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Development and Validation of an Innovative Solar Compact Selective-Water-Sorbent-Based Heating System

Periodic Reporting for period 2 - SWS-HEATING (Development and Validation of an Innovative Solar Compact Selective-Water-Sorbent-Based Heating System)

Reporting period: 2019-12-01 to 2021-05-31

Project Overview

the SWS project aims at further developing a novel storage technology of thermal energy in order to utilize solar energy in buildings in the most efficient way throughout the year maximising the fraction of thermal energy demand of a building that is covered by solar energy (i.e. the solar fraction).

The project concept is to develop an innovative Seasonal Thermal Energy Storage (STES) unit with a novel sorbent storage material embedded in a compact multimodular sorption STES unit. This will allow to store and shift the harvested solar energy available abundantly during the summer to the less sunny and colder winter period thus covering a large fraction of heating and domestic hot water demand in buildings.
The solar compact Selective-Water-Sorbent (SWS) Seasonal Thermal Energy Storage (STES) concept relies on applying an innovative class of water sorbing material embedded in an intelligent configuration with an advanced control system.

Solar heat will be used to provide domestic hot water (DHW) up to 60 °C, while the remainder will be used mainly to charge the seasonal storage unit during summer, allowing to store the heat for several months, bridging the summer-winter period. During winter, the stored heat will preferentially be used for space heating, and a backup heater will operate only when all stored heat has been fully discharged to cover the heat demand.
Including also heat storage for DHW, recovering heat produced during the charging cycle and making use of even the lowest grade heat amounts (within 5 – 30 °C), the system will allow to reach solar fraction currently not achievable at reasonable price and small space demands (e.g. with solar collectors and water-based Thermal Energy Storage). The detailed concept is presented in Figure 1, based on its two main operating modes (summer/winter).


The core objective of the SWS-HEATING project is to develop a new sorbent material of the SWS family with optimised sorption properties, matching the working conditions of a heat storage cycle with low temperature solar heat charging (70 – 95 oC) to allow efficient application also in northern European countries.
The work progress of the project during the second reporting period (December 2019 – May 2021) has been severely affected by the unprecedented COVID-19 situation with travel restrictions, lockdowns and shortage of materials, goods and services which caused delays in many crucial tasks scheduled for RP2. The flexibility of the consortium and the ability to provide alternative solutions to the raising problems enabled the achievement of most of the Objectives and Milestones planned for this period although in certain cases with significant delays that are inherited in the following and interconnected tasks.
In this period, the development of the novel SWS material (Study of sorption equilibrium and dynamics of the new sorbents & Analysis of the long-term stability of the new sorbents) as well as the SWS system configuration and the development of the new components needed for the SWS HEATING system (vacuum valves, new vacuum-tube solar collectors, vacuum insulated combi-storage tank, PCM buffer tank) were completed.
The activities regarding the development of an optimised control system and of a simulation platform have also been severely affected by COVID-19 but most importantly these activities have has been completed. The most important outcomes are the developed prototype smart control environment for the SWS system using Artificial Intelligence algorithms and the developed simulation platform of the whole SWS system in TRNSYS using available and own-developed models for the various components with integrated devoted occupant-building interaction models.
High performing heat exchangers acting as adsorber/desorber and evaporator/condenser within the SWS storage modules are key-components. Thorough experimental and numerical investigations have been carried out on two types of commercial plate heat exchangers as well as an own developed plate HEX by OTH which was selected as the most suitable for the SWS HEATING system. However due to the COVID situation the consortium was unable to find a reliable manufacturing partner, who would produce the heat exchangers according to these specifications and in order to move forward it was decided to use the commercial HEXs.
The SWS HEATING ambitions at different levels are summarised next while the envisage progrees from the conception of the idea to the commercialisation of technology is schematically presented in Figure 2.

• Reach compact solutions that can be integrated in new or existing buildings.
• Potential to reduce system cost by up to 20-30% compared to other solar seasonal storage units.
• Optimised system design and sizing for achieving very high solar fraction - i.e. the amount of energy provided by the solar technology divided by the total energy required for domestic heating and hot water throughout the year - of over 60% in south, central and north Europe.
• Validation of SWS-heating system in an intended environment (TRL5) and preparation of its further development stages.

At material level
• Develop a family of advanced SWS-materials with tailored sorption capacity adapted to typical working conditions of the sorption module.
• Achieve very high heat storage density, in the range of 1.1-1.3 GJ/m3 (over 30% higher than the best SWS sorbent material and 40-60% than state of the art for the same temperature range) that would allow reaching compact seasonal storage solutions.
• Validate the stability of the new sorbent material against hydrothermal cycling under severe working conditions, up to 1,000 cycles that correspond to more than 50 years of operation, with at least 10 charging/discharging cycles per year.
• Evaluate enhanced STES capacity of the developed SWS, and prove the low production cost potential to less than 5 €/kg at commercial level, according to estimated cost of raw materials (currently it is about 10 €/kg for the delivery of few quantities).


The targeted benefit of this next generation solar heating technology is to reach and overcome a seasonal solar fraction of domestic energy systems of 60% in central/north Europe, reaching 80% in the sunnier south of Europe, with a compact and high-performing STES system at low cost. The overall aim is to validate this new technology in an intended environment (TRL5) and to proof all challenging objectives.
SWS-heating concept and main operating modes (summer winter)
SWS HEATING: From idea conception to commercialisation