Thermal performance of an asphalt solar collector coupled to a water-source heat pump for domestic hot water: A system simulation

Abstract

Electrification is increasing the use of heat pumps for domestic hot water (DHW), but their performance remains strongly dependent on source temperature. This study evaluates whether an asphalt solar collector (ASC) integrated into a 600 m2 parking area can provide seasonal source-side preheating for a water-source heat pump in Kaunas, Lithuania (54.9°N). An hourly model was run for 2015–2024 using ERA5 meteorological data and measured DHW demand. Building and vehicle shadows were represented through clipped polygon unions, collector heat transfer and hydraulics through flow-regime-specific correlations, and heat-pump capacity and electricity use through a manufacturer-based EN 14511 performance map that included operating limits, cycling, backup heating, standby demand and hydraulic auxiliaries. Under fixed-flow control, the shaded collector delivered 25.06 kWh m−2 yr−1 and increased the heat-pump seasonal coefficient of performance from 3.58 to 4.01. After collector pumping was included, the system seasonal performance factor was 3.92 and net electricity use fell by 8.66%. Variable-flow strategies increased gross heat collection by 4.6–4.7%, but reduced net electricity savings to 6.60–7.04% because of higher pumping demand. Across collector areas of 300–900 m2 and storage volumes of 2.5–10 m3, the maximum electricity-saving ratio was 9.80%. Meaningful collection occurred mainly from April to October, and neither economic nor carbon payback was achieved within the assumed 25-year service life. The system should therefore be regarded as a seasonal efficiency measure rather than a year-round heat source, with stronger prospects in sunnier, less shaded locations or projects that can realise additional pavement-related benefits.