Waste Heat Recovery

How can the waste heat be used sensibly?

This is a question that many companies ask - and it's often not an easy one to answer. Many technologies are available for the utilization of waste heat, here is an exemplary selection:

• Heat exchangers
• Heat pumps
• Organic Rankine Cycles

In this project, the focus was on direct heat recovery by means of heat exchangers. Although technically the simplest case, the problem often turns out to be extremely complex in practice.

Evolved structures complicate the analysis

In most cases, the optimization of waste heat recovery is done for an existing plant. This is also the case here. Therefore, one encounters existing heat exchanger networks, fixed process sequences, various types of equipment and usually incomplete data acquisition.

In the first project phase, it is therefore extremely important to get an overview and to assess the heating and cooling requirements of all processes. The most important tool here is the Pinch Analysis, which summarizes the total demand of the plant in so-called composite curves.

Here, it is irrelevant whether the various processes are already interconnected or how the heat or cooling is provided. The existing structures are masked out, so to speak.  

Planning of a new heat exchanger network

The next step is the actual engineering. Based on the pinch analysis, a new heat exchanger network is planned in order to realize the most sensible and cost-effective waste heat recovery possible. Of course, the existing heat exchangers, pipelines and storage facilities must be taken into account.

Although there are heuristics for planning, in the end the local conditions always require individual adjustments, so that a lot of experience and know-how are required. The result is usually several variants for waste heat utilization. Here, it is ultimately an entrepreneurial decision to what degree the heat recovery should take place. In most cases, the heat recovery is not technically but economically limited.

Simulation creates confidence

Is the concept for waste heat recovery really mature? And can all operating conditions be met with the planned heat exchanger network?

Such questions can best be answered with a simulation. Especially in the case of dynamic load changes, storage integration and for the design of controllers, you can hardly get around a system simulation in the final project phase.

We normally use Modelica and our in-house TIL Suite for simulation. Standard components such as heat exchangers, hot water storage tanks, piping, valves, and controllers can thus be quickly combined to form a complete system.

By means of simulation, any operating situation can then be calculated, and the efficiency of waste heat recovery can be analyzed.

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