The technology employed in 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.
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 power sources such as coal, oil, nuclear, and natural gas. PVs are much less polluting than other fuels.
The primary obstacle to increased use of photovoltaic systems is their high initial cost, although price reductions are continuing. In some off-grid locations as short as one quarter mile from an electric power line, photovoltaic systems can be more cost effective than connecting to the grid when the costs of power line installation and monthly electric bills are considered.
Some utilities, including Austin's, have established centralized PV power stations. The City of Austin's electric utility (Austin Energy) has recently established a 'solar Explorer Program" which allows customers to pay a small fee on their monthly utility bill to construct additional PV panels to add more renewable energy generation to the City's overall energy production base.
PV can serve many power requirements. PV can provide for specific individual power requirements such as water pumping needs; power for irrigation systems and controls; power to help ventilate outlying buildings or animal stables; power for entry gates or communication devices such as emergency phones or lighting; and the list goes on. However, the focus in this document will be on PV for a building or home.
Of greater interest to building and homeowners is the potential of decentralized PV systems located at commercial buildings or residences, providing power directly to the user and to the centralized power grid when PV power exceeds the user's requirements. The grid provides power to the building when the PVs are not producing power.
To reduce the initial costs for PV system, there should be an initial focus on reducing the electric energy requirements of the building. Electric appliances such as, refrigerators, air conditioners, water heaters, ranges, electric dryers, and clothes washers are all large users of electricity. Alternative energy sources such as gas or solar appliance must be investigated. Any electric appliances must be energy efficient. The building envelope should be designed to minimize HVAC requirements as well.
Specific guidelines for PVs or private power producing systems are available in a report by Austin Energy called City of Austin Standard Interconnection Guidelines for Customer Power Production Interface . This report includes information relevant to all PV systems: stand-alone or grid-connected.
The following information is very basic to understanding photovoltaic systems. There are several excellent and highly recommended guides and sourcebooks listed in the Resources section.
Stand-alone system
Does not use electric utility power. Provides direct DC power when sunlight is available. If power is needed when sunlight is not available, batteries will be required to store power for the times when the sun is not shining.
The stand-alone system is termed a 'separate system" by Austin Energy. 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.
Grid-interface system
Uses power from the central utility when needed and supplies surplus home-generated power back to the utility. It is termed a "parallel" system by Austin Energy.
The following information presents a partial overview of the guidelines for interface with Austin Energy:
(a) Technical data and information must be supplied to the Utility. 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 the Utility if requested. Protective equipment must be checked by the customer every 2 years or as required by the Utility.
(d) Customer must provide their own protective devices for their system.
(e) Extra costs incurred by Austin Energy in the interface arrangement must be borne by the customer.
(f) The PV system can operate only after written approval is received from Austin Energy.
(g) The customer and Austin Energy must agree upon safety procedures.
The power produced can be metered so that when power is produced by the PVs and sent into the grid the meter will run backwards, thus allowing for a discount in consumption costs.
Either a grid-interface or a stand-alone system can be used to partially power the building.
Calculate the electrical load
Examine the building's energy usage in the areas of lighting, heating, cooling, cooking and refrigeration. Conservation opportunities can then be isolated in each category that can affect overall electrical consumption.
Thermal energy requirement for heating living spaces, water, and cooking
Best accomplished by non-electrical fuels such as solar, gas, and wood. 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 more efficiently in other forms such as passive and active solar space heating and solar water heating. For thermal loads, gas can be used more economically and efficiently than electricity.
| Quantity |
Appliance |
Hours of Use |
Wattage* |
Total Daily
Watthours used |
| | | | | | X 1.1 |
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| | | | | | X 1.1 |
|
| | | | | | X 1.1 |
|
| | | | | | X 1.1 |
|
Daily Energy Use =
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*Wattage is usually listed. If not, multiply the voltage times the amperage to obtain wattage. See the labels for the appliance/equipment to get this information.
Steps:
- List the appliances, lighting, equipment that will be operated.
- Circle the appliances that will operate on DC.
- Enter the quantity of appliances, estimated hours of daily use and their respective wattage.
- Multiply "Qty" (quantity) times "Hours of Daily Use" times "Wattage" and enter into the "Total Daily Watt Hours Used" column for each appliance. For each appliance that is not circled, multiply the "Total Daily Watt Hours Used" amount by 1.1 and enter that amount in the column.
- Add the "Total Daily Watt Hours Used" to get a total "Daily Energy Use."
- 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.
Lighting, appliance, and equipment operation
Use the most efficient lighting, appliances, and operating strategies. Consider incorporating daylighting strategies. Highly efficient lighting products are readily available and the energy efficiency of appliances can be easily compared for the best choices.
Refrigeration
Refrigeration consumes a proportionally large amount of electrical energy making PV power very costly.
There are gas refrigerators and two manufacturers of very high efficiency electrical refrigerators and freezers (see solar friendly products in the Resources listed at the end of this section).
Air conditioning
Air conditioning systems account for a major portion of the electric bill in both homes and businesses. High efficiency units are available for all types of buildings at cost-effective prices . As an alternative to electric, natural gas powered air conditioning is also commercially available.
Size of the PV System
Different sized PV panels will produce different amounts of power. The rated output wattage of a panel is the amount of watts the panel will create in one hour of direct sunlight.
For our area, multiply the rated wattage by 5.1 to get the average wh (watt hours) amount produced in one day. The 5.1 factor equals 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.
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 negative. Sunlight hitting the positive side activates the negative side electrons and produces an electrical current.
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 them.
There are two categories of crystalline cells, single crystal and polycrystalline. They perform similarly, and their efficiency is around 13 percent.
Amorphous silicon
Amorphous silicon is a recent technology for solar cells. They are cheaper to produce and offer greater flexibility, but their efficiency is half of that of crystalline cells and they will degrade with use. This type of cell 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 is the crystalline variety.
Inverters
Conventional appliances and equipment and utility-supplied power use alternating current (AC) power. PV systems produce direct current (DC) power.
Inverters are required to convert the power from the PVs from DC to AC. Recently- produced inverters are reliable and efficient. They are also a major cost for the project.
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.
Charge controllers
Charge controllers prevent overcharging of batteries by regulating voltage. They also prevent losses of power back through the panels at night. All components must be sized properly to match the system.
Wiring
Some direct current (DC) equipment may be desirable 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 long. Tables for determining wire size are available in the manuals offered by companies listed in the Resources section.
Electrical code requirements will apply to PV installations regarding fused disconnects, load centers, and proper grounding. Inverted power (AC) is wired normally as per code.
PV arrays must be placed where they will 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 ñ fixed, fixed with adjustable tilt angles, manual tracking, passive tracking , and active tracking. 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 installation.
Fixed mounts are the least costly and lowest energy producing mounting systems. A metal frame suited for outdoor conditions is best, as wood racks degrade much more quickly.
The fixed mount with adjustable tilt angles and manual tracking mounts will require 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 must be no obstacles to 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 panel output from 40-50 percent. Active trackers will improve panel output by as much as 60 percent. However, it is important to realize that the largest gains for the trackers occur during the longest days of summer. There are not large gains in the winter.
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.) A deep cycle battery is needed for PV applications. This type of battery can be discharged almost completely and recharged to full capacity. Daily load information will be needed for determining the battery sizing.
Steps for sizing the battery bank
(a) Divide the "Daily Energy Use" by the voltage of the battery (typically 12 volts). The result is amp-hours, the common manner of measuring battery capacity. For example, if the "Daily Energy Use" is 2,000 (watt-hours), divide 2,000 by 12 (167 amp-hours).
(b) 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 (668 amp-hours).
Batteries should not be completely discharged. A deep cycle lead-acid battery (the main battery option) will last longest if it is discharged only 50 percent. 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.
Selecting batteries
Car batteries are not suitable for PV applications, as they cannot handle the deep cycling that can occur.
"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.
Gel cellsealed 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 percent. 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-cadmiums are costly but can last a very long time if they are not completely discharged. A new type of nickel-cadmium battery, fiber nickel cadmium , has outstanding longevity at a 25 percent 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 up to the last moments before it is 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. |