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Research and Briefing

New Renewable Energy Sources for Net-Zero-Carbon Cities


Net-zero-carbon buildings and developments must tap into novel sources of heat and power, such as sewer networks, metros, or even coal mines filled with water. In the seventh edition of our construction magazine Building Sight, we look at how the construction industry is tackling environmental issues and aiding countries to reduce emissions and meet ambitious targets.

A new model for net zero is emerging. Energy-intensive buildings or developments in urban areas need to step outside the traditional site boundary and find new sources of local renewable energy.

“Ensuring sustainable, reliable energy sources is one of the biggest challenges our cities face, particularly as we look to incorporate more smart technology,” says Blair Chalmers, director, Marsh & McLennan Insights. “Local, renewable energy will become a very important piece in that jigsaw.”

Over 60 global entities, including 6 states and regions, 26 cities, and 31 businesses, have signed up to the World Green Building Council’s, Net Zero Buildings Commitment, promising to cut operating emissions in all buildings under their direct control to net zero by 2030, and to advocate that all buildings become net zero by 2050.

The voluntary standard aims to maximize the chances of limiting global warming to below 2 degrees, and ideally below 1.5 degrees, in line with the Paris Agreement.

Harnessing Waste Heat

Reliance on natural gas for heating is a major source of CO2 emissions. Global fossil CO2 emissions were projected to rise by 0.6% in 2019, according to the Global Carbon Project, despite falls in coal use because natural gas and oil consumption are growing.

This could be cut significantly by exploiting abundant natural sources of heat in rivers and lakes, or from metro tunnels or commercial and industrial facilities that discharge warm water.

“There’s a need to take a more holistic view of infrastructure, particularly on large mixed-use sites,” says Duncan Price, director of sustainability at engineering consultancy Buro Happold.

“In terms of heating, it means moving away from high temperature gas combined heat and power systems [that have become staple on projects in Europe over the past decade], to lower temperature heat networks with secondary heat sources, such as waste heat from metros or sewage systems.”

Developments that harness waste heat this way already exist.

The recently completed Museum of Bavarian History in Regensburg, Germany, incorporates a structure that can extract up to 70 liters of sewage per second from a main sewage collector.

Coarse solids are removed and the clarified water is pumped to two heat exchangers; these operate in combination with a heat pump to each produce 280 kilowatt for heating, or 500 kilowatt for cooling in the summer.

The London Borough of Islington is upgrading its district heating network at Bunhill, which serves heat to seven local sites, to recover energy from the London Underground via a heat pump.

Underground metros produce hot air when trains brake or accelerate; this can be harnessed to heat buildings when combined with other warm air in the tunnel and heat radiating from the ground.

Until recently, engineers have struggled to accurately calculate how much heat can be transferred from metros, but researchers at Switzerland’s Ecole Polytechnique Fédérale de Lausanne (EPFL) found a way in 2019 of doing just that.

Modeling Lausanne’s planned M3 metro line, engineers at EPFL found that fitting a heat-recovery system along 50%-60% of the route could produce enough heating for 1,500 standard 80-square-meters apartments, in the process cutting the city’s CO2 emissions by two million tons per year.

In Japan, the focus is on hydrogen, which produces zero CO2 at the time of combustion.

The Tokyo 2020 Olympic Games (currently scheduled to take place in 2021) will be a key demonstrator for the technology, using hydrogen produced by renewable electricity generated in Fukushima to power the athletes’ village.

Other R&D is exploring novel concepts for energy storage required to capture heat or power, and make it available at times of peak demand.

Channa Karunaratne, regional director for energy within the Sustainable Development Group at Aecom, says: “Eventually, we will have truly integratable development-scale energy storage systems. In the UK, the Coal Authority is looking at storing hot water in old disused mines across the country. The cavernous spaces would act as a massive thermal sink.”

The need to look beyond the site boundary to access novel energy resources is a tough proposition. It could increase project costs and result in elevated risks, delays, or contractual issues.

The risks are technical, commercial, contractual, and policy-based, says Price.

“There is the commercial issue of who pays for the time and effort to make the interconnectors and how to guarantee that energy is provided at a reasonable cost. It is necessary to look at who underwrites projects — is it the local authority running a municipal energy company, or a commercial operator such as an energy company?” adds Price.

“There is lots of richness and complexity with the potential to unleash a lot of value, but developers will need the right advice to get projects through to financial close.”