AC and DC power offer benefits to different solar power applications. Generally, domestic and small commercial installations are suited to DC systems, whereas large systems using high voltage and power volumes during daylight hours lean towards AC systems.
🔑 Key Takeaways:
1️⃣ AC is the most economical method to transmit electricity over distances up to 400 miles. Beyond that, high-voltage, direct-current transmission (HVDCT) systems are used.
2️⃣ DC solar power systems represent the most cost-effective and efficient solution for small businesses and homes requiring power twenty-four hours a day.
3️⃣ AC systems are generally easier to install, but each solar panel requires a micro-inverter, increasing the acquisition costs.
4️⃣ DC solar panels are considered more durable as they do not require micro-inverters that are exposed to the elements.
🎓 Definition Of AC And DC Power
The de-factor norm of electric currents inside households is Alternating Current (AC). You may not know, however, that an AC lightbulb will light up just as well (in fact brighter) if run on DC.
That begs the question, why was AC power chosen as the base, not DC power?
AC is the standard form of electrical current used for transmitting electricity and power over long distances. The reasons are historical and resulted from competing technologies designed by Thomas Edison and Nikola Tesla.
Edison’s transmission network was designed around DC current. The resultant loss in voltage reduced the effectiveness, and so prevented long-range transmission.
Nikola Tesla invented the AC transformer, which enabled current transmission over longer distances.
AC transformers increase the voltage on high-tension cables (up to one million volts) at the start of the journey and reduce it to 110 volts (through a series of local transformers) for the end user; this is a relatively low-cost method of transmitting power.
More recent technology has now proven that, for several reasons, DC is more efficient when transmitted over exceptionally long distances.
For very long-distance transmission, high-voltage direct-current transmission (HVDCT) systems are already being employed.
The standard AC found in American homes has the following metrics.
- Amplitude – 120v (Volts)
- Wave Type – Pure Sine Wave (smooth curve)
- Frequency – 60hz (Hertz)
What Does An Inverter Do?
Most renewable power generation systems produce direct current (DC), which has to be converted (inverted) to alternating current.
The inverter uses insulated Gate Bipolar Resistors (IGBT) transistors to achieve this.
The simplest inverters work as follows:
1️⃣ The Direct Current flows in one direction into the inverter.
2️⃣ A series of four IGBT transistors are placed in an “H-Bridge” arrangement.
3️⃣ In the following diagram, the S1, S2, S3, and S4 IGBT transistors convert the current to AC in the following manner.
4️⃣ The control of the direction of the current alternates through the electrical consumer “M.”
5️⃣ Two (S1+S4) switch ON, and two (S2+S3) switch OFF simultaneously, causing the electrons to flow one way through M.
6️⃣ Two (S1+S4) switch OFF, and two (S2+S3) switch ON simultaneously, causing the electrons to flow the other way through M.
The measurement “Hertz” indicates how many times the direction of the circuit is switched each second. For example, sixty hertz means that the direction of the current reverses 60 times every second.
⚖️ Advantages And Disadvantages Of AC And DC Power In Home Solar Systems
All Solar power systems found in residential and small commercial applications use a combination of AC and DC power elements. Despite this, there are two categories of solar power systems – AC and DC.
DC Solar Systems
The solar photovoltaic cells in the panels generate Direct Current (DC). In its raw form, the current from the panels is uncontrolled and constantly varies in value, dependent on the sun’s intensity. The solar panels’ current is passed through a charge controller to make this usable.
The charge controller used in most systems is MPPT (maximum power point) enabled. This maximizes the yield from a solar panel and ensures that the system is not overloaded.
The current is split between supplying the household needs and recharging the solar batteries.
The current required by the household circuits is sent through the inverter, which is converted to AC. The modified current is then distributed through the distribution board.
AC Solar Systems
Alternating current solar systems require microinverters to be installed with each solar panel, which are used to convert the current from DC to AC.
The Benefits Of AC Solar Systems
The advantages of AC solar systems are listed below.
- Simpler, smaller gauge wiring can be used.
- There is a lower chance of the cabling overheating and causing a fire.
- Specialized electrical knowledge is not required to connect the panels.
- Because each panel has an inverter, problem diagnosis is straightforward.
- There is no reliance on single (or multiple) large inverters.
- Larger strings of solar panels can be added to each inverter.
The Disadvantages Of AC Solar Systems
The disadvantages of alternating current solar systems are listed below.
- The cost of the system (with a micro inverter for each panel) is higher.
- This is further impacted by lifecycle costs which may be higher due to the need to replace the microinverters more often.
- The inverters are mounted on the solar panels and therefore exposed to the elements, reducing life expectancy.
- There is no protection between the solar panel and the inverter, so lightning strikes represent a real danger.
- A system with a large number of solar panels may have interaction problems being connected to a single BUS.
- Micro-inverters do not have the same efficiencies. For example, DC systems invert the current once compared to multiple times with each microinverter on an AC system; as a result, DC-coupled systems are up to 3% more efficient than AC systems.
How To Decide Between And AC Or DC Solar System
Until the AC inverters become more efficient and the costs of micro inverters reduce, we do not believe this technology is sufficiently mature or cost-effective to recommend for use in a home or small business.
Direct current systems are installed at a lower cost:
Pure DC panels (with no micro inverter) cost less, and only one inverter (or multiples thereof) is required.
If batteries are to be used, the current will have to be converted back to DC for storage, which adds another layer of cost and complexity.
Even though AC batteries are available, these are not truly AC as they have a built-in inverter converting the power from AC to DC and back again as required.
Some off-grid homes and commercial enterprises use AC-coupled systems for the following reasons:
There are advantages to using string solar inverters with higher DC voltages (up to 600V or higher).
Large DC systems that are coupled together require many MPPT charge controllers, which increases cost, while no charge controllers are needed for AC systems.
Instead, larger 3-phase commercial systems that need larger solar arrays can use an AC coupling with individual solar inverters.
🏡 AC vs DC Power In Home Solar System Design And Installation
Typical solar configurations comprise the following components.
DC Solar System
DC systems consist of the following parts:
- Solar photovoltaic cells within the solar panels collect the sun’s energy and convert this to DC.
- The solar panels are grouped into strings of four to six (power dependent) and are connected to each other in series.
- Each string is connected to the other strings in parallel.
- The current passes through a fuse to reduce the chance of a fire.
- The current is passed to a charge controller, which modifies it to ensure it does not overload the system.
- The current is passed to the inverter as required by the electrical loads in the home.
- Excess current (not required by the home) is sent to the battery and stored for later use.
- The DC current sent to the inverter is converted to AC and sent to the home distribution box.
AC Solar System
AC systems consist of the following parts:
- The AC solar panels have individual micro-inverters attached to each unit.
- These are connected through a busbar.
- The inverters control the current level and ensure that the home’s system is not overloaded.
- The current is sent directly to the home’s distribution box.
- If AC batteries (Tesla Powerwall2 is an example) are used, these also have a built-in inverter to convert the current to DC for storage and AC for subsequent consumption.
AC solar systems are well suited to higher power usage during daylight hours, for example, air-conditioning, swimming pool pumps, and hot water systems.
Is Household Electricity AC Or DC?
Household electricity is a combination of AC and DC.
The electricity arrives at the house from the utility as AC. However, some electrical consumers, such as cell phone chargers, computers, and some LED lights, convert the current to DC before they can be used.
With solar systems the Direct Current produced by the panels is converted to AC before being distributed to the house.
Which Is Safer, AC Or DC?
At small amperages, DC is safer than AC. In the historical dispute between Thomas Edison and Nikola Tesla, Edison tried to emphasize the dangers of AC by publicly electrocuting animals using AC current while simultaneously showing that DC was not as dangerous.
He even filmed the public electrocution of Topsy the elephant at Coney Island, New York City, on January 4, 1903. Truly horrific behavior!
While DC current is low, large levels are just as dangerous as AC.
🔑 Key Takeaways
DC solar systems are the most common configuration used in domestic and small commercial installations.
There are upfront costs and benefits to installing large solar arrays using AC systems, and the electrical skills required are not as complex.
While there are benefits with both systems until AC systems mature, DC will continue to be the typical protocol for homes and small businesses.
Next, I’d recommend exploring these other beginner topics to help you whet your solar appetite a little more!