Solar Products
Most people associate solar energy with remote installations and off-grid implementations. In those cases the choice may be an easy one, but in towns and cities there are still reasons to turn to solar energy as a supplement and alternative to electrical utility grids.
The best reason is the reduction in utility bills. Once your system is working, it requires no monthly fees and little or no maintenance (and most parts are warranted). While it runs, it also reduces your electrical bills. Eventually it will pay for itself and keep saving you money. Large systems may even make you money by giving you perpetual credit with your local utility company.
Not only is your investment going to save you money and pay for itself, but solar installations frequently raise property value in both industrial and residential settings. Adding a solar energy system to your home or business will also supplement the investment you've made in your property. Another great incentive--many states (and now the Federal government) offer rebates and incentives for implementing solar power systems. Check out the DSIRE USA database which lists all incentives for every state in the country.
Finally, solar energy is a clean source of renewable energy. It reduces dependence on fossil fuels in a practical and effective way, and helps keep our environment clean.
- Types of PV Technology
- Different PV System Types
- Module Performance
- Estimating Your System Size
- Solar Terms (Glossary)
How It Works
The solar panels produce DC electricity that run through an inverter to produce AC electricity. This energy flows into the AC power panel to power your home. Energy that is not utilized is diverted back to the utility grid, in which homeowners can receive credit through net metering programs from your local power companies.
Net Metering: The practice of exporting surplus solar power during the day (to actual power needs) to the electricity grid, which either causes the home owner electric meter to (physically) go backwards and/or simply creates a financial credit on the home owner's electricity bill. At night, the homeowner draws from the electricity grid in the normal way.
Types of PV Technologies
There are three general families of photovoltaic (PV) modules on the market today. They are single-crystal silicon, polycrystalline silicon, and thin film. This article will help you to understand the differences that are relevant to the system designer and owner.
Single-Crystal and PolycrystallineThese represent the "traditional" technologies. They can be grouped into the category "crystalline silicon." Single crystal is the original PV technology invented in 1955, and never known to wear out. Polycrystalline entered the market in 1981. It is similar in performance and reliability.
Single-crystal modules are composed of cells cut from a piece of continuous crystal. The material forms a cylinder which is sliced into thin circular wafers. To minimize waste, the cells may be fully round or they may be trimmed into other shapes, retaining more or less of the original circle. Because each cell is cut from a single crystal, it has a uniform color which is dark blue.
Polycrystalline cells are made from similar silicon material except that instead of being grown into a single crystal, it is melted and poured into a mold. This forms a square block that can be cut into square wafers with less waste of space or material than round single-crystal wafers. As the material cools it crystallizes in an imperfect manner, forming random crystal boundaries. The efficiency of energy conversion is slightly lower. This merely means that the size of the finished module is slightly greater per watt than most single-crystal modules. The cells look different from single-crystal cells. The surface has a jumbled look with many variations of blue color. In fact, they are quite beautiful like sheets of gemstone.
In addition to the above processes, some companies have developed alternatives such as ribbon growth and growth of crystalline film on glass. Most crystalline silicon technologies yield similar results, with high durability. Twenty-year warranties are common for crystalline silicon modules. Single-crystal tends to be slightly smaller in size per watt of power output, and slightly more expensive than polycrystalline.
The construction of finished modules from crystalline silicon cells is generally the same, regardless of the technique of crystal growth. The most common construction is by laminating the cells between a tempered glass front and a plastic backing, using a clear adhesive similar to that used in automotive safety glass. It is then framed with aluminum.
The silicon used to produce crystalline modules is derived from sand. It is the second most common element on Earth, so why is it so expensive? The answer is that in order to produce the photovoltaic effect, it must be purified to an extremely high degree. Such pure "semiconductor grade" silicon is very expensive to produce. It is also in high demand in the electronics industry because it is the base material for computer chips and other devices. Crystalline solar cells are about the thickness of a human fingernail. They use a relatively large amount of silicon.
Thin-Film TechnologiesImagine if a PV cell was made with a microscopically thin deposit of silicon, instead of a thick wafer. It would use very little of the precious material. Now, imagine if it was deposited on a sheet of metal or glass, without the wasteful work of slicing wafers with a saw. Imagine the individual cells deposited next to each other, instead of being mechanically assembled. That is the idea behind thin film technology. (It is also called amorphous, meaning "not crystalline.") The active material may be silicon, or it may be a more exotic material such as cadmium telluride.
Thin-film panels can be made flexible and lightweight by using plastic glazing. Some flexible panels can tolerate a bullet hole without failing. Some of them perform slightly better than crystalline modules under low light conditions. They are also less susceptible to power loss from partial shading of a module.
The disadvantages of thin-film technology are lower efficiency and uncertain durability. Lower efficiency means that more space and mounting hardware are required to produce the same power output. Thin film materials tend to be less stable than crystalline, causing degradation over time. The technology is being greatly improved however, so I do not wish to generalize in this article (written in late 1999). We will be seeing many new thin-film products introduced in the coming years, with efficiency and warranties that may approach those of crystalline silicon.
PV experts generally agree that crystalline silicon will remain the "premium" technology for critical applications in remote areas. Thin film will be strong in the "consumer" market where price is a critical factor.
Different PV System Types
Day Use Systems
Information:
The simplest and least expensive photovoltaic systems are designed for day use only. These systems consist of modules wired directly to a DC appliance, with no storage device. When the sun shines on the modules, the appliance consumes the electricity they generate. Higher insolation (sunshine) levels result in increased power output and greater load capacity
Consists of:
PV Array, DC Appliance
Direct Current Systems Power Alternating Current Loads
Information:
Photovoltaic Modules produce DC electrical power, but many common appliances require AC power. Direct current systems that power AC loads must use an inverter to convert DC electricity into AC. Inverters provide convenience and flexibility in a photovoltaic system, but add complexity and cost. Because AC appliances are mass produced, they generally offered in a wider selection, at lower cost, and with higher reliability than DC appliances. High quality inverters are commercially available in a wide range of capacities.
Consists of:
PV Array, Charge Controller, Storage Battery, Inverter, DC Load Center, AC Load Center
Direct Current Systems with Storage Batteries
Information:
To operate loads at night or during cloudy weather, PV systems must include a means of storing electrical energy. Batteries are the most common solution. System loads can be powered from the batteries during the day or night, continuously or intermittently, regardless of weather.
In addition, a battery bank has the capacity to supply high-surge currents for a brief period, giving the system the ability to start large motors or to perform other difficult tasks. This system�s basic components include: PV modules, charge controllers storage batteries, and appliances (the system�s electrical load).
A battery bank can range from small flashlight size batteries to dozens of heavy-duty industrial batteries. Deep-cycle batteries are designed to withstand being deeply discharged and then fully recharged when the sun shines. (Conventional automobile batteries are not well suited for use in photovoltaic systems and will have short effective lives). The size and configuration of the battery bank depends on the operating voltage of the system and the amount of nighttime usage In addition, local weather conditions must be considered in sizing a battery bank. The number of modules must be chosen to adequately recharge the batteries during the day.
Batteries must not be allowed to discharge too deeply or be overcharged- either situation will damage them severely. A Charge controller will prevent the battery from overcharging by automatically disconnecting the module from the battery bank when it is fully loaded. Some charge controllers also prevent batteries from reaching dangerously low charge levels by stopping the supply of power to the DC load. Providing charge control is critical to maintaining battery performance in all but the simplest of PV systems.
Consists of:
PV Array, Charge Controller, Storage Battery, DC Loads
Hybrid Systems
Information:
Most people do not run their entire load solely off their PV system. The majority of systems use a hybrid approach by integrating another power source. The most common form of hybrid system incorporates a gas or diesel powered engine generator, which can greatly reduce the initial cost. Meeting the full load with a PV system means the array and batteries need to support the load under worst-case weather conditions. This also means the battery bank must be large enough to power large loads, such as washing machines, dryers and large tools. A generator can provide the extra power needed during cloudy weather and during periods of heavier than normal electrical use, and can also be charging the batteries at the same time. A hybrid system provides increased reliability because there are two independent charging systems at work.
Another hybrid approach is a PV system integrated with a wind turbine. Adding a wind turbine makes sense in locations where the wind blows when the sun doesn�t shine. In this case, consecutive days of cloudy weather are not a problem, so long as the wind turbine is spinning. For even greater reliability and flexibility, a generator can be included in a PV/Wind system. A PV/Wind/Generator system has all of the advantages of a PV/Generator system, with the added benefit of a third charging source for the batteries.
DC Coupled
Consists of:
PV Array, Charge Controller, Storage Battery, Inverter/Charger, AC Load Center
AC Coupled
Consists of:
PV Array, Inverter/Charger, AC Load Center, Storage Battery
Grid Tied System
Information:
Photovoltaic systems that are connected to the utility grid (utility connected, grid tied, or line-tied systems) do not need battery storage in their design because the utility grid acts as a power reserve. Instead of storing surplus energy that is not used during the day, the homeowner sells the excess energy to a local utility through a specially designed inverter. When homeowners need more electricity than the photovoltaic system produces, they can draw power from the utility grid.
If the utility grid goes down, the inverter automatically shuts off and will not feed solar-generated electricity back into the grid. This ensures the safety of line persons working on the grid. Because utility-connected systems use the grid for storage, these systems will not have power if the utility grid goes down. For that reason, some of these systems are also equipped with battery storage to provide power in the event of power loss from the utility grid.
The Public Utilities Regulatory Policies Act (PURPA) of 1978 requires electric utilities to purchase power from qualified, small power producing system owners. The utilities must pay the small power producers based on their �avoided costs,� or costs the utility does not have to pay to generate that power themselves. Additional terms and conditions for these purchases are set by state utility commissions and vary from state to state. While this law allows homeowners in areas with utility power to purchase photovoltaic systems and sell their excess power to an electric utility, people contemplating doing so should remember that this is rarely a profitable venture at the present time.
Some utility companies offer �net metering� to their customers, where a single meter spins in either direction depending upon whether the utility is providing power to the customer or the customer is producing excess power. The customer or independent power producer pays or collects the net value on the meter. Net metering is very desirable to the independent power producer because he/she can sell power at the same retail rate that the utility charges its customers.
Without Batteries
Consists of:
PV Array, Inverter, AC Service Panel, AC Utility Meter, Power Lines
With Batteries
Consists of:
PV Array, Charge Controller, Storage Battery, Inverter, Back-up AC Load Center, AC Service Panel, AC Utility Meter, Power Lines
Off-Grid System
Information
By definition an off-grid power system is any system that provides power where utility power is unavailable. Off-grid systems typically make financial sense any place where the utility would have to run lines more than one half of a mile for grid connection. In addition, the new federal PV incentive does not distinguish between grid-tied and off-grid, so any system should be eligible for a federal tax rebate.
A typical off-grid system typically consists of an off-grid inverter, battery-bank, generator, and a DC power source (PV, Wind, Micro Hydro, etc.). If a PV array is used as a DC power source then a charge controller would also be used to harvest energy from the solar array and protect the batteries from overcharging.
System sizing is much more important on an off-grid system than a grid-tied system. Questions that need to be answered include:
- How many kWh to you expect to consume?
- How many hours/days of autonomy do you want to be able to run without PV (or other energy source)?
- What is the largest load that you need to run? How much power is required to start this load?
- What is your budget?
Off-Grid Inverters
Consider:
Tare Losses is the amount of power that is required to run the system in standby mode. Every watt is precious in an off-grid system and reducing power wasted is critical. This is a specification to look at very closely since there is a wide variance among different inverter manufacturers for tare losses. In addition some inverter companies have the ability to turn off inverters entirely in multiple inverter systems to further reduce tare losses.
Surge Capability
The ability of an inverter to surge to a higher level than its rated output for a short duration to start large loads like well pumps is critical. The specifications that should be looked at are the Maximum Output Current and the AC Overload capability. If there are large loads a good number to look for is a five second surge capability of at least 1.5 times the rated output of the inverter.
System Information
It is very helpful to have good, reliable information about your battery�s state of charge. In many systems, generators are started automatically when the batteries get down to a certain state of charge. Usually this is accomplished through an external DC monitor. The best systems give you true battery state of charge which is a more accurate reading of the capacity of your batteries than battery voltage.
Field Serviceability
Often systems are installed in very remote locations. The ability to service the product in the field without having to take down the system is very important
Consists of:
Off-grid inverter, battery-bank, generator, PV array
Module Performance
The total electrical power output (wattage) of a photovoltaic module is equal to its operating voltage multiplied by its operating current. Photovoltaic modules may produce current over a wide range of voltages. This is unlike voltage sources such as batteries, which produce current at a relatively constant voltage.
The output characteristics of any given module are characterized by a performance curve, called an I-V curve, which shows the relationship between current and voltage output. Most I-V curves are given for the standard test conditions (STC) of 1,000 watts per square meter irradiance (often referred to as one peak sun) and 24 degrees C cell temperature. It should be noted that STC represent the rare optimal conditions as a consistent means for measuring�rarely are these conditions recreated in outside environments. The IV curve contains three significant points:
1. Maximum Power Point (Vmp & Imp)This point, labeled Vmp and Imp, is the operating point at which the maximum output will be produced by the module at operating conditions indicated for that curve. In other words, the Vmp and Imp of the module can be measured when the system is under load at 25 degrees C cell temperature and 1,000 watts per square meter. The voltage at the maximum power point can be determined by extending a vertical line from the curve downward to read a value on the horizontal voltage scale. The wattage at the maximum power point is determined by multiplying the voltage at maximum power by the current at maximum power. The power output decreases as the voltage drops. Current and power output of most modules drops off as the voltage increases beyond the maximum power point
2. Open Circuit Voltage (Voc)
This point, labeled Voc, is the maximum potential voltage achieved when no current is being drawn from the module. Since no current is flowing, the module experiences maximum electrical pressure. The example at left displays an open circuit voltage of approximately 21 volts. The power output at Voc is zero watts. Open Circuit Voltage can be measured in the field in several common circumstances. When buying a module, it is recommended to test the voltage to see if it matches the manufacturers� specifications. When testing voltage with a digital multi-meter from the positive to the negative terminal, an open circuit is created by the meter which allows VOC to be measured. It is also common to see a module operating at Voc early in the morning and late in the evening.
3. Short Circuit Current (Isc)
This point, labeled Isc is the maximum current output that can be reached by the module under the conditions of a circuit with no resistance or a short circuit. The example to the left displays a current of approximately 2.65 amps. The power output at Isc is zero watts. When first purchasing a module, it is recommended to test the short circuit current to see if it matches the specification sheet. The short circuit current can be measured only when making a direct short across the positive and negative terminals of a module. Creating a direct short across more than one module at a time (or a module with voltage greater than 24V nominal) is not recommended and can be extremely dangerous. All Isc measurements should be taken when the module is not connected to other components in the system. Note: When testing modules with �quickconnects� it is recommended to use test leads to avoid leaving carbon deposits (which cause high resistance) on the module�s leads
Estimating Your System Size
Using the amount monthly kWhs from your utility bill, and the amount of sun hours your location receives, you can find the size of your system, in DC watts, needed to cover 100% of your usage
 |
< 3.0 | 3.5 | 4.0 | 4.5 | 5.0 | 5.5 | 6.0 | 6.5 |
| 50kWh | 0.7kw | 0.6kw | 0.5kw | 0.5kw | 0.4kw | 0.4kw | 0.3kw | 0. kw |
| 100kWh | 1.4kw | 1.2kw | 1.0kw | 0.9kw | 0.8kw | 0.8kw | 0.7kw | 0.6kw |
| 150kWh | 2.0kw | 1.7kw | 1.5kw | 1.4kw | 1.2kw | 1.2kw | 1.0kw | 0.9kw |
| 200kWh | 2.7kw | 2.3 kw | 2.0kw | 1.8kw | 1.6kw | 1.6kw | 1.4kw | 1.2kw |
| 250kWh | 3.4kw | 2.9kw | 2.5kw | 2.3kw | 2.0kw | 2.0kw | 1.7kw | 1.6kw |
| 300kWh | 4.1kw | 3.5kw | 3.0kw | 2.7kw | 2.4kw | 2.4kw | 2.0kw | 1.9kw |
| 350kWh | 4.7kw | 4.1kw | 3.5kw | 3.2kw | 2.8kw | 2.8kw | 2.4kw | 2.2kw |
| 400kWh | 5.4kw | 4.6kw | 4.1kw | 3.6kw | 3.2kw | 3.2kw | 2.7kw | 2.5kw |
| 450kWh | 6.1kw | 5.2kw | 4.6kw | 4.1kw | 3.6kw | 3.6kw | 3.0kw | 2.8kw |
| 500kWh | 6.8kw | 5.8kw | 5.1kw | 4.5kw | 4.1kw | 4.1kw | 3.4kw | 3.1kw |
| 550kWh | 7.4kw | 6.4kw | 5.6kw | 5.0kw | 4.5kw | 4.5kw | 3.7kw | 3.4kw |
| 600kWh | 8.1kw | 6.9kw | 6.1kw | 5.4kw | 4.9kw | 4.9kw | 4.1kw | 3.7kw |
| 650kWh | 8.8kw | 7.5kw | 6.6kw | 5.9kw | 5.3kw | 5.3kw | 4.4kw | 4.1kw |
| 700kWh | 9.5kw | 8.1kw | 7.1kw | 6.3kw | 5.7kw | 5.7kw | 4.7kw | 4.4kw |
| 750kWh | 10.1kw | 8.7kw | 7.6kw | 6.8kw | 6.1kw | 6.1kw | 5.1kw | 4.7kw |
| 800kWh | 10.8kw | 9.3kw | 8.1kw | 7.2kw | 6.5kw | 6.5kw | 5.4kw | 5.0kw |
| 850kWh | 11.5kw | 9.8kw | 8.6kw | 7.7kw | 6.9kw | 6.9kw | 5.7kw | 5.3kw |
| 900kWh | 12.2kw | 10.4kw | 9.1kw | 8.1kw | 7.3kw | 7.3kw | 6.1kw | 5.6kw |
| 950kWh | 12.8kw | 11.0kw | 9.6kw | 8.6kw | 7.7kw | 7.7kw | 6.4kw | 5.9kw |
| 1000kWh | 13.5kw | 11.6kw | 10.1kw | 9.0kw | 8.1kw | 8.1kw | 6.8kw | 6.2kw |
| 1050kWh | 14.2kw | 12.2kw | 10.6kw | 9.5kw | 8.5kw | 8.5kw | 7.1kw | 6.5kw |
| 1100kWh | 14.9kw | 12.7kw | 11.1kw | 9.9kw | 8.9kw | 8.9kw | 7.4kw | 6.9kw |
| 1150kWh | 15.5kw | 13.3kw | 11.6kw | 10.4kw | 9.3kw | 9.3kw | 7.8kw | 7.2kw |
| 1200kWh | 16.2kw | 13.9kw | 12.2kw | 10.8kw | 9.7kw | 9.7kw | 8.1kw | 7.5kw |
| 1250kWh | 16.9kw | 14.5kw | 12.7kw | 11.3kw | 10.1kw | 10.1kw | 8.4kw | 7.8kw |
| 1300kWh | 17.6kw | 15.0kw | 13.2kw | 11.7kw | 10.5kw | 10.5kw | 8.8kw | 8.1kw |
| 1350kWh | 18.2kw | 15.6kw | 13.7kw | 12.2kw | 10.9 kw | 10.9kw | 9.1kw | 8.4kw |
| 1400kWh | 18.9kw | 16.2kw | 14.2kw | 12.6kw | 11.3kw | 11.3kw | 9.5kw | 8.7kw |
| 1450kWh | 19.6kw | 16.8kw | 14. kw | 13.1kw | 11.7kw | 11.7kw | 9.8kw | 9.0kw |
| 1500kWh | 20.3kw | 17.4kw | 15.2kw | 13.5kw | 12.2kw | 12.2kw | 10.1kw | 9.3kw |
| 1550kWh | 20.9kw | 17.9kw | 15.7kw | 14.0kw | 12.6kw | 12.6kw | 10.5kw | 9.7kw |
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