Excess local production - how to manage

From Electrical Installation Guide

If the local production exceeds the load consumption — on weekends, for example — the excess electricity goes to the grid, for which financial compensation may also be possible. If, however, the contract with the energy supplier does not allow injection of electricity to the grid, local production should be curtailed at these times, managed by using storage or by shifting the loads to the period of local production.

When the locally produced power exceeds the loads consumption, there are several possible options for managing the excess power:

Fig. K72 – Local energy production (photovoltaic, for example) versus power consumption

Inject excess local production to the grid

Grid injection is one of the simplest ways to manage excess local production. When local production is higher than the loads consumption, the excess power will naturally go to the grid.

Although simple, this solution is usually not the most profitable. The injected electricity is bought at a lower price than the wholesale price, or not paid at all.

Fig. K73 – Excess local production (photovoltaic, for example) can be injected to the grid

Limit the local production

Limiting the locally produced power is used in particular when the injection to the grid is not allowed. The power limitation is done, for example, by controlling the photovoltaic inverters so that they reduce the DC voltage reference of the solar panels, to reduce the photovoltaic power output.

Fig. K74 – Excess local production (photovoltaic, for example) can be avoided by limiting the locally produced power

Store the excess local production for later use

Storage systems provide a highly effective solution: they can store the excess local production and make this energy available for later use. Today, however, this option is costly and often has a long payback period.

To improve the return on investment, storage can be associated with other use cases, such as providing a backup power supply, improving demand response, and avoiding peak power usage to reduce the power of the subscribed contract.

Fig. K75 – Excess local production (photovoltaic, for example) can be stored for later use

Shift some loads to the period of local production

Load management is a very attractive option which consists of simply making some loads operate during the period of local production, as much as possible. These loads must be flexible and manageable through a control system. Examples of such loads include electrical vehicle charging, water heating, and to a certain extent, HVAC (heating, ventilating, and air conditioning).

Fig. K76 – Shift loads to consume more during the periods of local production, in order to maximize the usage of locally produced power

The load management strategy has obvious advantages: it is cost effective, easy to implement, and guarantees a quick payback. Unlike storage, it does not require the installation of additional equipment and thus presents a better profitability.

When a load shifting strategy is not enough to absorb the total excess local production, it can be used in association with a storage system. In that case, load shifting offers the additional benefit of reducing the size — and optimizing the use — of the storage system.

Note: Shifting HVAC loads can be seen as an energy storage system ... hot/cold water storage ... or even the complete building can “store” some cold or heat by heating (or cooling) a bit more than needed when local energy production is available, and heating/cooling less later when not available.