Sustainability: Return air energy recovery

13/08/21

Having devoted an entire article to waste heat recovery in our previous blog it’s time to look at other ways of recovering energy. One of the most applied forms of energy recovery in HVAC systems is return air. There are multiple methods as we will see… But which is most suitable for your system?

The outside air brought in from an HVAC air handling unit is used for two main purposes: to create an indoor environment according to the specified conditions and to provide sufficient fresh air. The latter is a necessity as it prevents headaches and reduces the risk of infection and the spread of disease. In short, fresh air keeps the passengers and crew healthy and comfortable.

Before entering the cabins and rooms, outside air needs to be treated in the air handling unit and brought to the right condition. A lot of energy is used in this procedure and failing to reuse some of this energy is clearly a waste. Energy recovery from return air can be done in four main ways:

• Recirculation
• Crossflow heat exchange
• Twin-coil system
• Heat recovery wheel

Recirculation

One of the simplest ways of return air energy recovery is via recirculation. The air handling unit provides the vessel (or a section thereof) with the amount of treated air necessary for a comfortable and healthy environment. Part of this air volume is recirculated and the amounts of fresh air are based on the recirculation rate. A common ratio is 50/50, which indicates that 50% of the total air is recirculated while the other half is fresh. Even better results are gained with a 60/40 rate.

Crossflow heat exchanger

This method involves having a crossflow heat exchanger built inside the air handling unit. Before leaving the ship, return air runs through this heat exchanger which directs the conditioned air flow crosswise along the incoming air flow, separated by a thin ribbed metal plate. Heat transfer takes place through conduction to the incoming air stream. The efficiency of a crossflow heat exchanger is around 50%.

Twin-coil system

A twin-coil system consists of two water-fed elements called the cooler and economizer, located in the supply and discharge duct respectively. A built-in circulation pump transports the energy recovery medium taking heat from the economiser and transports it to the cooler. The efficiency of this system is also around 50%.

Heat recovery wheel

With a level of 70%, the highest efficiency rates are realised with a heat recovery wheel. This method brings incoming and outgoing air flows together in a wheel. The air currents enter the wheel in opposite directions, with both the warm and cold air filling half of the rotary heat exchanger. As the wheel rotates at a slow rate, the energy can be given off to the duct wall for half a revolution. The subsequent half revolution allows the duct wall to relinquish its energy to the incoming air stream.

Final thoughts

In the examples above we assume a summer situation where the incoming air is warm and therefore needs to be cooled. Naturally, all energy recovery methods also work in winter where the incoming air is colder and needs to be heated.

Furthermore, the methods mentioned are all applicable to full air systems where the central air handling unit provides air for both the conditioning of the cabins and ventilation purposes. This requires high air volumes and, therefore, bigger ducting systems.

Vessels with limited space can be equipped with a local cooling installation like a fan coil system. A central air handling unit only provides the fresh outside air. According to ISO 7547, a minimum of 0.008 m³/s per person of fresh outdoor air is required. Because the unit only provides fresh air, the ducting system will be a lot smaller compared to a full air system.

In this installation, all air (except the necessary fresh air) is recirculated. The conditioning of the cabin or room is done locally by a fan coil unit for example, which treat the recirculated air. Still, energy recovery systems like a crossflow heat exchanger, twin-coil and heat recovery wheel can be applied on the exhaust air from toilets and sanitary spaces to make it even more sustainable.

Like to know more about energy recovery and which method might best suit your project or vessel? Contact one of our engineers.

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