LOW CARBON ENERGY
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Electric hot water boiler
Air source heat pump
Electric panels/
Electric hot water boiler
Figure 1: Estimated difference in capital cost of system.
appropriately achieved has been considered and included in a review of three options, as described below. A gas-fired boiler option is included as a reference option, in order to demonstrate comparison against what is currently the most standard approach in the UK. The study used the energy consumption from the following systems as the basis of calculations when considering operational cost and carbon for each option, while cooling and lighting are excluded from the capital cost assessment: n Space heating. n Domestic hot water. n Cooling. n Lighting. n Auxiliary (pumps etc).
Option 1: electric boiler solution This is essentially very similar to a gas-fired solution, but utilises an electric boiler instead. The electric boiler operates at standard low temperature hot water (LTHW) temperatures of typically 80˚C, and distributes heating through a piped LTHW system. Domestic hot water (DHW) generation in this approach is via indirect hot water storage electric calorifiers fed by the LTHW system. Electric boilers are manufactured by a number of suppliers, and there are two main methods of heat generation in these systems: immersion element boilers and electrode boilers. Immersion
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element boilers pass current through metal-resistance heating elements immersed in water. The resistance generates heat, which in turn heats the water.
Electrode boilers pass alternating current between solid metal electrodes, with the water acting as the resistive element utilising the water resistance to heat the water. For the peak demands forming the basis of this study, immersion element boilers were used.
Option 2: air source heat pump Air source heat pumps (ASHP) transfer energy from the outside air, and feature in the building using a vapour compression refrigeration cycle. Heat output is typically in the region of 50˚C, which is lower than standard LTHW operating temperatures, and therefore requires consideration in heating system design. This includes measures such as increased pipework and emitter sizes, and location of ASHP systems as local to emitters as possible, which means more consideration of delocalised heat generation, especially for larger buildings.
This option is similar to Option 1, but utilising ASHP technology for heat generation. DHW generation in this approach utilises ASHP to pre-heat the DHW prior to uplift in heating by electric boilers.
While reversible ASHPs can be utilised to also provide space cooling, this requires
Electric hot water boiler
Air source heat pump
Electric panels/
Electric hot water boiler
Figure 3: Difference in heating fuel costs in the UK. 34 Health Estate Journal August 2021
Gas-fired boiler
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Gas-fired boiler
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Electric hot water boiler
Air source heat pump
Electric panels/
Electric hot water boiler
Figure 2: Estimated difference in annual maintenance cost.
separate distribution systems and more complicated system control and design. For the purposes of this study, the ASHP system is only proposed for heating.
Option 3: electric panel heaters/water boilers
This option consists of two separate heating systems, as follows: n Electric ceiling-mounted radiant heating panels to provide space heating, offsetting building fabric and infiltration losses.
n Electric hot water boilers to provide Low Temperature Hot Water (LTHW) heating to air-handling unit heating coils.
The electric panel heaters are typically ceiling mounted, with heating elements/ mats attached to a metal surface. Additional heating to the spaces is provided through the ventilation system. DHW generation in this approach is via indirect hot water storage electric calorifiers fed by the LTHW system.
Option 4: gas-fired boiler This is the most common heating system in the UK. Thermal energy is generated by a gas-fired boiler, utilising natural gas as the fuel for an LTHW distribution and heat emitter system. There are several gas-fired boiler types available, with the main ones including cast iron sectional, cast aluminium, steel sectional, and steel shell. DHW generation for the basis of this
Gas-fired boiler
Electric hot water boiler
Air source heat pump
Electric panels/
Electric hot water boiler
Figure 4: Difference in annual carbon emissions in the UK.
Gas-fired boiler
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