Present-day power conversion and conditioning systems focus on transferring energy from a single type of power source into a single type of load or energy storage system (ESS). While these systems can be optimized within their specific topology (e.g. MPPT for solar applications and BMS for batteries), the topologies are not easily adapted to accept a wide range of power flow operating conditions. With a hybrid approach to energy storage and power flow, a system can be designed to operate at its most advantageous point, given the operating conditions. Based on the load demand, the system can select the optimal power source and ESS. This paper will investigate the feasibility of combining two types of power sources (main utility grid and photovoltaics (PV)) along with two types of ESS (ultra-capacitors and batteries). The simulation results will show the impact of a hybrid ESS on a grid-tied residential microgrid system performance under various operating scenarios.
In developed regions, which make use of large energy distribution infrastructures (such as the U.S.), the energy demands fluctuate throughout the day. In most residential areas, the demand on the power utility grid follows a trend resembling a double-bell curve, with high demands early in the morning and late in the evening. A typical daily residential energy demand curve is presented in
In areas where the grid power is not reliable, an ESS provides continued power, grid stability and frequency regulation. Currently, the main types of ESS systems deployed in the US are intended for natural disaster/emergency situations or transient/remote operation situations. The ESSs used in these scenarios are typically gasoline or diesel electric generators. In recent days, however, some US states (such as Hawaii and California) are beginning to move towards large-scale battery installations for grid/renewable energy storage. While battery storage alone may be sufficient for a large-scale energy system, the focus of this paper will be on smaller-scale residential systems, where hybrid energy storage is more applicable. Researchers around the world are working on examining the pollution emissions and environmental impacts in the supply chain of renewable energy components (i.e. electrical power conditioning and energy storage systems components). A study led by Argonne National Laboratory [
The U.S. electrical network is made up of many utilities generating electricity (mainly from burning fossil fuels) and distributes this electricity to consumers over long distances. This network of utilities is known as the “utility grid” or “grid”. The utility grid provides single-phase power for small loads and three- phase power for larger loads (e.g. an electric stove). In areas where the grid is reliable, grid power is available throughout the day to support the daily energy consumption. Utilities companies have a rate structure (depending on the states they operate in) they use to bill consumer energy usage.
A grid-tied photovoltaic (PV) system can be either commercial (utility-scale PV) or residential (e.g. roof-top PV). This paper will only cover residential grid-tied PV systems.
Here, we can consider the grid as a way to store energy when the solar power exceeds the household demand and can provide power when the household demand is high.
A grid-tied PV system with an energy storage system (ESS) is necessary in areas where the grid power is not reliable. The grid generates energy mainly through
the use of large electric generators powered by steam which is heated by coal, natural gas, or nuclear fission. The grid can provide energy at all hours of the day, but due to the typical work schedule in today’s US society, the grid experiences its highest demands and stress during the early morning and mid-even- ing times for residential applications. Meanwhile, solar panels convert light energy (photons) from the sun into electricity through the photovoltaic effect within the Silicon junctions of the panel. The solar irradiance peaks during the middle of the day and is unable to supply the heavy demand during the peaking hours. Energy storage is needed to evenly distribute the energy coming from these sources. In the event of a utility grid failure, the ESS will provide back-up electricity to power the loads. An ESS in a grid-tied PV microgrid is typically used for emergency back-up or peak-load shifting (reducing energy load by shifting it from peak to off-peak hours). An example of peak-load shifting will be presented later in this paper.
Microgrid technologies (which are localized groupings of intelligently controlled sources and loads) have many challenges, ranging from grid stability to fuel supply disruptions. In [
In this paper, we discuss a method for modeling a hybrid battery/ultra-capacitor energy storage system as shown in
To model the grid in Simulink, the Simscape Power Electronics Toolbox was used. A small neighborhood model (consisting of a few homes) was constructed and connected to a 120 V split-single-phase step-down substation transformer.
Parameter | Value |
---|---|
Voltage frequency | 60 Hz |
Nominal transformer power rating | 500 kVA |
Primary winding voltage rating | 13.8 kV |
Primary winding resistance | 0.05 pu |
Primary winding leakage inductance | 0.05 pu |
Secondary windings voltage rating | 120 V |
Secondary windings resistance | 0.05 pu |
Secondary windings inductance | 0.05 pu |
Magnetizing resistance | 500 pu |
Magnetizing inductance | 500 pu |
Discharge model (i* > 0)
Charge model (i* < 0)
where E0 is constant voltage (V), Exp(s), exponential zone dynamics (V), Sel(s), battery mode (0 for discharge, 1 for charge), K, polarization constant (Ah−1) or polarization resistance (Ohms), i*, low frequency current dynamics (A), i, battery current (A), it, extracted capacity (Ah), Q, maximum battery capacity (Ah) and B, exponential capacity (Ah−1).
For modeling of the ESS, batteries can typically support high-energy loads but at low power. On the other hand, ultra-capacitors can handle additional power demands from high-current loads but have low energy storage capability. In this paper, the energy limitations were modeled using an integration of the power with saturation limits. The lower limit can be set to zero to indicate an empty system and the upper limit designates the maximum capacity of the system. If the lower saturation limit is reached, the allowed output power will become zero. If the upper limit is reached, the allowed input power will be zero.
The ESS power limitations are modeled using a simple dynamic equation together with a numerical saturation having a fixed minimum and maximum value. The battery storage system’s power reference is controlled using a low-pass filtered version of the household power demand. Conversely, the ultra-capaci- tor’s power reference control signal is the high-pass-filtered version of the hou- sehold power demand. The Simulink models of the two ESS systems are presen- ted in
where HLP describes the low-pass filter used for the battery power reference, HHP describes the high-pass filter used for the ultra-capacitor power reference, and ωc is the cutoff frequency of the filters. The cutoff frequency for both filters was set to 0.01 Hz.
A typical microgrid system can be either AC or DC-coupled, often referred to as an AC microgrid or DC microgrid, respectively. Both AC and DC microgrids have their pros and cons depending on applications [
In an AC microgrid, shown in
In a DC microgrid, shown in
Peak-shifting, as shown in
To verify the analysis of the proposed ESS control strategies, a solar microgrid system was simulated, having a daily household power demand as described by
To see the effects of the ESS on the demanded household power, three different simulation scenarios were created:
Case 1: A house with solar, plus battery storage,
Case 2: A house with solar, plus ultra-capacitor storage,
Case 3: A house with solar, plus a hybrid ESS using a battery and an ultra-ca- pacitor.
In each case, the original household demand power is compared with the demand presented to the grid, which is the combination of the household power, solar power, and the compensated power of the ESS.
The simulation output of the first case (having only battery storage) can be seen in
We can also observe from
Parameter | Value |
---|---|
Peak household power demand | 3 kW |
Peak solar power absorption | 2 kW |
Battery energy storage capacity | 10 kWh |
Battery peak-power capability | 2 kW |
Ultra-capacitor energy storage capacity | 2 kWh |
Ultra-capacitor peak-power capability | 2 kW |
power demands during the peak hours helps to lower the utility costs to the consumer. The direction of the power flow will depend on the size of the solar installation and the household-in this simulation, we might conclude that the solar installation is oversized, as the battery is discharging to the grid during the night-hours.
The simulation outputs for the second scenario (using only ultra-capacitors) can be seen in
Lastly in
This paper examined how hybrid ESS would work out in different residential grid-tied microgrid scenarios. A trade-off between an AC versus a DC microgrid
was presented. Emphasis was placed on system level performance based on real residential field data from [
Traore, A., Taylor, A., Zohdy, M.A. and Peng, F.Z. (2017) Modeling and Simulation of a Hybrid Energy Storage System for Residential Grid-Tied Solar Microgrid Systems. Journal of Power and Energy Engineering, 5, 28-39. https://doi.org/10.4236/jpee.2017.55003