Energy efficiency a good substitute for long-term storage, says NREL

From pv magazine USA

Oversizing renewable capacity and adding long-term storage are not the only ways to match renewable generation with electricity demand on a seasonal basis, according to a recent study by two researchers at the U.S. National Renewable Energy Laboratory (NREL).

Their model showed that reducing average building energy usage by about 60% would be key to achieving the least-cost 100% renewable grid with building electrification. The cost-optimal amount of building energy savings ranged from 52% to 68% across five selected climate zones in the United States.

For each climate zone, the model also found the cost-optimal levels of renewable capacity, allowing for oversizing, as well as long-term storage capacity and battery capacity. The model also chose an optimal mix of solar and wind generation, based on solar costs of $1,900 to $2,500 per kilowatt, wind power costs of $1,400 to $1,900 per kilowatt, and transmission costs.

Optimal oversizing of renewable capacity to handle year-round energy needs would be 3.2 times annual electricity demand in a climate zone stretching from Kansas to New York, the model showed. Solar in this case would contribute 15% of total renewable capacity with wind contributing 85%. Oversizing renewable capacity is associated with curtailment during mild seasons.

In a climate zone from Texas to Florida, optimal oversizing of solar would be 1.4 times annual demand, with solar providing all renewable capacity.

The study modeled long-term storage as being hydrogen storage in caverns, and using fuel cells to convert the hydrogen back to electricity. It used a capital cost of $161/kWh capacity. Optimal long-term storage capacity in each climate zone, measured in kilowatt-hours, ranged from 1 to 5 times annual average daily kWh consumption.

The authors said that most long-duration storage technologies “are either geologically constrained or still underdeveloped.”

Costs of a building’s energy efficiency improvements were based on case study data points found through a literature search. The authors reported those results as being “robust:” if the actual costs were 50% lower or higher, then changes in the optimal amount of energy efficiency would be within 5% of the reported results.Battery storage was modeled based on a capital cost of $380/kWh. Optimal battery storage capacity in each climate zone, measured in kilowatt-hours, ranged from 0.1 to 0.8 times annual average daily consumption.

The study also assessed the technical potential of buildings’ load flexibility, such as flexible air conditioning control, and building-sited thermal storage either in ice or in high-temperature ceramic brick. Load flexibility associated with such measures had the technical potential of reducing daily storage requirements by anywhere from 37% to 81%.

The authors said that many U.S. states, cities, and municipalities are developing plans to shift to 100% renewable energy.

The study, titled “Optimal strategies for a cost-effective and reliable 100% renewable electric grid,” was conducted by Sammy Houssainy and William Livingood, both with NREL.