Tree AirBattery: Modular CAES for Sustainable Mining
CAES is the renewable of the future
Executive Summary
Mining operations face steep energy costs and reliability risks. Energy can account for roughly 30% of a mine’s operating expenses, and off-grid power can cost up to $300/MWh. Traditionally this demand is met with diesel generators, which are expensive to run and maintain, expose mines to fuel-price volatility and carbon taxes, and generate significant emissions. Batteries (lithium-ion) offer emissions savings but carry safety and lifecycle downsides: they degrade over years and can ignite, releasing toxic smoke in confined spaces.
Tree’s AirBattery – a modular Compressed Air Energy Storage (CAES) system – offers a superior alternative. Using compressed air in durable steel tanks, it achieves about 75% round-trip efficiency (charge/discharge) and delivers power with no capacity fade over 40+ years. There are no flammable chemicals or rare metals, so fire risk is essentially zero. Life-cycle costs are low because air is free and the system needs minimal maintenance. This whitepaper compares diesel, batteries, and CAES for off-grid mining, highlighting how Tree’s AirBattery improves long-term ROI, resilience, and safety. Data from industry reports and case studies illustrate the cost and operational advantages of CAES in remote mine power systems.
Energy Challenges in the Mining Sector
High Energy Costs: Mines consume vast amounts of electricity (often for pumps, crushers, ventilation, etc.). Studies show energy can represent around 30% of cash operating costs for mining companies. In remote sites without grid access, generation costs skyrocket (fuel-only supply can exceed $300/MWh).
Diesel Dependence: The simplest off-grid solution is diesel gensets, but they introduce multiple pains: diesel fuel must be delivered and stored onsite (a logistical challenge and fire hazard), fuel prices fluctuate unpredictably, and rising carbon taxes and regulations will push costs higher. Diesel generators also emit CO₂ and pollutants continuously, undermining net-zero targets.
Maintenance & Downtime: Diesel engines require frequent maintenance (oil changes, overhauls) and skilled technicians. Breakdowns or refueling delays can halt production, causing costly downtime. In tight underground or remote operations, any interruption rapidly translates to lost output and revenue.
Battery Limitations: Lithium-ion (Li-ion) batteries are increasingly used for mobile equipment and backup power, but they have drawbacks in mining applications. Batteries degrade with cycling (losing ~20% capacity over ~5–10 years), necessitating expensive replacements. They also demand climate control (cooling/heating) and fire suppression systems. Critically, Li-ion packs can undergo thermal runaway: a battery fire releases thick, toxic smoke and high heat, especially dangerous underground. For instance, a 2018 incident at Newmont’s Red Lake mine involved a stack of Li-ion tool batteries catching fire and filling a shaft with smoke, forcing evacuation. Such safety risks raise insurance and operational costs.
Collectively, these issues – high fuel OPEX, maintenance overhead, emission liabilities, safety hazards, and downtime risks – drive mining firms to seek better solutions. Off-grid renewables (solar/wind) are part of the answer, but they require reliable storage to smooth variability. The question becomes: how best to store energy on-site?
Storage Technology Comparison: Diesel vs Li-ion vs CAES
Diesel Generators: The baseline for off-grid power. Diesel gensets are well-understood and robust, but their total cost is high. Fuel can easily constitute 80–90% of lifetime cost, and prices are exposed to market swings and carbon levies. Generators offer limited ramping flexibility (suiting steady loads) and virtually no storage capability. When load spikes occur, multiple gensets must idle or cycle on/off inefficiently. Emissions (CO₂, NOₓ, particulates) remain high. In summary, diesel is reliable but capital-intensive over time and environmentally unsustainable.
Lithium-Ion Batteries: Batteries provide instant response and smooth power delivery, complementing renewables or reducing genset runtime. They are clean at the point of use, but carry hidden costs. Li-ion packs degrade with each discharge cycle, typically losing ~20% capacity in under 10 years of cycling, which means frequent replacements for mining-scale systems. They also require sophisticated battery management and thermal controls. Crucially, safety is a major concern: Li-ion cells are flammable. As one industry review notes, CAES (compressed air) can match batteries in efficiency but avoid fire risk, and safety alerts warn that battery fires in mines produce “thick smoke and toxic gases”. Each of these battery replacements or failures can interrupt operations and add cost. While Li-ion is proven in many industries, its life-cycle cost and risk profile are significant in demanding mine environments.
Compressed Air Energy Storage (CAES): CAES systems use electricity to compress air into storage vessels, then later expand the air through turbines to generate power. Modern adiabatic CAES can achieve 65–75% round-trip efficiency, on par with pumped hydro and competitive with batteries. More importantly, CAES scales easily and lasts much longer. Analyses show CAES installations can exceed 40 years of lifespan (over 13,000 cycles) – far beyond battery systems. Operationally, CAES has only a few mechanical parts (compressor, expander turbine, and tanks), making it inherently reliable with low maintenance requirements. There are no toxic chemicals or fluids involved – the working medium is simply air – eliminating fire/explosion risk. A recent review concludes that CAES is a “sustainable and resilient alternative to batteries, with much longer life expectancy, lower life cycle costs, technical simplicity, and low maintenance”. In practice, CAES provides firm, dispatchable power (able to discharge for hours on demand), which is ideal for bridging gaps in renewables and reducing genset runtime.
Introducing Tree’s AirBattery: How Modular CAES Works
Tree’s AirBattery system implements CAES in a modular form designed for mining sites:
Energy Charging: When excess power is available (for example from solar panels or during off-peak hours), the AirBattery runs an electric motor/compressor to pump air into high-pressure storage tanks. The compression process heats the air, but Tree’s design (and most modern CAES approaches) capture or manage this heat to boost efficiency. Surplus renewables are thus efficiently stored as potential energy in pressurized air.
Energy Discharging: When electricity is needed – during peak loads, at night, or if the main power source fails – the compressed air is released from the tanks. It flows through Tree’s proprietary gearless expansion turbine, driving a generator to produce electricity. Because this conversion is fully reversible, the system provides power on-demand with fast response time.
Operation & Control: The generated electricity is automatically used to supply the mine’s loads, reducing or eliminating the need to run diesel generators. The AirBattery can provide uninterrupted power during outages or lulls in renewable output. Control electronics manage charge/discharge cycles to optimize round-trip efficiency (around 75%) and preserve tank pressure.
Key features of Tree’s design:
Modularity: AirBattery units are standardized in sizes (initially 50 kW, 100 kW, 150 kW modules) so multiple units can be installed in parallel to scale up for any site. This allows incremental deployment – a mine can start with one Air50 module and add more capacity as needed, aligning investment with project growth.
Long Life & No Fade: The core components (steel air tanks, motor/generator) are robust. Unlike batteries, compressed air does not degrade. Tree’s AirBattery is expected to deliver consistent performance for decades. In fact, industry analyses note CAES systems exceed 40-year lifespans. Over that time, a single AirBattery can replace several generations of batteries or multiple gensets.
Safety & Environmental: AirBattery contains no exotic materials – just air, water, and standard electrical/mechanical parts. There are no fire-prone chemistries, no thermal runaway. The system produces no emissions on discharge. Moreover, the compression cycle inherently separates moisture from the air, yielding condensate water – a useful byproduct for dry mining sites (providing cooling or potable water).
In summary, Tree’s AirBattery performs the same basic role as a battery (storing and releasing electrical energy) but with a safety-oriented, long-duration twist. Its high round-trip efficiency and flexibility make it suitable for providing spinning reserve and peak shaving, while its ruggedness makes it ideal for heavy-duty mining use.
Total Cost of Ownership Analysis
To illustrate economics, consider a hypothetical 10-year and 30-year TCO comparison for a fixed 1 MW power capacity at a remote mine. The table below (illustrative) compares diesel generation versus battery versus Tree’s AirBattery, including capital costs, fuel (diesel) or energy costs, maintenance and replacements:
Notes: These are sample estimates based on typical industry assumptions. Diesel scenario assumes $300/MWh fuel cost. Battery CAPEX is ~$300/kWh (so 1 MWh storage ≈ $300k, here scaled to $20M including installation), with 10-year lifespan (requiring replacements). AirBattery CAPEX is similar order ($293/kWh) but requires no replacements; only electricity to run compressors (assumed at average grid cost) and minor maintenance.
This example highlights that diesel fuel quickly dominates TCO: over 30 years, diesel’s fuel/O&M alone can reach $90M. Batteries save fuel but carry high upfront costs and replacement cycles ($75M over 30 years above). In contrast, Tree’s AirBattery has higher initial cost than a single battery pack but far lower ongoing costs. Because the AirBattery does not need replacement and incurs minimal fuel cost (electricity to charge can come from renewables or cheap off-peak), its 30-year TCO is dramatically lower.
BloombergNEF data supports these figures: it found capex ≈$293/kWh for CAES vs $304/kWh for 4-hour Li-ion systems, indicating similar upfront cost. However, the lifetime economics swing in favor of CAES due to the factors above. Fuel and replacement costs are only fully captured in a TCO view.
Key Benefits for Mine Operators
Tree’s AirBattery delivers several strategic advantages for mining decision-makers:
Lower Long-Term Costs: By replacing most diesel generation with stored energy, mines save on fuel purchases and maintenance. High round-trip efficiency (~75%) means more of the stored renewable energy is recovered, improving ROI. Industry analyses note CAES’s lower life-cycle cost compared to batteries. Over decades, these savings compound.
Extreme Reliability & Lifetime: The AirBattery’s components last decades – analysts report >40 years of service for CAES systems. In practical terms, a mine could install a Tree AirBattery once and avoid multiple future battery or genset replacements. This longevity also means higher uptime; the system can run over thousands of cycles without performance loss.
Safety & Regulatory Compliance: Using only air (no hazardous materials) means no risk of thermal runaway or toxic fire. This greatly reduces insurance and safety overhead, especially in underground operations. Also, because AirBattery discharge emits no CO₂ or pollutants, it helps mines meet strict environmental targets and net-zero commitments.
Resilience: The AirBattery provides uninterruptible power. It can ride through grid outages or low-renewable periods seamlessly, whereas diesels might suffer start-up delays or batteries could deplete unexpectedly. Its mechanical simplicity (no fragile chemistry) means it handles temperature extremes and dust with minimal issues.
Scalability & Flexibility: Modular design lets mines expand capacity in line with production growth. Upgrading is as simple as adding more modules. The AirBattery can integrate with solar, wind, or even existing generators, providing spinning reserve and load following. This flexibility protects against future demand increases (e.g. electrification of haul trucks or processing).
Byproduct Water Production: A useful side benefit of compression and expansion is water generation. Moisture condenses out during cycling, yielding clean condensate which can be harvested. In arid mining regions, this can offset water supply needs (for dust suppression or even potable use).
Taken together, these benefits translate into a strong ROI and risk reduction. While exact payback depends on site-specific factors, mines with high diesel burn can often see multi-year fuel savings outweighing the AirBattery cost. When coupled with carbon credits or local emissions regulations, the economics improve further. Importantly, the improved reliability and safety can prevent multi-million-dollar losses from unplanned shutdowns – a critical factor for continuous operations.
Getting Started with Tree
Tree Associates is gearing up for commercial deployment of the AirBattery. An Innovate UK grant has accelerated development, with commercial availability targeted ~12 months out. Three unit sizes (50, 100, 150 kW) are in production planning.
Pilot Programs: Tree welcomes mining partners to join early-access and pilot initiatives. By collaborating on a pilot installation, your team can evaluate the AirBattery under real mine conditions, gather data on performance and ROI, and shape the system for your needs.
Next Steps: To explore how Tree’s CAES can fit your operation, contact Tree Associates for detailed feasibility studies. A consultation will assess your site’s power profile, diesel usage, and growth plans, quantifying expected savings and scheduling a phased deployment. Visit Tree’s website or call our technical team to sign up for the AirBattery early access list.
By adopting Tree’s AirBattery, mining operators can drastically cut fuel costs, enhance power resilience, and advance decarbonization – all while ensuring long-term security of supply. The result is not just a lower-cost energy system, but a safer, cleaner, and more reliable one that strengthens both bottom-line and ESG performance.
