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New research shows that hybrid heat pumps that combine an electric heat pump and a gas-fired boiler represent a technically feasible and economically viable solution to partially decarbonise the heat supply in older homes.
An international research group has studied hybrid heat pumps (HHPs) as a technically and economically viable solution to decarbonise the heat supply in older residential buildings and found that they can achieve slightly higher energy savings than electric-only heat pumps. (E.H.P.).
The scientists described an HHP as a system that combines an electrically driven air-to-water heat pump with a gas condensing (GB) boiler, and stated that these devices would be particularly suitable for older homes equipped with high-temperature heat emitters.
“During colder weather periods, when higher heating loads occur along with associated lower EHP performance, HHPs offer the ability to switch to GB mode, which may be more favorable in such conditions,” they explained, adding that HHPs usually work in EHP mode when the outside temperature exceeds 3ºC. "The use of GB mode can also offer certain advantages over EHP mode when domestic hot water (DHW) is required, often at temperatures above 50ºC."
Using an energy simulation model, the group analyzed the performance of an HHP in a residential building retrofit scenario involving both the heating system and the building envelope. He then compared the performance of the HHP against that of a reference EHP and fossil fuel heating systems with different envelope rehabilitation measures and boundary operating conditions.
They validated the model in a south-facing house located in Dublin. It was built in 1999 and modernized with low-temperature aluminum radiators between 2014 and 2015. The building has an area of ??160 m2, a height of 2.5 m and a total area of ??139 m2 of exterior walls.
They considered three different rehabilitation scenarios: a base case scenario that takes into account the original construction; a minimum rehabilitation scenario based on the rehabilitation applied between 2014 and 2015; and a deep refurbishment scenario which assumes the building is renovated in accordance with current Irish national guidelines. "For all scenarios, the heating system was operated normally to maintain an average internal temperature of 20 C with a dead band of 0.5 C at all times," they specified.
They conducted their analysis assuming a heating system combining an 8 kW electric heat pump with a 33 kW gas boiler. It can work in three different modes: 1 – When the outside temperature is below 2ºC, the gas boiler supplies all the heat load; 2 – When the outside temperature is higher than 2ºC, the electric heat pump supplies all the thermal load; 3 – The heat pump and the gas boiler work together when the outside temperature is higher than 2 C and the required water temperature is higher than 45 ºC.
In their simulation they considered oil, natural gas and electricity prices of €0.088 ($0.095)/kWh, €0.069/kWh and €0.21/kWh, respectively.
The researchers found that all hybrid heat pump scenarios achieved slight energy savings compared to the respective electric heat pump scenarios. They also found that hybrid systems were competitive in terms of carbon emissions per unit area. “Between 41% and 80% of the capital cost of the heating system can be recovered in all scenarios with respect to a diesel boiler system,” they stated, noting that the economic performance of HHPs depends strictly on the prices of electricity and gas.
They presented the novel heat pump concept in the study “ Technical and economic assessment of a hybrid heat pump system as an energy retrofit measure in a residential building ”. in a residential building), published in Energy and Buildings . The research group is made up of academics from Dublin City University and University College Dublin (Ireland), as well as the University of Sassari (Italy). |
