Hybrid systems - the latest news

Hybrid, or bivalent systems, as they are sometimes known, are those that provide heat, water or energy from both a renewable source and a traditional one. For example, one might have a heat pump that is electronically interlocked with a traditional boiler: one controller decides when to use the heat pump and when to use the boiler to ensure both that demand is met and the most efficient energy source employed.

In this article I’m focusing on hybrid heating systems and some new developments that may mean they become eligible under government incentive schemes.

Such systems are ideal for remote properties where the rising cost of fossil fuels, particularly in off-gas areas, make an alternative source of energy attractive.
Yet heat loss in such properties is often too high for a single phase heat pump to be a good solution on its own, and the cost of installing 3 phase supply can be prohibitive: thus systems which use a heat pump in combination with another energy source are often the only solution.

Heat pumps can be sized in one of three ways:

  1. The first option is as a stand-alone renewable technology. “Mono mode” where the refrigerant circuit is large enough to supply all the heating demands of a system using the compressor to extract environmental heat. This is the most energy and carbon efficient option, but obviously requires a large heat pump for the task in hand, and this may mean a 3 phase supply is required for larger properties. It is also best run with a low temperature heating system or over-sized radiators. As already discussed where 3 phase supply is not available this option is sometimes unviable.
  2. Alternatively “Mono- energetic” mode is a kind of hybrid solution in which the compressor is sized to provide energy up to a given percentage of demand (or supply heating until the outside temperature drops below a given threshold) at which point the heat pump’s internal electric element will switch on to make up the shortfall. This also works best with low temperature heating systems or oversized radiators.
  3. Finally, there is the truly “Dual mode” in which a heat pump is paired with a fossil fuel boiler, again switching over to the boiler at a given point when demand exceeds the supply of renewable energy available. This option is often employed in retrofit situations where there is an existing central heating system in good condition and the property owner wants to reduce reliance on fossil fuels.
Debate about these latter two systems as regards their eligibility under the RHI and Green Deal has been ongoing for some years now. Whilst these systems are popular, and are how many of the first renewable technologies were employed in this country, they have been subject to some scrutiny. This was often due to the failure to manage customer expectations about the cost and carbon savings and consequent payback periods that these systems would offer.

However a bivalent heat pump remains the only solution available for a weak single phase supply and, if sized carefully, can undoubtedly save carbon and cash. The graph below shows how much of a contribution a heat pump will make to yearly heating requirements when sized at different percentages of the property’s heat loss. Counter-intutively, this is not a direct correlation. For example, a system with a heat pump sized at 50% of the heat loss will gain 80% of it’s requirements from the heat pump and have only 20% to make from it’s back- up system. In other words, a 20kwh property fitted with a 10kwh heat pump will have 80% of its heat delivered by the heat pump (the blue line on the graph shows this example).
Hybrid heat pump system sizing graph

Moreover, the problem remained that even if a bivalent heat pump system were to be included in an incentive scheme, such as the RHI, there was no easy way to count the contribution of a heat pump.

The reason I’m writing this piece now, is that I see a glimmer of hope on the horizon that we may be approaching a solution to both hybrid system eligibility under the RHI/Green Deal, and how to meter a contribution made by a heat pump.

In my capacity as Chairman of the Domestic Heat Pump Association, I have been working with DECC to find a way forward on both issues. Firstly, The new MIS 3005 legislation has placed a requirement on installers to ensure that customers are properly briefed on the contribution of a heat pump in an installation in dual mode:

“it shall be clearly stated by the contractor what proportion of the building’s space heating and domestic hot water has been designed to be provided by the heat pump. The figures stated (i.e. the proportion of the annual energy provided by the heat pump) shall be based only on the energy supplied by the heat pump and shall not include any heat supplied by a supplementary electric heater,” (MIS 3005 3.1.a).

The same legislation has laid down guidelines that state that where a heat pump is the sole source of heating it is always 100% sized for the property it is heating, preventing customers from being mislead into assuming an installation is in mono mode, when it is in fact mono-energetic.

The clarity that has thus been established around how hybrid systems are to be specified and sold to customers should help with industry and consumer confidence with this kind of approach.

By, the by, we have a very cool tool (even if I do say so myself) for calculating heating the annual energy provided by a heat pump in a bivalent system:

Heat pump contribution calculator

Secondly, with the publication in May of the outline strategy for Smart Metering (now in the consultation phase), the way forward for metering hybrid systems has become clearer and I see no barrier to them now being included in incentive schemes. (See  Smart metering information from DECC)

But the detail of the RHI agreement remains to be set, and I feel it is critical that we now approach those of you at the sharp end of the market for your thoughts. The question still remains at what point the incentive schemes will deem the contribution of a heat pump to a heating system to be sufficient to allow it to qualify. As the installed costs of a system are key in calculating whether a system is viable, it is those who have the best understanding of these costs (i.e. those who install them)who will be key in helping to decide these figures. Taking into the account the graph above, where do YOU see the sweet spot for bivalent systems? Where do install costs vs the renewable contributions made by a system mean it is viable? How does this differ for different types of system (eg air source or groundsource)?