Financial considerations play a significant role when you are thinking about adding photovoltaic (PV) renewable energy to any building. When evaluating the expected return, a building owner or investor naturally will base the decision to proceed on electricity rates, the amount being paid (tariff) for electricity exported to the grid, and available incentives. But once the choice has been made to move ahead with the installation, other questions must then be considered. Perhaps the most important of these is whether to export the PV electricity to the grid or to use it directly. However, as described in my previous post, the economics of PV energy are changing. In many areas tariffs for PV electricity have dropped below electricity rates, making self-consumption more profitable and leaving the export to grid model in the past.
To promote the transition to renewable energy and make citizens more active players in this movement, countries are adopting regulations—frequently associated with financial support mechanisms—that are more and more favorable to PV self-consumption. To help you, as a facility manager, design engineer, or contractor, better understand how self-consumption and grid export affect the building electrical installation, consider the schematic illustration of the two systems in figure 1.
Figure 1 – PV installations can be connected to export electricity directly to the grid (left) or for self-consumption (right).
WHEN EXPORTING TO THE GRID, YOUR PV INSTALLATION AND YOUR BUILDING INSTALLATION ARE SEPARATE AND INDEPENDENT
When the PV production is entirely exported to the grid, the PV installation is connected to the electrical distribution network without any connection to the building electrical system. Although part of the same physical infrastructure, the PV system and the building installation are two independent and autonomous electrical units. The PV energy injected to the grid and the energy consumed by the building are measured by two independent power measurement devices. The PV installation requires a minimum of control functions, usually handled by the PV inverters, and has no impact on the building control.
Indeed, the “PV production entirely exported to grid” model has attained a certain maturity and no longer raises questions or poses difficulties in the design or operation phase. The possible installation configurations are well known and standardized, the needed equipment well identified, and standards provide clear guidelines to ensure the safe operation of such installations.
One down side of this arrangement is that there is no provision for the building to operate offline, either by choice or when the grid falters, despite its proximity to locally harvested electricity.
WHEN SELF-CONSUMING, YOUR PV INSTALLATION IS PART OF YOUR BUILDING INSTALLATION
In the second case, the PV production is used in priority for self-consumption. When the PV production exceeds the building consumption needs, the excess PV energy may be injected to the grid provided an agreement has been set up with the energy provider.
Physically, the connection of the PV installation is done behind the utility meter. The PV installation is part of the building installation, and thus, its sizing, grounding system, and protection equipment are dependent on the building electrical installation. The integration of the PV installation may require modifications in the rest of the building electrical installation. The next post in this series will discuss the best way to integrate PV installation to your existing electrical system, including sizing and equipment selection.
Measuring the energy consumed by the building and the energy produced by the PV installation usually is done by two separate devices connected in series, although some energy providers accept a single bidirectional power meter.
In the self-consumption model, control needs can be quite different from the export-to-grid model. The minimum control functions include the PV produced power management and its optimized use. Control requirements can be much more important, especially if PV production exceeds the consumption at some periods of the day, and even more so if the building is required to be able to operate in off-grid mode, powered only by the local generation system.
Unlike the export-to-grid model, self-consumption arrangements are relatively new. Electrical installation configurations are varied and not always optimized. Regulatory guidelines and recommendations often do not exist, especially regarding the integration of PV system into the building electrical installation and how this integration affects the design and the sizing of the building installation as well as its operation, monitoring, and control.
The self-consumption model will at some point become standardized. But because today this approach is relatively new, the various organizations that develop design guidelines and regulations for such systems have issued only general requirements, if any at all.
For guidance on selecting the best protection and control components for a given PV system in residential premises, commercial buildings, and power plants that can be used both for self-consumption and grid export, download the application paper “Safe and reliable photovoltaic energy generation.” It also sets out guidelines for optimizing system operation, maximizing the safety of people and property, and ensuring continuity of service.
Click here to learn how Boston One Campus, Schneider Electric’s North American headquarters, is using local PV electricity production in an advanced microgrid environment to supplement power from the grid.
Subsequent posts in this series will explain how to choose a PV system architecture, installation rules for sizing, and choosing equipment to better inform those who might be considering a PV installation for self-consumption.
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