Session: 06-02 Power to Heat Solutions

Paper Number: 82920

82920 - High Temperature Heat Pump Cascade Systems With Water (R718) as Refrigerant 

The heat demand of industrial processes is still mostly covered by fossil fuels, especially at temperatures above 160°C. The installation of high temperature heat pumps (HTHP) that can operate at these temperatures is an effective way to reduce industrial CO₂-emissions and primary energy consumption. The German Aerospace Center (DLR) develops HTHP, one of which operates with water as refrigerant and uses its high critical point, evaporation enthalpy and environmental sustainability.

However, using water as refrigerant also requires a waste heat source of fairly high temperature, since any heat source below 100°C would result in sub-atmospheric evaporation, leading to more expensive equipment. If low temperature heat sources or even ambient heat should be utilized, the HTHP would consist of two coupled heat pumps, a cascade system.

In this study such a cascade system is investigated using the process modelling software EBSILON Professional. Since steam compressors have a limited compression ratio (compared to e.g. air compressors) multiple compression stages are necessary to achieve the desired temperature lift of up to 200 K. The HTHP in this study consists of up to 6 compression stages, 4 stages in the top-HP using water and 2 stages in the bottom-HP using R1336mzz(Z). In order to increase the coefficient of performance (COP) an interstage-cooling with spray injection is also included in the top-HP. The required number of compression stages  depends on the overall temperature lift and is automatically calculated using an advanced software tool that is described in the paper.

Changing the heat source as well as the heat sink temperatures in a parameter study we concluded, that for any combination of both a maximal COP can be found. This maximum is highly dependent on the temperature at which the bottom-HP transfers the heat to the top-HP, the coupling temperature. For R1336mzz(Z) the optimal coupling temperatures are usually between 90°C-130°C. The impact of additional parameters, like the maximal achievable stage compression ratio at the cycle configuration with maximum COP values and for different heat source and heat sink temperatures, are investigated in further parameter studies. The actual values of the COP are varying greatly dependent on the investigated temperature lift range. But even for sink temperatures above 200°C and source temperatures below 50°C COP’s of at least 2 are achievable.

In conclusion the combination of a water-based heat-pump with a suitable low-temperature refrigerant can greatly expand the applicability of the former towards heat sources of lower temperature and helps to avoid the use of vacuum equipment.

Presenting Author: Robert Hegner German Aerospace Center (DLR)

Presenting Author Biography: - Study of mechanical engineering at the University of Duisburg-Essen<br/>- PHD at the university of Duisburg-Essen on polygeneration in combustion engines<br/>- Research associate at the German Aerospace Center, Institute of Vehicle Concepts<br/>- REsearch Associate at the German Aerospace Center, Institute of low-carbon Industrial Processes

Authors:

Robert Hegner German Aerospace Center (DLR)
Omar Abu Khass German Aerospace Center (DLR)
Clement Moller German Aerospace Center (DLR)
Panagiotis Stathopoulos German Aerospace Center (DLR)