CAISO Issue Paper
Integration of Solar Energy into the Participating
Intermittent Resource Program (PIRP)
DRAFT August 10th 2007
Revised February 8, 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.
Physical Site Data
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
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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. The amount of
global solar irradiance that falls on the solar array (either photovoltaic or solar thermal)
accounts for about 90% of the variability of the power output of the solar array. So the
measurement and forecasting of this parameter is essential for any solar power (MW)
prediction system.
The total global solar irradiance that falls on a horizontal surface is from two sources,
direct and diffuse solar irradiance. During clear sky conditions, the dominate source of
solar irradiance is the direct form, which typically accounts for about 80-85% of the
variability of the power output of the solar array. The remaining variability would be
from the diffuse solar irradiance which is a measure of the 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. Diffuse solar irradiance accounts for much more of the
variability during cloudy sky conditions and under thick cloud it can account for 80 % or
more of the variability.
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
variability of output of the solar array.
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:





3
4
Real Time MW production
Global horizontal irradiance in watts/ m2, accounts for ~90% of variability3
Diffuse horizontal irradiance in watts/ m2 accounts for ~10% clear sky and up
to +80% of cloudy sky variability
Direct normal irradiance in watts/ m2 accounts for ~80% of clear sky and
~10% of cloudy sky variability 4
Ambient temperature at the array height in ◦C accounts for slightly less than
10% of the variability
http://www.freepatentsonline.com/20050039787.html
http://www.sandia.gov/pv/docs/PDF/viennaking2.pdf
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

Back panel temperature for PV-type arrays in ◦C accounts for less than 1% of
the variability
Wind speed and direction at the array height in m/s and degrees accounts for
less than 1% of the variability
The Global horizontal irradiance, diffuse horizontal irradiance and direct normal
irradiance measurements can be obtained with the use of two pyranometers. One
pyranometer for the global horizontal solar radiation and one for the plane of array solar
radiation.
The diffuse horizontal irradiance can also be calculated from the
measurements provided by these two devices.
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
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. It should be noted that the all cost
estimates are based on current costs and are subject to change and the cost of
communication equipment is not included in these estimates.
Table 1. Equipment needed and estimated costs to make required measurements5.
Units Var Equip
Element
Device (s)
Install
Needed
Cost
Costs
Global,
diffuse
horizontal
irradiance
& direct normal
irradiance
pyranometers for
global horizontal &
for plane of array
solar radiation
Ambient temperature (height to
be determined)
temperature probe
and shield for
ambient temperature
temperature probe
for back panel
temperature
Back panel temperature for PV
type arrays (height to be
determined)
W/m 2
~90%
$1600 - $2000
$800 - $1000
~10%
$225 - $300
$100 - $150
C
~1%
$90- $120
$100 - $150
◦
C
◦
Wind speed and direction
(height to be determined)
Cup anemometer,
wind vane and wind
mast
m/s
deg
~1%
$460 - $500
$460 - $500
Other Miscellaneous Equipment
Masts, mounting
hardware, surge
protectors etc.
N/A
N/A
$1050 - $1200
$1000 - $1100
5
Costs are provided as estimates and are neither a minimum nor maximum requirement.
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Note: It is recognized that the relevancy of each parameter will vary between CSP and
PV application and will be addressed with further stakeholder input.
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.
Solar Power Prediction System
The PIRP solar power prediction system will use a short term forecasting approach based
heavily on the use of satellite-based data and recent measurements from the solar
generation facility for the 0-6 hour portion of the forecast period and a numerical weather
predication (NWP) approach for hours six and longer of the forecast period. The
statistical methods will be configured in a manner that ensures a smooth transition the
predictions from those produced by the short-term prediction scheme to those generated
by the longer-term approach. This will be done in a manner that is analogous to the
transition to short to longer term approach in the PIRP wind forecast system.
The reason for the need for the different approaches is the result of the strengths and
weaknesses of the forecasting methods. Short term forecasting methods are best for
capturing short-term trends in the weather and power production rates. But such methods
tend to have a large error after about 6 hours. NWP has a problem for approximately the
first 4-6 hours due to the need for such physics-based models to have time to "spin-up",
but such models tend to perform quite well from after 6 hours. A key component to both
approaches is the incorporation of neural net and statistical forecasting methods that help
to find and compensation for any NWP model or method-based systematic errors that
may occur for any given solar plant.
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 . 6 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
6
http://www.solar2006.org/presentations/tech_sessions/t05-a243.pdf
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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
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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