Field of the Invention
The present invention relates to a grid-connected
power supply system in which a power generation apparatus
using natural energy, such as a solar power generation
apparatus or a wind power generation apparatus, is
associated with a commercial power grid.
Background of the Invention
As a conventional grid-connected power supply system,
there is provided, for example, a grid-connected solar power
generation system having a electricity storage device that
systematically interconnects a solar power generation
apparatus and a commercial power grid and can be charged by
both of them. The electricity storage device is charged
with power supplied from the commercial power grid in the
nighttime when the electricity rate is relatively low and
the power discharged from the electricity storage device is
supplied to one or more loads when the amount of power to be
generated by the solar power generation apparatus is
insufficient in the daytime when the electricity rate is
relatively high (see Japanese Patent Application Publication
No. H10-201129 or Japanese Patent Application Publication
No. 2002-369406). The conventional system supplies power,

charged in the electricity storage device, to the loads when
the amount of power generated by the solar power generation
apparatus is small and does not fulfill the power demand of
the loads, as in cloudy or rainy weather, thereby
suppressing costs (electricity costs) that are incurred by
the power supply from the commercial power grid.
However, the conventional electricity storage device
converts the AC power, supplied from the commercial power
grid, into DC power and then stores it, and needs to
reversely convert the DC power into AC power when supplying
power to the loads, so that there is a power loss
corresponding to the sum of power loss during AC-DC
conversion and power loss during DC-AC conversion. For this
reason, there may be cases where in fine weather in which
the solar power generation apparatus can generate sufficient
power, power (energy) savings and reduction in electricity
costs can be achieved by supplying power directly from the
commercial power grid, rather than compensating for the
deficit with the power that was charged in the electricity
storage device at the previous night.
Summary of the Invention
In view of the above, the present invention provides a
grid-connected power supply system that is capable of
achieving power (energy) savings while reducing power

(electricity) costs.
In accordance with one aspect of the present invention,
there is provided a grid-connected power supply system
including: a power generation apparatus configured to
generate power by using natural energy and connected in
parallel to a commercial power grid; a electricity storage
device configured to convert AC power, supplied from the
commercial power grid, into DC power to be charged with the
DC power, and to convert a discharged DC power into AC power
and supply the AC power to one or more loads; and a control
device configured to control charging and discharging of the
electricity storage device, wherein the control device
predicts an amount of power to be generated on a subsequent
day by the power generation apparatus and a power demand of
the loads and charges the electricity storage device with
power in a time zone in which an electricity rate of the
commercial power grid is relatively low if a predicted value
of the amount of power to be generated is lower than a
predicted value of the power demand, or does not charge the
electricity storage device with power during the time zone
if the predicted value of the amount of power to be
generated is not lower than the predicted value of the power
demand.
With such configuration, the electricity storage
device is charged during a time zone in which the
electricity rate of the commercial power grid is relatively

low if the predicted value of the amount of power to be
generated is smaller than the predicted value of the power
demand, thereby reducing electricity costs. In contrast,
the electricity storage device is not charged during the
time zone if the predicted value of the amount of power to
be generated is not lower than the predicted value of the
power demand, thereby eliminating power loss attributable to
the charging and discharging of the electricity storage
device and therefore achieving power (energy) savings.
Preferably, the control device may charge the
electricity storage device with power in a time zone in
which an electricity rate of the commercial power grid is
relatively low, and discharge power from the electricity
storage device in a time zone in which the electricity rate
is relatively high.
With such configuration, the electricity storage
device is charged with power during the time zone in which
the electricity rate of the commercial power grid is
relatively low and then the power is discharged from the
electricity storage device during the time zone in which the
electricity rate is relatively high, thereby reducing power
(electricity) costs.
Preferably, the grid-connected power supply system may
further include a manipulation reception unit for receiving
manipulation input related to the power demand of the loads,
wherein the control device predicts the power demand of the

loads based on the manipulation input received by the
manipulation reception unit.
With such configuration, the control device predicts
the power demand of the loads based on manipulation input
related to the power demand of the loads, for example, input
of the fact that the power demand is lower than normal due
to absence of person, or power demand is higher than normal
due to presence of many people, and therefore the power
demand of the loads in a situation different from a normal
situation can be predicted more accurately.
Preferably, the control device may compare an amount of
power dischargeable from the electricity storage device with
a predicted value of the power demand of the loads, and
provide notification if it is determined that the amount of
dischargeable power will be less than the power demand of
the loads within a predetermined time period.
With such configuration, the user is notified of, for
example, the fact that the power demand of the loads will
not be fulfilled by the discharging of power from the
electricity storage device after some hours, thereby
prompting the user to suppress the power consumption of the
loads.
Preferably, the control device may have a function of
controlling operations of electric devices as the loads, and
sequentially control the operations of the electric devices
in order of priority so that power consumption is reduced if

it is determined that the amount of dischargeable power will
be smaller than the power demand of the loads within the
predetermined time.
With such configuration, the power consumption of an
electric device is forcibly reduced, and therefore the
period for which power can be supplied from the electricity
storage device can be lengthened.
Preferably, when the predicted value of the amount of
power to be generated is smaller than the predicted value of
the power demand, the control device may make a schedule for
the power demand of the loads that prevents the predicted
value of the amount of power to be generated from being
smaller than the predicted value of the power demand, and
present the schedule to a user.
With such configuration, a user can be encouraged to
save power (energy).
In accordance with the present invention, it is
possible to achieve power (energy) savings while reducing
power (electricity) costs.
Brief Description of the Drawings
The objects and features of the present invention will
be apparent from the following description of embodiments
when taken in conjunction with the accompanying drawings, in
which:

FIG. 1 is a system configuration diagram illustrating
an embodiment of the present invention;
FIG. 2 is a flowchart illustrating the operation of
the control device shown in FIG. 1; and
FIG. 3 is an example of the schedule for the power
demand of the control device shown in FIG. 1.
Detailed Description of the Preferred Embodiments
Embodiments of the present invention will be described
in detail below with reference to the accompanying drawings
that constitute a part hereof. The same reference numerals
will be assigned to the same or similar components
throughout the drawings, and redundant descriptions thereof
will be omitted.
A grid-connected power supply system in accordance
with an embodiment of the present invention, which includes
a solar power generation apparatus as a power generation
apparatus using natural energy and is installed to a house,
will now be described with reference to the drawings in
detail. However, the power generation apparatus in the
grid-connected power supply system in accordance with the
present invention is not limited to a solar power generation
apparatus, but may be a power generation apparatus using a
different type of natural energy, for example, a wind power
generation apparatus. Furthermore, the place where the

grid-connected power supply system of the present embodiment
is installed is not limited to a house, but may also be a
multiple dwelling house or an office.
The grid-connected power supply system of the present
embodiment, as shown in FIG. 1, includes a solar power
generation apparatus 1 connected in parallel to a commercial
power grid AC, a electricity storage device 2 configured to
store (charge) AC power supplied from the commercial power
grid AC and supply charged AC power to loads 6, a control
device 3 configured to control the charging and discharging
of the electricity storage device 2, and a display
manipulation device 4. Furthermore, the solar cell array
(not shown) and the electricity storage device 2 of the
solar power generation apparatus 1 are installed outside a
house, while a power conditioner (not shown), the control
device 3 and the display manipulation device 4 of the solar
power generation apparatus 1 are installed in the house.
The solar power generation apparatus 1 is known in the
art, and includes a solar cell array including solar cells
and the power conditioner including an inverter for
converting DC power, output from the solar cell array, into
AC power. Furthermore, the solar power generation apparatus
1 may reversely supply (sell) the surplus of generated power
to the commercial power grid AC.
The electricity storage device 2 includes a storage
battery 20, such as a lead-acid battery, and a bidirectional

DC/AC power conversion unit 21 for converting AC power,
supplied from the commercial power grid AC and the solar
power generation apparatus 1, into DC power and charging the
storage battery 20 with the DC power, and converting DC
power, discharged from the storage battery 20, into AC
power. Furthermore, the bidirectional DC/AC power
conversion unit 21 may switch between a charging operation
(the conversion of AC power into DC power), a discharging
operation (the conversion of DC power into AC power), and
the stopping of the operation in response to control signals
transmitted from the control device 3 via a signal line Ls,
as described later. Furthermore, since loss occurs in the
bidirectional DC/AC power conversion unit 21 during the
conversion of AC power to DC power and during the conversion
of DC power to AC power, the power supplied to the loads 6
during discharging is less than the power supplied from the
commercial power grid AC during charging.
The distribution board 5 is connected to the
commercial power grid AC on the primary side thereof, and
includes, on the secondary side thereof, a main breaker (not
shown) connected in parallel to the solar power generation
apparatus 1 (the output terminal of the power conditioner)
and the electricity storage device 2 and branch breakers
(not shown) branched from the secondary side of the main
breaker. AC power is supplied from the commercial power
grid AC, the solar power generation apparatus 1 and the

electricity storage device 2 to the loads 6 via the branch
breakers. The loads 6 are, for example, electrical devices
such as an air conditioner, a television set, an electronic
cooking utensil, and a refrigerator. The operations of some
of the loads 6 can be controlled by using control signals
transmitted from the control device 3 via signal lines Ls,
as will be described later.
The control device 3 includes a control unit 30
configured to have a microcomputer as a main component, a
memory unit 31 formed of electrically rewritable
semiconductor memory (for example, flash memory) and
configured to store a program executable by the
microcomputer of the control unit 30 and a variety of data,
a control signal transmission unit 32 configured to transmit
control signals to-' the electricity storage device 2 or loads
6 via the signal lines Ls, and a network communications unit
33 configured to perform network communications with the
Internet 7 via a communications line Lx such as a telephone
line, a CATV coaxial cable or an optical fiber cable.
The control unit 30 controls, using control signals
transmitted from the control signal transmission unit 32 via
the signal lines Ls, not only the stopping of the operation,
charging and discharging of the bidirectional DC/AC power
conversion unit 21 of the electricity storage device 2, but
also the operations of some controllable loads 6.
Furthermore, the control signal transmission unit 32 may

radiation on the subsequent day which is acquired from a
website which provides weather information such as weather
forecasts (for example, the website of a central
meteorological agency or a district meteorological agency)
via the Internet 7. Furthermore, a method for predicting
the power demand for the subsequent day includes a method of
predicting the power demand based on history information
such as past power consumption history and the past weather
information.
The display manipulation device 4 includes a control
unit 40 configured to have a microcomputer as a main
component; a display device, such as an LCD monitor or an
organic EL display, a display unit 41 configured to
indicate (display) visual information, such as characters,
diagrams and symbols, by operating the corresponding
display device; a manipulation reception unit 42 formed of
a touch panel combined with the display device, and
configured to receive a variety of types of manipulation
input generated when a user (resident) touches the touch
panel and sends them to the control unit 40; and a network
communications unit 43 formed of the communications device
(LAN controller) that is the same as the network
communications unit 33 of the control device 3 and
configured to perform packet communications with the
control device 3 via the communications line Lx (LAN
cable) . Furthermore, in order to perform corresponding

packet communications, the control device 3 and the display
manipulation device 4 have been assigned respective unique
IP addresses (individual IP addresses).
Next, the control operation of the control device 3,
which is the gist of the present invention, will be
described with reference to the flowchart of FIG. .2. Here,
with regard to the commercial power grid AC, electricity
rates are set for respective time zones. For example, the
same electricity rate is set for a morning time zone of 7
a.m. to 10 a.m. and a evening time zone of 5 p.m. to 11
p.m., the highest electricity rate is set for the diurnal
time zone of 10 a.m. to 5 p.m., and the lowest electricity
rate is set for a late-night time zone of 11 p.m. to 7 a.m.
of the subsequent morning. That is, when the same amount of
power is supplied from the commercial power grid AC, the
electricity rate in the late-night time zone is lowest and
the electricity rate in the diurnal time zone is highest.
For example, when the time (11 p.m.) is reached from
which the time zone having the lowest electricity rate of
the commercial power grid AC starts, the control unit 30
predicts the amount of power PI to be generated on the
subsequent day by the solar power generation apparatus 1
based on the predicted highest temperature and the amount of
predicted solar radiation on the subsequent day which are
acquired from a website by the network communications unit
33 (step SI), also predicts the power demand P2 of the loads

6 on the subsequent day based on the history of past power
demand (step S2), and compares the amount of predicted power
P1 to be generated on the subsequent day with the predicted
power demand P2 for the subsequent day (step S3).
If the amount of power P1 to be generated on the
subsequent day is smaller than the power demand P2 for the
subsequent day, the control unit 30 controls the electricity
storage device 2 (the bidirectional DC/AC power conversion
unit 21) so that it stores the AC power supplied from the
commercial power grid AC during the time zone (11 p.m. to 7
a.m. of the subsequent morning) in which the electricity
rate is lowest (step S4). Meanwhile, if the amount of power
P1 to be generated on the subsequent day is not smaller than
the power demand P2 for the subsequent day, the control unit
30 does not perform the control that stores power in the
electricity storage device 2 during the late-night time zone
(step S5) .
As described above, in the present embodiment, the
control unit 30 of the control device 3 predicts the amount
of power to be generated on the subsequent day by the solar
power generation apparatus 1 and the power demand of the
loads 6 on the subsequent day, and compares the amount of
power to be generated with the power demand. If the
predicted value P1. of the amount of power to be generated is
smaller than the predicted value P2 of the power demand, the
electricity storage device 2 is charged during the late-

night time zone in which the electricity rate of the
commercial power g-rid AC is relatively low, thereby reducing
electricity costs. In contrast, if the predicted value P1
of the amount of power to be generated is not lower than the
predicted value P2 of the power demand, the electricity
storage device 2 is not charged during the time zone,
thereby eliminating power loss attributable to the charging
and discharging of the electricity storage device 2 and
achieving power (energy) savings.
In this case, since the memory unit 31 of the control
device 3 stores information, such as the history of past
power demand, and the control unit 30 predicts the power
demand for the subsequent day based on the history of past
power demand stored in the memory unit 31, there is concern
that the predicted value P2 of the power demand may be
considerably different from the actual power demand if the
situation is different, as in a situation where the power
demand is lower than normal due to absence of person, or a
situation where the power demand is higher than normal due
to presence of many people.
Accordingly, when the manipulation reception unit 42
of the display manipulation device 4 receives a manipulation
input related to the power demand of the loads 6, for
example, an manipulation input indicative of an irregular
increase or decrease in power demand such as that described
above, the corresponding indication is transferred from the

control unit 40 of the display manipulation device 4 to the
control unit 30 of the control device 3 via the network
communications unit 43. Meanwhile, the control unit 30 of
the control device 3 predicts the power demand of the loads
6 based on the indication received from the display
manipulation device 4, that is, information about the fact
that the power demand for the subsequent day is higher (or
lower) than normal, and therefore the power demand of the
loads 6 in a situation different from a normal situation can
be predicted more accurately.
Furthermore, if the amount of power to be generated on
the subsequent day by the solar power generation apparatus 1
is smaller than the predicted power demand of the loads 6 on
the subsequent day, the control unit 30 of the control
device 3 charges the electricity storage device 3 with power
in a time zone in which the electricity rate of the
commercial power grid AC is relatively low (for example, a
morning or nocturnal time zone), and discharges power from
the electricity storage device 3 during a time zone of the
subsequent day in which the electricity rate is relatively
high (for example, a diurnal time zone), thereby reducing
electricity costs.
Meanwhile, the bidirectional DC/AC power conversion
unit 21 of the electricity storage device 2 detects the
power level (the amount of dischargeable power) of the
storage battery 20 based on, e.g., the battery voltage of

the storage battery 20, and sends information about the
power level to the control unit 30 of the control device
3when receiving a control signal inquiring about the power
level from the control unit 30 of the control device 3.
Furthermore, when power is discharged from the electricity
storage device 2, the control unit 30 of the control device
3 may compare the amount of power dischargeable from the
electricity storage device 2 (the power level of the storage
battery 20) with the predicted value P2 of the power demand
of the loads 6, and, if it is determined that the amount of
dischargeable power will be less than the power demand of
the loads 6 within a predetermined time period (for example,
1 to 2 hours) , notifies the user (resident) of the fact by
displaying a relevant message, for example, a message
indicating that the supply of power from the electricity
storage device 2 will be impossible within some hours if no
change were to be made, on the display unit 41 of the
display manipulation device 4, thereby prompting the user to
suppress the power- consumption of the loads 6. Furthermore,
although only visual notification is provided, audible
notification as well as visible notification may be provided
in the present embodiment by installing a speaker in the
display manipulation device 4 and allowing the corresponding
speaker to issue a voice message.
Furthermore, if it is determined that the amount of
power dischargeable from the electricity storage device 2

will be less than the power demand of the loads 6 within the
predetermined time as described above, the control unit 30
of the control device 3 sequentially controls the operation
of the loads (electric devices) 6 in a predetermined order
of priority so that the power consumption of the loads 6 can
be reduced. For example, when the set temperature of the
air conditioner is changed or an air conditioner is stopped,
the power consumption of the electric device is forcibly
reduced and therefore the period for which power can be
supplied from the electricity storage device 2 can be
lengthened.
Furthermore, when the predicted value P1 of the amount
of power to be generated is less than the predicted value P2
of the power demand, the control unit 30 of the control
device 3 makes the schedule for the power demand of the
loads 6 that prevents the predicted value P1 of the amount
of power to be generated from being less than the predicted
value P2 of the power demand, and presents the schedule to a
user. For example, when the schedule for the power demand
for a normal day (one day) is represented by the solid line
A of FIG. 3, the control unit 30 makes the schedule for the
power demand represented by the dotted line of FIG. 3, and
creates web content that allows the newly made schedule for
the power demand to be represented as the schedule for the
power demand for a normal day and delivers it to the display
manipulation device 4. The display manipulation device 4

plays back web content, delivered by the control device 3,
using the web browser function of the control unit 40, and
displays the schedules for the power demand, such as those
shown in FIG. 3 (the solid line A and the dotted line B), on
the screen of the display unit 41. As a result, a user who
views the schedule for the power demand displayed on the
display unit 41 is encouraged to save power (energy).
Although the embodiments of the present invention have
been described above, the present invention is not limited
to these specific embodiments, but a variety of variations
and modifications are possible without departing from the
scope of the following claims and fall within the scope of
the present invention.

What is claimed is::
1. A grid-connected power supply system comprising:
a power generation apparatus configured to generate
power by using natural energy and connected in parallel to a
commercial power grid;
a electricity storage device configured to convert AC
power, supplied from the commercial power grid, into DC
power to be charged with the DC power, and to convert a
discharged DC power into AC power and supply the AC power to
one or more loads; and
a control device configured to control charging and
discharging of the, electricity storage device,
wherein the control device predicts an amount of power
to be generated on a subsequent day by the power generation
apparatus and a power demand of the loads and charges the
electricity storage device with power in a time zone in
which an electricity rate of the commercial power grid is
relatively low if a predicted value of the amount of power
to be generated is lower than a predicted value of the power
demand, or does not charge the electricity storage device
with power during the time zone if the predicted value of
the amount of power to be generated is not lower than the
predicted value of the power demand.
2. The grid-connected power supply system of claim 1,

wherein the control device charges the electricity storage
device with power in a time zone in which an electricity
rate of the commercial power grid is relatively low, and
discharges power from the electricity storage device in a
time zone in which the electricity rate is relatively high.
3. The grid-connected power supply system of claim 1 or 2,
further comprising a manipulation reception unit for
receiving manipulation input related to the power demand of
the loads, wherein the control device predicts the power
demand of the loads based on the manipulation input received
by the manipulation reception unit.
4 . The grid-connected power supply system of any one of
claims 1 to 3, wherein the control device compares an amount
of power dischargeable from the electricity storage device
with a predicted value of the power demand of the loads, and
provides notification if it is determined that the amount of
dischargeable power will be less than the power demand of
the loads within a predetermined time period.
5. The grid-connected power supply system of claim 4,
wherein the control device has a function of controlling
operations of electric devices as the loads, and
sequentially controls the operations of the electric devices
in order of priority so that power consumption is reduced if

it is determined that the amount of dischargeable power will
be smaller than the power demand of the loads within the
predetermined time.
6. The grid-connected power supply system of any one of
claims 1 to 5, wherein, when the predicted value of the
amount of power to be generated is smaller than the
predicted value of the power demand, the control device
makes a schedule for the power demand of the loads that
prevents the predicted value of the amount of power to be
generated from being smaller than the predicted value of the
power demand, and presents the schedule to a user.

ABSTRACT
A grid-connected power supply system includes a power
generation apparatus configured to generate power by using
natural energy and connected in parallel to a commercial
power grid; a electricity storage device; and a control
device configured. The control device predicts an amount of
power to be generated on a subsequent day by the power
generation apparatus and a power demand of the loads and
charges the electricity storage device with power in a time
zone in which an electricity rate of the commercial power
grid is relatively low if a predicted value of the amount of
power to be generated is lower than a predicted value of the
power demand, or does not charge the electricity storage
device with power during the time zone if the predicted
value of the amount of power to be generated is not lower
than the predicted value of the power demand.