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Development and evaluation of a loop thermosiphon heat exchanger as passive replacement to secondary loop for low GWP heat pumps.

Number: 2172

Author(s) : ROBINSON M., GOEDEKE S., MAXWELL T., PELLICONE D., BLATT M., BEAINI S.

Summary

Natural refrigerants such as propane (R-290) and ammonia (R-717) often have superior refrigerant characteristics with the additional benefit of low global warming potentials (GWP). These benefits come with drawbacks of flammability and toxicity. R-290 is classified as an A3 refrigerant (flammable) and R-717 as B2L (toxic and slightly flammable). To use these refrigerants in heat pumps for space conditioning, the refrigerant must be isolated from the air stream, and this is typically done by a hydronic system. This work presents an alternative passive approach using a loop thermosiphon heat exchanger (LTHX) to replace the secondary loop heat exchanger. The primary advantage of the LTHX is that it is passive and can operate at higher evaporator temperatures in cooling mode and lower condensing temperatures in heating mode. This work first briefly discusses the design of a heat pipe heat exchanger (HPHX) which is the fundamental operating principle of the LTHX. The HPHX is comprised of multiple heat pipes where the size, quantity, and spacing are determined to meet the thermal load, while minimizing air side flow restriction. When considering manufacturability and operation of the HPHX there are significant challenges. These challenges lead to the design of an LTHX which alleviates these problems. The LTHX was prototyped and evaluated in heating and cooling mode over a range of conditions that are suitable for space conditioning applications. The evaluation showed that the LTHX could effectively transfer heat with efficiencies up to 98%. The LTHX saturated in heating mode at around 5,300 Btu/hr (1.6 kW, 0.4 RT) while cooling mode did not appear to saturate with a measured capacity of 6,400 Btu/hr (1.9 kW, 0.5 RT). Maximum temperature differences were on the order of 10 °F (5.6 °C) between return and supply air when the HPHX was performing most efficiently.

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  • Original title: Development and evaluation of a loop thermosiphon heat exchanger as passive replacement to secondary loop for low GWP heat pumps.
  • Record ID : 30033063
  • Languages: English
  • Subject: Technology
  • Source: 2024 Purdue Conferences. 20th International Refrigeration and Air-Conditioning Conference at Purdue.
  • Publication date: 2024/07/17

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