Evaporative Cooling Of The Roof

Economy, Physics

   In hot summer periods, the external surface of roofs of industrial and commercial buildings may reach the temperature of 60-80 °C, causing three major problems: 

  1. The ascending internal heat (produced by machines and hot technologies), normally leaving the buildings through the roof, is trapped inside.
  2. The internal heat is even increased by the solar heat gain, with it’s peak intensity of 6-800 Watt / m2 on an everage sunny day. It exceeds 6-8 times the heating power of a radiant floor-heating system: reaching 50-60 MWh a day in case of a 10.000 m2 industrial roof. Added to the trapped internal heat, it makes the whole internal ceiling to act as a huge solar thermal panel.
  3. The continuous dilatation of the roof membrane (60-80 °C daytime; 8-10 °C at nights; 0 °C or below in a summer shower or hail) creates tears and cut-off points, causing leakage

Before Installing "Water-film Technology"

Past state - prior to installing Water-film Technology
(Data measured by GM Powertrain Hungary, Szentgotthard, Hun.)  

Evaporative roof cooling can greatly reduce all three problems. Naturally, it doesn’t substitute a full air-condition, but in most cases the effect of roof cooling is sufficient to reduce the internal heat-load under the critical level on machinery and employees as well, thus preventing 80-100 % of costly technical and staff problems normally caused by summer heat. At the same time, keeping the roof at an even temperature stops dilatation and leakage. 

Evaporative roof cooling prevents the overheating of the buildings, and keeps the microclimate of the roof - enveloping roof-mounted HVAC equipments - at a cool, low temperature. Thus greatly reduces the need of costly and energy-intensive air-conditioning.

History and Innovation - „Evaporative” Cooling in the World - and "Water-Film Technology" of Ontozes.hu Ltd.

   Roof spray systems were first introduced in the USA in 1934. These systems basicly sprayed excessive amounts of cool water over the surfaces, using water itself – rather than evaporation – for cooling. After World War II, many thousand hectars of roofs were equipped with this method, mainly in the textile – and quite a few in metal – industry. The introduction of cheap air-condition systems and low energy prices had cut the technology back, but following the energy crisis of 1973-74 in the US it became widely popular again.

   Today there is a large industry with many contractors and wide scientific literature based on „evaporative” roof cooling throughout the US, India, Japan, etc. Evaporation is much cheaper (using a small fraction of water compared to the old systems), much more energy- and environment-friendly, and at the same time much more effective - since evaporating water can carry away 7-10 times the energy distracted by cooling water. (Just a hint of Physics: Heat of vaporization of water: 2 257 kJ/kg - Specific heat capacity of water - 4.18 kJ/kgK. It means that mere vaporization of 1 kg = 1 liter of water pre-heated beforehand until its boiling point of 100 oC, takes 5.4 times the energy that of the heating process of the same amount of water from O oC to 100 oC.)

   With the use of fine-mist-producing spray heads and microprocessor-based controllers, the modern American systems started to use the effect of vaporization rather than cooling with water itself. But - probably due to technical problems caused by the harsh environmental and technical circumstances on these roofs - in the American and Asian systems, water amounts / frequency of wettenning and drying  are controlled merely by a pre-set „scheme” or simple temperature-based feedback. Unfortunately, actual EVAPORATION is much more influenced by relative humidity, air-movement, and quite a few other, weather-depending and ever-changing circumstances. So most of the times, roof-surfaces of these systems are either fully WET, needing „dwell time”, or they remain totally dry for almost full cycles of every sequent zones. That’s why these American and Asian systems can only make 0,1gal/ft2/day = 4,2 liter / m2 /day to evaporate – compared to the values of 10-15 liter / m2 / day of our system. Evaporation of 4,2 l/m2/day (assuming that there is no run-away-water loss at all - though that seems quite unrealistic in the case of this type of „control”) means less than 2 700 Wh / day / m2 of heat carried away. That is less than 1/2 of the sum of daily Solar heat load – and 1/3 of the cooling effect of our „Water-Film Technology”.

But even these „half-evaporative” American and Asian cooling technologies have not even traces in Europe.

Our Company, Öntözés.hu Ltd., co-operating with the Laboratory of Thermo-Physics of BME (Budapest University Of Technology And Economics), started a fundamentally new research and development on the issue of really evaporative cooling of large surfaces.

The first ACTUALLY evaporative roof cooling system of the World:
Öntözés.hu Ltd’s   „Water-Film Technology”™
„Water-Film Technology”™ -  Advanteges

The heart of our technology is, that

  • for each distinctive part of a roof
  • within a fraction of a second
  • we spray the exact amount of water that is possible to be evaporated
  • at the current conditions (determined by temperature, surface, relative humidity, wind, etc.) 


  • instead of producing closed surfaces of ponds or flowing  water (except the rear occasions when the efficiency-calculations of our central controller show flushing or the use of excessive amounts of cool water to be more effective then evaporation)
  • we produce, and continuously re-produce only a thin layer – a „film” – of water
  • doing so we multiplied the speed of vaporization (and at the same measure, naturally, the amount of heat-distraction), since evaporation of thin water layers is faster by orders of magnitude than that of a closed water-surface
  • and at the same time we reduced water-consumption by another 30…70%
What Does It Actually Mean For Your Building:

After installing Water-film Technology

Conditions Permanently Achieved - After Installing "Water-Film Technology" 
(Data measured by GM Powertrain Hungary - today: Opel - Szentgotthard, Hun.)  

Effects WITHOUT HVAC equipments: 

"Water-Film Technology" REVERSES the heat-transfer of a summer roof:

  1. The total sum of solar heat gain is prevented from entering the building
  2. Significant part of the internal heat (produced by machinery, electric devices, technology, lighting, etc) trapped under hot roofs can leave the building again
  3. These two effects combined provide a peak radiant temperature of 32 oC under the ceiling, instead of 45-52 oC. (Temperature of radiant surfaces has much deeper effect on thermal comfort of people than that of the air. This is the reason of the usage of thermal panels for heating cold industrial buildings.)
  4. Keeping the roof membrane at an even temperature, we avoid dilatation and cut-offs, stopping water-leaks, improving life expectancy of the roof-membrane by 40-50 %.
Effects on HVAC equipment:
  1. In new construction: Reduced necessary (planned) HVAC capacities - up to 20-40 %
  2. When planned in a retrofitted installation: remarkable reductions of the loads on HVAC equipments (running-time reduction) - 20-40 %
  3. Cooling the layer of air above the roof by 11-16 oC, we reduce the temperature of the micro-climate around roof-top HVAC equipment 
  4. It greatly enhances their thermal efficiency, reducing specific energy-consumption by as much as 60-80 (!!!) % 
  5. Malfunctions, automatic and/or emergency shutdowns under peak-load periods (related to over-heating) are eliminated
  6. So operating times AND specific energy-consumption AND reliability are all greatly improved! 
„Water-Film Technology”™ -  Costs, paybacks, environment protection

"Since the first energy crisis, where there is a sufficient supply of industrial water available, roof cooling spray systems proved to be the most effective and economic way of heat-protection in the US and in many other countries" - the ASHRAE statement is valid for the "half-evaporative" systems they know. Our system produces 3 times the cooling effects of these systems - for a lower price.

  • Operating costs (industrial water consumption: about 10-18 l /m2 / day on the hottest days + energy consumption: about 2…5 Wh /m/ day (!!!) are incomparably lower (<1-2%) than those of air-condition
  • A well-designed roof-cooling system substitutes about 20-40% of HVAC equippments needed – for a fraction of their investment and less then 1-2% of their operating costs.
  • Return of investment: according to ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers) researches, Roof Cooling Systems, in our climate, when planned in new construction, usually return 1.5-2 times the investments in the first year, in equipment savings and operating costs.
  • But in many cases, where there are high expenses due to technical, labour or productivity losses in hot summer conditions, return of investment – even in cases of retrofitting on existing buildings – can be reached in 1 or 2 DAYS.
Environment protection:
  • energy consumption is extremely low: 2…5 W / m2/ day (peak values - on hottest days!!!) 
  • the system neither uses nor produces environmentally dangerous materials 
  • by preventing thermal dilatations of the roof, it reduces or stops leaks without the use of further layers of environmentally hazardous plastic or asphalt, and – according to ASHRAE researches – it usually doubles the life expectancy of the roof

2010/31/EU DIRECTIVE OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 19 May 2010 on the energy performance of buildings

"Recent years have seen a rise in the number of air-conditioning systems in European countries. This creates considerable problems at peak load times, increasing the cost of electricity and disrupting the energy balance. Priority should be given to strategies which enhance the thermal performance of buildings during the summer period. To that end, there should be focus on measures which avoid overheating, such as shading and sufficient thermal capacity in the building construction, and further development and application of passive cooling techniques, primarily those that improve indoor climatic conditions and the micro-climate around buildings." (Preface, par. (25))

This is exactly, what "Water-Film Technology" does. We minimize the heat-transfer into the building - without warming up (what is more: even COOLING) it's micro-environment, reducing the need of energy-intensive cooling methods.

Conveniences, ergonomy:
  • With 2 professional maintenance a year (autumn drainage + flushing and inspection when activating the system in the springtime), the system usually needs no service or personnel.
  • the interactive central unit provides continuous control and self-correction
  • it provides full monitoring AND remote control/management possibilities for both the authorized local staff and our service as well.
  • practically maintenance-free troughout the harsh conditions on a roof in continental climate (against thundrestorms, - 30 °C at winter, + 80 °C in the summer, etc.)
  • self-controlled, but remote-controllable and remote-manageable

1.Dunapack Zrt., Csepel (August, 2004.)

  • 14 000 m2  (4000 m2 black/darkgrey bituminous roofshingle, 10 000 m2 rubber-elastomer  previously treated with heat-reflecting paint)  - an experimental and testing work, the development phase of  "Water-Film Technology"

2.  General Motors Hungary Ltd., Szentgotthard (2008. július)

  • 12.614 m2  - "Water-Film Technology"

3.  Opel Szentgotthard Ltd, Szentgotthard (2010. június) 

  • expansion of the system built in 2008 - "Water-Film Technology" for cooling another 2000 m2 of the same building.