The systems shown in figures below are the simplest representations of solar+storage installations. In reality, solar+storage systems may be composed of any number and variety of solar arrays, energy storage technologies, inverters, management systems, and additional backup generators.
But, regardless of complexity, each system will likely follow one of the two simplified designs already discussed and be composed primarily of the same basic core components: a critical load subpanel, solar panels, batteries, and at least one inverter.
Critical Load Subpanel
The critical load subpanel is arguably the most important component to consider when designing a solar+storage system. Which loads to include, and which not to include, will determine the optimal sizing of the rest of the system.
It is usually not practical to design a solar+storage system with the intent to support all of a facility’s electrical loads when the grid is down. At this time, installing a battery system large enough to serve all loads is not typically a cost effective solution. This is why it is necessary to install a critical load subpanel that is separate from the main distribution panel. The subpanel is very similar to a main circuit breaker panel, except that it only includes loads deemed critical to a facility during a central power outage. In the event of grid failure, only the loads connected through the critical load subpanel will be supported until power is restored.
Specific critical loads will vary depending on the needs of each facility, but generally include loads such as heating and cooling, emergency lighting, a few outlets, elevators, refrigeration, and water pressure pumps. Table 1 provides a sample of some common critical loads and their corresponding power needs.
The total expected critical load, the sum of all individual critical loads, can be used as a baseline to specify inverter and battery capacity requirements. In the case where a facility has one or more high-power critical load requirements that may not be well-suited for energy storage applications, an emergency backup generator can be employed to serve these individual loads while the solar+storage system supplies power to the remaining critical loads.
Solar panels are obviously an integral part of any solar+storage system. Each solar panel, also known as a solar module, is actually composed of a number of small units called photovoltaic (PV) cells. The cells themselves are composed of a semiconductor material, which is most commonly made of silicon. When a particle of light, or photon, hits a PV cell, an electron is knocked free. These free electrons generate a current. It is this current along with the cell’s voltage that defines the power that a PV cell can produce.
The combined power of all PV cells in a solar panel will be the rated power output of the solar panel. Peak rated power varies among manufacturers and solar panel designs, but tends to average around 200 watts per panel. Individual panels can then be connected to form a solar array. If an array is composed of 10 solar panels, each with a rating of 200 watts, the entire solar array will have the potential to produce 2000 watts (2 kilowatts) of power. Solar arrays can be located on rooftops, carports, and other structures or they can be stand-alone, ground-mounted arrays positioned in open areas.
There are many different types of solar panels available today. The most common are panels composed of either monocrystalline or polycrystalline silicon cells. The main difference is that monocrystalline cells are cut from a single silicon crystal, so that the entire cell is aligned in one direction; whereas, polycrystalline cells are made up of several bits of pure silicon, meaning that individual crystals may not be perfectly aligned. Monocrystalline cells are very efficient in bright, direct sunlight. And though polycrystalline cells tend to be less efficient in direct light, they can perform better in more diffuse and low light conditions.
There are numerous other solar technologies, such as thin film and hybrid panels, but when it comes down to it, a 10-kilowatt solar PV system will generate a very similar amount of energy regardless of the material it’s made of.
The efficiency of the solar panels and system design will ultimately determine how much area is needed to produce the desired power output.
Optimal sizing of a solar array depends on a number of factors. In many cases, array size is limited by the space available for solar panel placement. Ideally, solar panels in the northern hemisphere should point due south in order to maximize energy generation, though more westerly or easterly orientations can work as well and can be better situated to produce electricity at different times of the day. Shading of any part of a solar array should be avoided at all times. If even one cell is shaded, the power output of the whole interconnected array can be significantly impaired.
The inclination, or tilt, of a solar panel is another important consideration. The most efficient inclination occurs when panels are aligned at an angle as close as possible to the location’s latitude. Most solar panels are installed with a fixed position. The addition of single or double axis tracking capabilities allows solar panels to alter their orientation throughout the day in order to maximize production along with the changing position of the sun. While these systems have higher efficiencies than fixed-tilt arrays, they are more complex and expensive. Tracking systems are more common in large, utility-scale installations.
When space isn’t a determining factor, solar systems are typically sized to meet a facility’s average monthly or an-nual electricity needs. Electricity needs can be estimated by simply examining a facility’s electricity bills over a period to calculate average kilowatt-hour usage.
The other crucial factor in sizing a solar system to meet electricity needs is determining how much solar energy a location receives throughout the year. For example, on average Phoenix, AZ receives about 6.6 hours of peak sun per day while Ithaca, NY receives about 3.9 hours of peak sun per day. A derate factor must also be applied to account for all the potential losses in a solar system, such as energy conversion from DC to AC, wiring losses, temperature effects, shading, dust, and age of the system. A derate factor of about 75 percent is typical of most systems.
So, a facility in Phoenix, USA looking to offset 10,000 kilowatt-hours of electricity per year would need a solar array with a rated power of at least:
(10,000 kilowatt-hours per year) /
(6.6 hours per day * 365 days per year * 0.75) = 5.5 kilowatts
If the same facility was instead located in Ithaca, USA, it would need a solar array with a rated power of at least:
(10,000 kilowatt-hours per year) /
(3.9 hours per day * 365 days per year * 0.75) = 9.4 kilowatts
This type of calculation can provide a rough initial estimate for the size of solar array a particular facility might require. A number of good online tools are also available to help determine optimal solar array sizing. The National Renewable Energy Lab (NREL) has developed a particularly useful tool called PVWatts (available online at pvwatts.nrel.gov).
The Failure of Diesel Generators
Most of the critical infrastructure that communities depend on in an emergency—fire stations, hospitals, community shelters—rely almost exclusively on diesel generators when the grid goes down. Unfortunately, diesel generators aren’t always up to the task when called upon.
In fact, diesel generator failure is a commonly occurring theme when disaster strikes. High-profile failures at hospitals, resulting in life-threatening conditions for vulnerable patients, have been particularly widespread. Superstorm Sandy led to the evacuation of nearly 1,000 patients in New York when generators failed at two of the city’s busiest medical centers. Hospitals in New Orleans were crippled after Hurricane Katrina. Irene brought down hospitals in Connecticut. Tropical Storm Allison left Houston hospitals in the dark when generators failed. And failure is not just limited to severe weather. In 2003, a software problem led to massive blackouts across the Northeast, and diesel generators malfunctioned at hospitals throughout the region during the outage. The list goes on and on.
Generators can fail for any number of reasons: malfunctioning switches, overheating, lack of adequate fuel supplies, improper sizing for loads. A key issue with diesel generators is that they spend the majority of their time just sitting there, doing nothing. When a generator sits idle for too long, it tends to break. There are protocols in place for periodic testing of generators, but testing is generally infrequent and does not typically reflect real load conditions. This sporadic testing provides no guarantee that generators will perform properly in an emergency.
Despite their flaws, diesel generators still meet the minimum requirements specified by current backup power supply standards. Because of this, they remain the default choice in emergency power for many facility owners. However, with their inherent limitations and consistently high rates of failure, diesel generators fail to ensure the level of reliable, resilient power that other technologies can provide and that critical facilities demand.
The other main component of a solar+storage system is the energy storage technology, usually a battery. A battery is a device consisting of one or more electrochemical cells that converts stored chemical energy into electrical energy through chemical reactions.
Each cell in a battery has a positive terminal (cathode), a negative terminal (anode), and an electrolyte. The electrolyte allows ions to move between terminals, which generates a current that can flow out of the battery to perform work. While there are many different varieties of batteries with different chemical compositions, the two most common technologies for solar+storage applications are lead-acid and lithium-ion batteries.
Lead-acid batteries have been around since the mid-1800s and remain the workhorse of PV storage applications, though this may change as other technologies continue to evolve. Most people are familiar with the lead-acid batteries found in automobile engines; however, these are not the same as those used in energy storage systems. Car batteries are designed to be almost always at or near full charge, whereas those required for solar storage must be able to withstand frequent deep discharge. These are known as deep-cycle lead-acid batteries. Some of the disadvantages of lead-acid batteries for energy storage applications, such as lower energy density and shorter battery life, are now being addressed with the next generation of advanced lead-acid battery technologies.
Lithium-ion batteries are a much newer and still developing technology. First used for consumer products like laptops and mobile phones, lithium-ion batteries have a far greater energy density than lead-acid batteries, which means that a lithium-ion battery can weigh less and require less space while storing the same amount of energy as a lead-acid battery. The term “lithium-ion” actually refers to a wide array of different chemistries, all of which transfer lithium ions between electrodes during charging and discharging reactions. Primarily due to their use in electric vehicles, the cost of lithium-ion energy storage technologies has been decreasing at a rapid pace in recent years.
Each battery technology has its advantages and disadvantages. Deep-cycle lead-acid batteries are a proven technology that is widely available and relatively inexpensive. On the downside, they are quite large and heavy and tend to have a shorter lifespan than lithiumion batteries. Lithium-ion batteries are more compact and lightweight and are better suited for frequent cycling. Lithium-ion batteries also typically perform better at low temperatures than lead-acid batteries. For some applications, the increased lifespan and more robust cycling capabilities of lithium-ion batteries will make them a more cost effective choice. As the costs of lithium-ion batteries continue to drop, they are likely to become increasingly cost competitive. Hybrid battery systems are also being deployed that combine the use of lead-acid and lithium-ion batteries to capture the benefits of each technology.
Like solar panels, batteries can be wired together to achieve desired current, voltage, and power ratings. To determine the optimal size of an emergency battery storage system, a facility must know the maximum power needs of critical loads and how much time the battery may be required to supply critical power. If the system will be required to provide critical power for an exceptionally long period of time, the developer may want to consider adding a generator as a third source of power during extended outages.
Two other important factors must be considered when sizing the system: depth of discharge and inverter losses. Batteries can be damaged or have their lifespan significantly shortened if they are discharged too deeply too often. A good rule of thumb for sizing a lead-acid or lithium-ion battery system is to set a maximum depth of discharge of 80 percent. Battery system sizing must also take into account inverter and other losses incurred when DC power from the battery is converted to AC power. Battery system charge/discharge round trip efficiency can typically be assumed to be about 85 percent.
So, for a facility with a total critical load of 2 kilowatts that must be able to supply power for two days, the battery bank will need to have a power rating of at least 2 kilowatts and an energy storage capacity of at least:
( 2 kilowatts x 48 hours ) / ( 0.80 depth of discharge x 0.85 inverter efficiency ) = 141 kilowatt-hours
Again, this is just a rough estimate of the battery storage system size a facility with these needs might require. Some critical loads may not need to be powered 24 hours a day, allowing for the battery capacity to be reduced. Available battery capacity is also highly dependent on temperature and the rate of discharge.
Lower temperatures and higher rates of discharge will both result in less available battery capacity. Both variables should be considered when designing a system. Ultimately, capacity requirements and the choice of technology may also be limited by real world considerations, such as cost and space available for placement of the battery system.
Solar panels generate DC power and batteries discharge DC power; just about everything else in a building runs on AC power. That’s where inverters come in. Inverters can perform a number of useful functions, but their primary role is converting energy between DC and AC currents. There are many inverter manufacturers offering various technologies, but only two basic types are commonly needed in a solar+storage system: grid tied inverters and battery-based inverters.
If a building only has a solar PV system, it only needs a grid-tied inverter. These inverters are sometimes known as grid-direct inverters. They convert the DC power produced by solar arrays into the AC power used by appliances and distributed across the grid.
Grid-tied inverters are required to have anti-islanding protection, which ensures that they will shut down in the event of a power outage. This is why most solar systems don’t provide any power when the central grid goes down. Depending on the number of solar panels involved, a system may have one or many grid-tied inverters. Some solar panels even have their own built-in inverters, called micro-inverters. Micro-inverters typically increase the cost of a system, but can also improve the overall performance of a solar array. A system based on micro-inverters can optimize performance for each individual panel, so that one poorly performing panel won’t drag down the performance of the whole array. This can be particularly beneficial when shading or debris can be an issue. Grid-tied inverters should be sized to effectively pass energy from the PV system to building loads or the grid. If a solar array is rated to produce a maximum 4,000 watts of power, the inverter should be sized accordingly.
If a building is installing a solar+storage system from scratch, with no existing solar system, it may not need a grid-tied inverter, but most systems must have a battery based inverter. A battery-based inverter can be known by any number of names: grid-interactive, dual-function, hybrid, bi-directional, multi-function. No matter what you call it, it is responsible for converting energy flowing to and from the battery.
DC power from the battery must be converted to AC before it can be used to power loads, and AC power flowing in to charge the battery must first be converted to DC. Unlike the grid-tied inverter, the battery-based inverter includes an automated transfer switch that enables the system to disconnect from the central grid and continue to operate critical loads during a power outage. When the central grid is up and running again, the inverter will automatically switch back to grid interactive operation. Larger systems with multiple battery banks may require more than one battery based inverter.
In a DC-coupled system with no grid-tied inverter, the battery will usually be charged directly by the solar array. Because the solar array produces a DC current, no energy conversion is needed. A charge controller connected between the solar array and battery is responsible for regulating the DC transfer of solar energy to the battery in these systems. In some cases, the charge controller is integrated with the DC port of the battery-based inverter.
Battery-based inverters should also be able to handle the energy from a PV system. Additionally, they must be sized with the ability to supply power to all critical loads simultaneously. The battery-based inverter should be appropriately sized to accommodate the larger of the two values. If the array can produce 4,000 watts and critical loads may require up to 5,000 watts, the battery based inverter should be sized to meet the 5,000 watt need.
A whole lot more goes into making a solar+storage system work than just these few major components.
It takes fuses, circuit breakers, switchgear, cables and wires, mounting hardware, battery enclosures, and safety equipment like earth-grounding and lightning protection to properly get a system up and running. Added functionality like peak load reduction may require additional management software and hardware. For systems with large loads or extended outage requirements, it may be necessary to incorporate a backup generator.
There are myriad possible equipment combinations and sometimes getting all these pieces to work together can be a challenge. To help avoid compatibility issues, it can be beneficial to use hardware from a single manufacturer whenever possible.