The concept of heat pumps has been explored within multiple entries of the Decarb Guidebook. As a refresher - the technology operates solely on electricity, requires zero on-site combustion, and exceeds the energy efficiency of boilers. Heat pump variable refrigerant flow systems redistribute heat inside a building. Heat pumps can also provide heating and cooling from the ambient outdoor air.

Propane as a refrigerant (R-290) provides a couple of benefits. Notably, its thermodynamic properties allow more heat transfer per volume refrigerant than most commonly used refrigerants. Propane is also non-synthetic and has minimal adverse environmental effects with a GWP of 3.

However, Propane is flammable and current codes restrict the size of the refrigerant loop to below 150g of circulating refrigerant. A high performing propane heat pump with these restrictions may produce the heating capacity equivalent to that of a 50 MBh boiler. Resulting in a compact, high performing heat pump that is limited to use within small facilities.

Technical Description

Heat Pumps

Within a heat pump, the heat transfer fluid moves through a vapor compression cycle to move heat from one space to another. For more background knowledge on how heat pumps work, see other guidebook sections on Heat Recovery Chillers and Heat Pumps or Heat pumps for DHW.

Figure 1. Vapor-compression cycle (Top) and visualization of the Coefficient of Performance(COP) of an average heat pump (Bottom)

Advantages of using Propane

The latent heat of vaporization for propane is nearly twice as large as the latent heat of common HFCs; during the condensing phase, twice the heating effect can be achieved with the same refrigerant mass flow. Propane would be used as a pure fluid which serves to avoid additional considerations for heat exchangers operating with a mixed solution. As a refrigerant propane poses little threat to the environment as a naturally occurring molecule with a global warming potential (GWP) of 3 (as compared to synthetic refrigerants commonly used in healthcare equipment).

Table 1. The Global Warming Potential of some refrigerants commonly used in healthcare [1]

How does this decarbonize?

Heat pumps run on electricity and do not use any natural gas; by using electrical energy to move heat (vs. producing heat from combustion), a large reduction in emissions for space heating can be achieved. There are two sources of emissions associated with heat pumps. The first Is scope 2 - purchased electricity, ie. the emissions produced at the power plant. Studies aimed at using propane as a refrigerant aim to increase the performance of heat pumps to further decrease energy consumption. The second source of emission is scope 1 refrigerants used with the heat pump; which propane can dramatically decrease.

Research and Testing

The “Low Charge 150g” project [2], conducted by the Fraunhofer Institute for Solar Energy Systems in conjunction with multiple heat pump manufacturers, aims to standardize propane refrigerant circuits as well to reduce refrigerant charge - how much heating capacity can be achieved per gram of refrigerant. A maximum heating capacity of 12.8kW (43.6MBh)and a COP of 4.7 was achieved with 124 grams of propane.

A 34.2 (116.7 MBh) kW capacity, propane circulated heat pump was performance tested within a laboratory environment (San Ramon, CA) and compared to the performance of a simulated VRF system. The unit served as a hydronic space conditioning system with an outdoor propane loop transferring heat to an indoor hydronic distribution loop with multiple water to air heat exchangers. The research group noted that due to several limitations, the results should be considered as a proof-of-concept of distributed heating and cooling with secondary fluids rather than a display of available efficiencies. In comparison the propane heat pump closely matched the cooling capacity, power draw and efficiency of the compared R410a simulated Variable refrigerant flow system. Another attribute was strong lift potential in heating mode - maintaining a COP above 3 within an ambient temperature of 10 - 25 ℉.

Implementation

Barriers: Codes

Propane is an ASHRAE class A3 refrigerant as defined by ASHRAE standard 34. Its flammability limits its application to closed loop refrigeration circuits with a charge load less than 150g. UL 60335-2-89 safety standards were recently updated to allow a maximum charge load of 500g. The remaining safety standards - EPA snap approval and building codes - have not been updated. Currently, US EPA snap regulations [3] address the use of R-290 specifically within refrigerators and freezers and states “the maximum charge size for each separate refrigerant circuit must be no greater than 150g.” Current CA codes only regulate 150g or even 110g on charge loads.

Barriers: Technology

During the Bundgaard study referenced within the ‘Research and Testing’ section, a fire-rated 4-way valve was not commercially available for use within a reversible refrigerant circuit. As a substitute, multiple 3 way diverting valves were used to switch between heating and cooling.

The semi-hermetic automotive compressor used in the LC150 project prototype trial, conducted by Fraunhofer Institute, is not suited for the expected 20 year lifespan of the heat pump. A fully hermetic compressor with a longer service life is in development.