CAISO Issue Paper
Integration of Solar Energy into the Participating
Intermittent Resource Program (PIRP)
DRAFT August 10th 2007
Revised February 8, 2008
Revised April 17, 2008
Prepared by
Jim Blatchford
Sr. Policy Representative
CAISO
John Zack
AWS True Wind, LLC.
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Integration of Solar Into the PIRP
ISSUE PAPER, Revision 1, February 6, 2008
Introduction
The California ISO (CAISO) established the Participating Intermittent Resource Program
(PIRP) to ensure the successful integration of Intermittent Resources into the market and
operations of the California grid. The CAISO tariff's original focus was on wind
resources but also took into account other intermittent resources including solar energy
production. The CAISO tariff states:
Eligible Intermittent Resources other than wind projects that wish to become
Participating Intermittent Resources will be required to provide data of comparable
relevance to estimating Energy generation. Standards will be developed as such projects
are identified and will be posted on the ISO Home Page.1
Solar energy production is more predictable than wind energy production, but it does
have distinct components of intermittency. For instance, energy production is obviously
non-existent during the twilight to dark periods, but it will also be reduced due to cloud
cover, dust storms or even high winds that may affect the focus of the solar beam into
thermal troughs. Because of its intermittency, solar energy production is included in the
California Renewable Portfolio Standard (RPS) and the PIRP.
The purpose of this paper is to establish guidelines for the integration of solar energy
production (concentrated and photovoltaic) into the PIRP.
Eligibility
In order for a solar power producer to participate in the solar component of PIRP, they
must meet ALL criteria set forth in the Appendix Q EIRP of the CAISO Conformed
Simplified and Reorganized Tariff dated Apr 6, 2007, except for those paragraphs
directly related to wind production. To participate in the PIRP, solar energy production
cannot be augmented by fossil fuel generation devices. Although the California
Renewable Portfolio Standard2 allows for a de minimis amount of fossil fuels to augment
the production, because augmentation of the hourly energy production for a solar
Participating Intermittent Resource (PIR) could unfairly advantage the other PIRs 3 4.
1
Conformed Simplified and Reorganized Tariff as of April 6, 2007 Appendix Q
2
Ca RPS defines de minimus as 2% for new facilities and 5% for existing facilities measured on an annual basis of electricity
production. http://www.energy.ca.gov/2007publications/CEC-300-2007-006/CEC-300-2007-006-CMF.PDF
“SCE would support the incorporation of solar into PIRP as long as the CEC
requirements were enforced.”
3
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Physical Site Data
A solar farm must provide the CAISO with an accurate footprint of the site before a
forecast can be produced. The footprint must include (1) the location (latitude and
longitude coordinates), and elevation of meteorological collection devices, (2) the
location, elevation and orientation angles of arrays or concentrators, (3) the generation
capacity of the facility and (4) the type of solar generation technology employed at the
facility. For redundancy purpose, each solar farm must provide a minimum of 2
meteorological stations with an independent power source.
Meteorological and Production Data
As outlined in the PIRP, meteorological data must be provided to the CAISO via the Data
Processing Gateway (DPG) for accurate power generation forecasting. Global irradiance
(GHI) is composed of direct and diffuse components. For most flat plate energy
collection via PV, much of the output variability can be explained by global irradiance
alone (>90% in the case of fixed collector and >80% for trackers). For solar thermal
collection involving concentration and tracking, the direct component must be known and
global alone may not be enough to explain the variability.
Fortunately there are techniques and models to generate direct irradiance from global
using the time series of the latter. In fact, the recently released high-resolution satellitederived direct normal irradiance (DNI) data from NREL’s National Solar Resource Data
Base was generated using such a technique (George et al. 20075; Perez et al. 20026; Perez
et al. 19927). Therefore, on site GHI measurements could be used to extrapolate DNI with
an acceptable degree of accuracy. In addition, having tilted irradiance measurements
available could be used as needed as a fine-tuning step for DNI estimation using an
anisotropic diffuse model such as described by Perez et al. 19908.
The second most important factor in the performance of the solar array is the
temperature. For a photovoltaic (PV) array, it is the back panel temperature that is
particularly important. In general, temperature accounts for slightly less than 10% of the
“While there may be reasons for the development of augmented solar facilitates, PG&E does not support
the extension of the PIRP benefits to these facilities during periods of augmentation .”
4
5
George R., S. Wilcox, M. Anderberg and R. Perez, (2007): National Solar Radiation Database (NSRDB) 10 Km Gridded Hourly Solar Database. Proc. ASES National Conference, Cleveland, OH
6
Perez R., P. Ineichen, K. Moore, M. Kmiecik, C. Chain, R. George and F. Vignola, (2002): A New
Operational Satellite-to-Irradiance Model. Solar Energy 73, 5, pp. 307-317
7
Perez, R., P. Ineichen, E. Maxwell, R. Seals and A. Zelenka, (1992): Dynamic Global-to-Direct
Irradiance Conversion Models. ASHRAE Transactions-Research Series, pp. 354-369
8
Perez, R., P. Ineichen, R. Seals, J. Michalsky and R. Stewart, (1990): Modeling Daylight Availability and
Irradiance Components from Direct and Global Irradiance. Solar Energy Vol. 44, pp. 271-289
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variability of output of the solar array. For crystalline silicon that constitute the majority
of PV installations, for every 20 C ° change in temperature the power output changes by
approximately 10% 9 (the higher the temperature the lower power output). The exact
sensitivity to temperature is dependent on the type of solar technology. In general, thin
film technologies are less sensitive to operating temperature.
The third most important factor is the wind, but it accounts for less than 1% of the
variability of output from a solar array. The primary impact of the wind is the ventilation
factor of removing heat away from the array.
Considering the elements and factors that influence the performance of the array, the
required data from the production site should include:








Real Time MW production
Global horizontal irradiance in watts/ m2, which accounts for ~90% of
variability of flat plate systems variability. But global horizontal irradiance
typically only accounts for 40-50% of a concentrating system's variability
unless the global horizontal irradiance is converted into direct normal
irradiance through a model that accounts for the strong non-linear relationship
between the two. If such a model is used the direct normal irradiance can
account for 90% of the system's performance variability.10
Plane of array irradiance recommended for flat plat technologies
Direct normal and diffuse irradiance measurements should not be NOT
REQUIRED for flat-plate PV plants
Direct irradiance accounts for >95% of concentrating system’s variability.
Diffuse irradiance, the difference between the two above components, is often
used for quality control purposes when both quantities are measured. 11
Ambient temperature at the array height in ◦C accounts for slightly less than
10% of the variability – ambient temperature is used to estimate panel
temperature, unless this is directly measured on the back of the panel
Wind speed and direction at the array height in m/s and degrees accounts for
less than 1% of the variability
Direct and diffuse irradiance can be modeled from global irradiance with a degree of
accuracy that is sufficient for non-concentrating applications. The plane of array
irradiance recommended for flat plat technologies can be obtained by the use of a
calibrated reference cell or a pyranometer. For concentrating solar technologies, it is
necessary to measure direct irradiance, either directly with a pyrheliometer, or indirectly
using a rotating shadowband pyranometer. For all above measurements, calibration
accuracy is an important concern.
The ambient and back panel temperature will require two separate temperature probes. In
addition the ambient temperature probe will require a probe shield to protect it from the
9
E.g., Menicucci D.F., and J.P. Fernandez, (1988): User's Manual for PVFORM. Report # SAND85-0376UC-276, Sandia Natl. Labs, Albuquerque, NM
10
http://www.freepatentsonline.com/20050039787.html
11
http://www.sandia.gov/pv/docs/PDF/viennaking2.pdf
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direct solar radiation. The wind speed and direction requires a mounting mast and
standard cup anemometer and wind vane device.
There will be other miscellaneous pieces of equipment required such as masts mounting
hardware and surge protectors. Table 1 gives the devices needed along with the units and
costs associated with the installation of the equipment for the flat plate technologies.
Table 2 gives the devices, units and costs associated with the installation of the
equipment for the concentrating technologies.
It should be noted that all cost estimates are based on prices that are current as of the date
of this paper but are, of course, subject to change and the cost of communication
equipment is not included in these estimates.
Table 1. Equipment needed and typical associated costs to make required measurements
for Flat Plate Technologies.
Units Var Equip
Element
Device (s)
Install
Needed
Cost
Costs
Plane-of-Array Irradiance
Global
Horizontal Irradiance
Calibrated Reference cell
or Pyranometer
Pyranometer
Ambient temperature at the
array height
Back panel temperature for
PV type arrays
Temperature probe &
shield for ambient temp.
Temperature probe for
back panel temperature
Wind speed and direction at
the array height
Cup anemometer, wind
vane and wind mast
Other Miscellaneous
Equipment
Masts, mounting
hardware, surge
protectors etc.
W/m 2
~90%
$300 - 1,000
$300-1,500
W/m 2
~90%
$300 - 1,000
$300-1,500
~10%
$225 - $300
$100 - $150
C
~1%
$90- $120
$100 - $150
m/s
deg
~1%
$460 - $500
$460 - $500
N/A
N/A
$1050 - $1200
$1000 - $1100
◦
C
◦
Table 2. Equipment needed and typical associated costs to make required measurements
for Concentrating Technologies.
Units Var Equip
Element
Device (s)
Install
Needed
Cost
Costs
Option 1: Direct and global
horizontal irradiance
Option 2: Direct and global
horizontal irradiance
Ambient temperature at the
array height
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Full station with a
Normal Incidence
Pryheliometer (NIP) for
direct irradiance and
tracking disk for global
and diffuse. .
Rotating shadow band
and pyranometer
temperature probe and
shield for ambient
temperature
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W/m 2
~90%
$26-30,000
*$14 -16,000
W/m 2
~90%
$11-13,000
*$5 - 7,000
~10%
$225 - $300
$100 - $150
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C
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Back panel temperature for
PV type arrays
temperature probe for
back panel temperature
Wind speed and direction at
the array height
Cup anemometer, wind
vane and wind mast
Other Miscellaneous
Equipment
Masts, mounting
hardware, surge
protectors etc.
◦
C
~1%
$90- $120
$100 - $150
m/s
deg
~1%
$460 - $500
$460 - $500
N/A
N/A
$1050 - $1200
$1000 - $1100
* Option 1 and Option 2 include initial 6 months costs.
Production and meteorological data will be collected for a minimum of 60 days before
the farm is considered in the PIRP. This data needs to be collected in advance in order to
train the forecast models (e.g. artificial neural networks) responsible for producing the
power production (MW) forecast for each site. The forecast service provider requires
high quality, continuously streaming data to provide an accurate forecast.
Anticipated Forecast Error
Studies have shown that the accuracy of a solar power prediction system is highly
depended on the types of local weather conditions and the time of day . 12 In general
such solar irradiance prediction systems are quite accurate with an MAE of 3- 4 % of the
solar irradiance for clear sky or consistent cloud cover that persist for more than an hour.
However, the MAE increases to as high as 15% of actual irradiance for situations when
small scale convective cloud elements develop that have life cycles on the order of 15 20 minutes. This type of condition is most likely to occur during mid to late afternoon
during the warm season.
Outage Data
If the solar farm is reducing its production from its stated maximum production value
(pMax), it is the responsibility of the solar farm (or its Scheduling Coordinator) to
provide the CAISO with plant outage information via the CAISOs Scheduling Logging
for the ISO of California (SLIC) reporting system. This data is needed to ensure the
MW forecast does not exceed the plants derated capability.
Explanation of Terms
12
http://www.solar2006.org/presentations/tech_sessions/t05-a243.pdf
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Global solar irradiance is a measure of the rate of total incoming solar energy (both
direct and diffuse) on a horizontal plane at the Earth's surface.
Direct (normal) solar irradiance is a measure of the rate of solar energy arriving at the
Earth's surface from the Sun's direct beam, on a plane perpendicular to the beam.
Diffuse solar irradiance is a measure of the rate of solar energy arriving at the Earth's
surface that is the result of scattering of the Sun's beam due to the various atmospheric
constituents.
Satellite-derived measurements of solar irradiance (both global and direct solar
irradiance) are possible through the use of computer models. A model of solar irradiance
uses radiation measurements from the visible-radiation channel and visible cloud imagery
from geostationary meteorological satellites to estimate ground level global and direct
irradiation. Diffuse irradiance can be calculated by using the relationship: diffuse =
global – direct. For the current generation of geostationary meteorological satellites, the
ground resolution is about one kilometer for the visible-radiation sensors. Studies have
concluded that satellite-derived measurements of solar irradiance are more accurate to
use than ground based observation if the ground based observing site is more than 25 km
away from the site of interest.
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