Waste heat recovery

Situation

Interest in reducing energy consumption and all its resulting consequences is constantly evolving. Among the possible options, heat dissipated from processes represents a great opportunity. Indeed, it is not uncommon for production activities to have the need to dispose of process heat, an operation that, depending on the case, may require the consumption of additional energy and materials. Recovering this heat allows for covering part of the energy needs related to the activity that produces it, as well as ancillary activities such as, for example, office heating or domestic hot water.

Objective

To effectively exploit recovery heat, two conditions are necessary: the presence of a thermal user and the temperature compatibility of the recovered heat with the specific requirements. The PCM thermal battery guarantees high flexibility in the recovery process because it is able to store heat at a well-defined temperature, close to that of the source, to be reused when necessary; the physical principle of phase change, with energy release at a constant temperature, allows maintaining the high useful heat content (exergy) supplied to the user.

Some examples of the use of recovered heat are: preheating in the process or activity that generates the waste heat, efficient valorization of the recovered heat using a heat pump (also for high temperatures), space heating, domestic hot water, and covering peak thermal demands.

Design

The first step towards implementing a recovery system is the quantification of available heat: it is therefore necessary to evaluate the thermal flows involved and their usage profile in order to determine the amount of energy that can be stored and the times when it needs to be used. Subsequently, the fraction of recoverable heat relative to the total is evaluated, depending on the available thermal level, the process characteristics, and the recovery method.

At the end of these evaluations, it is possible to proceed with the sizing of the thermal storage battery, the material to be used, and the operating schedule.

Proposal and expected results

The case study involves a plant that produces mechanical parts through hot stamping, with air cooling of the pieces at the end of the process.

Following the design phases, it is estimated that the heat available at the end of the entire plant's processing exceeds 2 MWh per day; only a portion of this can actually be recovered. To quantify this value, it is necessary to define how the heat will be recovered and estimate productivity based on the thermodynamic characteristics of the system.

High-temperature objects are stored inside metal bins; therefore, to avoid heavily revising the current production process, the bins are enclosed in an insulated metal box containing air/water heat exchangers. Following experimental tests conducted directly at the plant, thermal recovery proved promising, allowing for the collection of information on the technical specifications to be adopted for the final version.