Bivalent energy: How much, when and where

Bob Long, MD Eco Innovate Ltd
Bob Long, MD Eco Innovate Ltd
Heat pump trouble shooter Bob Long outlines the importance of accurate control in bivalent systems to avoid expensive fuel bills

When speaking of heat pumps, bivalent-energy refers to the supply of additional energy to supplement the output of the heat pump, as energy demand dictates.

Heat pump manufacturers often embed an electrical heater element in the water-output manifold of the heat pump, which adds bivalent energy to the water stream leaving the heat pump.

Bivalent energy sources are generally expensive to operate, particularly when compared to the p/kW output of a good quality heat pump.

Bivalent energy can be derived from a choice of dependable sources, but the source must be accurately controlled, able to quickly deploy energy, and cease immediately when not required.

Often, bivalent energy from embedded heaters is controlled by a climatic compensation thermostat and can be quite a crude method of control.

Climatic compensation control is a pre-requisite of the MCS installation criteria regarding air source heat pumps, and is designed to prohibit the use of bivalent energy until the outdoor climatic condition reaches a pre-determined low of approximately -2°C (Differs slightly according to geographic location).

Air source heat pumps experience significant reductions in energy output when outdoor temperatures fall and this drop usually means the system will need bivalent support.

Over-supply of bivalent energy will produce higher running costs and can negatively impact the overall economic operation of the heat pump itself.

A simple example would be that of a 3kW embedded electrical water heater, adding energy to a heating system that requires perhaps only 1kW of support. In this instance, 2kW of expensive bivalent energy would be wasted.

Bivalent energy is usually derived from relatively expensive sources, and it is therefore important for the energy to produce the best effect possible.

The most effective delivery point for bivalent support is directly into the emitter circuit, after the thermal store, and as close to the intended point of use as possible, eliminating costly thermal losses between the heat pump and the heat emitter.

Accurate control of bivalent energy is paramount to the heating system’s overall economic performance and a bivalent energy source should be capable of constant modulation according to ever-changing energy demand.

The energy demand of a heating system is not only affected by changing climatic conditions. The energy requirement is also influenced by indoor events for example occupancy levels or use of energy emitting devices such as cookers/stoves, tumble driers, hair driers, lighting, etc. which all have a positive effect on the heat load.

Alternatively, ventilation, domestic hot water use, or simply the number of times exterior doors are opened and closed, will have a negative effect on the heat load.

With so many unquantifiable factors influencing the energy requirement, accurately controlled bivalent support is an essential feature of any heat pump system.

A bivalent energy source needs to have the ability to respond to all variations in energy demand, and in doing so, should provide the exact amount of support required, and no more.

Too little, and the system will not reach the desired temperature, too much, and valuable energy will be wasted.

Irrespective of the energy source chosen, the cost per kW/hr of supplementary energy supplied will be more expensive than energy supplied by the heat pump, making accurate control of the bivalent energy a paramount consideration.

Proportional, modulating control of the bivalent energy source can be accomplished quite accurately and will be discussed in next month’s column.