DIY Cooling via Evaporative Roof Water
Preface — Business and Environmental Ethics
This is a “secret” invention Sidd and I developed some time ago.
I call it secret not because it is proprietary in the usual sense, but because we tend to apply a fairly strict internal filter before releasing anything into the wild. Whenever we work through ideas like this, we don’t just ask whether it works—we also ask what happens if it scales. What happens if everybody does it? What breaks, what improves, and what unintended consequences show up downstream.
That second question usually slows things down more than the first.
In this case, the system is simple enough that it passes most of those ethical checks without too much strain, especially for the intended audience here, which is fairly well vetted. Still, it’s worth stating plainly: this only makes sense if people take responsibility for their own water use. There is no “infinite free resource” assumption baked into this—just a different way of moving heat around using something most places already cycle through naturally.
The motivation for sharing it is straightforward. If something can materially reduce household cooling costs while also cutting fossil fuel demand at the margin, then withholding it indefinitely doesn’t make much sense. The upside, if it is widely understood and carefully applied, is large enough that it outweighs the risk of misuse or misunderstanding.
As always, the ethics are part of the engineering.
“The Waterfall House”: Passive Solar-Driven Heat Rejection Using Phase-Change Energy Transfer
Authors: Daniel Brouse and Sidd Mukherjee
Date: 2026
Abstract
This paper evaluates a passive cooling strategy using evaporative water films on building surfaces. The system leverages the latent heat of vaporization of water (~2.26 MJ/kg) to remove heat from structures. Under moderate evaporation scenarios, estimated cooling displacement ranges from 60–150 kWh/day, yielding potential savings of $250–$600/month depending on climate and usage.
1. Introduction
Residential cooling demand is increasing globally due to rising temperatures and air conditioning adoption. Typical vapor-compression systems operate at COP values of 2.5–4.0.
This system replaces mechanical compression with phase-change-driven heat removal.
2. Physical Principle: Latent Heat of Vaporization
The latent heat of vaporization of water is approximately:
2.26 MJ/kg (at 100°C) ~2.45 MJ/kg at ambient temperatures ~970 BTU/lb
Water density:
1 cubic foot = 62.4 lb
Energy absorbed per cubic foot:
62.4 lb × 970 BTU/lb ≈ 60,500 BTU ≈ 61,000 BTU per cubic foot evaporated
3. Air Conditioning Energy Equivalence
1 kWh = 3,412 BTU (thermal equivalent). With typical COP ≈ 3:
1 kWh ≈ 10,000 BTU cooling
Cooling displacement:
61,000 ÷ 10,000 = 6.1 kWh per cubic foot evaporated
4. Economic Value
Electricity price (July 2026): $0.11759/kWh
6.1 kWh × 0.11759 ≈ $0.72 per cubic foot
Result: Each cubic foot evaporated saves approximately $0.72
5. Household Scaling
Assumed evaporation: 10–25 ft³/day
| Scenario | Daily Savings | Monthly Savings |
|---|---|---|
| Low (10 ft³/day) | $7.20 | $216 |
| High (25 ft³/day) | $18.00 | $540 |
6. Thermodynamic Interpretation
This system operates as a solar-driven phase-change heat pump where the atmosphere acts as the heat sink.
7. Key Constraints
- Humidity strongly limits evaporation efficiency
- Requires substantial water supply
- Performance highest in dry-hot climates
- Potential biological/structural concerns
8. Comparative Energy Pathways
System Comparison
| System | Mechanism | Energy Source | Efficiency Driver |
|---|---|---|---|
| Air Conditioning | Vapor compression | Electricity | COP 2.5–4 |
| Waterfall House | Phase change evaporation | Solar + ambient heat | Latent heat of vaporization |
Addendum
As temperatures rise, cooling demand increases sharply. This drives a cascading set of system stresses:
- higher electricity demand during heat waves
- strain on generation and transmission infrastructure
- increased water demand in some regions for cooling and supply stability
- and, in grids still reliant on fossil fuels, increased emissions during peak demand periods
This creates a reinforcing sequence:
more heat → more cooling demand → higher energy use → higher emissions → further warming → more heat
In practice, what emerges is not a single isolated feedback loop, but a coupled network of reinforcing systems—biophysical (permafrost thaw, forest stress and mortality, wildfire regimes, hydrological intensification) and socioeconomic (energy demand, infrastructure constraints, and grid response). These systems can interact nonlinearly, particularly under sustained warming and extreme heat conditions.
The key point is that these feedbacks are already operating, but their magnitude, interaction strength, and long-term dominance relative to human emissions vary by region, sector, and timeframe. Reducing risk ultimately depends on rapidly reducing greenhouse gas emissions, especially from fossil fuel combustion, while adapting infrastructure to rising heat extremes.
References
- U.S. DOE – Energy Efficiency in Air Conditioning: https://www.energy.gov
- EIA Electricity Prices: https://www.eia.gov
- IAPWS Thermophysical Properties: http://www.iapws.org
- ASHRAE Handbook – Fundamentals: https://www.ashrae.org
- Moran & Shapiro, Fundamentals of Engineering Thermodynamics
