Technology and Hybrid Heating Plants
Heat pumps operate by moving heat from a lowertemperature area to a higher-temperature area . They typically take advantage of an electrically driven compressor and a closed vapor compression cycle . Commercial-grade heat pumps come in many forms and can provide useful heating , useful cooling , or simultaneously provide useful heating and cooling . Heat pumps are rated using their coefficient of performance ( COP ), which measures their useful output to the required energy input . The COP for heating ( COPH ) is the ratio of the useful heating energy divided by the energy input . The COP for cooling ( COPC ) is the ratio of the useful cooling energy divided by the energy input . For dual-mode heat pumps that can provide useful cooling and heating , the unit will have a combined COP — the ratio of the sum of heating energy and cooling energy to the energy input .
There are many ways to integrate a heat pump into an existing building heating system . The designer should consider the actual building heat load and the potential to apply the useful cooling available from heat pumps . Buildings with concurrent heating and cooling loads can be good candidates for heat pumps , such as water-to-water heat recovery chillers ( HRC ). It ’ s important to understand the presence of simultaneous heating and cooling loads in a building can indicate building inefficiency . Designers should first reduce wasteful simultaneous heating and cooling before sizing any heat pump system . Large buildings and campuses made up of many buildings will have simultaneous heating and cooling requirements that justify using an HRC . Heat recovery chillers can be used to cover the majority of a building heating load while supplying chilled water to the campus chilled water loop . To illustrate this concept , the simplified heating hot water piping diagram in Figure 3 shows a modular HRC piped in series with a variable-flow , gas-fired , condensing boiler plant to provide preferential loading of the HRC ( Peterson , 2019 ).
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FIGURE 3 : A sample preferential loading heat recovery chiller ( HRC ) heating hot water ( HHW ) piping diagram ( Peterson , 2019 ).
Heating Coil Performance at Reduced Water Temperatures
As discussed , the maximum capacity available from existing heating hot water coils will be limited by the original design building pipe sizes and the construction of the coil , i . e ., rows , fin spacing , circuiting , and the quantity of coil passes . Heat exchanger effectiveness can be a useful concept in assessing the capacity of an existing coil against lowering water inlet temperatures . Heat exchanger effectiveness is the ratio of the actual heat transfer rate to the maximum possible heat transfer rate in a counterflow heat exchanger of infinite surface area with the same flow rates and inlet temperatures ( ASHRAE , 2021 ). Suppose performance data of a heating coil , i . e ., data from a drawing schedule , is known .