Decoding AC Coupled vs DC Coupled Batteries

In the realm of renewable energy systems, the choice between AC coupled and DC coupled batteries is a crucial decision that can significantly impact your energy needs. Understanding the differences and advantages of each approach is essential for making an informed choice. This article will decode the concepts of AC coupled and DC coupled batteries, highlighting their characteristics, benefits, and suitability for various energy requirements.

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1. AC Coupled Batteries:

AC coupled batteries are integrated into the alternating current (AC) side of a solar power system. They are typically connected to an inverter, which converts the direct current (DC) produced by solar panels into AC electricity for immediate use or feeding into the grid. The key features of AC coupled batteries include:

Flexibility: AC coupled system allows for retrofitting existing solar installations, making them a versatile option for upgrading or expanding your energy storage capacity.

Independence: AC coupling enables independence between the solar PV system and the battery system. This means that even if the PV system fails, the batteries can still provide power during outages.

Compatibility: AC coupled batteries are compatible with a wide range of inverters, making them suitable for various solar power setups.

2. DC Coupled Batteries:

Unlike AC coupled batteries, DC coupled batteries are connected to the direct current (DC) side of a solar power system. They are integrated with the solar charge controller, which regulates the charging process. Here are the main characteristics of DC coupled batteries:

Efficiency: DC coupled systems are known for their high efficiency, as they minimize energy losses during the conversion process. Since the batteries directly receive the DC power generated by the solar panels, fewer conversions are required.

System optimization: DC coupling allows for more precise control over the charging and discharging of batteries, optimizing the overall system performance.

Cost-effectiveness: In some cases, DC coupled batteries can be more cost-effective, especially for new solar installations, as they eliminate the need for an additional inverter.

AC Coupled vs DC Coupled: Making the Right Choice

When deciding between AC coupled and DC coupled batteries, it's crucial to consider your specific energy needs and system requirements. Here are some factors to consider:

Existing system: If you already have a solar power system in place, AC coupling might be a more suitable option for expanding your energy storage capacity.

Efficiency: If maximizing system efficiency is a top priority, DC coupled batteries are worth considering due to their streamlined DC-to-DC connection.

Cost: Assess the overall costs of installation, including inverters and charge controllers, to determine which option aligns with your budget.

The choice between AC coupled and DC coupled batteries depends on various factors, including system requirements, flexibility, efficiency, and cost considerations. Both approaches offer distinct advantages, and understanding their differences will help you make an informed decision that meets your energy needs. Whether you opt for AC coupled or DC coupled batteries, leveraging renewable energy storage solution is a step towards a sustainable and reliable energy future.

North America currently has over 50 gigawatts of installed utility scale PV generation with many more gigawatts under construction. Utility scale solar will only reach its fullest financial and energy generating potential with the addition of energy storage, however. The majority of the utility scale PV base could expand energy production and increase revenues with the addition of energy storage. Choosing the right topology is critical to maximizing the impact of coupling energy storage with utility scale solar installations.

In this post, we will examine the coupling of energy storage with utility scale PV by defining and comparing three principle methods: AC coupled, DC coupled, and Reverse DC coupled. We will also consider all possible revenue streams of solar plus storage and their availability based on available systems for coupling storage.

In addition to this overview, Dynapower has detailed papers on each topology to help installation developers and owners determine which approach will provide the greatest ROI when adding energy storage.

Solar Plus Storage Revenue Streams

The addition of energy storage to an existing or new utility scale PV installation allows system owners and operators the opportunity to capture additional revenues only available with the addition of energy storage. These include:

CAPACITY FIRMING

Turn Solar into a Dispatchable asset

For certain time periods during the day the availability of storage gives the system operator the ability to bid firm capacity into merchant markets. That is storage makes PV generation a dispatchable revenue generating asset.

Depending on the available local energy market, this may translate to higher kWh rates for firm capacity during dispatchable periods.

ENERGY TIME SHIFTING

Utilize Generated PV Energy When Its Value is Highest

Energy Storage allows bulk energy shifting of solar generation to take advantage of higher PPA rates in peak periods, or to allow utilities to address daily peak demand that falls outside periods of solar generation.

CLIPPING RECAPTURE

Maximize Value of PV Generated Energy

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Given common inverter loading ratios of 1.25:1 up to 1.5:1 on utility-scale PV (PVDC rating: PVAC rating), there is an opportunity for the recapture of clipped energy through the addition of energy storage.

Using a simplified system for illustrative purposes, consider a 14MWDC PV array behind a total inverter capacity of 10MWAC. Depending on your location and type of racking, the total clipped energy can be over 1,000,000 kWh per year.

With storage attached to the array, the batteries can be charged with excess PV output when the PV inverter hits its peak rating and would otherwise begin clipping. This stored energy can then be fed into the grid at the appropriate time.

Without energy storage, these kWhs are lost and revenues stunted.

CURTAILMENT & OUTAGE RECAPTURE

Continuous Uptime and Revenue Generation

When storage is on the DC bus behind the PV inverter, the energy storage system can operate and maintain the DC bus voltage when the PV inverter is off-line for scheduled or unplanned outages. When the PV inverter is offline the energy from the array can still flow to the batteries via the DC-DC converter ensuring energy can be harvested for later use.

The same uptime capabilities apply when a large utility-scale array is curtailed by the ISO or utility. Curtailment is sometimes seen in areas of high solar penetration — such as California — when there is overall excess production on the grid. With a DC-coupled energy storage system, energy production can continue with energy being stored and available for discharge when curtailment ends.

LOW VOLTAGE HARVESTING

Make Money On The Edges

PV inverters typically require a minimum threshold DC bus voltage to operate. On a 1,500VDC nominal system, this ‘wake up’ voltage may be around 500VDC. As a result of this minimum voltage threshold, the available generated energy in the morning and evening when voltage on the array is below the PV inverter ’wake up’ threshold is not captured.  Adding energy storage through a DC-to-DC converter allows for the capture of this generated energy from the margins.

This phenomenon also takes place when there is cloud coverage. In both cases, this lost energy could be captured by a DC-coupled energy storage system.

RAMP RATE

CONTROL

Modulate Power for Continuous Grid Connection

Ramp rate control is often required by utilities and ISOs for PV and wind systems to mitigate the impact of a sudden injection of power onto the grid or a sudden loss of generation due to the intermittent nature of both generation sources.

A storage system coupled with PV can monitor PV inverter output and inject or consume power to ensure the net output remains within the ramp requirements allowing for continuous energy injection into the grid.  Additionally, with this ramp rate control benefit, energy otherwise lost when a PV inverter would self-regulate during a ramp up (by manipulating the I-V curve to curtail power output) can now be stored for later use.

Solar Plus Storage System Options

AC-COUPLED SOLAR PLUS STORAGE SYSTEMS

In AC coupled systems there are two inverters, one for the battery and another for the solar PV system. With this system configuration, the power to the grid can be maximized by discharging both the battery and PV at maximum power. They can be dispatched independently or together.

This configuration is integration challenges for microgrid operations. Dynapower offers AC coupled energy storage inverters and fully integrated energy storage systems for both behind the meter and utility scale applications.

DC-COUPLED SOLAR PLUS STORAGE SYSTEM

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Primarily of interest to grid-tied utility scale solar projects, the DC coupled solution is a relatively new approach for adding energy storage to existing and new construction of utility scale solar installations.

Distinct advantages here include reduced cost to install energy storage with reduction of needed equipment — one set of inverters, MV switchgear and other balance of plant costs, higher efficiency than both AC and Reverse DC coupled options, and increased PV energy generation — from clipping recapture and low-voltage harvest. DC coupled solar plus storage allows for increasing the panel to inverter (DC/AC) ratio to much higher levels than solar-only plants. Solar plus storage can be contracted under a single PPA.

REVERSE DC COUPLED SYSTEMS

Reverse DC coupled solar plus storage ties a grid-tied bi-directional energy storage inverter with energy storage directly to the DC bus. The PV array is coupled to the DC bus through a DC to DC converter. The reverse DC coupled configuration allows you to operate in off-grid (microgrid) mode by virtue of the AC interface being a microgrid-capable storage inverter.

With a Reverse DC coupled solar plus storage system, you enjoy the CAPEX, efficiency and revenue advantages of DC-coupling while enabling a microgrid application with battery backup power traditionally only possible with an AC-coupled configuration. For microgrids connected to the electric grid and power markets, Reverse DC coupled systems can also unlock several value streams during times of grid connection including frequency regulation, arbitrage and bulk shifting.

Solar Plus Storage System Comparison

Dynapower has successfully delivered all three system architectures for our customers in both grid-tied and microgrid applications. When evaluating which of the system architectures is optimal, the options must be compared across a variety of metrics including cost, efficiency, reliability, and flexibility.

With over a decade of experience in solar plus energy storage and over 900MWs of inverters/converters deployed worldwide, Dynapower can assist you in selecting the
optimal system for your existing or new PV array.

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