Economize air conditioning system by pre-cooling fresh intake air

02012


Introduction
Air conditioning systems are inevitable in today's world.They play a prominent role in almost every industry.Apart from just offering comfort for residential purposes it has various other applications, including commercial, industrial, automotive, and refrigeration.Additionally, air conditioning is crucial for maintaining the proper temperature and humidity levels in server rooms and data centres.Air conditioning is an essential technology for creating comfortable and healthy indoor environment.
A study conducted by IEA (International energy agency) [1] says that electric fans and cooling systems alone contribute to nearly 20% of the total energy consumption as of today and this value can only rise in the future.By 2050, around 2/3 of the world's household could have an air conditioner.China, India and Indonesia will together account for half of the total number, says the agency.So, air conditioning system surely does have an economic impact.
Since more than 75% of our electricity comes from coal and other renewable energy sources, there is a humongous amount of carbon emission during the generation of electricity.As HVAC (Heating ventilation and air conditioning) stands high in the consumption of electricity, we can easily see that they are one of the major contributors to carbon emissions.A research carried out by NREL (National renewable energy laboratory) [2] predicts that air conditioning is responsible for the equivalent of 1,950 million tons of carbon dioxide released annually.So our project aims to reduce the electricity consumption of air conditioning system and thereby control the carbon emissions as well.
This research work considers conserving energy in air conditioning systems by precooling of fresh air.In India, air conditioners (AC) are operated mostly during the summer season, and therefore, the fresh air temperature is high.This will make the cooling coil and the refrigeration system to work harder, and will consume more power.Pre-cooling of fresh air using recirculated air in a non-contact plate heat exchanger will help to reduce the temperature of fresh air and hence reduce the load on the cooling coil.In this work, a huge auditorium in Thanjavur, India was considered for comfort air conditioning.Heat infiltration through walls, and windows were considered.Likewise, heat load from lighting and humans were considered.Based on heat load, an economical air conditioning system was proposed.
Various research papers which share resemblance to our project have been studied and the one closest was submitted by Khaled et al. [3], where heating of fresh air is accomplished by hot exhaust air of winter air conditioner.A prototype was built and upon testing it was verified that power savings of roughly 110 W could be achieved during cold climatic conditions.In contrast, the current project involves cooling of fresh air by cool indoor air.Khaled et al. [4] established a prototype to study the effects of precooling of fresh intake air during hot summer and preheating of fresh intake air during summer.Based on experiments, it was inferred that the capital cost of the equipment with precooling could be recovered in 1.5 years, if the equipment was operated nonstop during hot summer.Likewise, it was inferred that the capital cost of the equipment with preheating could be recovered in 6.3 years, if the equipment was operated nonstop during cold winter.Swan et al [5] conducted a detailed research to understand the cost savings associated with using condensate to pre cool condenser air.Chen et al. [6] conducted a detailed experimental study to understand the cost savings associated with water cooled air conditioners in residential applications in Hong Kong.Elsherbini and Maheshwari [7] studied the impact of placing a shade on air cooled condensers.From experiments it was observed that shading doesn't have a considerable impact on energy savings.

Design of Air-conditioning Process
The main objective of this mini project is to save energy consumption in air conditioning systems.Clean and fresh air with ample oxygen concentration is required for healthy living.Purity of air is also of concern.Likewise, maintaining optimal humidity is required for healthy living.Improper humidity levels in the living space affect the respiratory system.Therefore, maintaining adequate fresh air, proper humidity levels along with comfortable temperatures are required for proper air conditioning.For air conditioning the auditorium, the following must be considered: Energy savings through pre cooling of intake (fresh) air is presented in this work.The first step in air conditioning the auditorium is to determine the heat load.Heat load in the auditorium is from heat infiltration and heat generated in the room itself.Heat infiltration is the heat flow through walls and windows and whereas, heat generation is the heat flow from lighting and human beings.Both sensible and latent heat is considered in the design of air conditioning process.Sensible heat may be seen as dry heat that is emitted in the absence of moisture.Since humans perspire, humans emit latent heat, which is the heat associated with moisture content.In regular offices, classrooms, residences, etc. where humans are not physically active, sensible heat is more.On the contrary, in the case of factories, wellness centres, etc. where humans are physically very active, the latent heat is more.
First step towards calculating room (auditorium) heat load is measuring the dimensions of the auditorium.Thereafter, the heat load through walls, and windows were computed using the law of conduction heat transfer [9].The auditorium was assumed to be maintained at 28°C and whereas the ambient conditions were assumed at 40°C.
Where Tsi and Tso refers to temperature of inner and outer surfaces of the wall (or windows) designated in °C, L refers to the thickness of wall (or glass window) in m, K corresponds to the thermal conductivity of material in W/mK, and A is the area normal to the direction of heat transfer in m Consider a basic air conditioning layout to provide necessary cooling for the auditorium.Psychrometric chart is used in the design of air conditioning process.

Fig. 1. Basic air conditioning layout
As described in the above layout, initially mixing of fresh air with recirculated air is considered.Fresh air is designated as point 1, and recirculated air is designated as point 4.
Different mixing proportions were considered such that fresh air/person is maintained at least at 10 CFM as described in Table 1.Typical dry summer conditions of 40°C with 20% RH was assumed for fresh air, and 28°C with 50% RH was assumed for recirculated air.By trial and error method, it was noted that at least mixing of 30% fresh air with 70% recirculated air was needed to satisfy the fresh air criterion.By plotting in the psychrometric chart, the mixed condition, designated as point 2, was estimated to be 31.5°Cwith 37% RH.The mixed condition, h2, has an enthalpy of 60 kJ/kg.Figure 2 describes mixing of fresh air with recirculated air.
The amount of heat that must be removed from the air conditioned space, Qair, is given in Table 2. Specific enthalpies h3 and h4 are directly estimated from the psychrometric chart.The load on the cooling coil may be determined as Qcc= mair*(h3 -h2) Assuming a standard coefficient of performance (COP) for the refrigeration unit, the power consumed by the compressor (Wcomp) can be determined through the following relationship The details of basic design of air conditioning process is provided in Table 3.After estimating the volumetric flow rate of fresh air, which is found to be 4138 CFM, it is determined that each individual will receive 10.3 CFM of fresh air.These calculations show that the supply conditions to auditorium meet the comfort requirements specified by ASHRAE standards.
Next, precooling of fresh air was considered before the mixing process.Precooling is intended to reduce the temperature of incoming fresh air and thereby reduce the load on cooling coil and compressor power.In this configuration, ambient air is directed through a non-contact plate heat exchanger (pre-cooler) prior to entering the mixing chamber, resulting in a decrease in its temperature.The layout is given in figure below.

Fig. 4. Air conditioning layout with pre cooling
Capacity rate ratio [9] for the plate heat exchanger may be assumed as 1.0 as flow rate of incoming fresh air will be the same as the excess air drawn from the air conditioned space.
Where Cmin and Cmax corresponds to minimum and maximum capacity rates respectively.Heat exchanger effectiveness may be given as [9] Ɛ = NTU/(NTU+1) Where NTU is a dimensionless parameter termed as number of transfer units.Heat exchanger effectiveness may also be given as the ratio between actual and maximum possible heat transfer [9] Ɛ = Qactual/Qmax (8) = (Thi -Tho)/(Thi -Tci) NTU varies mostly between 0.1 to 10 for conventional heat exchangers, and in most applications, NTU doesn't exceed 3.0.Also, from previous studies it has been seen that indefinitely increasing the NTU beyond 2.5 doesn't serve much purpose.Assuming a NTU of 2.5 the effectiveness may be determined as follows: The temperature of hot fluid leaving the plate heat exchanger was found out to be 31.42°CWhen the incoming fresh air is pre cooled using a plate heat exchanger, the temperature of fresh air has fallen from 40°C, 20% RH to 31.42°C, 32% RH; a temperature drop of 8.58°C.
On repeating the same design exercise for mixing, estimation of supply condition, cooling coil load, and compressor load, the following results were obtained for air conditioning design with precooling.For brevity, the design procedure is not detailed again in the manuscript.The results are presented in Table 5.

Conclusion
The project considers economizing the air conditioning system.A huge auditorium was considered for this study.The heat load in the auditorium was first estimated and an air conditioning process was designed using ASHRAE standards.Therein, pre-cooling of fresh air was considered to reduce the load on the cooling coil, which in turn reduces the load on the compressor.Pre-cooling of fresh air is accomplished by introducing a plate heat exchanger, where fresh air exchanges heat with room air (recirculated air).For the situation considered, pre-cooling of fresh air provides 13% reduction in energy consumption as compared with standard air conditioning system.
With over 10% of world's energy being utilized for air conditioning systems, pre cooling of fresh air certainly provides considerable savings in energy utilization.Likewise, in the design of the refrigeration cycle, water cooled condenser may be considered.Water has a much higher heat absorbing capability than air and thus produces better condenser cooling.Water cooled condensers typically employ a cooling tower to reject heat.It should be noted that irrespective of weather, earth's temperature is considerably lower and remains constant throughout the year at a certain depth.The negative geothermal gradient that exists for the first 100 m underground has a huge potential for cooling.This is yet to be understood and be utilized to its maximum potential.More ground surveys need to be conducted at many parts of the world to develop more accurate empirical relationships for estimating ground temperature.The negative geothermal ground potential can be used to reject heat from the condenser cooling water and further enhance energy savings.

Table 1 .
Conditions to be maintained in auditorium[8]

Table 2 .
Total heat load in the auditorium 2. Per 2001 ASHRAE fundamentals handbook [8], the representative heat load from humans involved in light work would be 225 W (82.5 W sensible heat and 142.5 W of latent heat).The auditorium can accommodate about 400 humans.The total estimated heat load in the auditorium is presented in the table below.

Table 3 .
Basic design of air conditioning process

Table 4 .
Effect of NTU on pre-cooling

Table 5 .
Design of air conditioning process with pre cooling