The comfort and convenience of the indoor built environment are undeniable, and historically, so are the costs of maintaining it.
Through heating, ventilation, and air-conditioning (HVAC) systems, spaces are conditioned for human occupancy. When it is hot in the summer, people enjoy the effect of air conditioning. In the cold winter months, heating systems keep people safe and cozy.
Most people expect at least a base level of comfort, and although few notice truly exemplary climate control in a space, its lack is obvious to almost anyone. Conditioning built environments for human comfort are, therefore, a necessity, but must we expend so many resources to achieve this?
The million-dollar question
How can we effectively reduce the monetary cost and environmental impact of heating and cooling indoor spaces for human occupancy?
As demand for energy grows and the greenhouse effect persists, this question has led to many initiatives that call for increased efficiency of HVAC system operations to lower overall energy consumption.
Preventing unwanted heat transfer in the first place is a major step towards achieving these goals. This means keeping additional thermal energy, or heat, from entering a cooled space in the summer and alternately preventing thermal energy from escaping from a heated space in the winter. This significantly decreases the overall amount of energy that must be removed from or added to the space.
The concept of passive cooling and heating build on this idea. Passive cooling methods not only reduce heat gain but also drive heat loss, whereas means of passive heating retain heat through insulation and induce additional heat flow into the space.
To cut utility costs as well as reduce CO2 emissions, a team from Peking University and the Beijing Graphene Institute (BGI) used this concept of passive thermal management to further existing research on materials used for solar heating and radiative cooling.
Opposed to many previous methods which primarily addressed only one mode, either summer cooling or winter heating, the team developed and tested a two-sided, or dual-function, fabric for thermal energy management. The new material can be used in both summer and winter, and outperforms single-function materials that do not adapt to dynamic environments with changing ambient temperatures and thus consume more energy by acting in a counterproductive mode for part of the year.
Making an adaptable material for thermal management
The team designed a material that is reversible so that it may adapt to both the cooling and heating needs of a space simply by flipping it over. Composition was challenging because the needs of the fabric in cooling mode versus heating mode are opposite. Ultimately, the team composed this advanced dual-function fabric of vertical graphene (VG), graphene glass fiber fabric (GGFF) and polyacrylonitrile (PAN) nanofibers sandwiched together.
On the cooling mode side, the PAN nanofibers are on top. This makes the fabric highly reflective to prevent solar radiation from heating the space, and it also lends to high emissivity which means that the material is good at releasing energy as thermal radiation to dissipate heat from within the space. Conversely, the heating mode side of the material leaves the VG layer exposed, and solar energy is concentrated by reflecting between the vertical structures within this layer to heat the space. In this mode, the PAN layer acts as an insulator to prevent heat loss.
By adjusting the PAN layer thickness, the solar reflectivity can be adjusted because the additional fibers scatter more light, making it even more difficult for solar radiation to penetrate the nanofibers. More fibers also add more insulation in heating mode. This level of configurability will allow the fabric to be specialized for the requirements of different climate zones with varying cooling requirements.
For example, arctic environments with extremely cold winters would require a thick PAN layer to prevent heat loss from the indoor environment, whereas more temperate climates would benefit from a thinner PAN layer to allow some heat loss and avoid overheating.
How this dual-function fabric is useful
As a whole, the fabric is very flexible and lightweight yet durable. The result is a portable material that performs well when exposed to the elements. These characteristics make the fabric highly versatile, and the team envisions it being useful in many applications, such as roof coatings, car covers, tents, and more.
Recording data during both summer and winter outdoor conditions, the research team tested the fabric as a roof covering and as a car cover in Beijing, China. In both scenarios, the fabric helped manage the interior temperature of the space through passive cooling and heating, and the team concluded that the innovative material can achieve year-round dynamic thermal management if the mode is switched between the heating and cooling seasons.
The future of this dual-function fabric for thermal management appears bright. The fabrication of the material demonstrates high scalability with commercially abundant raw materials, and the team believes there is promising potential for mass applications.
Reference: Hao Yuan, et al., Scalable Fabrication of Dual-Function Fabric for Zero-Energy Thermal Environmental Management through Multiband, Synergistic, and Asymmetric Optical Modulations, Advanced Materials (2023) DOI: 10.1002/adma.202209897