ICR – Korean study of solar hybrid CO2 heat pump systems

By R744.com team, Sep 07, 2011, 15:50 2 minute reading

On Wednesday 24 August at the seminal International Congress of Refrigeration (ICR) research on a CO2 solar hybrid heat pump for residential applications was presented in a technical session on “Solar and waste heat cooling”. The research paper showed the importance of heat pump operating temperature and outdoor operating temperature on solar collector efficiency, solar collector heat, and heating COP.

South Korea is the 10th largest energy consumer in the world, largely dependent on imported energy sources to meet its needs. With almost no domestic reserves of raw materials and cut off from overland supply routes because of North Korea, investment in alternative energy sources could dramatically improve South Korea’s energy security. To this end researchers at Chosun University undertook a study of a residential heat pump, analysing its performance.
Study on a solar hybrid heat pump system using carbon dioxide, Won-Seok K., Jaekyeong O., Chasik P., Honghyun C., Chosun University, Korea
Given that the reliability of CO2 heat pump systems depends on operating conditions researchers at Chosun University analysed heat pump performance data against a series of varying conditions. The objective is to eventually develop an optimal control method. Kim Won-Seok, from Chosun University in gave a 5-minute poster presentation of the simulation study.
The system modelled included the following components:
  • Solar collector: 8 concentric evacuated tubes 
  • Heat storage tank
  • Heat pump: CO2 system with a double-pipe heat exchanger, fin-tube heat exchanger, electric expansion valve (EEV) and a reciprocating compressor of semi-hermetic type
The simulation model included the following assumptions:
  • Indoor space of 66.25 m2
  • Daily hot water usage 280l for a four-member family (ASHRAE standard 116)
  • Maximum solar fraction of 24.5%
Experimental results
System performance of the CO2 heat pump was found to vary significantly according to the operating conditions. One of the most important factors affecting performance was the EEV opening and its effect on mass flow rate and compressor discharge pressure. 
In turn the optimal EEV opening was linked to the heat pump operating temperature and outdoor temperature. Overall, system performance and the optimal EEV opening benefited from an increase in outdoor temperature and a decrease in heat pump operating temperature. For example, as the heat pump operating temperature increased from 40°C to 48°C, the optimal EEV opening decreased from 32% to 24%. What is more, the collector efficiency and collector heat were reduced by 7.6% and 2.7 kWh respectively, with an increase in heat pump operating temperature from 40°C to 48°C.
On average, results showed a reduction in system performance of 5.7% with every 2°C increase in heat pump operating temperature. On the other hand system performance improved by approximately 5.2% as the outdoor temperature increases by 2°C. As the outdoor temperature rose by 2°C, the heat pump heating coefficient of performance (COP) increased by about 6.5%, whilst the heating load decreased by about 7.6%.


By R744.com team (@r744)

Sep 07, 2011, 15:50

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