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By 2050, more than 970 cities will experience average summer temperatures of 35 °C (95 °F). Cities already consume about 78% of the world’s energy, and account for more than 60% of global greenhouse gas emissions according to the United Nations. Yet cities are also playing a vital role in developing, implementing and overseeing clean energy transition policies, as highlighted in the report Building Sustainable Cities, published by consultancy firm PWC.
Excess heat is increasingly viewed as the world's largest untapped energy source, according to many experts. Energy harvesting is part of the solution required for urban sustainability and climate resilience. According to this renewable energy specialist, “By tapping into the abundant energy sources present in cities, from pedestrian footfall to vehicular motion, we can transform urban landscapes into vibrant hubs of renewable energy production.”
Energy harvesting is the process of capturing energy from a system's environment and converting it into usable electric power. The global energy harvesting systems market is projected to grow at 10,3% a year between now and 2031 when it will reach USD 1 billion with smart buildings and infrastructure expected to register the highest compound annual growth rate (CAGR) in that period.
Here are some of the innovative technologies and collaborative efforts governments and municipal authorities can harness for a more resilient urban future.
Walking on sunshine
Across the European Union’s 27 member states, there are around 60 billion square metres of available roof space and a similar amount of currently unused surfaces on buildings. This is considered prime real estate for energy harvesting. While photovoltaic panels are the main means of energy capture from roofs, a variety of other materials and methods are being investigated to harvest the near-infrared (NIR) solar radiation from façades.
The EU funded ENVISION project based in the Netherlands is developing four technologies, including solar heat collectors based on NIR absorbing coloured coatings. The energy collected is used for heat pumps. This technology has been trialled at a school gym hall in the Netherlands.
Smart ventilated glass could also be used to absorb NIR solar radiation with the energy stored or used directly to heat inside the building. Trials using a combination of technologies have taken place in Eindhoven and Helmond (Netherlands), in Austria at the factory of a major glass manufacturer and at the University of Genoa (Italy).
Streets and pavements to harness electricity
The kinetic energy from roads and pavement surfaces in urban areas can also be converted into electricity. According to a UK-based specialist, kinetic energy recovery systems (KERS) technology is particularly effective in environments with heavy traffic, such as intersections and parking lots, where vehicles frequently decelerate and accelerate.
Beneath the surface of pavements on London's Bird Street and Dupont Circle, Washington DC, for example, lie a network of sensors and generators that capture the pressure and movement caused by walking. As individuals traverse these pavements, their footsteps cause slight deflections, activating the piezoelectric materials that produce electricity to power nearby street lighting.
These energy harvesting technologies require international standards to be used safely and efficiently and that’s where the work of the IEC comes in. The IEC 62830-1 series, prepared by the IEC technical committee which develops standards for semiconductor devices, includes methods for evaluating the performance of vibration-based piezoelectric energy harvesting devices.
Standards for piezoelectric technology are developed by IEC TC 49, which addresses piezoelectric, dielectric and electrostatic devices. This includes IEC TS 61994-5, which gives the terms and definitions for sensors, intended for manufacturing piezoelectric elements, cells, modules and the systems.
Photovoltaic modules are also being developed for pavements, roads and other parts of urban infrastructure. Having tested their design in Shanghai and 255 other Chinese cities, researchers in Hong Kong found that electricity potential could range from 0,70 kWh/W to 1,83 kWh/W. A Spanish startup has developed panels for locations such as hotel terraces, office buildings, sports centres, bike paths and will be ready for installation later this year.
IEC TC 82 develops standards for photovoltaic energy systems and is working on a publication which measures the flexural strength of crystalline silicon photovoltaic cells.
Wastewater heat recovery
Billions of litres of heated water end up in the sewers every day of which the heat content is completely wasted. When this heated water is discharged from dwellings all year round, its temperature is in the range of 25 to 30 °C and by the time it reaches wastewater treatment plants, it’s normally between 12 to 10 °C.
The same research also found that the average temperature of sewage wastewater remains relatively steady throughout the year. That means heat pumps continue to operate efficiently even on cold days when heat demand is highest – providing a constant source of renewable energy.
An increasing number of municipalities are harnessing this form of excess heat to help decarbonize their energy use. A neighbourhood of Vancouver, Canada, is one of them.
Here, heat is captured from sewage at around 20 °C before it reaches the local treatment plant. Heat pumps concentrate that heat to produce hot water which can be as high as 80 °C.
According to the project’s manager, the heat recovery system operates at efficiencies of over 300%, meaning that for every unit of electricity used to run the heat pump, it returns three units of thermal energy.
An 8,3 km thermal grid distributes the recovered heat back to the district's 6 210 apartments. In each building, heat exchangers transfer heat from the water system into the buildings' heat system and domestic hot water pipes.
The IEC prepares standards for heat pumps, including thermoelectric devices. IEC 60335‑2‑40 specifies safety requirements for electrical heat pumps.
Other wastewater heat recovery projects are underway around the world, including in Norway and in Denmark where the potential in excess heat from wastewater treatment plants is calculated to heat 20% of all households in a country of 5,9 million. A similar system in Beijing also produces biogas from digested sludge – which is in turn used to fuel public transport networks.
All these intelligent ways of harvesting power could very well become the norm in our future smart cities, as they attempt to reduce heat and become more sustainable. More than ever, international standards are required to pave the way forward.
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