Heat recovery – finding the next energy-saving opportunities

Traditional heat recovery systems in heating, ventilation and air conditioning have been servicing us well, however, they do come with limitations dictated by the laws of physics. While we've managed to achieve high efficiency in ventilation heat recovery through systems like thermal wheels, cross-flow heat exchangers, and run-around coils, there’s a sense that we've nearly maximised their potential. As engineers, however, we find ways to push the boundaries.

As we look to the low carbon future, we see the next frontier in heat recovery lies in tapping into other overlooked sources of waste heat. This evolution is already underway, with applications expanding into areas such as data centers and metro systems. And there are even more opportunities on the horizon in industrial processes, sewage systems, and commercial buildings.

Heat recovery in data centers and metro systems

One of the most promising areas for heat recovery is data centres. These facilities, which are critical to our digital infrastructure, generate significant amounts of heat through their operations. Forward-thinking companies are already repurposing this waste heat for district heating systems, providing a dual benefit of energy efficiency and sustainability.

A notable example is the Odense Data Center in Denmark, operated by Meta (formerly Facebook), where process heat is captured and redirected to the local district heating network. This approach not only reduces the energy footprint of the data centre but also provides a reliable heat source for the community.

But data centres are not the only urban infrastructure ripe for heat recovery. Metro systems, particularly older ones like the London Tube, were historically designed with minimal ventilation, let alone heat recovery. However, even relatively modern systems, such as the Warsaw Metro, which commissioned its first line just 30 years ago and has a second line currently under construction, are facing increasing heat challenges.

As trains operate, the brakes and electrical engines generate heat, which accumulates in the underground environment, exacerbating discomfort during the summer months. To address this issue, there is growing interest in implementing heat recovery systems based on air source heat pumps (ASHP) in metro systems. Recently, the Warsaw Metro signed an intention letter with Veolia, the city's district heating provider, to explore such solutions. Cundall, as a consultancy involved in the project, is participating in the tender process, and we have a keen interest in the technical details of this experimental approach.

The Warsaw Metro presents a particularly intriguing case. Most of its stations were constructed using the "cut and cover" method, leaving ample space above the tracks for installing ASHP systems. Furthermore, Poland’s widespread district heating infrastructure means that integrating these systems into the broader heating network should be relatively straightforward.

Industrial process heat recovery

The industrial sector, especially in Poland, faces significant decarbonisation challenges. External factors, such as the EU Green Deal, rising energy costs, and the geopolitical impact of the war in Ukraine, are driving a shift toward more sustainable practices. This includes investing in systems that can recover as much waste heat as possible.

In the past, efforts focused on lowering heating system supply temperatures to accommodate technologies like condensing gas boilers. However, the next step in this evolution is to eliminate gas boilers altogether and transition to even lower temperature, all-electric systems. This shift would enable the use of ground, air, or water source heat pumps, in conjunction with advanced heat recovery systems.

One of the critical areas for improvement lies in the recovery of process heat energy released during industrial operations, heat which often goes to waste. By capturing this heat and redirecting it back into the production process or into district heating networks, industries can significantly reduce their carbon footprints and improve energy efficiency. The potential for process heat recovery is vast, especially in energy-intensive industries like manufacturing, where high temperatures are a byproduct of essential operations.

Sewage heat recovery: an untapped resource

Sewage heat recovery is an area with enormous potential that remains largely unexplored in commercial applications. However, large-scale projects offer a tempting opportunity to investigate this technology further. One of the first implementations of sewage heat pumps is set to occur in Wrocław’s sewage system, which could serve as a model for other cities and large infrastructure projects.

The concept of sewage heat recovery involves extracting heat from wastewater as it leaves buildings. Given that wastewater temperatures are relatively stable year-round, this provides a consistent and reliable heat source. By using heat exchangers and heat pumps, this otherwise wasted energy can be captured and used to preheat water in district heating systems or even directly supply heat to nearby buildings.

An exciting application of this technology could be in the Central Communication Port (CPK) project - a new airport near Warsaw, which is initially planned to serve 32 million passengers per year. The sheer scale of such a project presents an ideal opportunity to implement sewage heat recovery on a large scale, turning what is typically seen as waste into a valuable resource.

Commercial buildings: challenges and opportunities

Commercial buildings, while traditionally slower to adopt unconventional heat recovery systems, represent a significant area for future growth. Currently, the trend in commercial architecture is to incorporate more passive systems, such as natural ventilation, while reducing heat gains through thoughtful façade design and limiting glazing. These strategies help mitigate the need for extensive mechanical cooling, thereby lowering energy consumption.

However, there are still areas within commercial buildings where heat recovery systems could make a substantial impact. One example is the typical tenant fit-out spaces, often equipped with direct expansion (DX) systems for cooling. These systems, designed to operate independently 24/7 due to the steady heat gains from IT equipment, are traditionally not connected to the building's central HVAC system, which typically includes heat recovery. As buildings move towards using heat pumps as their primary source for heating and cooling, it becomes increasingly beneficial to integrate all spaces into a unified system that includes heat recovery.

The cost and technical challenges associated with installing systems like sewage heat recovery in commercial buildings can be perceived as barriers. However, as technology advances and the pressure to reduce carbon emissions increases, we can expect to see more innovative solutions being developed to overcome these obstacles. For example, modular heat recovery units that can be retrofitted into existing buildings without significant disruption could become a game-changer for the industry.

Hidden energy: turning waste into power

The future of heat recovery lies in exploring innovative applications across various sectors, expanding beyond traditional systems. As we advance towards a more sustainable and energy-efficient world, the potential for heat recovery to make a significant impact is immense. Capturing heat from diverse sources such as data centers, metro systems, industrial processes, sewage systems, or commercial buildings presents numerous opportunities for energy savings and sustainability. By integrating these systems into our existing infrastructure, we not only reduce energy consumption but also contribute significantly to climate change mitigation.