Rotational moulding with zero greenhouse emissions, zero energy costs

LightManufacturing, a US-based solar technology company, has developed a technology that replaces the use of fossil fuels in plastic rotational moulding.

Solar rotational moulding (SRM) uses sun-tracking mirrors called heliostats to concentrate solar thermal energy on a rotational mould. The heat from the sun directly warms the mould, melting the plastic with zero greenhouse gas emissions at zero fuel cost.

The idea to use concentrated solar to power rotational moulding started through a previous venture, LightManufacturing’s founder and CEO Karl von Kries told Sustainable Plastics. Another of his companies, called Technomad, had a contract with the US government to manufacturer loudspeakers, for which it was using rotomolded shells as building blocks. At one point, the team wondered whether it could replace natural gas by reflecting heat from the sun and concentrate it onto a mould.

Not having been able to find any evidence in the literature of past attempts, the team put the idea to the test, although feeling ‘certain it was going to fail’, von Kries recalled. But it did not fail, and although the first rotomolded loudspeaker shells were ‘terrible’ and ‘small’, von Kries said, the team improved its moulds and eventually graduated to manufacturing more sought after rotomolded products like water tanks, boats, furniture, and toys.

“We then started applying the technology to water tanks and we learnt a lot of tricks and techniques for making these larger objects. Pretty soon, we were moulding 1,200 litre, 2,000 litre, even 4,000 litre tanks using the technology,” von Kries explained.

Factory in a box

LightManufacturing’s rotational moulding system is a ‘factory in a box’ consisting of two moulds inside a container. The off-grid system does not require any infrastructure for gas, grid electricity, or water cooling.

“We can take these things and we can drop them quite close to customers or population centres, closer than where you could place a traditional factory, which also reduces transportation costs” von Kries explained. “With a traditional factory you have to get a whole bunch of permits; they're huge and expensive. Our systems are small, compact, and deployable,” he contrasted.

Manufacturing process

SRM can use any typical rotational moulding resin, including polyethylene, polypropylene, and nylon. LightManufacutring has so far mostly manufactured polyethylene products.

A key difference between SRM and traditional rotational moulding is that SRM heats the mould by radiative heating, which directly heats the metal, rather than convective heating, which heats the air around the metal.

This means the process can achieve desired moulding temperature at much lower surface/air temperature which, in turn, allows the system to place electronics inside the moulding chamber.

“The difference with us is that, because we're radiatively heating the mould, we can build a lot of advanced technology right in the moulding chamber because it's not being exposed to 300 C air but rather around 170 C,” von Kries explained. “That means our systems can be highly automated and use sensors and computers inside the moulding chamber which is not possible with traditional rotational moulding. That means we have a much more automated process than is typically possible: it can adapt to changing solar conditions and help the operator.”

Automation technology frees operators during the long rotational moulding heating cycles, of around 30 minutes for products like water tanks. Sensors placed inside the moulding chamber check internal and external temperature and feed that data onto a control system that monitors the heating process. If the sky turns cloudy for a period, the system automatically extends the heating cycle until the mould is at the desired temperature.

Cooling is also automated. Once the first mould is at temperature, the system automatically stops the heating cycle and reroutes the heliostats to direct heat at the second mould, starting a second heating cycle. In the meanwhile, cooling fans are automatically turned on in the first mould. Once the whole process is finished, a buzzer alerts the operator to remove the finished products from the moulds.

In general, having sensor technology inside the moulding chamber allows LightManufacturing to adapt the heating cycle to changing external conditions, thereby outputting a higher quality product.

Cost

Besides the environmental benefits of moulding plastic products with zero greenhouse gas emissions, solar rotational moulding also delivers big savings on cost. The technology cuts product costs in 15% to 30%, according to von Kries, because energy costs to power the process are reduced to zero.

“Many people thought that there were hard limits on how you could save on rotor moulding,” von Kries said. “It was viewed as a fully understood, commodified process. And now we're taking that pie that has labour, materials, and energy, and suddenly one of those pie slices just goes away completely. If you're in a high energy cost environment, taking out natural gas or even kerosene in some cases results in massive savings.”

The cost reduction refers to operating expenses (opex) only. To fully compare solar rotational moulding with a gas-fired process, the capital expenditure (capex) for the heliostats, containerised moulding system, and photovoltaic panels that power the mould rotation would have to be compared with infrastructure used in the standard process.

Location and efficiency

Because the moulds are heated by concentrated thermal energy from the sun, SRM is most efficient in areas with high solar irradiation. The system also works under cloudy conditions, although efficiency will of course be impacted. LightManufacturing says that 49% of the Earth’s land is ideal for deploying its technology (pictured red below). It already has two systems in place in the United States, one in California and another in Hawaii.

In terms of efficiency, solar rotational moulding converts around 75% of available solar energy to useful heat, in contrast with the standard 20% to 25% efficiency of solar photovoltaics in converting energy to electricity.

This efficiency, combined with LightManufacturing’s heat management, positioning, and process control software allows mirror arrays as small as 30 sqm to generate sufficient heat to mould objects 1 m x 1 m in size. Put another way, 35 heliostats reflect enough heat to mould a 2,000 litre water tank.

Other applications

LightManufacturing is currently focused on applying its technology to rotational moulding, but von Kries revealed ‘plans’ to eventually start covering vacuum forming and compression moulding applications.

Concentrated solar could also be used as a pre-heating or partial heating input for injection or blow moulding, von Kries added. Given that those manufacturing processes tend to operate 24/7, concentrated solar would only act as a heating ‘boost’ during the day.