Performance study on a hybrid solid desiccant-vapor compression air conditioning system for hot-humid ambient conditions
R. Venkatesh1 Madhu Ganesh2 R. Rudramoorthy3
1. Department of Mechanical Engineering, PSG College of Technology, Coimbatore, India venkiram88@gmail.com
2. Department of Aerospace Engineering, Karunya Institute of Technology and Sciences, Coimbatore, India. madhumini@gmail.com
3. Department of Production Engineering, PSG College of Technology, Coimbatore, India. drrrresearch@gmail.com
Abstract
The paper focuses on the simulation and testing of a hybrid solid desiccant vapor compression air conditioning system under different hot-humid climates. The simulation is carried out by a BLUEJ programming framework. Air at the lowest achievable air temperatures from a solid desiccant cooling system is supplied to a standard vapor compression air conditioning system. The cooling capacities of the hybrid system under three modes indicating different supply air conditions to the cabin, are reported in the paper. From the performance study, it is inferred that the hybrid system provides significant energy savings compared to a standard air conditioner, especially in hot-humid ambient conditions. The solid desiccant cooling system thus establishes itself as an effective pre-cooler unit to a standard vapor compression air conditioning system.
Introduction
The rapid increase in the emission of greenhouse gases (GHG) and ozone-depleting gases such as carbon dioxide, methane, nitrous oxide, and halogenated compounds, since the 19th century, because of increased human activity has been established beyond doubt by many research studies as reported by Fadnavis et al [1]. The rise in global temperature is predominantly caused by greenhouse gas emissions, aerosols, deforestation, etc. and the rate of rise also has increased in proportion to the acceleration of industrial activities. The study also reported that by the end of the 21st century, the average ambient temperature rise in India would touch anywhere between 4.7°C and 5.5°C and the frequency of occurrence of warm days and warm nights would increase by 55% and 70% respectively. The brisk changes in India’s climate are creating more pressure on India’s ecosystems, agricultural activities, and freshwater resources creating damage to biodiversity, food, water, energy security, and public health. These findings were corroborated by Biardeau et al [2] and predicted that the higher ambient temperatures, increased occurrence of extreme weather events, and soaring variations in climate would lead to a high level of risk of occurrence of heat strokes, cardiovascular and neurological diseases, and psychological disorders. Since more than 70% of the energy requirements of India are met by thermal power plants, the increase in demand for space cooling would in turn increase greenhouse gas emissions also.
Cooling has now become essential not only for comfort but also for survival and health in most parts of the world [3]. The adoption of air conditioners is aided by the increase in electrification in all the lower and middle-income countries over the last 20 years. This increase threatens the reliable operations of power grids and negates all efforts to combat climate change. Air conditioners liberate significant heat that increases the ambient temperature. The advancements and adaptation of sustainable air conditioning technologies could reduce carbon dioxide emissions by 50 billion tons [3].
Desiccant-assisted air conditioning offers an edge over other alternate technologies as it offers independent handling of sensible load and latent load, has fewer moving parts, uses natural fluids like water, uses non-corrosive working fluids, avoids crystallization in the equipment, and integrates easily with waste heat sources and solar collectors [4].
The solid desiccant air conditioning system uses materials like silica gel, zeolites, activated carbons, titanium oxide, and metal-organic frameworks for the removal of moisture from the humid air. The air becomes dry and hot after undergoing dehumidification by solid desiccant materials. The hot-dry air is further cooled by sensible and evaporative cooling heat exchangers to achieve the desired cooling temperature inside the cooling space. The solid desiccant cooling system is further classified as
Literature survey
Energy-efficient vapor compression air conditioners would be an inevitable choice to provide basic cooling in developed countries and standards have become stringent over the years. Further, the urgent deployment of sustainable cooling technologies in both residential and commercial spaces is much needed for protecting the planet [3]. A combination (HSDVC) of a solid desiccant-based system (SDAS) and a standard vapor compression air system (VCAS) has been investigated widely to address this. Such a system was evaluated by H Liu et al [5] and was found to provide significant energy savings by addressing 52% of the latent load and 76% of the total cooling load.
An HSDVC evaluated by A.E. Kabeel [6] et al offered benefits such as no corrosion, no crystallization, fewer moving parts, and reduced CO2 emissions. The potential savings of carbon dioxide emissions by the hybrid solid desiccant- VCAS was 34%.
Jani et al [7] reported that an HSDVC eliminates the energy costs associated with dew-point cooling and reheating of supply air. Artificial neural networks and TRNSYS were found to be effective simulation tools for HSDVC systems. The regeneration temperature of the desiccant wheel was found to be up to 100°C for high latent loads. Humidity reduction of up to 66% could be achieved by this hybrid system with a potential energy savings of 10% to 63%. Similar results were reported by Jia et al [9] Hussain et al [10], Luo et al [15], and William M. Worek et al [16]. Ukai et al [11] reported that the HSDVC gave a maximum COP of 1.35 when the chilled water was supplied at 14°C from the chiller unit.
Lee et al [12] simulated a hybrid system coupled with direct and indirect evaporative coolers using MATLAB-Simulink. This system gave desired comfort cooling at the regeneration temperatures of 70°C and 90°C for residential air conditioning.
Sohani et al [13] simulated a building-integrated photovoltaic system along with an SDAS on TRNSYS for the city of Tehran. The results showed that the system gave savings in cooling load of 60% with a payback period of 2.87 years. Singh et al [14] simulated an HSDVC using ENERGY PLUS software for hot-humid ambient conditions. Their results showed that the system gave an annual electric savings of 5% and the system would be the best fit for large-scale buildings.
Jyun-DeLiang et al [17] investigated a ground source-based HSDVC. Their results showed that the system when used as a pre-cooler gave potential energy savings from 33% to 73%. The system developed the highest COP of 4.10 with a savings of 31.7% in energy when it is operated in the summer.
Most of the research works have addressed the savings of energy and maximum possible COPs that could be obtained from HSDVC systems. The working of those systems is primarily to supply air at a comfortable temperature of 25°C and 50% RH.
Only a few investigations reported the potential reductions in the cooling capacity of the vapor compression air conditioning system. Moreover, such reductions reported were not for a supply air temperature and relative humidity substantially closer to the comfort condition. For an air conditioner to be operated in an energy-efficient mode, the sensible heat ratio should not be less than 0.75. For hot-humid ambient conditions, a VCAS would operate below a sensible heat ratio of 0.75 resulting in high power consumption.
The objective of the research work is to find the possible reductions in the cooling capacity of a VCAS when it is made to operate with a solid desiccant cooling system in a hybrid cooling mode. This is primarily done for hot-humid ambient conditions. This research work necessitates the framework to have a sub-system to take a substantial portion of the latent load of the VCAS with a solid desiccant in hybrid mode. This is also crucial for an energy-efficient operation of a VCAS in hot-humid climates.
Solid desiccant – Vapor Compression Hybrid Air Conditioning System
The working of a VCAS is presented in the psychrometric chart given in Figure 1. The hot-humid air is given as point 1. The air is cooled sensibly till point 2 and dehumidified till point 3. Then the air is heated to point 4 to the comfort condition of 22°C and 50% relative humidity. The processes undergone by the air in standard air conditioning equipment are cooling, dehumidification, and heating. All the processes undergone by the air are highly energy-intensive.
\({\text{Cooling}\ \text{capacity}}_{\text{VCAS}}=\ =\ \dot{m_{a}}\ \times\ \left(h_{\text{amb}-1}-h_{\text{supply}-3}\right)\)(1)