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All you need to know about: Energy Recovery
An overview of the assets and drawbacks of the most common heating and cooling recovery methods in air handling units (AHUs).
Efficiency
There are several heating and cooling recovery methods in Air Handling Units (AHUs), all offering its own unique range of benefits. It is important for customers to be aware of the individual solution’s advantages and disadvantages to be in the best position to choose the ideal option for their respective projects.
This starts with having a good understanding of which parameters and features should be taken into consideration. In this document, we will be discussing the four main systems:
Plate heat exchanger
Rotary heat exchanger
Run-around coil system
Compressor system
Plate heat exchanger
Plate heat exchangers are one of the most common units available in the market. According to Eurovent Market intelligence, units with plate heat recovery were responsible for 34 % of total AHUs sold in Europe in the year 2020.
Principle of operation: How does a plate heat exchanger work?
A heat exchanger package consists of aluminum plates with a specially profiled surface. The plates are so situated that each plate has a warm and a cold side. The plates’ distance depends on the unit size, requested efficiency and pressure drop. In such systems, the possibility of leakage is less than 0,5 % at 250 Pa pressure difference for Eurovent-certified product models without humidity transfer.
Plastic plate heat exchangers
The traditional material used in heat exchangers is aluminum. In some applications, plastic heat exchangers can be an option. In comparison to aluminum, the features of plastic such as good elongation, low weight, and lower price per kilogram makes the use of plastic attractive. However, plastic has lower strength and lower heat transfer coefficient compared to aluminum. The choice of material should consider the total requirements, considering the advantages and disadvantages.
Difference between efficiencies with and without condensation
The efficiency of a plate heat exchanger is dependent on the type of exchanger, plate distance, profiled surface design, and condensation in the exchanger. The efficiency is typically referred to in the following expressions:
Temperature efficiency dry, which shows the efficiency without condensation
Temperature efficiency wet, which shows the efficiency with condensation at the actual data for the airflows
Ice buildup in a plate heat exchanger
A typical concern related to the plate heat exchanger is ice buildup, which always starts in the “cold corner” of the exchanger. This can be avoided by the following methods:
Advantages of plate heat exchangers
High efficiency of up to 85 %
No leakage or, at the most, minimal leakage
Disadvantages of plate heat exchangers
Freezing problems especially during wintertime
Usually no humidity transfer
Large length of unit at bigger airflows
Difficult to clean
Systemair produces the plate heat exchangers it uses in-house.
Rotary heat exchanger
The rotary heat exchanger is one of the most common units available in the market. According to Eurovent Market Intelligence, units with rotary heat recovery were responsible for 32% of the total AHUs sold in Europe in the year 2020. In Northern Europe, 62% of the AHU market is with rotors.
Principle of operation: How does a rotary heat exchanger?
A rotary heat exchanger ensures both heat and moisture recovery. The rotor consists of every second layer waved aluminium foil and plain aluminium foil winded on a core.
Rotor type definitions (according to Eurovent)
Condensation rotor
Enthalpy / Hygroscopic rotor
Sorption rotor
A sorption rotor demonstrates a humidity efficiency which is at least 70 % of the temperature efficiency under all tested conditions with nominal airflow rate. Units having a lower humidity efficiency can only be certified in the class “hygroscopic RHE = enthalpy hygroscopic RHE”.
Sorption rotors offer several advantages, outlined below:
Lower cooling capacity for AHU
Energy saving in summer conditions
Higher humidity recovery in wintertime
Lower risk of freezing
In view of these advantages, sorption technology is best suitable for projects meeting the following parameters:
Installations where cooling is required
Installations where peak load management is a key issue
Whenever humidification of supply air is required
Existing installation where chiller's capacity is limited and there are problems during the summer period
Remember!
Acceptance of the system is set to further increase with the introduction of Eurovent Certification’s Energy Efficiency Classes for Summer Application (EECS) label, which underscores the contribution of sorption wheels in terms energy savings. The new Eurovent “summer label” reflects energy saving for warmer weather conditions and has the added benefit of acknowledging humidity recovery and its contribution to overall energy efficiency performance. The new label also gives a realistic representation of the usefulness and necessity of a certain energy recovery efficiency, with the air resistance that this may cost at a certain fan efficiency.
Rotor leakages: Why does it happen?
Leakage is sometimes a concern regarding rotors. Leakages can occur due to the following reasons:
Through sealing (central beam + periphery) from one air stream to another
By-pass leakage through sealing in the same air stream
Through the purge sector
Carry over air carried in storage matrix from one air stream to another
Leakage of casing to/from outside in the case of a stand-alone unit
Eurovent defines two different rotor leakages:
Outdoor Air Correction Factor (OACF)
Air going from outdoor to exhaust through sealing and/or purge sector
This leakage increases exhaust fan power consumption
Exhaust Air Transfer Ratio (EATR)
Air going from extract to supply through sealing and/or in storage matrix
Leakage carried in storage matrix is also called CARRY OVER
It is not possible for both the OACF and EATR to be zero. When pressure difference increases, OACF increases, EATR decreases, and vice versa.
Advantages of rotary heat exchangers
High efficiency of up to 85%
High humidity efficiency possible
Nonfreezing risk in most regions
Low space needed
Self-cleaning feature
Can be delivered in all AHU sizes
Disadvantages of rotary heat exchangers
Moving parts, motor, belt drive which require maintenance
Leakage, carryover risk 0-3%
Requires considerations on fan positions. It is important to ensure the AHU manufacturer designed the unit optimally, otherwise the unit will have high risk of leakage. There are instances where fans are misplaced because of either lack of knowledge or to save space. Therefore, customers must be aware that the fan is positioned correctly within the system.
Run-around coil system
Run-around coil systems are not as widespread compared to its rotary and plate heat exchanger counterparts. According to Eurovent Market intelligence, units with run-around-coil heat recovery were responsible for only 4% of the total AHUs sold in Europe in the year 2020.
Principle of operation: How does a run-around coil system work?
The system works owing to water with anti-freeze additive being used in the circuit, which is pumped between two coils, thus transferring energy from one coil to the other.
Advantages of run-around coil systems
Supply and exhaust air can be in separate places
No leakage, fully separated air flows
Different materials can be used in supply and exhaust air
Can be used to recover energy from processes and toxic air
Can be delivered in all AHU sizes
Disadvantages of run-around coil systems
Difficult to get high efficiency
No humidity transfers possible
Glycol leakage
Expensive pipelines and pump systems
Compressor system
According to Eurovent Market Intelligence, units with a compressor were responsible for only 4% of the total AHUs sold in Europe in the year 2020. The main market for this system is northern Europe where it represents 11% of the market. In AHUs with integrated reversible heat pump you gain ventilation, heating, and cooling all in one unit.
Principle of operation: How does a compressor system work?
In principle, a heat pump is using a compressor to compress and move refrigerant around in a closed system. There are four main components in a heat pump that are very good to know the basics of:
Compressor
The compressor compresses and moves the refrigerant gas around in the heat pump system.
The compressors can regulate capacity to meet the temperature and airflow demands of the AHU control system.
Condenser
The condenser is the coil that receives the warm and compressed refrigerant gas from the compressor.
This coil is always hot and will heat up the airflow in the air handling unit. The refrigerant gas condensates during the passing through the condenser coil because the airflow in the AHU will cool the gas, and at the same time absorb heat to the airflow that supplies the building.
Expansion valve
The expansion valve is always placed between the condenser coil and the evaporator coil.
The valve is equipped with a spring so refrigerant gas and liquid mixture are relieved of pressure when passing through.
Evaporator
The evaporator is the coil that receives cooled liquid refrigerant after the expansion valve.
The liquid refrigerant evaporates into gas and this evaporation draws energy from the surrounding coil material, so the coil will become cold. This coil is always cold and will cool down the airflow in the air handling unit.
Advantages of compressor systems
High total system efficiency 80-95%
Cooling or heating for comfort levels can be added into the building (total climate solution)
Built in heat pump saves space compared with external heat pump system
Disadvantages of compressor systems
Moving parts, motor, belt drive which require maintenance
Leakage in the rotor, carryover risk 0-3%
Requires larger power installation because of compressor
Service on refrigerant system needed