By Jason Deign
Phase-change materials (PCMs) are boldly going into an energy storage realm where even the most modern battery technologies have failed to deliver: saving lives.
Dulas, a Welsh renewable energy technology company, is using PCMs in place of batteries as an essential component of solar-powered direct-drive refrigerators for off-grid vaccine storage in developing countries.
On Monday the company announced a contract to supply 345 of its VC200 fridges to health and aid agencies working in Yemen, Sierra Leone and Nigeria.
The company said the deal represented “a significant expansion” of its partnerships with the World Health Organisation (WHO), the United Nations Children’s Fund (UNICEF) and the Institute of Human Virology in Nigeria.
Dulas will be sending 60 fridges to the Institute of Human Virology, 143 to the WHO in Yemen and 142 to UNICEF in Sierra Leone. “There is the potential for further orders in the near future,” said the company.
Fridges delivered to three projects
The fridges are due to be delivered within eight weeks on all three projects, to be used in programmes helping to fights AIDS, tuberculosis and malaria.
Non-governmental organisations such as the WHO are interested in Dulas’s off-grid solar direct-drive fridge design because it provides significant advantages over traditional versions, which have tended to use lead-acid batteries.
The batteries, which are preferable to lithium-ion because of their greater temperature tolerance, allow vaccine fridges to work overnight with stored solar power, but tend to run down and need replacing after about five years.
This can be a problem for remote healthcare centres, which may not have the funds to invest in new batteries… or the skills to recognise when a battery needs changing.
Just as importantly, however, battery-based fridges rely on traditional evaporators to stay cool. This means there can be parts of the fridge where the temperature is less than 0ºC, so there is a risk of freezing a vaccine.
Improving vaccination rates
Once frozen, a vaccine is useless.
And it can be hard to tell whether a vaccine has been frozen at some point, so in order to improve vaccination rates it is important that the whole of the fridge be maintained at a constant temperature between 2º and 7ºC.
The Dulas design, developed with funding from a joint European and Welsh government research and development project, overcomes this problem by having a lining with a paraffin wax-based PCM that freezes at 5ºC.
During the day, rooftop solar arrays are used to cool the PCM down to its freezing point.
At night, in the absence of solar power, the PCM melts and in the process absorbs heat, without any rise in temperature, until it has all turned into liquid. This makes sure the vaccines stay within their optimum temperature range.
10,000 cycles with minimal degradation
Another advantage of the PCM design is that the thermal storage medium can go through around 10,000 cycles with minimal degradation. This equates to a lifespan at least twice as long as that possible with batteries.
Dulas senior technical consultant David Elliot told Energy Storage Report the fridges were expected to last at least a decade but up to 15 years might be more likely.
“The design and capabilities of Dulas’ vaccine refrigerators are a major step forward,” said Opute Ifeoma Rosemary, associate director of procurement at the Institute of Human Virology, in a press release.
The design is “allowing us to more efficiently cool and freeze samples for preservation in turbulent areas or those that are particularly difficult to reach,” she said.
Dulas currently offers three fridge designs, each with a solar array to match. The smallest unit has capacity for 50 litres of vaccine and comes with a 300w, 1m by 1.5m PV array.
Room for 132 litres of vaccine
A larger product has room for 132 litres of vaccine and requires up to 500w of solar power.
Finally, Dulas has a version that combines a vaccine storage space with a freezer unit that is used to make ice for when vaccines need to be carried far afield, in cool boxes. This model requires 750w of PV power.
In all cases, said Elliot, the solar arrays are oversized compared to the 80w-per-compressor maximum power requirement of the fridges because of the need to keep the PCM cool for as long as possible.
“The secret is about managing the energy coming in,” he said, “and getting 365 days of the box staying cold.”
To this end, the solar arrays are big enough to start delivering the fridges’ full energy requirements early in the day, and late into the afternoon.
Excess energy from PV panels
During the daytime, excess energy from the PV panels is made available through a ‘solar socket’ that can be used to charge phones and other appliances, or even top up batteries for electrical applications.
Dulas, which has been making solar vaccine fridges since 1982, launched the PCM-based design in 2014 and sold 1,000 units last year, at between €2,750 and €4,750 per unit depending on size.
Apart from Dulas, “there are only five or six companies doing direct-drive fridges,” Elliot said. “It’s not a huge market.”