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Greening the Local Grid

Smart solutions for distributed renewables.

Fortnightly Magazine - February 2011

rating and loading indicates it can support 30 MW of DG. Rather than allowing just 30 MW of combined wind, PV or other renewable resource to be connected, given the diversity and variability of the resources, utilities need to look at new applications that will allow them to connect a greater amount of each—potentially up to 30 MW of each, so that during the night and windy conditions, it allows up to 30 MW of windmill DG to be generating and during the day it dispatches up to 30 MW of PV. Or it can be a mixture of the available resources. This implies that the distribution operating company must constantly monitor generation resources and exercise some control over DGs in order to ensure the grid isn’t adversely affected in the rare event that generation from all available sources is operational and being maximized.

All of this means that producers will encounter difficulty if they seek to offer accurate generation forecasts to the utility grid system operators in order to support the 1-hour, 1-day, or 7-day and beyond forecasts. Since the grid operator (whether the utility or a regional independent agency) is responsible for matching anticipated demand with expected generation resources, any less-than-reliable forecasting of renewable energy production might result in the grid operator maintaining a higher reserve margin than ought to be needed—at least until better experience and tools are available.

Physical Back-up

There are physical mechanisms to address the impact that natural intermittency has on the value that renewable resources provide to the electric grid. These solutions also must offer immediate reactive response to sudden changes in generation capacity. A survey of the marketplace produces a variety of available solutions to this dilemma, from adding spinning reserve to demand response and the expansion of system interconnections.

Additional operational (or spinning) reserve can be added to the system to address the intermittency of renewables. While a technically acceptable solution, this comes with potentially high costs—both financial and environmental—because spinning reserve plants are often gas fired turbine engines. Their low use factor gives them a high per-MW and per-MWh cost, and so they are designed primarily for fast ramp and dependability—not low emissions.

Demand response (DR) and energy storage both help to address intermittency. DR refers to the solutions that allow utilities to curtail load based on preset conditions and agreements with customers. This can be direct load control (DLC), advanced pricing programs, emerging enabling technologies or a combination. Energy storage technologies allow renewable energy to be captured during periods of low demand or high generation and fed back into the grid when demand is high or renewable output is low. Energy storage technologies also provide stability during ramping periods.

Another approach uses complementary power sources to combine the particular renewable facility with a resource that has opposing generation patterns. An example would be combining wind and solar resources, or wind and hydro. Ideally, these complementary resources would also be renewable, but the challenge is to overcome the inherent geographical distribution of different renewable energy supplies.

Expanding the existing system interconnections with