Heat pumps: gas or electric?

Air source heat pumps (ASHP) are now widely available, powered at point of use by two basic energy sources, gas or electricity. The question is which system is more energy efficient and/or cheaper to run?
How gas driven heat pump’s work
A gas air sourced heat pump (GASHP) unit (using a vapour compression cycle – not to be confused with Absorption Chillers) is very similar to its electrically-driven counterpart, the difference being that to drive the compressor, GASHP uses a fairly conventional internal combustion engine, fuelled by natural gas, rather than petrol or diesel. As a consequence, these units are always open-drive or semi-hermetic machines – very similar to those used in cars, but bigger, with the associated operating characteristics. It is more common for electrically-driven ASHP to use hermetic compressors and hence refrigerant leakage is less likely.
Comparing the efficiency
The following calculation is designed to help specifiers make informed choices, analysing the relative efficiency to demonstrate the point when both systems are as efficient as each other. A comparison can be calculated using either emissions or cost.
CO2 emissions
To generate 100 kW of cooling in the building: Electric ASHP: 100 kWhr/ 2.80 EER (Energy Efficiency Ratio)* = 35.7 kWhr; x 0.432 kg CO2/kWhr electricity = 15.4 kgCO2 Gas ASHP: 100 kWhr/ 1.19 EER* = 84 kWhr; x 0.193 kg CO2/kWhr gas = 16.2 kgCO2
*corrected for pipe separation losses at 60m run
Cost of fuel
To generate 100 kW of cooling in the building: Electric ASHP: 100 kWhr/ 2.80 EER* = 35.7 kWhr; x 0.09 £/kWhr electricity = £3.21 Gas ASHP: 100 kWhr/ 1.19 EER* = 84 kWhr; x 0.035 £/kWhr gas = £2.94
*corrected for pipe separation losses at 60m run
As EER’s, CO2 emissions and energy costs change, then these new figures can be substituted into the calculations above and the results re-analysed.
This calculation shows that there is actually very little difference in the cost to the pocket, or the environment, between gas and electrically powered ASHP’s. GASHP comes into its own if the waste heat the system produces is utilised.
The majority of electricity is generated by gas at an efficiency of around 40%, in which case the heat energy lost is at the power station. With GASHP the gas is supplied direct, with the internal combustion efficiency taking place on site, at around 35-40%. While this figure is much the same, the waste heat energy can potentially be harnessed, changing the whole energy/cost balance completely. Such heat can be used to heat domestic hot water, but only during the cooling cycle.
Benefits of GASHP

  • No defrost cycle so constant heat input
  • Full heat output down to -15°C
  • Potential for recovering heat generated by the engine cooling system, normally lost up the cooling towers, especially in power stations.

The Colt Caloris WRF (Water Refrigerant Flow) system can be supplied with heating and cooling from a GASHP system. Better still, because the secondary system already incorporates heat recovery between units, heating and cooling at the same time, the Caloris system needs only a changeover GASHP unit and not a heat recovery version, which would be required to operate a VRF (Variable Refrigerant Flow) system. A refrigerant-to-water heat exchanger is used with the Caloris system passed directly through. In this case the refrigerant pipe run-losses are minimal and the EER rises by some 10-15%!

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