In the paper “Technical and economic working domains of industrial heat pumps: Part 1 – Vapour Compression Heat Pumps,” by Ommen, Jensen, Markussen, Reinholdt and Elmegaard, low sink temperature glide R717 heat pumps demonstrated the best performance, whilst transcritical R744 heat pumps had good performance with a higher sink glide.
Comparative study analyses technical, thermodynamic and economic constraints
The study was designed to reveal the possible operation areas for several natural working fluids when considering temperature lift and sink temperatures, using the one stage vapour compression heat pump cycle. In the study, four heat pumps using natural refrigerants and one using HFC 134a were compared according to technical, thermodynamic and economic constraints, and taking into consideration heat pump performance and investment. The five types of compressors considered were:
- R134a: with pressure limit bar of 28, with capacity (1500 RPM) m3/h of 5-280, applicable for HFC working fluid
- R290: with pressure limit bar of 28, with capacity (1500 RPM) m3/h of 5-280, prepared for flammable environments
- R717-LP: with pressure limit bar of 28, with capacity (1500 RPM) m3/h of 5-180, equipped for R717 specially
- R717-HP: with pressure limit bar of 50, with capacity (1500 RPM) m3/h of 90-200, with high pressure
- R744: with pressure limit bar of 140, with capacity (1500 RPM) m3/h of 6-25, high pressure for transcritical process
The study assumptions included the following:
- Sink outlet temperature upper limit is 100°C
- Effects such as pressure drop in pipes, the extent of non-useful superheat and subcooling, and compressor heat losses are assumed to be of similar magnitude between the five heat pumps
- The lubrication maximum temperature is 180°C for all heat pumps
- No consideration is given to possibilities for improved performance such as internal heat exchangers
- Only full load steady state operation is considered
- The study assumes heat transfer fluid is water, which is pressurised, to avoid phase change
- 3,500 annual operating hours, and QHP=1000kW
R744 is the optimal solution in the areas where R717 is constrained by discharge temperature
When compared to the fuel cost of a natural gas burner, the net present value is positive in all five heat pumps. Among the five heat pump studied, R717LP, R717HP and transcritical R744 have the best performance when considering all presented constraints. In a low sink temperature glide, R717 heat pumps demonstrated best performance and transcritical R744 performed better in a higher sink glide, when R717 is constrained by discharge temperature. Therefore, according to the authors, optimal heat production may then be considered as a trade off between capacity and performance.
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