ANALYZING OWR LOSSS AND THIR FFCTS IN COMLX OWR SYSTMS S. Stoll, U. Konigorski Institute of lectrical Information Technology, Clausthal University of Technology, Leibnizstr. 28, 38678 Clausthal-Zellerfeld, Germany hone: +49 5323/72-2689, Fax: -3197; e-mail: stoll@iei.tu-clausthal.de ABSTRACT The key prerequisite for improving system performance is understanding operating characteristics including losses. Therefore, a method has been developed that is especially designed to analyze the performance of power systems. The focus is on the system s power flows power sources and sinks that connect to the environment and power conversion mechanisms for power transfer between different system parts. Included in the analysis are power loss mechanisms that are required for prediction of system efficiency and power storage that compensates for temporary imbalance. The performance data from an operating V/Hybrid ower System is used to introduce the analysis method. The straightforward analysis allows visualization of power flows and power losses and the results match closely with actual performance. With a performance-certified model, different scenarios of configuration or operation changes can be analyzed. INTRODUCTION The authors are active in modelling, analyzing and controlling power systems for renewable energy and automotive applications. In their work, they have discovered that no suitable analysis tools are available to qualify the performance results of different operation strategies in regard to system efficiency. Therefore, a modelling approach for power systems has been developed to fill this gap. Results show that the developed method can provide accurate performance predictions as well as serve as a test bed for evaluating operational scenarios. The approach is described here. TH MODLLING AROACH ower systems can be described by few principal functions if the focus lies on the analysis of the system performance. Modules, here referred to as ower Cells, may be combined to represent even complex systems containing various types of power. In the system model, a power cell represents the mechanisms related to one form of power like electric DC power or thermal power. In figure 1, a power cell is shown with the focus on the external interfaces, while the internal mechanisms are only indicated. In figure 2, the internal mechanisms are represented in more detail. The entire system is represented by a combination of power cells that are connected to each other by the power transfer lines that go in vertical direction. They are arbitrarily defined as in (top) and out (bottom) and may be uni- or bidirectional. The power transfer mechanisms on the left side represent
in L out Figure 1: A ower Cell the power flow across the borders of the entire system s boundary. The transfer to the outside are loads, and the transfer from the outside are power generators. To the right, the power losses L that occur inside the power cell are shown (the internal connections are left out in the power cell to avoid confusion). These losses may be power flows to the system s outside (thus similar to a load) or they may affect other system parts. To the right also, the energy level as a measure of the stored power is shown. In contrast to the other connectors, it does not interact with other parts but serves as an important indicator of the inside system state. in age L stor L load L L gen eration ersion L conv out Figure 2: A ower Cell Details ach power cell contains the principal processes including the loss mechanisms related to one form of power (fig. 2). The and eration blocks represent the functions how the power is finally transferred to and from the outside. The power flow between power cells is controlled by the ersion which could be represented in either of the connected power cells. Often, the conversion is clearly associated with one form of power while otherwise the choice is arbitrary. The sum of the powers (the circle) can be considered a bus where all power flows from individual power cell come together. In case of an imbalance of in- and outgoing powers, the difference is stored / retrieved from age. The energy level shows the amount of stored power that potentially can be taken out. ach component may represent one or several different devices and in case the function is not used in a particular system, it may be left out. MODL OF A V/HYBRID OWR SYSTM The SunWize 1200-Watt V Hybrid System (SW1200) located at the research facility of the Southwest Technology Development Institute in New Mexico, USA had been analyzed and tested for several years
[2]. Collected performance data from October 1997 1 are used for demonstration of the proposed method. The V Hybrid System The SW1200 is a compact, skid-mounted, stand-alone electrical power supply that consists of a Siemens 1200W V array, an AT power center with charge controller, a Trojan 25 kwh battery bank (1042 Ah at 24 V), a Trace ngineering 4 kw bi-directional sine-wave Inverter and a Kohler 6.3 kw propanefueled generator. Controllable resistive ac loads are used to impose different load profiles. erally, it is loaded with about 6 kwh per day to represent a small residential application in the USA. Typically, the annual load is met with about 70% of the energy from solar and about 30% from the generator. The energy management system is designed to operate the V/hybrid system as an efficient uninterruptible power supply. The goals are to maximize use of photovoltaic energy, minimize backup generator run-time, and maintain battery SOC within a range that will produce maximum battery life. Advanced energy management strategies are currently under design [1] using the presented method for evaluation. The SW1200 Model The model of the SW1200 is shown in figure 3. The system itself consists of the DC and AC subsystems. A fictitious air-conditioning system is used as an example for detailed load modelling. Cool House L Aircon Aircon L L rec L inv L AC Inv AC Bat L bat Bat V L V L DC Figure 3: Model of the SW1200 DC Figure 4: Graphic Representation of ower Flows in the SW1200 The DC power cell consists of the V generator and the battery storage. The V system is modelled as a constant factor that is related to the V array efficiency. Then, the generated V power is the incoming irradiation power times this factor. The battery is an integrator of the incoming / outgoing power with conversion losses caused by the internal resistance in the form: const 2 and gassing near 1 The data from October 1997 have been selected because only very few hours of data are missing. Using data of other months give comparable results
full state of charge in the form e const ( n) when charging given the nominal battery size n and the actual amount of stored power (SOC) 2. The AC power cell consists of the inverter (conversion), the backup generator (here referred to as generator), and the AC loads. The AC part does not have its own storage (at least no significant one). Therefore, the inverter balances the power by transferring power to the DC cell and using the battery as storage. The inverter as an interface between AC and DC could be included in either power cell, AC is chosen arbitrarily. The inverter is described by a 50W constant loss and an 87% inverting / rectifying efficiency in any operation point. During generator operation, the fuel use is arbitrarily considered to be a linear function between 1 fuel unit when idling and 2 fuel units at rated power. The operation strategy is not regarded in this model. The AC load block represents the AC loads with one exception, the fictitious air- conditioning system, which is regarded separately for demonstration. The air-conditioning system is modelled structurally. It contains a converter that converts AC power to cooling power, thus it is the air-conditioner itself. The storage is the cooling storage capacity of the object that is to be cooled (e.g. a house) and the load is the transfer of the cooling power through the isolation, openings, etc. which means the mechanisms that heat up the system. RFORMANC ANALYSIS 3 A visual summary of the power flows and power losses over one month of operation (10/1997) is shown in figure 4. The air-conditioning system is excluded as the analysis has been done without it. In the figure, the principal power flows are shown that take place inside the system where the thickness of the lines represents the relative amount of power flowing. The monthly total of the AC load is 195kWh which is supplied by the V system (198kWh) and the backup generator (80kWh). The system efficiency may be calculated as 71% (the differences in the battery SOC in the beginning and in the end of the month are accounted for). It can be seen, that the generator power is in part directly consumed without losses - shown by a direct (thin) arrow. If the generator power is not directly consumed, it is converted / rectified to DC power which includes losses from rectification L rec. On the DC side, the power is stored in the battery where losses L bat occur. At another point in time, the power is taken out and inverted (with inverter losses L inv ) before supplying the load. Thus the operating sequence recharging the battery with the backup generator includes three consecutive procedures with losses. The direct load supply by V includes inverting losses (direct path to the load) or if the supply is indirect, battery losses occur additionally. What special information is provided using this method? The simulation shows that more than 80% of the system losses resulted through inverter operation while battery losses have been comparatively small. About half of these losses are caused by the 50W permanent loss that can hardly be avoided while an improved operation strategy may reduce the other losses. An analysis of generator operation options shows that increased system efficiency does not automatically mean increased fuel efficiency..g. a direct load supply by the generator when the battery is fully charged results in low losses in the electric system while the generator may only be partially loaded and thus inefficient. 2 The models represent just the principal functionality and a more detailed analysis (as is generally done) could be provided for performance predictions. For this purpose, though, the models proved sufficiently accurate simulated and measured values stayed within measurement tolerance. 3 Details of the analysis will be published soon.
The route of power flows and losses can be visualized. This may be useful especially in more complex systems to help detect inefficiencies. The amount of stored power given as energy levels is known for analysis. This way, system efficiency can be calculated more accurately accounting for the different energy levels. In this case, the difference of the battery SOC between the beginning and the end of the regarded time period is known and can be considered when calculating efficiencies. By calculation and visualization of scenarios, behaviour on hardware or operation changes may be predicted with a good accuracy. Scenarios As example, several possible scenarios of power flow to supply a 100Wh load (one hour operation of a 100W light bulb) are shown in addition to normal operation. Of course, during normal operation the scenarios will not occur as distinct as may be suggested but with their knowledge, e.g. preferable options may be extracted for performance enhancement. For simplified comparison, all losses are considered to be 20% of the power going through in the cases shown in below example scenarios. The visualization can be done in a very similar way as shown for the performance analysis (fig. 4). The backup generator supplies the load directly so that no losses take place (direct connection between and ). The backup generator provides the power indirectly so that the generator power is rectified (20% loss), stored in the battery (20% loss) and inverted (20% loss) before reaching the load. As result, the generator has to produce 195Wh. Ideally, V power is used directly for the load supply which results in only 20% losses (by inverting) or a necessary production of 125W. A worst-case scenario for the operation of the light bulb may be as follows: First, the backup generator provides the load with the power temporarily stored in the battery (195Wh). In addition, the light bulb is considered to be 20% efficient which means 80Wh power losses by heat. If the house is cooled by an air-conditioning system, this heat energy may have to be dissipated, as well. Then, the generator needs to supply additional 80Wh plus losses which would result to 195Wh. So, 390Wh electric power have to be produced just to receive 20W light. CONCLUSION A method for the performance analysis of power systems is presented in this paper. This modular approach focuses on power flows and losses as well as the storage of power inside the system. The method is introduced using a V/Hybrid system for which very detailed analysis data are available. Results of a performance analysis using this method indicate its strengths compared to the classic approach. In addition, the method allows to study and visualize scenarios when changes in hardware or operation take place. While the results may seem rather obvious for the example system, the method allows good insight when dealing with more complex systems. RFRNCS 1. Wiles, J.; Stoll, S. Final Test Report...: Sunwize nergy Corporation 1200 Watt Hybrid ower System. SWTDI, Las Cruces, NM, USA, 2000 2. Stoll, S.; Konigorski, U.; Wiles, J.; Risser, V. Rapid-rototyping Methods for the design of nergy Management Strategies 17th uropean V Solar nergy Conference, WI, München 2001