INTEGRATING HEAT PUMPS INTO HEAT NETWORKS
Josh Collins, Heat Networks Manager at Altecnic, considers the challenge of integrating heat pumps into communal heat networks.
Integrating a heat pump into a communal heat network needs two sets of operating requirements to work together. The heat pump must receive the water temperatures and flow rates it needs, while the wider distribution network benefits from low return temperatures and a wide temperature differential (ΔT). The challenge is to accommodate the selected heat pump without compromising the performance of the wider network. Many heat pumps operate with a water temperature rise of around 5-10K. A heat network, however, may be designed around a much wider differential, for example, 55°C flow and 25°C return. These are separate design requirements. For a given heat duty, a narrower ΔT requires greater water flow. Reducing a network from a 30K to a 5K differential requires six times the flow. Across a heat network, this can mean larger pipework and pumps, increased energy and greater demands.
Maintaining low return temperatures also helps reduce heat losses from return pipework. But a 30K differential should not be seen as a universal target. What can be achieved will depend on the system, the DHW demand and operating conditions. The objective should be the lowest practical return temperature without compromising the service.
A narrower ΔT can be managed locally rather than designing the entire network around the same conditions. A heat pump operating at 55/50°C can serve a network operating at 55/25°C. A correctly engineered recirculation arrangement can mix some of the 55°C outlet water with the cooler network return to provide the 50°C inlet temperature required. Recirculation adds local pumping duty and raises the heat pump’s entering water temperature, so designers should compare this with equipment capable of accepting wider temperature differences.
Hydraulic separation and buffer storage also need careful consideration. Flow paths, sensor positions and controls all influence how the system behaves, while poorly controlled mixing can dilute the network supply or disrupt useful stratification within buffer storage.
The type of heat pump selected can also change the priorities for the network design. R290 heat pumps can deliver temperatures above 55°C, so this should not be considered a refrigerant limit or universal efficiency threshold. However, increasing the required delivery temperature can increase compressor lift and reduce COP. If 55°C is enough to meet the network duty, routinely producing 60°C needs to be justified. Seasonal performance should be considered using manufacturer data across the source temperatures and expected loads. www.altecnic.co.uk