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Enthalpy wheel in a heat recovery unit
Energy Recovery Wheels | HVAC Enthalpy & Heat - As the cost of energy rises engineers are looking for ways to improve and increase energy efficiency. Aside from HVAC equipment with higher energy efficiency rates, improving structures for less heat loss and heat gain, and building automation systems giving the equipment precision control for the
better efficiency of the equipment, there are other ways to increase energy efficiency inside buildings. One of the ways to increase energy efficiency inside buildings is to add energy recovery wheels to the economizer systems. A US Department of Energy study concluded energy recovery wheels can save over 15% of the energy used in commercial HVAC applications.
HVAC Enthalpy & Heat - Energy Recovery Wheels Benefits
Using outside air is mandatory by mechanical code. Changing the inside air with outside air makes the air inside sealed structures healthier for people to breathe. People, office machines, furniture, and carpet inside structures produce volatile organic compounds including CO2. The things that are not good for health in high concentrations. Too much CO2 inside a structure will make people less productive so increase the fresh air with lower CO2 levels and worker productivity increases.
In buildings that have high CO2 levels, occupants complain about headaches and fatigue among other health problems caused by high CO2 levels. How much fresh air is good? ASHRAE has recommended levels of CO2 to maintain healthy conditions inside structures.
To keep the levels of CO2 and other volatile organic compounds diluted or ventilated the building needs mechanical or natural ventilation to decrease the amounts of CO2 and volatile organic compounds contained in the air inside buildings. Many buildings have mechanical ventilation in place as required by code but many buildings do not effectively control the mechanical ventilation nor do the HVAC systems condition the outside air that comes into the building.
Energy Recovery Wheels | Conditioning and Energy Savings
An energy recovery wheel conditions the air that comes into the structure from the outside using the relief or exhaust air leaving the building. Additionally, a building that uses CO2 control for outside air ventilation will save money on energy costs by precisely controlling the outside air introduced to the structure. Using CO2 control with an energy recovery wheel will offer added efficiency for buildings while also meeting requirements of changing the air inside buildings according to code requirements.
How the Energy Recovery Wheel Works
In the summertime when the temperature of the outdoor air is high and the control system calls for more outdoor air to be introduced to the structure (if the HVAC system is equipped with controls for outdoor air) The outside air duct is ducted through an energy recovery wheel while the air leaving the building is also ducted through the energy recovery wheel in a different duct The wheel turns and absorbs the heat and moisture taken from the outdoor air that comes through the part of the duct which supplies the outside air to the building.
The air leaving the building, which is cooler and partly conditioned, is ducted through the energy recovery wheel. The moisture and heat absorbed by the energy recovery wheel in the incoming duct is absorbed in the leaving building air and expelled outside. This process can drop the temperature of the outdoor air or incoming air by more than 10° F. plus the humidity absorbed adds to the comfort level indoors and reduces the amount of mechanical cooling need to dehumidify in the summer.
Energy Recovery Wheels | The Bonus the Energy Savings
This is a bonus when you compare it to an HVAC system that does not utilize an energy recovery wheel to condition the outside air before it is introduced to the HVAC ductwork inside the building. So the desiccant wheel saves energy by absorbing heat and moisture (sensible and latent heat) and transferring that to the leaving air. In the wintertime, this process works in reverse by using the exhaust air vented to the outside through the energy recovery wheel to heat the air coming into the building via the outside air duct via the energy recovery wheel.
The temperature of the outside air can be conditioned to more than 40º F. by the same transfer method described above for summer operation except for the process for the energy recovery wheel is reversed in the winter. One can quickly grasp how much energy savings can be realized by utilizing an energy recovery wheel where outside air is conditioned using exhaust or relief air. Basically, an energy recovery wheel utilizes the partially conditioned exhaust air leaving the building to condition the incoming outside air.
Energy Recovery Wheels | Other Benefits
- Removes moisture from outdoor air which reduces or prevents moisture and humidity problems and it will also reduce mechanical cooling in the summer
- Increases indoor air quality
- Improved health of the occupants of the building resulting from better indoor air quality. Better health of the occupants of the building translates to higher productivity for businesses
- Saves energy
Energy Recovery Wheels | HVAC Enthalpy & Heat and CO2 Control Can Save Energy - Conclusion
By utilizing precise control that CO2 control offers to a building’s HVAC system and by pre-conditioning the outside air using the energy contained in the exhaust air by using an energy recovery wheel a building can be optimized to utilize less energy. Using less energy saves building owners more money, utilizes less of our finite natural resources, and is good for the environment. In new construction projects and in retrofitted projects a building project aiming for LEED status can be given LEED points for using energy recovery wheels in the HVAC system for the building.
Desiccant Wheel Heat Recovery Unit
This is a desiccant wheel in a heat recovery unit. Mainly used in HVAC commercial applications where codes require a specified amount of fresh air for buildings. The old way was to pull the unconditioned fresh air and then condition the air using the mechanical equipment installed in the building. Using a heat recovery wheel will save energy by conditioning the air before it is introduced into the building and the mechanical equipment used to condition the air inside the building.
Exhaust air flows through the desiccant wheel one way while fresh outside air flows through the wheel going the other way. The exhaust, while containing conditioned air, transfers energy to the wheel. The wheel turns slowly and the incoming outside air absorbs this energy or conditioned air. In the summer, the wheel reduces the humidity and temperature of the incoming outside air while in the winter increases the temperature of the incoming outside air.
Energy Recovery Wheels
