CO₂ heat pumps best option to heat low energy houses

Combined space and hot water heating R744 heat pumps outperform state-of-the-art units using conventional refrigerants in low energy houses, tests by SINTEF have found.

In low energy and passive houses, where Domestic Hot Water (DHW) heating accounts for 50-85% of the total annual heating demand, an integrated CO₂ (R744) heat pump system will outperform the most energy efficient HFC units on the market, according to tests by SINTEF in Norway. R744 systems, achieving the highest Coefficient of Performance (COP) during operation in DHW mode and the combined space/hot water heating mode, would therefore become the best option in buildings with low space heating load where more than 50% of heating is used for the production of hot water.

Tests performed with a 6.5 kW prototype brine-to-water CO₂ heat pump for combined space and hot water heating found that a counter-flow tripartite gas cooler in combination with an external single-shell hot water tank and a low-temperature heat distribution system would enable the production of hot water in the required temperature range from 60 to 85°C while achieving the highest possible COP for the heat pump unit. SINTEF also confirmed that integrated CO2 models can use different heat sources, including bedrock, ground, exhaust ventilation air, ambient air, or a combination of these last two.

Test Setup & Results

The prototype CO₂ heat pump unit was equipped with a hermetic rolling piston compressor, a tripartite counterflow tube-in-tube gas cooler and a counter-flow tube-in-tube suction gas heat exchanger. An expansion valve and a low-pressure liquid receiver were used to control the pressure in the tripartite gas cooler. The integrated heat pump system was tested in simultaneous space and hot water heating (combined mode), hot water heating alone (DHW mode), and space heating alone (SH mode). The system was tested at 40/35°C, 35/30°C or 33/28°C supply/return temperature in the SH system, and 60°C, 70°C or 80°C in the DHW system.

Comparing its performance to a conventional HFC system, Dr. Jørn Stene, Research Scientist at SINTEF who conducted the tests, found that the CO₂ unit achieves a higher Seasonal Performance Factor (SPF) than the HFC system, reaching a COP of up to 3.9 for the combined mode at 35/30°C and 10/60°C. Key to reaching these efficiency levels is that the temperature level in the heat distribution system for space heating is sufficiently low (<35°C), since the application of a high-temperature radiator system will significantly reduce the COP of the system. Investigations with an improved CO₂ system showed that further COP improvements (up to 4.3 for the combined mode) can be achieved by using a more energy-efficient compressor, optimizing the tripartite gas cooler, or replacing the throttling valve by an ejector. The latter alone is capable of increasing the COP by 10-20%.

CO₂ promising for next-generation buildings

In existing buildings with a high share of space heating (70-80%), R744 systems are not yet as efficient as current systems. Therefore, Jørn Stene is clear:

“We have to use different heat pump technologies in different application areas, and residential CO₂ heat pumps for combined space heating and hot water heating do represent the most energy efficient option in low-energy houses and passive houses. Since there is an increasing focus on this kind of buildings all over Europe, CO₂ heat pumps are a very promising solution due to the high energy efficiency and the application of a natural and environmentally benign working fluid.”

IEA Heat Pump Conference to discuss latest findings

The issue of “Heat Pump Applications to Low Energy Buildings” will also be discussed during a Technical Session at the IEA Heat Pump Conference – a world-leading event focusing exclusively on heat pumps under the auspices of the International Energy Agency. More specifically, the use of CO₂ for domestic and commercial use in Europe and Asia will be the subject of different presentations, including a lecture by SINTEF focusing on “CO₂ as a way to reduce greenhouse gas emissions”. R744.com will report about major outcomes of this conference, taking place from 20-22 May.