Wrinkled surface for faster boiling

Björn Palm, professor at the Department of Energy Technology at KTH

KTH researchers have developed a new surface coating that can substantially cut energy consumption by heat pumps and cooling devices.

Most refrigerators, heat pumps and air conditioners rely on vapour-compression technology, which transfers heat by compressing a refrigerant gas to liquid state and then pumping the liquid away to release the heat elsewhere. Key to the efficiency of this process is the temperature differential required to convert the refrigerant from its liquid to gas and back to liquid again.
A KTH research team led by Professor Björn Palm in the university’s Department of Energy Technology has discovered that coating the inside surfaces of the pipes carrying the refrigerant with tiny wrinkles lowers the temperature at which vapourisation begins.

"A smooth surface must be several degrees warmer than the refrigerant fluid to facilitate the formation of the first tiny bubbles that emerge as vapourisation begins. With our micro-porous surface, bubbles form with a lower temperature difference," Palm says.

"Since about 15 per cent of all electricity produced in the world today is used to run heat pumps and cooling devices, our new technology can save huge amounts of electricity," he continues. The team estimates that the energy efficiency of heating and cooling systems can be increased by around 10 per cent.

"Applied globally, that amounts to electrical energy savings of 360 TWh per year, or three times the consumption of the entire country of Sweden, with a value of around SEK 500 billion (USD 79 billion) annually," Palm says.

The technology creates a surface structure consisting of large and small pores that provide an orderly flow of liquid and gas, which the scientists believe contributes to a more efficient boiling process. A reduction of one degree in the temperature difference between the heat source and the evaporating liquid lowers the power consumption of the system by 2 to 3 per cent.

The technique works for both heat pumps and cooling devices, since both applications are based on the same underlying principles. The surface coating can be used for small refrigerators in households as well as for large-scale refrigeration in industry.

"Our technology can also be more efficient for the cooling of electronics, where the refrigerant is allowed to pass over small heat exchangers in with the components. More efficient cooling allows the CPU to work faster, and that improves performance," says Palm.

To benchmark the energy gains from the surface coating, the researchers have built a demonstration plant with two heat exchangers, one with a micro-porous surface and one with a smooth surface.

"Our patented technology will provide a fantastic business opportunity for producers of heat exchangers," Palm says, and he and four other researchers from KTH have founded a new company, Micro Delta T, which has a prototype facility ready for the technology.

Established in 2007, Micro Delta’T has attracted considerable interest from investors. A Chinese entrepreneur has invested USD 315,000 in the project, and in 2009 Vinnova, the Swedish government agency that administers state funding for research and development, contributed USD 190,000.

Palm gives credit to KTH Innovation, which supports commercialisation of the university’s research, with helping bring the idea to the verge of market introduction. "KTH Innovation has been an important business consultant to our multi-disciplinary team from the start," he says.