Distributed Renewable Energy Operating Impacts and Valuation Study

Technical Value – Power Supply Capacity & Energy
5.1.3 Solar DE Impacts on Reliable Operations
In addition to avoiding resource expansion and marginal energy production, solar DE resources
may have an impact on system requirements for certain ancillary services. Ancillary services
can generally be described as the amount of generating capacity that APS must have up and
running (committed) but constrained from running at full or optimum output to ensure that
sufficient capacity reserves are available, on a nearly instantaneous basis, to meet contingencies
that may occur on the system. For instance, if a generating resource trips off-line due to a
mechanical problem, the load it was serving still exists and, therefore, the power the failed
generator was providing must be produced from the reserves available.
A key concern for solar DE resources is that ancillary services may increase as solar DE
implementation increases. For instance, a cloud passing over a solar PV array would cause the
array to virtually stop producing electricity until the shadow of the cloud cleared the array. If the
PV array is a solar DE resource (i.e., connected to and affecting an APS customer’s load), then
during the transient cloudy period when the array stopped producing power, APS would
experience an increase in load, followed by a sudden reduction in load once the shadow cleared
the array. Such fluctuations in load could require APS to provide for additional generating
reserves anytime the solar DE resource was expected to be operating.
As discussed in Section 4.4, solar DE resources are not expected to contribute significantly to
APS requirements for spinning or operating reserves. This is due to the highly improbable
nature of double contingency events that would necessitate additional spinning or operating
reserves for solar DE. However, APS requirements for regulation reserves may be affected by
solar DE installations when solar DE is installed in significant quantities.
5.1.4 Single-Axis Sensitivity
Power supply planning and development generally occurs on a system-wide level. Electric
utilities plan to meet the load and reserve requirements of the total electric system peak demand
with the total of all their generating units and power purchases. On rare occasions, when a
transmission network is constrained from serving a load pocket and there are no reasonable
alternatives to upgrade the transmission system, an electric utility may decide it is cheaper to
build a generating resource inside the load pocket rather than upgrade the transmission system to
fix the constraint. However, APS is not planning for any such conditions and there are no
generating assets that can be avoided though strategically located solar DE installations.
Other possible strategic considerations for power supply planning would include solar DE
resources that are more readily matched to the system peak demand hour and could provide
greater capacity value. Such technologies could involve solar DE systems that have a westward
orientation, solar tracking facilities, and solar storage facilities. Configuring solar facilities with
a westward orientation could prove valuable to APS, but the diminished energy output from
these facilities would potentially reduce the economic value to the customers and, hence,
possibly the level of customer adoption. Solar tracking, on the other hand, is reasonably
comparable to a fixed plate installation on a total economic basis. A solar tracking facility may
have a higher cost than a fixed plate configuration, but typically produces enough additional
energy value to compensate for the increased cost.
With regard to battery storage, it is possible that customer installations could prove valuable to
APS. However, from the perspective of power supply planning, APS is generally indifferent as
Distributed Renewable Energy Operating Impacts & Valuation Study R. W. Beck, Inc. 5-3