08-02-2013, 09:41 AM
Steady-State Performance of a Grid-Connected Rooftop Hybrid Wind–Photovoltaic Power System with Battery Storage
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Abstract
This paper reports the performance of a 4-kW gridconnected
residential Wind–Photovoltaic system (WPS) with battery
storage located in Lowell, MA. The system was originally designed
to meet a typical New-England (TNE) load demand with a
loss of power supply probability (LPSP) of one day in ten years as
recommended by the Utility Company. The data used in the calculation
was wind speed and irradiance of Login Airport Boston
(LAB) obtained from the National Climate Center in North Carolina.
The present performance study is based on two-year operation.
(May 1996–Apr 1998) of the WPS.
Unlike conventional generation, the wind and the sunrays are
available at no cost and generate electricity pollution-free. Around
noontime the WPS satisfies its load and provides additional energy
to the storage or to the grid. On-site energy production is undoubtedly
accompanied with minimization of environmental pollution,
reduction of losses in power systems transmission and distribution
equipment, and supports the utility in Demand Side Management
(DSM). This paper includes discussion on system reliability, power
quality, loss of supply, and effects of the randomness of the wind
and the solar radiation on system design.
INTRODUCTION
IF ENVIRONMENTAL concerns keep growing, and restrictive
guidelines constrain the use of the pollutant sources,
wind and solar power can be considered as viable options for
future electricity generation. Besides being emission-free, the
energy coming from the wind and the sunrays is available at no
cost. In addition, they offer a solution for power supply to remote
areas that are not accessible by the Utility Company, and
to developing countries that are poor in fossil-based resources.
The interest in renewable energy forms is indeed growingworldwide.
Today, more than 28 000 wind turbines, and more than
100 000 off-grid PV systems are installed all over the world.
Since 1970, the PV price has continuously dropped. In 1970, the
installed PV peak watt cost $100, during the 80s, it fell to $10; at
the present time, the price is around $4 per installed peak watt.
The Power Conditioning Units
The PV and the synchronous machine voltages follow the
variability in the wind speed and in the insolation. The modules
produce dc power and the wind generator produces ac power.
This requires the need to condition the power at a fixed dc or
ac voltage.Amicroprocessor-controlled Maximum Power Point
Tracker (MPPT) maximizes the solar generator output; it separates
the array terminals from the battery voltage and sets the
solar generator at its optimum operating voltage at each insolation
level.
SYSTEM ANALYSIS
The Energy Production
Fig. 5 shows typical profiles of the TNE load and the WPS
energy production for each season. For part of the day, the production
exceeds the demand (typically 8 hours in spring, 8 hours
in fall, 6 hours in summer and 5 hours in winter).
These diagrams display also the important role of the battery
storage. In most cases, between 9:00 AM and 3:00 PM, the
system is able to produce more power than required by the
load. The energy surplus is stored in the batteries in order to
be used during time of insufficient production, or sold to the
utility. Reserved energy in the batteries is instantaneously used
as backup, if the supply is significantly reduced by random
passage of clouds and/or absence of wind. In addition, the
storage can also help reduce the utility peak hours requirements
by storing energy from the utility in time of low demand to
use it when the residential load needs it in peak hours, thus
reducing the scheduling of expensive generation units at time
of peak demand.
CONCLUDING REMARKS
This paper has discussed the experience with a residential
wind–solar power system with storage during two years of
operation. The conclusions drawn from the analysis are the
following:
• Hybrid wind–solar power generation with battery storage
form a complementary system: the wind is a more dynamic
source than solar; it also provides energy during
periods of little or no sunshine; the battery storage allows
for the displacement of the energy by storing at a favorable
time and then using the excess energy when necessary.
This complementary feature is favorable to system
reliability.
• In utility-interactive mode, the residential storage can also
help reduce the burden of the utility during peak hours
by storing energy from the utility during period of low
demand and retrieve it to the load in time of peak demand,
reducing the need for expensive generation units.
• If data of the nearest meteorological station is to be used
in system designing, the degree of similarity of both sites
must be taken in to account.