Data centers are some of the biggest energy guzzlers in the world. Buildings that house the hardware and software required to run cloud applications use as much as 1-2% of global energy demand. This figure is expected to zoom to 20% or more by 2025, even after accounting for improvements in energy efficiency.

Almost 100% of this energy is dissipated as heat, requiring the use of expensive and energy-intensive cooling systems that further contribute to data centers’ increasing carbon footprint.

Given the inevitability of building more data centers as computing needs grow, sustainability experts and energy companies have for years been trying to recycle and put the (typically wasted) heat generated by data centers to good use.

Around the globe, Amazon and other big tech companies have begun experimenting with waste heat recycling at their data centers. In Seattle, an Amazon campus is being heated by waste heat released by a neighboring Westin data center, through giant cooling towers. In Odense, Denmark, where temperatures go as low as negative 1 degree Celsius in the winter, Meta’s 50,000-square meter data center provides heat energy to a district heating system, which is then redistributed to residents. Microsoft is running a similar initiative in Finland, as well.

And in Ireland, energy efficiency agency Codema is partnering with an Amazon data center in Tallaght to capture the waste heat they emit and use it to heat homes and council buildings. The initiative is part of the Tallaght District Heating Scheme (TDHS), the first large-scale district heating network of its kind in Ireland.

Construction kicked off in May 2021, and the new system has been in operation since early 2022. Months later, experts and locals have differing opinions on the energy efficiency, financial implications and climate change impact of the project.

“There’s a lot of data centers in Ireland – that’s a huge amount of waste heat,” says former Dublin mayor and city councilor Andrew Montague, who spearheaded some of the city’s sustainability initiatives including a public bicycle rental program. “If we were to use that for district heating, we wouldn’t have to insulate homes and buildings. That’s a big deal because it saves billions of euros that would otherwise be spent on insulating homes.”

Ireland’s 70 data centers use up as much as 14% of all metered electricity –far higher than what all rural Ireland homes use, put together. That indicates any data recycling initiatives would prevent some energy waste and make substantial amounts of energy available to use.

A paper authored by John O’ Shea, an energy systems analyst who heads the Tallaght district heating project at Codema, suggests that energy derived from unconventional waste heat sources, including data centers and other heat-emitting operations, could supply as much as 10% of the EU’s total energy demand for heat and hot water.

The heat captured from these data centers goes to a district heating system that distributes heat to buildings and homes through a network of insulated underground pipelines. As part of Codema’s project, this heat is being used in the South Dublin County Council buildings and the TU Dublin-Tallaght campus. There are also plans to expand the project to provide heating in low-cost housing units.

But the overall ‘green-ness’ of these initiatives may not be as significant as it appears. Experts express concerns over the efficiency of the process (only a percentage of the heat released actually makes its way to homes), and worry about the additional electricity required to transport this heat.

Anthony Robinson has concerns over the system using heat pumps, which require additional electricity. He is an associate professor of mechanical, manufacturing and biomedical engineering at Trinity College Dublin, and has over 20 years of experience in heat transfer, fluid mechanics and applied energy research.

Since captured heat from data centers only reaches temperatures of 35 to 40 degree Celsius, which is insufficient for space heating, heat pumps are often used to bring temperatures up to 60 degree Celsius or higher. This raises the amount of electricity being used, Robinson notes.

“For every unit of electricity that you put in, you might end up with 2 or 3 units of heat going into the homes. So it’s a feasible idea. But what we don’t want to do is use more electricity,” he says.

“My opinion as an academic is that this is not the future,” Robinson says. “It’s an example of what can be done and it is positive in the sense that it shows that there was a willingness to do it. But my opinion is that this is not the solution that we’re going to see 10 or 15 or 20 years from now.”

Robinson’s other concern is that transporting heat through air, as in this initiative, is vastly inefficient and results in large heat losses, because “air isn’t a very good medium for transporting heat.” He argues that systems using water to transport this heat could end up being 300% more efficient.

Even though waste heat recycling might not be as efficient as it needs to be, experts say it has immense potential in offsetting the carbon footprint of data centers, and is a solution other countries could benefit from – especially cold ones. With advancements in research and processes, more efficient ways of capturing this heat and putting it to use could emerge.

Ireland is among the biggest carbon emitters in the European Union, and with mean winter temperatures as low as 4 degrees Celsius, the energy used to heat homes and businesses makes up about a third of the country’s carbon emissions.

Recycling waste heat from data centers and other by-products of electricity usage could account for 3,579 GWh of heat per annum and heat up to 1.6 million homes. This would also cut down on the heat used by data centers to cool their systems, and offset part of carbon emitted by data centers, which accounts for 16% of total carbon emissions.

District heating systems work best in densely populated areas, and are especially viable in buildings requiring larger amounts of heat, such as swimming pools, greenhouses and swimming pools. The closer these buildings are to the data center, and to each other, the more viable the heating system.

Codema estimates that about 70% of ‘small areas’ in Ireland are viable for this form of heating. But this doesn’t work so well in rural and sparsely populated areas.

“Once you get outside the city, it’s harder to create a network of pipes that reach everybody’s homes,” says Montague. “When people are living in a more dispersed area in small villages, or even isolated houses, it becomes a more difficult proposition.”

An investment analysis of waste heat from data centers indicated that the process of reusing waste heat can provide a positive return on investment to companies, and is financially viable.

However, this heat doesn’t necessarily come free for citizens. Under the Tallaght District Heating Scheme, this heat is only being used in local governmental buildings such as the council headquarters at County Hall, the local library and TU Dublin’s Tallaght campus. The city council plans to extend this to 133 affordable apartments and an innovation center to be built on public land in Tallaght.

The scheme will also eventually serve other homes, and can provide heating to up to 2,000 to 3,000 apartments over its lifetime. This heat, while not free, is expected to be available at the same price as gas, but could come cheaper than fossil fuels in the future.

“It would likely be a lot cheaper than the normal heating system that we have now. And what’s more it wouldn’t be subject to the ups and downs of the price of gas or the price of oil,” says Montague.

The district heating system providing low-cost heat could also do away with the need for retrofitting, the process of updating a home to make it more energy efficient.

Montague says that though retrofitting was once seen as the key to reducing emissions in Ireland, this new method of reusing data center heat is a “a very viable way of providing heat and it would save huge amounts of money in retrofitting, especially within the city of Dublin and presumably within other cities in Ireland.”

Retrofitting is an expensive proposition, costing as much as 50,000 euros for a single house. And although it brings down heating costs, a heating system is still required. The bigger the house, the more it costs to retrofit it. Under the National Housing Retrofit Scheme, Irish residents have access to government grants and funding to cover part of the cost of retrofitting.

However, critics point to inequities in the system.

“It meant that the people who had the biggest houses (often the wealthiest) were getting the most money to help them with their homes, so it wasn’t a very equitable system but it was seen as the only way to achieve our carbon emissions targets,” Montague says. The new system of district heating, using waste heat from data centers and other sources, would not depend on the size of one’s home.

“Everybody would get their heating, and it will be a much fairer system,” he says.