PV*SOL 5.0 standalone Simulation of a Stand-Alone AC System Dipl.-Ing. Miguel Carrasco miguel.carrasco@valentin.de Dipl.-Ing. Rainer Hunfeld rainer.hunfeld@valentin.de Dr. Valentin EnergieSoftware GmbH Stralauer Platz 34 10243 Berlin Germany Tel: +49 (0)30 588 439-0 Fax: +49 (0)30 588 439-11 www.valentin.de Keywords: Off Grid System, AC Coupling, Simulation, Battery Model, Battery degradation. 1. Introduction Stand-Alone (Off-Grid) systems currently represent a small portion of the global photovoltaic market. Because the price of components has dropped significantly, this is expected to change in the next few years. Based on this trend, Valentin Energysoftware is further developing its wellknown PV*SOL program for the design and calculation of Off-Grid photovoltaic systems to implement a completely new concept. The focus of this paper will be the description of AC systems. When simulating Off-Grid systems, the behavior of the battery plays a key role. The new PV*SOL incorporates a new battery model which calculates battery degradation and its effect on the system. The user is able to design the system based on a realistic simulation. In this paper the new PV SOL calculation engine for Off-Grid systems and its battery model are described. A simulation is validated with measures of a real system.
2. AC Coupling AC-Coupling is a solution for solar systems which consists of building an AC island grid where both generation and consumption take place. Power can be generated directly in AC (a diesel generator for example) or in DC (PV modules). In the latter case, an inverter converts this power into AC current and feeds it into the AC grid. When the generators produce more energy than the consumer needs, a battery stores this energy, so that it can be used at a later moment if the generators cannot meet the needs of the consumers due to weather conditions for example. The batteries are connected to the AC grid through an off grid inverter which incorporates a battery charger. The system showed in Figure 1 corresponds to one of the SMA Off-Grid solutions and is a good example of AC-Coupling. This system was monitored and the measures provided by SMA will be used in this paper to assess the precision of the simulation. It is a three phase AC system consisting of PV generators, diesel generator, AC loads and 3 clusters. Each cluster is made up of 3 Off-Grid inverters (one for each phase) and one battery pack. Figure 1: System Topology
3. Battery Model The central component in the simulation of Off-Grid systems is the battery. The simulation requires a good battery model in order to calculate the energy balance of the system over a period of time. This battery model has to be able to reproduce two key aspects of a real battery so that the other simulated components of the system can react properly: the voltage answer and the State of Charge (SOC). To achieve this, the battery temperature and State of Health (SOH) must be calculated as well. The model structure is showed in Figure 2. Figure 2: Structure of the battery model
3.1 State of Health The State of Health (SOH) is an indication of the condition of a battery. For a new battery which delivers the specified performance the SOH = 1. After its lifetime, when the battery is no longer usable, the SOH = 0. Simulating this indicator we have an estimation of the degradation of the battery under the operating conditions of the simulated system. We can also compare different systems and their impact on the life expectancy. The well known post processing models are suitable to calculate the degradation of the battery by counting the number of cycles. A cycle would be a complete discharge followed by a complete charge. Since this definition doesn t fit the real operating conditions of the battery, new counting methodologies must be implemented. The post processing models present two more disadvantages. On one side, they don t take into account the operating temperature and currents which have a great impact on the battery life expectancy. On the other side, counting the cycles is only possible when the simulation is over. Yet an important requirement on a degradation model is to provide a good estimate of the state of health during the simulation, due to the negative effect of degradation on the self-discharge, internal resistance and charge efficiency between others. This requirement is met with the new performance degradation model developed for PV*SOL, which takes into account not only SOC but temperature and operating current as well and provides the SOH for each simulation step.
3.2 State of Charge The State of Charge is an indication of the energy remaining in a battery. The first step to achieve this estimation is to run a balance between current in and current out over time, and taking into account the different losses. This amount of energy must be related to a reference, for example the rated capacity. But the fact that the capacity has several strong dependences (operating current, temperature, SOH) makes it more interesting to take the real capacity at a given time as the reference rather than the rated capacity, only valid under certain operating conditions for a new battery. In order to have a reliable SOC at every moment, so that the behavior of charge controller and other components of the system can be properly simulated, a new capacity model has been developed and integrated in the battery model in PV*SOL. 4. Simulation and Results The system presented in (3) can be now simulated with the new calculation engine integrated in the future versions of PV*SOL. The focus of this paper is the validation of the new battery model with measures of the real system. The system was measured during a 6 hour uninterrupted time frame. The batteries used in this system were Hoppecke OPzS 2000 Ah and most of the input parameters for the simulation model were provided by the manufacturer. The measured operating current is the input signal for the simulation (Figure 3).
Measurement -15,0 0 5000 10000 15000 20000-16,0-17,0-18,0-19,0 I (A) -20,0-21,0-22,0-23,0-24,0-25,0 Time (s) Figure 3: Measured operating current The initial state of health (SOH) of the battery was unknown. Since it is a key variable to reproduce a precise and realistic behavior of the battery, this initial SOH was calculated fitting the simulated capacity with the capacity estimation made by the measurement equipment from SMA, included in the provided measures (Figure 4). The fitted initial SOH was 0.455, where SOH=1 would mean a fresh battery and SOH=0 a dead one. Simulation Measurement 1800 1750 Capacity (A.h) 1700 1650 1600 1550 1500 0 5000 10000 15000 20000 Time (s) Figure 4: Simulated and monitored capacity of the battery
The measured and simulated battery voltages are shown in the Figure 5. The two signals represent a good correlation to each other and the reliability of the voltage model is confirmed. Simulation Measurement 63 62 U (V) 61 60 59 58 0 5000 10000 15000 20000 Time (s) Figure 5: Simulated and measured battery voltage. One of the most important simulation results is the SOC, since it is the indication of the remaining energy in the battery after the simulated period of time. The measurement equipment from SMA provides an estimation of the SOC of the battery as well. Figure 6 shows both the simulated SOC and this estimation. The correlation between the two signals is good, showing the simulation a more conservative behavior, since the simulated SOC is slightly lower than the estimation.
Simulation Measurement 100 90 80 70 SOC (%) 60 50 40 30 20 10 0 0 5000 10000 15000 20000 Time (s) Figure 6 : Simulated and monitored State of Charge of the battery (SOC). 5. Conclusions The new PV*SOL calculation engine for Off-Grid systems and the new integrated battery model were described. The results of the simulation were compared with the measures of the monitored AC-System, the correlation between the signals is good and confirms the PV*SOL simulation as a useful and reliable approach to designing an Off-Grid system.