Swapping gas-fired systems with heat pumps

Heat pumps, essential for generating hot water, have operating parameters that differ significantly from conventional systems, requiring modifications to existing infrastructure such as risers, pumps, and main distribution systems, to handle lower temperatures and higher flow rates. The transition is particularly complex in healthcare facilities, where complete re-piping to optimise heat pump performance may be impractical, often leading to sub-optimal operation. This article underscores the importance of distinguishing between heating and domestic hot water systems, as they operate at different temperatures, which impacts their energy efficiency and overall system performance.

Integration of heat pumps

The all-electrification process in most instances requires the use of heat pump-based technologies to generate the hot water. When considering system replacements, the installed systems in existing hospital systems have different requirements. The integration of heat pumps into legacy systems is not just a case of swapping one heat source for another. Heat pumps invariably have different operating parameters to ‘traditional’ gas-fired heating water and domestic hot water systems. These parameters are not always compatible with the embedded infrastructure within an existing facility. When swapping gas for electricity using heat pumps, the design needs to consider the system and its operation, not just the swapping out of the heat source.

If the design parameters are altered to suit common heat pump performance specifications (lower temperatures and higher flow rates), then it’s likely this will result in changes to the existing infrastructure, such as risers, pumps, and main distribution. So, when considering integration into an existing healthcare facility, it may have to be accepted that in most buildings and systems the heat pumps will be operating sub-optimally, since it may well not be practical to completely re-pipe the existing systems in the building to accommodate common heat pump systems’ optimal flow rates and temperatures. Such integration may also introduce other factors, such as the need for enhanced infection control measures.

The distinction between heating and Domestic Hot Water (DHW) is important, as the different temperatures mean that the coefficient of performance (CoP) of the two systems can vary significantly. Design conditions are key — for example at 3 °C ambient, the CoP of a heat pump supplying 60 °C DHW is around 3.5. For a Low Temperature Hot Water (LTHW) system at 80 °C, with a return temperature of 60 °C, the CoP is closer to 2,
i.e. much less efficient in the use of energy to create heat.

New-build installations

Choosing system-appropriate heat pump technology that is able to provide the correct temperatures and flow rates for buildings should not present too much of a challenge for competent healthcare building services designers, as they have full control over the primary and secondary water circuits, and incoming power requirements, and should be fully aware of the activities undertaken in the building. Swapping gas out of existing facilities is, however, a very different proposition.

Existing installations

• Estimating the demand

In most hospital facilities the systems that serve the buildings have evolved over time — starting from an initial coherent design, and slowly morphing into systems that have been extended, cut back, or repurposed. The result of this evolution is that the building systems are often performing in a manner distinctly different from that envisaged in the original design parameters. Establishing the required duty is the first challenge in retrofitting a heat pump.

• Availability of power

The second challenge in retrofitting a heat pump will be the availability of power from the local plant level mechanical switchboard. When the system uses gas as the primary thermal fuel, the mechanical switch board (MSSB) only supplies the power to the pumps for circulation and controls to enable the equipment to function. When a heat pump is used to replace the gas appliance, the thermal energy is required to come from the electrical supply. Most existing MSSBs will not have sufficient spare capacity to feed the new heat pumps, and in a large installation it is equally likely that the sub-mains supplying power to the plant rooms will have insufficient spare capacity to accommodate a large heat pump installation without upgrading. There can also be knock-on effects on the emergency power supplies to the plant rooms, depending on whether the heat pumps are deemed a critical service.

• Spatial requirements

Heat pumps need space, and lots of it. A 200 kW gas boiler needs around 1.5 m² of floor space for the boiler and maintenance access. A 200 kW water-to-water heat pump would need 12.2 m² in the plant room, and 8 m² of heat rejection outside the plantroom. A 200 kW air-to-water heat pump would need 24 m², and to be located outside the plantroom. Solutions to all the above can be found with time and money, but the remainder of the limitations come down to basic thermodynamics, so what are the technical limitations with boiler replacement heat pumps?

What are the technical issues in achieving an 80/60 °C boiler replacement heat pump?

The battle with natural gas is over (at least in Victorian healthcare), and a comparison of emissions of electrically-powered solutions must have a new benchmark. Electricity will be the fuel into the short to medium term, and the grid is provided with power from both renewable and non-renewable sources. Under power purchasing agreements, the ‘green power’ is sold separately from the non-renewable power; thus from the grid it is possible to be allocated green power only. Having solved the emissions issue, the green power is limited, and the best use of this then allows other facilities to benefit from the green power available.

A new benchmark becomes to maximise the lifetime CoP of the solution. Resistive heating — the simple, reliable and cheap solution, has a CoP of 1. When all electricity is zero carbon, then the only environmental impact is the leakage of the refrigerant. Currently, where the greenness of the grid is a point of debate, these emissions need to be balanced against the energy efficiency of the overall system.

The refrigeration cycle

In the following paragraphs there are several factors that — by necessity — have been glossed over as regards the details of the heat transfer etc. Most of these are additional small losses that would need to be added to arrive at the actual performance of any real-world system.

The refrigeration cycle is the same for heating as it is for cooling. The term ‘reverse cycle’ comes from a valve that swaps the condenser and the evaporator flows, and allows one compressor to use either heat exchanger as the condenser, and the other as the evaporator.

Thus, the plot on a pressure-enthalpy diagram (see Figure 1) is the same for an air-conditioner and a heat pump. The conventional refrigeration cycle on a P-H diagram resembles what is shown. The P-H diagram shows the energy, kJ per cycle; the refrigerant circulation in kg/s is required to resolve the heat transfer in terms of kW of heating.

• Heat exchangers

There are two heat exchangers in the cycle in Figure 1, with the evaporator represented by the lower blue horizontal line, where the saturated liquid evaporates to saturated vapour and the condenser the upper red line, where superheated vapour cools to saturated vapour, and subsequently condenses to a mix of mostly saturated liquid with some saturated vapour at a high pressure.

The horizontal lines in Figure 1 are at a constant pressure, and the process is boiling to the right and condensing to the left. For a given pressure boiling occurs at a constant temperature. The lines of the diagram are the conditions inside the pipes. The temperatures available to the heating system depend on the coils or plates of the evaporator and the condenser, while the size and arrangement of the heat exchangers determine how close the air or water temperatures are to the refrigerant temperature.

• The compressor

The height of the vertical axis in Figure 1 relates to the difference in pressures in the condenser and the evaporator. The difference in pressure between the cold and the hot side needs to be provided by a compressor using reliable technology. The Pressure Axis on the P-H diagram is always logarithmic. A small change higher up the pressure axis is very large in comparison with the same change lower down.

A small amount of ‘superheat’ (extension to the right from the saturated vapour line) is necessary to make sure that liquid does not enter the compressor. Compressors are being pushed to deliver higher differential pressures, presenting challenges both for the compressor and the quality of the pipework assembly. The overall build quality required for the complete system is related to the absolute pressure in the system. With higher pressures comes the need for better joints to prevent leaks. The higher the pressure, the greater the chances of equipment failure if the equipment is not regularly maintained.

The difference between the evaporator pressure and the condenser pressure (plus the amount of super heat) needs to match both the temperatures required from the hot and cold sides. With boiler replacements these are fixed further apart than normal, the hot side at 80 °C, and the cold side at winter ambient temperatures.

• Expansion valve

The near vertical line to the left in Figure 1 is the operation of the expansion valve; these components have become much more sophisticated in recent times, regulating the flow through the circuit, and allowing the pressures to be maintained at the target values in the condenser and evaporator. The slight inclination of the line represents the parasitic thermal losses at this point in the circuit.

• The critical point for refrigerants

The top of the curve (in Figure 1) is the ‘critical point’ — the pressure above which there is no latent heat of evaporation. Above this there is no liquid to vapour transition. Substance in this region is referred to only as fluid. There is also a specific critical temperature for each substance. Conventional refrigeration circuits are sub-critical, i.e. the entire circuit is below the critical pressure.

The curve to the left (in Figure 1) is the saturated liquid condition, where the substance is completely liquid at the boiling point, while the one to the right is the saturated vapour line where the substance is completely vapour at the boiling point. For pure substances and azeotropic blends the transition from saturated liquid to saturated vapour is a horizontal line from the left side to the right. The latent heat of vaporisation is the property that determines the length of the line from the left to the right of the curve.

Critical temperatures for some refrigerants used, or capable of operating in the 80 °C condenser range, are shown in Table 1.

• Visualising CoP

The CoP for the system is the ratios of the enthalpy for each of the parts of the cycle (accounting for the compressor efficiency). For heating this would be the horizontal length of the red line, divided by the horizontal length of the green line, divided by the compressor efficiency.

Design conditions

The external ambient design conditions for a heating system would be more onerous than for a DHW system. Heating systems are likely to need to be able to run 24/7 (in the winter), and the maximum need for heating would be coincident with the coldest ambient. Heating systems would need to work at the design ambient for critical systems (possibly with a margin for climate change).

However, the usage of DHW systems is normally low in the middle of the night, and all heat pump systems for DHW have storage. Thus, it is acceptable to do some of the following:

• Delay the recharge until the CoPs are more favourable.
• Run the system with a reduced rate of recharge.

DHW systems could be designed for carefully considered, less onerous external design ambient temperatures. Most of the heat must come from the air. Heat pumps are devices that upgrade the available heat (they expand temperature differences, making the heat more useful); rather than producing the ‘heat’, they simply move it from a cold source to a hot one.

All refrigeration cycles (including heat pumps) need two heat sinks — one ‘cold’ the other ‘hot’. With the cold side, unless there is a low-grade waste heat source from another reliable process, this would normally be the ambient atmosphere. To transfer heat out of the cold source the refrigerant in the evaporator must be at a lower temperature than the cold source temperature (ambient air). This, for each given location, fixes the lower horizontal line in the P-H diagram.

To illustrate the source of the heat, Figure 2 shows a two-stage system. Currently the most likely solution for a boiler replacement heat pump, it illustrates that the number of kW going into the system at the cold side is only slightly less than the number of kW transferred at the hot side. The additional kW coming from the ‘work done’ by the compressors is added to the heat in the system. The CoP of this system would be:

Transcritical cycles

Carbon dioxide has become a common refrigerant for DHS systems, yet its critical temperature is only 31 °C. With a conventional refrigeration cycle the hot side would be limited to something like 25 °C to get a reasonable enthalpy difference across the saturation conditions. This application uses a transcritical cycle, where the evaporator operates in the sub-critical region as normal, and the hot side (no longer actually a condenser, as there is no liquid/vapour transition) operates above the critical point.

Outside the saturation envelope the temperatures (isotherms) are no longer horizontal, and changing enthalpy at a constant pressure means a related change in temperature. This temperature change is dependent on the water return temperature in the hot side.

The problem is that a DHW system is not designed to lose heat into a process; heat losses at all points in such a system are parasitic. If the water return temperature is too high (say 55 °C in the case of CO2 at 100 bar), then the expansion of this will still be mostly liquid (at 30 bar) as illustrated in Figure 3 by the central vertical line. As can be seen in Figure 3, the CoP at this point is very low, and possibly less than unity.

To get the best CoP out of a CO2 heat pump (in DHW applications), the cold feed needs to provide the lower temperatures at the hot side. The DHW system must be arranged to heat the coldest water last, and produce the lowest temperature after the ‘condenser’. This is illustrated in the outer loop on the diagram in Figure 3.

What does the solution look like now?

Manufacturers are using cascading heat pumps with different refrigerants to achieve a low-grade heat source, with an air-to-water heat pump feeding into a water-to-water heat exchanger to get the standard boiler temperatures. This solution is practical, and provides an improvement against resistive heating, even with a low night-time temperature. Typically, as the ambient rises, the CoP of the first stage increases vastly, thus providing a real alternative to resistive heating.

We can achieve the 80/60 and thus re-use the existing secondary systems and pipework. The CoPs will not be that great, as this would most probably be part of a cascading system, and each system would have a compressor. The total kW input would be higher than if a single stage system was practical, but this will only ever be a Band-Aid for ageing buildings. New buildings will be built around the most efficient, lower temperature heat pumps. Who needs to solve the 80/60 boiler replacement heat pump?

Potential options

Looking at the available refrigerants, there are quite a few capable of operation at sub-critical cycles with a hot side at 80 °C. The number of stages and the resulting CoP aside, it is possible to do, with either a single machine or a cascade of two heat pumps.

The question becomes what is the incentive for manufacturers to concentrate on this solution? There is no requirement for such a solution in new buildings, as the more efficient, lower temperature machines that can provide cooling as well would be the obvious choice. The other building components would be designed around these. So, such systems would only be for existing building stock, and indeed many such buildings would continue to use gas-fired plant for some considerable time. For many, a shutdown for a complete refurbishment — including the necessary pipework and coil upgrades in order to work with the new temperatures, is a viable option, especially if the refurbishment is undertaken alongside other substantial changes in the building, such as change of use. In addition, in the northern hemisphere many heating systems have been designed using condensing boilers operating in the 40 to 50 °C range, and such systems are well suited to a boiler / heat pump swap without much in the way of alteration.

This leaves only buildings (or sites) that have a specific need to move away from gas, that equally, cannot go offline for an extended period. So, the question will be whether a large enough sector exists for manufacturers to specifically focus on? Our feeling is no.

In summary

Physics means that it is not practical to remove a boiler and plug in a heat pump in its place. There is a practicality gap, and as you increase in size from systems that are measured in kW, to those measured in MW, that practicality gap becomes exponentially larger the bigger the system becomes. In a complex health environment, hot water is a key element in critical healthcare, and underperforming heat pumps can easily introduce unforeseen risks to hot water systems such as increased incidence of Legionella; this is something healthcare designers must be cognisant of when retrofitting heat pumps. In Australia, building services engineers will be working with heat pumps that are available now, towards solutions that involve multiple machines with different strengths and weaknesses, and each site will have a bespoke solution driven by the understanding of its history and the skill of the individual designer.

Acknowledgment

•  This article — based on a paper given at the IHEA’s Conference in June 2024 in Melbourne, and titled ‘So, you want to install a heat pump?’, first appeared in the Spring 2024 issue of Healthcare Facilities, the magazine of the Institute of Healthcare Engineering, Australia. HEJ acknowledges the help of the author, the IHEA, the event organiser, Iceberg Events, and the publisher of Healthcare Facilities, Adbourne Publishing, in allowing its publication here in slightly edited form.