Aquifer thermal energy storage, demonstrating underground water heating and cooling. (Image: PrusarooYakk/Adobe Stock)

In my last blog I suggested that when we approach a particular application that requires the storage of energy, we should consider all the possible methods. So, now I’d like to take a deeper dive “into the weeds” by considering one of the most important applications for energy storage: reducing the reliance on burning fossil fuels for our supply of electricity. The natural resources wind and sunshine are becoming viable alternatives, but the problem with them is that they are variable — therefore energy storage is critical. I decided to see what some of the literature says about this.

The balance of available renewable resources not only varies throughout the course of a day, but also as seasons change throughout the year. So, to enable the transition to renewables, we need practical and economically viable “long duration energy storage (LDES).” A study reported in the July 21, 2021 issue of Joule  addresses the technologies that can be used to “support high-variable renewable energy grids” and compares their relative costs using a measure called the levelized cost of energy (LCOE). This measure considers a multitude of factors, including initial capital investment. The study is based on one of the three U.S. grids, the U.S. Western Interconnection, and the assumption that 85 percent of the energy would come from renewable sources.

Their conclusion was that among the “least-cost options for durations longer than 36 hours” are pumped thermal energy storage and a proposed system that would use stationary versions of the existing hydrogen fuel cells that are used to power heavy-duty trucking. More expensive, but viable, technologies would include Li-ion batteries, vanadium redox flow batteries, and pumped hydropower storage.

A study in the journal Energy & Environmental Science  focuses on thermal energy storage. According to the authors, “Buildings consume most of the world’s electricity, and as much as 50 percent of their consumption goes toward meeting thermal loads.” Since, as the Joule study indicated, thermal storage is one of the least-cost options, I thought it would be worthwhile to look at it in detail.

First of all, the authors point to the limited life and safety issues of Li-ion batteries as barriers to their use for heating buildings. There is therefore a detailed discussion of how to best provide thermal energy to buildings using renewable energy with non-lithium storage.

The researchers divide buildings’ electrical loads into thermal and non-thermal (for example, lighting and electronics). They assume that with 100 percent solar energy, excess generation would be available during the daytime and with 100 percent wind energy, excess generation is available during the evening and nighttime. Their research finds that all of the typical non-thermal loads can be directly sourced from a combination of solar and wind without storage.

The researchers conclude that “to really make a dent in greenhouse gas emissions, we will need to electrify building heating using renewable heating.” And the best way to achieve that is to use heat pumps, combined with thermal storage.

They found that electrochemical (battery) storage is a more costly way to provide sustainable heating and cooling than thermal. With batteries, the inputs and outputs are both electrical — the electrical output drives the compressor in the heat pump to provide the thermal output. However, with thermal storage, while the input is electric, the output is directly thermal. And for new construction, the thermal output can even be embedded in the walls, a construction that takes advantage of a building’s thermal inertia to smooth out temperature fluctuations.

As I mentioned in my previous blog, the thermal storage medium could be water in an electric heater, or water directly heated by the sun to be stored in an insulated tank for future use. Or it could be a geothermal heat pump , which doesn’t depend on sunlight because it uses the earth as the storage medium. And there are other techniques being studied such as heated gravel or sand.

Another problem with increasing reliance on variable renewable energy sources is that sudden changes in load can cause the AC frequency to shift, which can be extremely detrimental. Generally, this is handled by the inertia of spinning generators that continue to produce output for the few seconds required for mechanical relays to shift grid connections. However, although renewables have no inertia, storage systems like pumped hydroelectricity, compressed air, and liquid air  do.

I am not suggesting there is no role for Li-ion batteries, but I also think it’s important to consider alternatives for different applications, such as some that I have sampled here.