At the beginning of the project, installation of the CO2 heat pump with a heated room experimental platform was done. Afterwards, extensive measurements and data logging was performed. A proportional integral (PI) controller was used to maintain the desired discharge pressure by adjusting the opening of the electronic expansion valve. The dynamic performance of the CO2 heat pump in the typical discharge pressure of 7,200 to 8,400 kPa were analyzed. The results indicated that the method of using the PI controller to adjust the opening of the electronic expansion valve could effectively maintain the desired discharge pressure of the CO2 heat pump in the experimental conditions. Experimental cases in which the compressor rotational speed of 1,100 rev/min and 1,300 rev/min were conducted. Operation of the CO2 heat pump was observed for different gas cooler water inlet temperatures. COP was calculated by measuring the compressor power and heating capacity in the gas cooler. The analysis on the water to-water CO2 heat pump COP in different measured cases was depicted.
The quasi-dynamic model of the CO2 heat pump was developed by integrating dynamic and steady-state models. Model validation results demonstrated that average relative error for the outlet fluid temperature of evaporator and gas cooler were 7.8% and 1.4%, respectively. This indicated that the developed quasi-dynamic model was reliable. Modelling results indicated that COP for the heat pump was 3.99 when the inlet water temperature in gas cooler and expansion valve opening were 24oC and 28%, respectively. The proposed model-based sizing method could effectively determine the optimal gas cooler area enlargement ratio in different compressor and expansion valve features. The results showed that higher priority to the operation and higher COP value, would favor larger gas cooler area, while higher priority on the investment cost would prefer lower gas cooler area.
Based on the developed model and measurements performed in the laboratory about the expansion valve characterists, the effect of valve features on the sizing of the CO2 heat pump might be not obvious but seems to be crucial. Therefore, future work should focus to define more accurate valve properties and thereby enable easier sizing of the expansion valve. This research work is useful to properly design and operate the CO2 heat pump.
The most important results from the project are:
1) detail performance analysis of the CO2 heat pump connected to the heated room with quantification how COP of the heat pump may be changed. Specifically, the focus was given to the inlet temperature to the CO2 heat pump gas cooler or the return temperature from the heating system.
2) development of the quasi-dynamic model of the CO2 heat pump for design and performance analysis of the CO2 heat pump. This model allowed us to identify the most critical issues in design the CO2 heat pump, such as area of the gas cooler and the expansion valve properties.
To encourage results exploitation and dissemination, the following actions were done with the project:
1. Nine publications were published (five journal paper and four conference papers). One journal paper about the quasi-dynamic model of the CO2 heat pump is still under review, but it passed two reviews until now.
2. Plant promotion under Researcher Night for the secondary school pupils at NTNU in September 2021 was hold. An informative lecture was given, and the laboratory plant was presented to the secondary school pupils. See the program here:
https://www.ntnu.no/forskningsdagene/night/live(si apre in una nuova finestra). The promotion lecture was going live for those who could not met due to the COVID-19 restrictions.
3. The laboratory plant was included in the specialization course Thermal Energy and Indoor Environment for the 5th year students. A research-based laboratory assignment was developed.
4. A four-hour course about the plant was given to the practitioners within heating, ventilation, and air-conditioning (HVAC) of buildings. The focus was to show operational issues and how to improve operation of the CO2 heat pump. 40 participants follow the course. The course was given on-line in collaboration with the national HVAC fellowship NEMITEK. Most of the participants were experts from contractor companies and building operation and maintenance.
