The Large-scale Wind Power Integration Using The Integrated Heating Load and Heating Storage Control

The Large-scale Wind Power Integration Using The Integrated Heating Load and Heating Storage Control Yulong Yang1, Kai Wu1, Xu Yan2, Jianchao Gao1, Hongyu Long31. State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi’an, China;2. Electric Power Research Institute of Guangxi Power Grid Co., Ltd., Nanning, China;3. College of Engineering and Technology, Southwest University, Chongqing, China;yougyokuryuu@Abstract—Aiming at the challenge of large-scale wind power integration, a new method for integrating the heating load and heating storage control is proposed based on district energy system with a core of CHP. In the new method, thermal energy storage (TES) can increase the flexibility of CHP’s heat output and power output to follow the fluctuation of wind power. The distributed electric heat pumps (EHPs) are used for heating load control to increase the capability to balance the wind power and electricity demand. In addition, the building thermal inertia and human thermal comfort are taken into account to make the power consumption of EHPs to follow the wind power. At last, the simulation results show the effect on balancing the wind power and electricity demand as well as the influence of space heating load characteristic.Index Terms—Wind power, heating load, heating storage, combined heat and power (CHP).I.I NTRODUCTIONThis paper aims at the difficulty in large-scale wind power integration in China. In last decade, China has a rapid growth for wind power generation under the support of national policy of renewable energy, especially in northern China [1]. However, the intermittence and volatility of wind power generation has become the obstacle for the integration of large-scale wind power, especially in the area with the high penetration of coal-fired combined heat and power (CHP) units in northern China. Currently CHP technology is supported by national policy, and the gross installed capacity of CHP units accounted for 14.05% of the total installed generating capacity [2]. However, because CHP units are excluded for electricity peak regulation with restrict of space heating load, lots of wind power must be curtailed in winter. Approximately 20% of the wind power could not be integrated into the power network and must be abandoned in China [3].Aiming at the co-existence of large-scale wind power and district energy system based on CHP units, the problem of wind power integration is important for the stable operation of electric power system. According to some researches [4-7], the flexible operation of CHP with introduction of electric heat pumps (EHPs), electric boilers and thermal energy storages can compensate the wind power fluctuation more easily. In northern China, the district heating system is widely used for heating in winter from early on, but the distributed EHPs such as air-conditioner has also been installed in recent years and is nearly only used for cooling in summer. In addition, many researches about distributed CHP plants equipped with thermal energy storages have been carried out [8-10], but the thermal energy storages are seldom used in large-scale CHP plants in China. With respect to the space heating demand, the thermal characteristics of demand-side such as the thermal inertia of buildings and the thermal comfort of end users [11-14] have seldom been realized by researchers in the field of electric power system.Therefore, the authors present a new method for integrating the heating load and heating storage control to facilitate the wind power integration based on the district energy system. Firstly, the centralized thermal energy storage is used for cooperating with CHP’s operation. Secondly, the distributed electric heat pumps (EHPs) widely used by city dwellers are introduced into the district energy system as the heating source. At last, the thermal characteristics of demand side such as the thermal inertia of buildings and thermal comfort of end users are taken into consideration. Thus, the heating load and heating storage control for day-ahead plan can be achieved through the district energy system as Fig.1, so as to mitigate the negative impact of wind power on the powergrid.Figure 1. District energy systemThis article is configured in six sections. Besides this section that serves as introduction, the methodology andmathematical model are put forth in Section II and Section III. Section IV is devoted to a numerical simulation. Results and analysis are given in Section V. Finally, Conclusions are drawn in Section VI. II. M ETHODOLOGY The heating load and heating storage control is applied for the district energy system, which consists of large-scale CHP unit, thermal energy storage and distributed EHPs. The district energy system can utilize the thermal energy storage and distributed EHPs with consideration of building thermalinertia and human thermal comfort to increase the flexibility of heating supply. This makes it possible to provide peakingservice for power grid to help wind power integration in a day-ahead plan by means of CHP and EHPs. In detailed, thermal energy storage (TES) such as hot-water tank is low-cost and can be large enough for storing lots of hot water to match with the large-scale CHP unit. The thermal energy storage can store heat from CHP unit at theoff-peak heating load, and then release heat to compensate the on-peak heating load. This can increase the flexibility of CHP’s heat output to enlarge the adjustable range of power output as Figure 2, so as to compensate the off-peak windpower.Figure 2. P-Q chart of CN300/200-16.7 extraction condensing turbine CHPThe distributed EHPs such as air-condition are commonlyused in Chinese cities and can be used to replace parts of heating load assumed by CHP units currently. This makes itpossible to utilize surplus wind power for space heating. In addition, this can also increase the flexibility of CHP’s heating and electricity supply, and thus increase the capability tobalance the wind power and electricity demand.The buildings have a considerable heat storage capacitydue to the thermal inertia of building materials. Heat is stored in buildings and the indoor temperature rises when the heating supply is greater than the space heating load; on the contrary,heat is released from buildings, and the indoor temperature drops. Considering the thermal comfort of end users, the indoor temperature is allowed vary in a certain range. Thus, the heat output of CHP and EHPs can be adjusted to changethe heat balance of buildings and make the power consumption of EHPs and power output of CHP to follow thewind power.III. M ATHEMATICAL MODEL A. Objective Function In general, the peak-valley difference and step changes of load will increase with the integration of large-scale wind power. Thus, the optimization objective is set to reduce thestandard deviation of equivalent load as equation (1):(1) where, the adjusted equivalent load Equivalent load ()Pt (MW) expressed as equation (2), and Equivalent load Pis the average of adjusted equivalent load (MW): Equivalent load load Wind CHP EHPs ()=()()()()P t P t P t p t p t −−+ (2) where, P load (t ) is the electricity load (MW), and P wind (t ) is the available wind power (MW). CHP ()p t is the power output of CHP unit (MW) . EHPs()p t is the power consumption of EHPs(MW).B. Constraint of CHP UnitThe power output of CHP units can vary between minimum and maximum power output, as described by equation (3):min max CHP CHP CHP ()()()p t p t p t ≤≤ (3)min CHP ()p t and maxCHP ()p t are constrained by the heat output of CHP CHP ()h t (MW) through equation (4)~(6):min min min CHP CHP CHP CHP ()()p t l h t n =⋅+ (4)max max max CHP CHP CHP CHP ()()p t l h t n =⋅+ (5) max CHP CHP 0()()h t h t ≤≤ (6)C. Heating Storage Control The cumulative heat storage at step t TES ()endC t (MW·h) can be get through equation (7): TES.ini TES.in TES.out TES TES TES.in TES.outC +(()())(=1)()=(1)+(()())(1)end t h t h t t C t C t t h t h t t Δ⋅−⎧⎨−Δ⋅−>⎩ (7) where, TES.ini C is the initial heat storage in a day (MW·h).TES.in ()h t is the heat input of TES(MW) .TES.out ()h t is the heat output of TES (MW). In addition, the TES ()endC t , TES.in ()h t , TES.out ()h t are constrained by equation (8) and (9): maxTES TES 0()end C t C ≤≤ (8)maxTES.in TES.inmax TES.out TES.out()()h t h h t h ≤≤ (9)where, max TESC is the maximum capacity of TES (MW·h). maxTES.in h is the maximum heat input of TES (MW). maxTES.outh is the maximum heat output of TES (MW).D. Heating Load Control The total heat power of end users end ()h t (MW) is supplied by the hot water water ()h t from CHP system (MW) and thermal power EHPs ()h t from EHPs (MW):end water EHPs ()()()h t h t h t =+ (10)where the hot water water ()h t from CHP system is not onlysupplied by CHP unit but also involve the heat input and output of TES as equation (11): water CHP TES.out TES.in ()()()()h t h t h t h t =+− (11)meanwhile, the thermal power from EHPs of end users, which is constrained by the installed capacity capacityEHPsH (MW) and the coefficient of performance (COP) of EHPs through equation (12) and (13): capacityEHPs EHPs 0()h t H ≤≤ (12) EHPs EHPs ()()h t COP p t =⋅ (13)Where, EHPs ()p t is the power consumption of EHPs.E. Thermal Inertia and Thermal ComfortThe building can be regarded as a thermal storage device because of the thermal inertia of building materials. The thermal inertia can be expressed through equation (14) according to the law of conservation of energy:out end air air out end 1()=exp()((1)()())1(()()K F T t t T t T t h t c V K F T t h t K Fρ⋅−⋅Δ⋅−−−⋅⋅⋅⋅++⋅⋅ (14) where, T (t ) is the indoor temperature (°C),out ()T t is theoutdoor temperature (°C), end ()h t is the heat consumption of end users (MW). K is the average thermal conductivity of buildings (W·m -2·°C -1), F is the external surface area (m 2) of buildings, V is the buildings volume or the indoor air volume (m 3), air c and air ρ are the specific heat (kJ·kg -1·°C -1) and the density of indoor air (kg·m -3). Δt is the time step.Lastly, taking account of the human thermal comfort, the indoor temperature should vary in a certain range betweenminimum thermal comfort temperature down T and maximum thermal comfort temperature up T of end users: down up ()T T t T ≤≤ (15)IV. N UMERICAL SIMULATIONThe simulations for new method and original method without regard to EHPs and TES are carried out respectively.NLP problem is implemented in GAMS/MOSEK. The specific parameters in the case studies are showed as follow: A. Wind PowerA wind power prediction system based on numericalweather prediction and artificial neural network has been usedto forecast wind power for the dispatching center of power grid in China [15]. According to the specification of windpower forecasting in China, a wind power curve with 15-minutes interval form 0 to 24 o’clock is predicted one day inadvance. Thus, a prediction wind power curve in a typicalwinter day is used as simulation example as shown in Figure 3.0501001502002503003504000:004:008:0012:0016:0020:00W i n d P o w e r (M W )Time(Hour)Figure 3. Prediction wind power curveB. Electricity Load10001100120013001400150016000:004:008:0012:0016:0020:00E l e c t r i c i t y L o a d (M W )Time(Hour)Figure 4. Electricity loadIn this report, the forecasted electricity load curve is shownas Figure 4.C. Heating LoadTemperature difference between day and night brings about the variation of space heating load. In general, the daily temperature difference is the typical characteristic of daily temperature. Thus, three typical space heating load are used in this paper as shown in Figure 5 based on the daily outdoor temperature differences.0501001502002503003504004505000:004:008:0012:0016:0020:00H e a t i n g L o a d (M W )Time(Hour)High Medium LowFigure 5. Space heating loadD. District Energy SystemTABLE I.T HE PARAMETERS OF BUILDINGSParameters ofbuildingsAverage thermal conductivity K (W·m -2·°C -1)Specific heat of indoor airc air (kJ·kg -1·°C -1) Density of indoor air ρair (kg·m -3) Value 0.51.0071.2energy storage and end users. Firstly, a set of CHP unit is assumed to be in the district energy system, and the P-Q chart of CN300/200-16.7 extraction condensing turbine CHP is illustrated in Figure 2; secondly, a large-scale hot water tank with the capacity of 2000MWh is assumed to be equipped for CHP, and the maximum heat output and input are set to be 500MW; lastly, The parameters of buildings are listed in TABLE I, and the minimum thermal comfort temperature andmaximum thermal comfort temperature of end users are set to be 18 °C and 22°C.V.R ESULTS AND ANALYSISA. Electricity DispatchFigure 6 shows the original power output of CHP, the newpower output of CHP and the power consumption of EHPs. Itcan been seen that all of the three power curves can follow theequivalent load that needs adjusting in Figure 7, this makes itpossible for peak regulation and reducing the negative impactof wind power. However, comparing with the original scheduling plan, the fluctuation range of power output of CHPin new method is bigger due to the heating load and heatingstorage control, but the power output is smoother on thesmaller time scale, this is because of the compensation of power consumption of EHPs that can be adjusted moreflexibly than CHP.0501001502002503003500:004:008:0012:0016:0020:00E l e c t r i c a l P o w e r (M W )Time(Hour)Power output of CHP(new)Power output of CHP(original)Power consumption of EHPsFigure 6. The Electricity dispatch for CHP and EHPsFigure 7 shows the original adjusted equivalent load, the new adjusted equivalent load and the equivalent load that needs adjusting. The equivalent load is the difference between the electricity load and wind power, the peak-valley difference and fluctuation of which are larger than electricity load due to the integration of wind power. We can see that the adjusted equivalent load in new method is almost a straight line and smoother than original method, because the heating load and heating storage control enlarge the adjusted range of CHP’s power output, as well as the power consumption of EHPs can reduce the fluctuation of equivalent load.60070080090010001100120013000:004:008:0012:0016:0020:00L o a d (M W )Time(Hour)Equivalent loadAdjusted equivalent load(original)Adjusted equivalent load(new)Figure 7. The adjusted equivalent loadB. Heating Load and Heating Storage DispatchFigure 8 shows the heating load dispatch for CHP and EHPs in new method. The heating load dispatch brought about the indoor temperature variation as shown in Figure 9. Comparing with the original scheduling plan, on the one hand EHPs can share parts of space heating load to increase the flexibility of CHP’s power output; on the other hand, heating power can follow the fluctuation of equivalent load, this resulted in the indoor temperature variation.Figure 10 shows the heat input and output of TES. The heat input into TES from CHP at the time of low heating load and then the heat output from TES to compensate CHP’s hot water at the time of high heating load, this can increase the flexibility of CHP’s power output. Therefore, it can be seen that the heat input and output of TES followed the hot water of CHP in Figure 8.Furthermore, the heating load control for EHPs and heating storage control for TES can cooperate with each other very well, e.g. the high heat output of EHPs between 12 o’clock to 16 o’clock reduced the need for CHP’s hot water, this make spare hot water of CHP for heat input to TES. Thus, at the time of low equivalent load, the EHPs can not only compensate the load but also promote the heat input to TES, and then at the time of high equivalent load, the heat stored in TES can increase the power output of CHP to follow theequivalent load.01002003004005006007000:004:008:0012:0016:0020:00T h e r m a l P o w e r (M W )Time(Hour)Heat output of EHPs Hot water of CHPHeating loadFigure 8. The heating load dispatch for CHP and EHPs171819202122230:004:008:0012:0016:0020:00I n d o o r T e m p e r a t u r e (℃)Time(Hour)Figure 9. The variation of indoor temperature0501001502002503003504000:004:008:0012:0016:0020:00T h e r m a l P o w e r (M W )Time(Hour)Heat input Heat outputFigure 10. The heat input and output of TESC. Influence of Space Heating LoadFigure 11 shows the adjusted equivalent load for three typical space heating load. It can be seen that the fluctuation of adjusted equivalent load reduced gradually from Low case to High case, especially at the time of low equivalent load (12 o’clock to 16 o’clock). This is because that the power consumption of EHPs is restricted with the low space heating load, so that the equivalent load cannot be compensated well.80082084086088090092094096098010000:004:008:0012:0016:0020:00E q u i v a l e n t L o a d (M W )Time(Hour)High Medium LowFigure 11. The adjusted equivalent load for three typical space heating loadThus, the power consumption of EHPs reduced from High case to Low case due to the restriction of space heating load as listed in TABLE II. In addition, as the reduction of EHPs’ power consumption, the range of CHP’s power output is enlarged to compensate for the lack of EHPs’ power consumption as second line of TABLE II, this can attribute to the increase of TES’s heat output as third line of TABLE II.TABLE II.T HE DIFFERENT RESULTS FOR SPACE HEATING LOADHigh Medium Low Power consumption of EHPs(MWh)1564.53 1266.66731.60 Peak-valley difference of CHP'spower output (MW) 126.22 135.68 155.90 Heat output of TES (MWh)868.12943.221056.56D. Abondoned Wind Power Wind power have to be curtailed sometimes due to insufficient transmission line capacity or insufficient capability to balance the power supply and demand. Thus, Figure 12 shows the abandoned wind power for different balance capabilities of power grid and space heating loads. In this paper balance capability can be represented by the minimum power output of all the generators in power grid except CHP unit.Comparing with the original scheduling plan, theabandoned wind power is reduced significantly due to theheating load and heating storage control, especially the abandoned wind power is zero in high case and medium case.In addition, the abandoned wind power increased from highcase to low case in new method, and this trend is different from the original method. Therefore, we can see that the lower space heating load is good for increasing the flexibility of CHP, but not for the utilization of EHPs, especially whenwind power is high.50100150200250300350High Medium LowA b a n d o n e d w i n d p o w e r (M W h )Space heating load850MW(original)850MW(new)900MW(original)900MW(new)Figure 12. The abandoned wind powerVI. C ONCLUSIONSIn this paper, the integrated heating load and heating storage control method is proposed to promote the integration of large-scale wind power for day-ahead plan of power grid based on the district energy system, which consists of large-scale CHP unit, thermal energy storage and distributed EHPs, and meanwhile the building thermal inertia and human thermal comfort are considered. A case study of proposal is carried out, and the following results are obtained:1) The adjusted range of CHP’s power output can be enlarged through the integrated heating load and heating storage control, meanwhile the power output is smoother on the smaller time scale due to the flexibility of EHPs’ power consumption.2) The power consumption of EHPs can follow the equivalent load well with consideration of building thermal inertia and human thermal comfort, and this caused the indoor temperature variation.3) The heating load control for EHPs and heating storage control can cooperate with each other very well: the utilization of EHPs can promote the heat stored in TES, andthis is good for utilizing TES to compensate hot water of CHP to increase its power output in the next moment.4) The power consumption of EHPs is restricted with the space heating load. 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