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Procedía
ELSEVIER
Procedía Engineering 205 (2017) 4060-4066
www.elsevier.com/loeate/procedia
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China
Experimental study on heating characteristics and control strategies of ground source heat pump and radiant floor heating system in an
office building
In building energy consumption, the proportion of heating in energy consumption is very large. From the point of view of energy saving and thermal comfort, radiant floor heating system needs to be popularized and applied. This study takes the ground source heat pump and radiant floor heating system of an energy-saving demonstration building as the experimental plat-form. The indoor temperature of the operating system is obtained by the instrument at different heights. Indoor temperature fluctuation and distribution and the heating characteristics of the system can be obtained. Results show that the indoor temperature of the heating system meets human thermal comfort, and the indoor temperature of room which receives the solar radiation can still be maintained more than 18 °C in work time within two days when the sys-tem is closed. Moreover, valley electric operation and storage tank system operation mode makes the ground source heat pump and radiant floor heating system more efficient. The sys-tem is worth widely being used.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
Keywords: Building energy consumption; Ground source heat pump; Radiant floor heating system; Thermal comfort
* Corresponding author. Tel.: +86-139-0531-6324 E-mail address: zhth0015@sdjzu.edu.cn, jxl83@sdjzu.edu.cn
1877-7058 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and
Air Conditioning.
10.1016/j.proeng.2017.09.890
Linhua Zhangabc'*9 Xiaokai Huanga, Li Liangd, Jiying Liuab;
aSchool of Thermal Engineering, Shandong Jianzhu University, Jinan, China 250101 Key Laboratory of Renewable Energy Technologies for Buildings, Ministry of Education, Jinan, China 250101 cShandong Key Laboratory of Renewable Energy Technologies for Buildings, Jinan, China 250101 dShandong Huanneng Design Institute Co., Ltd., Jinan, China 250101
Abstract
1. Introduction
From the present energy situation and development trend in China, the sustainable development of construction industry makes energy consumption increase consistently. According to the statistics and calculation by 2012, the building energy consumption has been reaching 910 million tce and accounts for about 26% of the national terminal energy consumption. If the current situation can't be controlled effectively, building energy consumption will rise to 150 million tce in 2020 [1-3]. Heating energy consumption takes up large proportion in building energy consumption, and Long and Bai point out that the air conditioning energy consumption will account for 40% of total building energy consumption in 2020 [4]. From the perspective of energy saving and human thermal comfort, low temperature hot water radiant floor heating system is paid more and more attention. Compared with traditional convection heating system, the indoor temperature distribution using radiant floor heating system is equality uniformity. In contrast with the warm air heating, the advantages of radiant floor heating system are draught sensation, avoiding dust, maintaining indoor health, matching human body physiology regulation characteristics and making human body feel more comfortable [5]. Moreover, the design temperature of radiant floor heating system will be 2-3°C lower than the convection heating system, if the same thermal comfort wants to be reached, which saves 10%-15% of building energy consumption, meanwhile, the system takes up little installation space [6]. Radiant floor heating system can select the heat source flexibly, and utilize the heat pump, solar energy, geothermal energy and other low-grade synthetically [7]. Based on the field experiment of an energy-saving demonstration building in Jinan, the variation law of indoor temperature with outdoor temperature is obtained, and the heating characteristics and control strategy of radiant floor heating system can be analyzed.
2. Methods
2.1. Description of the building
The energy-saving demonstration building is located in Jinan, Shandong province, and the height is 20.7m, in which five floors above the ground, and one floor under the ground. The construction area of building is 5453 m2, and air conditioning area of 3815 m2. The building is concrete structures, and the exterior windows use heat insolation low-e hollow glass with aluminum alloy, meanwhile the exterior wall is assumed to be adiabatic in this research. The average temperature for air conditioning is 0.7°C in the heating period in winter, meanwhile, the outdoor design dry-bulb temperature is 7.7°C and the indoor design temperature is 18°C [8]. This system combines low grade energy utilization technology with low additional energy consumption technology, to strengthen the adjustability of air conditioning system and reduce operation cost, which can achieve dynamic energy saving [9].
2.2. Introduction of the heating system
The heating system of this paper is a ground source heat pump combined with radiant floor heating system, including ground source heat pump, the energy storage tank and buried pipes under the floor. The ground source heat pump unit is selected as heat source, which can product hot water with low temperature of 40°C. The working process of the system is that heat pump units transfer energy from soil to circulating water, and then low temperature hot water is supplied to buried pipes and the energy storage tank. The heated floor offer heat to room in the form of radiation and convection, when the electricity reaches its peak, the energy storage tank will become an auxiliary heating to maintain indoor thermal comfort.
Fig. 1. The system diagram of radiant floor heating system.
Heat pump units generally run in the valley-electricity period at most time, and the valley-electricity time in Jinan is from 23:00 to 7:00, the electricity price at current average time is 0.55 Chinese Yuan, but valley-electricity price is 0.35 Chinese Yuan. The heat pump units of this system usually run from 21:00 to 07:30 next day, to make full use of the valley of electricity, which not only staggers the peak period of power consumption, but also reduce the system operation cost. Heating load is small at night, so the heat can be stored in the floor and palisade structure, during the day, the heat stored can be released to heating. In the periods of peak load, energy stored in the tank play a supplementary role [10]. This control strategy is designed to reduce running cost, and realizes energy saving operation of the total system.
2.3. Measurement setup
The main instruments in the experiment include the GAM series multichannel temperature and humidity measurement recorder, Apresys intelligent temperature and humidity recorder, and so on.
Fig. 2. Measured instruments for vertical temperature distribution [11].
In this study, an open office area and a northward office are selected as experimental site, and each area of them
is 321.5m2 and 28.6m2, respectively. The vertical distributions of indoor temperature were measured. Eight measuring points was set up in the open office area, which are located at 0.1 m, 0.3 m, 0.9 m, 1.1 m, 1.3 m, 1.5 m, 1.7 m and 1.9 m above the ground, respectively. A set of data were recorded every 15 minutes by multichannel temperature and humidity measurement recorder. This test chose the northward office as the topical room due to the southward office, which is strongly influenced by solar radiation. The five testing points are respectively chosen at locations of 0.1 m, 0.6 m, 1.1 m, 1.7 m, and 2.3 m above the ground. The test used the multichannel temperature and humidity measurement and control instrument every 10 minutes to record a set of data.
The experimental arrangement of measuring points is shown in Fig 2. The measuring period is selected from December 29th, 2015 to January 4th, 2016, totally 158 hours. Specially, the date from Jan. 1st, 2016 to Jan. 3rd, 2016 is a national legal holiday, and the energy-saving operation and normal running of the system are taken into account. Therefore, the system should run continuously from Dec. 29th, 2015 to 17:00 in Dec. 31st, 2015. In this period these two heat pumps stored heat from 21:00 at night to 7:00 in the morning next day. While, the system closed on 17:00 in Dec. 31st, 2015. Before the holiday ends, a heat pump opened at 21:00 in Jan. 2nd, 2016. The floor circulating water pump ran at 8:00 in Jan. 3rd, 2016 and two heat pumps operate normally at night.
3. Results
3.1. Indoor temperature variation in the open office area
From Dec.29th to Dec.31th at 17:00, when the heating system runs continuously, indoor temperature changes from 18.56°C to 29.06°C. ISO7730 puts forward that when people mainly engages in light work sitting in a fixed position, the thermal comfort recommended temperature is from 20°C to 24°C, and the indoor temperature is mainly above 20°C from 8:30 to 16:30. Kang and Shen shows that the value of PMV in radiant floor heating room of 16°C is same with radiator heating room of 18°C [12]. It can be seen that the indoor heating temperature can be reduced by 2°C when the radiant floor heating system is adopted. Therefore, the indoor temperature above 18.56 can satisfy person's need for thermal comfort.
Two heat pumps were opened in the night of Jan. 3rd to prepare hot water to the water storage tank and the floor water circulating pump opened at 8:00 in Jan. 4th. By the record of multichannel temperature and humidity measurement and control instrument, indoor air temperature rose slowly, and the average temperature was up to 18.29°C before 9:00 in Jan. 4th when staffs started work. Meanwhile, the temperature of 0.1m above the ground where is position of the ankle was 18.19°C, and the measuring point temperature of breath area that 1.1m above the
12/29 12/30 12/31 1/1 1/2 1/3 1/4 Date
Fig. 3. Vertical temperature distribution in the open office area.
ground was 18.38°C. Therefore, the temperature in the office can meet people's needs for normal heating, the human body and work comfort [13].
3.2. Indoor temperature variation in the northern office
The indoor temperature variation in the northern office is shown in Fig 4. The comparative results between Fig.3 and Fig. 4 show the indoor temperature of the open office area fluctuated clearly with outdoor temperature variation, but the northern office rose slightly in the midday when the outside air temperature is the highest. In the other period for the northern office, the temperature difference was few at the different heights.
20 18 16
6 1 •s
2 0 -2
! ! I--4 12 --1-1-1 ■ 1 i-'-1-■-1 1—1-1----
12729 12/30 12/31 1/1 1/2 1/3 1/4 Date
Fig. 4. vertical temperature distributions in the northern office.
the indoor highest temperature was 19.84°C at the beginning of closing the system, while the indoor minimum temperature was 17.96°C that appeared from 5:30 to 7:40 in Dec. 29th and at the height of 0.6m. The maximum temperature difference in northward office was 1.88°C, which is lower than the open office area. The system closed from 17:00 in Dec.31st, 2015 to 8:00 in Jan.3rd, 2016, and the minimum temperature was 15.39°C, which is lower than the condition of the open office area obviously. Therefore, the fact can be concluded that solar radiation has a great effect on temperature, which is the main reason the temperature in the northern office is relative steady and the indoor temperature of open office area will be steady and uniformity in vertical direction if the solar radiation is not strong.
3.3. Temperature variation in northern office
This study took the day of Dec.30th as an example. The temperature difference between 0.12 and 4.56 °C was measured at different height points at the same time, and the biggest temperature difference was 7.88°C. All the indoor temperatures in the day are above 18.56°C. Fig 5 shows that the ground temperature tended to be constant, and each measuring point temperature fluctuation was not evident before 8:00, however, the indoor temperature began to rise from 8:00. The highest outdoor temperature appeared at 12:00 when the solar radiation is the strongest. But due to the thermal resistance of structures envelopes, as well as the temperature wave decayed and delayed, the highest indoor temperature was 26.44°C appeared at 14:00. Measured by the instrument, the temperature order of different points at that time was that 0.9m>0.3m>1.3m>1.5m>0.1m>1.7m>1.9m>ground temperature, and at other times the temperature of each measuring point is basically the same. Overall, the indoor temperature uniformity of radiant floor heating system is better than other heating methods.
Fig. 5. The day temperature variation of open office area on Dec.30th, 2015.
4. Conclusions
• Radiant floor heating system has obvious advantages than other forms of heating system. When the design temperature takes 18°C, the indoor air temperature satisfies the requirement of thermal comfort of human body in normal working hours.
• Solar radiation has bigger influence on the indoor temperature. Northward office is not directly impacted by solar radiation, so the temperature fluctuation is not evident, which is good for the cooling in summer, and the indoor temperature will be stable if the solar radiation is removed.
• For the open space office, which has sufficient solar radiation. when radiant floor heating system is closed for two days, the tank has residual heat, the floor is still release heat by the residual heat stored in tank, to make sure the indoor temperature up to the demand of thermal comfort. According to the heating characteristics, the system operation and control can be optimized.
• The temperature is stratified vertically, and the temperature differences are between 0.12°C to 4.56°C. In addition to the strong solar radiation time, the temperature appears clear stratification at noon, the temperature differences of other time is under 3°C, which is correspond to the ISO7730 standard. Meanwhile, the indoor temperature distribution is uniform, so the air conditioning system can better meet the requirements of thermal comfort of the human body.
• The operation mode of valley power operation, peak power closing, the water tank storing energy of the ground source heat pump with radiant floor heating system is energy saving, operation cost saving, and also meet the needs of the indoor thermal comfort.
Acknowledgements
This study was sponsored by Natural Science Foundation of China (NSFC, No.51176104), Natural Science
Foundation of Shandong Province (ZR2016EEB08), Science and Technology Plan Project of University in
Shandong Province (J16LG07), Science and Technology Development Plan in Shandong Province
(2012GGX10416). Authors would like to thank graduate students for their help with the onsite data collection.
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