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Fighting Air Conditioning's Peak Demand With Thermal Energy Storage

As we head into the hottest part of summer in the Northern Hemisphere, in what could be the hottest year on record, we barely give a thought to what using air conditioning does to our electricity grid. And what it will do to a hotter world in the future.

According to a recent Department of Energy report, the world is gearing up to install 1.6 billion new air conditioners by 2050 (see figure). While air conditioning is a common luxury in the United States, in most other countries it is not that common. As these countries become wealthier and grow in population, they will install large amounts of air conditioning. Air conditioner sales are already rising in India, Brazil and Indonesia by about 15% per year.

Of course this is a good thing. Air conditioning is essential for overall health and well-being and actually saves lives during extreme weather. However, air conditioning is the main contributor to peak electricity demand in the summer. During extremely hot days or heat waves, this jump in energy demand leads to blackouts and brownouts, keenly felt in the most populated parts of the country. Energy prices also jump during these times where supply and demand combine to as much as quadruple the costs.

Although the energy use is dependent on a region’s climate, a central air conditioner will generally run 3 to 7 months of the year, using between 3 and 5 kW of power every hour for around 9 hours a day during the hotter months. This averages to about 3,000 kWhs per year. So 1.6 billion new air conditioners will require an additional 4.8 trillion kWhs of electricity every year – the output of 400 new GenIII nuclear plants, 1000 new coal plants or a million wind turbines.

Unfortunately, much of this electricity will be concentrated during peak hours, necessitating the use of peaker plants. Peaker plants operate only to make up the difference between base load and peak load. Our current energy grid makes these plants necessary, but they come at a steep price. Peaker plants are usually natural gas or coal, but can also use jet fuel, oil and diesel.

Peakers usually run for short periods of time and so are less efficient and dirtier than conventional base load fossil fuel plants. Though natural gas is certainly cleaner than coal, natural gas does have some environmental issues from fracking, to pipelines, to fugitive emissions of methane. And lots of CO2 emissions, if you care about that.

The commercial sector accounts for about a third of the total energy consumption in America and, according to data from the EIA, air conditioning use makes up 10 to 15 percent of that total consumption. In some southern regions, cooling can make up 25 percent of energy use in a large building.

To combat this hot strain on the grid, commercial buildings can take advantage of thermal energy storage. In fact, they must take advantage of this strategy or we will have some major problems in the coming decades.

Thermal energy storage allows cooling to be created at night, when there is excess capacity from the utility, and stored for use the next day during peak times. During off-peak hours, cold water, or even ice, is made using this energy and stored inside some type of energy storage tank. The stored ice is then used to cool the building’s occupants the next day. Alternatively, the facility may run on energy storage or just the chiller depending on cooling demands or the availability of onsite renewable resources such as solar and local tariffs. This provides greater flexibility to smarter and zero energy buildings. Thermal storage systems have the ability to store renewable energy like wind, which mainly blows at night.

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