This annex project will develop energy-efficient heating, cooling and air purification strategies by using novel smart materials, especially advanced sorbents, such as metal-organic frameworks (MOFs) and their related composites, through cross-disciplinary international collaboration. The project will gather the existing scientific knowledge and data on novel sorbent materials for heating, cooling/dehumidification, pollutant removal, and thermal energy storage; and study current and innovative use of these materials in heating, air-conditioning, air purification, and thermal storage systems. It will also identify and bridge the knowledge gaps by establishing links between different disciplines. In the project, experts from building science, materials chemistry, mechanical engineering, material sciences, and environmental health will work together with other stakeholders to accelerate the development of better and more energy-efficient heating, cooling, and IAQ control systems by using advanced materials.

Background

The fast-growing use of heating, ventilation, and air-conditioning (HVAC) systems in buildings worldwide has become one of the main drivers of global energy demand. Heating is currently responsible for around 45% of building emissions and still relying on fossil fuels for supplying more than 55% of its final energy consumption [1], whereas cooling is primarily provided by compressing volatile fluorinated gases. The basic technology of mechanical cooling hasn't changed much since it was invented 100 years ago. However, these conventional technologies are neither efficient nor friendly to the environment. Taken together, space and process heating and cooling represent the biggest contribution to the world's energy consumption, and the biggest source of greenhouse gas emissions [2]. Transformation of heating and cooling technologies by using new functional materials and new physical-chemical processes can significantly reduce HVAC systems' energy demand, improving indoor air quality (IAQ) and minimizing the negative impacts on the environment and climate.

Numerous novel functional materials have been developed by chemists and material scientists in recent decades. Many of them have exceptional sorption performances and hygrothermal properties, such as metalorganic frameworks (MOFs), polymer hydrogels, etc. They can be integrated into either building structures or HVAC systems to passively/actively control the indoor hygrothermal conditions and IAQ, and reduce the energy demand for air-conditioning and ventilation. MOFs are a recently developed class of crystalline micro or meso-porous materials [3,4]. The organic-inorganic hybrid characteristics lead to a unique steep uptake of water vapor. Their exceptional chemical and structural diversity allow them to tune on demand the hydrophilic/ hydrophobic balance and is associated in most cases with an easy release at low relative pressures and moderate temperatures (50-80°C), together with high working capacities (up to 2g of water vapor per g of MOF) under these conditions while their cyclability under long term testing for heat reallocation or dehumidification has also been established. This makes MOFs highly attractive, promising materials for energy-efficient applications in adsorption devices for humidity control (evaporation and condensation processes) [5-7] and heat reallocation (heating and cooling) [8-10] by utilizing water as benign adsorptive and International Energy Agency low-grade renewable or waste heat. Emerging MOF-based process applications covered are dehumidification [11,12], heat pumps/chillers [13,14], air conditioning [15], air cleaners [16,17], thermal energy storage [18,19], water harvesting [20], etc.

Thermo-responsive polymer hydrogel is another type of new moisture sorbent [21]. By combining hygroscopic materials and hydrophilicity-controllable polymers at a molecular level, the new hydrogel has exceptionally high adsorption capacity for water vapor (6.7 g/g @ 90%RH [22]), and can ooze the absorbed moisture as liquid water with a small temperature increase (less than 25 oC) [21]. This unique property reveals the possibility of applying thermo-responsive gels as high-energy efficiency materials for condensing gaseous water to liquid water. Both MOFs and novel hydrogels and their related composites have the potential to form a basis for revolutionary paradigm shifts in air-conditioning technology (e.g., avoiding vapor compression mechanical cooling). However, few have yet been introduced or applied to the building industry for indoor environment control due to the knowledge gap between different disciplines. Cost and scaling are also potential barriers.

Several previous IEA annexes and tasks have carried out investigations and have obtained results that will be relevant to this Annex, Such as EBC Annex 41, 59, 68, 85, and 86, and Energy Storage ES TCP Task 40, 41, and Solar heating and cooling TCP SHC Task 67. 

IEA ECBCS Annex 41 ‘Whole Building Heat, Air and Moisture Response’ has investigated the impact of different hygroscopic materials on the indoor environmental conditions (e.g., temperature, relative humidity, IAQ, etc.) in residential buildings by using HAM (Heat, Air and Moisture) analysis approaches. Some laboratory tests were carried out to study the adsorption and desorption mechanisms of porous materials. However, annex 41 mainly focused on traditional and conventional hygroscopic building materials (e.g., wood, concrete, etc.) IEA EBC Annex 59 ‘High Temperature Cooling and Low Temperature Heating in Buildings’ has developed a temperature humidity independent control (THIC) air-conditioning system by using a separate desiccant system to handle the latent load. The efficiency of the THIC system is higher than the conventional cooling and dehumidification system. However, the desiccants used in the study are either conventional solid sorbents (e.g., silica gel and zeolite) or liquid salt solutions. Both types of sorbents have some inherent defects.

IEA EBC Annex 68 ‘Design and Operational Strategies for High IAQ in Low Energy Buildings’ has proposed an integrated approach to the modeling and simulation of chemical indoor air pollution and energy consumption. It included a procedure for generating parameters of material emission source models from standard chamber tests, and an investigation of the temperature and RH on material emissions. Due to its focus on the integration of energy, HAM, and chemical pollutant modeling, the annex was limited to considering how novel materials can be integrated into building systems for energy-efficient  indoor environment control.

IEA EBC Annex 85 ‘Indirect Evaporative Cooling’ has investigated the possibility of integrating evaporative cooling with a desiccant system for energy-efficient cooling in hot and humid climates. However, the efficiency of this system is still limited due to the high heat of adsorption and high regeneration conditions of conventional desiccants. Development and application of novel smart sorbents (e.g., MOFs) with low heat of adsorption and mild regeneration conditions shall significantly improve the efficiency of the desiccant + evaporative cooling system. 

IEA EBC Annex 86 ‘Energy Efficient Indoor Air Quality Management in Residential Buildings’ aims to improve the energy efficiency of the IAQ management strategies in operation and to improve their acceptability, control, installation quality, and long-term reliability. Subtask 3 identifies opportunities to use the building structure and (bio-based) building materials (focusing on hemp concrete) and the novel functional materials inside it to actively/passively manage the IAQ, for example, through active paint, wallboards, textiles coated with advanced sorbents or hemp concrete, and quantifies their potential based on the assessment framework developed in the annex.

IEA SHC Task 67 and ES Task 40 ‘Compact Thermal Energy Storage Materials within Components within Systems’ focus on compact thermal energy storage (CTES) technologies by using phase change materials (PCM) and thermochemical materials (TCM). The main objectives are to have a better understanding of the factors that influence the storage density and the performance degradation of CTES materials. The sorbent materials studied in this task are natural zeolites, silica gel, molecular sieves, clays, etc. Most of them are traditional or conventional sorbents. This proposed EBC Annex will establish close collaboration with SHC Task 67 and Task 40 to use MOFs as a new type of CTES material and test the performance of MOFs for thermal energy storage. 

However, none of the previous Annexes and Tasks have carried out an in-depth investigation of smart materials, especially advanced sorbents, and their applications for energy-efficient indoor environment control. The proposed Annex will establish a cross-disciplinary international collaboration platform to develop breakthrough heating, cooling and air-cleaning technologies by using novel materials.

Aim, Objectives and Scope

The annex project aims to develop energy-efficient heating, cooling and air purification strategies by using novel smart materials, especially advanced sorbents (MOFs and hydrogels) and their related composites, through a cross-disciplinary international collaboration.

Key objectives

The Annex has the following specific key objectives:

• To establish a cross-disciplinary international collaboration platform to develop breakthrough cooling/heating technologies by using smart materials.
• To review, analyze, and evaluate novel sorbent materials suitable for energy-efficient heating, cooling, and air purification. Selection criteria will be set up for different applications.
• To develop or further improve the performance of the selected materials for specific applications in different climates.
• To develop suitable shaping methods of the best sorbents to adapt to the criteria of the different applications.
• To identify or further develop innovative cooling systems using new materials, which avoid conventional vapor compression refrigeration.
• To develop innovative air purification systems using new sorbent materials. Both the active system and passive approaches will be studied.
• To develop innovative heating and heat storage systems using new sorbent materials.
• To carry out laboratory tests to measure the performance of the new solid cooling, heating, and air purification systems. Numerical modeling and optimization will also be conducted.
• To develop guidelines regarding design and control strategies for novel cooling, heating and air purification systems using novel sorbent materials.
• To identify or further develop models and tools that will be needed to assist designers and managers of buildings in using the guidelines.
• To identify and investigate relevant case studies where the above-mentioned performances can be examined and optimized.
• To disseminate each of the above findings.

Relation to the EBC Strategic Plan 2019-2024

• This Annex focuses on one of the high-priority research themes of IEA EBC - the further development of energy-efficient cooling in hot and humid, or dry climates, avoiding mechanical cooling if possible. The Annex will develop novel cooling technologies using advanced sorbents, particularly flexible MOFs to realize isothermal dehumidification and barocaloric cooling/heating, which can avoid using traditional refrigerants and mechanical cooling, and thus significantly improve the cooling efficiency.

Scope

The Annex will focus on the development and application of smart materials, especially advanced sorbents, (MOFs and polymer hydrogels) and their related composites, for energy-efficient heating, cooling and air cleaning in residential buildings. Both passive and active approaches will be investigated.

Link with other ongoing IEA Annexes

The proposed Annex will establish links with some ongoing annexes, such as 85, 86, and IEA SHC Task 67, and ES Task 40, ES Task 41.

• EBC Annex 85: Indirect Evaporative Cooling. One outcome of Annex 85 is a guideline for designing indirect evaporative cooling systems for different types of buildings under various climates. The MOF-based desiccant system developed in the proposed Annex can be used as a pre-processing phase for the evaporative cooling phase, making the system work in hot and humid climates.
• EBC Annex 86: Energy Efficient Indoor Air Quality Management in Residential Buildings. This annex is focused on the use of novel hygroscopic materials (materials that have the ability to actively or passively influence both moisture and IAQ in the space) and smart ventilation (as defined by AIVC VIP nr. 38) as novel IAQ management strategies. The results from Annex 86 will be a value input for the proposed annex.
• SHC Task 67 and ES Task 40: Compact Thermal Energy Storage Materials within Components within Systems. The proposed annex will establish close collaboration with Task 67 & 40. We will provide novel thermochemical materials (e.g., MOFs) for testing in the system (e.g., optimized heat exchangers and reactors, etc.) designed in Task 67 & 40. We will also test the materials studied in Task 67 & 40 in our cooling and dehumidification systems.
• ES Task 41: Economics of Energy Storage. ES Task 41 aims to carry out economic analysis of different energy storage technologies. Success stories and difficult cases of energy storage under different scenarios will be identified. The proposed task will benefit from Task 41 with key performance indicators or evaluation methods for economic analysis of the developed heat storage. 

Cooperation with these Annexes will involve organizing shared expert meetings, coordinating sessions at relevant conferences, building networks and institutional relationships among experts involved in the projects, and facilitating joint experimental tests.

Target audience

The target audience can be categorized into five groups:

• Manufacturers (both materials and HVAC equipment manufacturers need design guidelines)
• Building designers and consultants (need design and operating guidelines)
• Building managers, and users (need information on performance, operation, benefits, challenges, and how smart materials are integrated into cooling, dehumidification, and air cleaning systems in residential buildings)
• Researchers and professionals (materials scientists, building energy and IAQ experts who are not participating in the Annex are secondary stakeholders for the proposed project)
• Policy, regulatory and standards bodies (revision of standards for building energy conservation and IAQ control)

For policymakers and regulatory bodies, including standardization bodies, the outcomes of the annex will include a clear framework that allows them to assess the performance of smart materials in terms of both energy efficiency and IAQ. This will enable them to avoid reverting to overly-prescriptive requirements that block innovation. Building owners and occupants, facility managers, environmental health professionals, and researchers not participating in the annex are secondary stakeholders for the proposed annex.

Means

Methodology

A range of methods will be used to accomplish the work in this annex, ranging from literature review to experimental lab work, and to system performance modeling and simulations. There will be a considerable amount of numerical modeling to analyze the behavior of the selected materials at different scales
(microscopic to macroscopic), and the performance of these materials in cooling, air purification, and energy storage systems under different conditions. There will also be in-person consensus workshops for the development of the guidelines for MOF selection and applications.

Research Strategy

The general research strategy of the annex is a cyclical process of consensus formation. The five subtasks will proceed in parallel, each with their specific work plan, methods, and goals, but coordinate closely where needed to achieve the overall goal of the Annex. Subtask A will conduct fundamental research on new MOF materials and will provide materials samples and characterization data for other subtasks. Subtasks B, C, and D will work largely in parallel, but their work is closely interconnected, with the results from the different tasks feeding into the work of others. This requires an iterative process of interaction with the whole group. Contributing from their own activities, the subtasks work together towards a set of 4 common deliverables. The biannual meetings (in person or via Internet) are used to share progress updates across the different subtasks and monitor the overall alignment of the work. At the end of these meetings, the work plans for the next 6 months are discussed in a plenary session and updated based on the latest findings in other subtasks.