Italian researchers have combined a solar-powered air source heat pump with a water tank to store thermal energy to produce hot water in a residential building as a retrofit solution. They claim that the proposed system configuration can achieve the “desired” heat output while reducing annual electricity consumption.
The system is expected to replace traditional electric boilers, and its purpose is to decouple energy production and demand. The scientists said that the novelty of their approach consists in creating two levels of thermal energy storage to exploit the full potential of the photovoltaic system.
“The first storage level is provided by a large centralized water tank equipped with a programmable control system that uses the water tanks as a buffer and allows the heat pump to operate under the desired conditions,” they explained. “Another level of energy storage is provided by innovative thin and modular distributed tanks, referred to below as e-Tanks dedicated to the production and storage of DHW provide better thermal efficiency and, at the same time, greater user autonomy thanks to local storages installed in each apartment.
e-TANK consists of a water tank and a water module designed by the Austrian expert Pink GmbH. Both components are mounted on the wall in a steel frame. The tank is also equipped with an internal coiled heat exchanger connected to a 2-pipe hot water network. This network can be used either for charging hot water heaters or for heating purposes. It can operate at high temperatures during charging periods during the day. It has lower heat losses from the pipeline and lower consumption of circulating water pumps compared to traditional boilers.
Through the TRNSYS software that is used to simulate the behavior of transient renewable systems the research team simulated the proposed system configuration in a residential building located in Catania, Sicily, southern Italy, and assumed that it could operate with a 13.5 kW PV system consisting of 36 monocrystalline modules.
The electricity produced by the PV system could be either by a reversible air-water system with a heating power of 26.0 kW and a coefficient of performance (COP) of 3.10, or stored in a 20 kWh battery. “The maximum temperature of the water supplied by the heat pump is 65 °C when the outside temperature is between 5 and 19 C,” the researchers said.
The researchers compared the energy efficiency of the proposed retrofit solution to the existing system in the pilot building and found that the system achieved the “desired” thermal performance in both the loading and unloading processes.
“The result obtained by dynamic simulations showed that e-The TANK system reduces the annual energy consumption of HVAC production by more than 7,200 kWh compared to the current domestic water system, they explained. “It was shown that a typical centralized system requires about 11 times more electrical energy for a circulation pump than a circulation pump e-TANK solution.”
They also found that the new solution has a 6.1% lower life cycle cost (LCC) over 20 years compared to traditional boilers. “Furthermore, it turned out that applying the proposed PV-BESS system to power generation reduces the required primary energy by 7.1 MWh/y compared to generating power from the grid,” they emphasized.
The team presented the new solution in the magazine “Technical-economic analysis of a new hot water system retrofit solution: a comparative study,” published Energy conversion and management. “The results of this pilot building study are expected to be extended to a significant part of the current building stock in Europe,” it said.