HVAC Systems Encyclopedia

A comprehensive encyclopedia of heating, ventilation, and air conditioning systems

Energy Recovery Systems for HVAC Engineers

Energy Recovery Systems for HVAC Engineers

Energy recovery reduces ventilation loads by transferring energy between exhaust and outdoor air streams. Proper selection and sizing can reduce HVAC energy consumption by 20-40% in high-ventilation applications.

Energy Recovery Types

Heat Recovery Ventilator (HRV)

Transfers: Sensible heat only

Applications: Cold/dry climates (avoid over-humidification in winter)

Effectiveness: 60-80% sensible

Energy Recovery Ventilator (ERV)

Transfers: Sensible + latent heat (total energy)

Applications: Hot/humid climates, high latent loads

Effectiveness: 60-80% total, 50-70% latent

Technology Types

Rotary Wheel (Air-to-Air Exchanger)

Characteristics:

  • Rotating desiccant-coated wheel
  • Transfers sensible + latent (ERV)
  • High effectiveness (70-85%)
  • Cross-contamination: 1-5% (purge sector reduces)

Advantages: Highest effectiveness, lowest pressure drop Disadvantages: Moving parts, cross-contamination, requires maintenance

Plate Heat Exchanger

Characteristics:

  • Stationary plates separate airstreams
  • Transfers sensible only (unless with permeable membrane)
  • Effectiveness: 50-75%
  • Zero cross-contamination

Advantages: No moving parts, reliable, no cross-contamination Disadvantages: Lower effectiveness, higher pressure drop

Heat Pipe

Characteristics:

  • Refrigerant-filled pipes transfer heat
  • Sensible only
  • Effectiveness: 45-65%
  • Zero cross-contamination

Advantages: Passive (no power), no moving parts Disadvantages: Limited effectiveness, orientation-sensitive

Run-Around Loop

Characteristics:

  • Glycol loop connects coils in exhaust and supply ducts
  • Allows separated air handlers
  • Effectiveness: 45-65%

Advantages: Flexible placement, no cross-contamination Disadvantages: Pump energy, glycol maintenance

Effectiveness

Sensible effectiveness:

$$\epsilon_s = \frac{T_{supply,leaving} - T_{OA}}{T_{exhaust} - T_{OA}}$$

Total effectiveness:

$$\epsilon_t = \frac{h_{supply,leaving} - h_{OA}}{h_{exhaust} - h_{OA}}$$

Energy Savings

Annual heating energy recovery:

$$Q_{recover} = 1.08 \times CFM \times \epsilon_s \times HDD \times 24$$

Annual cooling energy recovery:

$$Q_{recover} = 4.5 \times CFM \times \epsilon_t \times CDD \times 24$$

Typical payback: 3-8 years depending on climate, utility rates, ventilation rates

Frost Control

Problem: When exhaust air moisture freezes on cold wheel/plates

Occurs: Outdoor air < 15-25°F with high indoor humidity

Strategies:

  1. Preheat outdoor air: Electric or hot water coil before wheel
  2. Bypass outdoor air: Route some OA around wheel
  3. Wheel rotation modulation: Slow or stop rotation
  4. Exhaust air recirculation: Reduce exhaust airflow (warms wheel)

Pressure Drop

Typical:

  • Rotary wheel: 0.4-0.8 “w.g. (each side)
  • Plate exchanger: 0.6-1.2 “w.g.
  • Heat pipe: 0.3-0.6 “w.g.

Fan power penalty must be considered in energy analysis

Applications

High-benefit applications:

  • High outdoor air rates (100% OA systems, DOAS)
  • Long operating hours (24/7 facilities)
  • Extreme climates (very cold winters or hot/humid summers)
  • Healthcare (high ventilation codes)
  • Schools (high occupant density)

Low-benefit applications:

  • Low outdoor air rates (< 20%)
  • Mild climates
  • Short operating hours
  • Where cross-contamination unacceptable (unless using plate/run-around)

Practical Design

  1. Sizing: Match outdoor air CFM to ensure balanced flows
  2. Effectiveness: Select 65-75% for cost/performance balance
  3. Frost control: Required in climates with winter design < 20°F
  4. Maintenance: Plan filter access, wheel cleaning (annually)

Related Technical Guides:

References:

  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 26: Air-to-Air Energy Recovery Equipment
  • ASHRAE Standard 84: Method of Testing Air-to-Air Heat/Energy Exchangers
  • AHRI Standard 1060: Performance Rating of Air-to-Air Heat Exchangers for Energy Recovery Ventilation Equipment