Experimental study on double pipe PCM floor heating system under different operation strategies
Introduction
With the rapid increase of energy consumption and greenhouse gas emissions, environmental problems have currently attracted widespread attention [1]. In China, carbon emissions caused by rural coal burning are greatly underestimated [2,3], an important factor that causes severe haze in northern China winters [2]. The Chinese government has proposed replacing rural bulk coal combustion with clean energy [4]. However, some clean energy sources can only be used at a specific time. For instance, solar energy can only be used during the day, while off-peak electricity can only be used at night. The mismatch between energy supply and demand thus limits the promotion of clean energy in rural areas.
The application of phase change materials (PCM) in buildings is an effective solution to solve the mismatch between energy supply and user load demand [5,6]. Due to its advantage of high energy storage density and near isothermal storage process [[7], [8], [9], [10]], researchers have incorporated PCM into various building components, such as walls [[11], [12], [13], [14]], ceilings [[15], [16], [17], [18]], roofs [19,20], windows [21,22], and floors [23,24]. Unlike other heating systems, the underfloor heating system can distribute heat more evenly, conducive to the usage of low-grade energy such as solar hot water [25,26]. Previous studies have confirmed that floor heating systems containing PCM can store more heat and provide smaller surface temperature oscillations [24,27,28]. Consequently, various studies have focused on the PCM floor in space heating.
In previous studies, researchers most adopted the single operation strategy, such as nocturnal and daytime energy storage. Due to considerable economic benefits of using off-peak electricity, the nocturnal energy storage strategy has been widely applied [[29], [30], [31], 32, [33], [34], [35], [36]]. Ye et al. [29,30], Lin et al. [31,32], and Cheng et al. [33] integrated shape-stabilized PCM (SSPCM) into the floor heating system to store heat from electric heating film at night and release heat during the day. Yamaguchi et al. [34] and Mazo et al. [35] prepared hot water by heat pump at night for PCM floor space heating. And in the study by Li et al. [36], the investment could be recovered in 3.2 years, using off-peak electricity 7 h per day. Additionally, Huang et al. [37] adopted a daytime heat storage strategy, and 47.7% of solar energy was stored and released by PCM. Lu et al. [38] numerically and experimentally studied the coupling effect of PCM floor and solar collectors. The results showed that the experimental buildings with PCM floor can save 5.87% on the energy consumption when the room temperature was maintained at 20 °C. Among these studies [[29], [30], [31], 32, [33], [34], [35], [36], [37], [38]], the adoption of single operation strategies can gain economic benefits and energy conservation. However, due to the changes in outdoor climate and energy policies, the single operation strategy was not flexible enough to guarantee long-term operation effect. For instance, when solar energy is insufficient, it is unrealistic to rely solely on solar collectors for heating.
Besides, several researchers have applied the automatic control method in PCM floor heating systems. Barzin et al. [39] proposed a control method based on power price control. In the five-day experiment, a total energy saving and cost saving were 18.8% and 28.7% respectively. In his extended research [40], the application of weather forecast combined with price control was experimentally studied. Results revealed that the energy saving effect was remarkable, reaching up to 90% in one day. And Paul Devaux et al. [41] used the EnergyPlus software to simulate the operating condition of the PCM floor under an automatic control method. The above studies show that the application of automatic control methods have achieved good results in energy conservation and cost saving. However, the effectiveness of the control method depends on accurate weather forecasts [40]. Automatic control methods combined with inaccurate weather forecasts can even lead to the increase of energy consumption and cost [40]. And sometimes the indoor temperature fluctuated violently, exceeding 30 °C [39]. In such a scenario, the current automatic control methods may not be reliable enough to ensure the operation effect.
It can be concluded from above literature reviews of PCM floor heating systems that there remain some limitations on single operation strategy and automatic control method. And few researchers consider adopting multiple operation strategies to cope with changes in working conditions. Besides, in previous studies, the PCM floor had only one heating module, which was not conducive to fine operation. Thus, this paper has proposed a double pipe PCM floor heating system with three independent heating modules. And the operation effect of the system under four different strategies has been studied through experiments.
Section snippets
System principle
As shown in Fig. 1, unlike previous studies, the double pipe PCM floor consists of three parallel heating loops. The end of the loop was connected to the water separator and the water collector. Depending on the operating strategy, hot water can flow through different loops at different times. Additionally, the PCM can store and release heat at different times, providing feasibility adjustment of the system. In this experiment, Module 1 was close to the inner wall and the Module 3 was close to
Operation effect of the system under strategy 1
In the early stage of heating, Strategy 1 was implemented for five days Fig. 9 shows the variation of the outdoor dry-bulb temperature, Room 1 (reference room) temperature, Room 2 (test room) temperature, Room 2 floor surface temperature, and the floor surface heat flux density. Table 5, Table 6 outline the specific data of the above parameters. From Fig. 9, the outdoor temperature is observed to fluctuated around 0 °C, with Room 1 temperature continuing to drop as Room 2 temperature stays
Conclusions
This study proposed a double pipe PCM floor heating system with three independent heating modules. A 26-day experiment was conducted to study the operation effect of the system under four running strategies. The primary conclusions can be drawn as follows:
- (1)
In the early stage of heating, the average temperature of the experimental room was 18.9 °C, and the daily temperature fluctuation was controlled within 2.1 °C, meaning that the heating equipment running at partial power could achieve indoor
Acknowledgements
This study was supported by “National Key R&D Program of China” (Grant No. 2018YFD1100700).
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