PDF Publication Title:
Text from PDF Page: 007
1264 J. Stene / International Journal of Refrigeration 28 (2005) 1259–1265 10 and 60 8C, respectively, 35/30 8C supply/return tempera- tures in the space heating system, and a charging period of 0.5 h (17 l charging volume), the average drop in COP during operation in the Combined mode and DHW mode was about 25 and 15%, respectively. At a charging period of 2 h (70 l charging volume), the COP reduction was about 12 and 8%, respectively. Consequently, the smaller the charging volume, the higher the relative reduction in the COP for the integrated COP heat pump unit. In addition, mixing of hot and cold water during the tapping and charging periods would have led to a further reduction in the COP. In order to achieve a high COP for integrated CO2 heat pump systems, it is essential to develop tailor-made DHW storage tanks that minimizes the conductive heat transfer and mixing of hot and cold water inside the tank. Possible designs include the use of adequate diffusers at the inlet pipelines, application of a tank with a small diameter to height ratio, utilization of a movable insulating plate that separates the hot and cold water volume in the tank [7] and a system using two separate tanks for hot water storage and cold city water. 6. Seasonal performance factor (SPF) comparison The seasonal performance factor (SPF) for the prototype CO2 heat pump unit and a high-efficiency residential brine-to-water HFC (R407C or R410A) heat pump unit was estimated [7]. It was assumed constant inlet brine temperature for the evaporator (0 8C), and constant temperature levels in the space heating system (35/30 8C) and the DHW system (10/60 8C). The COPs for the HFC system was gathered from TNO–MEP in the Netherlands and the Heat Pump Test Centre (WPZ To ̈ss) in Switzerland. An improved CO2 heat pump unit with 10% higher COP than the prototype was also investigated in order to demonstrate the future potential of the CO2 system. Higher COP can be achieved by, among other things, using a more energy efficient compressor, optimizing the tripartite gas cooler or replacing the throttling valve by an ejector [4]. For the CO2 heat pump systems, the exergy losses in the DHW tank were not included. The HFC-system used a shuttle valve for prioritized DHW heating, and electric immersion elements were used for reheating of the DHW from 53 to 60 8C. It was assumed 2 K temperature difference between the water from the heat pump condenser and the hot water in the DHW tank. Table 2 shows the measured and estimated COPs for the heat pump systems. Table 2 demonstrates that the CO2 heat pumps and the HFC heat pump have reversed COP characteristics, i.e. the CO2 units achieve the highest COP during operation in the combined mode and DHW mode, whereas the HFC unit achieves the highest COP in the SH mode. In Fig. 7, the estimated SPFs for the three heat pump systems during monovalent operation are presented as a function of the seasonal DHW heating capacity ratio. In monovalent heat pump systems the heat pump unit covers the entire space heating demand and no auxiliary (peak load) heating system is required. At low seasonal DHW heating capacity ratios, the HFC heat pump unit was more energy efficient than the CO2 units due to their poor COP during operation in the SH mode. At increasing seasonal DHW heating capacity ratios, the SPF of the CO2 systems were gradually improved, since an increasing part of the heating demand was covered by operation in the combined mode and DHW mode. On the other hand, the SPF for the HFC heat pump unit dropped quite rapidly at increasing seasonal DHW heating capacity ratios, since the COP during operation in the DHW mode was about 30% lower than that of the SH mode. At the actual operating conditions, the break-even for the prototype CO2 unit occurred at a seasonal DHW heating capacity ratio around 35%, whereas the break-even for the more energy efficient CO2 unit was about 10% points lower. Conse- quently, in existing houses where the DHW ratio typically ranges from 10 to 25%, an HFC heat pump unit will be more energy efficient than an integrated CO2 heat pump unit. However, in well-insulated houses or in low-energy houses, where the DHW ratio typically ranges from 25 to 45%, an optimized integrated CO2 heat pump system may achieve the same or higher SPF. 7. Conclusion An integrated residential brine-to-water CO2 heat pump system may achieve the same or higher SPF than Table 2 The measured and estimated COPs for the analysed heat pump systems [7] Prototype CO2 heat pump Improved CO2 heat pump HFC heat pump COPZ3.0—SH mode at 35/30 8C COPZ3.8—DHW mode at 10/60 8C COPZ3.9—combined mode at 35/30 and 10/60 8C COPZ3.3—SH mode at 35/30 8C COPZ4.2—DHW mode at 10/60 8C COPZ4.3—combined mode at 35/30 and 10/60 8C COPZ4.6—SH mode at 35/30 8C COPZ3.1—DHW mode at 10/55 8CPDF Image | Residential CO2 heat pump system space and hot water heating
PDF Search Title:
Residential CO2 heat pump system space and hot water heatingOriginal File Name Searched:
Article_IIR-Journal_STENE_2005.pdfDIY PDF Search: Google It | Yahoo | Bing
CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
Heat Pumps CO2 ORC Heat Pump System Platform More Info
CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com (Standard Web Page)