Energy Storage Research

Energy storage research addresses peak demand reduction, load shifting, renewable energy integration, and economic optimization of HVAC systems through thermal and electrical storage technologies.

Thermal Energy Storage Fundamentals

Thermal energy storage (TES) systems store heating or cooling capacity for later use, decoupling energy production from consumption. Storage enables cost reduction through time-of-use rate arbitrage and improves grid stability by shifting electrical loads.

Storage capacity equation:

Q = m × c × ΔT (sensible storage) Q = m × hfg (latent storage)

Where:

• Q = stored energy (Btu or kJ)
• m = mass of storage medium (lb or kg)
• c = specific heat (Btu/lb·°F or kJ/kg·K)
• ΔT = temperature change (°F or K)
• hfg = latent heat of fusion (Btu/lb or kJ/kg)

Efficiency metrics:

ηstorage = Qout / Qin × 100%

Practical storage efficiencies range from 85-95% depending on configuration, insulation, and operating conditions.

Ice Storage Systems

Ice storage produces ice during off-peak hours using chillers operating at low loads and higher efficiency. Ice melts during peak periods to provide cooling without chiller operation.

Ice-on-Coil Systems

Polyethylene tanks contain coils submerged in water. Glycol solution at 18-22°F circulates through coils, freezing surrounding water.

Charge characteristics:

• Ice builds uniformly around coils
• Complete freeze time: 6-10 hours
• Ice thickness: 0.5-1.5 inches
• Glycol supply temperature: 18-20°F
• Glycol return temperature: 22-24°F

Discharge characteristics:

• Melt water circulates through external heat exchanger
• Leaving water temperature: 34-36°F
• Discharge rate: variable based on demand
• Complete melt time: 4-8 hours

Ice Harvesting Systems

Ice forms on evaporator plates, then releases into storage tank as loose ice. Harvesting cycle repeats continuously during charging.

Operational sequence:

1. Freeze cycle: 20-30 minutes, ice builds to 0.25-0.375 inch thickness
2. Harvest cycle: 1-3 minutes, warm refrigerant releases ice
3. Ice falls into insulated storage tank
4. Process repeats until tank is full

Performance parameters:

• Ice production rate: 15-25 tons per harvester module
• Storage density: 25-35 lb ice/ft³ tank volume
• Harvest efficiency: 92-96%
• Subcooling during harvest: 2-4°F

External Melt Ice-on-Coil

Warm return water from building flows over external surface of ice-filled tubes, melting ice from outside. Internal glycol circuit remains isolated from building loop.

Advantages:

• Direct interface with chilled water system
• No glycol in building loop
• Lower pumping energy
• Simpler controls

Design considerations:

• Tank inlet/outlet manifold design critical for uniform flow
• Typical discharge approach: 2-3°F
• Water velocity over coils: 1-2 ft/s
• Stratification control required

Chilled Water Storage

Stratified chilled water storage tanks maintain temperature separation between warm and cold water layers through density differences and careful inlet/outlet design.

Stratification Maintenance

Richardson number criterion:

Ri = (g × β × ΔT × H) / V²

Where:

• Ri = Richardson number (>10 for stable stratification)
• g = gravitational acceleration (32.2 ft/s²)
• β = thermal expansion coefficient (1/°R)
• ΔT = temperature difference between layers (°F)
• H = thermocline height (ft)
• V = inlet/outlet velocity (ft/s)

Diffuser design requirements:

• Radial diffusers with octagonal or circular geometry
• Inlet velocity: 0.1-0.3 ft/s through diffuser openings
• Froude number: Fr < 1.0 for stability
• Multiple inlet levels for variable volume operation

Tank Configuration

Storage capacity sizing:

Vstorage = (Qpeak × tpeak) / (ρ × c × ΔT × ηstorage)

Where:

• Vstorage = required tank volume (gal or m³)
• Qpeak = peak cooling load (tons or kW)
• tpeak = peak period duration (hours)
• ρ = water density (8.33 lb/gal or 1000 kg/m³)
• c = specific heat (1.0 Btu/lb·°F or 4.186 kJ/kg·K)
• ΔT = temperature differential (°F or K)

Operating Strategies

Full storage: Chiller off during peak hours, all cooling from storage. Requires larger tank, smaller chiller.

Partial storage: Chiller operates during peak hours at reduced capacity, supplemented by storage. Optimizes first cost and operating cost balance.

Load leveling: Constant chiller operation over 24 hours, storage absorbs load variations. Maximizes chiller efficiency.

Phase Change Materials (PCM)

PCM research focuses on materials with high latent heat capacity at useful temperature ranges for HVAC applications. Materials store 5-14 times more energy per unit volume than sensible storage.

Organic PCMs

Paraffin waxes:

• Melt temperature range: 32-140°F
• Latent heat: 75-90 Btu/lb
• Chemical stability: excellent
• Subcooling: minimal (1-2°F)
• Thermal conductivity: low (0.1 Btu/hr·ft·°F)
• Cost: moderate ($1.50-3.00/lb)

Fatty acids:

• Melt temperature range: 40-120°F
• Latent heat: 80-100 Btu/lb
• Chemical stability: good
• Corrosivity: low with proper containment
• Thermal cycling stability: >10,000 cycles

Inorganic PCMs

Salt hydrates:

• High latent heat: 100-140 Btu/lb
• Lower cost: $0.50-1.50/lb
• Supercooling issues: requires nucleating agents
• Phase separation over cycles: requires thickening agents
• Corrosivity: moderate to high

PCM Integration Methods

Encapsulation:

• Macro-encapsulation: pouches, tubes, panels (>1 inch dimension)
• Micro-encapsulation: spherical particles (1-1000 μm)
• Encapsulation prevents leakage during phase transition
• Shell material must withstand thermal cycling

Heat transfer enhancement:

• Metal foam matrices increase effective conductivity by 10-50×
• Graphite additives improve conductivity to 1-5 Btu/hr·ft·°F
• Finned tubes increase surface area by 5-20×
• Trade-off: reduced PCM volume fraction

Battery Energy Storage Integration

Battery systems enable electrical load shifting, demand charge reduction, and renewable integration for electrically-driven HVAC equipment.

Battery Technologies for HVAC Applications

Control Integration

Peak shaving algorithm:

Pbattery(t) = Pload(t) - Pthreshold

Battery discharges when building load exceeds threshold, maintaining flat demand profile below utility demand charge breakpoint.

Time-of-use optimization:

Battery charges during low-cost periods (typically nighttime), discharges during high-cost periods (afternoon peak). Optimization considers:

• Time-of-use rate structure
• Demand charges
• Battery degradation cost
• Round-trip efficiency losses

Renewable integration:

Battery buffers solar PV output, enabling:

• Peak demand offset from time-shifted solar
• Voltage regulation
• Frequency response
• Reduced grid interaction

Hybrid TES-Battery Systems

Combining thermal and electrical storage optimizes total system performance:

Chiller + ice storage + battery:

• Battery handles short-duration peaks (<1 hour)
• Ice storage handles extended afternoon peak (4-8 hours)
• Chiller sized for average load, not peak
• Battery protects against demand charges from auxiliary loads

Heat pump + water storage + battery:

• Battery enables heat pump operation during low-cost hours
• Water storage provides heating/cooling during peak hours
• Combined system coefficient of performance (COP) 3-5
• Payback period: 4-8 years in high time-of-use differential markets

Advanced Storage Research Directions

Solid-state thermal storage:

• Ceramic and concrete storage media for high-temperature (300-1000°F) applications
• Volumetric heat capacity: 30-50 Btu/ft³·°F
• Applications: solar thermal integration, industrial waste heat recovery

Thermochemical storage:

• Reversible chemical reactions store energy as chemical potential
• Energy density: 200-300 Btu/lb (2-3× PCM)
• Long-term storage without losses
• Challenges: reaction kinetics, material degradation, cost

Underground thermal energy storage (UTES):

• Borehole thermal energy storage (BTES): seasonal storage in soil/rock
• Aquifer thermal energy storage (ATES): warm/cold water storage in aquifers
• Capacity: 1000-100,000 MWh thermal
• Recovery efficiency: 50-90% depending on geology and duration

Cryogenic energy storage:

• Liquefied air energy storage (LAES)
• Air liquefaction during off-peak hours (-196°C)
• Expansion through cryogenic turbine generates power
• Integration with HVAC for cold recovery during regasification
• Round-trip efficiency: 50-70%

Economic Analysis

Life cycle cost analysis for storage systems:

LCC = C₀ + Σ[(Cenergy + Cmaint - Csavings) / (1 + d)ⁿ]

Where:

• C₀ = initial capital cost
• Cenergy = annual energy cost
• Cmaint = annual maintenance cost
• Csavings = annual savings from demand reduction and TOU arbitrage
• d = discount rate
• n = year number

Typical payback periods:

Storage economics improve with:

• Higher utility demand charges (>$15/kW-month)
• Larger time-of-use rate differentials (>$0.12/kWh)
• Longer peak periods (>6 hours/day)
• Available incentives and rebates
• Participation in demand response programs (revenue: $50-200/kW-year)

Sections