Solar Panels

When Solar Radiation Encounters GI Science

- Solar Assessment in SMUD Service Area -

Doina Brownell

- Geography 350 -

American River College, Sacramento, CA

Abstract

The nation’s leader in energy, California, established a more ambitious clean energy standards this year, as reaching one third of its power renewable by 2020 (Associated Press, September 23, 2010). The standards also require utilities to reach 20% renewable standard by next year, and hopefully, will create thousands of “green jobs”.

The project canvasses the potential for development of solar resources using GIS (Geographic Information Systems) technology, in Sacramento County, Sacramento Municipal Utility District (SMUD) service area.

The project result might be used by solar private industry, local consulting companies and SMUD to plan and promote residential solar development roof tops.

Introduction

The project objectives are:

} 1. Identification of rooftops in the Pilot Area

} 2. Creating solar potential maps (SPM) of Sacramento County

} 3. Estimate the photovoltaic energy of annual solar radiation potential in the Pilot area and Study Area, from new solar panels on residential roof tops.

Integrating GIS analysis tools, such as Solar Analyst, in data sources, as orthographic aerial photos and land parcel information could produce solar potential maps which present corridors of high and low solar access.

Sacramento County (or SMUD service area) was considered the Study Area (Fig. 1). Inside of it, an area with 20 selected houses was considered the Pilot Area (Fig.2).

Fig. 1 - SMUD Distribution Area in Sacramento County Fig. 2 - Study Area and Pilot Area locations

Background

All over the world, governments are setting ambitious goals to produce green energy and solve the present energy crisis with a renewable energy created by a never-ending source. Several studies were created in order to evaluate possible solar areas in different counties. Germany, Chile and United States are only a few countries where the renewable energy is a prime concern and were GIS specialists try to find viable solutions for the local energy crisis.

ArcGIS® Spatial Analyst is an optional extension to ArcGIS Desktop which provides powerful tools for comprehensive, raster-based spatial analysis. Also, the solar radiation analysis tools in ArcGIS Spatial Analyst enable mapping and analyzing the effects of the sun over a geographic area for specific time periods.

Data Acquisition

The imagery used in the project was comprised by a digital orthographic image which covers Sacramento Area, collected from City of Sacramento web site (http://generalmap.gis.saccounty.net/Default.aspx). The limitation of the orthographic imagery used in the solar potential map creation is due to landscape capture or changes in vegetation and lead to map inaccuracies. This is why, as input for Solar Analyst tools was used a raw digital elevation map (DEM), downloaded from USGS Seamless Data Warehouse web site (http://data.geocomm.com/catalog/US/61069/1318/group4-3.html). The map, a 7.5-minute (full-quad) Digital Orthophoto Quadrangle in native format cast to the UTM projection and referenced to NAD83.

Method

ArcGIS Spatial Analyst is fully integrated with ArcGIS Desktop and provides more than 150 tools and functions that users can access in the same environment as the more than 200 other ArcGIS Desktop tools.

From City of Sacramento web site I could provide a better view of the Pilot Area, which is located in south Natomas, between Pebble Wood Drive and Oak Bluf Way, (http://generalmap.gis.saccounty.net/Default.aspx), as in the image below:

Fig. 3 – Pilot Project closer look

In the Pilot Area were selected 20 houses with rooftops oriented South, West or/and Southwest, each classified from a Land Use point of view as single family residential unit:

Fig. 4 – Pilot Project location with selected rooftops

For each house and each potential solar panel was calculated its area, considering an average of 30% rooftop pitch and recorded in a table (see Table 1).

The solar radiation is modified as it travels through the atmosphere by topography and surface features. On the Earth’s surface the sun radiation is comprise of direct, diffuse, and reflected components. The main radiation is the direct one, intercepted unimpeded, in a direct line from the sun. Diffuse radiation is scattered by atmospheric constituents, such as clouds and dust. Reflected radiation is reflected from surface features. The sum of the direct, diffuse, and reflected radiation is called total or global solar radiation.

ArcGIS Spatial Analyst extension tool allows solar radiation calculation and analyzes the effects of the sun over a geographic area. The criteria to be considered as well are: atmospheric effects, site latitude and elevation, steepness (slope) and compass direction (aspect), daily and seasonal shifts of the sun angle, and effects of shadows cast by surrounding topography.

The DEM was used as input data for the Study Area SPM:

Fig. 5 - Digital elevation map covering the entire Sacramento County

Continuing the solar spatial analysis I aimed to determine how solar energy resources can be optimized by placing photovoltaic panels on rooftops in Sacramento area.

The solar radiation potential for the whole Sacramento County, using the Area Solar Radiation tool of Spatial Analyst was created for the entire year 2010, using monthly data based on hourly sampling. Fig. 6 represents areas of high solar potential – around 4,400 kWh/m² per year, indicated by light/yellow shading, while areas of low solar potential – around 1,900 kWh/m² per year, indicated in dark/purple shading.

Fig. 6 - Solar potential radiation in Sacramento County

Analysis:

According to the California Climate Zone Descriptions of June 2001 Sacramento Area is considered part of Zone 12, with the coordinates and main characteristics presented below:

Fig. 7 - California climate zones Fig. 8 - Average monthly temperatures

Latitude: 37.54 N Basic Climate Conditions (F):

Longitude: 121.15 W Summer Temperature Range 35

Elevation: 22 ft Record High Temperature (1972) 114

Record Low Temperature (1963) 19

The final output took in consideration the effects of shadows cast by surrounding topography and the South, Wet and southwestern orientation of the residential rooftops in the Pilot Area, as shown below:

Fig. 9 – Final solar analysis result in the Pilot Area

However, knowing that in a sunny climate like California, a one square meter panel can generate up to 100 W at peak output and on a yearly average generates about 18% of peak output, and that the average SMUD tariff for a KWh is $0.11, I calculated the potential customer cost savings per year from the surface area of rooftops in the Pilot Area:

House No.	Surface (sq.m.)			Potential Electrical Energy per year	Annual Electric Bill Savings
House No.	Final			(kWh)	($)
1	S1	63.05	63.05	9942	1094
2	S2	39.77	39.77	6271	690
3	S3	49.6	49.6	7821	860
4	S4	29.71	84.13	13266	1459
4	S5	54.42	84.13	13266	1459
5	S6	93.92	93.92	14809	1629
6	S7	35.78	101.94	16074	1768
6	S8	66.16	101.94	16074	1768
7	S9	81.74	81.74	12889	1418
8	S10	45.93	45.93	7242	797
9	S11	56.8	56.8	8956	985
10	S12	44.44	44.44	7007	771
11	S13	43.48	81.79	12897	1419
11	S14	38.31	81.79	12897	1419
12	S15	54.2	54.2	8546	940
13	S16	38.37	38.37	6050	666
14	S17	64.76	64.76	10211	1123
15	S18	50.52	50.52	7966	876
16	S19	70.34	70.34	11091	1220
17	S20	73.5	73.5	11589	1275
18	S21	106.3	106.3	16761	1844
19	S22	43.99	89.69	14142	1556
19	S23	45.7	89.69	14142	1556
20	S24	73.39	73.39	11572	1273
Total	-	-	1364.18	215,104	23,661

Table 1 – Solar energy cost efficiency in the Pilot Area

For the Pilot Area the annual savings for all 20 houses would be $23,661, representing 215 MWh.

However, extrapolating for the whole SMUD territory, considering 500,000 households, the final result would have to take in consideration several criteria, as 30% loss due to lack of interest in solar panels and/or losses on the distribution wires, 30% due to financial constraints, and 20% due to shaded rooftops, or lack of appropriate rooftops orientation (S, SW or W), it might be possible to return to the utility service grid an amount of 1,613 GWh, representing $177 million each year.

Conclusion:

The project succeeded to demonstrate that Sacramento area has an important potential for solar radiation. The SPM has a broad applicability in the areas of resource assessment, outreach and technical assistance, being a beneficial tool for community development and solar education programs.

Applying ArcGIS tools in Sacramento County I was able to identify a potential of 1,613 GWh from solar radiation, which could supply SMUD smart grid for a later usage. In the same time, the population could save on energy bills and participate in the green energy incentive as promoters of the California renewable energy plans.

From each solar panel, due to an inverter the solar radiation is transformed in AC power, which supplies each house main electric panel and further supplies all the household electrical needs. The surplus may go in the local electrical utility company, for a later usage. Also, the amount of green power generated by households is aimed at encouraging consumers to play a part in a "clean energy revolution" and also to bring cash back in their pockets. An explicit diagram is presented below:

Net Metering Diagram - Clean Energy Cashback

Fig. 10 - Clean energy cashback diagram

Figures, Tables and Maps:

Fig. 1 - SMUD distribution area in Sacramento County

Fig. 2 - Study Area and Pilot Area locations

Fig. 3 - Pilot Project closer look

Fig. 4 - Pilot Project location with selected rooftops

Fig. 5 - Digital elevation map covering the entire Sacramento County

Fig. 6 - Solar area radiation in Sacramento County

Fig. 7 - California climate zones

Fig. 8 - Average monthly temperatures

Fig. 9 - Final solar analysis result in the Pilot Area

Fig.10 - Clean energy cashback diagram

Table 1 - Solar energy cost efficiency in the Pilot Area

References:

1/ ArcGIS Spatial Analyst - Advanced Raster Spatial Analysis, ESRI Online, http://www.esri.com/library/brochures/pdfs/spatialanalystbro.pdf

2/ Boston Showcases Solar Power Potential with Web GIS, GIS Best Practices in Renewable Energy, January 2010, http://www.esri.com/library/bestpractices/renewable-energy.pdf

3/ Collin, C., Interpolating Surfaces in ArcGIS, Education Services, ArcUser, July-September 2004, http://www.esri.com/news/arcuser/0704/files/interpolating.pdf

4/ GIS for Renewable Energy, ArcGIS News, ESRI, Vol. 31, Nr.3, Fall 2009, http://www.esri.com/news/arcnews/fall09articles/files/31_3/arcnews-fall09.pdf

5/ Kelly, M., GIS to Meet Renewable Energy Goals, ArcNews Online, Fall 2009, http://www.esri.com/news/arcnews/fall09articles/gis-to-meet.html

6/ Mapping the Solar Potential of Rooftops, Germany's SUN-AREA Research Project Uses GIS, GIS Best Practices in Renewable Energy, January 2010, http://www.esri.com/library/bestpractices/renewable-energy.pdf

7/ National Renewable Energy Laboratory Measurement and Instrumentation Data Center (NREL MIDC) Solar Position and Intensity (SOLPOS) Calculation, 2008, available at: http://www.nrel.gov/midc/solpos/solpos.html

Acknowledgements:

The project was supported by the SMUD GIS Department of Transmission and Department of Contracts and Acquisition, and Black and Veatch consulting company.