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Heat networks – a hybrid approach

Pete Mills, commercial technical operations manager at Bosch Commercial & Industrial, explores where renewables can fit in within a heat network

Pete Mills of Bosch Commercial & Industrial

With the trend for fourth generation heat networks managing to squeeze operating temperatures down towards their limits, system designers have more options for how heat will be generated sustainably, whilst keeping capital cost and operating costs on target.

Over the last decade, CHP has been the bedrock of heat generation within heat networks. However, with the rapid decarbonisation of the electricity grid and changes to Building Regulations on their way, heat pumps have become a natural replacement. However, for the operator of a heat network, the returns from the electricity generated from CHP often tipped the financial balance in favour of a scheme going ahead. Without this revenue, the pressure to keep capital costs under control can become much more important. At the same time, it is well known that since heat networks operate 24/7 over decades, the factors affecting the operating costs can be the most significant. So the system designer has to weigh up the options carefully and this is something that CIBSE’s CP1 Heat Networks: Code of Practice for the UK has long since pointed out.

There is a temptation for heat pumps to be the sole heat source for a new heat network project. On the face of it, this looks a simple option. Aligned with this there is an understandable desire from clients to do without a gas connection following the announcement from Government that its intention is for no new fossil fuel gas use in new build domestic homes from 2025. This could lead to a significant uplift in capital costs for heat networks and concerns that it could cap the number of heat network projects being delivered. 

A collaborative solution

Hybrid solutions between heat pumps and peak load boilers can offer a practical option to keeping capital costs under control, whilst still delivering significant carbon savings. Experience has shown that with around 50% to 60% of the peak demand covered by heat pumps, over 80% of the kWh’s can typically be provided through heat pumps. Heat networks typically operate below 25% of their peak demand for over half of the year, which is well suited to a heat pump. On the small number of days each year when temperatures are coldest, demand can be taken up by the peak load boilers. This makes even more sense where air source heat pumps are used, since it is on these days with low external temperatures that their operating efficiency will be at its lowest. Heat generating plant redundancy is also not optimised by having expensive heat pumps waiting to kick in on the rare occasion that another heat pump goes down. Far better to meet this need through lower cost boilers and further reduce capital costs.

Let us not forget plant replacement costs though, as heat networks typically will be designed to have service life of the order of 40 years or more. It is important that a plant replacement strategy be thought about from the outset. The energy landscape may evolve significantly within the timescales of the plant, particularly if we are considering large-scale industrial equipment. One of the unique abilities of a heat network is its straightforward adaptation to multiple forms of heat that may become available in the mid to long term. One key energy transformation that is looking more and more likely is the decarbonisation of the gas grid to hydrogen blends and ultimately 100% hydrogen. Without an existing gas supply, this option would be far more costly to implement later. 

Waste heat is often cited as the catalyst that sparked the very first heat networks to be built. Using a supply of heat that would otherwise be discarded makes perfect sense; something that has not gone unnoticed by policy makers. We are seeing, and are likely to see more policy moving towards incentivising this valuable resource. However, currently the trend is for smaller unconnected heat networks to be built; not ideal for the use of waste heat that is better introduced into a larger distribution network. Leaving strategic connection points that would aid future hook up to a wider district heating scheme makes sense and represents minimal capital outlay at the start of a project. If, at a later date an opportunity arises to connect to a much larger district heating scheme, the transition is a much simpler prospect. However, keep in mind that many larger schemes will have strict guidelines about connection arrangements and in particular the return temperatures that can be accepted. Leaving plant room space for associated plate heat exchangers and hydraulic control equipment will be necessary.

Finally, a technology that has sadly dwindled in recent years, namely solar thermal. It seems to have been put into the too difficult to do bucket, which I think is a pity. Admittedly, the financial argument is harder to justify, but in carbon terms, it can have a valuable contribution. Even for a small heat network, there can generally be found sufficient roof space for a reasonable size collector area that could contribute typically around 6% of the annual kWh’s. The good steady loads of a heat network suit solar thermal and it is straightforward to integrate if considered at the outset. Keep in mind that this technology is robust if maintained and sized correctly, meaning it can have long service life without significant replacement.

So, as you can see, there are a number of options for renewables to be included in hybrid heat networks. If used effectively it will greatly help large developments and properties reduce their carbon emissions and ultimately help the UK on its path to net zero 2050.


22 December 2020


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