PERFORMANCE OF ONCE RUNNING AND PROSPECT OF THERMAL ENERGY STORAGE IN ROAD HYDRONIC SNOW-ICE MELTING

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1 PERFORMANCE OF ONCE RUNNING AND PROSPECT OF THERMAL ENERGY STORAGE IN ROAD HYDRONIC SNOW-ICE MELTING Qing Gao, Yong Huang, Yan Liu, Mi Lin, Ming Li Jilin University Changchun , China Tel: Xiaobing Liu ClimateMaster Inc. USA ABSTRACT The recent work focuses on the hydronic snow-ice melting and classifies the structure of module and physical model for the further numerical simulation, and strengthens theoretical understanding. The example results show that in Changchun city, the north eastern of China, under the condition of the statistical weather data from historical record, a normal heat consumption rate in the snow-ice melting was about 246W/m 2 and COP (coefficient of Performance) of heat pump unit was over 4. It is recognized that, when the ground heat exchanger is used for heating, the heating will result in a gradual drop of the ground thermal equilibrium temperature or the initial underground temperature of next heating season annually. As a result, by using the support of the seasonal UTES with solar collecting, the underground temperature can be increased. Therefore, the solar collection actually supplements the thermal energy to the ground and provides a sustainable development. 1. INTRODUCTION A hydronic snow-ice melting (HSIM) system coupled with slab solar collection (SSC) and thermal energy storage (TES) is a new method of snow melting (Gao et al. 1997; Eugster et al. 2002; GAO 2007), which uses solar energy storage in summer to melt the snow on the road in winter, and the ground energy can be kept thermal balance. The research works of hydronic snow-ice melting system on road and bridge has been carried on since 1998 under the leadership of J.D. Spitler (2002; 2004) in Oklahoma State University (OSU) of America. The experimental system was the largest one in the world, a full-scale bridge snow melting system. They studied a numerical model and a set of boundary conditions, that allow treatment of various surface and weather conditions associated with storm events. A model was established by Owczarek et al. (2003), which combined with heat energy gained from solar energy in road imbedded pipe. They studied non-transparent solar collector and transient heat transfer of solid collector with large heat capacity. Furthermore it took into account influence of instantaneous shadow of the vehicles and moving convection. In China Gao (1997), a scholar of Jilin University (JLU), firstly conceived and presented a scheme of snow-ice melting with solar collector and energy storage in the northern. Hereafter, researchers have been studying the utilization of earth energy, the underground heat transfer, ground source heat pump (GSHP), underground thermal energy storage (UTES) and snow-ice melting, etc (Gao et al. 2007; 2006). At same time a few other different thermal snow-ice melting was explored, such as the electrically conductive concrete with carbon fiber

2 investigated in Wuhan University of Technology (Tang et al. 2004), soil energy storage of solar heat studied in Tianjin University (Zhu et al. 2005) and the electrically heating cable tested in Beijing University of Technology (Li et al. 2006). A HSIM is a very complex system which couples with SSC and UTES, and involves transient heat and mass transfer. Once the facilities of heat transfer are set up, it is very difficult to rebuild and amend. Therefore, the recent research works already make more emphases on the theoretical analysis and numerical simulation all around the world. This paper focuses on the hydronic snow-ice melting and classifies the structure of module and physical model for the further numerical simulation and strengthens a theoretical understanding. 2. CONFIGURATION OF HSIM-SSC-TES Process Analysis The integrated system is very complicated because it involves transient heat and mass transfer. Therefore, the configuration of the HSIM-SSC-TES frame, the structure of every partial system and its module play an important role in the practical analysis and numerical simulation. According to the heat transfer analysis of all processes, the whole system is divided into three units, namely, HSIM, SSC on the road and the seasonal UTES, as shown in Figure 1. Basic Components Fig.1. Modularization of parts A HSIM-SSC-TES system consists of underground heat exchanger (GHE), heat pump, circulating pump, road heating piping, etc. The heating piping is composed of a series of parallel or circular pipe loops that embedded in the road near the upper surface. The system can be separated into two terminals, one is the loading terminal which is composed of the condenser of heat pump, road heating piping and circulating pump for snow-ice melting and collecting solar heat, another is the pooling source terminal which is composed of GHEs, the evaporator of heat pump and circulating pump for storing energy. In summer, the road is insolated and the surface temperature can get up to 60~70. Then, the fluid passes through the GHEs and the heating piping and carries the solar energy into the ground for storage. So the energy to snow-ice melting comes from not only the earth energy but also the energy stored which replenishes the loss of earth energy in winter. Both terminals are shown in Figure 1. As to the circulation, a HSIM-SSC-TES system mainly includes two circulations, one is SSC circulation, and the other is HSIM circulation. In each circulation, the module and its physical and mathematical model are set up individually according to the manner of heat transfer. This paper mainly discussed the snow-ice melting process in winter. 2. HEAT TRANSFER ANALYSIS AND PHYSICAL MODEL OF HSIM The unit of HSIM takes an important role of target in the system, which mainly relates to the transient heat and mass transfer. Snow is a kind of porous media which is composed of ice crystalloid, air and vapor, and a blend of melted-snow and thawed-ice during most of time. As we know, many factors impact the HSIM. Objective factors include road, ambient temperature, humidity, heaven high-altitude temperature, solar radiation intensity, wind speed and direction,

3 rainfall, snowfall, underground temperature, road materials, road structure, situation of road, etc. Subjective factors connect the capacity of snow-ice melting, heating load, inlet and exit temperature, on/off time of the system, control strategies of operation, etc. Snow-ice melting stages A snow-ice melting process can be divided into six stages, as shown in Figure 2. 1-solar radiation; 2-heaven thermal radiation; 3-snowfall; 4-convection; 5-conduction; 6-evaporation; 7-thaw; 8-flow away. Fig. 2. Heat transfer phenomena of melting snow and deicing The first stage: When it is snowing, HSIM system isn t turned on and the road is covered by snow. In this stage, there is only dry snow layer over the road. The second stage: When it is snowing, HSIM system has turned on and the circulating fluid heats up the road. The snow on the road begins to melt, and a part of snow on the bottom gradually gets in the state of saturation and become a layer of ice slush. So all snow layer is composed of dry snow layer and ice slush layer. The third stage: By the time of continuing to heat the fluid, snow on the bottom comes into water gradually. One part of water flows away by the slope of road and another part of water infiltrates into the non-impregnated roadbed. So all snow layer is composed of dry snow layer, slush layer and water layer. The fourth stage: Along with continuing to melt snow, dry snow layer disappears gradually, both slush layer and water layer are always left. The fifth stage: All snow on the road melts away. After the infiltration and evaporation of the water from snow-ice melting, the road tends to be dried out, or the transform constantly is going on with falling snow. The sixth stage: The road is the same as to the situation before falling snow, or the road recovers to the initial state. Situation of snow-ice melting and mesh generation of roadbed In winter the situation on the surface of road can be divided into some cases, and it generally relates to the objective factors of climate and environment closely. Summarizing previous studies (Rees 2002; Liu 2004; Lin 2007), it can be simplified as follows: 1 Dry state, the surface isn t covered by snow, and its temperature is probably higher than zero, or below zero; 2 Humid state, the surface temperature is higher than zero, and its humidity is originated from rainfall and snow melting; 3 Dry snow state, the surface temperature is below zero, and snow doesn t begin to melt; 4 Slush state, the road is covered with saturated humid ice crystals and the surface temperature is about zero; 5 Snow and slush state, dry snow is on the top, slush is at the bottom, the surface temperature is about zero;

4 6 Dry ice state, the surface covered by dry ice and the surface temperature is below the critical temperature of ice melting. Slush or snow water will refreeze and recrystallize; 7 Frost state, the surface covered by the frost and the surface temperature is close to the critical temperature of snow melting, but the environment temperature is so lower as to result in frosting. Basic Relation Equations The mathematic description of transient model in snow-ice melting process mainly depends on the mass balance equations and heat balance equations. The mass balance equations are based on the ice-water conversion process between dry snow and melting snow. And heat balance equations are based on the several layers of different melting stages, such as the heat transfer among dry snow layer, slush layer, roadbed and the ambient areas. The basic heat transfer equations are mainly composed of mass balance equations, heat balance equations and heat transfer flow equations in snow melting process. The detail equations refer from the literature (Lin, 2008). 3. EXAMPLES AND ANALYSIS Situation and condition Changchun city, locates in the northeast of China was taken as an example to discuss the snow melting behavior and characteristics. The meteorologic record data in some day of history were used to assume an objective analysis conditions. The size of road construction was selected for the length of 20m, width of 3.5m of reinforced concrete structure of a unilateral road. Roadbed buried with high-density polyethylene pipe, which outside diameter was m, inside diameter was m, spacing was m. The fluid initial temperature of ground heat exchanger and the initial underground temperature were 10. The snowfall began at 3:00AM and ended at 11:00AM, persisting 8 hours; a same snowfall intensity of 7.5 mm/h, up to 6cm snow depth. Figure 3 includes the curve of snowfall intensity and ambient air temperature. Five situations of snow thickness on the road, 30mm, 60mm, 90mm, 120mm and 150mm were selected for the discussion of influence of snowfall and the intensity on the system of heat pump. The simulation calculation made it a condition that the snowfall intensity was same, i.e. transient snowfall depth was 7.5mm/h, and ambient temperature was keeping in the dynamic change among 0 ~-9 ( seeing Figure 3-2 ). Road surface and pipe temperature It is 4:00 AM after one hour of snowfall that snow melting system was started. The working fluid circulated through the condenser and was heated up and then warmed the road bed for snow melting. Figure 4 describes average temperature on the surface of road and ambient air temperature. At about 6:00 AM, the average surface temperature reached 0 and the snow on the road began melting. As a part of the heat raised the roadbed temperature, another part of the heat was thawing the snow. At the beginning without heating, the temperature difference between the average surface temperature and ambient air temperature keeps in 2 and the surface temperature went below the rising ambient temperature due to the cold inertia of roadbed in the lower climate temperature before. And then the average surface temperature varied with the ambient air temperature trends. On heating, the surface temperature increased faster and faster, and exceeded the ambient air temperature at 5:00AM and got to 0 at 6:00AM. After that time, the rising speed of average surface temperature slowed down due to consuming more heat in snow melting. Thereinto, because the complex transformation of snow melting situations, such as dry snow, slush, and water evaporation etc, the temperature behaved itself with a

5 fluctuate. At 11 AM snowfall ceased, heat pump system was shut down, but the working fluid was circulating to make use of the residual heat. Up to a later period of time, especially after A dot (Figure 4), the temperature rise occurred obviously. It is indicated that the road snow melting were going to be finished and a reduced heat requirement leaded to the more obvious raise of the surface temperature of the road. After this, the surface temperature began to decline until that was similar to air temperature trend. Fig. 3 Snowfall and ambient temperature Fig. 4 Average temperature on the road surface In snow melting operational process, the surface temperature value was set in the range of 2 ~3. When the surface temperature was lower than 2, all the heat pumps were turned on. When the surface temperature was higher than 3, some heat pumps wre turned off and the heat load was reduced gradually according to the linear decrease until the temperature was near 2. Figure 5 showed that the fluid temperature in the inlet and outlet of heating system in snow melting process. The inlet temperature can be up to about 28, and the outlet temperature about 15. Obviously, in the beginning period of heating, fluid temperature raised fast, and then the rising decelerated unto the top value due to the increase of heat consumption of snow melting. After this the temperature began to fall down. Underground heat exchanger and heat pump Snow melting heat load is related with snowfall, climate, road conditions and surrounding environment. The load requirement of snow melting is depended on heat pumps and ground heat exchangers. Like this in the hybrid system the subsystem of snow melting is linked with both subsystems of heat pump and ground heat exchanger by the heating supply. In the subsystems of heat pump some heat pumps are needed to meet the heat load demand of snow melting and its change according to the controllable quantity of heat pumps. In the example, there are a total of 8 heat pumps. In this snow melting process, the quantity of activated heat pumps was shown in Figure 6. Every heat pump producted a certain heat load and needed a certain power, so the quantity of working heat pumps implies the variaty of total heat load and power consumption. In the initial period there were all heat pumps to be activated for maximal heating capacity, then after 3 hours the quantity of working heat pumps was reduced gradually. This is due to the road in the coldest state in the initial, it need much more heat. At last 1 hour only one heat pump was working. The total power consumption related to the quantity of working heat pumps. The more quantity of heat pumps are activated, the more power consumption, and the more heat absorption from underground heat exchanger. But because the diminution of the underground temperature, heat extracted from ground decreased accordingly, and the outlet fluid temperature of underground heat exchanger also be lowered. According to calculating results, the total heat pump power consumption was 28kWh, the total heat extracted from underground was about 110kWh, so that the value of COP (coefficient of Performance) of

6 heat pump unit was Meanwhile the average heat consumption rate of road in the snow melting was 1.971kWh/m 2, which is equivalent to 246W/ m 2. Fig. 5 Hydronic temperature of pipe imbedded Fig. 6 Quantity of activated heat pumps Generally, the heat consumption rate of snow melting directly related to the requirement of snow melting rate. The quicker is snow melting rate, the higher heat consumption rate. It needs more and more heat for use. Therefore designers need to evaluate overall situation and determine the rate by combining with road traffic condition, capital investment, energy efficiency and other circumstance factors. Above discussion, it is indicated that a lot of element such as road situation, climate environment, underground constructure, unit of heat pumps and some circumstances had markedly impact to the heating system of heat pump. It is important to focus on the more researching basic problems during the coming demonstration application, such as investigation of mechanisms, characteristics and performance of the unsteady and transient heat transfer in a complex snow melting with ground source heat pump (GSHP) and underground thermal energy storage (UTES). These analysis will strengthen theoretical and practical understanding and facilitate more extensive application. Ambient temperature influence In order to analyze the influence of different ambient temperature, outdoor surrounding temperature was selected during snow melting by a fixed average value, 0,-5,-10,-15 and -20. The simulation calculation made it a condition that the snowfall intensity was same, i.e. transient snowfall depth was 7.5mm/h, and the snow thickness kept a same 150mm, and the initial underground temperature was 10. Figure 7 shows the results of different ambient temperature around the road. The results of Figure 7 show that those hot-working character factors related with the heat consumption of snow melting, the heat extracted from the ground heat exchangers and the power consumption of heat pump system decreases with the rise of ambient temperature. In the very low ambient temperature these factors change very serious and there is a big ascending grade. In this illustration, it is indicated that the value of COP falls down by 6.7% when the ambient temperature changes from 0 to -20. Undoubtedly; when the ambient temperature is low, because of heat load increasing, the power consumption increases concomitantly. Although the COP value variation is not observable, both heat load and power consumption go up hugely. The lower ambient temperature will bring out more heat dispersion to the surrounding, and it becomes more and more important part of heat consumption. One of the solutions is to provide more heat pumps or use larger scale heat pump. It is demonstrated that the engineering design not only relies on the regional snowfall from history statistics, but also on ambient temperature. Actually the ambient temperature is a dominant factor.

7 (a) power and heat Fig. 7 Effect of ambient temperature Initial underground temperature (b) COP As an example, five initial underground temperature 5, 8, 10, 12 and 15 were selected to analyze the influence of underground temperature and the effect of underground thermal energy storage. The simulation calculation made it a condition that the snowfall intensity was same, i.e. transient snowfall depth was 7.5mm/h, and the snow thickness kept in 150mm, and ambient temperature was keeping in 0. Figure 8 shows the (influence) results of different initial underground temperature. (a) power and heat (b) COP Figure 8 Effect of underground temperature Under certain snowfall intensity and ambient temperature, the load requirement gets to be stable, so the variation of those hot-working character factors is based on the underground temperature directly. The heat extracted from the ground will increase with the incremental of the initial underground source temperature. Thus the heating capacity enhanced will bring a boosting ability of snow melting and shorten the thawing time. Therefore, raising initial underground temperature will bring underground source heat pump a vast potential to improve the efficiency. The underground thermal energy storage will supplement the heat and increase the underground temperature. Thus, the hydronic snow-ice melting on the road with slab solar energy collection and seasonal underground thermal energy storage will have a prospective significance of application. 4. CONCLUSION From the perspective of physics and heat transfer, physical phenomena of snow-ice melting were analyzed and also some stages and situation of heat transfer were defined. Designed and sketched the modular structure of all system was presented. The example results show that in Changchun city, the north eastern of China, under the condition of mean meteorologic data of historical record, the average heat consumption rate of road in the snow melting was about 246W/m 2. The value of COP (coefficient of Performance) of heat pump unit was more than 4. Designers should pay more attention to the tradeoff among character factors to improve the

8 COP of the heat pump system. As a result, the support of underground thermal energy storage will supplement the heat and increase the underground temperature. Thus, the hydronic snow-ice melting on the road with slab solar energy collection and UTES will be significant.. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the NSFC (National Natural Science Foundation of China) under the grant No We would like to express heartfelt thanks to the Center of Policy Research, Ministry of Construction of the People s Republic China, and also to Professor Jeffrey D. Spitler, Oklahoma State University USA, and Dr. Y.Y. Yan, University of Nottingham, UK. REFERENCES Qing Gao, Ming Li, Zhehao Xuan,Jeffrey D. Spitler. Practice and Task Developing Underground Thermal Energy Storage in China, Proceedings of the 10th International Conference on Thermal Energy Storage-Ecostock 2006, Pomona,, New Jersey, USA, June 2006 Qing Gao, Ming Yu and Xiaobing Liu, Prospects and Investigation of Snow-Ice Melting System on the Road by Thermal Energy Storage, Highway,No.5,2007,P Yiping Gao. Snow-ice Melting System on the Road by Solar Energy. Journal of China & Foreign Highway, Vol.17, No.4, 1997:53-55 Mi Lin, Hybrid System on Solar Energy and Underground Thermal Energy Storage for Engineering Applications, A Dissertation for the Master Degree of Jilin University, 2007 Liu, X. and J.D. Spitler. Simulation Based Investigation on the Design of Hydronic Snow Melting System. Proceedings of the Transportation Research Board 83rd Annual Meeting. Washington, D.C. January, 2004: 5-6. Li Yan-feng, Wu Hai-qin, Wang Guan-ming, Zhu Bin, Shi Bo-wei. Experimental Study on the Electrical Road Heating System for Snow Melting. Journal of Beijing University of Technology, Vol.31, No.3, 2006: Mariuse Owczarek,Roman Domański. Application of dynamic solar collector model for evaluation of heat extraction from the road bridge. 9th International Conference on Thermal Energy Storage. Warsaw in Poland, Vol.II, 2003: Rees S.J., J.D. Spitler and X. Xiao. Transient Analysis of Snow-melting System Performance. ASHRAE Transactions. Vol.108, No.2, 2002: Tang Zuquan, Li Zhuoqiu, Qian Jueshi. Application of Carbon Fiber Reinforced Conductive Concrete for Melting Ice and Snow on Road Surface. Journal of Building Materials, Vol. 7, No.2, 2004: Walter J. Eugster, Jürg Schatzmann. Harnessing Solar Energy for Winter Road Clearing On Heavily Loaded Expressways. Proceeding of XIth PIARC International Winter Road Congress, January 2002, Sapporo, Japan Zhu Qiang, Zhao Jun and Liu Yiqing. Application of Solar Energy and Soil Thermal Storage in Melting Snow on the Road. Construction Science and Technology, No.4, 2005: Lin Mi, Gao Qing, Ma Chunqiang, Liu Xiaobing, Li Ming, Liu Yan. Analysis of Heat Transfer on Hydronic Snow-Ice Melting on the road. Proceedings of the 14th Conference of Chinese University Society of Engineering Thermophysics, B-08048, Xia Men, in China, May 2008