06-09-2013, 04:55 PM
A Hybrid Wind-Solar Energy System: A New Rectifier Stage Topology
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Abstract
Environmentally friendly solutions are becoming more
prominent than ever as a result of concern regarding the state of our
deteriorating planet. This paper presents a new system configuration
of the front-end rectifier stage for a hybrid wind/photovoltaic energy
system. This configuration allows the two sources to supply the
load separately or simultaneously depending on the availability of
the energy sources. The inherent nature of this Cuk-SEPIC fused
converter, additional input filters are not necessary to filter out high
frequency harmonics. Harmonic content is detrimental for the
generator lifespan, heating issues, and efficiency. The fused multi-
input rectifier stage also allows Maximum Power Point Tracking
(MPPT) to be used to extract maximum power from the wind and
sun when it is available. An adaptive MPPT algorithm will be used
for the wind system and a standard perturb and observe method will
be used for the PV system. Operational analysis of the proposed
system will be discussed in this paper. Simulation results are given
to highlight the merits of the proposed circuit.
I.
INTRODUCTION
With increasing concern of global warming and the
depletion of fossil fuel reserves, many are looking at
sustainable energy solutions to preserve the earth for the
future generations. Other than hydro power, wind and
photovoltaic energy holds the most potential to meet our
energy demands. Alone, wind energy is capable of supplying
large amounts of power but its presence is highly
unpredictable as it can be here one moment and gone in
another. Similarly, solar energy is present throughout the day
but the solar irradiation levels vary due to sun intensity and
unpredictable shadows cast by clouds, birds, trees, etc. The
common inherent drawback of wind and photovoltaic systems
are their intermittent natures that make them unreliable.
However, by combining these two intermittent sources and by
incorporating maximum power point tracking (MPPT)
algorithms, the system’s power transfer efficiency and
reliability can be improved significantly.
PROPOSED MULTI-INPUT RECTIFIER STAGE
A system diagram of the proposed rectifier stage of a
hybrid energy system is shown in Figure 2, where one of the
inputs is connected to the output of the PV array and the other
input connected to the output of a generator. The fusion of the
two converters is achieved by reconfiguring the two existing
diodes from each converter and the shared utilization of the
Cuk output inductor by the SEPIC converter.
Thisconfiguration allows each converter to operate normally
individually in the event that one source is unavailable. Figure
3 illustrates the case when only the wind source is available.
In this case, D1 turns off and D2 turns on; the proposed circuit
becomes a SEPIC converter and the input to output voltage
relationship is given by (1). On the other hand, if only the PV
source is available, then D2 turns off and D1 will always be on
and the circuit becomes a Cuk converter as shown in Figure 4.
The input to output voltage relationship is given by (2). In
both cases, both converters have step-up/down capability,
which provide more design flexibility in the system if duty
ratio control is utilized to perform MPPT control.
ANALYSIS OF PROPOSED CIRCUIT
To find an expression for the output DC bus voltage, Vdc,
the volt-balance of the output inductor, L2, is examined
according to Figure 6 with d2 > d1. Since the net change in the
voltage of L2 is zero, applying volt-balance to L2 results in (3).
The expression that relates the average output DC voltage
(Vdc) to the capacitor voltages (vc1 and vc2) is then obtained as
shown in (4), where vc1 and vc2 can then be obtained by
applying volt-balance to L1 and L3 [9]. The final expression
that relates the average output voltage and the two input
sources (VW and VPV) is then given by (5). It is observed that
Vdc is simply the sum of the two output voltages of the Cuk
and SEPIC converter. This further implies that Vdc can be
controlled by d1 and d2 individually or simultaneously.
MPPT CONTROL OF PROPOSED CIRCUIT
A common inherent drawback of wind and PV systems is
the intermittent nature of their energy sources. Wind energy
is capable of supplying large amounts of power but its
presence is highly unpredictable as it can be here one moment
and gone in another. Solar energy is present throughout the
day, but the solar irradiation levels vary due to sun intensity
and unpredictable shadows cast by clouds, birds, trees, etc.
These drawbacks tend to make these renewable systems
inefficient. However, by incorporating maximum power point
tracking (MPPT) algorithms, the systems’ power transfer
efficiency can be improved significantly.
CONCLUSION
In this paper a new multi-input Cuk-SEPIC rectifier stage
for hybrid wind/solar energy systems has been presented.
The features of this circuit are: 1) additional input filters are
not necessary to filter out high frequency harmonics; 2) both
renewable sources can be stepped up/down (supports wide
ranges of PV and wind input); 3) MPPT can be realized for
each source; 4) individual and simultaneous operation is
supported. Simulation results have been presented to verify
the features of the proposed topology.