efficient design, and it is the disturbance on building outlook.
Water-flow window which combines both active and passive solar designs can compensate the above-mentioned disadvantages [10]. It is composed of two parallel glass panes with water flowing in the middle. Positioned in external wall, the window is capable of collecting incident solar energy while reducing indoor cooling load substantially. Additional advantages include the feasible materials and low cost, which encouraged exploring investigations in the past decade [11]. Experiments were carried out to verify the effectiveness of solar energy absorption and indoor cooling load reduction [11]. Numerical model was built up which output results in good accordance with experiment recordings [12]. Since then, validated program was used to predict the environmental and economical superiority of water-flow window over conventional air-sealed window [[13], [14], [15]], to improve and to optimize designs and configurations in order to enhance system efficiency [16,17], to control energy storage and usage pattern [18], to cope with the freezing problem [19] and to utilize warm water in the window cavity for room heating [20]. The results from these studies proved the application potential and research value of water-flow window as high-performance building envelop.
To further improve the energy performance of water-flow window, geothermal energy was proposed to be incorporated for indoor cooling/heating. The performance was verified through experiments carried out in Spain with text boxes [21]. On the other hand, the possibility of enhanced shading by utilizing colored water within water-flow window was brought up [22]. This coincides with the worldwide trend of adaptive building envelop, especially the window part. So far, the optical properties, its influence on indoor thermal/visual environment and the energy performance were investigated [[23], [24], [25]]. By combing these two improvements in single set, water-flow window can serve as solar thermal collector, heating/cooling terminal, transparent envelop and adaptive shading at the same time. This is the topic of present study, i.e. the double-circulation water-flow window with one water circulation aiming at solar energy utilization and the other water circulation providing indoor heating/cooling.
In the work of [26], performance of double-circulation water-flow window, or the so-called “FLUIDGLASS façade”, as part of a district heating system was evaluated with the aid of software TRNSYS and Building Controls Virtual Test Bed (BCVTB). However, the system setting, the window configuration and the simulation methodology was not introduced. Results were given in forms of year-round prime energy saving and CO2 emission reduction. Whereas in real projects, precise prediction on hourly, monthly and year-round performance of the window as shading device, solar collector and indoor cooling/heating terminal is necessary to convince the building owners and green building engineers to adopt this innovative double-circulation water-flow window. Up to date, this important information is still missing and cannot be obtained with existing building simulation software, which hinders the project application.
To summarize, the application potential of double-circulation water-flow window is promising, especially in buildings with large demand of warm water and high density of indoor heating/cooling load. The prediction of its performance under different running modes is worthy of study, which is carried out in the present investigation with self-developed FORTAN program. Comparative cases are set to figuring out system upgrading methodology, in which water flow rate and shading rate are taken into consideration. The results can assist engineers to further boost the performance of green buildings, near-zero energy buildings or up-to-date zero energy buildings.
The structural composition of present paper is as follows: Recent development in high-performance windows and the concept of double-circulation water-flow window is introduced in Part 1. System structure and simulation cases are illustrated in Part 2. The comparative cases are set with different water flow rates and shading rates. In Part 3, detailed numerical model and the development of program is introduced. Year-round performances of comparative simulation cases are presented in aspects of solar collection efficiency, total/beneficial indoor heat gain/loss and geothermal energy utilization in Part 4. The findings from present study are discussed in Part 5 and conclusions are illustrated in Part 6.
Section snippets
System structure
The schematic structure and summer/winter running modes of the double-circulation water-flow window system are shown in Fig. 1 and Fig. 2. There are in total 4 layers of glass panes and two layers of flowing water. The external flowing water layer (f2) and its adjacent glass panes (g1, g3) serve as solar collector, with water circulates between the window cavity and a heat exchanger. The water circulation is close, and is shortened as Cir1 in the following discussion. Solar thermal energy is
Numerical model
The numerical model of the double-circulation water-flow window is built up based on 2 scenarios: the division of incident solar radiation within the window, and the heat balance of each glass pane and flowing water layer. The boundary conditions includ the indoor and outdoor temperature, the flow rate of water circulations, the inlet water temperatures at the entrance of the window cavities. The parameters of concern are the temperature distribution within the window, the related indoor heat
Water temperature variations within the window cavities
The water temperature increments within window cavities are decisive factors in the calculation of solar collection and evaluation of the energy performance. Water temperature variations in both water layers during typical winter week (22nd-28th January) and typical summer week (8th-14th July) of Case 1 are presented in Fig. 4.
In Fig. 4(a), Tf2-in and Tf2-out represent the water temperatures at the inlet and outlet of external water layer f2 in Cir1. Tf2-in is preset the same as ambient
Solar energy utilization
By actively collecting solar thermal energy and passively reducing the unfavorable solar-related indoor heat gain, double-circulation water-flow window helps building energy conservation as high-performance building envelop. Throughout the year, the water-flow window collects 12.77% of the incident solar energy with 0.0025 kg/s water flow rate in Cir1. This efficiency is much higher than the research of [12], in which the annual solar efficiency of 8.22% is calculated under Hong Kong (which is
Conclusions
The present investigation focuses on double--circulation water-flow window, which realizes the combined utilization of renewable solar energy and geothermal energy in single transparent building envelop. This design provides an particular method to cope with the global energy shortage and environmental pollution related to power generation. With self-developed FORTRAN program, the numerical simulation is carried out to predict and evaluate the energy performance of this innovative window under
Declaration of competing interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Evaluation on year-round performance of double-circulation water-flow window”.
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