For most DC-coupled off-grid systems it really comes down to four main components – solar panels, charger controller, inverter and the battery bank. There is a lot more that can go into a solar system setup, but those are the four main pieces that will be discussed in this article.
Solar Panels & Mounting
Starting with the most obvious part of an off-grid solar system are the solar panels. Currently, the most cost-effective solar panels are those made up of 60, 72, 120, 132, or 144 cells. Solar cells are the small squares that make up the entirety of the panel itself. These days, monocrystalline panels are the industry standard in most systems.
A typical 60 cell monocrystalline solar panel will be around 68” x 40” in size and output in the range of 300-375 watts, whereas a 72 or 144 cell panel will measure 80” x 40” usually and output around 375 watts and above. Determining the size of the solar array is one of the most significant calculations in Off-Grid system design.
After this we determine the appropriate array configuration. The solar panels are connected in series strings (limited by the max input voltage), then the various strings of solar panels can be connected in parallel to create a substantial array (limited by power or current). This method simplifies the solar array output down to as few conductors as possible.
So why use monocrystalline instead of polycrystalline panels? It really comes down to availability and cost. Most of the time monocrystalline panels are used in off-grid solar systems as the industry has shifted to manufacturing cost effective monocrystalline modules. In the early days, affordability gave the advantage to polycrystalline panels because they were cheaper to manufacture. Now, monocrystalline has become mainstream, much more efficient, and affordable, so there isn’t a real advantage to using polycrystalline any longer.
Solar module mounting is the next consideration.
There are three common methods for mounting solar modules and the selection is usually dependent on application or available mounting space:
- Roof Mount - Mounting the solar array on a home or structure
Roof mounts use parallel rails secured to the roof system with feet secured to roof trusses or cross members with the solar panels set on top of these rails and secured with a clamp type system. Roof mounting solar panels have the advantage of using an existing flat roof area. Roof mounts can have the disadvantage of not optimizing the solar panel angle in relation to the southern horizon thus reducing the potential energy production or the array.
- Pole Mount - Mounting the solar array on pole secured in the ground by concrete
The top of pole mounts uses a gimbal that is attached to the top of a vertical steel pipe. The solar panels are then secured to several rails that are attached to the gimbal. The top of pole mounts can attach a single panel to as many as 12 solar panels on a single pole. These mounts can optimally tilt the solar panels from completely horizontal to 45 deg. Top pole mounts are more easily cleaned without the need to climb a roof and they also shed snow very effectively.
- Ground Mount - Mounting the solar array on concrete piers closer to the ground for stability
Linear ground mounts involve a lattice of vertical and horizontal steel poles with parallel rails – usually aluminum. The solar panels are then secured to the aluminum parallel rails. The panels are arranged row and column style and the entire array can be angled towards the southern horizon optimizing for maximum energy production. Linear ground mounts can be cleaned easily like top of pole mounts and can also be cleared of snow more easily than roof mounted arrays. Linear ground mounts can be used for large solar arrays with the only limitation being the available ground space.
Charge Controller
The charge controller is the device that manages the flow of energy from the solar panels to the battery. Charge controllers make sure batteries are charged properly and are not overcharged, which is important for the longevity of the battery bank. There are two main types of charge controllers, MPPT (Maximum Power Point Tracking) and PWM (Pulse Width Modulation).
MPPT charge controllers are different in the fact that the input voltage from the solar panels needs to be 30% over the voltage of the battery voltage (up to the limit of the charge controller), so it doesn’t matter as much what voltage solar panel is used with the system.
MPPT charge controllers are more efficient due to their ability to track the maximum point of power coming from the solar panels and deliver it to the batteries. It takes a higher voltage / lower current input and converts it to lower voltage / higher current output for the same amount of power. Given this fact, MPPTs very accurately control the amount of power that is sent to the batteries which is important when batteries get full and try to satisfy system loads. The main selling point of using an MPPT controller is their ability to capture the most power from the solar array at any given moment contrary to limited input of a PWM controller. It is possible for a PWM to deliver as much power as an MPPT, but it will never deliver more power than an MPPT. For those reasons, MPPTs are usually the norm when choosing a charge controller for a solar system design.
PWM charge controllers use pulse modulation to turn on and off the rate at which the energy from the solar panels is being sent to the batteries. When using PWM charge controllers it is essential for the nominal voltage of the panels to match the nominal voltage of the batteries. For instance, if the system is using 12 volts panels the battery bank needs to be 12 volts. There is not a lot of control on managing the power coming from the panels using a PWM, it’s dumping the power into the batteries. PWMs offer limited input compared to an MPPT controller.
Inverter
The next component in an off-grid solar system design would be an inverter. In nearly all off-grid solar systems, the inverter is a battery-based inverter. The inverter’s purpose is to take DC power that is stored in the battery bank and convert it to usable AC power and send it to your loads so it can be used in the same manner as plugging into an AC outlet in a home. Inverters come in varied sizes which can accommodate smaller loads or larger loads depending on the off-grid loads required. Another consideration is making sure the inverter can handle all the loads running simultaneously in the system.
When all the system loads that are present in the off-grid system are added up, it will determine that maximum amount the inverter needs to be able to handle. Watch the following video below to learn how to calculate system loads.
Learning how to calculate the system loads for a specific system will allow our team to design a system that can handle all the loads required.
Another important fact is that the inverter needs to match “voltage-wise” the system in which it is being used. For instance, a 12-volt inverter cannot be used with a 24-volt battery bank – it must be used with a 12-volt battery bank. Unlike charge controllers, the voltage on an inverter cannot be changed as it is fixed and must be matched with the battery voltage of the system.
Given that information, it’s important to choose an inverter wisely when designing a system especially if expanding the system is in the plans. Choosing an inverter is a crucial decision to make in the beginning of your planning process.
In most off-grid systems we choose to use inverter chargers. Notice we said inverter “charger”. So, we already know what a regular inverter does. What does an inverter charger do? The inverter charger acts the same as a regular inverter, but doubles as a charger. That means that the inverter not only has an output, but it also has an input.
This is important because this allows the system to use an external power source such as a gas generator to power the system loads and stops drawing power from the battery bank. Once the system loads are satisfied the excess power that is being input into the system from the external power source is then being used to charge the battery bank. Going with an inverter charger allows redundancy in the system which is needed if there a several cloudy days and the solar array cannot provide enough power to charge the battery bank.
Hybrid Inverter System
Most hybrid inverters are all-in-one units, meaning there are inputs for solar, grid, loads, generator, and battery all built into one inverter. A hybrid inverter system integrates the best of both MPPT charge controller and Inverter/charger worlds for a very custom and flexible solution. These types of systems are commonly referred to as hybrid inverters or ESS - energy storage system. Hybrid inverters are often used in applications where a simple easy to install and all-inclusive product is desirable. They can manage PV production and battery charging like a charge controller but will also provide power output from batteries and/or PV just like an off-grid inverter. Given the flexible nature of hybrid systems, this is often the best choice for flexible and dynamic solutions. Being that these solutions are also quite modern, they work very cohesively with lithium battery solutions. Most will also allow for charging batteries from a generator (or Grid where applicable).
Batteries
The last main component in the solar system is the battery bank, which is one of the most important considerations and the most expensive. In the solar power industry, there are two common battery chemistries – lead acid and lithium.
Lithium Batteries
Lithium Iron Phosphate (LiFePO4) is the chemistry makeup of most lithium batteries used in the solar power industry. Lithium batteries are significantly different than flooded lead-acid and AGM batteries in several different ways, not only in size/weight, but also in how they can be charged and discharged. Lithium Iron Phosphate is an extremely safe chemistry, it does not off-gas and can be stored without the need for ventilation. Lithium batteries are completely maintenance-free and do not need to be fully charged, unlike lead-acid batteries. LiFePO4 chemistry is also designed specifically for a significant amount of charging cycles. These characteristics make lithium batteries extremely advantageous for off-grid solar applications. Another advantage is that lithium batteries have a built-in BMS (battery management system). The BMS is constantly monitoring the operating state of the battery. This means if the battery is being over-discharged or if the battery is too hot or cold, the BMS will force the battery to shut down until those parameter violations have been resolved. Think of BMS as a level of protection for the batteries, which makes it difficult to damage them.
Another advantage of lithium is that you can stack or expand an existing battery bank without affecting the lifespan of the existing bank. Adding batteries to an existing lead acid battery bank will ultimately result in premature failure of the entire battery bank. Lithium batteries can also be purchased in 12v, 24v and 48v variations so you can parallel them easily with conventional system voltages. This is important because if a battery is forced into shut down mode by the BMS, the entire bank does not necessarily have to shut down.
In all facets, lithium batteries are significantly superior to lead acid batteries. Depth of discharge, the number of charge cycles, safe chemistry and a built-in BMS deal a knockout blow to lead acid batteries in the long run. Not to mention lithium batteries also charge faster and deliver a substantial amount of power continuously without damaging the battery. Also, all reputable manufacturers are offering warranties on lithium batteries for around 10 years which is substantially more than warranties given on lead acid batteries. One more advantage is that the space needed and weight of a lithium battery bank is much less than that of a lead acid battery bank.
Lead Acid Batteries
The two main types of lead acid batteries used in solar are flooded lead acid batteries and sealed AGM batteries.
Flooded Lead Acid Batteries
A flooded battery is a standard wet cell lead acid battery which is usually the most cost-effective battery upfront. The batteries themselves are relatively inexpensive, but there comes necessary maintenance that is required to prolong the life of the battery. Maintenance such as checking water levels in the battery, and checking the specific gravity is required to prevent the battery from being ultimately destroyed. Also, regular equalization charges should be done as well to reduce stratification of the electrolyte and to help loosen build-up that may have become solidified and adhered to the plates within the battery. Another consideration when thinking about using flooded lead-acid batteries is off-gassing. Under certain conditions when lead acid batteries are being charged, hydrogen gas is produced as a byproduct which requires ventilation for the battery bank. Lack of ventilation can pose a dangerous situation when dealing with hydrogen gas fumes, so it must be taken seriously. Because these types of batteries are cost-effective many people tend to use them for their solar applications. Lack of maintenance or excessive usage is often the cause of most premature lead acid battery failure. If a maintenance-free battery is desired and a cost-effective lead-acid battery option is needed, AGM batteries may be the perfect fit.
AGM Lead Acid Batteries
AGM means Absorbed Glass Mat – which refers to the fiberglass mats between the plates where the electrolyte is absorbed. These batteries are completely sealed and there is little to no maintenance involved. Life expectancy, charge cycles, and size/weight in an AGM battery are consistent with those of a flooded lead-acid battery. Due to the lack of maintenance of AGM batteries, there is an increase in price compared to flooded batteries which offset the concern of destroying the batteries due to lack of maintenance in their flooded counterpart. One of the drawbacks of an AGM battery is that if it is abused there isn't much that can be done to revive the battery once the damage has been done. AGM and flooded batteries offer a cheaper price point up front than lithium batteries, but their lifespans are significantly shorter.
There are many critical decisions that need to be made when considering the component makeup of an off-grid solar system design. If you need help in making these decisions for your off-grid solar system, contact one of our solar sales engineers by calling (800) 383-0195 or email us at sales@solar-electric.com.
For a more detailed conversation about off-grid solar systems, check out the interview below. Be sure to subscribe to the NAZ Solar Electric YouTube Channel to be notified of more videos when we post them.
