SandVault

The sun sets, the heat stays. SandVault: Smart sand batteries turning renewable surplus into 24/7 heat. Eliminating waste and reinforcing energy independence.

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Inspiration

It all started with a frustrating conversation about solar panels at home. One of our founders realized a massive blind spot in residential energy: during the day, our solar system produced a massive surplus of energy that we were forced to sell back to the grid for pennies. Yet, at night, when we actually needed to heat our water and homes, we had to buy energy back at premium prices.

We quickly realized this wasn't just a personal problem, it’s a massive market inefficiency affecting millions of households and small businesses.

Determined to find a solution, our team started researching how the most energy-progressive nations handle this paradox. That is when we discovered the Nordic approach. Countries like Finland are years ahead in sustainable infrastructure, using massive, industrial-scale sand batteries to store heat for entire villages.

That was our turning point. The physics were brilliant, but the technology was locked behind giant, expensive industrial applications. We saw the perfect opportunity to bring this highly advanced thermal storage to a broader market and democratize it.

We decided to take this proven Nordic concept, shrink it down to fit residential and commercial spaces, and, most importantly, give it a brain. By adding an intelligent predictive algorithm to autonomously manage thermal charging based on real-time market prices (OMIE API), consumer habits, and weather forecasts, SandVault was born. We are turning a frustrating household energy problem into a smart, profitable, and green solution.

What it does

SandVault is a scalable thermal energy management system that bridges the gap between when solar energy is generated and when thermal energy is actually consumed. Designed for scale, it acts as a centralized thermal battery to replace traditional gas boilers and expensive lithium batteries in condominiums and Energy Communities.

The system operates in four core pillars:

1. The Charging Phase (Power to Heat)
During peak sunlight hours, instead of exporting unconsumed solar energy to the grid, SandVault redirects this surplus electricity to internal heating resistors. These resistors are embedded in a heavily insulated core filled with common sand, safely raising its temperature up to 600°C.

2. The Discharging Phase (Heat to Water)
When residents demand hot water for showers, appliances, or central heating, water does not go inside the sand tank. Instead, a smart ventilation system drives air through the superheated sand core. This incredibly hot air then passes through an external air-to-water heat exchanger, safely and efficiently transferring the energy to the building's water supply at the perfect temperature.

3. The Smart Grid Integration (The Brain)
To guarantee 24/7 reliability, SandVault acts as an active energy trader. It runs an underlying predictive logic that continuously monitors weather forecasts, internal thermal reserves, and consumer habits. If the system anticipates a lack of solar energy (e.g., a cloudy week approaching), it automatically scans the OMIE electricity market index. It then schedules grid-charging exclusively during the absolute cheapest hours, ensuring the building always has hot water at the lowest possible cost.

4. The Scalability and Business Model (The Revenue)
SandVault is engineered for a B2B2C (Business-to-Business-to-Consumer) market approach. Thanks to the Square-Cube Law of thermodynamics, as we scale the tank up for a condominium, its energy capacity grows exponentially faster than its surface area (heat loss). But our true innovation lies in the revenue model: we combine a one-time hardware installation (CapEx) with a SaaS (Software as a Service) subscription. Real estate developers and management entities buy the core infrastructure, and we charge a recurring monthly fee for the predictive algorithm that continuously optimizes their OMIE market bidding. Even with this SaaS fee, the net financial gain for the families is massive—they stop buying expensive grid energy for evening heating and stop losing money by selling their solar surplus for pennies. This ensures scalable, recurring revenue for us, while delivering drastically cheaper, 100% decarbonized heating to the end consumer.

How we built it

To prove that SandVault is a viable, scalable business and not just a theoretical concept, we split the development into two fronts: building a physical hardware prototype to prove the thermodynamics, and designing an interactive simulation dashboard to validate the economic model and logic.

1. The Physical Prototype (Hardware & Physics)

We needed to prove the fundamental heat transfer formula: Q = mcΔT. While the full-scale B2B2C system uses vacuum insulated panels and air-circulation, for this prototype, we built a scaled-down closed-loop system:

  • The Core: We constructed a thermal unit using two nested containers, heavily insulated with rock wool. Inside, we packed 13 kg of sand embedded with electrical heating resistors.
  • The Electronics: We wired temperature sensors to an ESP32 microcontroller to monitor the sand's internal temperature in real-time.
  • The Heat Transfer: We used a water pump to continuously circulate water through copper and silicone tubing buried directly inside the heated sand.
  • The Result: The system quickly pushed the sand to 90°C. When the loop was activated, it raised the water temperature from 20°C to 45°C in roughly three minutes, proving the phenomenal thermal conductivity and retention of the sand.

2. The Simulation Dashboard (UI/UX & Logic)

To prove the viability of SandVault, we built a fully functional Simulation Dashboard. This is not just a visual interface (UI/UX); it runs on a robust physics and economic logic engine that calculates thermal dynamics, financial savings, and environmental impact in real-time.

A. The Baseline Parameters

Every calculation in the dashboard is grounded in real-world data:

  • Consumption: 2.5 kWh/day per resident.
  • Sand Density: 160 kWh/m³.
  • Solar Efficiency: 0.2 kWp/m².
  • Weather Multiplier: Solar generation varies dynamically (Sunny = 5h; Cloudy = 2.5h; Rainy = 0.5h).
  • OMIE Market Tariffs: 0.16 €/kWh (Normal Grid) vs. 0.04 €/kWh (Off-peak/Madrugada).
  • Environmental Impact: The fossil grid emits 0.20 kg CO₂/kWh; planting 1 tree offsets 20 kg CO₂.

B. The Core Physics Engine

The central battery responds directly to the user's inputs based on strict thermal formulas:

  • System Sizing: The required volume automatically scales with the number of residents to guarantee a safety baseline of 1.5 days (36 hours) of autonomy.

$$Volume = \frac{Residents \times 2.5 \text{ kWh} \times 1.5}{160 \text{ kWh/m}^3}$$

  • Temperature & Autonomy: The core temperature (Tcore) and the Estimated Autonomy are directly tied to the State of Charge (SoC). We assume a base temperature of 15°C at 0% and 500°C at 100%.

$$T_{core} = 15 + \left(\frac{SoC}{100} \times 485\right)$$

C. The Impact Outputs

The metrics on the right side of the dashboard update dynamically to showcase the system's value:

  • CO₂ & Trees Planted: Calculates the clean energy generated, multiplies it by the grid's emission factor, and converts the saved emissions into the equivalent number of "trees planted".
  • Financial Savings: Calculates the cost difference between relying 100% on the standard grid versus using free solar energy (and occasional off-peak grid charging).

D. The Smart Manager Alert (Predictive Algorithm)

This is the brain of the system. It cross-references the current SoC with a 3-day weather forecast to make autonomous decisions:

  1. Standby (Green): If the 3-day solar forecast covers the residents' consumption, the system blocks grid usage. 100% free energy.
  2. Predictive Optimization (Yellow): If rainy days are coming and a negative balance is predicted, the algorithm schedules an automatic grid charge for 03:00 AM, locking in the 0.04 €/kWh OMIE off-peak tariff.
  3. Emergency (Red): If the SoC drops below 10% or autonomy falls under 2 hours, it overrides all pricing rules and immediately injects grid power to ensure users never run out of thermal energy.

E. OMIE Market Integration

The bottom chart provides visual proof of the Iberian Electricity Market (OMIE) behavior over 24 hours, graphically justifying why our Smart Manager specifically targets the 02:00–06:00 AM window for optimization.

Challenges we ran into

Building a hybrid hardware-software system in a short timeframe brought a lot of real-world hurdles, especially when moving from theoretical thermodynamics to physical prototyping:

1. Bridging Theory and High-Voltage Hardware: Transitioning from code to physical electrical circuits was a steep learning curve. Wiring the components, safely handling transformers, and managing power distribution for the heating resistors taught us critical, hands-on lessons in electrical safety and circuit architecture.

2. Sensor Noise and ESP32 Telemetry: Programming the ESP32 microcontroller was complex, but getting accurate high-temperature readings was initially a nightmare. Our sensor data kept showing erratic spikes and random values. After hours of hardware debugging, we identified the bus interference and engineered the circuit by adding a precise 4.7kΩ pull-up resistor, which beautifully stabilized our data logging.

3. The Breadboard Fragility: Cable management became a constant struggle. The sheer number of jumper wires connected to our breadboard meant that the initial prototype was extremely sensitive. A slight movement could pop a wire out, forcing us to trace connections back to their right pins. This frustration was actually a great lesson—it highlighted exactly why our main focus for this new hackathon is to build a much more robust, structurally sound, and cleanly wired physical prototype.

Accomplishments that we're proud of

We are incredibly proud of turning a complex industrial concept into a working physical reality in just a few days. Bridging theoretical thermodynamics with a functional hardware prototype was a massive milestone for our team.

  • Building a Working Thermal Battery: Seeing the physical prototype actually work was a massive win. Watching the water loop temperature rise from 20°C to 45°C in just a few minutes proved that the fundamental physics and our scaled-down design were completely viable.
  • Mastering the Hardware Integration: Successfully debugging the ESP32 sensor noise by engineering the pull-up resistor solution. Going from an unstable telemetry bus to a highly reliable, live-reading microcontroller felt like a huge victory and proved our capacity to solve real engineering bottlenecks.
  • Designing the Smart Manager Dashboard: We are extremely proud of the software simulation. Translating complex thermodynamics, OMIE market prices, and the 3-day predictive algorithm into a clean, interactive UI/UX in Figma proves that SandVault is not just a science experiment, but a viable, user-centric product.

What's next for SandVault

Our immediate focus for this upcoming competition is twofold. First, we will build a more robust, investor-ready physical prototype, moving past our early PoC to reliably demonstrate our thermal efficiency on the spot. Second, and just as importantly, we want to aggressively refine our business model. We aim to validate our B2B2C pricing strategy, ensure financial scalability, and craft a solid pitch that proves SandVault is a highly profitable, market-ready solution.

Beyond the competition, the next step for SandVault is scaling up to a real-world B2B2C pilot. Our goal is to build a full-scale centralized thermal battery specifically designed for a condominium or a local Energy Community, allowing us to truly capitalize on the thermodynamic efficiency of the Square-Cube Law.

Simultaneously, we plan to transition our Figma simulation into a fully functional software backend, integrating real machine learning models that can learn a building's specific water consumption habits down to the minute to further optimize our predictive OMIE market purchasing.

Ultimately, our vision is clear: we want to completely eliminate residential reliance on fossil fuels like gas boilers, replacing them with a sustainable, circular-economy solution that doesn't just save the planet, but actively puts money back into the consumer's pocket.


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