Custom «Direct and Indirect Evaporative Cooling Using Ceramics» Essay Paper

2.0. Introduction

Evaporative cooler allow cooling of air by forcing warm air over a wetted pad. It results into evaporation of water in the pad causing removal of heat from the air while moisture is added. According to Soares (2008), there are several advances in technology that have enabled viability of evaporative cooling systems become an alternative to conventional cooling systems in commercial setups as well as other areas of the country. Interests of manufacturers have shifted to a two-stage evaporative cooling system, which offers a greater capacity for cooling in a range of geographic settings where evaporative cooling can be used.

2.1. Direct Evaporative Cooling

It is the most commonly used evaporative cooling system in residential areas and ensures cooling of air by evaporation to increase the level of moisture in the air. Typical residential systems use evaporative cooling systems shredded aspen fibres of thickness between 1 to 2 inches. The effectiveness of these systems ranges from 55 to 70 per cent.

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In the above equation, TDB refers to the outdoor dry-bulb temperature, TWB refers to the outdoor wet-bulb temperature, and TSAT refers to the supply of air temperature coming from the evaporative cooler.

According to Bom (1998), the thickness of the media and speed of the air contribute to effectiveness. Direct evaporative cooling systems are mostly used in hot and dry climates, where design wet-bulb temperatures are below 68 degrees Fahrenheit.

Evaporative cooling of any evaporative cooler is limited by the amount of outdoor wet-bulb temperature. The table below shows the supply air temperatures that can be achieved with direct and indirect evaporative coolers.

Table 1. Evaporative Cooling Effectiveness

According to the table above, it is assumed that the mean effectiveness is 85% for direct evaporative coolers, and 110% for indirect evaporative coolers.

2.2. Indirect Evaporative Cooling

According to Bainbridge & Haggard (2011), in indirect evaporative cooling systems, an air to air heat exchanger is used to remove heat from the primary air stream without adding moisture. In one design, external hot dry air is passed through the series of horizontal tubes that are humid in their outer surfaces. A secondary air is blown over the external coils resulting into exhaustion of the warm, moist air to the outdoors. The external air is cooled without addition of moisture as it passes through the tubes. The effectiveness of indirect evaporative cooling is about 75 per cent.

2.3. Two stage direct evaporative cooling system

In the first stage of this system, warm air is pre-cooled indirectly without adding humidity by passing heat exchanger which is cooled by evaporation outside. Cherry & Duxbury (2009) explain that in the case of direct stage, the precooled air is passed through a water-soaked pad, and it collects enough humidity as it cools. Since the air available to the second stage is precooled, there is reduced addition of water to the air because there is less moisture held by cooler air compared with warm air. It results in a cooler air with relative humidity between 50 and 70 per cent based on climate compared to traditional systems that produce approximately 80 per cent relative humidity air.

The advanced stage of this system uses 100 per cent outdoor air and a variable speed blower that circulates the air. According to Mcdowall (2007), the advantage of this type of evaporative cooler is that it reduces energy consumption by 60 - 70 per cent compared to conventional air conditioning systems.

2.4. Two stage indirect evaporative cooling system

According to Bom (1998), indirect cooling is paired with an evaporative cooling stage that cools the air further while adding moisture to the air. These two stage systems can ensure that cooling loads are accomplished in buildings in arid and semi-arid climates. These systems provide cooler air supply at reduced relative humidity compared to direct evaporative coolers. In the first indirect evaporative cooling stage, the supply air is cooled without any increase in relative humidity. Since the air is cooled, there is reduced capacity to hold moisture. It is followed by passing the air through a direct stage resulting in cooling of the air further while moisture is added. The effectiveness of indirect cooling systems is typically between 100% and 115%, which cools the air to temperatures slightly below the outdoor air wet-bulb temperature.

Givoni (1994) explains that for commercial applications, indirect evaporative systems are usually coupled with conventional DX stage that meets cooling demands during hot and humid environmental conditions. Since the systems use external air for cooling, they can also be paired by use of heat recovery to capture a section of energy that is not utilized in the exhaust air stream and reduce level of ventilation cooling load. It is explained visually in the following diagram.

2.5. Development in direct and indirect evaporative cooling systems

There are certain advancements in evaporative coolers that can contribute to their effectiveness, mainly in terms of controls. Some units of evaporative coolers use variable speed drives, two-speed or commutated motors that control the space temperature to the right set point. Advanced features of evaporative coolers that have been incorporated recently could include the use of sensors that monitor performance of systems. Measurements of temperature and humidity would result in estimation of temperature effectiveness. Static pressure gauges are also used to determine pressure drops across media and can be used when maintenance of the media is needed.

Since usually there are higher temperatures than the traditional air conditioning systems, there is a need to use much more air to provide the right level of cooling. Since 100% outside air is used, there is the need for provisions for venting of the air without the need for opening of windows. In residential use, vents are installed in the ceiling to open when the room is positively pressurized. Right amount of attic ventilation must be provided to then exhaust the ventilation air to the outside.

The table below shows a comparison of energy use for various cooling systems 

Table2. Comparison of Energy Use for various Cooling Systems.

2.6. Application of evaporative cooling systems

There are a number of applications where evaporative cooling systems are used in residential and commercial markets. Soares (2008) explains that direct evaporative cooling is mainly used in very hot and dry areas and homes. Indirect evaporative cooling is mainly used in California. Since 100% of the air used is external, there are significant secondary benefits that are derived in application with outdoor air requirements such as classrooms. Also, there have been recent developments in indirect and direct evaporative cooling for residential and light commercial markets.

Indirect evaporative cooling can be accompanied with conventional DX cooling to reduce refrigeration loads. A system that uses indirect evaporative cooling with DX cooling can be characterized by a lower total cooling load compared with a recirculation system that uses only DX cooling.

Water is used in evaporative systems to assist in replacing the evaporated water and to remove dissolved minerals that collect during evaporation of water. Between 5 to 10 gallons of water is used per hour of operation of residential systems. The consumption of water is offset by reduction in power consumption in the evaporative cooling units as reduced loads result in reduction in water consumption at the power plant.

CHAPTER 4

4.1. Methodology

Methodology will involve review of literature, computer modelling, and lab tests.

Review of Literature

Most of ceramics building products such as cladding and panels are produced by extrusion. However, most ceramic products such as pots and jars are produced by casting. The process of casting enables creation of a container of water, but there are limitations imposed by this technique of production such as size and shape of the container. Tests performed in laboratories have indicated that better performance can be generally attained by the direct system and full-scale prototypes of two different components showed that it was possible to obtain a better performance when tested in a climate chamber at the University of Nottingham. Both systems demonstrated similar values of specific cooling when compared at similar volume flow rates.

Computer Modelling

I. Calculation of thermoelectric module performance

A computer model will be constructed where five variable parameters that can be applied to thermoelectric modules that affect the operation. The modules that will be used include:

I- It will represent the input current to the module and will be expressed in amperes

Vin – It will represent the input voltage to the module and will be expressed in volts

Th- It will represent the temperature of the hot side of the module and will be represented in degrees Kelvin.

Tc-It will represent the temperature of the cold side of the module and is expressed in Kelvin

Qc- the heat input to the module and is expressed in degrees Kelvin

Calculation of module performance will be accomplished by setting three different variables to specific values. The calculations will involve either fixing the values of Th, I and Qc or fixing the values of Th, I and Tc.

In computer modelling case, a straightforward calculation routine will be developed to incrementally set through a series of fixed values that allow production of an output of module performance in a range of operating environments.

II. Single-Stage Module Calculations

Certain mathematical models will be used to understand performance of a single stage thermoelectric module. In this model, temperatures will be converted into degrees Kelvin. The mathematical calculations will be performed in the following order:

Temperature difference across the module is given by

Heat pumped by the module is determined by the equation

Input voltage to the module is determined by the equation

Electrical input power is determined by the equation

Heat rejected by the module is determined by the equation

The mathematical modelling presented in the above paragraphs provides mathematical models of thermoelectric modules that can be incorporated into calculations of simulations of overall thermal performance of the system.

4.2. Future work

It will involve tests that will be performed in porous ceramic. In this set up, prototype porous ceramic rectangular unit measuring 315 mm X 165mm X 35mm of nominal 5mm thickness. Holes will be created on the sides to let in water and sensors. Ceramic evaporators will be produced at three different temperatures: 1110, 1130 and 1170 degrees Celsius to give three different porosities.

The table below will be filled from the values of porosity observed at each temperature.

Table 3. Porosity prototypes

The dry weight (D) of each prototype will be measured. Then, they will be soaked overnight in water. The weight under water (l) will be measured. Each will be lifted out and porosity will be calculated using the equation. 

The results of the equation will give variation of measured values of porosity within each batch. The effect of firing temperature on porosity will be measured. Experiments will be done within environmental chambers enabling proper control of humidity and temperature during the tests. The ceramic evaporators will be placed in a 2530 mm high, vertical duct of rectangular cross-section with an external insulation of 70mm polyurethane. The duct used will be 340mm X 210mm in cross-section internally and will be able to accommodate a single evaporator or two placed side by side.

Air will be taken through the top of the duct from the chamber at a controlled environmental temperature and relative humidity. The air will be passed through a honeycomb straightener to enable disturbance free flow to enable measurement of velocity. Then, the air will be passed through ceramic evaporators and allowed to exit through the bottom of the chamber. Water will then be supplied at two selectable pressure levels of 0.1 and 0.4 m head from a tank by a siphon system. Seeping water will be collected at the bottom of the duct.