  The output of renewable energy fluctuates significantly depending on weather
conditions. We develop a unit commitment model to analyze requirements of the
forecast output and its error for renewable energies. Our model obtains the
time series for the operational state of thermal power plants that would
maximize the profits of an electric power utility by taking into account both
the forecast of output its error for renewable energies and the demand response
of consumers. We consider a power system consisting of thermal power plants,
photovoltaic systems (PV), and wind farms and analyze the effect of the
forecast error on the operation cost and reserves. We confirm that the
operation cost was increases with the forecast error. The effect of a sudden
decrease in wind power is also analyzed. More thermal power plants need to be
operated to generate power to absorb this sudden decrease in wind power. The
increase in the number of operating thermal power plants within a short period
does not affect the total operation cost significantly; however the
substitution of thermal power plants by wind farms or PV systems is not
expected to be very high. Finally, the effects of the demand response in the
case of a sudden decrease in wind power are analyzed. We confirm that the
number of operating thermal power plants is reduced by the demand response. A
power utility has to continue thermal power plants for ensuring supply-demand
balance; some of these plants can be decommissioned after installing a large
number of wind farms or PV systems, if the demand response is applied using an
appropriate price structure.
