‘Hybrid’ solar system proves cheaper than photovoltaics

A ‘hybrid’ rooftop solar system that produces hydrogen can create more useful energy at a lower overall cost than photovoltaics, according to researchers at Duke University.

They recently conducted a trial of their system, which uses glass tubes filled with water, methanol and catalyst, based on its exegetic performance — a measure of how much of a given quantity of energy can theoretically be converted to useful work.

‘The hybrid system achieved exegetic efficiencies of 28.5 per cent in the summer and 18.5 per cent in the winter, compared with five to 15 per cent for the conventional systems in the summer and 2.5 to five per cent in the winter,’ said Nico Hotz, assistant professor of mechanical engineering and materials science at Duke.

The team’s hybrid device looks similar to a traditional solar collector but actually contains a series of copper tubes coated with a thin layer of aluminium and aluminium oxide, and partly filled with catalytic nanoparticles. A combination of water and methanol flows through the tubes, which are sealed in a vacuum.

This setup allows up to 95 per cent of the sunlight to be absorbed with very little being lost as heat to the surroundings, facilitating temperatures of higher than 200ºC within the tubes, according to the team.

Once the evaporated liquid achieves these higher temperatures, tiny amounts of a catalyst are added, which produces hydrogen that can be directed to a fuel cell to provide electricity to a building during the day, or compressed and stored in a tank.

Three systems were examined in the recent trial: a standard photovoltaic cell that converts sunlight directly into electricity to then split water electrolytically into hydrogen and oxygen; a prototype version of Hotz’s system; and a system in which photovoltaic cells turn sunlight into electricity that is then stored in different types of batteries (with lithium ion being the most efficient).

‘We performed a cost analysis and found that the hybrid solar methanol is the least expensive solution, considering the total installation costs of $7,900 [£4,900] if designed to fulfil the requirements in summer — although this is still much more expensive than a conventional fossil-fuel-fed generator,’ Hotz said.