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In today’s life sciences sector, pharmaceutical and life sciences companies are looking for ways to become more energy efficient and lower facility emissions without compromising exacting research and product safety requirements.
In laboratory settings, where there are significant sources of contamination, high ventilation rates are often required to maintain acceptable indoor air quality to meet regulatory safety requirements, maintain product quality and provide optimal environments for equipment and personnel. Energy recovery is a critical strategy to mitigate additional energy penalties resulting from the high ventilation requirements.
Energy recovery is the process of recovering heat and/or moisture between two or more air streams at different conditions. Energy that would otherwise be discharged as waste from exhaust systems can be recovered, minimising cooling and heating demands, operating costs and associated greenhouse gas emissions.
It can be difficult to decide which system would work best for your unique project. As a leading engineering firm designing and delivering laboratories for research facilities and major pharmaceutical companies, we’ve seen which environment each system lends itself to best.
In this article, we’ll provide an overview of key energy recovery technologies, compare their functions and decide which work best for certain projects.
Where do I start?
An initial evaluation will need to be made to determine the suitability of an energy recovery system for your project. Here are the key preliminary considerations you need to make:
- Local code requirements
In Australia, the 2022 National Construction Code (NCC) requires outside air treatment in certain climate zones where outside air flow rates exceed minimum criteria to include either modulating control or an energy reclaim system with minimum heat transfer effectiveness of 60%.
- Cross-contamination risk
A risk assessment will need to be undertaken to determine whether leakage is permitted between the supply and exhaust air streams and, if permitted, the amount of leakage allowed. We’ve provided a system comparison in this article which shares additional detail on leakage between air streams.
- Assessment of local climate conditions
Energy recovery is most suitable where there are significant requirements for cooling (cooling hours > 1000 hours per year) and/or heating (heating hours > for 800 hours per year).
In cold environments, where freezing of heat exchangers can occur, additional mitigation measures will also need to be considered.
- Assessment of quantity of outside air
Laboratory HVAC systems with high exhaust air requirements resulting in a significant outside air to supply air ratios (> 60%) would benefit from energy recovery systems.
- Assessment of available plant area
Energy recovery systems have additional spatial requirements and will need to consider existing building constraints and/or plant spatial allowances negotiated with the architect.
- Assessment of return on investment (ROI)
Energy recovery systems should be designed for the maximum cost benefit or least life cycle costs expressed either over the service life or annually with an acceptable payback period (nominally between 5-10 years).
A review of common energy recovery systems
Technology 1: Plate air-to-air heat exchangers
Plate air-to-air heat exchangers have separate passages that allow the two air streams to pass by each other, without mixing. They are typically made of aluminium or composite materials.
A counterflow heat exchanger allows the two air streams to move in opposing directions, requiring a larger footprint but offering improved heat transfer effectiveness up to 75%.
A crossflow heat exchanger is a cube with the two air streams at right angles to each other. The unit is more compact but can be up to 15% less effective compared with counterflow type heat exchangers due to the reduced thermal contact.
It should be noted that for both cross and counterflow heat exchangers, the plates are not perfectly sealed and care should be taken if 100% air separation is required. Air separation with reduced exhaust cross contamination risks could potentially be achieved by:
- Ensuring supply side fan(s) are located such that the supply side passage of the heat exchanger is positively pressurised (supply fan(s) upstream of the heat exchanger)
- Ensuring the exhaust side fan(s) are located such that the exhaust side passage of the heat exchanger is negatively pressurised (exhaust fan(s) downstream of the heat exchanger)
- Additional space within the plantroom
Figure 1: Cross flow vs Counterflow heat exchangers (Source: Swegon)
Most plate heat exchangers only have sensible heat transfer. If latent heat transfer is required and cross-contamination risks are not critical, some heat exchangers can use a membrane to separate the airstream that is permeable to moisture.
An enthalpy plate can also allow moisture movement from the humid airstream to the dry air stream. It generally requires a larger core to provide enough surface area for moisture transfer. Where stringent control of cross-contamination is required, the risk of using of enthalpy heat exchangers should be reviewed.
Plate air-to-air heat exchangers typically have a sensible heat transfer effectiveness between 50-75% when supply and exhaust air streams have (near) equal air flow rates. Membrane plate heat exchangers typically have a latent heat transfer effectiveness of 25-60% when supply and exhaust air streams have (near) equal air flow rates. The higher energy recovery effectiveness is offset by the additional fan energy required to overcome fan pressure losses from the heat exchanger. Typical plate heat exchanger air side pressure losses can be around 100-300 Pa. Pressure drops across the heat exchanger can be reduced by utilising a lower face velocity across the heat exchanger.
For plate heat exchangers to be worthwhile, the energy consumed to recover the sensible/latent heat (fan power) must be greater than the energy saved from energy recovery.
Some advantages of plate heat exchangers include:
- Higher sensible heat transfer effectiveness compared to run around coils
- Option for latent heat recovery utilising permeable plate membranes (where cross-contamination is not a critical risk)
- Less mechanical components and moving parts. There is no pump within this arrangement and therefore pump failure will not affect energy recovery
Some disadvantages of plate heat exchangers include:
- Potentially higher air side pressure drops and fan power requirements
- Supply and exhaust air paths need to be within close proximity
- Potential for leakage between supply and exhaust air streams (can be minimised through appropriate supply and exhaust air placement)
- May require a longer air handling unit compared to run around coils and rotary heat exchangers
- In colder climates, frosting of the heat exchangers could be in issue.
Technology 2: Run around coils
Run around coils use a fluid (water or water-glycol solution) to transfer heat from one coil to another. The fluid is typically pumped from a coil in the exhaust path to a coil in the supply path. Run around coils offer sensible heat recovery only and have typical heat transfer effectiveness between 45-65% when supply and exhaust air streams have (near) equal air flow rates.
Figure 2 Run around coils during winter operation (Source: Swegon)
For run around coils to be worthwhile, the energy consumed to recover the sensible heat (fan, pump and ancillary power) must be greater than the energy saved from heat recovery. Typical run around coil air pressure losses can be around 70-400 Pa.
Some advantages of run around coils include:
- Run around coils guarantee full separation of supply and exhaust air which can be critical in physical containment lab applications (no risk of cross-contamination)
- Fan location is not critical
- Supply and exhaust air paths do not have to be in close proximity
- Multiple exhaust and supply coils can be on a common circuit
- Lower fan power requirements compared to heat recovery ventilators. Run around coil air pressure drops are typically lower than heat recovery ventilators
Some disadvantages of run around coils include:
- Lower sensible heat transfer effectiveness when compared to plate heat exchangers. For process applications, a dispensation from a Building Surveyor could be required where heat transfer effectiveness is below 60% (as required by the National Construction Code)
- No option for latent heat recovery
- More mechanical components such as chemical dosing pots, expansion tanks and auto-refill units which have ongoing operating and maintenance costs. Additional pump redundancy may also be required if pump failure affects the indoor design conditions
Technology 3: Rotary wheel heat exchangers
Rotary wheel heat exchangers use a revolving cylinder with an air-permeable medium to transfer heat and/or moisture from adjacent supply and exhaust air streams. Heat transfer mediums may be selected to recover sensible heat only or total heat (sensible plus latent).
Because rotary exchangers have a counterflow configuration and normally use small-diameter flow passages, they are quite compact and can achieve high sensible and latent transfer effectiveness between 50-80%.
Figure 3 Rotary wheel heat exchanger (Source: ASHRAE Handbook)
Cross leakage can occur in rotary wheel heat exchangers via seal leakage and carryover. Air separation with reduced exhaust cross contamination risks could potentially be achieved by:
- Ensuring supply side fan(s) are located such that the supply side passage of the heat exchanger is positively pressurised (supply fan(s) upstream of the heat exchanger)
- Ensuring the exhaust side fan(s) are located such that the exhaust side passage of the heat exchanger is negatively pressurised (exhaust fan(s) downstream of the heat exchanger)
- Additional space within the plantroom
A purge section can also be installed to reduce carryover to less than 0.1% of the exhaust air flow but cannot be eliminated. Where stringent control of cross-contamination is required, the risk of using enthalpy rotary wheels should be reviewed.
Some advantages of rotary wheel heat exchangers include:
- Higher sensible heat transfer effectiveness when compared to run around coils
- Option for latent heat recovery utilising desiccant transfer mediums
- Highest latent heat recovery
- Lower air side pressure drop compared to plate heat exchangers
- Air handling units are typically shorter compared to plate heat exchangers
Some disadvantages of plate heat exchangers include:
- Supply and exhaust air paths need to be within close proximity
- Potential for leakage between supply and exhaust air streams (can be minimised through appropriate supply and exhaust placement and utilisation of purge section)
- Air handling units are typically wider compared to plate heat exchangers / run around coils
- More moving parts compared to plate heat exchangers which have ongoing operating and maintenance costs
A comparison of plate air-to-air heat exchangers, rotary wheel heat exchangers and run around coils
Plate Air-to-Air Heat Exchangers | Rotary Wheel Heat Exchangers | Run around Coils | |||
Fixed Plate | Membrane Plate | Heat Wheel | Enthalpy Wheel | Run around Coils | |
Airflow arrangement | Counterflow/Crossflow | Counterflow/Crossflow | Counterflow | Counterflow | - |
Typical sensible effectiveness (%) | 50 to 75 | 55 to 75 | 65 to 80 | 65 to 80 | 45-65 |
Typical latent effectiveness (%) | N/A – No latent heat recovery | 25 to 60 | N/A – No latent heat recovery | 50 to 80 | N/A – No latent heat recovery |
Pressure drop (Pa) range | 100 to 300 Lower end – systems with wider channels and lower airflow rates. Upper end – systems with more compact plate and higher airflow rates. | Similar to fixed plate | 50 to 300 Lower end – Larger wheels with slower rotation and lower air flow rates. Upper end – Smaller, faster rotating wheels with higher air flow rates. | Similar to heat wheel | 70 to 400 Lower end - systems with larger coil surface area and lower air velocities Upper end – systems with smaller coil areas or where the fluid flow requires high resistance to achieve sufficient heat transfer. |
Exhaust air transfer ratio (%) | 0 to 2 | 0 to 5 | 0.5 to 10 | 0.5 to 10 | 0 |
Cross contamination risk | Low. Can be reduced with good fan placement. | Low to Moderate. Dependent on amount of latent transfer, moisture born contaminants can carryover from exhaust stream. | Moderate to high. Small amounts of exhaust can become trapped in wheel. Can be reduced with good fan placement and use of purge. | Moderate to high. Dependent on amount of latent transfer, moisture born contaminants can carryover from exhaust stream. | Zero. No direct air to air contact. |
Temp range (°C) | -60 to 800 | -40 to 60 | -55 to 800 | -55 to 800 | -45 to 500 |
Spatial requirements | Cross flow arrangement is compact. Counterflow arrangement is slightly longer. | Similar to fixed plate. | Compact to high. Large diameter wheels require larger spatial requirements. | Similar to heat wheel. | Moderate. Requires space for at least two coils. Coils can be distributed in different areas to take up less space. |
Capital costs | Lower upfront costs compared to rotary wheels and run around coils due to fewer mechanical components. Approx range $500 to $5,000 per unit. Larger systems or specialised materials can increase costs. | Similar to fixed plate. Additional costs for specialised materials required for latent transfer. | The capital costs for rotary wheels are typically higher than fixed plate heat exchangers due to the inclusion of moving parts, larger housing and motorised components. Approx range: $2,000 to $20,000 per unit. | Similar to heat wheel. Additional costs for specialised materials required for latent transfer. | Most expensive. Approx range: $5000 to $25,000 per system depending on size, complexity and fluid handling requirements. Larger systems with multiple coils can increase costs. |
Embodied carbon | Moderate mainly due to use of metals like aluminium / stainless steel. | Moderate to high. Membranes may require energy intensive process to manufacture. | High mainly due to complexity and use of specialist materials and moving parts. The energy needed to manufacture components may add significantly to embodied carbon. | Similar to heat wheel. | Moderate to high mainly due to use of metals like copper and aluminium. Additional embodied carbon for pumps and piping. |
Energy reduction | High energy reduction. Up to 12% reduction compared to standard system with no heat recovery | Similar to fixed plate but with additional latent energy recovery. | Highest energy reduction. Up to 16% reduction compared to standard system with no heat recovery | Similar to heat wheel but with additional latent energy recovery. | Lowest. Up to 7% reduction compared to standard system with no heat recovery |
Operating costs | Low due to passive operation and minimal maintenance. Main operating cost is slightly higher fan energy due to increased pressure drop. | Similar to fixed plate. | Moderate due to motor energy usage and more frequent maintenance. This is partially offset by higher energy savings from heat recovery | Similar to heat wheel. | High. Due to pump energy usage, lower heat transfer effectiveness and maintenance of moving parts. |
Maintenance costs | Low maintenance costs. No mechanical parts that require regular serving. Maintenance primarily involves cleaning the plates to prevent fouling. | Similar to fixed plate. | Moderate maintenance costs. Require more maintenance than fixed plate exchangers. Bearings, belts and motors require period maintenance. The wheel’s surface requires periodic cleaning to maintain effectiveness. | Similar to heat wheel. | High maintenance costs due regular maintenance required on pumps and periodic inspection of coils. |
Advantages | - High sensible effectiveness - No moving parts | - Allows some latent transfer - No moving parts | - High sensible effectiveness - More compact at large size | -Highest latent effectiveness - More compact at large size | - Full separation of supply & exhaust (no risk of cross-contamination) - Fan location not critical - Supply and exhaust paths can be located in different areas |
Limitations | - Relatively high pressure drop - Supply and exhaust streams need to be collocated. -Potential cross leakage of air between supply and exhaust streams | - Relatively high pressure drop - Supply and exhaust streams need to be collocated. -Potential cross leakage of air between supply and exhaust streams | -Cross leakage of air between supply and exhaust streams - Additional moving parts. | -Cross leakage of air between supply and exhaust streams - Additional moving parts. | - No latent recovery. - Lower sensible effectiveness. - Additional maintenance costs. |
Note 1) Regular maintenance and monitoring are essential for optimal performance in each type of energy recovery technology.
Note 2) Capital costs and embodied carbon associated with heat recovery equipment are often offset by reducing sizing of other HVAC components (eg. AHU coils and chiller plant).
In laboratory settings, where high ventilation rates are required to maintain indoor environment quality, energy recovery technologies play a key role in reducing energy consumption and operating costs.
This article has provided an overview of the available energy recovery technologies and a simple flow chart to determine the suitability of energy recovery for your project.
In Australia, energy recovery systems have been deployed on various high containment (PC2/PC3) research facilities, where energy reductions between 6 to 16% were estimated compared to systems without energy recovery. These technologies have been important in helping our clients meet their 2030 net zero emission targets.
If you’d like to discover how energy recovery systems can work for you on your unique project, please reach out to us through the contact link below.
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