Scientists in Spain have simulated a system that uses both power-to-heat-to-power thermal batteries and lithium-ion batteries for energy storage. The hybrid system reportedly achieved a 7% lower LCOE compared to a PV system relying solely on lithium batteries, while simultaneously increasing PV self-consumption by up to 20%.
Researchers from Spain’s Technical University of Madrid have designed a hybrid system that combines PV, lithium-ion (Li-ion) batteries, power-to-heat-to-power thermal batteries (PHPS), and heat pumps.
“The high capital cost per unit of energy storage of Li-ion batteries often results in systems with relatively small storage capacities, leading to low self-consumption ratios,” the group explained. “Thermal batteries with power generation capacity, such as PHPS, leverage the significantly lower cost of thermal energy storage to increase the overall storage capacity of the system. In addition, PHPS systems generate heat as a by-product during the energy conversion, which can be used directly in the building to supply its heating demand.”
The research team simulated a fully electrified building via PVSyst and EnergyPlus, with price and weather data relevant to Madrid.
The PHPS and Li-ion (LP system) include PV panels, Li-ion battery system, low-temperature energy storage (LTES), and PHPS system. The latter included a power-to-heat (P2H) converter, high-temperature energy storage (HTES), and a heat-to-power (H2P) converter.
A reference L system without the PHPS unit was used for comparison. Both systems were tested with either an electric heater with a coefficient of performance (COP) of 1 or a heat pump with a COP of 2.7.
“The priorities of the energy use for the LP system are (1) electricity demand is prioritized to be met by PV generation, followed by the PHPS system, and finally by the Li-ion battery; (2) heating demand is preferentially covered by the LTES, then by PV, PHPS, and finally Li-ion; (3) when there is a surplus of PV generation, this is preferably used to charge the Li-ion battery, then the PHPS, and finally the LTES. Any remaining excess PV generation is injected into the grid without remuneration,” the academics explained. “That also applies also for the L-system configuration, removing the PHPS.”
Image: Universidad Politécnica de Madrid, Applied Energy, CC BY 4.0
The yearly total energy demand of the system was set to 20 MWh, with electricity demand accounting for 4 MWh and heating demand for 16 MWh. The lifetime of all technologies in the configuration was assumed to be 25 years, with the cost per power capacity of PV being €800 ($910.2)/kW. The weighted average cost of capital was assumed to be 4%, and inflation was set at 2%.
Through their analysis, the research team found that hybridizing Li-ion batteries with a low-cost PHPS system is generally preferable over a single Li-ion battery installation, regardless of variations in the costs and COP of the heat pump. Furthermore, they ascertained that the low cost per energy and input power capacity of the PHPS system makes it ideal for storing high peak power from PV generation and supplying base-load power demand over longer periods. “Contrarily, optimal Li-ion battery specializes in supplying high peak power demand for shorter durations,” they further explained.
The scientists also found that the levelized cost of energy (LCOE) for an LP system with a heat pump was €76/MWh, while it was €147/MWh for an LP system with an electric heater. In comparison, the L system with a heat pump had an LCOE of €77/MWh and of €149/MWh when using an electric heater. The optimal size of the PV unit was 7 kW for the L system with heat pump, and 10 kW for all other systems.
The PV self-consumption rate was found to be 68.3% in the LP case with the heat pump, 68.1% for the L case with the heat pump, 79.5% for the LP system with an electric heater, and 68.6% for the L system with an electric heater.
“Under the assumptions of this study, the hybrid solution reduces the levelized cost of consumed energy by 7% compared to a system relying solely on Li-ion batteries, while simultaneously increasing PV self-consumption by up to 20%,” the group concluded.
The hybrid system was presented in “Integrating lithium-ion and thermal batteries with heat pumps for enhanced photovoltaic self-consumption,” published in Applied Energy.