ELECTRICAL FUNDAMENTALS AND PRINCIPALS BEHIND SOLAR DESIGNS
Electrical Fundaments (Volts, Amps & Watts)
Electricity and plumbing have a lot in common. Instead of flowing water, electricity deals with flowing electrons. Instead of pipes, electrons flow through wires. Instead of reservoirs, electricity can be stored in batteries.To further visualize these similarities consider the diagram below showing a water tank (or a battery.)
At the bottom of a full tank of water there will be a certain pressure caused by the weight of the water that would push water out of the tank of an opening were to form. In a fully charged battery there will be certain voltage (similar to pressure) caused by a chemical process that would cause electrons to flow if a path (or circuit) was created.
When an opening is formed at the bottom of a tank, the water flows out at a certain rate depending on the resistance of the pipes and pressure behind the water. Similarly, if an electrical circuit is formed, electrons will flow at a certain rate depending on the voltage and resistance of the load. The flow rate of electrons is referred to as current also known as Amps, A, or in some cases the letter “I”. The resistance of the load is expressed in Ohms or with the symbol “Ω”.
Energy is the flow rate times the pressure, or volts times amps. Energy is expressed in units of Watts.
Volts = Amps x Ohms, V=IR
Watts = Amps x Volts, W=AV, W=RI2
Kilowatts = 1000 x Watts
POWER VS. ENERGY (KWH VS. WATTS)
It is very common for people to confuse Watt hours (Wh) and Watts (W). The “h” isn’t just a longhand way of expressing Watts, it is very significant. A Watt hour is a unit of energy and a Watt is a unit of power. Watt hours equals Watts times Time.
For example, a 65W light bulb has an instantaneous electrical consumption of 65W of energy. If you run that light bulb for 2 hours it will have consumed 130Wh of power (2h x 65W = 130Wh).
Another way to highlight differences between Watts and Watt hours is to compare them to Speed vs. Distance. Watts are to Speed as Distance is to Watt hours. If someone drove down a highway at a constant speed of 65mph for 2 hours they will have travelled 130 miles.
In terms of calculus, if you were to plot Watts over Time the area under the curve would be the Watt Hours.
In a solar system, the number of panels is proportional to the kWh used over a given period (so is your electric bill). The inverter in an off-grid system is sized based on the maximum instantaneous Wattage needed by the user.
Series and Parallel Connections
Solar panels and batteries are often connected in parallel, series or combinations of the two to achieve a desired voltage or current.
If both series and parallel will work, series is preferred because line losses will be less and parallel connections can be expensive.
Strings – Refers to the number of panels in series. If someone were to describe an array configuration as having three strings of twelve modules, they are saying that the system has three parallel strings of twelve panels in series.
HOW SOLAR WORKS
A thorough explanation of how solar works involves some very complicated physics.
Light, which consists of photons, strikes the silicon atoms on a solar cell. When this happens, energy is added to the atom which causes the electrons orbiting the nucleus of that atom to move into higher energy shells (orbits). Solar cells are made of two types of silicon layered one on top of the other. One layer has a higher electron density than the other. Because of this layering, electrons from the top layer can be captured by the bottom layer when they are excited into higher energy shells. Another feature of the layering is that the captured electrons cannot move back to the layer they originally came from. This surplus of electrons on one layer and deficit on the other creates a voltage (force) of electrons being attracted back to the top layer. If wires are attached to each layer (one positive and one negative), connected to devices that run on electricity, completing a circuit, the electrons will flow down those wires, power electrical devices along the way and return to the top silicon layer. This process repeats continually as long as photons strike the silicon atoms.
Calculating Solar Production
Converting the Watts of a solar array to actual kWh that reduces your power bill requires a computer simulation that takes into account latitude and climate factors that would control how much sun exposure your panels get.
AC vs. DC, 120V, 240V, Single Phase vs. Three Phase
Electrical current is transmitted in two main ways, Direct Current DC and Alternating Current AC. Direct Current is when the current maintains a relatively constant flow without changing directions. DC current is produced by solar panels and stored in batteries.
Alternating Current changes directions in a sinusoidal pattern several times per second. In the United States, the frequency is 60hz, meaning that it goes from positive to negative 60 times per second. In Europe and many other countries the frequency is 50hz. Alternating Current is what the utility grid distributes and what inverters produce from DC sources.
When transmitting current over long distances Alternating Current is preferred because of the more dramatic voltage losses that take place when using Direct Current. The reason for this is that with DC, each electron has to travel from the source to the destination. AC on the other hand, is just electrons vibrating into each other.
The standard voltage of AC power coming out of an electrical outlet is 120V, but when it is transmitted over the grid it is at a much higher voltage, at least 240V depending the particular section of the grid. The higher voltage is preferred because it reduces line loses.
In single phase AC there are two conductors and an optional ground. These two conductors oscillate opposite of each other. Since one full oscillation is 360 degrees, the two conductors would be 180 degrees out of phase. In three phase power, there are three conductors and an optional ground. Each conductor is 120 degrees out of phase from the other two conductors. Three phase is commonly used for large electric motors.
System Design: Charge Controllers
Charge controllers go between panels and batteries. Their purpose is to take power from panels and feed it to batteries without over charging the batteries. When selecting a charge controller or determining what panels to put on a charge controller there are four rules that should be followed.
1) Make sure the voltage of the solar panel array going into the charge controller is at least a couple volts higher than the battery bank voltage over a wide range of temperatures and charge levels. For example, a 12.0V panel (if there was such a thing) would not charge a 12V battery because there would no “pressure” differential that would cause current to flow. The voltage of the panel would have to be at least 14.0V to charge a 12V battery bank.
2) Make sure the open circuit voltage of the solar array is not above the voltage limit of the charge controller. For example some Sunforce charge controllers have a voltage limit of 25V. Because of this, you would not be able to use our 250W panel because it has an open circuit voltage of 37.7V. Also, Outback charge controllers have a voltage limit of 150V. Because of this you would not want to put more than three250W panels in each series string because on a cold day (panel voltage increases as temperature decreases) their combined voltage would exceed 150V.
3) Make sure the current going from the charge controller to the battery bank does not exceed the charge controller current rating. The calculations for current have nothing to do with the current coming off the array since current coming into a charge controller and going out of a charge controller are not necessarily equal. To calculate current, take the total wattage of the solar array, multiply it 90% to take into account losses and real world solar irradiance, then divide it by the voltage of the battery bank. For example, two 250W panels on a 12V battery bank would produce about 37.5A (2 x 250W x 0.90 / 12V = 37.5A). This would not work on a charge controller rated for 15A.
4) Use MPPT when there is a large differential between the panel voltage and the battery bank voltage. There are two types of charge controllers, PWM and MPPT. PWM charge controllers feed the batteries with a direct connection to the solar panels. MPPT charge controllers utilize a transformer to step down the voltage and increase the current before being fed onto a battery bank. For example, even though the Xantrex C35 charge controller can handle the voltage of a 250W panel and is compatible with 12V systems, it uses PWM and would force the panel to operate at 12V instead of its more efficient 30.7V. An MPPT charge controller would be a better choice and result in much better production.
System Design: Inverters
The inverter is the component that converts DC power into the AC that runs most electronics. The two main types of inverter we deal with are Grid-tied and Off-grid. Grid-tied inverters take power directly from the solar array and feed it onto the utility grid. Off-grid inverters take power from a battery bank and power devices.
Grid-tied inverters are selected based on the wattage of the solar array. The panels are organized in combinations of series and parallel to match the voltage and current handling characteristics of the inverters. For example a system consisting of twenty 250W panels for a total of 5000W would use a KACO 5002xi inverter. If all the panels were in series, the voltage would be too high. If all the panels were in parallel, the voltage would be too low. By using string sizing software we can determine that there should be two strings of ten panels to operate efficiently with the inverter.
Another type of inverter is the micro inverter. This is when each panel has its own inverter and inverters are connected to each other in parallel. For these inverters, only certain panels can be used that match the voltage and current requirements of the inverter. Since grid-tied inverters only feed power to the grid and not to certain devices, the size of your load does not matter in system design.
Off-grid inverters draw their power from the battery bank, therefore it doesn’t matter how many or what type of panels are used in the system. Off-grid systems are designed around the total wattage of all the devices they would power. For example, if you have a 1200W microwave and 800W of lights, you wouldn’t want to use an inverter with an output of less than 2000W.
Another important factor to consider when selecting an off-grid inverter is whether you need pure sine or if modified sine will work. Pure sine is identical to what comes out of an electrical outlet, with smooth oscillations from positive to negative. Modified sine is a square wave with instantaneous transitions. If the device that the inverter is powering has moving parts like motors or refrigerator compressors, pure sine is needed. Otherwise, if the inverter only powers lighting or chargers, modified sine will be sufficient.
System Design: Battery Banks
A battery serves as an energy buffer between the power producing panels and the power consuming load. Since grid-tied systems feed directly to the grid and do not provide backup power, no battery is needed. But, off-grid and grid-interactive systems do require batteries. A common question is “Do I really need batteries if I only want to work during the day?” The answer, 99% of the time is yes. In some rare cases special DC pumps can be connected directly to panels, so can lighting. The reason for needing batteries is that without them the current being produced would have to exactly match the current being consumed, which is not possible with small scale conventional solar power systems.
As a general rule of thumb, we recommend that the Ah rating of a battery bank should be at least half of the panel wattage rating. For example, a 200W solar kit should have a 100Ah battery bank. On an average day, a 200W system will take a 100Ah battery from emptyto full. If a smaller battery bank was used, the battery would reach full charge and the panels would be disconnected from the battery by the charge controller before sunset, leaving expensive panels doing nothing.
A bigger battery bank is always better. Bigger battery banks provide more backup time and the lifespan of the batteries is also improved. A system with a large battery bank would likely be less deeply discharged than a system with a small battery bank. If a battery is only discharged 20% each day it will last longer than a battery discharged 50% each day.
To convert battery Amp hours (Ah) to usable Watt hours (Wh) take the nominal voltage of the battery, multiply it by the Ah rating, and multiply that by 50%. For example, a 100Ah 12V battery has a usable capacity of 600Wh (12V x 100Ah x 50% = 600Wh.)
INCENTIVES, REBATES AND CREDITS
Incentives, rebates and tax credits can come from the federal government, the state government, the utility company and even the city. This means that each town may have a different program. To add to the complexity, these programs are constantly changing. Because of this, it doesn’t make sense for Grape Solar, a company with a customer base spanning continents, to become experts on the various programs. If you need information on a specific area we can connect you with one of our expert local installers. In the meantime, these two websites can be very useful:
The IV Curve
A graph of solar panel current vs. voltage is referred to as the IV Curve. The optimum combination of voltage and current for producing the most energy from a solar panel is at the point along the curve where the slope equals -1A/V. The current and voltage at this point are referred to as Impp and Vmpp respectively. The “mmp” stands for Maximum Power Point.
When a solar panel gets warmer the curve shifts to the left, producing less voltage.
When a solar panel gets exposed to more light the curve shifts upward producing more current.