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.
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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.
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
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
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
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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.
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
SPrimarily 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.
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.
robbob said:The BCS is optional. I have set up a triple XW system with no BCS. (This was before the BCS was even available.) That system is still purring along with no issues whatsoever. That one is off-grid with a genset auto starting when batteries get low. Now, we did create an external transfer using contactors with a timed voltage sensing relay that would transfer loads in a way that avoids putting stress on the relays in the XWs.
When they are pointing out it not being an "on grid" they are no saying it can't be grid tied, but they seem to be saying they aren't meant to be directly connected to the grid before the main panel. All the diagrams show it being fed into breakers unless the BCS is used. But even then it isn't like the Sol-Ark and EG4 which connect to the grid feed on one connection and the load feed on another... so it requires the BCS to combine into a 200amp relay.
Prior to this I was thinking you could just split the grid feed into 4 (for a 4 XW install) using a small box and 60 amp breakers and wires to each XW. Makes me wonder if that would work and each XW would switch their own 60amp relays when grid power is lost.
So are they trying to sell the BCS or is it required for grid tie with more than 60 amp service?Watching this video at 19 minutes they are talking about the BCS which is connected to the load and is basically a 200amp auto transfer switch "to keep from overloading the 60 amp relays" ... and their diagram is of directly connected to the grid.When they are pointing out it not being an "on grid" they are no saying it can't be grid tied, but they seem to be saying they aren't meant to be directly connected to the grid before the main panel. All the diagrams show it being fed into breakers unless the BCS is used. But even then it isn't like the Sol-Ark and EG4 which connect to the grid feed on one connection and the load feed on another... so it requires the BCS to combine into a 200amp relay.Prior to this I was thinking you could just split the grid feed into 4 (for a 4 XW install) using a small box and 60 amp breakers and wires to each XW. Makes me wonder if that would work and each XW would switch their own 60amp relays when grid power is lost.So are they trying to sell the BCS or is it required for grid tie with more than 60 amp service?
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The BCS is optional. I have set up a triple XW system with no BCS. (This was before the BCS was even available.) That system is still purring along with no issues whatsoever. That one is off-grid with a genset auto starting when batteries get low. Now, we did create an external transfer using contactors with a timed voltage sensing relay that would transfer loads in a way that avoids putting stress on the relays in the XWs.Anytime you stack inverters, you run a risk of burning up transfer relays. Schneider specifically recommends not doing more passthrough amps than a single XW's passthrough rating (60A), even when stacking units.What the BCS does is controls the connection to grid via a large contactor. When the inverters sense grid power (sensing is done at gen terminals) they synchronize up to the grid source, then give the signal for the BCS to pull the contactor. That connects grid into the loads panel, and then the XWs shift from inverting to charging via the wires from load terminals to panel. This eliminates any internal transfer, and all transfer make/break load gets managed by the contactor in the BCS.This setup can be done using a contactor and your own controlling, but the BCS makes it way easier! We did a 3 phase setup this way once. The only thing that we did not set up was the WattNode meter (this is integrated in the BCS). Without the WattNode meter, you can't limit the amount of draw coming from grid.the WattNode meter you could probably even do a "limited to home"/ zero export type of operation, since the WattNode meter tells the inverters (via Insight Home or Insight Facility) how much current is coming from, or going to, the grid.Here is a screenshot of the drawing on the datasheet that shows the general gist of the power connections.Here is a page from the XW Pro Multi-unit Design Guide:And here are drawings showing how it is done with an external contactor, instead of the BCS.You can install up to 4 XW Pros on 120/240V split phase, with or without BCS or external contactor. The inverters will work fine and they can transfer internally all together, but you will very likely end up with burnt relays eventually.Here is a page showing the issue with parallel units and passthrough current:
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