Integrated DC Energy Management System

Transcription

1 Global Science and Technology Journal Vol. 3. No. 1. March 2015 Issue. Pp Integrated DC Energy Management System 1 Marufa Ferdausi and 2 K. M. A. Salam A complete, cost effective DC energy management system does not yet fulfil the demand. Using backup system can contribute to improved and higher quality advancement in the use of a DC bus signalling. We studied the integration of Vanadium Redox Battery (VRB) & DC backup system in a grid through simulation. MATLAB was used to do the simulation. The simulation results revealed that grid integrity of VRB is better than Lead Acid Battery and the system has 99.5% efficiency and a wireless power transfer system connectable to the grid with a capability of power 25% of emergency load. Field of Research: Renewable energy 1. Introduction In recent years, the energy storage of battery which is integrated to the DC network in a renewable energy system is widely studied (Barote et al. 2008, Li, Joós & Bélanger 2010, Locment, Sechilariu, & Houssamo 2012, Pang, Lo & Pong 2006 and Sun et al. 2011). Recently, Vanadium Redox battery (VRB) has also become popular for energy storage (Barote et al. 2008; Kear, Shah and Walsh 2011; Li, Joós and Bélanger 2010; Oudalov, Cherkaoui and Beguin 2007; Skyllas-Kazacos and Menictas 1997). Vanadium Redox Battery (VRB) technologies have demonstrated their ability to provide large-scale energy storage for applications including back-up power supplies, largescale energy storage for applications including remote area power supplies (RAPS), distributed power generation and power quality optimization. Although most of these applications are at the kw power scale, both MW and GW-scale stationary batteries have the potential to contribute to the improvement of energy efficiency as well as grid stabilization of power derived from renewable energy-based sources. In this paper, DC energy management system for emergency situations has been studied through simulation approach. 2. Literature Review Uninterruptable power systems have incorporated battery technology to allow smooth power feeding switch-over in the case of a power failure. In such systems lead-acid batteries have commonly been used until generators come on-line or for safe computer shutdown (Skyllas-Kazacos and Menictas 1997). 1 Department of Electrical and Computer Engineering, North South University, Dhaka, Bangladesh, ferdausi.09@gmail.com 2 Department of Electrical and Computer Engineering, North South University, Dhaka, Bangladesh, asalam@northsouth.edu

2 A Lead Acid Battery uses a combination of lead plates or grids. It also uses an electrolyte consisting of a diluted sulphuric acid to convert electrical energy into potential chemical energy and back again. Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) oxide (Pb0 2 ) in an electrolyte of about 37% wlw (5.99 Molar) sulfuric acid (H 2 S0 4 ). In the discharged state both electrodes turn into lead (II) sulfate (PbS0 4 ) and the electrolyte loses its dissolved sulfuric acid and becomes primarily water. Figure 1: Lead-Acid Battery Third Order Model The Fig.1 model is consisted of two main parts: a main branch which approximated the battery dynamics under most conditions, and a parasitic branch which is accounted for the battery behavior at the end of a charge. Where: Em is the main branch voltage, R1 is the main branch resistance 1, C1 is the main branch capacitance, R2 is the main branch resistance 2, I (V pn ) is the parasitic branch current, R0 is the terminal resistance. With no solid phase changes, the all-vanadium flow cell, containing the V 2+ /V 2+ (hypovanadous/vanadous ion) and VO 2+ /VO 2+ (vanadyl/vanadic ion) redox couples, is comparatively simple and will operate with high-cell and stack-energy efficiencies. It is a dual electrolyte system where the separation of the redox couples is usually achieved using a cation (proton, H + ) exchange membrane. 53

3 Figure 2: All Vanadium Redox Flow Battery Model A vanadium redox battery model is shown in Fig. 2. With a standard electrode potential (E o ) of approximately +1.00V versus the standard hydrogen electrode (SHE), the characteristic reaction of the all-vanadium battery at the positive electrode, which involves soluble V(IV) and V(V) species, is as follows: From Eqn. 1, the reversible reaction at the negative electrode involves V(II) and V(III) species and has a standard potential of V versus SHE: Vanadium Redox Battery has been analysed with simulation. The VRB has approximately 79% efficiency, i.e. 15% internal and 6% parasitic losses, at the operating point of 20% state of charge (SOC) and rated discharging current. 54

4 Numerous researches have been done to compare the advantages of VRB over LAB like: i. The VRB is a more flexible system because the energy storage and power can be separately designed, whereas in the LAB, the power and energy storage are functions of each other (i.e., based on the number of battery cells). ii. It has a higher efficiency. iii. It can accept a larger depth of discharge. iv. It has a slightly higher energy density. v. The maintenance cost of the VRB is lower and easy maintenance. vi. The VRB can withstand a significantly larger number of charge/discharge cycles, and thus has a longer life span. This is because the active materials are kept in solution and are not lost nor degraded by battery cycling. In addition, a life-cycle assessment approach of the environmental impact of both the VRB and LAB for use in stationary applications indicates that the VRB contributes between 7-25 % of emissions of key environmental impact components (CO 2, SO 2, CO, CH 4, NO x ) during its life cycle, when compared with lead-acid batteries. It is evaluated that the VRB has a lower cost per kwh of energy storage using the following equation: (8) A VRB system has a high speed response and is not aged by frequent charging and discharging. The battery efficiency actually increases when the charge/discharge period becomes shorter, unlike the LAB battery. It has also a short duration overload capacity and a long service life. Thus, this relatively new electrochemical technology is well suited for enhancing the utilization of renewable energy. Table I shows the key performance targets for grid storage applications. A report is prepared by the Nexight Group based upon a workshop convened by Sandia, PNNL, and the Minerals, Metals, and Materials Society (TMS) for the US Department of Energy, suggests the following cost performance targets for key utility applications, and identify cost targets for VRB of $250/kWh in capital costs in 2015, decreasing to $100/kWh by 2030 (Weber et al. 2011). 55

5 Table 1: Key Performance Targets for Grid-Storage Applications of VRB Application Purpose Key performance targets Area and frequency regulation (short duration) Reconciles momentary differences between supply and demand within a given area Service cost: $20/MW Roundtrip efficiency: 85 90% System lifetime: 10 years Discharge duration: 15 min 2 h Renewable grid integration (short duration) Transmission and distribution upgrade deferral (long duration) Load following (long duration) Electric energy time shift (long duration) Offsets fluctuations of short-duration variation of renewable generation output. Accommodates renewable generation at times of high grid congestion. Delays or avoids the need to upgrade transmission and/or distribution infrastructure. Reduces loading on existing equipment to extend equipment life. Changes power output in response to the changing balance between energy supply and demand. Operates at partial load (i.e., increased output) without compromising performance or increasing emissions. Stores inexpensive energy during low demand periods and discharges the energy during times of high demand (often referred to as arbitrage) Response time: milliseconds Roundtrip efficiency: 90% Cycle life: 10 years Capacity: 1 20 MW Response time: 1 2 s Cost: $500/kWh Discharge duration: 2 4 h Capacity: MW Reliability: 99.9% System life: 10 years Capital cost: $1,500/kW or $500/kWh Operations and maintenance cost: $500/kWh Discharge duration: 2 6 h Capital cost: $1,500/kW or $500/kWh Operations and maintenance cost: $250 $500/kWh Discharge duration: 2 6 h Efficiency: 70 80% Response time: 5 30 min In IEEE Std (IEEE 1995) the supply of uninterrupted power is discussed. In emergency situations power supply to critical loads should be ensured. VRBs are ideal for emergency back-up applications, particularly as a diesel generator replacement (Skyllas-Kazacos and Menictas 1997). Low voltage DC supply needs some special considerations. There are some losses regarding the converters. If AC-DC conversion is needed, the loss will be more as additional components will be required. For DC-DC conversion, loss will be low and no 56

6 additional component will be required. Another consideration is of appliances. If we are using only DC appliances, what should be the conversion process is also a problem. There have been researches regarding low voltage DC system (Anand, Fernandes and Guerrero 2013; Kinn 2011). In this paper, we have studied the comparison of Vanadium Redox Battery (VRB) and Lead Acid Battery (LAB). We also integrated VRB in a complete DC network. Then we integrated a complete DC energy management system to the existing grid and analyzed its practicality. 3. Methodology Figure 3 shows the DC distribution system. Usually, it is isolated from the public grid. Figure 1. DC distribution system. Figure 3: DC Distribution System Figure 4 shows the block diagram of a three phase system with battery integration. Measurement blocks are also included. The main library used for system modeling was SimPowerSystem. Figure 4: Three Phase Model in Simulink 57

7 A complete DC network is integrated with VRB and LAB via a DC bus to the public grid. A PV system is also integrated to charge the batteries and it is also functioning for backup energy storage. Since the elements and devices in this system can be summarized into three categories: energy generation unit, energy storage unit and loads, shown in Fig. 4 which can be considered as a typical case of DC microgrid with renewable generation and energy storage. The control strategies developed for this system can also be applied on DC microgrids. The block diagram of the model describes all the components of the network. Here the grid is represented by a three phase source and a 100 km line is also represented in the diagram. The AC load is represented by the three phase series RLC load. The AC voltage is converted to DC using an AC/DC converter. The converter output is together with two outputs from LAB and VRB. We can see that the two batteries are connected to the PV source and the output is taken through a charge controller. The charge controller controls the current to the batteries and also the charge characteristics of the batteries. The DC bus voltage was taken to be 220V. So, VRB and LAB output was adjusted to the value of 220V. The output was measured in simulation. The signals from the simulation determine the output of the DC bus. The DC bus output is the voltage that the DC loads and low voltage AC loads will receive. This output has to be less defective. That is the voltage the consumer receives. So, this voltage should not be intermittent. Fig. 5 shows the simulation results of the three phase model. The bus consisted of VRB, LAB and grid. The bus output was 220V. From the figure it is clearly visible that the Vanadium Redox Battery is charged faster than the Lead Acid Battery. This proves that the same voltage can be achieved faster than a regular battery by using VRB. So, VRB energy storage is more efficient than the LAB energy storage. The three phase source output was taken and rms value was measured. The AC three phase values were converted to DC to match the DC bus. Output and the values are shown in Fig. 6. Figure 5: DC Bus Output (Voltage Vs. Time) 58

8 Figure 6: Three Phase Rms Signal At Network And Converted DC Signal (voltage vs. time) It is also mentioned that VRB is better than the Lead Acid Battery for a PV system. Available commercial backup systems have complex configuration and additional setup component for safety operation. In our model, the following parameters are known to the consumer and also any malfunction is easily detectable for individual component (battery, appliance) given in Table 2. Table 2: Parameters for Each Component PV Battery Critical load Inductive load 48V 48V (each) 13.8kV 15kΩ The proposed system has been modeled and simulated using the Matlab/Simulink environment. Fig. 7 shows the three phase model in simulink. The signals from the oscilloscope graphs are the output of the DC bus. Figure 7: Three Phase Model in Simulink 59

9 In normal condition, battery power will be supplying power and the backup will not be active. When there is no power from the grid or battery, the backup will be activated. Only critical loads and inductive loads will be activated. Fig. 8 shows the output of the DC bus. Battery is functioning so that backup battery will not be activated and will not supplying any power to the loads. Figure 8: DC Bus Output The emergency loads are assumed to be 13.8kv. This measurement is divided for the critical and inductive loads. The inductive loads are supplied AC power. Upon decibel conversion if inductive charging is supplying power to 15kΩ load, the power for critical and inductive loads is shown below in Fig. 9 and Fig. 10. Figure 9: Critical Load Power vs. Time 60

10 Figure 10: Inductive Load Power vs. Time 4. Discussion The experimental setup in the laboratory is shown in Fig. 11(a). The supply voltage was 24V. The output was consistent with the input. It is clearly visible from Fig. 11(b) that the output was 24V. For any variation of the supply voltage the output was the same. We calculate the cost of our system is around $20 whereas commercially available ones are around $3,000. Figure 11: a. Experimental Setup, b. Oscilloscope Output Most commercially available HVDC appliances are 24V but many researchers are interested in a 48V appliance system which shows fourfold efficiency. In a 24V system the loss we calculated for only one converter which is 0.5% for a DC-DC converter efficiency of 99.5%. There are DC-DC converter having efficiency of 95%, 97% and 99.5% (Starke, Tolbert and Ozpineci 2008). Also, for the wireless transfer system, inductive charging devices available commercially in markets are capable to charge a variety of devices including lamps, laptops, mobile devices and medical implant systems consisting 25% emergency loads. Assuming 13.8 kv backup load and depending on converter efficiency, losses were calculated for kv critical load. Table III represents the loss comparison between 95%, 97% and 99.5% efficient DC-DC converter.table IV shows the list of commercially available 24V appliances. 61

11 Table 3: Losses for kv Critical Load 95% 1 97% % 1 Voltage [V] Voltage [V] Voltage [V] represents DC-DC converter efficiency Table 4: DC Appliances of 24 Volt Appliance Appliance current (A) indelb Cruise 195 Fridge/freezer 1.38 Summit Kettle C DC Airco 4400RM Air conditioner Freeview receiver 2.00 Uses of the backup system can be various. For example, in households, industry applications, hospitals & defence etc.. 5. Conclusion Integrated energy storage of Vanadium Redox Battery in a PV system has been studied. From the comparison results, Vanadium Redox Battery is better than Lead Acid Batteries. It is not only helpful for increasing efficiency but also for reducing electricity bill. This paper also shows an integrated PV system with complete DC network which uses DC energy management. It also considers real time values to get a more real time simulation result. From the results, we can evaluate the current and voltage for grid, the AC loads, energy storage and the power consumed by the DC loads. Wireless power transfer is easily connectable to the existing grid and power is calculated for the inductive loads. All the calculations can be contributed to an improved DC bus signaling for a complete DC home or a low voltage DC network grid. The loss calculations including the line loss, active and reactive power loss have also been studied. However, there should be an analysis of real time demand response and reliability. Still there are some limitations regarding the HVDC loads and wireless power transfer efficiency to overcome the above mentioned limitations, it might be a study of real time demand response and reliability. Emerging new technologies including low voltage DC and wireless power transfer need to be more studied. So, this simulation shows the implementation of some new commercially available technologies in the practical scenario. 6. Acknowledgment: The authors would like to express their heartfelt gratitude to their parents and their family for invaluable help and support all over this work. 62

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