How thermal destratification can increase energy savings and improve comfort
How thermal destratification can increase energy savings, and improve heating and cooling comfort levels is divulged by JOHN BRODIE of Vento Australasia.
The Carbon Trust (UK) rates thermal destratification as one of the top three carbon reduction initiatives for buildings, and the list of overseas businesses that have identified energy and carbon savings via destratifying or equalising temperatures is long and growing all the time. Major UK retailers, such as Morrisons, Sainsbury’s, Tesco, John Lewis and Waitrose, use thermal destratification in many of their stores, and BJ’s Retail, one of the US’s largest retailers, uses thermal destratification in its stores in hot climates, such as in Florida.
The US Department of Defence uses thermal destratification in many of its facilities across the US. In addition, a major retailer in Australia is currently conducting a trial in one of its stores to gauge the effectiveness of thermal destratification. The use of thermal destratification fans is a cost-effective solution to reducing cooling and heating loads. It is a simple but effective solution to a simple but hitherto unrecognised problem.
Hundreds of different building types – from indoor pools to warehouses, manufacturing plants, offices, atriums, gymnasiums, veterinarians, hospitals, churches, cool rooms, aged-care facilities, pharmaceutical manufacturing and storage, car showrooms and food processing plants – can use thermal destratification to reduce heating and cooling bills. In many cases, heating and cooling bills are reduced by up to 50 percent and some businesses have noted paybacks in less than three months.
WHAT IS THERMAL DESTRATIFICATION?
Thermal stratification is the temperature gradient between the floor and ceiling due to thermal buoyancy. Hot air rises, while cold air falls. Most people will have gone swimming and felt hot and cold stratification levels at different depths of the ocean or the pond or pool. Air and water are both fluids and unless mixed this stratification will remain.
This temperature variance creates heat loss through ceilings, cold spots on floors and varying temperature between walls. This exposure to a fluctuation in temperature leads to discomfort for the building’s occupants and increased HVAC running costs due to attempts to counteract this discomfort.
The energy efficient solution to this problem is via destratification using low energy destratification fans. In conditioned buildings, the air-conditioning or heating and cooling equipment is continually over-delivering to compensate for the hot air sitting up under the ceiling or the cold air sitting on the floor. If this conditioned yet wasted air is used in a building then significant energy savings can be made.
ACHIEVING INCREASED ENERGY SAVINGS AND IMPROVED COMFORT
Thermal destratification is suitable for both hot and cold climates. It offers a greater level of thermal comfort in both summer and winter in non-conditioned spaces and improves the efficiency of air-conditioning and heating systems. We have all experienced sitting in an auditorium, for example, and feeling very cold even though the heating is going. This is because all the hot air is rising up to the ceiling and sitting up there where it is of no value. In winter, the hot air up under the ceiling increases the temperature there and, via physics, as the difference in temperature (Delta T) is greater between inside and outside, there is an acceleration of the hot air leakage from the building.
In addition, a large majority of people find the cold aisles in supermarkets a disincentive to buy cold aisle products. Hanging around the cold aisle is not a common occurrence and many people complain about it. Thermal destratification can negate that effect and measurably increase sales. How much more comfortable would a space be in summer if all the hot and cold spots were removed by maximised air circulation. Many building types, including offices and retail stores, suffer from this problem where one area is too hot and another is too cold. This is a function of air-conditioning design and, to alleviate this, a lot of ducts and then a large amount of fans to move the air through those ducts are needed. Or, a simple destratification fan could be used.
Gentle, non-turbulent airflow in summer will improve cooling and the use of a good destratification fan will circulate the air for a very low energy cost while also maximising comfort and ensuring that the energy in the air-conditioning system is minimised. This happens because a good destratification fan system will move the air in a certain direction as required, with no turbulence and for very little energy. If you put enough horsepower behind a fan, you can move large quantities of air in any direction over long distances. The trouble is you will pay for it in electrical energy.
The biggest user of energy in air-conditioning systems is fan power. The key is moving air non-turbulently in one direction using very low amounts of energy. Most fans are not designed to move air in a direction with minimal turbulence. Take a typical ceiling fan, for example. These cannot move air very far without causing a lot of turbulence, resulting in loss of energy and more discomfort. A celling fan will use 60 watts and will only move the air a few metres, while a good destratification fan can move air a relatively longer distance with little or no turbulence for around 15 watts.
The way thermal destratification fans move air in columns that entrain the air around them to move in that same direction means that the fans in the air-conditioning system do not have to work as hard. In addition, the gentle airflow in the space means that cooling is achieved, which enables cooling set points to be raised, reducing the energy load on the units. The same works in reverse. If a warehouse or atrium has a heat-generating process inside or artificial heating, in winter, thermal destratification can be used to bring all the heat sitting up near the ceiling back down again.
Energy savings can also be achieved in heated pools through the use of thermal destratification. Pushing the air (heated as a by-product of the pool water heating system) back down to the pool surface reduces the heating load of the pool water, while the constant circulation changes the dew point, removing the condensation and, hence, the mould and corrosion off the internal surfaces. MLC Burwood girl’s school in Melbourne has significantly improved comfort in its indoor pool space in summer using thermal destratification.
In the ongoing scramble for carbon and energy saving initiatives in the built environment, the simple option of thermal destratification is not only effective, but also cost-effective to implement per square metre. Consideration of this type of solution should be given before embarking on more expensive and complicated solutions that will inevitably use more energy, have more maintenance and provide minimal improvement.
John Brodie is managing director of Airius (Oceania).