Modelling of wet compression in compression-resorption heat pumps

Modelling of wet compression in compression-resorption heat pumps

Brancaccio, Marina (TU Delft Mechanical, Maritime and Materials Engineering)

Hooman, K. (mentor)

Delft University of Technology

Mechanical Engineering | Energy, Flow and Process Technology

2023-10-02

The ever-increasing power demand, the high scarcity of fossil fuel resources and the growing environmental pollution have pushed the development on technologies related to sustainable energy systems. Maximizing the energetic performance of industrial processes is a key aspect in this quest for sustainable
energy management. A large share of waste heat below 100°C is rejected into the environment during industrial processes. This low temperature industrial waste heat can be a relevant heat source, reducing the consumption of fossil fuel and the emission of CO2 into the atmosphere. While these lower temperatures may not be suitable for direct industrial use and their capture may not be economically justified, the implementation of a heat pump allows for the elevation of these stream temperatures to levels that can support a wide range of industrial processes.
Heat pumps can be utilized to upgrade waste heat streams from low to higher temperature levels
which can then be used as energy supply for other industrial processes. State-of-the-art heat pumps are still restricted to temperature levels below 120°C due to long payback periods and technical limitations regarding the compressor. The application of heat pumps in industry could become more widespread if the technology would allow for a higher temperature output. Compression-resorption heat pumps (CRHPs) are a promising option to upgrade waste heat streams since they combine the advantages of absorption heat pumps (working with a mixture having non-isothermal phase transitions and low environmental impact) and vapor-compression heat pumps. CRHPs can operate both under dry and wet compression conditions. One of the main issues in reaching high temperatures with dry compression is the degradation of the oil, which leads to a reduction of the performance. Employing wet compression, the liquid can function as a lubricant avoiding both oil contamination and irreversibility caused by the superheating of the vapor. As pointed out by several researchers, a crucial point for the feasibility of high temperature CRHPs is a good value of isentropic efficiency for the compressor. As such, since a technological solution is currently not commercially available, the diffusion of CRHPs in the industrial processes is inhibited.
This research thesis develops a numerical model of the compressor in which the liquid phase and
the vapor phase are at non-equilibrium conditions. The model incorporates heat transfer between
the phases, yielding new insights and results. The validated model serves as a tool for analyzing
optimal operating conditions to maximize compressor efficiency. A case study within the dairy industry was considered. The results indicate a 60% reduction in operating costs and a saving of 104 tonnes of emissions if a single heat pump substitutes a traditional boiler. The substitution of fossil fuel fired boilers with heat pumps becomes increasingly necessary in light of the EU Renewable Energy Directive, ratified in 2023, which targets a 45% renewable energy share by 2030. The development of such next generation heat pumps could be a major breakthrough for optimizing energy management in industrial processes.

Heat pumps
Wet compression
oil-free compressor
twin screw compressor
ammonia-water
heterogeneous
modelling
double-phase flow
High temperature Heat Pumps
waste heat
Waste heat recovery
renewable energy

http://resolver.tudelft.nl/uuid:ef26f5de-461b-4477-b4a4-47f32d8a33ba

Student theses

master thesis

© 2023 Marina Brancaccio