WASTE HEAT – THE OVERLOOKED RESOURCE WAITING TO BE HARNESSED

In an era of energy innovation, it’s almost paradoxical that vast amounts of heat generated by urban infrastructure and industrial processes continue to go to waste. Supermarkets, data centers, large office buildings, subway stations, and even wastewater treatment plants constantly release valuable thermal energy into the atmosphere. Yet, despite the growing focus on sustainability, this untapped heat escapes largely unnoticed, dissipating into the air.

By Oddgeir Gudmundsson, Director, Climate Solutions, Danfoss A/S, Nordborg
Jan Eric Thorsen, Director, Climate Solutions, Danfoss A/S, Nordborg

Published in Hot Cool, edition no. 6/2024 | ISSN 0904 9681 |

Given the variety and sheer scale of waste heat sources surrounding us, a simple question arises: Why aren’t we capturing and repurposing this abundant energy resource? The potential is enormous, and the energy efficiency and sustainability benefits are equally compelling. The challenge now lies in recognizing waste heat as the asset it truly is and turning it into a cornerstone of our energy systems.

Why do we not capture the abundantly available waste heat and use it for something useful?

The answer to that question can vary depending on whom is being asked, but in many cases, the answer will be in the following directions:

1. It is not economical compared to using normal high-quality energy carriers, such as natural gas, coal, electricity, etc.
2. Waste heat recovery is not our business.
3. Once we have modernized our processes there will be so little waste heat that it is not worth capturing it.
4. There are no options to deliver it to anyone who can use it.
5. It is too complicated to capture it!
6. No need; decarbonized electricity is just around the corner…

Of course, there can be some merit in some of the answers, particularly if basing it on the past and turning a blind eye to the future.
But let’s think about them!

1) High-quality energy carriers, such as natural gas, coal, electricity, etc.

Clearly, this would represent a closed and ignorant mindset, as those readily available fossil fuels are not a long-term viable option. It would even be simple to argue that if their real cost to society were included, they would not have been a viable option for a long, long time.

Once decarbonization really takes off, it will become clear that utilizing waste heat sources will become essential for enabling an ecologically and economically sustainable transition of the energy system.

2) Waste heat recovery is not our business.

The traditional approach is, as the statement indicates, not to bother about the waste heat and treat it as an unavoidable waste product. Historically, there have been no monetary benefits from capturing and reusing the waste heat due to the low cost of primary fuels during the era of fossil fuels, and thus, there was no basis for taking up the business.

With the changing energy landscape and phase-out of fossil fuels, the focus on waste heat utilization is obvious. Nonetheless, the primary challenge is intact: waste heat utilization is not a core business of manufacturing companies. To facilitate waste heat utilization, creating new and motivating business models is necessary. 

Appropriate business models can, on the one hand, create the foundation for the waste heat owners to integrate waste heat recovery into their production processes and, on the other hand, provide the foundation for establishing specialized waste heat recovery companies that can bridge the gap between the waste heat owners and eventual heat users, for example, district heating utilities.

Typically, the business setup involves the waste heat owner, who can be considered a heat supplier or a cooling customer, and a counterpart, who can be either a heat customer or a cooling supplier. Where district heating networks are present, the utility is the obvious customer. 

Depending on the business model, the district heating utility could either buy the heat for a substitution price, which could reflect the lowest cost of an alternative heat source, or sell cooling to the waste heat owner, which could reflect the alternative cost of dissipating the waste heat. The choice of the business model will depend on various factors, such as waste heat temperature, accessibility, availability, investing entity, etc. 

A common denominator for waste heat is that the cost of the waste heat will be low, as it will always be measured against the lowest-cost alternative heat generation, which can either be existing or planned heat sources.

Due to the wide variety of waste heat sources and district heating system setups, the business opportunity should be investigated in each case to ensure long-term viability and minimum risks for all partners.

Today, many concrete examples of reusing waste heat in district heating systems exist. The learning from these examples provides a strong basis for adapting regulations and reporting obligations to ensure simple and favorable conditions for utilizing this important low-carbon resource.

3) Once we have modernized our processes, there will be so little waste heat that it is not worth capturing.

Everyone should embrace this statement and jump on the energy efficiency train. However, all processes have unavoidable waste heat—the last step should always be heat recovery for secondary usage!

The first step should always be to energy-optimize processes. The second should be to reuse as much waste heat as possible internally. The third should be to export/sell the remaining waste heat for other purposes. After all, if companies do not ensure that they have an economic operation, someone else will eventually outperform them and potentially put them out of business.

It is important to note that waste heat recovery is not only viable for large-capacity sources. Heat recovery from supermarkets is a perfect example of how even small-scale waste heat recovery is economically attractive. 

A great example of successful small-scale heat recovery can be found in many Danish supermarkets. The supermarket heat recovery cases clearly show that it makes good sense to capture even a small amount of waste heat and make it available for others to use .

4) There are no options to deliver it to anyone who can use it.

Yes, this is true in many cases, and it is complex and even time-dependent!
We would, however, argue that this is still a poor excuse, as one should expect that initiatives like mandatory heat planning and a requirement to assess the feasibility of district energy systems in urban areas in the European Union will be the norm around the globe in the future.

With proper heat planning, current and future local heat sources and heat demands are mapped to create a long-term holistic approach to fulfilling local thermal demands. As heat planning becomes more widely adopted, new opportunities will arise.

Opportunities can emerge either through establishing city-wide district energy systems or through area zoning and local microgrids, where smart city planners designate areas for industrial processes with low-temperature waste heat next to areas with industries with low-temperature heat demands.

While waste heat has a great potential for fulfilling building space heating and hot water demands other use cases could be onshore fish farming, greenhouses, spa facilities, etc.

5) It is too complicated to capture it!

Fortunately, the notion that heat recovery is complicated is, in most instances, wrong!
While it can be difficult to retrofit new means to capture the waste heat in existing constructions, whether in a tightly optimized industry production hall, an old subway station, or elsewhere, it might not be necessary.

In most industry processes, existing cooling systems ensure that the industry process is running smoothly and that products are produced with consistent quality. In essence, this means that the waste heat is already being captured; it is just not used for anything. 

In other instances, the waste heat is dissipated directly into the air, particularly if it has really high temperatures. Such examples can be found in metal smelting operations. Capturing such waste heat is a bit more complex, but even in these cases, there are commercially available methods.

Similarly, in any commercial cooling system, whether in a large building, supermarket, or data center, there is no way around having a chiller system that generates the required cooling. Again, this means that the waste heat from commercial operations that require cooling will be available in a central location.
In other instances, such as subway stations, the ventilation systems can be retrofitted with simple air-to-water heat exchangers to capture the waste heat.

The bottom line is that waste heat recovery is typically not complicated from a technical point of view. The complexities are generally on the organization side, where it can be narrowed down to the lack of knowledge of the potential value of the waste heat, complicated local regulations, unfavorable tax regulations, or complex reporting and documentation requirements.

F) No need; decarbonized electricity is just around the corner…

Which corner and at what cost?
The fact is that while electrification and decarbonized electricity are very big parts of the future solution, it truly matters how electrification takes place. It can be made sustainable and cost-effective with a minimal ecological footprint, or it can be made with a major ecological footprint and high cost.

Smart electrification is to utilize all available resources to minimize the final electricity demand, and by this, the investment in the generation and distribution capacity is also minimized. In this regard, waste heat sources and other renewable, low-temperature sources are immensely important. Another important aspect is ensuring as much decoupling of the electricity demand and generation as possible. 

In fact, decoupling is the number one measure for enabling sustainable and cost-effective electricity generation based on fluctuation renewables. This is where district heating and district cooling come into play, as no other technology can make a cost-effective decoupling of the supply and demand. 

The key features here are cost-efficient large-scale thermal storage, which come at a fractional cost compared to alternative energy storage, such as battery storage.

Heat recovery applications

Clearly, simply capturing the waste heat is not enough; it will also need to be made available to the waste heat user at the right temperature level.

Depending on the temperature level of the captured waste heat relative to the temperature requirement of the waste heat users, different temperature adaption options are available. These can be 1) direct heat exchange via heat exchanger, 2) a combination of a heat exchanger and a heat booster unit, could be a heat pump, or 3) a heat pump.

The following section will present the general heat recovery options with district heating in mind.

Case 1: Direct heat exchange via heat exchanger

In case the temperature of the waste heat is high enough to directly fulfill the temperature requirement of the district heating system, a simple heat exchanger solution should be applied:

Figure 1. An example of simple direct waste heat recovery using heat exchangers.

Case 2: Combination of direct heat exchange and a temperature booster

If the waste heat temperature exceeds the return temperature from the district heating system, the return flow should be preheated via a heat exchanger and subsequently boosted by a temperature booster. The temperature booster could either be a heat pump or any type of heat source, like a fuel boiler or direct electrical heater. The choice of the boosting application can depend on, for example, the required return temperature to the process or available waste heat capacity.

Heat pump booster application:

Figure 2. An example of a combination of direct waste heat recovery using heat exchangers and further temperature boosting using a heat pump.

Boiler booster application:

Figure 3. An example of a combination of direct waste heat recovery using heat exchangers and further temperature boosting using a fuel or electric boiler.

The boiler could, for example, be an existing boiler on the waste heat source premise, where typically, a smaller share of the capacity is boosting the recovered waste heat temperature.

Case 3: Heat pump boosting

Even if the waste heat temperature is lower than the return temperature of the district heating system, it can be hugely beneficial to the district heating system, as the waste heat source can ensure the high efficiency of the heat pump. 

Further, as waste heat sources tend to have stable temperature levels over the year, it will ensure that even during winter, the heat pump will have stable and predictable operations, unlike heat pumps that rely on ambient heat sources, air, or water sources.

Figure 4. An example of a low-temperature waste heat recovery using a heat pump.

Conclusions

Waste heat is a valuable resource that should not be wasted; it’s there and can be utilized!

District heating systems are key solutions for urban areas to take advantage of available waste heat sources and, by that, make our cities more sustainable, livable, and resilient.

Further, recycling waste heat reduces pollution, retains money in the local community, and creates local jobs.

And finally, it’s the basis for realizing that the future renewable and sector-coupled energy system will be smart and cost-efficient.

We have no excuse, as all necessary solutions are already commercially available!