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Optimal Phase Change Temperature for Energy Storage Based on Fluctuating Loads in Building Cooling Heating and Power System
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 75 (2015) 1360 – 1365
The 7 th International Conference on Applied Energy – ICAE2015
Optimal phase change temperature for energy storage based on fluctuating loads in building cooling heating and power system Fei Xiong, Yin Zhang, Xin Wang*, Yinping Zhang Department of Building Science, Tsinghua University, Beijing 100084, China
Abstract
Natural gas driven Building Cooling Heating and Power (BCHP) system is encouraged to develop as small-scale energy supply system in recent year. Integrating Energy Storage System (TES) with trigeneration system can improve the energy efficiency and reduce the installed capacity for energy supply equipment. However, it has never been an easy task to reasonably design the TES-BCHP system, in terms of that it is hard to determine the suitable installed capacity of energy supply devices considering the fluctuating loads and the influence of TES. In this paper, a thermodynamic model of BCHP system with phase change material (PCM) energy storage is established. According to practical user loads and considering the correspondingly hourly changing flow rate in charge and discharge processes, a simplified method is proposed to determine the optimal phase change temperature for BCHP-TES, aiming to minimize the primary energy consumption of BCHP-TES system. An illustrative example shows that the optimal phase change temperature can be obtained for given loads. It also indicates that compared to infinite NTU for PCM-TES, the optimal phase change temperature will increase as well as the corresponding minimal primary energy consumption for finite NTU. This work is of great importance for PCM optimization of BCHP-TES system design. keywords: T ri-generation , Energy storage , Phase change , Energy saving, Flow rate
1. Introduction Distributed energy supply system is of many advantages, such as high energy efficiency, low emissions, small-capacity, being close to the user and so on. The Building Cooling Heating and Power (BCHP) system based on the principle of cascade utilization is the main form of distributed systems1]. In most cases the operation mode of tri-generation is in variable working conditions because of the non-
* Corresponding author. T el.: 86 10 62796113; fax:86 10 62773461. E-mail address: [email protected].
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.214
Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
1361
synchronized and fluctuating loads, leading to the low system efficiency at part load conditions. To improve the system performance, it is an effective way to combine tri-generation system with energy storage system. In fact, the impact of energy storage on tri-generation system saving ratio is related to many factors and some BCHP-TES systems are not energy efficient [3]. For the typical form of BCHP, composed by a gas turbine and an absorption chiller, the main purpose of energy storage is to make the equipment run as efficiently as possible by improving the system performance in part load conditions. Phase change material (PCM) energy storage is widely used in engineering for high energy storage density [4], but there are also many problems in the BCHP-TES tri-generation such as how the system is running, when TES charges, when the TES discharges, how to determine the phase change temperature. In previous studies on BCHP-TES, the load is square or sine wave and the mass flow rate of gas for energy charge and discharge is constant [5]. The optimal phase change temperature is not available for an actual project. In this paper, the approach is based on actual load, and the mass flow rate of gas for energy charge and discharge varies according to the actual load hourly. For a given actual load, we can get an optimal phase change temperature and a minimum of primary energy consumption. The results are useful to the selection of PCM and real engineering equipment capacity. 2. System model 2.1. System form In summer working conditions, operation strategies are shown in Figures 1and 2. In the off-peak time when the user load is less than the average load, the exhaust gas from the gas turbine flow into the rpt absorption chiller (cooling load), and the extra heat goes into the TES to store energy. The mass flow determined ned by the use user load hourly, the mass flow rate m1 of exhaust gas entering the absorption chiller is dete rate m2 of gas flow for energy storage is determined by m1 that m2 m m1 . In the peak time, when the user load is higher than the average load, all the exhaust gas of gas turbine cannot meet the requirements of cooling load. Thus, it is needed to take out the heat which is stored in the TES in the off-peak time. The gas going out from the absorption chiller is used to take out heat from the TES and its temperature is the outlet temperature T a,o of absorption chiller. Two Tw strands of gas flow into sorp the absorption chiller together after mixing. The mass flow rate of discharge m3 is determined by the user load. m3 will not only affect heat consumption of absorption refrigeration but also affect the COP of absorption chiller.
Figure 1
In the off-peak period
Figure 2 In the peak period
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Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
The energy supplied for the user: In the off-peak period: ˄1˅
Qc =m1c p,a (T Ta,o )COP
In the peak period: Qc =Q COP
(m m3 )c p ,a (
mT m3Td ,o m m3
Ta ,o )COP
˄2˅
2.2. Gas turbine The calorific value of natural gas going into the gas turbine (power generation efficiency η) is Qg , Electricity generation directly supplied to users is P. The exhaust gas with temperature T and mass flow rate m flows into the absorption chiller’s generator, to drive absorption refrigeration cycle as a hightemperature heat source. For gas turbines, according to energy conservation (assume no energy loss): Qg P
mc p ,a (T T0 ) ˄˅
The efficiency of gas turbine reduces at part-load when the working condition of the gas turbine varies [6]. K P P P P P f ( ) 2.648 3.587( ) 2 2.816( ) 3 0.913( ) 4 Ke Pe Pe Pe Pe Pe ˄˅ where η e is the rated efficiency of the gas turbine while η is the actual one. P e is the rated power while P is the actual one. The actual temperature T1 of exhaust gas and the rated one T1e can be calculated according to the relationship between the exhaust heat. T1 1 K Ke P T1e 1 K e K Pe ˄˅ 2.3. Absorption chiller From the thermodynamic principles, an absorption chiller can be regarded as the combination of a heat engine and a heat pump. The COP of an absorption chiller is mainly affected by generation temperature, evaporation temperature, condensation temperature. To simplify analysis, we use the temperature of the gas turbine exhaust gas, the chilled water temperature, the ambient temperature takes the place of generation temperature evaporating temperature, evaporation temperature separately, then we can get the COP T T T T COP P (1 0 ) e P (1 0 ) e Tg Tc Te
T T0 Te
˄˅
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Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
where μ is the thermodynamic perfectness of the refrigeration cycle. When the gas turbine runs under the rated condition and the exhaust gas temperature is the rated temperature, the generation temperature of absorption is the highest and the COP reaches the maximum. Assuming Tg = T = 773K, T0 = Tc = 303K, Te = 280K, and the rated COP of absorption chiller is 1.2, we can get the thermodynamic perfectness μ = 0.163. If μ remains the same in variable conditions, we can find the actual COP of absorption chiller in part load conditions [7]. 2.4. Energy storage device In energy charge process, the relationship of exhaust gas of gas turbine and energy storage medium satisfies the equation:
T Tc ,o T Tm
1 exp NTUc
˄7˅
In energy discharge process, the relationship of fluid used to take out heat and energy storage medium satisfies the equation:
Td ,o Ta ,o Tm Ta ,o
1 exp NTUd
˄8˅
where T is the temperature of exhaust gas of the gas turbine, Tm is the phase change temperature, Tc,o and Td,o are the outlet temperature of energy storage fluid and energy discharge fluid respectively, Ta,o is the outlet temperature of flue gas at the outlet of absorption outlet. In addition, energy balance equation must be meet in one charge and discharge cycle for the TES device. NTUc and NTUd are the Heat transfer unit number of charge and dis charge. 3. Calculation method For ideal energy storage, the average of user load is used to divide the peak and off-peak, and the installed capacity is determined according to that average value. However, for PCM-TES, in the off-peak period, because the exhaust gas into the absorption chiller is of the high temperature, and the COP of the absorption chiller is 1.2 constantly. In the peak period, the flue gas into the absorption chiller is the mixed gas of flue gas through gas turbine and energy discharge fluid from the TES. The temperature will be lower than the flue gas temperature at the gas turbine outlet, then the COP of absorption chiller will be lower than 1.2. The specific value can be calculated according to Eq. (6). In addition, due to the reduced energy storage temperature difference, the phase change energy storage will be less than ideal one. Firstly we also select the average of user load at a typical day to divide the peak and off-peak, and then we can get the corresponding mass flow rate of gas turbine flue gas. Under this condition, the total of energy charge will be less than the total of energy discharge in one cycle. Difference of energy discharge and charge will come into being and it can be calculated. We can make the difference reduced to zero by increasing the mass flow rate m of gas turbine flue gas. The phase change temperature Tm of the TES and m are coupled. When only one phase change temperature makes the difference equal to 0, the primary energy consumption reaches the minimum. At this time, m is minimal and the phase change temperature is the best.
4. Illustrative Example
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Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
We select a hotel in Beijing in summer working condition and a typical day hourly load is known (Figure 3). Assuming the rated power generation efficiency of gas turbine η e=35%, the temperature of flue gas out of the absorption chiller Ta,o = 453K, the specific volume of flue gas cp, a = 1.2kJ / (kgg K), the temperatures of supply and return chilled water are 280K and 285K respectively . When NTU is infinite, the smallest gas turbine flue gas mass flow rate is 1.575kg/s and the optimal phase change temperature Tm is 586K. We can calculate the power generation efficiency of gas turbine, power volume and the COP of the absorption chiller according to the cooling load hourly. The sale of electricity will be determined according to the demand of electrical load, as shown in Figure 4, then the total primary energy consumption (natural gas calorific value) at the typical day can be calculated. Excessive electricity will be sold to large power grids, the corresponding primary energy consumption should subtract the earning brought by the sale of excess electricity. For comparison purpose, the primary energy consumption is converted into natural gas calorific value uniformly, which assumes that the large power grid is formed combined natural gas power generation cycle (efficiency W h natural gas at this time and the 55%). The minimum of primary energy consumption is 1.83h104 kW energy saving ratio is 12.7% compared to separate generation system. The relationship of primary energy consumption and phase change temperature is shown in Figure 5.
Figure 3 a typical day hourly load Figure 4 the sale of electricity When NTU is finite, the best phase change temperature and the minimum of primary energy consumption will increase. When NTU = 2, the phase change temperature Tm = 596K while the minimum of primary energy consumption is 1.85h kW•h.The maximum of energy saving rate is 11.9%. The relationship of primary energy consumption and phase change temperature is shown in Figure 5. When primary energy consumption is higher than the minimum, there will be two phase change temperature corresponding to one primary energy consumption on the left and right side of best phase change temperature.
Figure 5
The relationship of primary energy consumption and phase change temperature
Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
5. Conclusions In this paper, a new method is proposed to determine the optimal phase change temperature for energy storage on fluctuating loads in BCHP system with PCM. For a given typical day hourly load, the minimum of primary energy consumption and the optimal phase change temperature are calculated. Compared to separate generation system, the maximum energy saving ratio is 12.7% with infinite NT U while the maximum energy saving ratio is 11.9% with finite NTU (NTU=2 in our case study). When NTU is finite, the optimal phase change temperature and the minimum of PEC will increase. The curve which shows the relationship of primary energy consumption and phase change temperature. When PEC is higher than the minimum, there will be two phase change temperature corresponding to one primary energy consumption on the left and right side of best phase change temperature. It has a reference significance for selecting the actual project phase change material. Acknowledgements This research is financed by National Natural Science Foundation of China (51376098). References [1] Wu DW, Wang RZ. Combined cooling, heating and power: A review. Progress in Energy and Combustion Science, 2006,32(5-6):459-495. [2] Wang JJ, Jing YY, Zhang CF, et al. Performance comparison of combined cooling heating and power system in different operation modes. Applied Energy, 2011,88(12):4621-4631. [3] Khan KH, Rasul MG, Khan MMK, Energy conservation in buildings: Cogeneration and cogeneration coupled with thermal energy storage. Applied Energy, 2004, 77(1): 15-34. [4] Aceves-Saborio, S., Nakamura, H. and Reistad, G.M. (1994). ‘Optimum efficiencies and phase change temperatures in latent heat storage systems’, ASME J. Energy Res. Technol., 116,79-86. [5] Teng XG, Wang X, Chen YL, Shi WX. A simple method to determine the optimal gas turbine
capacity and operating strategy in building cooling,heating and power system, Energy and Buildings,2014,80,623-630 [6] Campos CA, Odriozola M, Sala JM. Implications of the modelling of stratified hot water storage tanks in the simulation of CHP plants. Energy Conversion and Management, 2011, 52(8-9):3018-3026. [7] Bailey JM, Davidson AW, Smith GR, et al. Evaluation of thermal energy storage and recovery for an electrical energy mediator system. Simulation Modelling Practice and Theory, 2011,19(4):1164-1174. Biography Fei Xiong is a Ph.D. candidate in department of building science, school of architecture, Tsinghua University. He got bachelor's degree of engineering in Huazhong University of Science and Technology 2013. His research interest centers on building energy efficiency.
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ScienceDirect Energy Procedia 75 (2015) 1360 – 1365
The 7 th International Conference on Applied Energy – ICAE2015
Optimal phase change temperature for energy storage based on fluctuating loads in building cooling heating and power system Fei Xiong, Yin Zhang, Xin Wang*, Yinping Zhang Department of Building Science, Tsinghua University, Beijing 100084, China
Abstract
Natural gas driven Building Cooling Heating and Power (BCHP) system is encouraged to develop as small-scale energy supply system in recent year. Integrating Energy Storage System (TES) with trigeneration system can improve the energy efficiency and reduce the installed capacity for energy supply equipment. However, it has never been an easy task to reasonably design the TES-BCHP system, in terms of that it is hard to determine the suitable installed capacity of energy supply devices considering the fluctuating loads and the influence of TES. In this paper, a thermodynamic model of BCHP system with phase change material (PCM) energy storage is established. According to practical user loads and considering the correspondingly hourly changing flow rate in charge and discharge processes, a simplified method is proposed to determine the optimal phase change temperature for BCHP-TES, aiming to minimize the primary energy consumption of BCHP-TES system. An illustrative example shows that the optimal phase change temperature can be obtained for given loads. It also indicates that compared to infinite NTU for PCM-TES, the optimal phase change temperature will increase as well as the corresponding minimal primary energy consumption for finite NTU. This work is of great importance for PCM optimization of BCHP-TES system design. keywords: T ri-generation , Energy storage , Phase change , Energy saving, Flow rate
1. Introduction Distributed energy supply system is of many advantages, such as high energy efficiency, low emissions, small-capacity, being close to the user and so on. The Building Cooling Heating and Power (BCHP) system based on the principle of cascade utilization is the main form of distributed systems1]. In most cases the operation mode of tri-generation is in variable working conditions because of the non-
* Corresponding author. T el.: 86 10 62796113; fax:86 10 62773461. E-mail address: [email protected].
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.214
Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
1361
synchronized and fluctuating loads, leading to the low system efficiency at part load conditions. To improve the system performance, it is an effective way to combine tri-generation system with energy storage system. In fact, the impact of energy storage on tri-generation system saving ratio is related to many factors and some BCHP-TES systems are not energy efficient [3]. For the typical form of BCHP, composed by a gas turbine and an absorption chiller, the main purpose of energy storage is to make the equipment run as efficiently as possible by improving the system performance in part load conditions. Phase change material (PCM) energy storage is widely used in engineering for high energy storage density [4], but there are also many problems in the BCHP-TES tri-generation such as how the system is running, when TES charges, when the TES discharges, how to determine the phase change temperature. In previous studies on BCHP-TES, the load is square or sine wave and the mass flow rate of gas for energy charge and discharge is constant [5]. The optimal phase change temperature is not available for an actual project. In this paper, the approach is based on actual load, and the mass flow rate of gas for energy charge and discharge varies according to the actual load hourly. For a given actual load, we can get an optimal phase change temperature and a minimum of primary energy consumption. The results are useful to the selection of PCM and real engineering equipment capacity. 2. System model 2.1. System form In summer working conditions, operation strategies are shown in Figures 1and 2. In the off-peak time when the user load is less than the average load, the exhaust gas from the gas turbine flow into the rpt absorption chiller (cooling load), and the extra heat goes into the TES to store energy. The mass flow determined ned by the use user load hourly, the mass flow rate m1 of exhaust gas entering the absorption chiller is dete rate m2 of gas flow for energy storage is determined by m1 that m2 m m1 . In the peak time, when the user load is higher than the average load, all the exhaust gas of gas turbine cannot meet the requirements of cooling load. Thus, it is needed to take out the heat which is stored in the TES in the off-peak time. The gas going out from the absorption chiller is used to take out heat from the TES and its temperature is the outlet temperature T a,o of absorption chiller. Two Tw strands of gas flow into sorp the absorption chiller together after mixing. The mass flow rate of discharge m3 is determined by the user load. m3 will not only affect heat consumption of absorption refrigeration but also affect the COP of absorption chiller.
Figure 1
In the off-peak period
Figure 2 In the peak period
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Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
The energy supplied for the user: In the off-peak period: ˄1˅
Qc =m1c p,a (T Ta,o )COP
In the peak period: Qc =Q COP
(m m3 )c p ,a (
mT m3Td ,o m m3
Ta ,o )COP
˄2˅
2.2. Gas turbine The calorific value of natural gas going into the gas turbine (power generation efficiency η) is Qg , Electricity generation directly supplied to users is P. The exhaust gas with temperature T and mass flow rate m flows into the absorption chiller’s generator, to drive absorption refrigeration cycle as a hightemperature heat source. For gas turbines, according to energy conservation (assume no energy loss): Qg P
mc p ,a (T T0 ) ˄˅
The efficiency of gas turbine reduces at part-load when the working condition of the gas turbine varies [6]. K P P P P P f ( ) 2.648 3.587( ) 2 2.816( ) 3 0.913( ) 4 Ke Pe Pe Pe Pe Pe ˄˅ where η e is the rated efficiency of the gas turbine while η is the actual one. P e is the rated power while P is the actual one. The actual temperature T1 of exhaust gas and the rated one T1e can be calculated according to the relationship between the exhaust heat. T1 1 K Ke P T1e 1 K e K Pe ˄˅ 2.3. Absorption chiller From the thermodynamic principles, an absorption chiller can be regarded as the combination of a heat engine and a heat pump. The COP of an absorption chiller is mainly affected by generation temperature, evaporation temperature, condensation temperature. To simplify analysis, we use the temperature of the gas turbine exhaust gas, the chilled water temperature, the ambient temperature takes the place of generation temperature evaporating temperature, evaporation temperature separately, then we can get the COP T T T T COP P (1 0 ) e P (1 0 ) e Tg Tc Te
T T0 Te
˄˅
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Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
where μ is the thermodynamic perfectness of the refrigeration cycle. When the gas turbine runs under the rated condition and the exhaust gas temperature is the rated temperature, the generation temperature of absorption is the highest and the COP reaches the maximum. Assuming Tg = T = 773K, T0 = Tc = 303K, Te = 280K, and the rated COP of absorption chiller is 1.2, we can get the thermodynamic perfectness μ = 0.163. If μ remains the same in variable conditions, we can find the actual COP of absorption chiller in part load conditions [7]. 2.4. Energy storage device In energy charge process, the relationship of exhaust gas of gas turbine and energy storage medium satisfies the equation:
T Tc ,o T Tm
1 exp NTUc
˄7˅
In energy discharge process, the relationship of fluid used to take out heat and energy storage medium satisfies the equation:
Td ,o Ta ,o Tm Ta ,o
1 exp NTUd
˄8˅
where T is the temperature of exhaust gas of the gas turbine, Tm is the phase change temperature, Tc,o and Td,o are the outlet temperature of energy storage fluid and energy discharge fluid respectively, Ta,o is the outlet temperature of flue gas at the outlet of absorption outlet. In addition, energy balance equation must be meet in one charge and discharge cycle for the TES device. NTUc and NTUd are the Heat transfer unit number of charge and dis charge. 3. Calculation method For ideal energy storage, the average of user load is used to divide the peak and off-peak, and the installed capacity is determined according to that average value. However, for PCM-TES, in the off-peak period, because the exhaust gas into the absorption chiller is of the high temperature, and the COP of the absorption chiller is 1.2 constantly. In the peak period, the flue gas into the absorption chiller is the mixed gas of flue gas through gas turbine and energy discharge fluid from the TES. The temperature will be lower than the flue gas temperature at the gas turbine outlet, then the COP of absorption chiller will be lower than 1.2. The specific value can be calculated according to Eq. (6). In addition, due to the reduced energy storage temperature difference, the phase change energy storage will be less than ideal one. Firstly we also select the average of user load at a typical day to divide the peak and off-peak, and then we can get the corresponding mass flow rate of gas turbine flue gas. Under this condition, the total of energy charge will be less than the total of energy discharge in one cycle. Difference of energy discharge and charge will come into being and it can be calculated. We can make the difference reduced to zero by increasing the mass flow rate m of gas turbine flue gas. The phase change temperature Tm of the TES and m are coupled. When only one phase change temperature makes the difference equal to 0, the primary energy consumption reaches the minimum. At this time, m is minimal and the phase change temperature is the best.
4. Illustrative Example
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Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
We select a hotel in Beijing in summer working condition and a typical day hourly load is known (Figure 3). Assuming the rated power generation efficiency of gas turbine η e=35%, the temperature of flue gas out of the absorption chiller Ta,o = 453K, the specific volume of flue gas cp, a = 1.2kJ / (kgg K), the temperatures of supply and return chilled water are 280K and 285K respectively . When NTU is infinite, the smallest gas turbine flue gas mass flow rate is 1.575kg/s and the optimal phase change temperature Tm is 586K. We can calculate the power generation efficiency of gas turbine, power volume and the COP of the absorption chiller according to the cooling load hourly. The sale of electricity will be determined according to the demand of electrical load, as shown in Figure 4, then the total primary energy consumption (natural gas calorific value) at the typical day can be calculated. Excessive electricity will be sold to large power grids, the corresponding primary energy consumption should subtract the earning brought by the sale of excess electricity. For comparison purpose, the primary energy consumption is converted into natural gas calorific value uniformly, which assumes that the large power grid is formed combined natural gas power generation cycle (efficiency W h natural gas at this time and the 55%). The minimum of primary energy consumption is 1.83h104 kW energy saving ratio is 12.7% compared to separate generation system. The relationship of primary energy consumption and phase change temperature is shown in Figure 5.
Figure 3 a typical day hourly load Figure 4 the sale of electricity When NTU is finite, the best phase change temperature and the minimum of primary energy consumption will increase. When NTU = 2, the phase change temperature Tm = 596K while the minimum of primary energy consumption is 1.85h kW•h.The maximum of energy saving rate is 11.9%. The relationship of primary energy consumption and phase change temperature is shown in Figure 5. When primary energy consumption is higher than the minimum, there will be two phase change temperature corresponding to one primary energy consumption on the left and right side of best phase change temperature.
Figure 5
The relationship of primary energy consumption and phase change temperature
Fei Xiong et al. / Energy Procedia 75 (2015) 1360 – 1365
5. Conclusions In this paper, a new method is proposed to determine the optimal phase change temperature for energy storage on fluctuating loads in BCHP system with PCM. For a given typical day hourly load, the minimum of primary energy consumption and the optimal phase change temperature are calculated. Compared to separate generation system, the maximum energy saving ratio is 12.7% with infinite NT U while the maximum energy saving ratio is 11.9% with finite NTU (NTU=2 in our case study). When NTU is finite, the optimal phase change temperature and the minimum of PEC will increase. The curve which shows the relationship of primary energy consumption and phase change temperature. When PEC is higher than the minimum, there will be two phase change temperature corresponding to one primary energy consumption on the left and right side of best phase change temperature. It has a reference significance for selecting the actual project phase change material. Acknowledgements This research is financed by National Natural Science Foundation of China (51376098). References [1] Wu DW, Wang RZ. Combined cooling, heating and power: A review. Progress in Energy and Combustion Science, 2006,32(5-6):459-495. [2] Wang JJ, Jing YY, Zhang CF, et al. Performance comparison of combined cooling heating and power system in different operation modes. Applied Energy, 2011,88(12):4621-4631. [3] Khan KH, Rasul MG, Khan MMK, Energy conservation in buildings: Cogeneration and cogeneration coupled with thermal energy storage. Applied Energy, 2004, 77(1): 15-34. [4] Aceves-Saborio, S., Nakamura, H. and Reistad, G.M. (1994). ‘Optimum efficiencies and phase change temperatures in latent heat storage systems’, ASME J. Energy Res. Technol., 116,79-86. [5] Teng XG, Wang X, Chen YL, Shi WX. A simple method to determine the optimal gas turbine
capacity and operating strategy in building cooling,heating and power system, Energy and Buildings,2014,80,623-630 [6] Campos CA, Odriozola M, Sala JM. Implications of the modelling of stratified hot water storage tanks in the simulation of CHP plants. Energy Conversion and Management, 2011, 52(8-9):3018-3026. [7] Bailey JM, Davidson AW, Smith GR, et al. Evaluation of thermal energy storage and recovery for an electrical energy mediator system. Simulation Modelling Practice and Theory, 2011,19(4):1164-1174. Biography Fei Xiong is a Ph.D. candidate in department of building science, school of architecture, Tsinghua University. He got bachelor's degree of engineering in Huazhong University of Science and Technology 2013. His research interest centers on building energy efficiency.
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