When Should We Consider the REZ Transmission Planning Process?

This page answers key questions that practitioners may have about the REZ Transmission Planning Process and its applicability to their current planning needs.

What is a renewable energy zone (REZ)?

A REZ is a geographic area that enables the development of profitable and cost effective grid-connected renewable energy. A REZ has high-quality renewable energy resources, suitable topography and land use designations, and demonstrated interest from developers, all of which support cost effective renewable energy development.

What is the REZ transmission planning process?

REZ transmission planning is a process to plan, approve, and build transmission infrastructure that connect REZs to the power system. The REZ process helps to increase the share of solar, wind, and other renewable energy resources in the power system while maintaining reliability and economics. The REZ process focuses on large-scale wind and solar resources that can be developed in sufficient quantities to warrant transmission system expansion and upgrades. These variable renewable energy resources are similar to large hydropower in that transmission systems must be brought to the location of the resource to connect them to the grid.

The REZ process is not an isolated activity of cataloging and mapping potential renewable resources for electricity generation. Additionally, it is not intended to be a process supporting the isolated procurement of renewable resources by individual utilities.

Does the REZ process consider all renewable energy resources?

The REZ process focuses on large-scale wind and solar development because other renewable energy resources (such as geothermal or mini-hydropower) are seldom found in sufficient concentration to warrant consideration as a REZ. However, when located within a designated REZ, these supplemental renewable energy resources may provide additional value to the REZ.

When should we consider the REZ process?

The REZ process is not ideal for the objectives set for all transmission expansion undertakings with a focus on renewable electricity generation. It is therefore vital to fully understand the process and its applicability to each specific context. Actors should consider the following questions when deciding if the REZ process is the right fit for their needs.

Is there a need to increase the share of renewable energy resources in the power system?

The REZ process does not necessarily focus on the zones with the highest-quality resources, but on the zones with the highest probability of commercial development in addition to high-quality resources. The REZ process is an effective tool to encourage private sector investment in renewable energy generation through demonstrated commitment to the construction of efficient transmission to promote the scale-up the share of renewable energy resources in the power system.

What are the current barriers to scaling up renewables?

The REZ process presented here applies to renewable energy expansion that is constrained by the lack of existing transmission. Traditional transmission planning may be ill-suited to the characteristics of renewable energy development because transmission planning decisions need to be made well in advance of renewable generation development decisions. Transmission system access to windy and sunny areas, which are attractive for wind and solar power development, may require 5–10 years to plan and construct; however, wind and solar generation projects only require 1–3 years to construct. Financing for these remote generation projects is not available without transmission access, but transmission lines cannot be built without a demonstrated need for service. Siting for conventional generation such as coal is seldom as constrained. Renewable energy planning that does not consider transmission expansion may limit countries to less economical renewable energy development.


Nate's newer version of timescale misalignment

Figure. Timescale misalignment


attempt to blow up circular dilemma image

Figure. Circular dilemma


In what situations is the REZ process not applicable?

The REZ process may not be applicable in situations in which other reasons limit renewable energy development, or if the existing transmission system already has capacity to accommodate new renewable energy development. Other constraints, such as congestion on existing lines that leads to curtailment of renewable energy generation, can be addressed through traditional transmission planning activities and do not require the REZ process.

Is transmission expansion planning the objective of the exercise?

The REZ process should be considered when a transmission expansion planning tool is required that promotes the development of high-quality and abundant renewable resource zones while also simultaneously expanding transmission networks.

What is the scope and time horizon of the planning activity?

The scope and time horizon of the REZ process varies depending on the local context. And, it can be aligned with power system planning activities such as integrated resource planning (IRP), power development plans (PDPs), and transmission development plans (TDPs). It is often an effective tool to avoid transmission congestion in the medium to long term, as penetration of renewable generation and demand in load centers increase.

The REZ process offers opportunities for reducing costs and amplifying benefits by evaluating the transmission needs of the system as a whole, as opposed to upgrading the system incrementally based on the needs of specific projects. For this it is not a short-term planning activity.

What are the desired outputs of the planning activity?

The specific outputs of the REZ process may vary depending on the context; however, they should include the identified REZs as well as additional knowledge and tools necessary to support transmission expansion planning activities.

Outputs often consist of a final transmission order from the system operator or regulator with the a geographic map of the selected REZs; identification of the major transmission improvements necessary to deliver the energy generated in the REZs in the most beneficial and cost-effective way; and an updated estimate of the maximum generating capacity of the REZs based on capacity upgrades.

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