Operating characteristic analysis on the ultra-thin low temperature floor-heating system

Abstract

Prefabricated ultra-thin radiant heating panel, as a new heating terminal type, is becoming a highlight in Yangtze River Valley area, China recently. However, there is a lack of operating characteristic research in this region, especially the energy consumption and operating mode are even less. To obtain these data, a heating system was set up in a duplex house in Chongqing. The test results show that the floor heating system could almost satisfy thermal comfort requirement at supply water temperature 45°C. But the preheating time was up to 4.5 h which was 1 h longer than that at supply water temperature 50°C. Meanwhile, the energy consumption at supply water temperature 50°C increased 0.10 Nm³/h, and the operating efficiency decrease about 2.6% compared to those at water temperature 45°C. Considering both the thermal lag and operating efficiency, a reasonable suggestion was proposed in this paper. That was, the standard families which just stay home at night should adopt the interim mode of partial room with part time. And the supply water temperature should be properly raised during the preheating period and lowered down in the steady heating stage.

Keywords

ultra-thin floor heating panel / the preheating time / thermal comfort / energy saving

Introduction

The inhabitants in the Yangtze River valley of China have a strong demand for heating because of the cold moist climate and the indoor rigorous condition in winter. With the development of economy and the improvement of living standard, residents have become interested in environment comfort, and thus heating technology has developed very quickly in this area. Nowadays, the residential buildings in this area are furnished with room heat pump air conditioner (HPAC) as the most common heating way, but the use of HPAC in residential building exists negative effects on the environment due to CFCs, and also has some disadvantages, such as uneven air temperature, high wind speed and local thermal discomfort [1,2]. The alternative radiant floor heating system can reduce the shortcomings. There are a few residential buildings using the traditional radiant floor heating system in this area [3]. The radiant heating system is considered to have a number of advantages over convective heating systems, such as minimal air temperature, higher thermal performance and air cleaner [4,5]. However, it has been found that the traditional radiant floor heating system has larger thermal lag and more loss of the storey height. Those shortages to a certain degree limit its application.

With the development of heating technology, different types of heating products spring up. The prefabricated ultra-thin radiant heating pane is becoming a highlight of heating products and attracts user’s attention recently, which owes to such its advantages as high thermal comfort, decreasing storey height loss, light weight, installation shortcut, and so on [6]. However, how about the operating characteristics of prefabricated ultra-thin radiant heating system in the residential buildings of Yangtze River valley area? Actual operating experiments should be carried out to study the system characteristics in order to find out energy-saving operating modes.

Experimental programs

Experimental system

A 155 m² duplex apartment located in Shapingba District of Chongqing was considered in this experiment. The heat transfer coefficient (W/m²·K) of external wall, roof and external windows are 1.0, 0.8 and 3.2 respectively. These values were subject to the local 50% energy-saving criteria in China [7]. The radiant heating floor from top to bottom consisted of 8 mm multiplayer solid wood floor, 12 mm ultral floor heating panel, 20 mm cement mortar, 30 mm insulation mortar and 120 mm reinforced concrete floor in turn, and its heat transfer coefficient (W/m²·K) was 0.93. When the indoor design temperature was 18°C, the total heat load with 140 m² heating area was 6820 W.

Prefabricated ultra-thin radiant heating pane was shown in Fig. 1. It was made of a 12 mm thick polystyrene boards with installation groove (8 mm × 8 mm), on which De7.2 × 1.1 mm PB pipes were inserted as the floor heater. The heating pipe space was 75 mm in general. The pipe was covered by a 5 μm thick aluminum plate in order to reduce the surface temperature non uniformity. In general, the heating elements were mass-produced in factories. Furthermore, this floor heating panels could be directly laid over the concrete floor, compared to the installation of the traditional floor heating system. And then the tiles or wood floor can be directly laid over them as well without concrete backfill.

A schematic diagram of the heating system was shown in Fig. 2. The heating source was an 8.9-24 KW capacity, 2.8 m³/h rated nature gas consumption dual-purpose wall-hung boiler. The boiler produced hot water and connected in series with the ultra-thin floor heating slabs. A 5 ~ 7 m head variable speed pump was installed at boiler outlet. And a multi-function calorimeter was set up at the supply and return water mains to measure heating capacity and hot water temperatures. The whole system was divided into 12 circulation loops, 5 downstairs and 7 upstairs. And each layer installs a set of manifolds with an individual thermostat so as to adopt hierarchical control mode. The total length of the serpentine coil of the floor slabs was 1030 m, and the water capacity was about 31 L.

Experimental instruments and contents

The experiments were carried out in early February 2012. The main testing parameters included outdoor air temperature, indoor air temperature and floor surface temperature with varied supply water temperature. A bedroom of this duplex apartment was chosen and tested. Its overall dimension was 3.4 m (depth) × 3.8 m (width) × 2.8 m (height). Its exterior envelopes included the east, north wall and roof. And there was a 1.5 m × 1.8 m double-glazed window facing north. Adjoining rooms were all heated. Indoor air temperature was measured by copper-constantan thermal couples and the data were recorded by a multi-channel acquisition. A total of 20 temperature points had been arranged as shown in Figs. 3 and 4, 16 for the floor surface and 4 for vertical space. The temperature of the wall inside surface was measured twice an hour by an infrared detection gun. Apart from these temperature parameters, measurements had also been made of heat capacity and gas consumption. The apparatus applied for measuring were shown in Table 1.

Results and discussion

The experimental system had been working for a week with the supply water temperature varying from 40°C to 55°C. The characteristics of the ultra-thin low temperature radiant floor heating system were measured and compared with each other. Two typical testing data, whose supply water temperatures were respectively 45°C and 50°C, were chosen to analyze because of their similar outdoor conditions, which were close to the local typical weather in winter. These details were presented in Table 2.

Analysis of temperature characteristics

The operative temperature (t_o) has taken into account convective and radiation heat transfer between the environment and human body, and is the better response to the radiant heating comfort. When indoor air velocity is less than 0.2 m/s, it can be calculated approximately as:

(1)t_{\mathrm{o}}={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}(t_{\mathrm{m}\mathrm{r}\mathrm{t}}+t_{\mathrm{i}\mathrm{n}})\approx {\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}(t_{\mathrm{w}\mathrm{p}}+t_{\mathrm{i}\mathrm{n}}),

(2)t_{\mathrm{w}\mathrm{p}}={\displaystyle \frac{{\displaystyle \sum A_i{t}_i}}{{\displaystyle \sum A_i}}},

where t_o was the operative temperature (°C); t_mrt was the mean radiant temperature (°C); t_in was the indoor air temperature (°C); t_wp was the mean internal surface temperature of envelopes (°C); A_i was the area of wall i (m²); t_i was the mean internal surface temperature of wall i (°C).

Figures 5 and 6 showed the hourly data of two group data respectively under the water supply temperate of 45°C and 50°C. And the duration of each test was 9 h. It was seen from Figs. 5 6 that temperature trends over time were almost similar, when the water supply temperature was different. But there were still some differences:

First, there were some differences during the preheating stage. Preheating time meant the time of heating up the room from the initial temperature to the setting temperature (16-18°C). It was an important operating characteristic to evaluate the radiation floor heating system. When the water supply temperature was 45°C, the indoor air temperature increased rapidly between 0 and 90min, and the heating rate was 2.17°C/h. The indoor temperature reached the room setting temperature 18°C after 270 min (see Fig. 5). In other words, the preheating time was 4.5 h basically equal to traditional radiation floor heating system’s preheating time of 4-6 h. It was apparent that the thermal inertia of ultra-thin floor heating panels was still big so as to show no significant advantage. Figure 6 showed that when the temperature of the supply water rose to 50°C, the preheating time was 3.5 h, 1 h shorter than the former. Hence, higher supply water temperature contributed to shortening the preheating time effectively.

Second, Fig. 5 showed that the indoor temperature could be kept at the setting temperature 18°C, and the operative temperature be maintained at about 16°Cat last so as to satisfy the comfort requirement when the supply water temperature at 45°C. When the supply water temperature was 50°C, the mean indoor temperature could increase by 0.3°C, and the mean operative temperature increase by 0.4°C, which demonstrated slightly high thermal comfort degree. Meanwhile, the mean slab temperature (t_f) reached about 22.85°C, which was 0.40°C higher than the former. However, the temperature of 22.85°C was still slightly less than the low limit of the mean slab temperature range in “Technical specification for floor radiant heating JGJ142—2004” [8].

Analysis of energy consumption

Energy consumption was one of the key factors to evaluate the performance of the radiant floor heating system and an important issue that users cared greatly. Table 3 showed the hourly operating energy consumption and the primary energy ratio (PER) of the heater when the temperature of the supply water were kept at 45°C and 50°C respectively. During the preheating stage, the gas consumption per hour under different working conditions was gradually decreasing with the increase of the time. And the decreasing tendency was almost similar. When the water temperature was 45°C, the average hourly energy consumption during the steady stage was 0.1Nm³/h less than that at supply water temperature 50°C, and the PER was 2.6% higher.

It also could be seen that when the supply water temperature was certain, the mean hourly energy consumption during the preheating stage was more than that of the steady-state. According to the data in Table 3, when the supply water temperature was kept at 45°C, the mean hourly energy consumption during the preheating stage was 0.11 Nm³ more than that during the steady stage, and the PER was 1.4% less. It was apparent that there was a high-energy consumption, low-efficiency phenomenon at the preheating stage, which was especially notable for the intermittent mode of “no man, no heating.” Therefore, in order to reduce the negative effect, the way of shortening preheating time was undoubtedly effective. Therefore, the way of improving supply water temperature could be used to shorten the period of low efficient performance and save energy.

Discussion of energy-saving operating strategy

The residences in Yangtze river basin mostly adopted household independent heat source with intermittent operating mode, which was generally considered as a energy-saving economic operating mode of worth promoting [9,10]. However, the intermittent mode required shorter preheating time to reach the room setting temperature quickly. But thermal inertia of the ultra-thin floor radiant heating terminal was slightly shorter than that of the traditional floor heating. So it was necessary to find a better operating strategy to shorten preheating time in order to save energy.

Based on practice and comparison above, it was suggested by considering both preheating time and the PER that the supply water temperature should be kept at higher level during the preheating stage, and reduce when the indoor temperature reached the setting condition. For example, if this ultra-thin radiant floor heating system was as well continuously running for 9 h, the supply water temperature first was maintained at 50°C during the preheat period of 3.5 h, and then decreased to 45°C during the steady heating stage. The total and mean hourly gas consumption were respectively about 11.25 and 1.25 Nm³/h, which were all falling in between two testing conditions (see Table 3). The results showed that the new operating strategy could better the comfort of intermittent operating mode, and provide effect on the energy saving. Therefore, this better operating strategy should be recommended in Yangtze river basin.

Conclusions

Based on the test and analysis of the ultra-thin floor radiant heating system, some valuable information can be obtained as follow:

1) Under similar outdoor conditions, when the supply water temperature kept at 45°C, indoor air temperature and operative temperature could reach respectively 18°C and 16°C, which could meet the requirement of thermal comfort. Furthermore, 4.5 h preheating time of the system almost closed to that of the traditional floor radiant heating system.

2) When the supply water temperature was 50°C, a slight increase of the indoor temperature had no obvious effect on the room comfort. And the corresponding hourly energy consumption was 0.1 Nm³/h more, the PER 2.6% less, compared to the former. But the preheating time of such heating system shortened 1h.

3) In the Yangtze Valley River area, the residence buildings equipped the ultra-thin floor radiant heating systems was suggested using an energy-saving operating way. That was the intermittent mode of partial room with part time. Meanwhile, it was advised to keep the supply water temperature higher during the preheating period and lower it down during the steady heating stage. Then the preheating time would be shorten so as to improve the PER and save energy.

References

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