How to power buildings with solar-driven reversible heat pumps, organic Rankine cycle

Researchers from Hebei University of Technology in China have proposed an integrated energy system that combines solar-driven reversible heat pumps with organic Rankine cycle (ORC) technology to provide efficient and flexible energy supply for buildings.

Their concept leverages the complementary operation of both systems: the reversible heat pump can deliver heating or cooling depending on seasonal demand, while the ORC system enables the recovery and conversion of low-grade thermal energy into electricity.

ORC is a thermodynamic process that generates electricity by using an organic working fluid with a low boiling point to convert low-temperature heat sources into mechanical work, which is then transformed into electrical power. Compared with the conventional Rankine cycle, it is better suited for low- and medium-grade heat recovery applications because the working fluids can efficiently vaporize at relatively low temperatures, enabling effective energy conversion from waste heat sources such as industrial exhaust, geothermal energy, or solar thermal systems.

“Our study focuses on the design, thermodynamic modeling, and annual performance evaluation of a switchable solar energy system combining heat pump and ORC cycles for residential buildings,” corresponding author Xiaohui Yu told pv magazine. “The system can switch between heating and power generation modes depending on seasonal demand, enabling year-round utilization of solar energy. A dual-function compression–expansion unit is introduced to simplify system architecture and improve operational flexibility.”

The proposed system integrates solar thermal collection, heat pumping, and power generation within a single flexible configuration. At its core is a dual-function unit coupled with a permanent magnet synchronous motor/generator, which can operate either as an expander for electricity production or as a compressor for heat pumping depending on seasonal demand.

Solar energy is harvested through a collector and transferred to an evaporator, where it heats the working fluid in an organic Rankine cycle loop. The vaporized fluid then drives the dual-function unit to generate electricity via the PMSM before being condensed and recirculated. A heat storage tank enables thermal energy buffering, supplying heating when solar input is insufficient. Waste heat is managed through a condenser connected to a cooling water loop and a cooling tower for heat rejection.

In heating-oriented operation, recovered thermal energy can be directed to the building through a dedicated heating water circuit. By adjusting multiple control valves, the system can switch between power generation, direct solar heating with storage, and heat pump operation for low-irradiation conditions. This multifunctional integration enables continuous energy supply and improved utilization of low-grade solar thermal resources.

Through a series of simulation, the research group found that system performance is strongly influenced by solar irradiance and operating temperatures. In the heating season, when solar irradiance increases from 300 to 650 W/m², the heat pump coefficient of performance (COP) rises from 3.2 to 4.1, representing an improvement of about 22%. This enhancement is mainly attributed to higher evaporation temperatures under stronger solar input, which improves heat absorption, even though compressor power consumption also increases.

In non-heating periods, the ORC electrical conversion efficiency exhibits a seasonal trend of being higher in spring, lower in summer, and partially recovering in autumn. Rising ambient temperature negatively affects performance: as it increases from 10 C to 30°C, the thermoelectric conversion efficiency drops from 6.68% to 2.27%, resulting in an approximate 66% reduction in net power output.

Sensitivity analysis further shows that optimal system operation occurs when solar irradiance is between 600 and 850 W/m², where the COP remains above 4.0. However, increasing the heating supply temperature from 50 C to 80 C significantly reduces COP from 5.43 to 3.12, a decline of around 42%.

“Compared with conventional electric heating, the proposed system achieves a payback period of about 14 years,” Yu said. “Compared with conventional electric heating, the system reduces operating costs by approximately 88%, demonstrating strong potential for low-carbon building energy systems.”

The proposed system was described in “Thermodynamic performance of solar driven heat pump and ORC switchable system for building energy supply,” published in Solar Energy.