Researchers at the Miguel Hernández University of Elche in Spain have designed an air-to-water heat pump system that can shift domestic hot water (DHW) production to central daylight hours, thus maximizing the utilization of PV power generation.
The novelty of the system lies in the use of two condensers instead of a single unit.
The researchers explained that a conventional compact domestic hot water (DHW) heat pump includes a compressor, evaporator, expansion valve and a condenser wrapped around the bottom of the storage tank, heating the full water volume through natural convection. The proposed dual-condenser configuration adds a second condenser at the top of the tank, combined with an optimized control system to select the operating mode, while retaining the standard components.
Both the lower and upper condensers consist of spiral tubes installed between the tank wall and the insulation layer. When the lower condenser operates, heat is delivered to the bottom of the 215-liter tank, promoting stratification and heating the entire volume. When the upper condenser is activated, only the top section of the tank is heated, enabling more targeted operation and lower energy storage.
The prototype was developed from a commercial split-type air-to-water heat pump equipped with a 600 W scroll compressor and R134a refrigerant. The original 2,400 W electric resistance heater was disconnected to ensure operation exclusively in heat pump mode. The system had a manufacturer-rated coefficient of performance (COP) of 3.17 at 14 C. Modifications included integrating the second condenser, redesigning the refrigeration circuit and upgrading the control system for testing under realistic DHW and PV operating conditions.
The experimental setup was designed to replicate real household DHW demand using a closed-loop system to avoid water waste. It comprised two climatic chambers, the dual-condenser heat pump, a 600 W PV installation and a controlled hydraulic circuit. The heat pump was connected to both the grid and the PV system, with no financial compensation considered for surplus electricity fed into the grid.
An auxiliary tank, circulation pump and water chiller maintained the inlet water temperature at 10 C to simulate mains supply conditions. An Arduino Mega controller managed pumps, valves, the chiller and the heat pump to enable automated testing. The system was also equipped with 30 temperature sensors, flow meters and electrical monitoring devices, with data recorded at one-minute intervals.
The researchers evaluated three configurations at an ambient temperature of 18 C: a conventional single-condenser heat pump, the same system coupled with PV, and the dual-condenser heat pump with PV. Tests followed an EN 16147-based DHW consumption profile, ensuring supply temperatures above 45 C.
Results showed that the dual-condenser configuration improved stratification control, reduced overall energy consumption and maintained DHW service quality while significantly increasing PV self-consumption.
The analysis found that the average seasonal COP of the heat pump reached 3.55 in the single-condenser configuration and 3.65 when combined with PV.
“As expected, both values are similar, since there is no difference in the operating mode between them,” the research team emphasized. “In the third test, with two condensers and an improved control strategy that allows operation with a lower water temperature, this efficiency rises to 3.71. This trend is more pronounced when analyzing the efficiency of the DHW service, where the results are 3.08 and 3.12 for the first two operating modes and 3.37 for the configuration with two condensers and PV panels. As the tank is colder in the configuration with two condensers, there are less heat losses.”
Solar energy self-consumption with the dual-condenser system, meanwhile, rose from 9.9% to 55.5%.
“The results also highlight the need to consider instantaneous self-consumption, using a calculation base of at most minute-by-minute rather than hourly or daily, as the latter result in unrealistically high solar contributions,” the academics concluded. “Considering the energy supplied by the PV panels, the performance of the HP can be reevaluated leading to a COP of 3.46 when working with one condenser and of 7.59 when working with the dual condenser configuration.”
The system was described in “Experimental assessment of a novel photovoltaic heat pump design with dual condenser,” published in Solar Energy.
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