Increased efficiency through flexible design and operation of a heat pump system in paper drying
The use of heat pumps in industrial processes offers great potential for increasing efficiency. Often, high temperature lifts are required leading to a low Coefficient of Performance (COP) and thus high operating costs. Additionally, requirements on the heat supply (e.g. steam pressure) may change depending on the product (e.g. paper type). To reduce operating costs, achieving the highest possible COP at every operating point is crucial. Consequently, a flexible design of the heat pump system can be essential to ensure economical operation. So-called cylinder dryers are often used in paper manufacturing. In these dryers, the energy required for drying is supplied by steam-heated cylinders over which the paper web is transported. This process produces moist exhaust air with a high relative humidity of nearly 100%. A heat pump can be integrated in these processes to utilize the humid exhaust air to provide steam for the steam cylinders. In these drying systems, the steam requirements (pressure and mass flow) and the available waste heat (temperature, mass flow, and moisture content) generally vary depending on the actual product. If a heat pump is used, the heat pump system should be designed in a way that it can be operated as efficiently as possible at different operating points. In this contribution, different heat pump system designs are compared with each other based on typical operating points of a paper machine. The results are compared in terms of energy saving potential. Moreover, it will be shown how flexible operation in response to electricity prices can affect economic efficiency of the system. For this analysis, a second technology, biomass boilers, is considered for periods with high electricity prices. The aim is to show that both a flexible design and flexible operation of a heat pump system offer advantages for economic feasibility.