Total Energy Recovery: Sensible and Latent Transfer

Total energy recovery (TER) systems recover both sensible heat and latent heat (moisture) from exhaust air streams, maximizing energy efficiency in ventilation applications. Unlike sensible-only heat recovery, TER systems transfer enthalpy, reducing both heating/cooling loads and humidification/dehumidification requirements.

Fundamental Principles

Enthalpy Transfer Mechanism

Total energy recovery operates on the principle of simultaneous heat and mass transfer. When two air streams at different temperatures and humidity ratios flow through a TER device, energy transfer occurs in two forms:

Sensible heat transfer follows temperature difference: $$Q_s = \dot{m} \cdot c_p \cdot (T_1 - T_2)$$

Latent heat transfer follows moisture content difference: $$Q_l = \dot{m} \cdot h_{fg} \cdot (W_1 - W_2)$$

where $\dot{m}$ is mass flow rate, $c_p$ is specific heat of air, $h_{fg}$ is latent heat of vaporization (approximately 1,060 BTU/lb or 2,465 kJ/kg), and $W$ is humidity ratio.

Total enthalpy transfer combines both: $$Q_t = \dot{m} \cdot (h_1 - h_2)$$

where $h$ represents specific enthalpy of the moist air stream.

Total Effectiveness

Per ASHRAE Standard 84-2020, total effectiveness quantifies the device’s ability to transfer enthalpy between air streams:

$$\varepsilon_t = \frac{h_{supply,out} - h_{outdoor}}{h_{exhaust,in} - h_{outdoor}}$$

This can be decomposed into sensible and latent components:

$$\varepsilon_t = \frac{\varepsilon_s \cdot c_p \cdot (T_{exh} - T_{oa}) + \varepsilon_l \cdot h_{fg} \cdot (W_{exh} - W_{oa})}{h_{exh} - h_{oa}}$$

Typical total effectiveness values range from 60% to 85% for commercially available systems.

Total Energy Recovery Technologies

Energy Wheels (Enthalpy Wheels)

Energy wheels consist of rotating matrices of hygroscopic materials that absorb and desorb both heat and moisture.

graph TB
    subgraph "Enthalpy Wheel Operation"
        A[Exhaust Air<br/>75°F, 50% RH] -->|Heating & Humidifying<br/>Wheel Material| B[Energy Wheel<br/>Rotating Matrix]
        B -->|Releasing Energy<br/>& Moisture| C[Outdoor Air<br/>95°F, 60% RH]
        C -->|Cooling & Dehumidifying<br/>Wheel Material| B
        B -->|After Energy Transfer| D[Supply Air<br/>82°F, 54% RH]
        A -->|After Energy Transfer| E[Exhaust Air Out<br/>88°F, 56% RH]
    end

    style B fill:#f9f,stroke:#333,stroke-width:4px
    style A fill:#bbf,stroke:#333,stroke-width:2px
    style C fill:#fbb,stroke:#333,stroke-width:2px
    style D fill:#bfb,stroke:#333,stroke-width:2px
    style E fill:#fdb,stroke:#333,stroke-width:2px

Construction characteristics:

Desiccant-coated aluminum or synthetic substrate
Rotation speeds: 10-20 rpm typical
Matrix depth: 200-500 mm
Wheel diameter: 0.6-4.0 meters

Performance parameters:

Total effectiveness: 70-85%
Sensible effectiveness: 75-90%
Latent effectiveness: 50-75%
Pressure drop: 0.4-1.2 in. w.g. (100-300 Pa)

Cross-contamination control: Energy wheels inherently allow minimal air transfer between streams (typically 1-5%). Purge sectors can reduce carryover to less than 1%.

Membrane Energy Exchangers

Fixed-plate or counter-flow membrane exchangers use semi-permeable materials that allow water vapor transmission while preventing air mixing.

Membrane types:

Polymer membranes - Polyethylene, polypropylene with molecular sieves
Treated paper - Cellulose-based hygroscopic materials
Synthetic composites - Multi-layer engineered materials

Operating principles:

flowchart LR
    subgraph Membrane_Transfer["Membrane Energy Exchanger"]
        direction TB
        EA[Exhaust Air<br/>72°F, 45% RH] --> M1[Membrane<br/>Layer]
        M1 -.Sensible Heat.-> M2[Heat Conduction]
        M1 -.Latent Heat.-> M3[Vapor Diffusion]
        M2 --> OA1[Outdoor Air]
        M3 --> OA1
        OA[Outdoor Air<br/>35°F, 30% RH] --> M1
        OA1[Conditioned Air<br/>58°F, 40% RH]
    end

    style M1 fill:#fcf,stroke:#333,stroke-width:3px
    style EA fill:#bbf,stroke:#333,stroke-width:2px
    style OA fill:#fbb,stroke:#333,stroke-width:2px
    style OA1 fill:#bfb,stroke:#333,stroke-width:2px

Performance characteristics:

Total effectiveness: 60-75%
Zero cross-contamination (complete separation)
No moving parts (higher reliability)
Pressure drop: 0.3-0.8 in. w.g. (75-200 Pa)
Requires periodic cleaning

Desiccant-Enhanced Systems

Advanced systems incorporate liquid or solid desiccants to enhance moisture transfer capacity, particularly valuable in high-humidity climates.

Configuration types:

Rotating desiccant wheels with enhanced hygroscopic coatings (silica gel, molecular sieves, lithium chloride)
Liquid desiccant loops using lithium chloride or calcium chloride solutions
Hybrid membrane-desiccant combining technologies

Enhanced moisture removal: Desiccant systems achieve latent effectiveness exceeding 80%, compared to 50-70% for standard energy wheels.

Climate-Specific Performance

Cooling-Dominated Climates

In hot-humid conditions (Miami, Houston), total energy recovery significantly reduces dehumidification loads:

$$Q_{total,saved} = \dot{V} \cdot \rho \cdot (h_{oa} - h_{supply}) \cdot \varepsilon_t$$

Summer design example (1,000 CFM):

Outdoor: 95°F, 70% RH (h = 46.5 BTU/lb)
Exhaust: 75°F, 50% RH (h = 28.2 BTU/lb)
With 75% total effectiveness: Supply at 81°F, 58% RH (h = 32.8 BTU/lb)
Energy recovered: 102,000 BTU/hr (30 kW)

Heating-Dominated Climates

In cold-dry conditions (Minneapolis, Calgary), TER prevents excessive indoor humidity loss:

Winter design example (1,000 CFM):

Outdoor: 0°F, 60% RH (h = 0.9 BTU/lb)
Exhaust: 70°F, 30% RH (h = 19.7 BTU/lb)
With 75% total effectiveness: Supply at 56°F, 32% RH (h = 15.6 BTU/lb)
Energy recovered: 110,000 BTU/hr (32 kW)

Mixed Climates

Year-round operation in moderate climates (San Francisco, Seattle) requires bypass dampers or modulating controls to prevent over-recovery during mild conditions.

ASHRAE Standards and Code Requirements

ASHRAE Standard 84-2020

Establishes testing and rating procedures for air-to-air heat exchangers:

Test conditions:

Balanced flow (supply = exhaust CFM)
Standard airflow rates: 500, 1000, 2000 CFM per test
Multiple temperature/humidity combinations
Frost accumulation testing for total energy recovery

Reported performance data:

Sensible effectiveness ($\varepsilon_s$)
Latent effectiveness ($\varepsilon_l$)
Total effectiveness ($\varepsilon_t$)
Pressure drop for supply and exhaust streams
Cross-contamination percentage

ASHRAE Standard 90.1-2022 Requirements

Section 6.5.6.1 mandates energy recovery for systems meeting specific criteria:

Climate Zone	Design Outdoor Air	System Operating Hours
1-2 (Hot)	≥ 5,000 CFM	≥ 8,000 hr/yr
3-4 (Moderate)	≥ 5,000 CFM	≥ 8,000 hr/yr
5-8 (Cold)	≥ 3,000 CFM	≥ 8,000 hr/yr

Minimum effectiveness requirements:

Total effectiveness ≥ 50% in all climate zones
Higher requirements for larger systems (60% for systems ≥ 20,000 CFM)

Exceptions:

Laboratory hoods with hazardous exhaust
Systems serving spaces with high particulate loads
Applications where outdoor air fraction exceeds 70%

System Selection Considerations

Application-Specific Factors

Energy wheels preferred for:

High effectiveness requirements (75-85%)
Balanced sensible/latent loads
Commercial office buildings, schools

Membrane exchangers preferred for:

Sensitive applications requiring zero cross-contamination
Hospitals, laboratories, food processing
Corrosive exhaust streams

Desiccant systems preferred for:

High-humidity climates (coastal, tropical)
Applications requiring deep dehumidification
Indoor pools, spas, museums

Economic Analysis

Payback period depends on:

Operating hours per year
Outdoor air fraction
Local utility rates
Climate severity

$$\text{Simple Payback (years)} = \frac{\text{Installed Cost}}{\text{Annual Energy Savings} - \text{Annual Maintenance}}$$

Typical installed costs range from $1.50 to $3.50 per CFM for commercial applications.

Maintenance and Longevity

Energy Wheel Maintenance

Inspect rotation mechanism: Quarterly
Clean wheel matrix: Annually or bi-annually
Check belt tension and drive motor: Quarterly
Replace desiccant coating: 10-15 years typical life

Membrane Exchanger Maintenance

Inspect for blockage: Quarterly
Clean membrane surfaces: Annually (pressure washing or chemical cleaning)
Check gaskets and seals: Annually
Replace membrane cores: 15-20 years typical life

Proper maintenance ensures sustained effectiveness throughout the equipment lifecycle, maximizing return on investment for total energy recovery systems.