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The Energy Storage Device We Haven't Considered: Our Homes

Dr. Marissa  Hummon's picture
Senior Energy Scientist Tendril

Marissa Hummon is a senior energy scientist with Tendril, a provider of customer-facing software to the energy industry, based in Boulder, Colorado.Previously, she spent five years at the...

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  • Oct 20, 2016

In the first quarter of 2016, grid-interactive energy storage installations surged by 127 percent year-over-year, according to GTM Research.

It was no accident.

With solar becoming firmly settled into the consumer market, its early adopter population saturated and its prices down to generally mainstream levels, storage is the next frontier for energy providers. Utilities are particularly interested: 40 percent of energy executive respondents to the 2016 Utility Dive State of the Electric Utility survey said deploying distributed energy storage was a new revenue stream they will pursue, and 65 percent said storage was an opportunity in which they should invest more.

But the storage market is where solar was a few years ago—the tools may be there (think Tesla Powerwall) but they’re not familiar or accessible to the average consumer. In fact, GTM found that despite all of the interest, only approximately 450 US homeowners installed batteries to store electricity last year.  The early adopters, the ones “generally unconcerned with system economics,” writes Brett Simon at GTM,[1] are jumping on board, but the typical residential utility customer living on a budget or concerned mainly with their home’s comfort level isn’t quite there yet.

One thing utilities and their customers could do is wait. They could wait for storage technology—electric batteries specifically—to evolve a bit more, for prices to come down, for the market to be ready.

Or, all utilities and nearly all residential utility customers could begin to dabble in storage right now, saving money and power, all while using a battery that’s already at their fingertips: the home.

The Home as a Battery: an introduction to thermal mass

There are 140 million homes in the United States, which means, in theory, that there are 140 million batteries up and running and ready to store energy.

The home works as a battery by storing thermal energy. That energy is stored in the thermal mass, which is the stuff in the building - furniture, flooring, etc. Dense materials like concrete and brick have high thermal mass, while lightweight materials like wood have low thermal mass. The thermal mass of a house can store energy because of the insulation that separates the outside environment from the inside environment.

When harnessed effectively, thermal mass can help moderate the internal temperatures of a home to reduce cooling bills in the summer and lower heating bills in the winter. During the warm months, the thermal mass of a well-designed home can store cool air overnight. Later in the day, as the outdoor temperature rises, the cooled thermal mass keeps the indoor temperature comfortable.  In the cooler winter months, thermal mass can have the opposite effect: the home gathers and absorbs heat from the sun during the day, releasing that heat into the home at night. The thermal mass of the home works like a rechargeable battery, charging itself with heat from the sun, holding that charge until the home needs it, then releasing the heat out of or into the home depending on the time of year. This process repeats each day.

Thermal Mass and HVAC in Concert

For the home to perform as an effective battery, both in terms of cost and comfort, thermal mass can’t work alone. Its ability to function depends on coordinated use of the home’s HVAC system. Because modern homes have insulation (as they must) and other design elements in play, temperature regulation depends on much more than outside air temperatures. On the hottest days of summer a cool breeze at night isn’t enough to counteract a 100+ degree day, even in a home with high thermal mass and good insulation; the home requires air conditioning to properly cool down. Likewise, in the winter, reaching 32 degrees on a sunny day won’t provide enough heat to warm a home after a below-freezing night, no matter the thermal mass of the structure; a heater still has to contribute.

An optimized relationship—one that provides the most comfort at the lowest cost and highest energy efficiency—between a home’s HVAC system and its thermal mass depends on finely-tuned data analytics. The right technology will calculate the thermal mass and the susceptibility of that thermal mass to the outside environment and then determine the best way to run the home’s HVAC system. It can program the system to precool the home in the summer, turning on the air conditioning in the morning before temperatures rise. Then, the system will rely on the thermal mass of the home to keep the heat out and trap the cool air in through the hottest hours of the day. Conversely, in the winter, the software can program the heater to run during the day, enabling the thermal mass of the home to absorb the heat and hold on to it, releasing it back into the rooms during the chilly night. By running the HVAC system at off-peak times and letting thermal mass do the work of temperature regulation during peak times, such technology ensures residents stay comfortable at the lowest possible energy cost.

In addition to maximizing customers’ comfort and efficiency, this software-enabled connection between thermal mass and HVAC benefits utilities. In taking thermal mass into account it leverages a resource that’s already there, built into the homes utilities already service. There’s no investment in expensive batteries required, and utilities can meet their regulatory demands while improving customer satisfaction at the same time by guaranteeing comfort at a lower cost.

Beyond Storage to Demand Management

Using the home as a storage device paves the way to revolutionize current approaches to demand side management. Traditional top-down methods in which utilities alert customers of peak-time events leave customers powerless, literally and figuratively: customers end up in situations that compromise their comfort or even their safety—losing air conditioning during a suffocating heat-wave is no small sacrifice. With thermal mass in play, enabling the home to act as a battery, the possibility exists for demand management without inconvenient demand response events. Harnessed intelligently, thermal mass can help homes realize continuous demand management through constant micro-adjustments that happen so subtly they never disturb the home’s occupants yet they let utilities manage load cost effectively and efficiently.

Putting thermal mass into action for load management is a complex process that requires sophisticated technology. But the potential is there, the opportunity is large and the cost is affordable. As data analytics continue to make the grid and the home smarter, thermal mass will take on a larger role in the energy orchestra, influencing energy management on multiple levels. Stay tuned.


Dr. Marissa  Hummon's picture
Thank Dr. Marissa for the Post!
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