Periodic Reporting for period 1 - ROCOCO2HP (Real-time optimal control of the CO2 heat pump system for residential use)
Reporting period: 2020-03-01 to 2022-02-28
The project aim was to enable proper operation and high performance of a CO2 heat pump for residential heating. To achieve the project aims, this project focused on the following scientific objectives:
- Establish data-driven model and simulation platform for the CO2 heat pump system;
- Understand the CO2 heat pump performance to enable their improvements;
- Suggest design and operational improvements of the CO2 heat pump for residential heating to enable continual high performance through operation.
Heating of residential building is taking a significant share in total building energy use internationally. In many countries, building heating is solved by use of fossil fuels. In our project, the idea was to show how an environmentally friendly solution, CO2 heat pump, could be used for space heating and domestic hot tap water heating. In general, the idea was to show how to increase the CO2 heat pump performance by optimizing interaction with the building heating system. This work is important for the society because it will encourage faster acceptance of the CO2 heat pump for residential heating systems.
The overall objective was to estimate CO2 heat pump performance when integrated with the building heating system and to implement reliable controllers to encourage the overall energy performance of the system.
- Establish data-driven model and simulation platform for the CO2 heat pump system;
- Understand the CO2 heat pump performance to enable their improvements;
- Suggest design and operational improvements of the CO2 heat pump for residential heating to enable continual high performance through operation.
Heating of residential building is taking a significant share in total building energy use internationally. In many countries, building heating is solved by use of fossil fuels. In our project, the idea was to show how an environmentally friendly solution, CO2 heat pump, could be used for space heating and domestic hot tap water heating. In general, the idea was to show how to increase the CO2 heat pump performance by optimizing interaction with the building heating system. This work is important for the society because it will encourage faster acceptance of the CO2 heat pump for residential heating systems.
The overall objective was to estimate CO2 heat pump performance when integrated with the building heating system and to implement reliable controllers to encourage the overall energy performance of the system.
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(opens in new window). 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.
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(opens in new window). 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.
The most important results beyond state of the art are the following:
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.
Potential impacts from the project are related to better understanding of the CO2 heat pump performance when connected to the heating system. In our project, we showed how critical may be the inlet temperature to the gas cooler or the return temperature from the heating system. Therefore, the wider impact of this results on the building heating system focusing to obtain lower return temperatures in the heating system to enable higher COP of the CO2 heat pump. Further, the results from the developed model clearly indicated the critical issues of the gas cooler size and the expansion valve properties. Our results showed that the size of the gas cooler may significantly change the performance of the CO2 heat pump. The expansion valve may significantly change the CO2 heat pump performance, because it is controlling the CO2 flow and the compressor power. Therefore, these last results related to the gas cooler size and the expansion valve may be very relevant for the component developers and engineers to choose the right components and thereby increase the CO2 heat pump performance.
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.
Potential impacts from the project are related to better understanding of the CO2 heat pump performance when connected to the heating system. In our project, we showed how critical may be the inlet temperature to the gas cooler or the return temperature from the heating system. Therefore, the wider impact of this results on the building heating system focusing to obtain lower return temperatures in the heating system to enable higher COP of the CO2 heat pump. Further, the results from the developed model clearly indicated the critical issues of the gas cooler size and the expansion valve properties. Our results showed that the size of the gas cooler may significantly change the performance of the CO2 heat pump. The expansion valve may significantly change the CO2 heat pump performance, because it is controlling the CO2 flow and the compressor power. Therefore, these last results related to the gas cooler size and the expansion valve may be very relevant for the component developers and engineers to choose the right components and thereby increase the CO2 heat pump performance.