| HEAT PUMPS 46 January 2021 www.cibsejournal.com CIBSE Jan21 pp45-48 CPD 174 v3.indd 46 CO2 content of heat 300 250 Gas boiler 200 CHP 37%/40% 150 Heat pump COP = 3 100 Heat pump COP = 2.5 50 0 -50 -100 -150 Electrode boiler 0 100 200 UK 2019 average 241g.kWh-1 a heating water loop under the control of a building management system (BMS). This heating loop then feeds individual heat interface units (HIUs) in each dwelling, which provide heating and hot water on demand. The HIU includes a plate heat exchanger with accompanying valves and pumps to distribute the heat to the heating system in each dwelling, or to heat the hot water, as required. The design of such systems apply diversity to the size and mix of the heat generators in the plantroom to ensure that there is always sufficient heat available while also meeting the local statutory requirements for system efficiency. This as with all diversity calculations is subject to degrees of uncertainty, which inevitably leads to some oversizing to ensure no deterioration in service. The 2015 DECC report2 indicated that the losses for schemes that deliver heat to the individual customer (each dwelling or apartment) were significant, with one scheme from the selection of 14 existing large heat-network schemes selected as representative of UK installations recording losses of 43% (with the overall sample average being 28%). Evidence suggests that high losses can be experienced through internal distribution pipework within buildings, the report stated. These losses will not only reduce the overall efficiency of the systems, but are also likely to add to overheating problems in summer, as building heat networks operate to meet the need of individual apartment HIUs to deliver domestic hot water (DHW). Such overheating is, reportedly, particularly prevalent in communal areas - such as corridors, where heat-network pipework traditionally runs in ceiling voids which can require separate ventilation measures or temporary fans and comfort air conditioning to deal with the issue. The DECC report noted that, although operation costs associated with the heat network appear to be low, there are higher operation costs associated with other elements of the heat-network scheme, such as heat sources and energy-centre buildings. The highest operation costs appear to be associated with the HIU and heat-meter maintenance, as well as meter reading and billing activities. The recently published London Energy Transformation Initiative (LETI) Climate Emergency Design Guide5 illustrates indicative opportunities for reducing energy consumption in new residential developments to transform energy use in terms of energy use index (EUI) kWh.m-2.y-1 from current practice to one that delivers a low-energy CO2 content of heat (g.kWh-1) CPD PROGRAMME 300 400 500 600 Biomass boiler Power station extraction * electricityinfo.org/real-time-british-electricity-supply Electricity emission factor (g.kWh-1) Figure 1: CO2 content of heat (Source: DECC2) design, as shown in Figure 2. Among the basket of measures most of them associated with good building practices approximately 40% of EUI reduction could potentially be from the application of heat pumps and, to a smaller extent, mechanical ventilation and heat recovery (MVHR) and hot-water efficiency. Among several other feasible options, LETI5 identifies water source heat pumps in apartments that are connected to a tempered ambient loop as having the potential for zero carbon once the electricity supply grid is decarbonised (or if the systems are powered by onsite renewables). The promise of the Future Homes Standard, together with established methodologies such as CIBSE TM596, and tools such as those developed by the Good Homes Alliance,7 all aim to identify and mitigate overheating risks in new homes using passive means before mechanical cooling solutions are considered. However, homes particularly those in urban, densely occupied areas may require active cooling systems to maintain the comfort, health and wellbeing of occupants. Without appropriate system design at the planning stage, this will inevitably lead to the further proliferation of the portable air conditioner, which is increasingly promoted as a standard home appliance in the UK.8,9,10 Residential heat pumps, together with whole-home ventilation systems employing heat recovery, may be designed to effectively provide cooling (as well as heating and DHW). Employing a water source heat pump for each residential unit, exchanging heat with an appropriately designed and controlled ambient communal water loop, can provide heat recycling within a building for example, between heat pumps that are providing space cooling and those that are generating DHW) and can connect to district heat networks so that heat rejected from cooling systems is redistributed to buildings with heat demands. The use of an ambient network (as opposed to fourth-generation) provides greater opportunity to recover waste and employ renewable heat sources, such as solar water heating, seasonal thermal storage and heat recovered from sewage. In a system such as that illustrated in Figure 3, the individual heat pumps are connected to a single communal ambient heat loop, nominally maintained at 25C THE MATURATION OF HEAT NETWORKS Heat networks are typically categorised in terms of generations, which are necessarily broadbrush descriptions: First- and second-generation networks were popular until the mid-20th century employing steam and, latterly, high-pressure hot water and were notable for high heat losses. Beyond large, managed sites, such as hospitals and institutional estates, these are rarely used today. Third-generation networks have been the prevalent system in recent years, circulating hot water 70C to <100C, but many reportedly13 suffered from poor control. Most existing UK heat networks use this technology. Fourth-generation networks operate at lower temperatures of around 40C to 60C. These are more able to use low carbon heat sources and result in lower heat losses. Fifth-generation, or ambient, heat networks employ water at, or close to, ambient temperature, reducing or eliminating heat loss and reducing the reliance on pipe insulation significantly. These are used in conjunction with heat pumps to raise temperature to deliver heating and hot water. 18/12/2020 14:59