SYSTEM COMPONENTS
Grid tie interface systems are rather simple, only requiring solar modules and an inverter. Whereas, stand alone and hybrid systems require solar modules, trackers and mounts, wind generators, hydro generators, charge controllers, inverters, batteries, battery chargers, and monitors.
SOLAR MODULES
The solar module is the main component in a solar electric system, its purpose is to convert sunlight to electricity.
POWER CHARACTERISTICS: The current and power output of photovoltaic modules are approximately proportional to sunlight intensity. At a given intensity, a modules output current and operating voltage are determined by the characteristics of the load. If that load is a battery, the battery’s internal resistance will dictate the module’s operating voltage.
A module which is rated at 17 volts will put out less than its rated power when used in a battery system. This is because the working voltage will be between 12 and 15 volts. As wattage (power) is the product of volts times amps, the module output will be reduced. For example: a 50 watt module working at 13.0 volts will produce 39.0 watts (13.0 volts x 3.0 amos = 39.0 watts). This is important to remember when sizing a PV system.
An I-V curve as illustrated to the right is simply all of a modules possible operating points (voltage/current combinations) at a gven cell temperature and light intensity. Increases in cell temperature increase current but decrease voltage. Maximum power is derived at the knee of the curve. Check the amperage generated at your batteries operating voltages to better illustrate the actual power developed at your voltages and temperatures.
MIXING SIZES AND BRANDS OF MODULES: In most cases mixing dissimilar modules in the same array is not a problem. When paralleling units of different amperage ratings, the output of the array will be the sum of the combined amperages. When paralleling units of different voltages, the lower vooltage units will begin to taper off sooner as high battery voltage is reached. If used for array direct power, the array voltage will be the approximate avaerage module voltage.
When series connecting string of dissimiliar modules, however, the amperage will be approxamately that of the weakest module in the string. It pays then, to pay attention to matching the modules connected in series.
SHADING: PV modules are very sensitive to shading. Unlike a solar thermal panel which can tolerate some shading, many brands of PV modules cannot even be shaded by the branch of a leafless tree. Once a solar cell or a portion of a cell is shaded it becomes a load adn draws power instead of producing it. Watch the amp meter of your system when a hand id passed over a module and you will see a substantial drop in output.
Some solar modules offer protection from partial shading. The advanced design of these include a diode between each cell, reducing partial shading problems. Rule of thumb - make sure no shading occurs between 9:00 am and 3:00 pm around solar noon. Shading early or late is not much of a problem because these are low power producing hours anyway.
REVERSE CURRENT PROTECTION: PV modules will leak power back from your batteries during no sun periods if not protected. This leakage is very small but over long, no-sun periods, this loss can accumulate. To prevent this we install a diode or protecting circuitry in the controller. All controllers that we sell have reverse leakage protection. The circuits open over periods of no sun, allowing the charging circuit to stop reverse flow. A diode can also be used. This unit acts as a one way check valve-letting power flow in one direction to the batteries but not back to the PV module.
TRACKERS & MOUNTS
Where are we going to put all of these PV modules? Track rack, ground mount rack, roof mount rack, pole top rack?
MODULE MOUNTING: Solar modules perform best when peependicular to the sun’s rays. Becasue tracking the sun is not always possible, we usaually mount the modules facing due south. Remember that true south and magnetic south vary upon your site’s declination. Call you local airport or us if you do not have this figure.
Becasue the position of the sun in the sky varies throughout the year it is a good idea to provide for seasonal adjustment. The rule of thumb goes: latitude plus 15 degrees angle in winter and latitude minus 15 degrees in summer. Your lattitude can be found on any good map of your area. If you wish to permanently mount the modules with no adjustment, it is recommended to mount your modules in winter angle. This offers maximum output in the winter when the days and solar gain is at a mininmum.
TO TRACK....OR NOT TO TRACK?: Sun tracking racks (tracker) increase the total daily output of PV modules by keeping them perpendicular to the sun throughout the day. A tracker can increase the the output of the modules by 25 to 40 percent in the summer and 10 to 15 percent in the winter.
WIND GENERATORS
IS WIND GENERATION FOR YOU?: Wind generated electricity can be used directly, for pumping water, or it can be stored in batteries. Wind generators can be used alone or as part of a hybrid system combined with PV, hydro or a fossil fuel powered generator. Hybrid systems are especially useful for winter backup of home systems where cloudy weather and windy conditions occur simultaneously.
The most important factor when considering wind power is determining wether your site has enough wind to supply your needs. Wind can be intermitant and seasonal.
The power available from the wind varies as the cube of the wind speed. If the wind speed doubles, the power of the wind increases eight times. For example, a 10 mile per hour wind has one eighth the power of a 20 mile per hour wind. (10 x 10 x 10 = 1000 versus 20 x 20 x 20 = 8000).
One of the effects of the cube rule is that a site which has an average wind speed reflecting wide swinigs from very low to very high velocity may have twice or more the energy potential of a site with the same average wind speed which experiences little variation. This is because the high wind speed packs a lot of power in a short period of time.
Wind speed data is often available from local weather stations or airports, as well as the US Dept. of Commerce, National Climatic Center in Asheville, NC. You can also do your own site analysis with an anemometer or totalizer and careful observation.
HYDRO GENERATORS
IS HYDRO POWER AN OPTION?: Hydro power has one requirement, head. Head is the measurement of vertical drop in a column of water. For hydro power to be a viable resource you must have a steady flow of water with some vertical drop.
CHARGE CONTROLLERS
DO YOU NEED A CHARGE CONTROLLER?: The main function of a charge controller or regulator is to allow a battery to fully charge without overcharging. If a solar array is connected to batteries with no control of over voltage, battery damage may occur. Basic controllers use relays to open an close the charge circuit at set voltages. More advanced chargers use charging sequences and stages, like pulse width modulation (PWM), to assure that the battery is getting fully charged. The latest chargers are using maximum power point tracking (MPPT) to maximize the PV output and charge the battery more efficiently. Some of these controllers even allow you to run a PV array at a higher nominal voltage than the battery. The bottom line is that the first 70% to 80% of battery charge is easily replaced, the last 20% to 30% is the difficult part. That is where these newer controllers shine.
INVERTERS
INVERTER BASICS: An inverters main function is to convert low voltage DC current that is generated by a solar electric system into higher voltage AC current that is found in common house hold outlets. Older inverters were rather inefficient and unreliable. The inverters of today run at 85% to 94% efficiency peak and are very reliable. Lots of inverters also work as battery chargers if there is an available AC source, a generator or grid power.
Most inverters output 120VAC but not 120/240 VAC. Most appliances will run on 120 VAC but if 240 VAC is required for pump motors or large tools, then a transformer or a second inverter (in some applications) can be used.
There are two types of inverters available, true sinewave and modified sinewave. Modified sinewave inverters are good for nearly any application except the most delicate equipment. True sinewave inverters put out a very smooth power curve but cost more per watt.
When installing inverters it is important to make sure that you have all the proper equipment for a safe and reliable installation. First off, you will need a DC disconnect and overcurrent protection. This is usually a switch and a fuse or a circuit breaker. Second, is to determine the proper cable size for the DC disconnect and inverter you are using. You also might want to consider puting a shunt inline for monitoring amperage and keeping track of amp-hours stored. Finally you will need some kind of overcurrent protection for the AC output of the inverter. This disconnect is usaually some srt of breaker box or fused disconnect.
BATTERIES
THE HEART OF THE SYSTEM: Of all the components of a solar electric system, the batteries require the most attention. Batteries are fairly simple and with proper matainance can last and be reliable for years. Without proper maintainance batteries can deteriorate quickly. It was once said, "few batteries die a natural death, most are murdered."
FLOODED LEAD ACID BATTERIES: There are several types of batteries. Some of which are flooded lead acid, sealed gel cell, nickel cadmium, nickel metal hydride. The info in this section deals primarily with flooded lead acid batteries, care and matainance for other types of batteries can differ greatly.
CYCLING - Deep Versus Shallow: A cycle in the battery is when a battery is discharged and then recharged to the same point. Different batteries have different discharge capacities, known as the depth of discharge. There are two types of batteries, shallow cycle and deep cycle. A shallow cycle battery is designed for use of about 20% of the charge at a rapid discharge rate. These batteries are generaly used for automotive applicatioins. The second type of battery is the deep cycle battery which has a charge depth of about 80%. Deep cycle batteries are used in solar electric systems due to their deep discharge capability, it get us through those cloudy days.
The depth of discharge has a good deal to do with determining a batteries useful life. Even batteries designed for deep cycling are "used up" faster as depth of discharge is increased. It is common practice for a system to be designed with deep cycle batteries even though the daily or average discharging amounts to a relatively shalllow depth of discharge. Shallow cycle your deep cycle battery for the most cycles.
TEMPERATURE EFFECTS: The speed of the chemical reaction in batteries is affected by temperature. The warmer the teperature the quicker the reaction. Optimum operating temperature for batteries is 77 degrees Fahrenheit. It is best to keep batteries in a ventilated space that maintains a temperature between 55 and 80 degrees.
SELF DISCHARGE: Self discharge happens due to impurities in the chemicals used in their construction. Self discharge rates vary due to construction type, temperature and the age of the battery. Self discharge can also occur from the film that forms on the top of batteries. This film can act as a resistive load and put a small drain on the batteries. This can be avoided by keeping batteries clean.
BATTERY VOLTAGE AND BATTERY STATE OF CHARGE: Battery state of charge is determined by reding either terminal voltage of the specific gravity of the electrolyte.
Volatage meters can be used to read battery state of charge. The problem with relying on the voltage reading is due to the high degree of battery voltage variation throughout the working day. Battery voltage is greatly reactive to charging or discharging. A charging battery can have an artificially high voltage reading whereas a dischaging battery will have an artificially low voltage.
The measurement of density or specific gravity of the electrolyte can also be used to tell state of charge. The density of the electrolyte varies with state of charge, and temperature. A hydrometer with temperature correction can be used to tell state of charge of flooded lead acid batteries.
MONITORING AND MAINTAINANCE: Monitoring the battery state of charge is the number one responsibility of the system owner or caretaker. The battery should not be drawn below 50% of its capacity to maximize battery life.
Keep the batteries electrolyte level to the appropriate level and never let the level drop below the top of the plates. Exposing the plates can cause serious damage. Always use distilled water, never tap water when filling batteries. It is not recommended to water your batteries when they are discharged. Water is the only fluid used by your battery, you should never have to add acid. Over watering will dilute the acid and can cause fluid discharge while charging.
BATTERY CONNECTIONS: Battery interconnections and load/charging connections are crucial. Terminals should be clean and treated with an anti-corrosive compound. Properly torqued connections are also important to avoid variations in resistance. This is also why it is preferable to minimize the number of parallel strings in the battery bank. More resistance in one string of batteries results in less charge to that string and consequently shorter life. Also, placing the negative and postive on opposing corners of the bank can help. The goal is to keep the variation of resistance from one cell to another a minimum.
CORROSION: A slight acid mist is formed as the electrolyte bubbles upon charging. This mist is highly corrosive, especially to the mettalic connectors to the batteries. Periodically check all connections for corrosion and clean as needed with baking soda and water. Corrosion causes resistance which in turn shortens battery life and waste of power.
OVERCURRENT PROTECTION: Batteries have the potential to discharge incredible amounts of power over a very short period of time. This can cause melted conductors and even fires. That is why it so important to use proper over current protection.
In a typical solar electric system we deal with both AC and DC power. Standard equipent is designed for AC use only and is fine for the inverter output. But DC overcurrent devices are required for connections between the batteries and inverter, charge input and DC loads. They are generally more heavy duty and more expensive.
BATTERY ENCLOSURES: It is best to install your batteries in a warm, dry location. The optimum temperature range for batteries is 55-80 degrees F. Operating batteries at higher or lower temperatures can diminish performance significantly.
When batteries are charging they produce a dangerous mixture of hydrogen and oxygen. Becasue of this it is important to provide adequate ventilation to prevent buildup of gasses. Since hydrogen is lighter than air it has the tendency to rise, so the vent should be placed at the top of the battery enclosure and and have rise all the way out.
Take note, if installing a battery box in a basement, venting might cause air to flow in rather than out. In this situation a power vent is recommended.
BATTERY EFFICIENCY: Batteries are not 100% percent efficient. When new they are about 80% to 90% effiecient. But for system sizing purposes we use a 70% efficiency factor to allow for wear and tear due to normal use.
BATTERY CHARGING: A typical 12 volt lead-acid battery must be charged to 14.2 to 14.4 volts before it is fully charged. If taken to a lower voltage, not all the sulfate deposits that form during discharge will be disappated. Over time these deposits will lessen the amp hour storage of the battery. Once the battery reaches it fully charged point it should then be held at a lower volatge to maintain its charge, typically 13.2 to 13.5 volts. Higher voltage levels will "gas" the batteries and boil off electrolyte, shortening battery life.
Some battery chargers, inverters with battery chargers and charge controllers have good charching algorthims that allow the batteries to maximize their charge. These algorothims include paramaters for bulk charge voltage, float voltage and equalization. The bulk voltage setting takes the batteries to the fully charged point. The float setting holds the batteries at a constant voltage, lowe then bulk, to ensure full charge. Equalization is actually reaching the stage of overcharging. This is done to give the battery plates a good "scrubbing", helping remove sulfation deposits.
EQUALIZATION: Equalization is the controlled overcharging of a fully cherged battery. This overcharge mixes the electrolyte, evens the charge among varying battery cells and reduces permanent sulfation of the battery plates. It is energy invested in lengthening battery life. Even though batteries are cycled and gassed through normal discharge and recharge use it is important to equalize. Equalization will help lengthen the life of your batteries as well as enhance their performance. Equalization should be done every 30 to 90 days.
USED BATTERIES: Used batteries of all types can usually be found for sale. While some used solar equipment is an acceptable risk, used batteries are questionable. It is impossible to tell how a used cell has been treated. Has it been over discharged or been exposed to extreme overcharging? The only way to test a used battery is to load test it. Without a load test there is no way of determining the actual state of the battery. Watch out for batteries that have just been cleaned and recharged.
SYSTEM MONITORS
WHY SYSTEM MONITORING IS IMPORTANT: Proper monitoring of a system is the key to ensuring a properly operating system. Important aspects to monitor are batery voltage, input amperage and output amperage. With proper monitoring and keeping track of normal operation values can determine the baseline of a system. When this baseline varies, it often a sign that something might be wrong with the system. It also helps to keep the user aware of state of charge ot avoid over discharge.
DIFFERENT TYPES OF MONITORS: There are basically two different types of meters, analog and digital. Analog meters are the simple kind like old speedometers, a needlr that points to the value. Digital meters provide you with an actual number, like a digital watch.
Analog meters can monitor the basic values like voltage and amperage. Digital meters take it to the next step, many of them can remember the amount of power coming into and leaving a system. By keeping track of total power into and out of a system you end up with almost a fuel gauge type of situation. It can monitor percent full and give you a one look state of charge.
Most digital meters and some analog amp meters require a shunt to operate. A shunt diverts a minute amount of electricity from the system input/output circuit to the meter for monitoring. A shunt goes in the negative lead from the batteries to the rest of the system.
LIGHTNING PROTECTION
Lightning presents a potential hazard hazard for systems with exposed conductors and aluminum framing mounted on rooftops or adjacent to buildings. Direct and close hits can cause damage in electrical circuits and electronics due to the presence of static charges and electromagnetic fields. These forces can induce voltage spikes of all types especially if the system is not properly grounded and protected.
While no lightning protection is foolproof, practical counter-measures are available and include a lightning rod at the PV source, adequate system grounding, and surge protection on the incoming DC wires and the secondary AC wiring.