Why Grid-Tied Solar Power Systems Are Recommended Over Other Solar Systems?

(by the 3 Phase Associates – Engineering/Design Team)

Distributed Energy Resources (DERs)

The way we think about traditional power distribution systems is ever-changing with the power flowing one-way (source/generation to load/end-users) to power flowing bi-directional (two-way). Distributed Energy Resources (DERs) are comprised of multiple energy sources or generating devices tied to the same electrical circuit(s) or load(s); like, grid power from utilities, solar energy, wind energy, energy storage or battery storage, gas turbines, back-up generation, etc. DER may also be referred to as renewable energy systems.

With the continued improvements and efficiencies in research, development, design, and manufacturing, DERs continue to become more productive and economical, especially solar electric power systems. Solar energy costs have steadily declined over the last few decades, with major cost reductions within the last few years along with continued local, state, and federal government subsidies and tax credits (solar renewable energy certificates – SRECs) for end users, making it a more desirable and cost-effective energy source for many users in the government and private sectors. These users include operations such as: businesses, banking, hospitals, hotels, recreational facilities, retail, restaurants, public facilities, manufacturing, industrial, commercial, residential, and more. Solar electricity continues to lead the way for cleaner and more sustainable energy.

Solar Energy / Solar Electric Power Systems

Solar electric systems basically are comprised of sunlight (a free energy resource), solar photovoltaic (PV) modules (which convert sunlight into direct-current, DC energy), power inverters (that convert DC power into alternating-current, AC power), energy or battery storage (responsible for delivering DC power when little to no sunlight is available), and other backup power systems (utility grid, generators, wind energy, etc.). Because energy systems deal with electric power, proper electrical (NEC) and building codes must be followed by issuing detailed engineering design drawings (electrical blueprints) by professional electrical engineers to the planning and zoning departments for construction permit requests and inspection approvals. This is to ensure the latest codes, regulations, and zoning ordinances are being followed to maintain safety in protecting the public and human life.

Solar Photovoltaic (PV) Modules or Solar Panels

Solar PV modules or solar panels are made up of tempered glass and several solar cells all housed within a metal frame. The solar cells are comprised of silicon material cut into extremely thin wafers with some of them being doped to create an electron imbalance in the wafers. Light energy or “photons,” produced from the sunlight are absorbed by the solar cells, ultimately exciting the electrons to produce a DC electrical current (measured in amperes). This process is known as the “photovoltaic effect” (wikipedia.org).

Wikipedia – Solar PV Module

The more intense the sunlight, the more photons are absorbed by the solar cells, increasing the electron current flow (power output), whereas less sunlight decreases the power flow.

Think of solar PV panels or a solar array like another DC power source, like batteries. When the sun is shining, the panels are producing DC power like a fully charged battery. When electric vehicle batteries are fully charged, electric vehicles (EVs) can travel long distances based on the amount of energy that exists and dependent on the rate that the batteries are discharged. With a solar array, the sunlight is the energy source that causes the solar cells to produce DC power at a rate related to light energy intensity. This power is affected by multiple environmental factors, such as the time of day, the angle of the sun, cloud coverage, shading, and the cleanliness of the panel itself.

Like batteries, solar modules can be connected in series or parallel or a combination of both. When connected in series (solar string), the DC source voltages are added while the current/amps remain the same. When connected in parallel (multiple solar strings) of same voltage, the voltage remains the same while the currents are added or summed. The combination of solar PV panels connected (both in series and in parallel) are known as a solar array. The total power in watts can be calculated for the complete solar array by multiplying the voltage by the current as shown in the figure below.

Solar PV Array (Series & Parallel Connections)

3 Main Types of Solar Electric System Configurations

Solar PV modules can be configured into three main types that offer different requirements to accommodate the end user. Additional configurations can be derived from various combinations, but these three shown below remain as the most common. All these configurations, except one, require the existing or traditional utility meter (one-way meter) to be upgraded to a new net meter (two-way meter), allowing the power to flow in both directions.

  1. Grid-Tied Solar System
  2. Grid-Tied Solar System with Backup
  3. Off-Grid Solar System

The grid-tied solar system (aka “grid-connected,” “grid-interconnected,” “grid-inter-tied,” “utility-connected”) type is the simplest, most reliable, least expensive, least maintenance-intensive, and most common configuration in use today for most commercial and residential users. This system type will save you money every month on your electric utility bill. With this configuration, you are still connected and dependent on the utility grid but less dependent during the day while the sun is shining, and your solar panels are producing surplus energy. This surplus can be sold back to the utility at a somewhat reduced rate controlled by the local utility. Some state legislatures do not allow or participate in end users selling back surplus energy. This system requires less upfront costs for the initial investment and allows the break-even or payback duration to be the shortest when adding up annual cost savings of lower monthly utility bills. The grid-tied system almost always pays for itself before PV equipment warranties expire, making it a win-win! (Try Out Our FREE Online Calculator Tool for Grid-Tied Solar Systems)

This is the reason that grid-tied solar systems, for the most part, are favored over the other systems. Another benefit is that you have the utility grid as backup if there is required maintenance to be performed on the PV system during a scheduled solar system outage. The major drawback to the grid-tied system is that when the utility power is interrupted or disconnected from your configuration, the grid-tied inverter shuts down resulting in a total power loss. Power is not restored until the inverter senses the utility grid power has been re-energized. If your loads are critical and cannot sustain a power outage, such as with freezers or home-based medical equipment, then it is recommended to choose the next configuration system, the grid-tied solar system with backup power.

Grid-Tied Solar System

The grid-tied solar system with backup power (aka “hybrid system,” “grid-interactive”) type is like the grid-tied system, but with the added reliability of having another backup power source other than being solely dependent on the electric power utility. This configuration protects against power interruptions and outages compared to the grid-tied system which has no backup power, but has the additional cost of batteries, generator(s), fuel, etc. Also, there is the added cost of maintaining the battery storage or backup generator source to ensure no power interruptions are experienced when the utility power is interrupted. This system type will also save you money every month on your electric utility bill, but the main disadvantage is that the added up front cost of installing a battery bank with charge controller and/or backup generator system will take much longer to pay back or to reach break-even when compared to the grid-tied system without backup power. The PV equipment warranties will more than likely expire prior to reaching the investment’s break-even period.

Grid-Tied Solar System with Battery Backup

The off-grid solar system (aka “stand-alone,” “self-sufficient,” “electric independence”) type is the most complex and costly, relying heavily on maintenance, and is less common compared to the other systems. This system is either the best option or the only option for rural areas where buildings or dwellings are too far away from the utility power lines. This configuration is totally independent of the grid, meaning there is no utility backup power connection. The only backup connections are the battery bank and/or a backup generator. This configuration requires a much larger battery system (proper amp-hour ratings with charge controllers) to supply all loads when the sun is not shining and is much more costly since it requires a large enough battery bank to handle the estimated days of autonomy for mostly cloudy days or during any required downtime for PV maintenance. More PV modules will also be required to properly recharge the larger battery bank by the appropriate time or before the sun goes down. Likewise, a larger kilowatt (kW) inverter will be essential to handle the larger system. Another disadvantage is that most battery systems are only warrantied for 7 to 15 years with their life expectancy (Depth of Discharge – DOD) not lasting much longer. Batteries also become much less efficient as they age over time. Most batteries are also susceptible to environmental factors like temperature, and must be stored in a well-ventilated, fire-rated area. This configuration will more than likely require replacing the battery bank at least once if not twice during the life of the PV modules, and will also require careful storage and monitoring of the batteries themselves. The main benefits of this system are energy independence and no more electric utility bills, but these benefits come with a hefty price tag.

Off-Grid Solar System with Battery & Generator Backup

Contact us if you need consultation or design help with implementing your solar electric system. 3 Phase Associates has professional electrical engineers (licensed in multiple states) ready to support your renewable energy needs.

(Try Out Our FREE Online Electrical Engineering Calculator Tools, especially the Grid-Tied Solar System Calculator3 Phase Associates)