The neverending Solar Glossary
I’m starting a glossary of solar electrical terms that I will continuously add to as long as new terms that seem worth defining pop up. It may seem that the terms used are just to confuse you, but really, any technical endeavor acquires jargon so that professionals can understand exactly what is meant by relatively short phrases. Solar electrical terms take that a step further by reusing old electrical terms in ways that have no relationship to their original use. So yeah, maybe they are just trying to confuse us. This glossary will grow as we continue through the project. If you don’t understand a term, check back here–if I used it and it’s not immediately obvious what it means, I probably defined it in the glossary. While I’m add it I’m going to shorten the term “Solar Power Systems” to just solar systems or PV systems with the assumption you know I’m not talking about stars and planets or Palos Verdes.
AC Coupling. In electronics and electrical terms that usually means connecting two circuits together through a capacitor so only the AC can pass, blocking the DC currents. In solar electrical terms, AC coupling means solar panels are connected to the electrical system through separate inverters, either microinverters or string inverters. In order to charge a battery the AC power needs to be converted back to DC of the correct voltage. In addition, most AC coupled systems require an AC voltage to synchronize. The voltage is generally the electrical grid but it could be a microgrid or the output of an inverter.
DC Coupling means connecting the solar panels to a solar controller–typically an MPPT controller–that charges a battery or directly supplies power to an inverter. In the case of all-in-one inverters the MPPT controllers are internal and can directly power the inverter or charge the battery. For modern off-grid or hybrid all-in-one inverters that usually means a high voltage series string of panels with the string voltage in the range of 140 to 450VDC.
While we’re talking about AC and DC lets define those terms though I’m sure you know them.
AC Alternating current is current that reverses flow direction, passing through zero volts as it changes direction. On the electrical grid, AC is produced by large electromagnetic generators which produce a smoothly flowing sine wave, typically at about 60 cycles per second in the USA. Solar inverters need to produce an AC output that is synchronized to the grid frequency, voltage and phase in order to be “grid tied”.
DC (Direct Current) is electrical current that does not reverse the direction of flow and generally remains at a constant voltage.
Battery Management System (BMS): We won’t bother writing about “the old days” except in rare occasions where it helps explain concepts. But in the old days charging a battery from a solar panel didn’t require anything but a solar panel and a battery. The panel’s output was something close to 12 volts, and was weak–like a few watts. The batteries were lead-acid and supported small loads. If the average load and the panel’s capacity weren’t well matched the battery would overcharge and could slowly boil dry, but it usually worked OK if the loads were predictable. Now the panels are huge and powerful, most output about 40 volts, and the batteries are lithium-based (generally LIFePO4 chemistry). These require some protective circuitry to ensure they stay healthy. Generally a good BMS will stop charging if the battery is full, will stop discharging is the battery is at a set limit for either voltage or state of charge (SOC), will limit the charging and discharging current to a level appropriate to the battery, will balance the voltage of the cells comprising the battery, and keep an eye on the temperature, cutting off charging if the temperature is too low, and disconnecting the battery completely if the temperature is too high. Some BMS’s have additional features that handle communications and control to make the work well with the rest of the system, and some do less.
Bypass Diode: A diode is a semiconductor device that only permits current to flow in one direction. Bypass diodes permit shaded or under-performing PV cells or panels to be bypassed to prevent them from limiting power production of the other panels in the same series string. Virtually all modern PV panels have bypass diodes. It might help to think of them as a check valve placed between the inlet and outlet of a pump which is part of a series string of pumps. If a pump is running the check valve is held closed because the outlet of the pump is at a higher pressure than the inlet. If the pump is shut off it would block the flow of the other pumps in the series, but the bypass check valve now opens, because the downstream pressure is now lower than the inlet pressure, and the system continues to flow though it does so at a lower pressure since one of the pumps contributing to pressure isn’t running.
Clipping: If the PV output wattage exceeds the capacity of the inverter, or the system can’t use any more power (for example, the battery bank is fully charged or is at it’s maximum chage rate and load is met) the power is lost. The term has nothing to do with Hockey, and little to do with the usual use in electronics.
Grid-tied Solar System: In legacy terms this means systems that use the electrical grid as a storage system, feeding excess power generated from PV panels to the electrical grid and then retrieving power from the grid when the solar generation doesn’t meet load requirements. This worked well when utilities played nice with solar generation, some systems installed in the early days of solar power have net-metering agreements where they can receive energy back from the grid on a on-to-one basis, but unscheduled, un-managed solar power is almost useless to utilities, and they don’t want to pay for it. So feed-in Tariffs (the rates utilities will pay or give credit for power you feed into the grid) have steadily declined to the point of being almost unworkable. The amount utilities will pay for power you put into the grid is a small fraction of what they charge for energy you take from the grid. From the utility (and the regulatory bodies that set rates) perspective this is perfectly reasonable: You put power into the grid regardless of whether it’s needed or not, and you take power whenever you want. But the low rates make the grid expensive as a storage device and makes battery prices more attractive.
Ground Mount System: A solar installation using mounts at ground level instead of on roofs. The advantages are maintenance access, no roof damage, and potentially more power if the roof is shaded or improperly oriented. But of course you need unshaded land to locate the mounts.
Grounding and Bonding: No, no one is getting tied up. Grounding and bonding in solar electric systems are crucial for safety, preventing electric shock, and ensuring the system operates correctly. Grounding connects the system to the earth, while bonding ensures all metal parts are at the same potential and provides a path for fault currents.
Grounding: In solar electric systems involves connecting parts of the system to the earth. This is typically achieved using a grounding electrode, such as a copper rod driven into the ground. There are two primary types of grounding:
- System Grounding: This involves connecting a current-carrying conductor, such as the neutral wire, to the earth. In photovoltaic (PV) systems, the DC negative is sometimes bonded to ground internally within the equipment. This type of grounding helps stabilize the voltage levels in the system and provides a reference point for the electrical system. Most electrical codes require that AC grounding only be done in one place–typically at the connection to the utility. This prevents “Ground loops” where ground wires provide an unintended path for current.
- Equipment Grounding: This involves connecting all non-current-carrying metal parts, such as frames, racks, and enclosures, to the earth. This ensures that any metal parts that might become energized due to a fault are safely grounded, reducing the risk of electric shock and fire
Bonding: refers to the process of connecting all the metal parts of the electrical system together so they have the same electrical potential. This is achieved by connecting them to an equipment grounding conductor (EGC), which is then connected to the grounding electrode. The key purposes of bonding include:
- Creating a Path for Fault Currents: Bonding ensures that all metal parts are connected, providing a low-resistance path for fault currents to flow back to the source, which helps in tripping circuit breakers and preventing electric shock
- Preventing Voltage Differences: By bonding all metal parts, any potential voltage differences between them are eliminated, reducing the risk of electric shock.
- Compliance with Codes: Proper bonding and grounding are required by electrical codes, such as the National Electrical Code (NEC), to ensure safety and compliance during inspections
Inverters: Inverters convert The DC output of solar panels to AC for use with common home appliances. There are three primary types of inverters–Lower frequency transformer inverters, high frequency transformer inverters, and transformerless.
- Low Frequency transformer inverters use electronics to convert DC to low frequency AC, and then a large transformer to adjust the voltage to the typical 120 VAC or 240 VAC split-phase. All the transformations of the power happen at the line frequency, which in the USA is generally 60 cycles per second (CPS) or 60 hertz. The low frequency requires a heavy core to deliver substantial power.
- High frequency transformer inverters take advantage of the fact that higher frequency transformers don’t need as massive a core in the transformer, so they can be made lighter and cheaper. Unfortunately not all electrical appliances can operate with high frequency AC.
- Transformer-less inverters use electronics and microcontrollers to transform DC to AC at line voltage and frequency. The output is synchronized to the line frequency (typically 60 hertz) or in the case of off-grid use, the frequency is selected when the inverter is configured. Since Transformerless Inverters don’t have a transformer they are lighter, more efficient, and generally less expensive. Transformer inverters inherently isolate the DC side of the inverter from the AC side since there is no direct connection across the transformer. Modern transformerless inverters compensate for this disadvantage with various fault detection circuits that prevent energizing the DC side with line AC, but Transformerless Inverters require more attention to proper grounding and bonding, though grounding and bonding best practices are required for safe operation in either case. It’s also more important to provide lightning arrestors with transformerless systems since lightning striking on the panels and frames can be fed through to the AC side. With more systems being built with higher voltage DC coupled series strings of PV panels the transformerless inverters are becoming more popular since they suit that use case more readily. It’s common for several MPPT controllers to be part of transformerless inverters since the circuitry can be integrated into the inverter more readily than in a transformer-based inverter.
KWh-Kilowatt hour: a measure of electrical storage capacity or electrical generation over time. A 100 watt bulb burning for ten hours uses 1KWh.
Mini split heat pumps: An inverter-based air conditioner or heat pump with an outdoor unit that houses the compressor and condenser with an indoor unit with an Evaporator Coil. During cooling mode, it absorbs heat from the indoor air, and during heating mode, it releases heat into the space. Mini-splits typically use either washable or replaceable air filters. A mini-split heat pump for a 400 square foot space would likely use around 9,000 BTUs of heating/cooling capacity with power consumption would in the range of 600-900 watts per hour when running.
MPPT Solar Controllers: Maximum Power Point Tracking (MPPT) controllers are sophisticated devices that significantly improve the efficiency and flexibility of solar power systems, particularly in challenging conditions or with more complex system configurations. Modern MPPT controllers use microprocessors to continuously monitor and adjust power output for optimal charging. They act as DC-to-DC converters, taking the higher voltage output from solar panels or series strings of solar panels and convert it to a lower, more appropriate voltage to charge batteries.
- Optimization: They continuously track the maximum power point of solar panels, adjusting for factors like time of day, cloud cover, and panel temperature to extract the most power possible.
- Efficiency: MPPT controllers are more efficient than simpler PWM (Pulse Width Modulation) controllers, typically harvesting 5-30% more energy depending on conditions. PWM controllers are generally extinct.
- Voltage flexibility: They allow the use of higher voltage solar panels or series strings (with perhaps 450VDC) with lower voltage battery systems (with perhaps 12, 24 or 48VDC), providing more flexibility in system design. High voltage series strings enable solar systems to deliver high power without heavy conductors since conductor size requirement is a function of current. Ten 40 volt panels in series might deliver 10 amps of current at 400 volts for 4000 watts, while the same ten panels in parallel would deliver 100 amps of current at 40 volts for the same 4000 watts. The series string would perform fine with fairly long runs of 10 AWG solar wire, while the parallel string would melt the same wire. The connection between the MPPT controller, which would convert that 400 Volts to charge the 48VDC battery would need to handle the roughly 100 amps the controller would output, which means the controller should be close to the battery and use appropriate cable.
- Performance in varying conditions: MPPT controllers perform better in cold weather and cloudy conditions when panel voltage may be significantly higher than battery voltage.
- Power conversion: They optimize both voltage and current to maximize power transfer from panels to batteries, unlike PWM controllers which only adjust current.
- Oversizing capability: MPPT controllers can handle oversized solar arrays by limiting current intake during peak production periods.
NEC (National Electric Code): A comprehensive set of electrical construction codes used as the basis for local code enforcement in all 50 states of the USA. The codes are updated every three years and not every update is applied in every jurisdiction. Adoption is not universal or consistent, local requirements take precedence. The most common code system PV and ESS systems are NFPA 70, or the National Electrical Code (NEC). PV systems have requirements that span multiple Code articles, so technicians need to navigate throughout the NEC to install code-compliant PV and ESS systems.
NFPA (National Fire Prevention Association: The NEC one of the NFPA manuals The NEC is the manual called NFPA 70, a delightful and expensive little tome that hits your desk with a thump. It’s also available for free online for a three-month trial period. PV systems have requirements that span multiple code sections within the manual, so you will see references like 2020:690.12 which means the 2020 edition of the NEC, section 690 part 12.
Split Phase: Residential power in the USA is generally split phase 120/240VAC from a center-tapped transformer connected to a single phase of the three-phase distributed power. Also called a single-phase three-wire system, this is generally the standard for residential and light commercial applications in North America. The center tap is connected to neutral and ground. Any load connected to one end of the transformer and the center tap s 120VAC. Loads connected to both ends of the center tap get 240VAC. While sometimes referred to as “two-phase,” this is not technically accurate. It’s a single-phase system with a center tap, hence the term “split phase”.
Watts: a unit of electical energy. The voltage of a system times the current flow equals the wattage. For example, 48 volts at 100amps = 4800watts
Various PV specifications:
kWh or KWh – Kilowatt-hour. KWh is the amount of power a system produces, while KW-Kilowatt is the amount of energy.
Imp – Maximum Power Point Current, the specific current value at which the solar panel produces its maximum power output when paired with the corresponding voltage at the maximum power point (Vmp).