Storage and inverters: seamlessly integrating with solar


By Conor Trujillo, Assistant Director of System Design.

Read original article at Solar Power World. 

The way we produce and consume energy is changing rapidly. With the price of solar panels declining by 99% over the last few decades, more and more customers are turning to distributed renewable energy (DRE) systems to meet their needs. Accordingly, the market for DRE storage systems is growing fast, with Navigant Consulting predicting a 10-fold increase over the next three years. As the market for solar and storage expands, the capabilities of inverters have evolved as well. Optimizing storage technologies with inverters during system design will be critical for companies looking to compete in what may soon become a $2.8 billion market.

Types of storage
While Tesla’s “Powerwall” home energy solution recently made headlines, various types of storage capabilities have been around for a long time. The two most basic methods of storage are mechanical and chemical. Mechanical storage systems work by moving physical mass to increase potential energy. The most commonly used method, pumped-storage hydroelectricity, operates by pumping water from a lower holding tank to a higher one when energy demand is low, later releasing the water back to the lower tank to create energy. The majority of solar-powered storage systems, however, rely on electrochemical storage in the form of batteries. The oldest and most commonly used battery solution in DRE is lead-acid, which is also the least expensive at a cost of $150 to $200 per kWh. Lead-acid batteries hold charges for long durations and are more commercially available, hence their ubiquity in the marketplace. However, lead-acid batteries tend to be heavier and take up more space, giving them a comparatively low energy density.

Alternatively, lithium ion batteries offer a solution with a longer overall lifetime and higher energy density. These batteries also work better in a range of environments. However, the technology behind lithium-ion is still emerging, making it more expensive at $500 to $700 per kWh. The use of lithium-ion is best for customers who need to deploy high amounts of power over a short amount of time but will require the use of a separate battery management system (BMS) to manage thermal runaway, among other things. With the emergence of Tesla’s Powerwall and other lithium-ion technologies, the use of lithium-ion batteries with renewable systems is expected to increase significantly in the coming years.

The role of inverters
There are two main categories of inverters: grid-tied (sometimes referred to as grid-direct) and battery-based. Grid-tied inverters allow power to flow from the installed solar system to the grid. Energy created by the solar array powers the loads directly, with any excess being sent to the utility, resulting in net metering. Due to this interaction with the grid, inverters are required to have anti-islanding protection, meaning they must automatically stop power flow when the grid goes down.

Battery-based inverters can either be one directional, taking DC power from the batteries and converting to AC power, or bi-directional, meaning they can invert DC to AC, as well as take incoming AC power and use it to charge the batteries. These inverters enable systems to be grid independent, so they can function with stand alone storage systems (where there is no grid connection), as well as grid-interactive, meaning they regulate energy flowing between the loads, the batteries and the grid. As with grid-tied inverters, battery-based inverters that are grid-interactive must have anti-islanding to disconnect from the grid during a power outage, however in this case, power will still be available for use via the battery storage system, thus increasing the energy security and independence.

Design challenges
When designing a solar-with-storage system, there are far more factors to consider than with a grid-tied solar project. These include the battery voltage and capacity, battery chemistry, AC output voltage and phase, average and peak loads and grid-interactivity. More often than not, off-the-shelf battery inverters are designed for lead-acid chemistry, although more manufacturers are developing products that can work with lithium ion as well. Project cost is another major consideration. Though added energy security seems like a no-brainer in most cases, the additional cost of energy storage and corresponding power electronics must be taken into account.

Market trends
As the utility grid ages and rates from conventional resources continue to go up, we will continue to see substantial and exponential investment in DRE systems. At the same time, there is downward pressure on pricing for storage and battery-based inverter systems, meaning in the next few years, the number of solar with storage installations will continue to rise, as these systems are deployed to residents and commercial businesses worldwide. Since much of the discussion around solar and storage hinges on the capabilities of battery-based inverters, new technologies will begin to emerge to improve the ease of installation and increase the functionality, improving the customer’s return on investment and energy independence.