Available online at ScienceDirect. Procedia Engineering 146 (2016 )

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1 Available online at ScienceDirect Procedia Engineering 146 (2016 ) th International Cold Climate HVAC 2015 Conference, CCHVAC 2015 Analysis on the Heat Transfer Process of the Burning Cave -- a Traditional Heating System in Rural Houses of Northern China Xueyan Zhang, Bin Chen* Dalian University of Technology, Dalian, China Abstract Nowadays, inhabitants living in rural areas of northern China still rely on traditional heating methods, such as kangs (bed-stoves), radiators heated by mini stoves, and burning caves in cold winter period. Burning caves are increasingly used due to free fuels from crop wastes, simple structures, and better heating effect than kangs and mini stoves. In this study, field measurements of indoor environment in several rural houses with burning caves located in northern China were carried out in 2010~2011. The results show that indoor temperatures maintained at above 18, and it was felt more comfortable than houses heated by kangs and mini stoves. By comparing analysis on exergy efficiency, using burning cave and kang is the highest to 55%, and the thermal efficiency was up to 42% The The Authors. Published by Elsevier by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of CCHVAC Peer-review under responsibility of the organizing committee of CCHVAC 2015 Keywords: Burning cave, Radiant floor heating, Heat transfer; 1. Summary Burning cave heating system is widely used and more popular, because of its continuous radiant heating, biomass energy resources, indoor environment improvements, and easy for integration with house. Through a long-term study on traditional heating system in rural houses of Northern China, indoor thermal environment could be improved in a house heated with burning cave, which could take the average indoor air temperature maintain at 15.5, and the temperature difference between the day and night could be reduced to 6. In this paper, indoor thermal environment, thermal comfort, and exergy efficiency has been analysed by experimental researches. It * Corresponding author. Tel.: address: chenbin@dlut.edu.cn The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of CCHVAC 2015 doi: /j.proeng

2 460 Xueyan Zhang and Bin Chen / Procedia Engineering 146 ( 2016 ) illustrates that the average indoor air temperature could be more than 16, which is more comfortable than the room heated by kang. 2. Introduction For thousands of years, Chinese people live in harmony with the nature and take actions that suit local circumstances to adapt the climate change, which has been proved by house construction modes, energy consumption modes, and indoor environment control strategies. With times changing, the forms of dwellings for residents living in were constantly updated developed. At the same time, in order to adapt to the harsh outdoor climatic and environmental changes, the reasonable optimized approaches of rural houses design is a great challenge for people to resist the cold winter, but also the constant practice of human wisdom. As shown in Fig. 1, the original form of Chinese residential northern cold region could be traced back to ancient times, people had always lived in the rocks or caves for social life, relying on a simple way of direct firing heating in winter [D. Lu.]. In order to increase the privacy of a family, civil engineering structures began to develop gradually until Shang Dynasty, at the same time, using the high temperature gas flames to heat the heating facilities have begun to emerge, such as heating-wall, Di-Huolong. Besides, kang was birthed gradually to make the heating surface warmer and better heating effects on human bodies, and the direct contact area has been much bigger [H.G. Ren.]. Chinese people s living styles and history cultures have been significantly affected by traditional heating approaches such as kangs, smoke heated walls and grounded kang (burning cave) which utilize biomass (crop wastes collected from farmland and excrements from animals) to heat the rooms in a long and severe cold winter in vast northern China. Different kangs are taken as the prototype of radiation heating systems, have been used since 10,000 B.C [Building Energy Research Center of Tsinghua University]. They are not only domestic heating systems, but also places for cooking, eating, sleeping and communications. In order to achieve the needs of modern variety architectural space layouts, square-shaped, L-type, and other forms of kangs have appeared in different houses, and have been in use ever since. Thus, technological improvements closed to people s daily lives have been carried out on these original heating systems, which reflect people s wisdom. A large amount of field surveys, experimental and theoretical research on the forms, constructions, and integrated application modes of the traditional heating systems like kangs and burning caves have been conducted [B. Robert, X.Y. Zhang, B. Chen, W.Z. Tian, Z. Zhuang, Y.G. Li, etc.]. Cavern Crude thatched cottages Palace with civil structure Palace with post and panel structure Stone structure Stone structure Brick-concrete structure Ancient time Yao and Shun age From Shang to Jin age Sui and Tang age Liao and Jin Period Qing dynasty and Manchu Timeline Modern times Bonfire Chinese fireplace Hot wall heated by smoke Smoke flues constructed under ground Kang like a Chinese Character Wan Ground kang Kang and radiators heated by mini-stove Fig.1. Original heating systems developed with the evolution of architectural forms In recent years, burning cave, an ancient traditional heating method, has been attracted much more attention in some rural areas of northern China. Although its form is original, it s still popular because of its simple technology, low cost, and better heating effect. Especially, many demands for integrated houses with improved burning cave heating system based on craftsman experience are increasing day by day, account for the continuing improvement

3 Xueyan Zhang and Bin Chen / Procedia Engineering 146 ( 2016 ) on structure of heating systems and integrating design of the house. This paper mainly describes the heating effect of the burning cave, optimized approaches and thermal performance of the house with burning cave heating by means of investigations. 3. Methods In order to discuss the heat transfer process, the unsteady heat flux has been measured and recorded by using the high-temperature multi-circuit monitoring system. The parameters of thermal environment has been measured, and the heat flux has been calculated by Laplace transform method for analyzing the unsteady heat transfer through the cover plate on the burning cave. 4. Experimental study While using a burning cave for heating a house, the problems of low efficiency, severe heat loss must be solved. As shown in Figure 2 (a) (b), the burning cave has been designed integrated with a kang, which could be heated by the flue gas exhausted from the burning cave. The total heat of the fuels was all distributed into the room. The indoor air temperature and relative humidity have been measured by thermo recorder TR-72U. The record interval is 10 mins. Chimney Bedroom on the second floor Filling door Burning cave Kang Room on the first floor Kang Air temperature (a) (b) Fig. 2. Experimental devices and objects ((a)the heating house integrated with burning cave and kang; (b)the sketch map of gas flue) 5. Results and Analysis Indoor thermal environment As can be seen in Fig. 3, while the outdoor air temperature is below -6, and the heat flux of the burning cave is from 300 W/m 2 to 500 W/m 2, the indoor air temperature of the room heated by a burning cave is from 20 to 25. At the same time, the average indoor air temperature of the living room is about 16, which was affected by the adjacent room heated by a burning cave. Both of this two room are more comfort than the room heated only by kang. It illustrates that the temperature difference between the adjacent rooms was about 8.5. Therefore, the heat of the burning cave should be efficiently used and could be for heating the whole house. When the biomass fuels was smoldering in the burning cave, one part of heat was used for heating a room on the up floor like a radiant floor heating, the other part of heat was used for heating a room on the first floor like a heating wall. The surface temperature of the heating wall is between 14 ~16, the indoor air temperature of this room is between 6 ~8.3, which is shown in Fig. 4.

4 462 Xueyan Zhang and Bin Chen / Procedia Engineering 146 ( 2016 ) Temperature of room1 (Heating with burning cave) Temperature of the living room(without heating) Temperature of room2 (heating with Kang) Outdoor temperature Heat flux of the cover-plate on the burning cave N Temperature / C Heat Flux / W/m /3/14 04: /3/15 04: /3/16 04: /3/17 04:10 Time Fig. 3. Indoor and outdoor air temperature on the second floor inside the experimental house Fig. 4. Surface temperature of the heating wall and the indoor air temperature on the first floor inside the experimental house Comfort analysis As shown in Fig. 5, the burning cave has been taken as a popular heating system in other rural houses, and measured from DEC to JNU The average temperature of the room heated by a burning cave is about 5 ~8 higher than the room heated by kang. Fig.6 shows that, the thermal comfort of the room heated by a burning cave is exceed the comfort zone in national design standard of energy-efficient rural housing, and achieve to the adaptive comfort zone in ASHRAE Standard

5 Xueyan Zhang and Bin Chen / Procedia Engineering 146 ( 2016 ) Fig. 5. Indoor and outdoor air temperature in measured houses in a heating period. Fig. 6. Compariment with thermal comfort zone of the experimental house integrated with burning cave and kang

6 464 Xueyan Zhang and Bin Chen / Procedia Engineering 146 ( 2016 ) Exergy efficiency analysis Using exergy efficiency calculation to evaluate on the utilization rate of biomass energy in rural houses integrated with traditional heating systems was beneficial for judging the energy-saving potential. A proper measure for energy conservation could be carried out according to the approaches of exergy losses. Therefore, the main thermodynamic target is "Exergy efficiency", which is defined as follows: η e=e eff / E exp=effective exergy used / total exergy consumed (1) The equation (1) has been used for calculating the exergy efficiency of each heating system, and the results are listed in Tab. 3. Through theory analysis, the irreversibility of the fuels combustion could lead to exergy loss. As comparison in Tab. 3, the exergy efficiency of the burning cave and kang is highest to 55%, but the thermal efficiency is only 42%. While thermodynamic process is deviated from the corresponding reversible process, the exergy loss is increasing bigger, which should be controlled reasonably in the actual energy conversion. Table 3 The exergy efficiency of the above three heating systems Heating system Temperature of fules ( 0 C) Combustion degree (%) Temperature of exhaust gas ( 0 C) Thermal efficiency (%) Exergy efficiency (%) Stove and kang Radiators heated by mini-stove Burning cave and kang Conclusions In order to study the heat performance of the burning cave, which was comparied with a kang has been taken out. There are some significant conclusions has been taken as follows: (1) The average temperature of the room heated by a burning cave is about 5 ~8 higher than the room heated by kang. (2) the thermal comfort of the room heated by a burning cave is exceed the comfort zone in national design standard of energy-efficient rural housing, and achieve to the adaptive comfort zone in ASHRAE Standard (3) By comparing analysis on exergy efficiency, using burning cave and kang is the highest to 55%, and the thermal efficiency was up to 42%. So that, in the actual energy conversion process, the energy loss must be controlled and lead to a reasonable use. Nomenclature E eff E exp η e Effective exergy used The total exergy consumed The exergy efficiency of each heating system

7 Xueyan Zhang and Bin Chen / Procedia Engineering 146 ( 2016 ) Acknowledgements This work was financed by a NSFC (No ), the key projects in the National Science & Technology Pillar Program in China during the Twelfth Five-year Plan Periods (No. 2012BAJ02B05), and a China Postdoctoral Science Foundation(No. 2015M571306). References [1] D. Lu. The acculturation research of northeast manchu residential. Master dissertation of Harbin Institute of technology, 2008 (in Chinese). [2] H.G. Ren. Design of kang technology in rural dwellings of northeastern area. Master dissertation of Harbin Institute of technology, 2008 (in Chinese). [3] Building Energy Research Center of Tsinghua University Annual report on China building energy efficiency. China Architecture and Building press, 2014, pp 4-5 (in Chinese). [4] B. Robert, B.W. Olesen, K.W. Kim. History of Radiant Heating & Cooling System: part I. ASHRAE Journal, January [5] X.Y. Zhang, B. Chen, Y.Q. Liu, X.L. Han. Analysis on the Relationship between Typical House Mode and Heating Energy Consumption in Cold Rural Areas of Eastern China. Materials Science and engineering. May, 2013, 368,pp [6] X.Y. Zhang, B. Chen, J.R. Zhao,.et al. Optimization of Thermal Performance in a Chinese Traditional Heating System - Burning Cave. Energy and Buildings, 2014, 68,pp [7] W.Z. Tian, B. Chen, X.Y. Zhang, et al. Study on technology utilization of burning cave for heating in rural houses in cold areas in China. Journal of Central South University (Science and Technology), supplementary issue, 2012, 4(43), pp (in Chinese). [8] X.Y. Zhang, B. Chen, W.Z. Tian. Experimental investigation on the hot-water heating system based on burning cave. Acta energiae solaris sinica. 2013, 34(7), pp (in Chinese). [9] Z. Zhuang, Y.G. Li, B. Chen, et al. Chinese kang as a domestic heating system in rural northern China a review. Energy and Buildings, 2009, 41(1), pp [10] Z. Zhuang, Y.G. Li, B. Chen. Smoke flows in Chinese kangs. Indoor and Built Environment, 2009, 18 (3), pp [11] X.Y. Zhang, B. Chen. A Simplified Calculation Method for Predicting the Thermal Performance of a Burning Cave in Rural Houses of Northern China. Energy and buildings. 2016, 117(1), pp