Salt and water: enablers of high-density thermal energy storage
The Sun’s energy is practically infinitely renewable and without geopolitical barriers to access. Currently, commercialised residential active solar systems for heating, cooling and domestic hot water include a short-term heat storage tank. The tank can store heat for a few hours, extending operation briefly if clouds roll in or when night falls. However, it cannot accommodate areas with less sun throughout the year, including central and northern Europe, or the increasingly rainy weather even in historically sunny regions. Seasonal thermal energy storage (STES) is required to unlock the full potential of solar energy. The EU-funded SWS-HEATING project has developed an innovative, cost-effective STES system that can store heat for months. It will support widespread adoption of active solar systems for residential use anywhere in the world, regardless of the weather or climate.
Selective water sorbents for STES
According to project coordinator Sotirios Karellas of the Laboratory of Steam Boilers and Thermal Plants at the National Technical University of Athens, “one simple STES concept under development relies on large-capacity underground water tanks but high manufacturing costs and large space requirements limit applications, particularly for residential energy needs. Latent heat and electrochemical seasonal energy storage are in their infancy and face challenges related to stability and costs.” SWS-HEATING focused on selective water sorbents (SWSs), using an inorganic salt impregnated in a porous host matrix to trap (adsorb) water molecules. Many host matrices and salts are possible to tailor properties to applications. Further, SWSs have several advantages over other sorbents: minimum heat loss – perfect for long-term (seasonal) storage applications; high heat storage density; and low-cost production.
A new SWS and single-plate heat exchanger
“SWS-HEATING developed a novel SWS material with a storage density in lab testing over 20 % greater than existing SWS materials. It was not degraded nor did it lose adsorption capacity after 1 000 ageing test cycles under working conditions more severe than real ones,” notes Karellas. A single-plate heat exchanger (HEX) for the SWS designed by SWS-HEATING can be used as an adsorber/desorber as well as an evaporator/condenser. This technological feat will significantly reduce costs since only one type of HEX will be produced, mounted differently for each application. Critically, “the SWS adsorber/desorber HEX has nearly three times greater adsorbent volume relative to heat transfer fluid volume (2.73 relative to 1.1-1.4 for commercial HEXs). This means much more adsorbent to trap water molecules, enhancing storage density and performance,” Karellas adds.
Residential solar power all year round, anywhere in the world
“Remarkably, fruitful cooperation among the consortium members and the European Commission’s project officers enabled us to overcome the materials and suppliers’ availability and costs, challenges and quarantines of the COVID-19 pandemic,” notes Karellas. Despite the odds, the project successfully designed and manufactured the holistic SWS-HEATING prototype including its SWS, HEX and a smart control system, enabling proof of concept. “Our STES technology with minimal heat loss can cover up to 60 % of a residential building’s annual energy needs and will enable solar power exploitation in regions with limited solar availability in the winter. It could be a breakthrough for the residential sector, enabling significant reductions in energy costs and emissions, and support EU energy policy and independence,” Karellas concludes.
Keywords
SWS-HEATING, energy, solar, STESselective water sorbent, seasonal thermal energy storage, single plate heat exchanger, adsorbent, solar energy
