Waste Heat Recovery

During industrial processes, an estimated 20 to 50% of the supplied energy is lost, i.e., by dumping the exhaust gas into the environment [1]. The waste heat losses and the potential work output based on different processes including but not limited to the ones shown in Figure 1. Does it REALLY have to be thrown away? Sometimes yes, other times no. In this blog post, we will focus on the “no” through a process  called “Waste Heat Recovery”.

Waste heat losses and work potential of different process exhaust gases - Image 1
Figure 1: Waste heat losses and work potential of different process exhaust gases [US Department of Energy [2]]
Some well-known examples of waste heat recovery processes are found in turbochargers in cars or a heat recovery steam generator. One simple structure of application is when a heat exchanger is fed with the exhaust gas of a turbine, therefore being cooled down before being released into the air. This heat exchanger is part of a secondary (bottoming) cycle where another turbine provides additional power output without having to burn additional fuel. This heat exchanger is part of a secondary cycle where another turbine provides additional power output.

Advantages and Disadvantages

Waste heat recovery has its advantages and disadvantages. One benefit is the efficiency is increased by extracting more energy from the fluid than without this recovery. Another plus is the decrease of air pollution, because the exhaust gas has a lower temperature and therefore the impact on the atmosphere is mitigated [3].

The disadvantages depend more on the scale of the primary (top) cycle. Due to the added components, the cost for the entire setup increases (along with the maintenance costs). On smaller systems, the sum of the initial and maintenance costs can be more than the value of energy savings from the recovered heat. It is therefore important to evaluate if recovering this waste heat is financially viable [4].

Nevertheless, the design a waste heat recovery cycle can be difficult. Difficulties include  accurately modeling real fluid properties, designing the turbine, selecting the working fluid, evaluating back-work ratio, complying with current and potentially future environmental regulations, etc. There are several fluids which can be used but each have different effects on the turbomachines design and their efficiency. Utilizing software programs such as AxCYCLE™ can make it much easier. Using AxCYCLE™ in combination with the AxSTREAM ION™ can automate your design process and help you select the most efficient and cost-effective turbine design along with the best working fluid for your application.  This can be further extended to the design of the pump and the parametrization of the heat exchangers.

The first step in the automated process is to build the investigated recovery cycle with its boundary conditions in AxCYCLE™. This functions as a blueprint, shown in Figure 2.

Figure 2: ORC Recovery Cycle

Figure 2 shows a generalized recovery cycle which takes the exhaust conditions of the waste heat source (left side of the heat exchanger) as fixed input parameters. The recovery cycle consists of a heat exchanger, a pump, a condenser, a turbine, and a generator. When we have a successful calculated the blueprint, the next step is to setup the AxSTREAM ION™ scheme to integrate, streamline and automate the process presented above. Figure 3 shows the corresponding workflow in AxSTREAM ION™.

Waste Heat Recovery in ION
Figure 3: ION Scheme

As shown in Figure 3, the scheme consists of processes to set input values, assign data, and has several loops. One of these loops deals with the iterations required to match the turbine efficiency values between AxCYCLE™ and the automated turbine design in AxSTREAM® based on the cycle boundary conditions. When these values match, the next step is to obtain the cycle performance for the selected fluid. A new calculation and turbine design is started for each desired fluid. The procedure is finished when all fluids have been evaluated. We tested three fluids for this example. The results of one study are presented in Figure 4.

Working Fluid Results
Figure 4: Results

The results show that R245fa delivers the highest efficiency for the examined recovery cycle of the three fluids investigated.

To sum things up, waste heat recovery systems are a very useful addition to a primary cycle. They can increase the overall efficiency and reduce the environmental impact. However, finding the right turbine and the right fluid can be time consuming when done manually. So, we went through the process of automating this procedure with AxCYCLE™ and AxSTREAM ION™. By combining the two programs, it is easy to generate an automation loop which will extract the cycle efficiency of different fluids, and speed up your overall design process. Once you have your preliminary design of the turbine (and other considered elements) for each fluid, it will give you insights regarding cost, equipment footprint, etc.

Are you working with waste heat recovery cycles? Are you interested in automating your design process? Reach out to us at info@softinway.com to learn more about AxCYCLE™, AxSTREAM ION™ and more tools to expedite your workflow.

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References:

  1. https://www.energy.gov/eere/amo/articles/waste-heat-recovery-resource-page
  2. https://www.energy.gov/sites/prod/files/2015/02/f19/QTR%20Ch8%20-%20Waste%20Heat%20Recovery%20TA%20Feb-13-2015.pdf
  3. https://www.zgindustrialboiler.com/news/i/advantages-and-disadvantages-of-waste-hea.html
  4. https://www.tlv.com/global/US/steam-theory/waste-heat-recovery.html

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