The Next Frontier: Energy Storage

by Paul Greenwalt, EIT

This article is part of Wood Harbinger’s newsletter series.

We have made great strides toward reducing energy consumption and increasing energy efficiency in building equipment and systems. Revisions to energy codes and building standards, metrics like LEED, and the economic incentive of energy costs savings have helped forge this path. At the same time, electricity-driven technology will continue to become more ubiquitous in our day-to-day lives and more pervasive on a global scale, as populations grow and developing countries become more technological. As a result, our overall energy usage will rise. To maintain the gains that we’ve made toward more efficient use of the Earth’s resources, we’ll need to use alternative energy generation sources like solar, wind, tidal, and geothermal more widely.

The downside to some of these energy sources is their reliability—solar energy can only be harvested when the sun is above the horizon. There’s also a supply and demand component to consider with solar—supply (during the day) doesn’t always line up with demand (residentially, more heavily required after sundown). Wind can only generate energy when it’s active. Tidal energy can be generated around the clock with great predictability, but is only viable where there are bodies of water.

Energy storage is the key to maximizing widespread and reliable alternative energy use adoption. More specifically, high-density energy storage. The ubiquity of computers has increased as size has decreased, and the size decrease is directly related to advancements in the density of data storage. The more energy we can store in a smaller (and cheaper) medium, the more we will be able to rely on alternative energy sources.

The Obvious Option: Batteries

What’s the first thing that comes to mind when you think of an energy storage technology? I’m going to bet it’s batteries. Battery tech has come a long way in terms of storage density. Lithium ion batteries have an energy density much greater than lead-acid—90 to 190 watt-hours per kilogram (Wh/kg), depending on the catalyst, for lithium ion, compared to 30-50 Wh/kg for lead-acid. And the costs are starting to come down for larger scale building applications. Devices like Tesla’s Powerwall, Sonnen’s sonnenBatterie, Schneider Electric’s EcoBlade, and others are providing options while creating market competition.

There are limitations to using batteries. While lithium-ion batteries may be more efficient than lead-acid batteries, the technology still has a long way to go before its energy density can compete with a fuel, like natural gas, to supply energy for building systems. There are some new options on the horizon that may increase battery density, such as graphene batteries. Graphene batteries (also called graphene supercapacitors) are not yet to the point of mass production, so not yet a real option for consideration.

You’d need a lot of batteries to store enough energy on large scale, so it can be cost and space prohibitive. Current iterations of building energy storage batteries don’t have a terribly long lifespan, and battery development and disposal isn’t the most sustainable practice, so couple this with limited lifespan and the overall sustainability of your system degrades.

Fire and Ice: Molten Salt and Frozen Water

 I was first introduced to the concept of molten salt as an energy storage medium while researching my Senior Synthesis paper at Seattle University. It is used in tandem with a solar energy concentrator system. Mirror arrays concentrate the sun’s energy on a single focal point, creating temperatures that reach upwards of 1,000 degrees F. A fluid is passed through this focal point until it heats up and is then run through a heat exchanger to generate steam from water. The expansion of water during its phase change from liquid to steam creates high pressure and is piped to a steam turbine, which spins and powers a generator, creating electricity. When there is an excess of electricity generated, the heat energy from the collection system can be diverted to a storage system. The heat can be transferred to molten salt, stored in a highly insulated container, and can continue being used to create solar-generated energy even after sunset. SolarReserve’s Crescent Dune Solar Energy Plant in Nevada uses molten salt as their storage medium.

Ice storage is a similar concept to molten salt, just on the opposite end of the thermometer. It also takes advantage of phase change, capturing a greater amount of energy as liquid water changes to solid ice than just the temperature change alone. Again, one can store the ice in a highly insulated container and use the energy as demand requires.

Thermal Mass for Radiant Heating and Cooling

Thermal mass refers to a material’s ability to gain and retain heat. Concrete, tiles, rock, and adobe plaster are examples of high, thermal-mass materials. Thermal mass can be used as storage for solar energy—gaining heat during the day and dissipating it at night through radiant heating. Thermal mass is good for radiant cooling as well. If the mass is chilled or kept away from a heat source (say, in the shade out of direct sunlight), it will dissipate its chill as the heat from the air transfers into it. This is why you often see tile and adobe plaster used extensively in hot climates. Interior concrete walls, columns, and floors can be used for radiant heating or cooling.

As a means of mass scale energy storage to make alternative energy sources more reliable, it’s not the most effective. But it can be cost effective for use with energy from the grid or a utility district that utilizes a varying rate schedule, based on demand. For example, our local utilities Puget Sound Energy and Seattle City Light offer lower overnight rates for energy, when demand is low. Using a thermal-mass system, you can heat or chill water during the night, when energy costs are reduced, and store it in pipe loops in floors, walls, or columns. The energy will radiate into the space during the day, like ice cubes in a glass or a radiant space heater. Pretty cool! Or hot!

Out on a Limb: Artificial Photosynthesis for Energy Generation and Storage

My colleague Matt Woo first discussed the bleeding-edge concept of artificial photosynthesis in his article, “A Tour of Solar (Energy) Systems,” in March 2016. The goal is to create a device that can mimic the photosynthesis process of plants, using sunlight to produce energy in the form of hydrogen. Since the publication of Matt’s article, more breakthroughs in this potentially game- changing technology have been made.

Harvard research scientist David Nocera, who debuted a prototype “artificial leaf” in 2011, published new results with colleague Pamela Silver in June 2016 using a bio-engineered microorganism bacteria that converts hydrogen and carbon dioxide into liquid fuel. Their process has a 10% efficiency rate using pure carbon dioxide, and 3-4% efficiency with carbon dioxide from the air. The efficiency rate is based on what percentage of the sun’s energy is converted to fuel. For comparison, natural photosynthesis is about 1% efficient.

The benefit of Nocera and Silver’s new approach, say other scientists, is its integration of the chemistry and biology fields to produce new achievements in the energy field. Overall, using photosynthesis to produce a fuel rather than direct electricity opens the storage options to using existing, viable methods that are also being improved.

Exciting stuff!

Alternative energy generation and storage is an exciting arena of innovation that we’re watching closely as it will be a game changer in the built environment. Some methods are readily available today and others are still in the development stages. As with all innovations, cost effectiveness will drive adoption. The other question will be how to deploy these new technologies on a large scale. Innovations in energy storage will play a pivotal role in positioning us for a stable energy future.