MAINTENANCE, REFURBISHMENT & REGENERATION


Incorporating heat pumps into low temperature heat networks


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As the UK pushes to decarbonise its heating systems, the take-up of heat pumps is gathering pace. The challenge now facing specifiers and contractors is how to effectively integrate heat pumps into heat networks without compromising reliability or comfort for end users, while addressing the relationship between heat pump and HIU. Alex Bradley, commercial director at Modutherm, looks at how advances in HIU technology can help bring these systems to the fore


hen looking to supply residential or commercial spaces with heating and hot water, a communal system, or heat network, is often regarded as the


most cost-effective and least carbon intensive option, compared to multiple individual alternatives.


A heat network, distributes heat energy generated from a central location to individual end users located within a building or buildings. The heat energy is delivered to the end users as low temperature hot water (LTHW) at < 90°C via a network of pipes. This energy is used to produce domestic hot water and space heating. Indeed, a well-designed heat network can deliver both cost and energy savings relative to the installation of multiple localised heat sources. More importantly, heat networks can better facilitate the integration of low carbon and/or renewable heat sources, reducing harmful CO2 emissions.


Assessing the options


So, what are the options available today? Fourth generation heat networks have become prominent, as they can operate at low flow temperatures circa 55-70°C. They enable a more cost-effective transition away from fossil fuels by the greater utilisation of renewables (such as heat pumps) and waste energy to maximise output and minimise carbon emissions. In fact, we have seen significant growth in the amount of heat networks incorporating commercial heat pumps on the primary system. These are then connected via the heat network to next generation, ‘smart’ heat interface units (HIUs) installed within each apartment. These fourth generation systems are further supported in the latest revisions to CIBSE’s Guide to Heat Networks (CP1 2020). The document sets the minimum requirement for best practice, as well as guidance on improving overall efficiency. CP1 2020 also focuses on efficiency at low temperatures caused by the expected rapid increase in the dominance of heat pump technologies. This has led to the recommendation that heat networks should be specified and developed with low-temperature and smart controls in mind. Alternatively, fifth generation systems reduce flow temperatures further, in most cases, down to ambient. Individual heat pumps are then utilised in dwellings to create heating and DHW. However, ambient systems are not a replacement for fourth generation systems, they are an alternative – especially as they may not be suitable for all projects. Indeed, for applications with solely apartment buildings, a fourth generation system is usually the best option.


Design decisions


From a design perspective, there are many factors that affect a networks efficiency network. Three key focus points would be:


1. Selecting the most efficient terminal units (such as HIU’s with a low VWART).


2. Keeping the number of network bypasses to a minimum.


3. Controlling the network pumps via index located differential pressure sensors thereby ensuring energy availability across the network. It will also ensure that the turndown ration between peak and ‘idle’ flow is maximised, reducing pump energy use and maximising delta T. Today’s high efficiency pumps can typically operate at less than 10% of their peak flow.


It is important that the high network delta T is not ‘wasted’ at the plant-room. The hydraulic connections need to ensure that there is no circulation through the thermal store back to the energy sources and that stratification is occurring. To achieve this, it is recommended that the store is connected in a two pipe, bi- directional flow arrangement. Thermal storage capacity also needs to be maximised. This creates flexibility, resilience and increases the ‘worth’ of the renewable energy sources. On heat pump networks this becomes even more important. Large networks can operate with significant differential pressures and it is imperative that the HIU control valves are capable of operating, with good authority, against these pressures.


A measure of performance


If heat interface units (HIUs) are to be considered heat pump ready, their efficiency and performance needs to be compatible with typical heat pump performance parameters. The HIU will need to produce the required apartment output at the lower


12 BUILDING SERVICES & ENVIRONMENTAL ENGINEER DECEMBER 2023


network flow temperatures. These lower temperatures would typically result in higher return temperatures and reduced delta T. However, a heat pump ready HIU, will be optimised to still deliver a very low return temperature even under these conditions. BESA (British Engineering Services Association) has created a UK test regime for HIUs that calculates a particular HIU’s efficiency (or VWART). This single value, shows the expected annual average return temperature, from a particular HIU, installed on a heat network. The lower this figure, the more efficient the HIU and the larger the network delta T will be. This reduces network flow rates and annual volumes while delivering the same kW delivery. This allows smaller pumps, pipes, fittings and valves to be used, reduces network losses and pump energy use lowering capital and operating expenditure. (At Modutherm, our MTA PLUS TWIN HIU achieved top marks in the BESA standard, registering the lowest VWART tests of any heat interface unit, with values of 26 (high) and 24 (low) respectively.)


Optimal heat network efficiency will only be achieved with careful and correct matching of the heat pumps, thermal store(s) and HIUs. It should be remembered that much of the network delta T will be dictated by the HIU’s and that different units can give rise to widely different delta T’s. A manufacturer that knows its products can advise on the achievable network return temperature and required flow rate to achieve the peak network demand. The heat pump manufacturer can then calculate the recommended thermal store size for their heat pumps So, there are many factors to consider when incorporating heat pumps into low temperature heat networks. Not only do HIUs need to be compatible but they need to produce return temperatures as low as possible. Thankfully, by selecting the best performing HIUs on the market, truly decarbonised heat networks remain firmly within specifiers and contractors’ reach.


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