Bivalent energy is the term describing secondary-energy, added to the heating system by an energy source other than the heat pump. The purpose is to ensure sufficient energy is available to meet the prevailing heat load.
Periodic requirements for bivalent energy exist in both air source and ground source heat pump installations, but the requirement is generally more pronounced with air source systems.
The predominant method of adding bivalent energy is by electrical heating elements, located inside the heat pump unit itself.
Characteristically, an air source heat pump will deliver its best efficiency and maximum output when ambient temperatures are higher. As temperature falls, so does the heat pump output and efficiency.
Climatic compensation
Heat pump manufacturers are aware of this fact and many models of heat pump are supplied with climatic compensation as a standard fitting.
The bivalent energy source is generally an electrical resistance heater element, embedded in the heat pump unit and turned on-and-off according to ambient temperature.
Climatic compensators are usually about 3kW in capacity, but larger heat pumps could be equipped with heaters of significantly higher output.
As ambient temperature declines, output also declines, simultaneously the heat load increases, and an energy deficit within the system is created.
The energy deficit is un-quantified, and often over-catered for by the heater, resulting in a higher than necessary operating cost.
Although some form of bivalent support is inevitable in every heat pump system, defining the precise amount of energy required is more difficult. Too little energy does not solve the problem, and too much is economically impractical.
So, how do we know when and how much bivalent assistance is required?
Energy deficit
The first indication of an energy deficit is the precise moment that the heat pump identifiably fails to maintain the minimum designed water temperature.
At this precise moment, the materialising energy deficit will be small, but as the differential between actual and adequate water temperature gets wider, the need for energy input increases proportionally.
To ensure the highest efficiency and best economics, supplementary energy input should never exceed the energy deficit, resulting in an overall reduced coefficient of performance.
System efficiency can also be compromised by injecting the supplementary energy into the system at the wrong location.
Supplementary energy should be introduced directly into the emitter circuit, after the thermal store/buffer cylinder, ensuring thermal losses from pipe work located outside the property are not responsible for wasting this high value energy.
Bivalent energy added directly to the emitter circuit has much less possibility of an overall negative effect on heat pump efficiency.
Ideal solution
The ideal solution for quantifying and adding just the right amount of bivalent energy is achieved through “Intelligent Temperature Trimming”.
By constantly tracking the emitter circuit water temperature, an intelligent device will define as and when climatic conditions deteriorate or improve, adding energy proportionate to the impact these changes have on the system.
It should be noted that all supplementary energy devices need to be prevented from operating until ambient temperature falls below a minimum value.
The actual temperature varies according to geographic region but ranges from -1 ̊C to -3 ̊C.
There is a compelling argument to have bivalent devices operating irrespective of ambient conditions, but MCS guide lines currently state this is not acceptable.
In the next issue, I will discuss other bivalent energy sources such as LPG, natural gas, or even oil, and economic considerations for the production of domestic hot water in heat pump systems.