- Stand-Alone vs. Grid Interface
- Steps in designing a PV system
- PV Subsystems – Inverters, Controllers, and Wiring
- Mounting PV panels
- Photovoltaic Cells
The word “photovoltaic” combines two terms – “photo” means light and “voltaic” means voltage. A photovoltaic system in this discussion uses photovoltaic cells to directly convert sunlight into electricity.
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, and many are including a “green power” option which allows customers to pay a small fee on their monthly utility bill which will be used to construct additional renewable energy installations (PV, wind, biomass, etc.) 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 PVs are not producing power in this case.
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 PVs. The use of PVs is much less polluting than other fuel choices.
The primary strategy for use of PVs 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.
|Satisfactory in most conditions|
|Satisfactory in Limited Conditions|
|Unsatisfactory or Difficult|
Reliable and well tested. PV subsystem componentry are greatly improved but installation and equipment selection generally requires engineering for each individual application.
SUPPLIERS and INSTALLERS:
In recent years many more installers have been trained and new companies have sprung up in many areas. However they are still not common in some locales, either due to legal constraints, low interest in the technology, or lack of trained installers.
Overall system costs are greatly influenced by installation and subsystem costs, as well as any available local, state, or federal rebate programs. With inexperienced local suppliers or subcontractors, this expense will dominate the system’s costs; tens of thousands of dollars can be spent on setting up a residential PV system for the complete electrical needs of a conventional home.
Lenders can sometimes be reluctant to finance a PV system when grid-connected power is available, though the availability of rebate programs has helped bring the costs down and bring the idea of PV systems more into the mainstream. A “stand alone” PV system will have a better chance for financing if backed up with a generator.
In some cases it is possible to lease a PV system. Much like leasing a car, the home or business owner has a lower up-front cost and a contract for a fixed monthly price for the system (typically lower than the expected cost of grid-produced power) for a specified number of years. There are currently (2014) some legal questions being raised about the status of some leases when the original homeowner sells, so be sure that such details are spelled out in advance.
People generally accept the idea of PVs as nonpolluting, “free” electric power from the sun. However, many people consider PVs futuristic and feel that there may be more technical developments needed other than price reductions. Common workable understanding of PV systems is improving.
National Electrical Code (NEC) requirements apply to PV systems. Article 690 of the NEC specifically addresses PV systems. There are other sections that also apply to PVs but when there is a conflict Article 690 takes precedence. Article 480 deals with battery safety along with Article 690.
The City of Austin Electric Utility Department (EUD) regulates PV systems in the following areas:
- Compliance with Laws – This refers to applicable NEC and National Electric Safety Code rules and any other applicable laws and ordinances.
- Compliance with Installation Rules – Compliance with Installation Rules and Standards for Electric Service for the City of Austin service area is needed.
- 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.
- Inspection – All systems must be inspected for safety code compliance.
Specific guidelines for PVs or private power producing systems should be available from your power provider. There may be specific information relevant to all PV systems – stand-alone or grid connected.
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 PVs sometimes limited and additional education on the topic beyond this presentation is useful.
2.0 Two approaches for using PVs: 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 or grid-tied system
Uses power from the central utility when needed and supplies surplus home-generated power back to the utility. Often termed a “parallel” system by the utility.
The following information presents a partial overview of the guidelines often needed to interface with the grid:
- Technical data and information must be supplied to the power company. 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.
- Interconnection equipment is installed and maintained by the customer.
- Maintenance records must be provided to power company if requested. Protective equipment must be maintained by the customer every 2 years or as required by power company.
- The customer must provide their own protective devices for their system.
- Extra costs incurred by the power company in the interface arrangement must be borne by the customer.
- The PV system can operate only after written approval is received from the power company.
- The customer and the power company 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 PVs 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. This is called “net metering”. Either approach (stand-alone or grid interface) can be done partially; with PVs 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
|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:
- List the appliances, lighting, and equipment that will be operated.
- Mark the appliances that will operate on DC
- Enter the quantity of appliances, estimated hours of daily use, and their respective wattage.
- 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.
- 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 PVs 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.
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
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 PVs 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.
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 (Austin, TX) , a 45-degree slope to the panels with a south orientation was long thought to be best. However, in recent years this has been amended: it is now considered better to orient the panels toward the sun’s position at 4pm to 5pm, when electricity demands are often highest. Some power providers charge more for electricity consumed during peak periods. The goal is to avoid more of the air pollution that comes from gas-fired generators at conventional power plants by reducing the demand at peak use times. More westerly-facing panels generate about 20 percent less power overall, but can boost energy production by 50 percent or more between the hours of 2 p.m. and 8 p.m.
A steeper 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.
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:
- 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).
- 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.
- 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 or nickel-iron 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
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.