Research progress of high-efficiency, low-temperature hot water systems

The energy used to heat urban buildings accounts for 27% of total building energy consumption in China. Current hot water production systems are mainly based on direct use of fossil fuels or electricity, which is of low primary energy efficiency and produces severe air pollution. Based on the principles of “high-grade energy as driving source, low-grade heat for low-temperature demand, and natural energy fully used”, we have developed various high-efficiency, low-temperature hot water systems, including: (1) a fuel-driven, air-source heat pump system that uses existing energy as the driving power of the air-source heat pump to extract additional heat from the ambient air, achieving energy saving rates of 20%-40% and payback times of 3-7 years; (2) a fuel-driven, water-source heat pump system that can use an absorption heat pump to relieve underground thermal imbalance and reduce the required number of boreholes and consumption of the conventional electric heat pump; (3) a hybrid ground-source heat pump system with heat compensation that can store ambient heat in the ground efficiently via heat thermosyphon and heat pump modes, maintaining excellent system performance in long-term operation; (4) a hybrid solar air-source heat pump system that can make full use of solar energy of varying intensities and ambient heat with different temperatures, so as to improve system efficiency and reduce the initial investment of a conventional solar heat pump or air-source heat pump; (5) a hybrid air-source heat pump system aided by seasonal energy storage that combines advantages of solar energy and ambient heat, with enhanced heating reliability and economic feasibility relative to the normal air-source heat pump; (6) a hot water system based on heat recovery from data centers, which uses an integrated air conditioner with heat thermosyphon and heat pump modes to collect waste heat in a water loop, from which a water-source heat pump extracts heat to produce low-temperature hot water with an energy saving rate of 60%. All the above proposed hot water systems are assessed to have high energy efficiency, heating reliability and economy feasibility, which should be important in energy savings and emission reduction within the building sector.