Photovoltaic Systems

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Photovoltaics Contents:

CSI NUMBERS
DEFINITION
CONSIDERATIONS
COMMERCIAL STATUS
IMPLEMENTATION ISSUES
GUIDELINES

1) Introduction
2) Stand-Alone vs. Grid Interface
3) Steps in designing a PV system
4) PV Subsystems - Inverters, Controllers, and Wiring
5) Mounting PV panels
6) Batteries
7) Photovoltaic Cells

RESOURCES

PROFESSIONAL ASSISTANCE
COMPONENTS / MATERIALS / SYSTEMS
GENERAL ASSISTANCE

None

The technology employed in photovoltaic (PV) systems is well-developed and there are improvements and modifications occurring regularly, primarily in production processes. The systems are quite reliable and have been well tested in space and terrestrial applications.

The primary obstacle to increased use of photovoltaic systems is their high initial cost. Continuous price reductions have been occurring. In some off-grid locations as short as one quarter mile, photovoltaic systems can be cost effective versus the costs of running power lines into the property and the subsequent continual electric charges.

Some utilities, including Austin's electric utility, have established PV centralized power stations. The City of Austin's Electric Utility has also recently established Solar Explorer Program which allows customers to pay a small fee on their monthly utility bill which will be used to construct additional PV panels in order to add more renewable energy inputs for the City's overall energy production base. However, of greater interest to homeowners is the potential of decentralized PV systems located at residences providing power to the home and to the centralized power grid when PV power exceeds the home's requirements. The grid provides power to the home when the PV's are not producing power in this case. There is a pilot project oering for each individual application.

Electric power generation options are now starting to be compared on a basis that includes "externalities." Externalities are the "hidden" costs associated with a power source that are not accounted for in the price of the power produced. These hidden costs include damage to the environment caused by the sourcing, processing, transporting, using, and disposal aspects of a power source. The operational costs and externalities associated with the conventional fuel mix (coal, oil, nuclear, natural gas) used for generating electricity are not substantially less than the "full" costs associated with photovoltaic systems and, in many cases, exceed the costs of PV's. The use of PV's is much less polluting than other fuel choices.

The primary strategy for use of PV's as the electrical power source for a residence is reducing the need for electricity. Refrigerators, air conditioners, electric water heaters, electric ranges, electric dryers, and clothes washers are all large users of electricity. Highly energy conserving alternatives and gas appliances are available to greatly reduce electrical loads.

	Commercial Status				Implementation Issues
Photovoltaics

Legend
	Satisfactory
	Satisfactory in most conditions
	Satisfactory in Limited Conditions
	Unsatisfactory or Difficult

TECHNOLOGY:

SUPPLIERS:

COST:

FINANCING:

PUBLIC ACCEPTANCE:

REGULATORY:

The City of Austin Electric Utility Department (EUD) regulates PV systems in the following areas (see Resources, General Assistance):

1. Compliance with Laws - This refers to applicable NEC and National Electric Safety Code rules and any other applicable laws and ordinances.
2. Compliance with Installation Rules - Compliance with Installation Rules and Standards for Electric Service for the City of Austin service area is needed.
3. Applicability - Safety requirements apply to all PV systems (or any private power producing systems) whether the system is connected to the City of Austin's grid or not.
4. Inspection - All systems must be inspected for safety code compliance.

The following information is very basic to understanding the implementation of photovoltaic systems. There are several excellent guides and sourcebooks listed in the Resources section that are highly recommended. Local expertise in PV's is limited and additional education on the topic beyond this presentation is useful.

2.0 Two approaches for using PV's: stand-alone and grid-interface.

• 2.1 Stand-alone system

  Requires batteries to store power for the times when the sun is not shining.

  Does not use electric utility power.

  The stand-alone system is termed a "separate system" by the electric utility. However, a "separate system" in the utility's terminology can exist in a home that also has utility power as long as they are completely separated.

• 2.2 Grid-interface system

  Uses power from the central utility when needed and supplies surplus home-generated power back to the utility.

  Termed a "parallel" system by the utility.

  The following information presents a partial overview of the guidelines needed to interface with the City of Austin EUD:

  (a) Technical data and information must be supplied to EUD. This includes physical layout drawings, equipment specifications and characteristics, coordination data (this pertains to the parts that will achieve the link to the utility system), test data on the equipment, synchronizing methods, operating and instruction manuals, and maintenance schedule and records.

  (b) Interconnection equipment is installed and maintained by the customer.

  (c) Maintenance records must be provided to EUD if requested. Protective equipment must be maintained by the customer every 2 years or as required by EUD.

  (d) The customer must provide their own protective devices for their system.

  (e) Extra costs incurred by EUD in the interface arrangement must be borne by the customer.

  (f) The PV system can operate only after written approval is received from EUD.

  (g) The customer and EUD must have agreed upon safety procedures.

  The interface between the home produced power can be metered in a manner that when power is produced by the PV's and sent into the grid the meter will run backwards. When power is brought in from the grid the meter will run in the regular direction.

  Either approach (stand-alone or grid interface) can be done partially; with PV's being used in conjunction with a generator in a stand-alone system, or with the central grid power serving as a primary power source in a grid-interface system.

3.0 Steps in designing a PV system.

• 3.1 Calculate the Electrical Load

  Examine the uses of energy in a home in three categories (thermal or heat energy, electrical energy, and refrigeration), conservation opportunities can then be isolated in each category that can affect overall electrical consumption.

  • 3.1.1 Thermal energy requirement for heating living spaces, water, and cooking.

    Best accomplished by non-electrical fuels such as solar, gas, wood, and others. Electric space heating, water heating, and cooking require an enormous amount of electricity. It is not practical to use photovoltaics to create electricity for these purposes. Solar energy can be used in other forms such as passive and active solar space heating and solar water heating more efficiently. Gas can also be used for the thermal loads more economically and efficiently than electricity.

  • 3.1.2 Electrical loads (lighting, appliance and equipment operation)

    Should be done with the most conserving items that can accomplish the task. Highly energy efficient lighting products are readily available and the energy efficiency of appliances can be easily compared for the best choices. Best application for PVs is in this catagory.

  • 3.3.3 Refrigeration for air conditioning and food preservation.

  Consumes proportionally enormous amounts of electrical energy making PV power very costly for these tasks.

  Gas powered air conditioning is available as an alternative.

  For food preservation, there are gas refrigerators and two manufacturers of very high efficiency electrical refrigerators and freezers (see Resources in Energy Efficient Appliances section).

  The following is a worksheet to use in calculating the size of a residential PV system.

  
  

  Quantity	Appliance	Hours of Use	Wattage	Total Daily     Watthours used
  					X 1.1
  					X 1.1
  					X 1.1
  					X 1.1
  		Daily Energy Use

  Wattage is usually listed on the appliance. If not, multiply the voltage times the amperage to obtain wattage. See the labels for the appliance/equipment to get this information.

  Steps:
  

  1. List the appliances, lighting, and equipment that will be operated.
  2. Mark the appliances that will operate on DC
  3. Enter the quantity of appliances, estimated hours of daily use, and their respective wattage.
  4. Multiply the quantity times the hours of daily use times the wattage and enter into the Total Daily Watt Hours Used column for each appliance. For each appliance that is not DC, multiply the Total Daily Watt Hours Used amount by 1.1 and enter that amount in the column.
  5. Add the Total Daily Watt Hours Used to get a total Daily Energy Use.

  (we intend to make this table interactive...in the meantime, please print or copy this form and do the calculations by hand.)

  If batteries are used to store the PV generated power, multiply the "Daily Energy Use" total by 1.25 to account for battery inefficiencies. The final total is the amount of power that PV's need to provide to accomplish operation of the listed appliances for one day.

• 3.2 Size the PV System-

  Different size PV panels will produce different amounts of power. The rated output wattage of the panel is the amount of watts the panel will create in one hour of direct sun.

  For our area, multiply the rated wattage by 5.1 to get the average amount produced in one day. The 5.1 factor is the viable operating hours per day and accounts for the fact that there will be more sun available in the summer and less in the winter.

  • 3.2.1 Example

    If a panel is rated at 48 watts, multiply that figure by 5.1 to get 245 watt-hours per day. Use that figure divided into the "Daily Energy Use" that was calculated above and the resulting number will be the number of panels of that particular size you will need. If the "Daily Energy Use" figure above was 2,000 watts per day, 2,000 divided by 245 gives us 8.16, rounded up to 9 panels. (Note that there are tracking systems that will increase the effective hours of sunlight striking a PV panel beyond 5.1.)

    Panel Rating (48) x Avg. operating time (5.1) = panel watt-hours per day (245)

    Daily Energy Use (2,000) / Panel watt-hours (245)= number of panels (8.16), round up to even number = 9

4.0 PV Subsystems - Inverters, Controllers, and Wiring

• 4.1 Inverters

  Conventional appliances and equipment and utility-supplied power use alternating current (AC) power and PV systems produce direct current (DC) power.

  Inverters are required to convert the power from the PV's from DC to AC. Recently produced inverters are reliable and efficient. They are also a major cost for the project starting at over $1,000 for a size that will accommodate a residence.

  For practical reasons, including electrical code compliance and financing, it is best to have a conventional (AC) electrical distribution system in the house. This will permit the use of appliances, equipment, and lighting that is commonly available.

• 4.2 Charge controllers

  Regulate the voltage entering batteries to avoid overcharging the batteries.

  Available in different capacities and must be selected to match the system.

  Prevents losses of power back through the panels at night.

• 4.3 Wiring

  Some direct current (DC) equipment may be desirable to operate in a home. DC appliances and equipment, although initially more costly than their AC counterparts, will use less power to operate. In some cases, such as pumps, the DC motors are much more efficient.

  When DC wiring is going to be used in a home, a heavier wire is required. Generally, #10 wire is best for direct current applications but larger wire may be necessary if the wire runs are quite long. Tables are available in the manuals offered by companies listed in the Resources section.

  Electrical code requirements will apply to PV installations in regards to having fused disconnects, load centers, and proper grounding.

  Inverted power (AC) is wired normally as per code.

5.0 Mounting PV panels

PV arrays must be placed to receive the most sunlight. At our latitude, a 45-degree slope to the panels with a south orientation is best. The 45-degree slope will help offset the shorter winter day by bringing the panels closer to perpendicular to the lower winter sun.

There are several ways to mount the panels - fixed, fixed with adjustable tilt angles, manual tracking, passive tracking, and active trackers. All of these mounting approaches can be placed on the ground or on a roof except for some active trackers which are pole mounted and thus more suited for a ground mount.

Fixed mounts are the least costly and lowest energy producing mounting systems. A metal frame suited for outdoor conditions is best. PV panels will substantially outlive the best wood racks.

The fixed mount with adjustable tilt angles and manual tracking mounts will require manually changing the angle of the PV panels either several times a day (manual tracking) and/or seasonal adjustments to keep the panels as close to perpendicular as possible to the sun (tilt angle adjustments).

Trackers are useful if the site is appropriate. There needs to be no obstacles in the east and west that will block the sun since the trackers will orient the PV panels to face the sun from early morning to late afternoon. Passive trackers are typically freon activated to track the sun from east to west only (there is no automatic tilt angle change). Active trackers draw a very small amount of power from the PV panels (as low as one watt) and mechanically track from east to west and adjust to the proper tilt angle. The passive trackers will increase the panels output from 40-50%. Active trackers will improve panal output by as much as 60%. However, it is important to realize that the largest gains for the trackers occurs during the longest days of summer. There are not large gains in the winter.

6.0 Batteries

Batteries are the best method of storing energy from a PV system for the periods when the sun is not shining. (This is for stand-alone or non -grid connected systems.) The information from calculating the daily load will be needed for determining the battery sizing.

• 6.1 Steps for sizing the battery bank :

  (1) Divide the "Daily Energy Use" (derived from using the Chart on page 6) by the voltage of the battery (typically 12 volts). The result is amp-hours which is the common manner of measuring battery capacity. For example, if the "Daily Energy Use" is 2,000 (watt-hours), divide 2,000 by 12 to get 167 (amp-hours).

  (2) Multiply the daily amp-hours by the number of days that you want to have power in storage in case the sun is not shining adequately. Three to five days is recommended. For this example, we will choose four days. Multiply 167 amp-hours per day times 4 days to get 668 amp-hours.

  (3) Batteries should not be discharged excessively. A deep cycle lead-acid battery (the main battery option) will last longest if it is discharged only 50%. By dividing the total amp-hours from Step 2 (668) by .50, the optimal battery capacity is determined; 668/.50 = 1336 amp-hours at 12 volts.

• 6.2 Selecting batteries

  Car batteries are not suitable for PV applications as they can not handle the deep discharges that can occur with PV systems.

  "RV" or "marine" batteries can handle a deeper discharge than car or starter batteries and can be used in a beginning system. They will last 2 to 3 years.

  Gell cell sealed batteries can be used in limited conditions, but also will not handle deep discharges. Because they are sealed, they are suited to marine applications.

  Deep cycle batteries are available for golf carts, and include Industrial Chloride batteries. These batteries are the best choice for PV systems as they can be discharged 80%. The golf cart batteries will last 3-5 years. There are some larger capacity deep cycle batteries that will last 7-10 years. Industrial Chloride batteries will last 15-20 years.

  Non lead-based batteries such as nickel-cadmium batteries are costly but can last a very long time if they are not discharged excessively. A new type of nickel-cadmium battery, fiber-nickel-cadmium, has outstanding longevity at a 25% discharge rate. Nickel-cadmium (NiCad) batteries have different operating and maintenance characteristics than lead-acid batteries that must be considered. For example, it is difficult to measure the depth of discharge that is occurring with a NiCad battery since its output is constant right up to the last moments before being completely discharged. Check with the suppliers in the Resources section about the operation and maintenance characteristics of the NiCad batteries they offer.

  For large systems, the best battery choices will be the "true" deep cycle types. Caution in using batteries must be observed along with recognition of their characteristics in response to temperature changes (lead-acid batteries operate less efficiently in cold temperatures) and ventilation requirements.

7.0 Photovoltaic Cells

• 7.1 Description

  Semiconductor material, typically silicon, is used in thin wafers or ribbons in most commercially available cells. One side of the semiconductor material has a positive charge and the other side is negatively charged. Sunlight hitting the positive side will activate the negative side electrons and produce an electrical current.

• 7.2 Types of cells

  • 7.2.1 Crystalline silicon

    Crystalline cells have been in service the longest and exhibit outstanding longevity. Cells developed almost 40 years ago are still operating and most manufacturers offer 10-year or longer warranties on crystalline cells.

    There are two sub-categories of crystalline cells - single crystal and polycrystalline. They both perform similarly. The efficiency of crystalline cells is around 13%.

  • 7.2.2 Amorphous silicon

    Amorphous silicon is a recent technology for solar cells.

    It is cheaper to produce and offers greater flexibility, but their efficiency is half of the crystalline cells and they will degrade with use.

    These type of cells will produce power in low light situations.

    This technology is expected to improve application possibilities far exceeding crystalline technology.

  Currently, the best choice for solar cells will be the crystalline variety.

The following companies offer equipment and engineering support.


Solar Energy International
P.O.Box 715
Carbondale, CO 81623
(970) 963-8855
sei@solarenergy.org
workshops, education

Photocomm Inc.
7681 E. Gray Rd.
Scottsdale, AZ 85260
(916) 477-5121

some listed here may also offer engineering assistance

Eco-Wise
110 W. Elizabeth
Austin, TX 78704
(512) 326-4474
solar outdoor lighting

Jade Mountain
P.O. Box 4616
Boulder, CO 80306-9846
(800) 442-1972
www.jademountain.com
retail PV system supplier

Real Goods Trading Co.
555 Leslie St.
Ukiah, CA 95482-5576
(800) 762-7325
www.realgoods.com
retail PV, wind systems supplier

Alternative Energy Engineering, Inc.
Box 339
Redway, CA 95560
(800) 777-6609
www.alt-energy.com
retail PV, hydro, wind systems supplier

Southwest PV Systems & Supply
212 E. Main St.
Tomball, TX 77375
(281) 351-0031
PV supplier/installer serving Austin area

Sunelco
P.O.Box 787 (mailing address)
100 Sheels St.
Hamilton, MT 59840
(800) 338-6844
retail PV, wind, hydro systems supplier

BP Solar Inc
2300 N. Watney Way
Fairfield, CA 94533
707 428 7800
www.bp.com/bpsolar/index.html
manufacturer of solar panels

Remote Power Inc.
649 Remington St.
Ft. Collins, CO 80524
(303) 444-9507

Photocomm Inc.
9806 Mula Rd.
Stafford, TX 77477
(713) 933-1578

Solar Electric Systems Inc.
2700 Espanola NE
Albuquerque, NM 87110
(505) 888-1370

Apollo Energy Systems Inc.
POB 238
Navasota, TX 77868
(800) 535-8588

ENTECH Inc.
POB 612246, 1015 Royal Lane
DFW Airport, TX 75261 (214) 456-0900
manufacturer

Precision Solar Controls
2915 National Court
Garland, TX 75041
(214) 278-0553
manufacturer

Solar Contractors Inc.
POB 743064
Dallas, TX 75374
(713) 266-1723

Solartron International
POB 33249
Kerrville, TX 78029
(512) 895-5600

Solar Kinetics
10635 King William Dr.
Dallas, TX 75220
(214) 556-2376

Solar Electric Inc.
1450 Harbor Island Dr., Suite 204A
San Diego, CA 92101
(800) 842-5678


Sunnyside Solar
RD 4, Box 808
Green River Rd.
Brattleboro, VT 05301
(800) 257-1482

Siemens Solar Industries
P. O. Box 6032
Camarillo, CA 93011
manufacturer

Carrizo Solar Corp.
(505) 764-0345
recycled PVs

Zomeworks Corp.
P. O. Box 25805
Albuequerque, NM 87125
(800) 279-6342
trackers

American Sun King
P. O. Box 789
Blue Hill, ME 04614
(207) 374-5700
trackers

Array Technologies, Inc.
P. O. Box 751
614 2nd St. SW
Albuquerque, NM 87103
(505) 242-8024

Energy Efficiency and Renewable Energy Clearinghouse
PO Box 3048
Merryfield, VA 22116
(800) 363-3732

National Appropriate Technology Assistance Service (NATAS)
Box 2525
Butte, MT 59702
(800) 428-2525

American Solar Energy Society, Inc. (ASES)
2400 Central Ave., G-1
Boulder, CO 80301
(303) 443-3130

Texas Energy Extension Service
Center for Energy and Mineral Resources
Texas A&M University
College Station, TX 77843-1243
(800) 643-SAVE

National Renewable Energy Laboratory
1617 Cole Blvd.
Golden, CO 80401
(303) 231-7683

Texas Solar Energy Society
P. O. Box 1447
Austin, TX 78767-1447
(512) 326-3391
(800) 465-5049

State Energy Conservation Office
200 E. 10th St., Suite 206
Austin, TX 78701
(512) 463-1931

PV Network News
2303 Cedros Circle
Santa Fe, NM 87505
(505) 473-1067
"Solar Electricity Today" directory of periodicals, catalogs, organizations, dealers, distributors, manufacturers, etc.


City of Austin Electric Utility
Solar Explorer Program
721 Barton Springs Rd.
Austin, TX 78704
(512) 322-6290
Source of "City of Austin Standard Interconnection Guidelines for Customer Power Production Interface with City of Austin Electric Utility System"

Florida Solar Energy Center
1679 Clearlake Rd.
Cocoa, FL 32922
(407) 638-1000

Council for Photovoltaic Research
Institute of Energy Conv., Univ. of Delaware
Newark, DE 19716
(302) 451-6220

Solar Energy Industries Assoc. (directory of SRCC ratings)
122 C St. NW, 4th floor
Washington, DC 20001
(202) 383-2600


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