In electricity grids, demand and generation must be balanced at all times. Modern electricity is primarily generated by constant power sources, such as nuclear and coal, and quickly dispatchable sources, such as gas fired power plants, which can be adjusted based on small demand variations. However, as anthropogenic total CO$_2$ emissions already make up almost 75% of the atmosphere's total carbon content, governments are increasingly implementing renewable energy mandates. These mandates limit a country's CO$_2$ emissions and reliance on fossil fuels. Therefore the electricity generation market is moving towards renewable power sources, which have no CO$_2$ emission.
Many renewable energies, such as wind, solar, and hydropower, are variable power sources. These power sources have a different power production capability at each moment in time depending on natural inputs, and these input variations cause their generation to fluctuate unpredictably. For example, on a cloudy day, a solar power plant will be able to generate only a fraction of its installed capacity. The field of energy optimization arose with the goal of calculating how to ensure full demand coverage with the least production. The optimization of variable power sources is thus an important problem to figure out how we can rely on variable power sources, such as most current renewable energy sources, to meet demand.
Two of the most widely used renewably power sources, wind and solar, have a mismatch in the timing of energy supply versus demand. In order to reconcile this mismatch, either a third energy source or an energy storage system has to be included to fully meet demand.
This study will optimize a purely renewable energy system to fully meet demand for the town of Minot, North Dakota. The renewable system considers CO$_2$ Plume Geothermal, in addition to wind and solar, as both a third power source and as a method of energy storage.
Geothermal energy production is dispatchable, as a specific amount of power can always be generated. Earth's thermal energy is always available, and a geothermal power system can extract this energy as needed. A CO$_2$ Plume Geothermal power plant runs like a geothermal plant, but with CO$_2$ instead of brine as the primary working fluid. This proposed technology therefore both generates power and provides a use for CO$_2$ sequestered in the ground.
This research finds the optimum sizes of wind, solar and CO$_2$ Plume Geothermal power plants to fully meet demand in a stand-alone system. CO$_2$ Plume Geothermal with Energy Storage (CPGES) is then implemented to store surplus energy and meet energy demand without supplemental gas turbine power generation. We find that using CPG energy storage reduces renewable energy overproduction by 80%, system capital cost by 75%, and the system remains fully renewable.