20-07-2012, 02:29 PM
Dynamic Modeling and Simulation of Hybrid Power Systems Based on Renewable Energy
27. Dynamic Modeling and Simulation of Hybrid Power.pdf (Size: 566.33 KB / Downloads: 107)
Abstract
This paper describes dynamic modeling and
simulation results of a renewable energy based hybrid power
system. The paper focuses on the combination of solar cell (SC),
wind turbine (WT), fuel cell (FC) and ultra-capacitor (UC)
systems for power generation. As the wind turbine output power
varies with the wind speed and the solar cell output power varies
with both the ambient temperature and radiation, a FC system
with an UC bank can be integrated to ensure that the system
performs under all conditions. Excess wind and solar energies
when available are converted to hydrogen using an electrolyzer
for later use in the fuel cell. Dynamic modeling of various
components of this isolated system is presented. Transient
responses of the system to step changes in the load, ambient
temperature, radiation, and wind speed in a number of possible
situations are studied.
INTRODUCTION
Comparing with the nuclear energy and thermal power, the
renewable energy is inexhaustible and has non-pollution
characteristics. The solar energy, wind power, hydraulic power
and tide energy are natural resources of the interest to generate
electrical sources. Extensive and generalized usage of
renewable energy is very popular to reduce the pollutions we
have cause on earth. The wind and solar energy are welcome
substitution for many other energy resources because it is
natural, inexhaustible resource of sunlight to generate
electricity [1]. The main disadvantage of wind turbines is that
naturally variable wind speed causes voltage and power
fluctuation problems at the load side. This problem can be
solved by using appropriate power converters and control
strategies. Another significant problem is to store the energy
generated by wind turbines for future usage when no wind is
available but the user demand exists [1]. The solar cell depends
on the weather factors, mainly the irradiation and the cell
temperature.
DYNAMIC SYSTEM MODELS
Solar Cell
A solar cell module is the basic element of each
photovoltaic system. It consists of many jointly connected solar
cells. A number of solar cell models have been developed, but
the one diode electrical equivalent circuit is commonly used for
cell based or module based analysis. It consists of a diode, a
current source, a series resistance and a parallel resistance. The
current source generates the photo-current that is a function of
the incident solar cell radiation and temperature [4], [5]. The
diode represents the p-n junction of a solar cell. The
temperature dependence of the diode saturation current and
constant diode ideality factor are included in the modeling. At
real solar cells, a voltage loss on the way to the external
contacts is observed. This voltage loss is expressed by a series
resistance (Rs). Furthermore leakage currents are described by
a parallel resistance (Rsh). However, the series resistance is
very small and the parallel resistance is very large [6]. So we
can ignore Rs and Rsh.
SIMULATION RESULTS OF THE HYBRID POWER SYSTEMS
Simulation results with step changes in load demand, wind
speed, radiation, and ambient temperature are analyzed and
shown in Figs. 2-5. The initial wind speed is 10 m/s. Wind
speed increases, at t=10s, from 10 to 12 m/s and decreases to 8
m/s at t=16s. The solar cell initially supplies power at the
radiation 400W/m2 and temperature 25°. At 15s, the radiation
increases to 600W/m2 and temperature also increases to 28°.
The load demand changes from 375W to 225W at 10s. These
step inputs cause changes in available power and load
consumption. The power tracking performance of the hybrid
topology with respect to load demand change and
environmental variations is shown in Fig. 2. Associated
parameter variations in solar cell, wind turbine, fuel cell, ultracapacitor,
power converter output, and system performance are
analyzed.
SYSTEMS DESCRIPTION
The renewable energy based hybrid power system model in
Simulink is shown in Fig. 1. The system consists of a 75W
solar cell, a 400W wind turbine, a 500W proton exchange
membrane fuel cell, an ultra-capacitors, an electrolyzer, and a
power conditioner. The power conditioner includes a boost
circuit and a SPWM inverter. It is used to step up ultracapacitor
voltage to DC 200V and invert to 120Vrms, 60Hz AC.
The wind turbine adopted is Southwest Windpower Air 403.
When wind speed is 12.5m/s, the wind turbine produces the
maximum power 400W. Solar cell adopted is SIEMENS SP75
and its maximum power is 75W. Wind turbine and solar cell
are the main sources to supply load demand. Fuel cell model
includes a fuel cell module and a fuel controller. The fuel
controller consists of two PID controllers to limit the flows of
hydrogen and oxygen. The fuel cell is a accessory generator in
this system and supplies insufficient power. In order to keep
the supply and demand is balanced. When the supply is bigger
than the load need, the electrolyzer model electrolyzes water to
produce hydrogen and store it for further usage. Thus, the
system can circulate supply load demand and energy will not
be wasted.
CONCLUSION
In this paper, a novel renewable energy based hybrid power
system is proposed and modeled for a stand-alone user with
appropriate power controllers. The available power from the
renewable energy sources is highly dependent on
environmental conditions such as wind speed, radiation, and
ambient temperature. To overcome this deficiency of the solar
cell and wind system, we integrated them with the FC/UC
system using a novel topology. The voltage variation at the
output is found to be within the acceptable range. The output
fluctuations of the wind turbine varying with wind speed and
the solar cell varying with both environmental temperature and
sun radiation are reduced using a fuel cell. Therefore, this
system can tolerate the rapid changes in load and
environmental conditions, and suppress the effects of these
fluctuations on the equipment side voltage. The proposed
system can be used for off-grid power generation in noninterconnected
areas or remote isolated communities.