With increased focus on reducing energy consumption in buildings, the use of exhaust-air energy recovery in HVAC systems is becoming more common, especially in Dedicated Outdoor Air Systems (DOAS). Exhaust-air energy recovery technology provides a valuable opportunity for engineers to reduce the first costs and operating costs of buildings. The use of energy recovery reduces the use of non-renewable resources and promotes a cleaner environment. Therefore, whether mandated by state or local building codes, or not, proper application of energy recovery wheels and heat recovery in general is a win-win proposition.
Principles of Energy Recovery
Energy recovery involves a transfer of energy between an exhaust airstream and a supply airstream. As the two air streams mix, energy is transferred from the higher energy airstream to the lower energy airstream. This means that the exhaust air preheats the supply air in the winter and precools the supply air in the summer. Especially relevant to Asian environments, some systems use energy recovery wheels to reheat supply air after it has been cooled – an effective means of humidity control. The next post will look at the functions of energy wheels.
Some energy recovery devices transfer only sensible energy, while others transfer sensible and latent (i.e. total) energy.
- Sensible Heat Transfer:
When sensible heat is transferred, the dry-bulb temperature of the colder airstream increases and the dry-bulb temperature of the warmer airstream decreases. No moisture is transferred, so the humidity ratio of the two airstreams remains unchanged.
- Total Heat Transfer:
Latent heat energy is dependent on the amount of water vapour in the air and therefore total heat transfer can only occur when water vapour is transferred from one airstream to the other.
Preconditioning of Outside Air
A primary use of exhaust air energy recovery is to precondition outside air. During the cooling season, when it is hot and humid outside, the total energy wheel pre-cools and “pre-dries” (dehumidifies) the outdoor air by transferring both sensible heat and water vapor to the cooler, drier exhaust airstream. During the heating season, when it is cold and dry outside, this same device pre-heats and pre-humidifies the outdoor air by removing both sensible heat and water vapor from the exhaust air and transferring it to the entering outdoor airstream.
Tempering of Supply Air
Using an energy recovery wheel to temper supply air is an energy efficient way to control humidity. In the past, humidity control involved cooling the air below the temperature required to satisfy the sensible load and then reheating it. While this simultaneous heating and cooling provided fine temperature and humidity control, it wasted a great deal of energy.
Supply air tempering is used at the peak dehumidifying (i.e. latent) design load when the sensible load in the space is lower and the latent load is higher than the cooling design condition. Typically when the sensible cooling load decreases, the supply air temperature increases (in a constant volume system) or the supply air volume decreases (in a VAV system). However, due to the high latent load in the space, a temperature increase or airflow decrease results in unacceptable humidity levels in the space. To maintain acceptable humidity and temperature levels the supply air is “overcooled” to dehumidify it, then reheated to the temperature required to satisfy the sensible load. Supply air tempering accomplishes this process while reducing or eliminating the need for mechanical reheat, which for example satisfies the requirements of ASHRAE Standard 90.1-2004.
Efficiency of Energy Recovery
The efficiency of an energy recovery system is the ratio of energy transferred between the two airstreams compared with the total energy transported through the heat exchanger. The total amount of energy transferred by an energy recovery device is a function of the effectiveness of the device, the airflow volumes of the two airstreams and the difference in energy levels between the two airstreams, as shown in the following equation defined by ASHRAE Standard 84-1991.
In today’s world, nearly half of global energy is used in buildings. Energy recovery in HVAC is a cost-efficient and sustainable way to reduce global energy consumption and provide better indoor air quality and protect buildings and the environment.