24/7 device that captures and stores solar energy

Materials World magazine
,
6 Jan 2020

A new hybrid solar device can capture and store solar energy, releasing energy day and night. Shardell Joseph reports.

A solar device that acts as a 24-hour generation and storage hybrid can effectively recover 80% of stored energy throughout the night, offering greater capabilities for power delivery. Researchers from the University of Houston, USA, stated that the device can also be used to improve distillation and desalination processes.

The team claimed the device can help mitigate thermal losses in conventional solar panels or cells, due to separate harvesting and storage infrastructures, which cause long piping lines and high costs associated with complex systems.

‘In the device, both harvesting and storage of the solar energy occur in one system which reduces the cost and complexity,’ University of Houston Department of Mechanical Engineering Associate Professor, Hadi Ghasemi, told Materials World.

‘The conversion rate of solar irradiation to thermal energy in this device is higher than state-of-the-art technologies, leading to high efficiency. On the storage side, as the solar irradiation is stored in the molecular form, there is no temporal loss and the energy could be kept for a long time leading to high efficiency, even at night,’ Ghasemi said.

According to the paper, Full spectrum solar thermal energy harvesting and storage by a molecular and phase-change hybrid material, published in Joule, the hybrid system releases energy day and night. Further, stored energy throughout the night is recovered at 80% efficiency with even higher rates possible during the day, which is considered one of the core advantages over existing technology.

24-hour power

Existing solar panels or solar cells rely on photovoltaic technology for the direct generation of energy, and current thermal approaches rely on costly high optical concentration systems, leading to high heat losses.

Solar is becoming increasingly imperative for satisfying power demands, but effectively utilising the energy generated has proven difficult. According to the paper, for the 0.12MW the sun provides Earth, solar electricity accounts for only around 0.015% of the global energy demand, 0.3% of which is generated from solar thermal technologies. The team argued that this gap in efficient utilisation of solar energy justifies a need for disruptive technologies that can better harness thermal energy from the sun.

By combining molecular energy storage and latent heat storage, the full solar spectrum for long-term operation is captured. Therefore, the team claimed that harvesting and storing energy is potentially a 24/7 operation, reporting that harvesting efficiency ranged from 73% at small-scale operation to 90% at large-scale.

Deconstructing the layers

In the study, the researchers described the two layers that make up the device – molecular storage material (MSM) and the localised phase-change material (L-PCM). With silica aerogel sitting in-between, they are combined to give the full spectrum of solar energy day and night.

During the day, the L-PCM – the bottom layer – absorbs the incident solar irradiation, resulting in a phase transition from solid to liquid. To reach phase-transition temperature at low solar flux, solar heat localisation is implemented, minimising heat loss.

To aid heat localisation, carbonised rayon (CR) – a fibre with high absorption in the solar spectrum – is introduced in the PCM, creating a hotspot in the material structure.

The next layer is the silica aerogel, placed between the L-PCM and the MSM to ensure constant temperature, which is vital for daytime functionality of the hybrid system.

‘The silica aerogel is transparent in the visible spectrum allowing the solar irradiation to path through it to the underlying PCM and be harvested and stored,’ said Ghasemi. ‘On the other side, silica aerogel is a great thermal insulator leading to a temperature difference between the PCM and MSM. During the day, while PCM harvests energy and is at high temperature, the MSM should be kept at low temperature to be fully functional for storing of the energy.’

On the top sits the MSM, which consists of a photoisomer. During the day, the photoisomer absorbs UV radiation which simultaneously stimulates the process of the isomerisation of MSM and stores energy. The harvesting of the radiation requires low temperatures of less than 70°C in order to maximise efficiency.

During the night, the heat transfer fluid from the L-PCM flows to the MSM, providing the necessary threshold temperature to initiate thermal back isomerisation – a process that releases the energy stored from the UV radiation. This allows the hybrid system to absorb the complete solar spectrum and provide energy both day and night.

Ghasemi explained that once the materials are layered and then activated, the generation and storage process begins inititating the harvesting of the full spectrum of solar irradiation. ‘The MSM harvests UV spectrum of the solar irradiation and the energy from irridation will be stored in the form of molecular energy with conversion rate greater than 99%. Then the PCM harvests visible and infrared spectrum of solar irradiation and the irradiation energy will be stored in the form of thermal energy with efficiency of more than 90%.’

According to Ghasemi, the team is working with several industrial partners to develop a large-scale prototype of this hybrid technology for commercialisation.