GL2014 CO2 heat pumps session: how to improve system efficiency and performance in residential applications

By Pilar Aleu, Oct 03, 2014, 15:08 4 minute reading

The Gustav Lorentzen 2014 (GL 2014) Conference on Natural Refrigerants featured a CO2 heat pumps session that discussed the latest research on heat pumps employing R744 as refrigerant in both cold and warm climates. R744.com highlights three of these presentations including a study focused on a solar assisted CO2 GSHP, improving the CO2 cycle with double-throttling, double-compression, and split cycle, and peak shaving and load shifting demand side management str

The biannual GL Conference on Natural Refrigerants has a long track record in serving as a meeting place for HVAC&R stakeholders to discuss the latest developments in natural refrigerant-based technology. One of the sessions was centered on CO2 heat pumps, where experts from around the world presented their technology case studies in this area.

High efficiency and low energy consumption with ground source heat pump (GSHP) systems in cold and warm climates areas

Its high efficiency and low energy consumption makes the ground source heat pump (GSHP) system an attractive solution for addressing energy and climate challenges. However, in heating-only or cooling-dominated applications, load imbalance affects the heat transfer performance of the borehole and the performance over time. The paper titled “Performance and economic analysis of a solar assisted CO2 ground source heat pump with air-cooled gas cooler under different climate conditions” by J.Ye, T.M. Eikevik, P. Neksa, A. Hafner, G. Ding and H. Hu, analysed the performance of a new solar assisted GSHP with a CO2 air-cooled gas cooler system designed to solve the imbalance problem of the ground heat.

Two cities with different weather characteristics Trondheim (cold-climate) and Beijing (hot-climate) were investigated and depending on their climatic features, corresponding operation strategies were chosen and evaluated:

  • Trondheim: use of a solar collector to compensate the heat extracted from the soil
  • Beijing: use of a gas cooler to remove excess heat to the air instead of injecting to the soil

Using the TRNSYS program to simulate system operation on an annual basis, results showed that with the novel system, compared with a traditional CO2 GSHP, the imbalance can be less than 0.1% per year. What is more, the operation cost can be significantly reduced. In addition, this novel system suits different climatic cities as well as different building structures.

Seeking alternative CO2 transcritical systems for heating applications

In the paper called “Analysis and optimization of different two stage transcritical CO2 cycles for heating applications,” jointly written by M. Pitarch, E. Navarro-Peris, J. Gonzalez and JM. Corberan from the Polytechnic University of Valencia, the authors analysed alternative transcritical systems in order to implement a heat pump for the production of water at 80°C with a temperature lift of 40°C, so as to substitute boilers in existing heating installations in Europe using CO2 technology. This work is part of the project Next generation of heat pump technologies (NEXTGHP), funded by the Seventh Framework Program of the European Union.

Through the commercial software EES, the research investigated two different transcritical thermodynamic cycles working with CO2:

  • Double-Throttling, Double-Compression, Split Cycle (DTDC_SC)
  • Double-Throttling, Auxiliary Compressor cycle (DTAC)

After analysing the benefits and performance, both cycles were compared with a subcritical injection cycle working with R134a and a single stage CO2 cycle for the same application. Results revealed that the improved cycle with CO2 can represent a global improvement of 15% in terms of Coefficient Of Performance (COP), depending on the application and environmental conditions.

The study concludes that the:

  • DTAC system is probably the easiest to implement out of all the systems and that for moderated evaporation temperatures it could have a COP similar to other two stage systems.
  • DTDC_SC is the system with the higher COP and could work in a wider temperature range and under broader conditions than the DTAC system. Assuming that the system should work with only one compressor (the two stages are integrated in the same shell), its performance depends on the compressor design.

Study on CO2 heat pumps including two DSM control strategies

Taking into account the benefits of using CO2 as a refrigerant, A. Arteconi, E. Ciarrocchi, F. Polonara, J.F. Chen, S. Deng, R.Z. Wang presented the paper titled “Performance analysis of a carbon dioxide heat pump installed in a residential application” studying the behavior of an existing installation of a CO2 heat pump (COP < 3) located in an apartment for heating, cooling and hot water production.

The paper evaluates two control strategies for demand side management (DSM): a peak shaving strategy and a load shifting strategy. In particular, the heating mode and the energy consumption of the heat pump are scrutinised and the internal thermal comfort are analysed when these strategies are in action.

The results revealed that is possible to apply a peak shaving strategy to the heat operation while maintaining the internal comfort of the building by increasing the storage tank size up to 1m3. Nevertheless, difficulties arise in maintaining the domestic hot water (DHW) supply temperature to the requested set point. Therefore the application of this DSM strategy both to space heating and domestic hot water is not feasible.

The load shifting strategy is also not suitable for space heating because it asks for much more flexibility than that available in the system.

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By Pilar Aleu

Oct 03, 2014, 15:08




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