A CHEAP and green ventilation system could give office air conditioning the
cold shoulder. The system, which has been developed by researchers at the
University of Nottingham, uses a chemical heat sink to soak up warm air, pumping
cool air back into the building. It uses only a fraction of the energy of
conventional air conditioning and consequently vastly reduces emissions of
carbon dioxide.
鈥淚t could remove the need for air conditioning provided the climate isn鈥檛 too
severe,鈥� says David Etheridge of Nottingham鈥檚 Institute of Building Technology.
He says that the system is suited to countries with a climate like that of
northern Europe, where air conditioning is widely used despite the relatively
mild summers. 鈥淵ou鈥檙e talking about a two or three-degree temperature
reduction,鈥� says Etheridge, 鈥渨hich in comfort terms is quite significant.鈥�
Etheridge and his colleague David Rae invented the system, which couples
natural ventilation with compounds known as phase-change materials (PCMs), which
cool the air. PCMs are capable of storing vast quantities of latent
heat鈥攖he heat that is required to turn a solid into a liquid.
Advertisement
During the day, a fan draws warm air over an array of fluid-filled pipes.
These 鈥渉eat pipes鈥� use a cycle of evaporation and condensation to conduct the
heat along the pipe to a series of storage tanks containing a solid PCM. The PCM
absorbs the heat and slowly melts during the day. The cooled air is then pumped
back into the room.
At night the process is put into reverse. Ventilator shutters open, allowing
cool night air to enter the room. The fan reverses direction and the cold air is
sucked up past the heat pipes. The PCM cools and solidifies, warming up the air
inside the room, which is then dumped outside the building.
The energy is stored as latent heat, says Etheridge. 鈥淭he PCM鈥檚 temperature
isn鈥檛 changing much, it鈥檚 the latent heat capacity that鈥檚 important.鈥� For their
PCM, Etheridge and Rae chose sodium sulphate, which is also known as Glauber鈥檚
salt. Adding additives to the PCM changes its melting point, says Etheridge, so
the system can be fine-tuned according to the needs of the local climate.
There are other ways of cooling buildings with natural ventilation. But they
depend on increasing the thermal mass of the building, for example, by using a
large mass of concrete to soak up the heat. PCMs are ideal for cooling systems,
says Jonathan Gates, a researcher at the School of the Environment at Brighton
University. They massively increase the thermal mass of the building without the
need to add vast quantities of concrete, he says.
In tests, the researchers found that the system cooled the air just as
effectively as air conditioning but cost only one-sixteenth as much to run. This
lower cost is largely because the Nottingham system uses less energy to cool the
air. So power generators burn less fossil fuel, which cuts emissions of
CO2. 鈥淭he main incentive behind this is not the cost but a reduction in
CO2 emissions,鈥� says Etheridge.
A number of studies have linked poor maintenance of ventilation systems to
sick building syndrome, so natural ventilation could have other health benefits.
People like to open windows in hot weather, but this prevents air conditioning
from working properly. Office workers in buildings that have been fitted with
Etheridge鈥檚 system will be able to open windows without affecting its
performance.
Another advantage of the system is that people who have dressed in light
summer clothes because of the hot weather won鈥檛 end up freezing in a chilly
office. 鈥淭he whole point of natural ventilation is that people will modify their
clothing according to the weather conditions,鈥� says Etheridge. He now wants to
run a year-long trial of the system in a real office.
