Evaporative Cooling Technology

Knowledge Centre – Passive Strategies

Low-energy cooling system
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In India’s predominantly hot climate, space cooling constitutes nearly half of a typical commercial building’s energy demand. Modern evaporative cooling systems are great low-energy alternatives to conventional refrigerant-based systems. They are relatively inexpensive, clean, energy-efficient and highly versatile. In this webinar Mr. Shamkant Mirashi – Head of Arka Clean Technologies, discusses evaporative cooling technologies available in the market and their applications along with case studies.

The principle of evaporative cooling stems from several traditional Indian cooling techniques, like ‘matkas’ or earthern pots used for cooling water, and blinds made out of khus which when sprayed with water cool the surroundings due to evaporation. Evaporative cooling technology is conceptualized using the same thermodynamic principle. It is most effective in hot and dry climate since it adds humidity to the air thereby providing thermal comfort. There are two types of evaporative cooling systems used commercially- direct and indirect. Both systems use water as the refrigerant, are low-energy and highly energy-efficient.

In direct systems, the outside hot air is directly cooled when it meets a cooling media. Water is sprayed from the top of the cooling media thus cooling the outside air down due to evaporation. A blower then circulates cooled air in the room.

The second type is the indirect system. In this, hot outside air passes through one side of a heat exchanger plate while the other side is sprayed with water. Due to the indirect contact between the hot air and the cool plate, the hot air cools down and is then circulated in the room through a blower. This can be used in humid climatic conditions as it doesn’t contribute to the increase in the humidity levels.

An even better variation is the indirect – direct evaporative (IDEC) cooling system that combines both direct and indirect technologies to derive maximum efficiency. This system is capable of achieving a supply air temperature of 220C, even when the dry bulb temperature ranges (DBT) at 400C. The air is first cooled through the indirect system and then further cooled with direct evaporative cooling.

This system finds application in providing outdoor thermal comfort in regions where the temperature shoots up to 40oC during the summers. An example of a project using IDEC is Bihar Museum in Patna. For the same DBT, the supply air temperature by the IDEC system is 40C lower compared to the evaporative cooling systems. Moreover, the air quantity significantly reduces in IDEC allowing for smaller capacity sizes.

IEC-assisted HVAC, that combines the indirect-direct evaporative cooling system with a conventional HVAC, can very well be the future of low energy cooling. Take a scenario where the outdoor air temp at 400C is to be cooled down to 150C. A conventional HVAC would consume substantial energy to achieve this. Further, it would need to operate at full load, thereby reducing efficiency. However, if a two-stage IDEC is connected to the AHU, the outdoor air would first cool down by 10oC or more in the IDEC system and then be cooled further by the HVAC’s cooling coils. The overall energy consumption by the HVAC unit would thus reduce. The IEC-assisted HVAC system also presents unique latent heat recovery for additional savings. This technology finds application in many commercial establishments, star hotels, data centers, healthcare etc. where the cooling loads are high, fresh air is mandatory and conventional heat recovery could lead to cross-contamination.

The type of cooling system plays an important role in the overall energy efficiency, comfort, operational cost of a building and its environmental impact. To summarize, evaporative systems are a clean and low-energy alternative to conventional HVAC systems, especially in hot and dry climate. Modern systems can allow improved sensible cooling, control of humidity levels and fresh air supply.

This webinar was conducted on 17th July 2019. 

Speaker profiles

Shamkant Mirashi | Head – Arka Clean Technologies

Shamkant Mirashi, head of ‘Arka Clean Technology’, has over a decade’s experience of designing evaporative cooling systems for industrial-scale setups. A serial entrepreneur in energy efficient tech, he has perfected low-energy cooling systems for retrofit as well as greenfield projects, designed and built around an entirely home-grown heat exchanger.

Q&A

Q1. What is the refrigerant in an evaporative cooling system?
Water

Q2. Can a flat shape of a cooling plate be modified to some other shaped like a beehive for a larger area of heat transfer?
In direct cooling, where air and water have contact, it is already in the beehive shape. But in indirect cooling, where we intentionally keeping water and air apart, it must be a flat plate so that we can keep them separate.

Q3. Is the IDEC feasible in residences?
Technically, yes. Commercially, they are not there yet.

Q4. Are there any ASHRAE guidelines for the system? Does this system comply to ASHRAE comfort requirements?
ASHRAE standard 60.2 talks about ventilation and thermal comfort requirements. This system is 100% compliant with it.

Q5. How large is the cost difference between the direct evaporative cooling and the IDEC?
Initial cost difference is there because of the hardware. But the life cycle cost and ROI for IDEC is lesser.

 Q6. What is the limitation on the quality of water required?
Technically, water must be potable, 400-500 PPM, and neutral, pH 7. But otherwise does not demand any big water treatment processes or RO plants. In industrial environment we have seen water quality deteriorating even 700-800 PPM and the machines are working comfortably.

Q7. What is the humidity of the supply air at the outlet side of the machine?
The relative humidity of the air at the starting point of the outlet is around 90%.

Q8. Can the supply temperature from IDEC be below the wet-bulb temperature of air?
Absolutely. The biggest advantage of IDEC system is that the supply temperature is typically 1-2℃ below the wet bulb temperature.

Q9. How do you solve the humidity challenge in a kitchen in Mumbai?
In Mumbai, IDEC alone cannot address the humidity problem. It must be in combination with refrigeration and must have a cooling coil.

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