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HVAC SYSTEMS


Figure 2: A model implementation of a system check (left), and a CHP installation (right).


includes three gas-fired boilers, but John Henry explained that, ‘under scrutiny’, the way they were originally designed and set up, in terms of key parameters, ‘simply wasn’t optimal’. He said: “We weren’t fully aware of this until Hysopt undertook the modelling. There’s always an assumption when you inherit a building that the HVAC plant will have been designed in a way that meets its needs. However, it’s not until you really get under the skin of the design that you see there is a better way of doing things, and that, via fairly simple modifications, we should be able to achieve some significant carbon reductions.”


Key modifications


I asked what the key modifications suggested by the analysis of the Hysopt modelling were. Chris Davis responded: “The Trust’s original proposal was to apply heat pump technology to provide predominantly domestic hot water, and to use the produced cooling from the heat pump to contribute to the existing cooling distribution. That route would, we concluded, only give the organisation an element of its targeted energy cost and carbon savings. The modelling also showed that only 30 per cent of the cooling produced by the heat pump system could be utilised in useful cooling, with the remainder rejected to the atmosphere. The model was also indicating that the existing chillers will trip out under certain working conditions due to the hydraulic integration of the suggested heat pump. As we made a digital twin of the whole system anyway when we were going through the exercise, we reviewed the existing plant as well. “We could see that the existing gas boiler-based space heating system was based on a constant flow installation,” he continued, “where the warm water was being circulated around the whole building constantly, with the hydraulic control on the secondary side and in individual rooms relatively rudimentary. Thus the return temperatures to the plant room were very high, with very little delta T across the system. When we see such factors, we know there is an opportunity to improve the delta T, reduce return temperatures, and get the existing plant operating more


efficiently, as well to reduce pump energy consumption. If the hydraulic system connected to the pumps is not optimised, you can miss a huge electrical energy- saving opportunity.”


Potential savings confirmed Chris Davis went on to explain that – based on the ‘digital twin’-created software – the company was able to suggest some modifications to the Cardigan Building’s heating distribution circuits, including changing the way the circuits are connected in the plant room, and changing the existing pump operation and control. He said: “Based on these steps, we were able to show the Trust that potential annual energy cost savings of 21 per cent, and 16 per cent carbon emission reductions, were achievable for the Cardigan Building at about a fifth of the capital cost of installing a heat pump.” John Henry added: “Among the key steps proposed for optimising the operation and efficiency of the heating and cooling system are splitting the low loss header, installing a flow-controlled bypass, and replacing all the dividing circuits with passive mixing circuits – to lower return temperatures and reduce the pump energy consumption; changing the primary pumps from 11 kW fixed to variable speed 2.2 kW pumps; upgrading mixing circuits; replacing 36 diverting


circuits to throttle circuits, and changing six cross-coils to passive mixing circuits. The impact of these measures should be significant. For example, based on the Hysopt modelling, changing from constant to variable flow is expected to reduce pump energy consumption by 93 per cent.” In addition, circulating/operating hot water temperatures in the Cardigan Building will be reduced from 85/80˚C to 67/52˚C at peak load, and to as low as 40/30˚C in milder weather conditions. The modifications will also reduce the peak radiator capacity from 2.1 to 1.2 MW, and halve the flow from 121 m3 under 60 m3


/hour to /hour. Chris Davis, Hysopt’s UK Sales manager.


Oversizing of original plant Chris Davis interjected: “The model showed us that the required heat load in a building for which the original heating and cooling plant was significantly oversized wasn’t, in fact, 2.1 MW, but 1.2 MW, and the Trust will be able to use the oversizing of the pipework and radiators to deliver the same levels of comfort at much lower operating temperatures. This should not only make the existing heating/cooling system work more efficiently, but also opens the door for the future for the Trust to consider installing heating pumps to the heating side confident that it has a system able to operate at the lower temperatures that are more heat pump-compatible.” I asked what stage the project was now at. John Henry explained: “Clearly this is still a theoretical model at this stage, so now, working with Hysopt, we need to determine how we turn all this into a specification. What I’d like to do is use the ‘digital twin’ as the ‘spec’, so we know what is required, and what the flow rates and temperatures are at the various points. If we convert this detail into the specification to go out to tender, we have our design and measurable parameters, and will then not need to take the traditional route of employing a design consultant to go back to basic principles. We now need to identify how we turn the digital twin into a specification that contractors can price on, as well as to consider whether we specify the heating circuit pump types; I think we will need to do so.


September 2020 Health Estate Journal 77


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