Energy Savings In Refrigeration With Addition Of Fine Water Droplets

Humidifiers using fine droplets of water have invaded hypermarket fruit and vegetable display units and Parisian cafe terraces during heat waves. Today, the humidification technique has been extended to refrigeration installation air condensers to improve their energy efficiency. The key results: energy savings and fewer greenhouse gas emissions.

Did you know that more than 15% of the electrical energy produced in industrialized countries was consumed by cold production (12%) and air conditioning (3%)? Yet consuming kilowatts is increasingly expensive, reduces fossil fuel resources, and increases greenhouse gas emissions. In the majority of refrigeration installations, the operating condition of the air condenser, located at the back of the unit, in contact with ambient air, is a decisive element in the amount of power consumed by the refrigeration unit.

Cemagref research is being conducted to improve the energy efficiency of the condensers of cooling devices using humidification techniques. The objective is to develop an efficient and economical process that can be adapted to mid- and high-power refrigerating devices, as low as 10 kWh.

The humidification principle

We have all experienced a sensation of heat when passing behind a functioning refrigerator or air conditioner. The cause of this phenomenon is the air condenser, a heat exchanger made up of tubes with air fins attached to the back of the device. This is where the cooling fluid condenses by releasing its heat into the ambient air. To limit the system’s energy consumption, humidification processes were initiated. The principle consists of saturating the ambient air in contact with the exchanger by projecting fine water droplets. Humidification of the air intensifies the heat exchange on the air side and reduces the cooling fluid’s condensation temperature. This lowers the compression rate in the cooling cycle and improves the compressor’s consumption of electrical power.

Making gains at every level

The process empirically applied needed to be optimized in view of its application to air conditioning and industrial refrigeration. Research was initiated in this context in 2006 by the ENERFRI team at Antony. A preliminary mathematical model made it possible to estimate the energy savings that could be made with an optimized humidification process, reaching up to 20% of the total electrical energy consumed. In addition, by integrating the humidification system at the design stage, it was possible to reduce the loads of cooling fluid by 10%. Knowing that most of these fluids are suspected of contributing directly to the greenhouse gas effect, the direct gain for the environment is substantial.

Improving scientific knowledge

A pilot project is being constructed to understand the interaction between the humidity and a surface made up of tubes and air fins spaced 2–3 mm apart. The theoretical models will then be validated by varying the size and number of droplets measured by an optical galvanometer. What is the optimal water flow to prevent the condensers from producing drops that flow on the air fins and obtain a maximum effect for a minimal water consumption? What is the optimal droplet size? How far from the unit should the humidifier be installed? These are the questions that Cemagref researchers are attempting to answer so as to define the optimal conditions for efficient and economical humidification of air condensers.