Pre-Cooling Strategies for Peak Load Reduction

Pre-Cooling Strategy Fundamentals

Pre-cooling leverages building thermal mass as passive energy storage to reduce peak cooling loads during high-occupancy periods. This strategy operates by sub-cooling the building structure 2-4 hours before occupancy, allowing the cooled mass to absorb sensible heat gains during peak periods while reducing instantaneous cooling requirements.

The effectiveness of pre-cooling depends on three primary factors:

• Thermal mass magnitude: Concrete, masonry, and gypsum board heat capacity
• Coupling effectiveness: Surface area exposure and convective heat transfer coefficients
• Load profile characteristics: Timing and magnitude of internal heat gains

Thermal Mass Energy Storage

The energy storage capacity of building thermal mass follows fundamental heat transfer principles. The total sensible heat absorbed by structural mass during pre-cooling is:

$$Q_{storage} = \sum_{i=1}^{n} m_i c_{p,i} \Delta T_i$$

Where:

• $Q_{storage}$ = total thermal energy stored (kJ)
• $m_i$ = mass of component i (kg)
• $c_{p,i}$ = specific heat capacity of material i (kJ/kg·K)
• $\Delta T_i$ = temperature depression of component i (K)

For typical concrete structures, the effective thermal mass per unit floor area can be estimated:

$$M_{eff} = \rho_{concrete} \cdot t_{slab} \cdot A_{floor} \cdot f_{coupling}$$

Where $f_{coupling}$ represents the coupling effectiveness factor (typically 0.6-0.8 for exposed slabs, 0.3-0.5 for carpeted or covered surfaces). The specific heat of concrete is approximately 0.88 kJ/kg·K with density of 2300 kg/m³.

The rate of heat absorption from the space to the thermal mass during occupied hours follows:

$$\dot{Q}{mass} = hA(T{air} - T_{mass,avg})$$

Where h is the combined convective heat transfer coefficient (typically 5-10 W/m²·K for horizontal surfaces, 8-15 W/m²·K for vertical surfaces).

Pre-Cooling Sequence and Timing

The optimal pre-cooling sequence balances energy storage capacity against compressor energy consumption and occupant comfort requirements.

graph TD
    A[Night Setup: T_set = 24°C] -->|04:00| B[Pre-Cool Initiation]
    B --> C[Aggressive Cooling: T_set = 19-20°C]
    C -->|2-4 Hours| D[Target Mass Temperature Achieved]
    D -->|08:00| E[Occupancy Begins]
    E --> F[Elevated Setpoint: T_set = 23-24°C]
    F --> G[Thermal Mass Absorbs Loads]
    G -->|12:00-15:00| H[Peak Period Load Reduction]
    H -->|17:00| I[Return to Normal Operation]

    style B fill:#e1f5ff
    style D fill:#b3e5fc
    style F fill:#81d4fa
    style H fill:#4fc3f7

Pre-Cooling Timeline

Peak Demand Reduction Calculations

The instantaneous cooling load reduction during peak periods can be quantified by comparing heat absorption rates:

$$\dot{Q}{reduction} = \dot{Q}{mass} - \dot{Q}_{system,precool}$$

For a typical 10,000 ft² (930 m²) high-occupancy space with 150 mm concrete slab:

Without Pre-Cooling:

• Peak cooling load: 35-40 W/m² = 32.5-37.2 kW
• Peak electrical demand: 11-13 kW (COP = 3.0)

With Pre-Cooling:

• Peak cooling load: 25-30 W/m² = 23.3-27.9 kW
• Peak electrical demand: 7.8-9.3 kW
• Demand reduction: 25-30%

The thermal mass provides temporary cooling at an effective rate:

$$\dot{Q}{mass,effective} = \frac{m{eff} c_p \Delta T_{available}}{t_{peak}}$$

For a 4-hour peak period with 3°C available temperature rise, this provides sustained load offset.

Energy and Cost Optimization

Pre-cooling strategy effectiveness depends critically on utility rate structures. Time-of-use (TOU) rates create economic incentive for load shifting.

Energy Cost Analysis

Typical TOU Rate Structure:

The economic benefit calculation:

$$Savings = (kW_{reduction} \times Demand_{charge}) + \sum_{period} (kWh_{shifted} \times \Delta Rate_{period})$$

Typical demand charge savings range from $15-25/kW-month. For the example 3-4 kW peak reduction, monthly demand charge savings alone equal $45-100.

Implementation Considerations per ASHRAE Standards

ASHRAE Standard 90.1 Section 6.4.3.3 addresses precooling and optimal start/stop controls. Key requirements:

• Temperature limits: Pre-cooling setpoints must not violate health and safety requirements (typically not below 18°C)
• Humidity control: Pre-cooling may require enhanced dehumidification capacity
• Control deadbands: Minimum 1.1°C deadband between heating and cooling per Standard 90.1
• Optimal start algorithms: ASHRAE Guideline 36 provides control sequences for building mass precooling

ASHRAE Research Project RP-1404 demonstrated that pre-cooling strategies achieve:

• 15-30% peak demand reduction in buildings with exposed thermal mass
• 8-15% reduction in daily cooling energy consumption under appropriate utility rate structures
• Optimal pre-cool duration of 2-4 hours depending on mass exposure and occupancy load magnitude

Control Strategy Integration

Effective pre-cooling requires sophisticated building automation:

sequenceDiagram
    participant BAS as Building Automation System
    participant Utility as Utility Pricing Signal
    participant Weather as Weather Forecast
    participant HVAC as HVAC Equipment

    Utility->>BAS: Next-day pricing data
    Weather->>BAS: Temperature forecast
    BAS->>BAS: Calculate optimal pre-cool timing
    BAS->>BAS: Determine setpoint schedule
    BAS->>HVAC: Initiate pre-cool (04:00)
    HVAC->>BAS: Zone temperatures feedback
    BAS->>BAS: Monitor thermal storage state
    BAS->>HVAC: Adjust to occupied setpoint (08:00)
    HVAC->>BAS: Load response during peak
    BAS->>BAS: Calculate actual savings
    BAS->>BAS: Update predictive algorithm

The control system must integrate multiple data streams to optimize performance while maintaining occupant comfort within ASHRAE Standard 55 thermal comfort boundaries.

Practical Application

Pre-cooling proves most effective in:

• Buildings with significant exposed thermal mass (concrete, masonry)
• High-occupancy facilities with predictable load schedules
• Regions with substantial TOU rate differentials
• Climate zones with significant diurnal temperature swing

Marginal effectiveness occurs in:

• Light-frame construction with minimal thermal mass
• 24-hour operation facilities
• Regions with flat utility rate structures
• Spaces with dominant latent cooling loads