Self-Consumption PV Systems: Page 2 of 10
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Nonexporting PV systems have been around in the US for some time. Facilities deployed them to streamline interconnection for systems intended to offset some portion of a large facility’s daytime load. However, the real growth in self-consumption or self-supply systems took off first in Germany due to increasingly lopsided economics where energy exported to the grid under the feed-in tariff compensated suppliers at less than the retail rate of electricity. Grade-school economics tell us it’s not a desirable financial position to buy high and sell low, and this drove interest in increasing the portion of PV power consumed on-site.
This movement picked up steam in areas such as Australia and Hawaii, where interconnection constraints limited or eliminated the ability to export energy. The next factor is primarily a logistics challenge: most residential load does not align with solar production potential, so a curtailment-only system is limited in terms of how much benefit it can provide. Adding storage and an intelligent inverter substantially increases the potential benefit of the PV system, because the addressable load is now available 24/7.
—Philip Undercuffler, director, strategic platforms, OutBack Power Technologies
The first markets to reach grid parity (meaning identical cost of grid-supplied energy and PV-generated energy) were Australia and Germany. With regulatory reduction of PV feed-in tariffs, the cost (benefit) of PV-generated energy becomes significantly lower than the cost of grid-supplied energy. By consuming self-generated PV energy, customers can reduce their energy bills, resulting in amortization of the investment for the PV system and profits from the subsequent savings. Due to low PV system cost, amortization periods have become reasonably short, so customers can realize direct savings within a foreseeable timeframe. This is the main driver for self-consumption in these countries. In countries or US states where net metering still is a common way of benefiting from PV generation, the drive for increased or enhanced self-consumption remains low.
—Martin Volkmar, product manager, energy management solutions, business unit residential, SMA Solar Technology
What additional components and software are required for nonexport or curtailed residential PV systems without energy storage?
Is this equipment certified and available in the US market?
What are the basic modes of operation for these systems?
In theory, the systems and software are pretty simple. An energy meter at the interconnection measures the direction and quantity of energy in real time and feeds a controller that communicates with the inverters to maintain the energy import at or above zero. Unfortunately, I know of no off-the-shelf systems that can do this in the US. Installers can build custom control systems, but not at a cost that makes them affordable for the average residential system. Due to the load profile of the average residence, I do not see a cost-effective solution to implement a nonexport PV system without some type of storage, either thermal or electrical, since residential loads peak in the evening.
—Marvin Hamon, PE, principal, Hamon Engineering
A smart meter is the main element needed. It monitors the current flow in service entrance feeders. As soon as PV production exceeds load and the feeder current drops to near zero, the import meter sends the PV inverter a simple power-curtail command. All modern smart inverters can move on the power curve, raising voltage and reducing current exported to zero at near-Voc conditions. This level of control is available in the US today, but requires some custom integration by the system installer.
—Wes Kennedy, Fronius USA
In general, for PV systems to meet limited or nonexport requirements, the system must include a meter. However, each market or state has its own requirements. In Hawaii, for example, Rule 22 from the Hawaiian Electric Company requires the installation of a meter and an inverter that limits the amount of energy transferred to the grid. SolarEdge inverters are self-certified for Rule 22. In terms of operation, these systems limit PV production to a level no higher than the electrical demand of the site. If the system produces more power than the site can consume at the time, the energy is lost. To maximize the economic return of these installations, owners should try to match their demand profile with the PV production profile.
—Peter Mathews, SolarEdge