ITB-100 Thermal Battery

A 16.7 kWh Phase-Change Thermal Energy Storage System for Building Electrification

License: MIT Project Status: Concept Seeking: Builders

🎯 The Opportunity

As buildings electrify (driven by gas bans, heat pump adoption, and time-of-use rates), there’s a growing need for affordable thermal energy storage. Current solutions cost $8,000-17,000 installed. This design targets $4,500 installed at scale, using proven phase-change materials and simple manufacturing.

This is a complete design ready for validation testing. I’m sharing it publicly to find builders who can help prove (or disprove) the concept.

📊 Quick Stats

Specification Value Competitive Benchmark
Energy Storage 16.7 kWh thermal Sunamp: 14 kWh
Power Output 1.7 kW average Sunamp: 8 kW
Cycle Life 1,000+ cycles (target) Steffes: 30 years
Installed Cost $4,500 (target) Sunamp: $8,000
Technology Sodium Acetate Trihydrate (SAT) Various PCM
Temperature 58°C phase change Ice/PCM: 0-65°C

🔥 Problems This Solves

1. Heat Pump Shoulder Season Optimization

2. New All-Electric Construction (Gas Ban States)

3. Time-of-Use Rate Arbitrage

4. Solar Thermal Integration

🏗️ Design Overview

Core Technology

The ITB-100 uses Sodium Acetate Trihydrate (SAT) as a phase-change material:

System Architecture

┌─────────────────────────────────────────────────────────┐
│                    THERMAL BATTERY                       │
│  ┌────────────────────────────────────────────────┐    │
│  │  52× Aluminum Heat Exchanger Plates            │    │
│  │  (500×600×2mm, serpentine tubing)              │    │
│  ├────────────────────────────────────────────────┤    │
│  │  227 kg Sodium Acetate Trihydrate (SAT)        │    │
│  │  • 3mm slabs in HDPE pouches                   │    │
│  │  • Stabilizers: Na-PMAA (0.67%), Na₂HPO₄ (2%)  │    │
│  │  • Silver electrodes for nucleation trigger     │    │
│  └────────────────────────────────────────────────┘    │
│                                                          │
│  Insulation: 4" polyisocyanurate (R-25)                │
│  Container: Chest freezer (modified)                    │
└─────────────────────────────────────────────────────────┘
        ↓                              ↑
    Heat Out                      Heat In
 (Space Heating)              (Solar or Grid)

Key Components

  1. Heat Exchanger: 52× aluminum plates with brazed stainless steel tubing
  2. PCM Storage: 227 kg SAT in 3mm HDPE pouches (thermal epoxy bonded to plates)
  3. Nucleation System: 1.5V pulse across silver electrodes (triggers crystallization)
  4. Insulation: Commercial chest freezer shell (repurposed, R-25)
  5. Controls: Simple temperature-based charging/discharging logic

Performance Visualization

Discharge Performance: 9.3 hours of continuous heating at 1.7 kW average power

Discharge Performance

Charge Performance: Solar thermal charging profile

Charge Performance

Economic Comparison: Thermal storage vs. lithium battery + heat pump

Thermal vs Lithium Comparison

📐 Technical Specifications

Performance Metrics

Metric Value Notes
Capacity 16.71 kWh Total thermal energy (20°C → 65°C)
Discharge Power 1.7 kW avg Delivers hot water for 9+ hours
Charge Power 2.8 kW avg From solar thermal or electric
Round-trip Efficiency 90-95% Heat in → heat out
Cycle Life 1,000+ Target (needs validation)
Supercooling Time 24-48 hrs How long SAT stays liquid below 58°C

Physical Specifications

Parameter Value
Dimensions 1120 × 700 × 850 mm (W × D × H)
Mass (dry) 280 kg
Volume 0.67 m³
PCM Mass 227 kg SAT
Energy Density 25 kWh/m³

Operating Conditions

Parameter Range
Charge Temperature 65-90°C (input)
Discharge Temperature 45-55°C (output)
Ambient Temperature -10 to 40°C
Flow Rate 4.4 L/min (design)
Pressure <100 kPa

💡 Use Cases & Configurations

Configuration A: Heat Pump Assist (Shoulder Seasons)

Best for: Cold climate homes with dual-fuel systems (heat pump + furnace backup)

Application: Extend heat pump operating season
Solar Input: 12 m² evacuated tube collectors
Cycles/Year: 139 days (spring/fall/winter)
Annual Savings: $339/year
Payback: 22.7 years (with 30% federal ITC)

How it works:

  1. Solar thermal charges battery during day
  2. Battery provides daytime heating (6 AM - 3 PM)
  3. Heat pump avoids morning cycling stress
  4. Furnace backup handles coldest days (<35°F)

Economics:

Configuration B: Peak Shaving (TOU Rate Arbitrage)

Best for: All-electric homes with high peak/off-peak rate spreads (>$0.20/kWh)

Application: Charge off-peak, discharge on-peak
Heat Source: Electric resistance (grid)
Cycles/Year: 250+ days
Annual Savings: $600-900/year
Payback: 11-15 years

How it works:

  1. Charge overnight at off-peak rates ($0.08-0.12/kWh)
  2. Discharge during peak morning/evening ($0.25-0.35/kWh)
  3. Avoid peak demand charges
  4. Reduce strain on local grid

Economics:

Configuration C: Solar Thermal + DHW

Best for: Homes with existing solar thermal, looking to add storage

Application: Seasonal heating + summer DHW preheating
Solar Input: Existing collectors (8-12 m²)
Cycles/Year: 180 days
Annual Savings: $450-600/year
Payback: 6-8 years (battery only)

How it works:

  1. Integrates with existing solar thermal system
  2. Spring/Fall: Space heating support
  3. Summer: Domestic hot water preheating
  4. Maximizes solar utilization year-round

Economics:

Configuration D: New Construction Grid-Interactive

Best for: New all-electric homes in gas ban states (NY, CA, WA)

Application: Backup heating + grid services
Heat Source: Electric + solar thermal (optional)
Market: 580,000 homes/year by 2030
Value: $6,500 (customer WTP for grid independence)

How it works:

  1. Part of integrated HVAC system design
  2. Provides backup heating during outages
  3. Enables demand response participation
  4. Future: Virtual power plant aggregation

Economics:

🔬 Research Foundation

This design builds on decades of phase-change thermal storage research:

Key Papers & Prior Art

  1. SAT Stabilization (1990s-2010s)
    • Wada et al. (2003): “Sodium acetate trihydrate as a phase change material”
    • Dannemand et al. (2016): “Long-term thermal stability of SAT with additives”
    • Research showed Na-PMAA + Na₂HPO₄ prevents phase separation over 1,000+ cycles
  2. Electrochemical Nucleation (2010s)
    • Yamagishi et al. (2007): “Control of supercooling in SAT by electrochemical nucleation”
    • Liu et al. (2014): “Silver electrode nucleation for reliable crystallization”
    • Demonstrated 95%+ success rate with 1.5V DC pulse
  3. Commercial Applications
    • Sunamp (UK): Proprietary PCM, $8k retail, 10+ years market
    • Calmac (US): Ice storage for commercial buildings, 30+ years proven
    • Steffes (US): Ceramic thermal storage, 30-year lifespan demonstrated
  4. Recent Building Electrification Research
    • NREL: “Electrification Futures Study” (2023)
    • ACEEE: “Time-of-Use Rates and Thermal Storage” (2024)
    • NYSERDA: “Gas Ban Impact Analysis” (2024)

What’s New in This Design

✅ Open-source — Full BOM, assembly instructions, models released publicly
✅ Affordable manufacturing — Targets $1,000-1,500 cost at 1,000 units/year
✅ Proven chemistry — SAT with validated stabilizers (not proprietary)
✅ Simple assembly — No specialized tooling for DIY/small-batch production
✅ Heat pump integration — Designed for emerging cold-climate HP market

What’s still uncertain:

💰 Economics & Market Analysis

Total Addressable Market (2025-2030)

Customer Segment 2025 TAM 2030 TAM Key Driver
Cold Climate Heat Pumps 180k 760k 15% annual HP growth
New All-Electric Homes 85k 580k Gas bans (NY, CA, 8+ states)
TOU Rate Arbitrage 120k 450k 75% on TOU rates by 2030
Solar Thermal Storage 15k 35k Existing solar owners
TOTAL 400k 1,825k Compounding drivers

Realistic Market (25% penetration by 2030): 344,000 units/year

Competitive Positioning

Product Capacity Cost $/kWh Key Advantage
Sunamp UniQ 14 kWh $8,000 $571 High power (8 kW)
Steffes ETS 25 kWh $4,500 $180 Proven (30+ years)
ThermaStor Tank 20 kWh $3,600 $180 Simple (hot water)
ITB-100 (target) 16.7 kWh $4,500 $269 Open-source, affordable

Manufacturing Cost Scaling

At different production volumes (modeled):

Volume Mfg Cost Retail Price Margin Payback Period
1 (DIY) $2,700 N/A N/A 9.7 years*
100 $2,200 $5,500 $3,300 13.2 years
1,000 $1,500 $4,000 $2,500 11.8 years
10,000 $1,000 $2,800 $1,800 8.6 years

*vs. electric resistance heating, with 30% solar ITC

Critical insight: Cost reduction requires volume (chicken-and-egg problem). Open-source approach could accelerate adoption by:

  1. Enabling DIY builders to validate performance
  2. Attracting contract manufacturers with proven design
  3. Building community around standardized components

🚀 Getting Started

For Researchers / Experimenters

I recommend starting with a single-cell validation test before building the full system.

Benchtop Test Rig (~$180, 3 days build time)

Purpose: Validate SAT chemistry, nucleation trigger, and thermal performance

Scale: 1/50th of full system

Key validation questions:

  1. Does SAT cycle 50× without phase separation?
  2. Does 1.5V silver electrode trigger work reliably?
  3. What’s the measured thermal conductance (UA value)?
  4. Any pouch degradation after 50 cycles?

Success criteria:

If interested in building the test rig, see: docs/VALIDATION_TEST.md

For Builders / DIY Enthusiasts

Full system build:

Build documentation:

⚠️ Important: This is an unproven design. Build at your own risk. Not certified for commercial installation.

For Manufacturers / Entrepreneurs

If you’re considering commercialization:

  1. Validate first: Build prototype, run for 6-12 months
  2. Get quotes: Contact contract manufacturers for 100/1,000/10,000 unit pricing
  3. Pursue certification: UL 2596 (Energy Storage), CSA (Canada), CE (Europe)
  4. Test market demand: Talk to HVAC distributors, heat pump manufacturers
  5. Explore partnerships: Integration with Mitsubishi, Daikin, Carrier cold-climate heat pumps

Market entry strategy (from analysis):

Revenue potential (2030, value positioning):

📁 Repository Contents

itb-100-thermal-battery/
├── README.md                              # This file
├── LICENSE                                # MIT License
├── requirements.txt                       # Python dependencies
├── pyproject.toml                         # Python project configuration
├── claude.ini                             # Claude Code project context
├── docs/                                  # Documentation
│   ├── BOM.md                             # Complete bill of materials
│   ├── BENCHTOP_TEST_PROTOCOL.md          # Experimental validation protocol
│   ├── COMPLETION_CHECKLIST.md            # Project roadmap
│   ├── CONTRIBUTING.md                    # Contribution guidelines
│   ├── EXECUTIVE_SUMMARY_FINAL.md         # Strategic assessment
│   ├── GETTING_STARTED.md                 # Quick start for different user types
│   ├── MODELS_README.md                   # Model usage and validation guide
│   ├── PUBLICATION_READY.md               # Publication checklist
│   ├── SAFETY.md                          # Safety guidelines and warnings
│   └── VALIDATION_TEST.md                 # Single-cell test protocol
├── models/                                # Python models & analysis
│   ├── itb100_system_model.py             # Core thermal dynamics model
│   ├── heat_pump_assist_analysis.py       # Shoulder season economics
│   ├── itb100_market_analysis.py          # Market sizing & competitive analysis
│   └── thermal_vs_lithium_comparison.py   # Comparison with battery alternatives
├── assets/                                # Images and visualizations
│   └── thermal_vs_lithium_comparison.png
└── output/                                # Generated model outputs (created when models run)
    ├── discharge_performance.png
    ├── charge_performance.png
    ├── heat_pump_assist_analysis.png
    └── itb100_market_analysis.png

Note: CAD files (heat exchanger, frame assembly) and assembly guide are planned but not yet available.

🤝 Contributing & Collaboration

This project needs:

  1. Test builders — Validate the design in real-world conditions
  2. Researchers — Improve SAT formulation, optimize heat exchanger
  3. Manufacturers — Provide quotes for volume production
  4. Market feedback — HVAC installers, heat pump owners, solar thermal users

How to contribute:

I’m particularly interested in:

📜 License & Legal

License: MIT License (see LICENSE)

What this means:

Important disclaimers:

⚠️ Not certified: This design has not been tested or certified by UL, CSA, or any regulatory body. Do not use in commercial installations without proper certification.

⚠️ Build at your own risk: Working with phase-change materials, pressure vessels, and thermal systems carries inherent risks. Follow all safety guidelines.

⚠️ Permits required: Check local building codes before installing. May require plumbing permit, electrical permit, or HVAC contractor.

⚠️ Insurance: Verify with your homeowner’s insurance that uncertified thermal storage is covered.

📞 Contact & Questions

Best ways to reach out:

Want to stay updated?

🙏 Acknowledgments

This design builds on decades of research by:

Special thanks to the building electrification movement and the open-source hardware community for inspiration.

🌟 Vision

My hope for this project:

Buildings are responsible for 40% of global energy use. As we electrify heating (driven by heat pumps, gas bans, and renewable energy), thermal storage becomes critical infrastructure.

Today, thermal batteries are expensive ($8k-17k) and proprietary. This limits adoption to early adopters and commercial buildings.

By open-sourcing this design, I hope to:

  1. Accelerate innovation — Let researchers and builders improve the design
  2. Lower costs — Enable competition and volume manufacturing
  3. Expand access — Make thermal storage available to more homeowners
  4. Prove viability — Validate (or invalidate!) the concept publicly

If this design works, it could help millions of homes electrify affordably.
If it doesn’t work, the community learns what doesn’t work, and we iterate.

Either way, we move forward together.

🔄 Project Status & Roadmap

Current Status: Design Phase (Not Yet Validated)

Roadmap:

Milestone Target Date Status
Publish design Q4 2025 ✅ Done
Find 3-5 test builders Q1 2026 🔄 In progress
Single-cell validation Q2 2026 ⏳ Pending
Full system prototype Q3 2026 ⏳ Pending
12-month field test Q4 2027 ⏳ Pending
Certification (if viable) 2028 ⏳ Pending

💭 Final Thoughts

This is an intellectual exercise turned public offering.

I designed this system to solve my own heat pump assist problem, realized it might be commercially viable, but don’t have the time or energy to pursue it further.

Rather than let the design sit on my hard drive, I’m releasing it publicly in the hope that:

If you build this, please share your results. The goal is learning, not perfection.

Let’s see if we can make affordable thermal storage a reality.

⭐ If you find this project interesting, please star the repository and share with others who might be interested!

Last updated: October 30, 2025
Project status: Seeking validation builders