Retrofit domestic hot water solution based on PV-driven heat pump, water tank

Researchers at the University of Bologna in Italy have developed a new retrofit solution for domestic hot water (DHW) that utilizes a PV-driven air source heat pump system relying on a water tank for thermal energy storage.

The system is expected to replace traditional electrical boilers and is aimed at decoupling energy production and demand. The scientists said the novelty of their approach consists of creating two levels of thermal energy storage, in order to utilize the full potential of the PV system.

“A first storage level is provided by a large centralized water tank which is equipped with a programmable control system to use the water tanks as a buffer and let the heat pump operate under desirable conditions,” they explained. “The second level of energy storage is provided by innovative slim and modular decentralized tanks, called hereafter e-TANKs, devoted to DHW production and storage, providing a higher thermal efficiency and, at the same time, a greater users autonomy through local storages installed in each dwelling.”

The e-TANK consists of a water tank and a hydronic module designed by Austrian specialist Pink GmbH. Both components are wall-mounted on a steel frame. The tank is also equipped with an internal helical heat exchanger that is linked to a 2-pipe hot water network. This network can be used either for the charging of DHW storage tanks or for heating purposes. It can work at high temperatures during charging periods during the day. It has lower heat losses from the piping network and lower consumption of circulating pumps compared to conventional boilers. 

Through the TRNSYS software, which is used to simulate the behavior of transient renewable systems, the research group simulated the proposed system configuration in a residential building located in Catania, Sicily, southern Italy, and assumed it could be powered by a 13.5 kW PV system made of 36 monocrystalline modules.

The electricity produced by the PV array could be either by a reversible air-to-water system, with a heating capacity of 26.0 kW and a coefficient of performance (COP) of 3.10, or stored in a 20 kWh battery. “The maximum water temperature supplied by the heat pump is equal to 65 °C for the outdoor temperature between 5 C and 19 C,” the researchers said.

The academics compared the energy performance of the proposed retrofit solution with that of the existing system in the pilot building and found that the system achieved a “desirable” thermal performance in both the charging and discharging processes.

“The result obtained through dynamic simulations indicated that the e-TANK system reduces the annual energy consumption for DHW production more than 7,200 kWh, compared to the current DHW system,” they explained. “It was shown that a typical centralized system requires about 11 times higher electrical energy for the circulating pump than the e-TANK solution.”

They also found that, over a 20-year period, the new solution has a 6.1% lower life cycle cost (LCC) compared to traditional boilers. “Furthermore, it was turned out that applying the proposed PV-BESS system for electricity generation reduces 7.1 MWh/yr the required primary energy, compared to the case that the electricity is provided by the grid,” they stressed.

The team presented the new solution in the paper “Techno-economic analysis of a novel retrofit solution for the domestic hot water system: A comparative study,” published in Energy Conversion and Management. “The outcomes of the present study for the pilot building are expected to be extended to a significant portion of the existing European building stock,” it concluded.