Drying is a complex and energetically intensive process accounting for at least 15% of industrial energy usage and even higher in some specific sectors: 30-55% in medicinal plants processing, 50% in textile manufacturing and up to 70% in wood industry. Heat pump assisted dryers are known as efficient heat recovery devices due to the potential of saving up to 50% or more of the primary energy used. During the last few decades a lot of academic research was performed on heat pump dryers (HPDs) to reduce energy consumption. However, reliable industrial applications are still exceptional. HPDs are much more complex than each of their components separately. Hence, theoretical and experimental analyses of the overall system must consider the complex interaction between the drying process and the HP cycle.
Usually, the dryer exhaust air serves as the heat source for the evaporator of the heat pump (HP). After cooling and dehumidification, the working air can (partially) be recirculated and heated up to a higher temperature level in the condenser. In case of completely closed air-cycle advantages such as the avoidance of fire hazard, annoying odour or environmental load of contaminants can be exploited. Moreover, other gases than air can be applied in order to avoid the drawbacks of oxygen: explosion risk or product degradation. Another promising possibility is to utilize the condensate, but in some cases additional costs due to the need of cleaning before discharge could be caused.
The main target of this project is to integrate HPs into different industrial drying processes in order to exploit the corresponding advantages by energetic and environmental optimization. The focus is on convective dryers because they are used most frequently and can be most conveniently modified to integrate HPs. By adapting HPs to operate under varying temperature levels (by using fluid mixtures, hybrid and multi-stage HPs, HPs with variable pressure ratio etc.) and developing innovative control systems, HPs could better follow the drying process. Thus the average efficiency of the HPs would increase substantially.
As a first step, a state-of-the-art study will be performed and existing industrial applications will be summarized. Existing technology, used in other fields, e.g. refrigeration systems or geothermal HPs, will be investigated to transfer to industrial HPDs. Special attention will be dedicated to process integration, operation control and automation of HPDs. In order to accelerate the development of industrial HPDs further, set-up and testing of lab scale and pilot-plant HPDs will be carried out. This will provide both proof of concept and reliable experimental data to validate the theoretical propositions. It will also help students in understanding and experimenting with this innovative technology. Listing up branch-specific technical/economic conditions and limiting (environmental) factors forms another essential part of this project. A number of industrial case-studies will also be performed for the branches represented by the participating SMEs, for which valorization is the purpose. Finally, substantial attention will be directed to dissemination in various industry branches and academia to accelerate future implementations.
As drying is crucial in various production systems, the project results are expected to be welcomed among a broad spectrum of industrial branches proving it to be a trans-sectorial project. In the project consortium, RTOs with a strong track record on HP and/or drying technology are brought together with branch oriented SMEs having practical knowledge of the production methods and specific technological/economic/ecological prerequisites in the industrial sectors. Fields associated with wood, textile, agricultural product processing, brick and tiles as well as biotechnology are represented by German RTOs. Within the Flemish User Group, additional branches are represented by participating companies.