Desiccant-assisted sustainable cooling innovations in HVAC

Using desiccant adsorbents or materials with a natural affinity for water – the desiccant dehumidification and cooling method is a creative way to achieve dry air, deliberates  Dr. D.B. Jani, Professor, Government Engineering College, GTU, Gujarat.  When indoor air releases more moisture due to large human occupancy, a desiccant can absorb it without changing size or form.

When moist air passes through a desiccant, it becomes noticeably drier. This process occurs without sophisticated cooling, compression, or other controlled systems, avoiding the high-grade electrical energy consumption for such operations.

Once the drying task is complete, the desiccant is dried using hot air in a process known as regeneration. After regeneration, the desiccant is ready to dry more air. Desiccant dehumidifiers are highly energy-efficient. They can be reactivated using freely available renewable energy, such as solar or industrial waste heat. Due to these advantages, they are the most widely used method of dehumidification and sustainable cooling across various industries.

Solid desiccant cooling systems

The growing global population has increased the demand for energy in many different applications. Much energy is needed to run more machines for people and industrial applications for everyday use, which are growing rapidly.

People get uncomfortable from fatigue. The first thing they think is to turn on the air conditioner. Consequently, it is essential to analyse the cooling system performance while considering indoor thermal comfort. People use vapour compression-based traditional cooling systems to achieve adequate indoor thermal comfort as an air conditioner. VCR-based traditional air conditioners have helped people maintain appropriate indoor comfort in moderate climates. For the Heating, Ventilation, and Air Conditioning (HVAC) industries to remain viable, they need to consider a few issues, like indoor air quality and the ventilation airflow required. One of the issues is that, as fossil fuel availability declines over the coming decades, air conditioners can not rely on them for their power source. It might cause an impending decline in air conditioner sustainable energy performance.

There are some limitations in conventionally used vapour compression refrigeration systems (VCR). It urges the researchers to find a better replacement for a cooling system. One of the problems in the vapour compression cycle is that its system has been consuming a high volume of electrical energy to operate air conditioners. VCR could not reduce the cooling load by dehumidification. Instead, it only uses up power to cool down surroundings with coolants. Besides that, the VCR system has also been a factor in the discharge of carbon dioxide (CO2) and ozone depletion substances such as Chlorinated Fluorocarbon Compounds (CFCs) and hydrochlorofluorocarbons (HCFCs). They are considered potential ozone-draining gases. These ozone depletion coolants can indirectly cause skin diseases and asthma for users over the long term. A high concentration of these coolants has the potential to lower the quality of indoor air. VCR has caused some loss for air conditioner users due to rotting on surrounding furniture when surrounding relative humidity exceeds 60 percent. It happens because the system cannot control the environment’s humidity and temperature.

High humidity causes a high amount of water vapour in the air. This promotes mould on furniture and causes the user’s furniture to rot because the VCR cannot control the indoor humidity and temperature separately. This can be replaced by innovating a cooling system that can reduce the indoor humidity and temperature at the same time. One promising idea by researchers to provide better cooling quality is using a Solid Desiccant Cooling System (SDCS).

SDCS requires solid desiccant materials to adsorb the indoor moisture and keep indoor humidity under control by the dehumidification process. Dehumidified air can provide dry and cool air to indoor surroundings and reduce the risk of rotting indoor furniture. SDCS minimises the energy cost of air conditioners. The system is operated only under thermal heat from the processed ambient air. As the dehumidification done by SDCS will remove moisture, it will also be able to cool down the surrounding air to a certain extent. It will reduce the need for cooling because indoors will be more comfortable than usual conditions. It will reduce the workload of the air conditioner. So, the air conditioner will be able to reduce power consumption by cutting the running cost. In addition, SDCS does not require refrigerant, which is another bright side of this system. Therefore, high energy is not better for compressing refrigerant into the system for cooling purposes.

Working of system

According to its configurations, desiccant cooling is either an open or closed cycle. It uses a desiccant and thermal wheel to achieve both cooling and dehumidification. Solar thermal energy or waste heat can reactivate the desiccant. The thermal wheel is a rotary heat exchanger positioned within the supply and exhaust air streams to recover the heat energy, and the desiccant wheel works similarly, additionally reactivating the desiccant.

There are several variations, depending on the condition of ambient outdoor air, air change requirements and humidity control requirements. It offers high efficiency but requires large air flows and is limited in operational range. It can be used with a conventional vapour compression cycle or evaporative cooling. For the air cycle, the outdoor air penetrates through the desiccant wheel. It is dried and ramped up due to dissipating heat from water vapour adsorption.

Afterwards, the air is blown into a regenerative evaporative cooler (REC) and cooled in sensible heat exchanges (SHX). Then, the air is supplied to the evaporative condenser to condense the refrigerant (R-410A), and the heated air is exhausted.

In the regeneration channel, the air taken from the outdoors is heated to the regeneration temperature at the heating coil, desorbs the moisture from the desiccant wheel to regenerate it, and is finally exhausted.

Different components used in the desiccant cooling cycle.

The outdoor air moisture content gets minimised. Its temperature increases as it passes through the desiccant wheel. It is sensibly cooled through the thermal wheel, further cooled to the required supply temperature, with some moisture gain as it passes through an evaporative cooler. The return air from indoors, at 25°C., passes through an evaporative cooler. It enters the thermal wheel at a lower temperature and higher moisture content. As it passes through the thermal wheel, it is sensibly heated and further heated to reactivate the desiccant before exhausting. The bypass is monitored so that unnecessary heat is not applied. The thermal wheel is as shown in figure.

The desiccant wheel, the adsorption rotary regenerator, uses silica gel for optimal adsorption capabilities. The rotation mechanism, maintained by a maintenance-friendly chain drive, operates at a constant speed of around 10 to 15 rotations per hour. Radial and axial seals separate counterflows and surroundings, ensuring efficient and controlled dehumidification processes. The housing of our units is constructed with stainless steel, offering durability. It is insulated with mineral wool to enhance efficiency in challenging industrial environments when the ambient environment is hot and humid.

Innovation in desiccant-based sustainable cooling

Open sorption systems, called DEC systems, supply conditioned air to a building. The air is set to a specific temperature (the lowest temperature is 16°C) and humidity. The principle of open systems uses ambient air or a combination with recirculated building air for air conditioning instead of chilled water. With such open systems, building heat is removed by the airflow through the building, and additional fresh air is supplied.

Therefore, air conditioning and building ventilation are operated simultaneously. Solar heat is necessary for regeneration to ensure continuous operation of the sorption part (solid-based sorption wheel or liquid-based salt solutions) of such an open system. The advantage of a solar heat-driven DEC system is that it fulfils all the essential requirements of air conditioning (i.e., control of fresh air temperature, humidity, and volume flow).

In a developing country such as India, a high volume of waste heat generated annually could be used to produce heat throughout the year. This technology, which uses thermal energy to provide cooling, could be a solution to our rising energy crisis. In many buildings like hotels, hospitals, and industries, there is a demand for hot water along with air cooling. Such a scenario is well suited for the application of the Tri-generation concept.

A tri-generation system produces three forms of energy: electricity, heating, and cooling. With suitable equipment, it could generate power, hot water, and air conditioning. The principle of tri-generation relies on heat energy generation. Heat captured through burning waste, electricity production with generators, or heat generated through solar panels could generate hot water through heat transfer equipment or cold/ chilled water with absorption chillers. The potential for using tri-generation systems is identified to be nearly 500 to 1,000 MW in India.

Tri-generation technology, also known as Combined Cooling, Heating, and Power (CCHP), comprises a gas engine or a power system operated by burning waste, biofuel, or fossil fuel to produce electricity. The connected heat recovery system is a heat exchanger that recovers heat from the engine or exhaust. This recovered heat is used for heating applications like hot water or a regeneration process in absorption chillers. The electricity produced within the tri-generation process could meet the building loads or power chillers during the peak load period.  The thermal energy could be diverted to boilers. It can heat the water used in hospitals, hotels, and industries as an absorption chiller to heat the absorbent and refrigerant mixture and regenerate the absorbent.

Desiccant cooling systems are developed as an alternative to conventional vapour compression systems since the traditional vapour compression system consumes a large amount of energy and causes environmental problems. So, the desiccant systems become a priority because of the rise in energy demand, increasing cost, and climate change. Also, they do not use refrigerants, which harm the environment. Therefore, desiccant systems are better for the environment.

Hybrid desiccant cooling systems can be combined with direct, indirect evaporating or vapour compression-based conventional cooling systems. They require a smaller regenerative temperature for desiccant. It means that they need less energy. Additional energy, such as electricity or gas, is required for desiccant wheel regeneration. These systems have a sanitising effect on the air. Airborne microorganisms are killed by passing through the system. There is the possibility to regulate the comfort level of the relative humidity in the indoor air. There is no dew, so no mould and mildew. Therefore, these systems can prevent building-related illnesses. In addition, desiccant cooling systems can be applied in different climates.

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Expertise shared by-

Dr. D.B. Jani

Government Engineering College

Gujarat Technological University, Ahmedabad, Gujarat.