GEM overview

GEM stands for Generation Expansion Model and is a long-term planning model used to study capacity expansion in the New Zealand electricity sector. While it is most often used to conduct research on generation capacity expansion given a fixed transmission network, it can also be configured to simultaneously optimize generation and transmission capacity investment decisions.

GEM is formulated as a mixed integer programming (MIP) problem with a cost minimization objective function. Selecting from a large list of potential new generation facilities, the model determines which plant to construct and in which year each new plant is to be commissioned, all the while satisfying a number of technical, physical and economic constraints. Each potential new plant is characterised by parameters describing attributes such as location, technology, fuel type, capacity, capital cost, and operating costs. The model is typically run for a series of scenarios describing possible future outcomes for factors such as demand for electricity (energy and peak), hydro and thermal fuel availability, fuel prices, plant costs, and policies such as carbon pricing, renewables targets, or transmission pricing. Under each scenario, a build plan and a supporting set of prices is generated. The time horizon over which the model is operated is typically 20-40 years.

GEM is written and solved using the GAMS modelling software. Input data is supplied to the model in the form of GDX files. Model output is produced as a collection of GDX and CSV files.