As a burgeoning mode of heating, the electrically-heated floor is rising and accepted by people (Tong, 2005). With the increasing requirement on the living environment, the electrically-heated floor becomes major floor material in low-temperature geothermal heating system (Wang et al., 2016). Li et al. (2004) studied the heat transfer characteristics of an electric floor heating radiant panel using a combined electric radiant panel. It was found that the structural material and physical parameters of the radiant panel have an important influence on the surface temperature distribution and heating energy consumption. Li et al. (2007) studied the comparative experiment of phase change material applied to the electric heating floor heating system and the common electric heating floor heating system, and obtained the conclusion that the heat transfer performance of the phase change system is good; Chen (2014) studied the influence of different phase change materials' physical parameters and operating conditions on the performance of electric heating floor radiant heating system through the combination of experimental research and numerical simulation. It was concluded that electric heating floor radiant heating system can not only extend the life of electrothermal film can also achieve the thermal comfort and economical energy-saving effect of building heating.

This paper simulates the environmental conditions of electric heating, and tests and analyzes the temperature rise contrast variation of ordinary electric heating floor and RPIB, measures the temperature change rate of the two kinds of floorboards, and analyzes the different thermal conductivity insulation boards to solid wood composites. The effect of electric heating floor insulation performance and energy saving performance. Through thermodynamic analysis, a thermodynamic model of RPIB was established, and the influence of different surface decorative panel materials on the insulation efficiency of RPIB was studied.

1 Materials and methods 1.1 Materials of the floor

The test materials in this paper are provided by Harbin Huayi Wood Industry Limited Company. The structure of a common electrically-heated floor is a solid wood floor layer, an electrically-heated layer and a heat insulation layer from top to bottom(Cheng, 2007; Si, 2004). The test replaces the regular foam board of electrically-heated floor by rigid polyurethane insulation board, and the structure of two floors are shown in Fig. 1.

The common electrically-heated floor is composed of 8 mm-thick floor layers, plywood with phase change layer and heat layer and 4 mm thick insulation board (Meng et al., 2011). The solid wood composite polyurethane insulation board floor is composed of an 8 mm-thick decorative panel, a plywood board with a phase change layer and a heat layer, and a 4 mm-thick rigid polyurethane insulation board. The size of the two floors are both 950 mm×150 mm×15 mm.

The decorative floor is made of oak. Carbon fiber electrothermal cable covered with insulating plastic is used as electrothermal material. The electrode is connected by coating the conductive paste and the metal foil on the two sides of the electrothermal material, and it is connected to the external power supplied at both ends of the floor, in order to accomplish the electric heating of the floor(Yuan, 2015). Phase change material uses paraffin with good energy storage function and high density mixed polyethylene to prepare for a phase change material(PCM)suitable for thermal storage. Thus, the use of cheap heating at night enables PCM to melt and absorb heat, and the floor power is cut off during the day so that the PCM can be exothermic to heat indoors. The insulation layer adopts the rigid polyurethane insulation board, it makes use of the rigid polyurethane's advantage such as small thermal conductivity and good insulation properties, enhancing the heating and insulation effect of electrically-heated floor(Gao et al., 2001; Lu et al., 2005).

1.2 Thermal insulation test

In order to test the thermal insulation performance of RPIB, the temperature rise test of common electrically-heated floor and RPIB was compared and the difference of thermal insulation performance between them was studied. The experimental time is in the heating season of Harbin City. The dimension of the lab is 8 m×6 m×3 m. The Taiwan TES-1310 thermocouple contact type temperature measuring instrument was chosen to measure the temperature range on floor surface and bottom per unit time.

Room temperature is chosen at(20±2)℃ during the public heating, and the humidity in room is 22%±5%, and we use a humidifier to maintain the humidity stable after the electric heating(Yu et al., 2011). The common electrically-heated floor and the RPIB are respectively assembled with three pieces into a group, and the two groups are tiled on both sides of the laboratory (Liang, 2010). Measure the temperature of the surface and bottom of the middle panel in the two groups of the floor as the initial temperature. Then energize the floor and set the electric heating layer to 30 ℃. Measure the center floor temperature of the two groups every 5 min(minimize the loss of heat from both sides of the floor to ensure the accuracy of the results) and keep records until the last measurement is close enough with the previous one. The diagram of thermal insulation test is shown in Fig. 2.

1.3 Energy saving function text

In order to find out the energy saving function of the RPIB which has the rapid thermal conductivity, reliable heat storage, thermal insulation and safety. Make the common electrically-heated floor and solid wood composite rigid polyurethane insulation board into 2 independent cells with dimensions of 150 mm× 150 mm×150 mm. Place them in a room with 20 ℃ room temperature(Seo et al., 2011). The two models involve two same temperature measuring probes. Agilent 34970A data collector was used to monitor the temperature of two models in real time. We electrified the floor and set the electrically- heated layer at 50 ℃, rise the air temperature in the model to 40 ℃ and maintain 10 min, then cut off the electric and monitor the temperature range in two models at the same time. Then recorded the temperature every 10 min until the air temperature in the model drops to 20 ℃.

1.4 Insulation efficiency text of decorative panel material on floor

Considering the different kinds of wood and the unsteady parameters in the process of making floor, a series of basic assumptions must be carried out on some experimental conditions:

1) The wood used in the decorative panel is uniform, standard and ideal material with equal isotropy(Li et al., 2002).

2) The phase change layer and the electrically-heated layer of the two electrically-heated floors have the same performance and no difference.

3) The floor is firmly bonded between the floors, and the floor is identical with other floor on the processing technology(Li et al., 2004).

Take two pieces of the same electrically-heated floor and replace the decorative panel material of one of the floors with poplar. Heat the floor with two different decorative panel materials at room temperature, set heating temperature of the electrically-heated layer at 30 ℃ and measure the temperature of the surface every 5 min. The measurement stops after 30 min from the beginning(Kim et al., 2005).

1.5 Heat transfer analysis of phase change floor

The heat transfer process of phase change floor includes heat storage process and heat release process. Electrical heating completes the heat storage process. The phase change floor absorbs heat of q₀ when it is stored. The heat storage rate is q_c=q₀－q_inf－q_ind. When the phase change material is exothermic, heat exchange occurs in the room, and the exothermic rate is:

$ {q_{\rm{f}}} = {q_{\inf }} + {q_{{\rm{ind}}}}. $

(1)

In formula: q_inf is radiant heat transfer; q_ind is convection heat transfer.

This problem can be regarded as a dimensional phase change heat transfer of the floor and the electric heating layer. The whole area can be expressed in a unified form of governing equations:

$ \rho \frac{{\partial H}}{{\partial \tau }} = k\frac{{{\partial ^2}t}}{{\partial {x^2}}}. $

(2)

In formula: ρ is material density; k is material heat transfer coefficient(Li et al., 2011).

In this paper: ρ=550 kg·m^-3, k= 0.017 W·m^-1K^-1.

Boundary condition:

$ {q_{\rm{t}}} + \lambda \left( {{t_0} - {t_1}} \right)\left| {_{x = {X_1}}} \right. = k\frac{{\partial t}}{{\partial x}}\left| {_{x = {X_1}}} \right.; $

(3)

$ {p_0} = - k\frac{{\partial t}}{{\partial x}}\left| {_{x = 0}} \right.. $

(4)

In formula: q_t is the radiant flux on the surface of the floor(W·m^-2); p₀ is electric heating power of electric heating layer(W·m^-2), when heating stops, p₀=0; λ is convective heat transfer coefficient of floor surface and indoor air(W·m^-2℃^-1); t₀ is indoor air temperature(K); t₁ is upper surface temperature of indoor floor(K); X₁ is heat transfer thickness of floor.

After the relevant parameters are taken into account, the radiant heat transfer rate of the unit area floor q_inf is:

$ {q_{{\rm{inf}}}} = 5.67\left[ {{{\left( {\frac{{{t_{\rm{f}}} + 273}}{{100}}} \right)}^4} - {{\left( {\frac{{{t_{{\rm{fw}}}} + 273}}{{100}}} \right)}^4}} \right]. $

(5)

In formula: t_fw is average temperature of non-radiation structural surface, take 10 ℃(Wang et al., 2016).

The convective heat transfer rate per unit area of the floor q_ind:

$ {q_{{\rm{ind}}}} = 1.78{\left( {{t_1} - {t_0}} \right)^{1.32}}. $

(6)

The heat transfer of phase change floor consists of two parts: convection heat transfer and radiation heat transfer. The two kinds of heat transfer are determined by the 2 equations:

$ {Q_{{\rm{ind}}}} = 2.17{\left( {{T_{\rm{p}}} - {T_{{\rm{sj}}}}} \right)^{1.31}}; $

(7)

$ {Q_{{\rm{inf}}}} = 4.98\left[ {{{\left( {\frac{{{T_{\rm{p}}}}}{{100}}} \right)}^4} - {{\left( {\frac{{{T_{{\rm{fp}}}}}}{{100}}} \right)}^4}} \right]. $

(8)

In formula: Q_ind is convective heat transfer of phase change floor(W·m^-2); Q_inf is radiant heat exchange in phase change floor(W·m^-2); T_sj is design temperature of heating room(℃); T_p is average temperature of floor radiant surface(K); T_fp is average temperature of non-heating surface of floor(K)(Yang et al., 2007).

By formula(1)we can see that the parameters affecting the floor heat transfer are T_p and T_fp.T_sj is the design parameters and can be calculated as a known value. Therefore, the parameters affecting the floor heat transfer are only T_p:

$ {T_{{\rm{fp}}}} = {T_{{\rm{sj}}}} - 1.1. $

(9)

In order to facilitate calculation, the radiative heat exchange is converted into convective heat transfer by means of the conversion theory of local resistance equivalent length, and the fitting formula is obtained(Wang, 1983):

$ Q = 4.298{\left( {{T_{\rm{p}}} - {T_{{\rm{sj}}}}} \right)^{1.31}}. $

(10)

2 Results 2.1 Thermal insulation test result

The measurements of thermal insulation test are shown in Tab. 1.

According to the results, the temperature decreasing percentage of the RPIB compared with the common electrically-heated floor can be easily figured out, and the percentage curve of thermal insulation performance increases over time can be worked out. As shown in Fig. 3.

It can be seen from the experimental results, in the unit time, the floor temperature of the solid wood composite rigid polyurethane insulation board is slower than that of the common electrically-heated floor. After 35 min, the difference of bottom temperature between the two groups reached 2 ℃. The experiment shows that when we adopt the rigid polyurethane insulation board, the heat radiation performance of the electrically-heated floor is obviously improved and the resistance to downward heat dissipation performance is significantly enhanced, and comprehensive insulation performance compared to common electrically-heated floor is more outstanding.

2.2 Energy saving function text result

According to the energy saving function text, the temperature and the stop time was record and the temperature curve was drawn. As shown in Fig. 4.

It can be seen from the experimental results that the temperature of ordinary electrically-heated floor change from 40 ℃ to 20 ℃ which costs 37 min, and the RPIB costs 55 min. It shows that the low thermal conductivity of rigid polyurethane is more excellent in delaying heat transfer. Therefore, the RPIB has longer insulation time and better effect. At the same time, it can reduce the fluctuation of indoor temperature and improve the comfort of the environment. On the basis of the electrically-heated floor which use night trough storage energy, the energy through heating can be cut off during the day(Xue et al., 2000; Wang et al., 2014). Long term insulation performance virtually reduces the power consumption of the indoor air conditioning and other heating equipment, so that energy can be fully utilized and the burden of energy consumption on the natural environment can be reduced.

2.3 Result of insulation efficiency text of decorative panel material on floor

The temperature of oak decorative panel and poplar decorative panel was recorded due to the experimental. As the results shown in Tab. 2.

3 Discussion

Trough the heat transfer analysis of phase change floor, insulation efficiency of decorative panel material on floor can be found out.

In the experiment, the room temperature is 20 ℃, and after the calculation of formula (5)-(8) we can get:

Radiative heat transfer rate of oak surface:

$ {q_{{\rm{inf}}}} = 5.67\left[ {{{\left( {\frac{{24.3 + 273}}{{100}}} \right)}^4} + {{\left( {\frac{{10 + 273}}{{100}}} \right)}^4}} \right] = 79.27. $

Convective heat transfer rate of oak surface:

$ {q_{{\rm{ind}}}} = 1.78 \times {\left( {24.3 - 20} \right)^{1.32}} = 12.21. $

Radiative heat transfer rate of poplar wood floor:

$ {q_{{\rm{inf}}}} = 5.67\left[ {{{\left( {\frac{{23.3 + 273}}{{100}}} \right)}^4} - {{\left( {\frac{{10 + 273}}{{100}}} \right)}^4}} \right] = 73.34. $

Convection heat transfer rate of poplar wood floor:

$ {q_{{\rm{ind}}}} = 1.78 \times {\left( {23.3 - 20} \right)^{1.32}} = 8.6. $

Radiant heat transfer of oak surface:

$ {Q_{{\rm{inf}}}} = 4.98\left[ {{{\left( {\frac{{24.3}}{{100}}} \right)}^4} - {{\left( {\frac{{10}}{{100}}} \right)}^4}} \right] = 0.017. $

Convective heat transfer of oak sheets:

$ {Q_{{\rm{ind}}}} = 2.17 \times {\left( {24.3 - 20} \right)^{1.31}} = 14.67. $

Radiation heat transfer of poplar wood surface:

$ {Q_{{\rm{inf}}}} = 4.98\left[ {{{\left( {\frac{{23.3}}{{100}}} \right)}^4} - {{\left( {\frac{{10}}{{100}}} \right)}^4}} \right] = 0.013. $

Convective heat transfer of poplar wood surface:

$ {Q_{{\rm{ind}}}} = 2.17 \times {\left( {23.3 - 20} \right)^{1.31}} = 10.37. $

Through the experiment, we can see that the decorative panels of different materials have influence on the heat transfer performance of the RPIB. Using poplar as decorative panels make floor surface temperature rise relatively slowly, and heat insulation efficiency is relatively high. The convective heat transfer rate and convection heat transfer are lower than those of using oak as decorative panels. The heat transfer coefficient of oak is k=0.17 and poplar is k=0.10 as known. Therefore, the electrically-heated floor with oak with higher heat transfer coefficient as the decorative panel can lead to higher heat conduction efficiency and better utilization effect.

4 Conclusion

1) RPIB has excellent insulation performance, in the unit time, the floor temperature decreases slower than the floor temperature of the ordinary electrically-heated floor. It is a novel electrically-heated floor with practicability and reliability.

2) The low thermal conductivity of hard polyurethane material gives excellent performance in delaying heat transfer. Therefore, the RPIB has longer insulation time and better effect. In the aspect of energy saving and environmental protection, the electric energy is effectively saved, and the environment comfort is improved while the environmental effect caused by energy using is effectively reduced.

3) The electrically-heated floor of a decorative panel with a higher heat transfer coefficient will lead to higher heat transfer efficiency. The heat produced by the heat layer is more easily transmitted to the surface of the floor, thereby radiating heat into the air. It makes use of heat and energy better.