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Blowing Hot, Cold, and Green

Finding ways to heat and cool residential and commercial buildings while maintaining environmental and climate concerns is a balancing act that many contractors must deal with. Architects design; engineers, academics, and scientists research and develop; contractors build; and consumers hope the end results will be efficient and affordable.

And the research goes on. With about 12% of the total global energy demand coming from heating and cooling homes and businesses, the field is both dynamic and fluid. And fluid is a good word for it as a new study suggests that using underground water to maintain comfortable temperatures could reduce consumption of natural gas and electricity in this sector by 40% in the U.S. alone.

The approach, called ATES (aquifer thermal energy storage), could also help prevent blackouts caused by high power demand during extreme weather events. As the grid absorbs greater amounts of electricity generated by solar and wind, the go-to approach for energy storage has been batteries but researchers at Lawrence Berkeley National Laboratory experimented with geothermal energy storage because heating and cooling is such a predominant part of the energy demand for buildings.

In a study funded by the Dept. of Energy’s Geothermal Technologies Office, the authors found that with ATES, a huge amount of energy can be stored, and it can be stored for a long period of time. As a result, the heating and cooling energy demand during extreme hot or cold periods can be met without adding an additional burden on the grid, making urban energy infrastructure more resilient.

After building a comprehensive technological and economic simulation of an energy system, the authors found that ATES is a compelling option for heating and cooling energy storage that, alongside other technologies such as batteries, could help end our reliance on fossil fuel-derived backup power and enable a fully renewable grid.

Putting thermodynamics to work

ATES is a concept that leverages the heat-absorbing property of water and the natural geological features of the planet. You just pump water up from existing underground reservoirs and heat it at the surface in the summer with environmental heat or excess energy from solar, or any time of the year with wind. Then you pump it back down. 

The Earth is a pretty good insulator, so then when you pull the water up in the winter, months later, that water will still be hotter than the ambient air and it can use it to heat buildings. Or you can pull up water and let it cool and then put it back down in the earth and store it until you need cooling during hot summer months. It’s a way of storing energy as temperature underground.

ATES is not yet widely used in the U.S., though it is gaining recognition internationally, most notably in the Netherlands. One major perk is that these systems get “free” thermal energy from seasonal temperature changes, which can be bolstered by the addition of artificial heating and cooling generated by electricity. As such, they perform very well in areas with large seasonal fluctuations, but have the potential to work anywhere, so long as there is wind or solar to hook up to. 

ATES systems are designed to avoid impinging upon critical drinking water resources – often the water used is from deeper aquifers than the drinking water supply – and do not introduce any chemicals into the water.

To get some data on how much energy ATES could save on the U.S. grid, and how much it would cost to deploy, the researchers designed a case study using a computational model of a neighborhood in Chicago. This virtual neighborhood was composed of 58 two-story, single-family residences with typical residential heating and cooling that were hooked up to a simulation of an energy grid with multiple possible energy sources and storage options, including ATES. Future climate projections were used to understand how much of the neighborhood’s total energy budget is taken up by heating and cooling demands currently, and how this might change in the future.

Finally, a microgrid simulation was designed for the neighborhood that included renewable energy technologies and ATES to evaluate the techno-economic feasibility and climate resilience. The results showed that adding ATES to the grid could reduce consumption of petroleum products by up to 40%, though it would cost 15-20% more than existing energy storage technologies.   

ATES does not need above ground space compared with tank-based water or ice storage systems. ATES is also more efficient and can scale up for large community cooling or heating compared with traditional geothermal heat pump systems that rely on heat transfer with the underground soil.

Making lemonade

Another major benefit of ATES is that it will become more efficient as weather becomes more extreme in the coming years due to climate change. The hotter summers and harsher winters predicted by the world’s leading climate models will have many downsides, but one upside is that they could supercharge the amount of free thermal energy that can be stored with ATES.

ATES will also make the future grid more resilient to outages caused by high power demands during heat waves – which happen quite often these days in many high-population U.S. areas, including Chicago – because ATES-driven cooling uses far less electricity than air conditioners, it only needs enough power to pump the water around.

And other government laboratories are working on that aspect as well. A tool developed by Oak Ridge National Laboratory researchers give building owners, equipment manufacturers, and installers an easy way to calculate the cost savings of a heating and cooling system that utilizes geothermal energy and emits no carbon. Ground source heat pumps, or GSHPs, operate with a heat exchanger that extracts heat from the ground in winter and serves as a heat sink in summer to provide cooling.

ORNL’s free, web-based application identifies the benefits and implementation costs for GSHP installation in existing U.S. buildings. Users can modify utility prices for electricity, water, and natural gas. A techno-economic analysis is provided in simple charts.

And water isn’t the only natural way to provide comfort to building occupants. Scientists have reported, at a conference of The American Chemical Society, an eco-friendly alternative — a plant-based film that gets cooler when exposed to sunlight and comes in a variety of textures and bright, iridescent colors. The material could someday keep buildings, cars, and other structures cool without requiring external power.

The PDRC (passive daytime radiative cooling) is the ability of a surface to emit its own heat into space without it being absorbed by the air or atmosphere. The result is a surface that, without using any electrical power, can become several degrees colder than the air around it. When used on buildings or other structures, materials that promote this effect can help limit the use of air conditioning and other power-intensive cooling methods.

Some paints and films currently in development can achieve PDRC, but most of them are white or have a mirrored finish. A building owner who wanted to use a blue-colored PDRC paint would be out of luck — colored pigments, by definition, absorb specific wavelengths of sunlight and only reflect the colors we see, causing undesirable warming effects in the process.

However, there’s a way to achieve color without the use of pigments. Soap bubbles, for example, show a prism of different colors on their surfaces. These colors result from the way light interacts with differing thicknesses of the bubble’s film, a phenomenon called structural color. Following up on this approach, CNC (cellulose nanocrystals), which are derived from the cellulose found in plants, could be made into iridescent, colorful films without any added pigment.

Because of the underlying photonic nanostructure, the film selectively reflects visible light resulting in intense, fade-resistant coloration, while maintaining a low solar absorption (~3%).  Additionally, a high emission within the mid-infrared atmospheric window (>90%) allows for significant radiative heat loss. By coating such CNC films onto a highly scattering, porous EC (ethylcellulose) base layer, any sunlight that penetrates the CNC layer is backscattered by the EC layer below, achieving broadband solar reflection and vibrant structural color simultaneously.

Researchers experimented with films that were vibrant blue, green, and red colors that, when placed under sunlight, were on average about 7o F cooler than the surrounding air. A square meter of the film generated over 120 Watts of cooling power, rivaling many types of residential air conditioners. The most challenging aspect of this research was finding a way to make the two layers stick together — on their own, the CNC films were brittle, and the ethyl cellulose layer had to be plasma-treated to get good adhesion. The result, however, was films that were robust and could be prepared several meters at a time in a standard manufacturing line.

The researchers now plan to find ways they can make their films even more functional. CNC materials can be used as sensors to detect environmental pollutants or weather changes, which could be useful if combined with the cooling power of their CNC-ethyl cellulose films. For example, a cobalt-colored PDRC on a building façade in a car-dense, urban area could someday keep the building cool and incorporate detectors that would alert officials to higher levels of smog-causing molecules in the air.

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