HEATING TECHNOLOGY Energy Critical Point


Expansion Valve


Hot End Condenser (T Constant)


(T Constant) Cold End Evaporator


Reverse cycle is the same (nothing on the P-H Diagram goes backwards)


Enthalpy kJ/kg (AKA internal energy)


Figure 1: A normal refrigerant cycle.


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.


n 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?


Figure 2: An energy flow two-stage heat pump.


Qout


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-


Energy Flow Two Stage Heat Pump


Expansion Valve


EC2 QC2 EC1 QC1 Qatm


Hot End Condenser To System


Qout Heat Out


QC1 QC2


EC1 EC2


Qatm


Compressor 2 Heat to Cycle Compressor 1 Heat to Cycle


Compressor 2 Elec Power In Compressor 1 Elec Power In


Heat In kW, Electrical Power (E) , Heat Flow (Q)


Cold End Evaporator From Atmosphere


Heating Energy Cooling Energy


Technology Limits Apply


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.


n 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.


n 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


32 Health Estate Journal February 2025


Pressure (Pa)


Compressor


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