Solar-assisted CO2 heat pump shows 18.7% COP gain with rising irradiance

Italian researchers have demonstrated that a solar-assisted CO2 heat pump can increase its coefficient of performance by 18.7% when solar irradiance rises from 322 to 904 W/m².

How does irradiance affect CO2 heat pump performance?

A team at the University of Naples Federico II and ENEA tested a 10 kW dual-source prototype at a Rome research center. The unit switches between an air evaporator and ten south-facing photovoltaic-thermal (PVT) collectors tilted at 48 degrees. In solar mode, CO2 flows through copper tubes bonded to the rear of the PVT array, while a 300-litre tank simulates domestic hot water demand at 20–60 °C.

Across 143 solar-mode tests, raising irradiance lifted the refrigerant evaporation temperature from 4 °C to 12.9 °C. That cut compression work, boosting heating capacity 21.1% (8.75 kW to 10.6 kW) and COP 18.7%. Below a minimum irradiance threshold, however, air-mode remained more efficient because the air evaporator has lower thermal resistance.

What are the economic implications across different climates?

The researchers trained artificial neural networks to predict electricity use, COP and compressor frequency, then ran annual simulations for seven cities — Rome, Tenerife, San Diego, Toronto, Mexico City, Perth and Beijing — using five hot-water profiles and a 900-litre store. Source-switching saved only 100–120 kWh of electricity per year in the best cases. The dominant economic driver was the PVT field’s own electricity generation. Simple payback ranged from 6–10 years in high-sun locations to over 20 years where solar resource is weaker.

Ecoverse analysis

The 18.7% COP improvement corresponds to a rise from roughly 3.2 to 3.8 (assuming a typical baseline CO2 heat pump COP near 3.2 at 4 °C evaporation). That gain is meaningful for domestic hot water, but the modest switching savings highlight that PVT electricity output, not thermal COP alone, determines project viability.

Read the full study. For more on solar payback calculations, see How does a solar panel system pay for itself?.