Danish district heating provider Marstal Fjernvarme has chosen to replace a 10-year-old CO2 (R744) heat pump with a new ammonia/NH3 (R717) system, increasing its COP from just over 3 to an annual average of 5 to 5.5, according to the contractor on the project, Kristian Sørensen.
Sørensen is the owner of Natref Engineering, a new Danish industrial contractor specializing in heat pumps for district heating applications, and Marstal is a district heating provider on the Danish island of Ærø with 1,600 customers.
The solution chosen by Marstal includes two Sabroe 716 HPX ammonia heat pumps. The new heat pumps will be commissioned in December.
The significant increase in COP with ammonia heat pumps is possible because the heat source for the district heating network is water from a 75,000m3 (2,648,600ft3) covered reservoir heated by solar panels. This means that the source temperature varies with the amount of sunlight. In late summer and fall, the source temperature increases to 50°C (122°F), and the ammonia heat pumps are better at utilizing these high temperatures than the CO2 heat pumps, Sørensen said.
“CO2 heat pumps reach their maximum COP when the heat source is around 20-25°C (68-77°F), but as the source heat is 50°C (122°F) in the beginning of the winter, it was important to have a heat pump design that could benefit from the high temperatures,” he added.
The warm water from the reservoir is led to the evaporator side of the heat pumps. “In order to utilize the water temperature most efficiently, the suction pressure must also be increased,” Sørensen explained, “but the design of CO2 heat pumps means that the suction pressure cannot exceed 13-15°C (55-59°F).” The Sabroe ammonia heat pumps can maintain a 43°C (109°F) suction pressure, which is what creates the higher COP, he said.
Marstal decided to replace the CO2 heat pump both to achieve a higher COP and to better utilize the additional power they have gained by installing a new cover on the water reservoir. This means a reduction in the loss of heat, making more available for the heat pumps. With a larger pump capable of utilizing the additional heat, Marstal will be able to decrease its reliance on supplemental heating from biomass (wood chips).
The two Sabroe ammonia heat pumps are capable of raising the temperature of the stored reservoir water to the 75°C (167°F) required by the district heating system. The combined capacity of the heat pumps is around 4MW (1,137.4TR), and the COP 7, when the source is at 50°C (122°F). This declines to around 2MW (568.7TR) capacity, and a COP of 4, in spring when the source temperature drops to 15°C (59°F). This results in an overall annual COP of 5-5.5 and “huge” energy savings for the plant, Sørensen noted.
In the summer, the heat pumps do not run, as the solar panels provide enough heated water for the 1,600 customers, while simultaneously heating up the stored water in the reservoir for the winter season.
R744 vs. R717
Kim G. Christensen, the CEO of OEM Fenagy, which produces R744 heat pumps, recently said during the ATMOsphere Europe conference that he believed CO2 to be “really good” for district heating applications and the best available technology in the 0.5 to 5MW (142 to 1,422TR) capacity range.
Using the example of the COP gain achieved by Marstal with ammonia products from his company, Kenneth Sørensen, Head of Sabroe Nordic, argues that the situation is a bit more complex than that, and that each installation should be assessed individually to find the best natural refrigerant. “In my opinion, ammonia is much more energy efficient than CO2 in most cases for heat pumps, but it must always be checked from case to case,” he said.
“The existing CO2 heat pump plants are a commercial design with multi-compressor racks and a life span much shorter that the industrial designs with few large compressors used in ammonia heat pumps,” Kenneth Sørensen added.
“This can make an ammonia plant a better investment, even if the CO2 plant is less expensive at first cost. There [is] room for both solutions depending on the specific case and the demands from the customers.”
The importance of energy prices
Kristian Sørensen agrees that it’s complicated to name a “winner” in the race between CO2 and ammonia heat pumps. “It will be all about predicting, or guessing, the energy prices in the future,” he said. “As a rule of thumb, you can say that CO2 systems have lower COPs than ammonia, but CO2 systems are often also cheaper.”
“If you go back one year in time, when electricity was much cheaper, CO2 would have a bigger advantage compared with today, but with the electricity prices being so high, it is much more important to look at COP now,” Sørensen stressed.
“So, natural refrigerant-based heat pumps are the winners,” he concluded, “sometimes CO2, sometimes ammonia.”
““In my opinion, ammonia is much more energy efficient than CO2 in most cases for heat pumps, but it must always be checked from case to case,”Kenneth Sørensen, Sabroe
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