During manufacturing, a lot of waste heat is generated: thermal energy that is released into the environment without any practical use. However, recent technological advancements have made capturing and reusing waste heat more feasible – for generating thermal and electrical power, for example. If you work in an industry where thermal processes are part of the key activities, you have probably heard of waste heat recovery. However, many organizations still don’t use its full potential, especially when it comes to low-grade heat recovery. In this article we will show you how heat recovery can help you save energy, lower GHG emissions, and reduce operating costs.
What is industrial waste heat?
Waste heat is heat from industrial processes that is released into the environment without any practical use. Depending on your industry, this heat can be carried away by warm water, steam, combustion products, or in other forms. It is estimated that between 20 to 50% of industrial energy input is lost as waste heat.
In the Netherlands, for instance, industries that generate the largest amount of waste heat are:
- Chemical and petrochemical
- Iron and steel
- Food and textile
- Non-metallic minerals
Why you should consider waste heat recovery
Waste heat recovery is widely used in industry – and for good reason. The idea behind waste heat recovery is to capture heat created in one part of your facility and to reuse it in other applications. It’s one of the easiest and most cost-effective ways to boost the overall efficiency of your industrial processes. This comes with some big advantages:
You save energy and lower GHG emissions
Saving energy and lowering greenhouse gas emissions are becoming increasingly important. First of all, it is a well known fact that our world has limited resources and that we are facing a climate crisis. Therefore, using resources efficiently should be a priority for any business. Even if it’s just to prevent public backlash. It can also help you attract talented employees, since younger generations are increasingly concerned about their impact on the world. Regional regulations, which you may need to comply with, are another good reason to make saving energy and lowering emissions a priority.
You reduce operating costs
By recovering waste heat, you can improve the overall efficiency of your industrial processes. You will be able to create more output, with the same amount of resources. In some cases, the recovered heat allows for replacing other heat sources altogether. It may also eliminate the need for an additional waste heat treatment facility. And lastly, recovering waste heat can help you qualify for governmental incentives and subsidies. All these advantages combined can greatly impact your bottom line.
How waste heat is classified
Waste heat media are usually classified based on temperature
A large majority of waste heat media are classified as Low-Grade Waste Heat (LGWH). It is often the by-product of processes in the chemical, petrochemical, food, and textile industries. Compared to waste heat of higher temperatures, recovering low-grade heat is far more challenging, since heat transfer is driven by temperature differences between the source and the recipient. Nevertheless, this situation may change soon thanks to the continuous development of state-of-the-art heat recovery techniques.
The main sources of low-grade waste heat
Identifying waste heat sources is a crucial step towards a successful heat recovery effort. The following list provides several sources of low-grade waste heat found in various industries. If any of these are present in your industrial activities, it could be worthwhile to further investigate the possibilities for low-grade waste heat recovery.
- Wastewater from heat exchangers (≈ 60°C)
- Condensed water from drying machines (80-90 °C)
- Hot oil waste from cooking processes (100 – 175 °C)
- Wastewater from cleaning processes (≈ 60°C)
- Exhaust steam from cooking with fryers or ovens (150 – 200 °C)
- Exhaust air from drying with spray/rotary dryers (110 – 160 °C)
- Hot air discharged from clinker coolers during cement production (100°C)
How waste heat can be recovered
The selection of a suitable waste heat recovery approach mainly depends on two aspects: the waste heat sources and the potential users. The heat in hot, moist air, leaving an industrial dryer machine, can be recovered directly using a heat exchanger. Then, the recovered heat can be supplied into an absorption chiller to cool down the product after leaving the dryer.
When it comes to temperature you can assume that waste heat media:
- with a high temperature is directly usable or exchangeable
- with a medium temperature may need a heat upgrade
- with a low temperature always needs a heat upgrade
Not all heat can be repurposed. High-grade waste heat usually provides more energy when repurposed than low-grade waste heat. But new technological advances are making low-grade heat recovery increasingly possible.
How to recover low-grade waste heat
The possibilities for low-grade waste heat recovery are quite new, but lately some very promising technologies have entered the market. Some examples:
Heat upgrade or storage using adsorption techniques
Adsorption refers to the adhesion of molecules (typically in gaseous/liquid state) onto a surface of an adsorber. This process can be found, for instance, in the adsorption of water molecules onto silica gel beads, which are widely used for moisture control in product packaging. The adsorption process is generally exothermic, implying that heat is released to the surroundings, as vapor molecules saturate the adsorber surface. The opposite is also true, as the adsorber can be returned to its original state (regenerate) when it is heated.
- Simultaneous dehumidification and heat upgrade can be utilized in, for example, a drying process
- Dry adsorber can be used as a heat storage medium
- Water (vapor) or ammonia + adsorber (e.g. zeolites, silica gel, activated carbons)
- New adsorber materials are still under active development
- Simple setup with a minimal number of moving parts
- Adsorber bed can be adapted for long-term energy storage
- When applied in a closed system, the adsorber bed requires minimum maintenance
- Lower energy loss due to irreversibility since the adsorber does not need to circulate
- High scalability potential
Heat upgrade using absorption techniques
Absorption is a process in which molecules of a compound are dissolved into the volume of another, resulting in a change in concentration. For certain absorbent – absorber pairs, the absorption phenomenon also generates heat, which can be utilized for thermal processes. Lately, the absorption process is being employed in heat pumps, where it replaces the traditional mechanical compressor.
- Absorption heat pump can be used for recycling waste heat with low additional energy consumption
- A large temperature upgrade can be achieved using an absorption heat transformer
- Water + soluble compound (e.g. LiBr or phosphate salt)
- Hydrocarbon-based pairs are still under active development
- Simple setup with a minimal number of moving parts
- The system can be operated continuously
- Energy input to the solvent pump is insignificant compared to the classical vapor-compression system
- Using a suitable absorber-refrigerant pair, the temperature boost at the absorber can be significant
- High scalability potential
Heat conversion (Organic Rankine cycle)
Rankine cycle is a thermodynamic process for generating mechanical or electrical energy. This cycle has been adapted to utilize organic compound as a refrigerant. This allows it to operate at lower temperatures, enabling the usage of waste heat.
- An Organic Rankine Cycle (ORC) generator can be used to produce electricity from low-grade waste heat
- The shaft power produced by an ORC generator can be used to assist the operation of the compressor of another heat pump for heat upgrade
Organic compound, e.g. butane, heptane, etc.
- Compatible with low-grade waste heat, as the ORC generally does not require superheating
- Organic compound does not condense inside the turbine/expander
- Low turbine inlet temperature, which is beneficial for longevity
- ORC generators are relatively easy to be made compact
The best solution for your situation
The answers to the following questions will help you to select the best waste heat recovery system for your situation:
What kind of heat are you dealing with?
These variables will have an impact on how you build your waste heat recovery system:
- The temperature of the waste heat
- The quantity of the waste heat
- When and where the waste heat is available
Sometimes the combination of these variables could mean you can’t recover your waste heat, but often there’s great potential.
How will you use the recovered waste heat?
Before you choose the best solution for you, you’ll need to think about how you will use the recovered waste heat. Where does the heat need to go?
How do I know if it’s worth the investment?
Everything we told you in this article may sound great, but there’s probably one question lingering in your head:
Is this worth the investment?
That’s an important question, but unfortunately it’s not one we can answer for you in this article. The payback time depends on a lot of factors, such as:
- The lifecycle costs of the heat recovery system
- The lifetime of the heat recovery system
- The efficiency of the heat recovery system
- The potential savings in energy usage and needed equipment
- The governmental incentives and subsidies you’d qualify for
Every situation is different and we’d be happy to look into yours. Using measurements and models, we’re able to determine the energy balance of your industrial processes and predict the (positive) impact of energy recovery. We’ll make sure you have a clear idea of the potential ROI, before you invest in a waste heat recovery solution.
How we can help
1. Site Survey
We use measurements and our unique air technical modeling software (developed in-house) to get a clear view of your current situation. This way, you’ll know exactly where improvements can be made.
2. Identify possibilities & predict impact
Using the results of the site survey, we’ll determine the energy balance of your industrial processes and predict the (positive) impact of energy recovery. How could you use the recovered energy? And how will that impact your bottom line?
3. Integration of a customized waste heat recovery solution
We handle everything: the design, the building, and the implementation of the solution.
Find out how much energy and money you could save using waste heat recovery
Is waste heat a significant concern for your operations? Get in touch with us and we’ll help you find out exactly how your business could benefit from waste heat recovery.