The Seawater Air Conditioning Engineering Essay

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The feasibility of using cold seawater to directly cool buildings has been studied and analyzed for many years. At certain locations, successful installation and operation has occurred. The following is a brief recent history of seawater air conditioning which gives some facts and figures on SWAC:

In 1980, the Naval Material Command at Port Hueneme, California, conducted a study entitled : "Sea/Lake Water Air Conditioning at Naval Facilities.". The study concluded that at a hypothetical typical Navy facility, a SWAC system will use 80% less energy than conventional A/C, but the capital costs of SWAC systems are 60% greater.

In 1986, a joint project between the Canadian government and Purdy's Wharf Development, Ltd. demonstrated the use of ocean water as a source for building cooling to a 350,000 square ft. office complex along the waterfront in Halifax, Nova Scotia. Total investment for this project was $200,000. The project was very successful and savings were identified in the following areas: a saving of $50-60,000 per year in avoided electrical cost, fewer maintenance staff, reduction in fresh water, savings in water treatment, and savings in cooling tower maintenance and replacement. The financial result in terms of a simple payback period was two years.

In 1986, the Natural Energy Laboratory of Hawaii Authority, Keahole Point, Hawaii began the successful utilization of SWAC in their main laboratory building. Deep-water pipelines were already installed to provide cold, nutrient rich, seawater for research purposes in alternate energy and aquaculture. Since a cold water supply was already incorporated into the infrastructure, it was decided to utilize the cold water for cooling. This proved to be a very sound economic decision that resulted in monthly electric savings of $400. Today, the use of SWAC has been expanded to a new administration building and a second laboratory. Estimated monthly saving in electricity is $2000. There have been of course several other SWAC projects since 1986 up to now.

2.2 Heating Ventilating and Air Conditioning

HVAC stands for Heating, Ventilation, and Air-Conditioning-three closely related fundamental functions found in homes, offices, and other building structures. The HVAC system is also known as climate control. This is because these three functions are essential in maintaining comfort in every dwelling. The primary use of HVAC is to regulate room temperature, humidity, and air flow, ensuring that such elements remain within their acceptable ranges.

Effective control of such factors minimizes health-related risks. A very humid atmosphere impairs the body's ability to regulate body temperature as it prevents the evaporation of sweat. High humidity also decreases physical strength, which usually leads to fatigue. An unhealthy surrounding can also affect people's thinking abilities. Hypothermia, heat stroke, and hyperpyrexia, among others, are some of the illnesses that may also occur.

Three functions of HVAC Heating are significant in maintaining adequate room temperature especially during colder weather conditions. There are two classifications of heating: local and central. The latter is more commonly used because it is more economical. Furnace or boiler, heat pump, and radiator make up the heating system.

Ventilation, on the other hand, is associated with air movement. There are many types of ventilation, but they all function similarly. Ventilation is necessary to allow carbon dioxide to go out and oxygen to get in, making sure that people are inhaling fresh air. Stagnant air causes the spreading of sickness, usually airborne, and allergies. But it is also essential to maintain an efficient ventilation system, especially in the attics. Insufficient ventilation usually promotes the growth of bacteria and fungi such as molds because of high humidity. It will also decrease the effectiveness of rafter and roof sheathing insulation because of water vapor condensation.

The air-conditioning system controls the heat as well as ventilation. They often come in different sizes. Most air conditioners have large air ducts, so it is better to check out the building first to see if they can be installed. Or else, you can use the split system or remote coils. It is necessary, though, that air ducts are properly cleaned. Pathogens thrive in dirty air ducts. Return-air grills are also vulnerable to chemical, microbiological, and radiological elements. Thus, HVAC return-air grill height should be that it is not accessible but visible for any observation.

Hotel HVAC Systems

The various components that make up a complete HVAC system are shown in Figure 2.1. The chilled water pump and distributive water piping, the air handling devices and distributive duct work, as well as the various cooling coils, dampers, heaters and so on, are considered to be identical for all three options.

It is important to design each alternative air conditioning option to mimic the heat transfer characteristics of an evaporator in a typical vapor-compression air-conditioning system. By doing so, standard chilled water components, as well as cooling coils can be used throughout the hotel. It is assumed that any benefits that could be gained by straying from typical chilled water entering and leaving temperatures in a cold sea water heat exchanger, would not outweigh the high costs associated with specialized HVAC components throughout the HVAC and chilled water systems.

Figure 2.1: Schematic of a typical commercial air-conditioning system [Source: Adapted from CSW Air-Conditioning, 2010]

2.3 Air Conditioning Systems

2.3.1 Conventional Air Conditioner (Vapor Compression Refrigeration System)

The most common form of meeting a large building air-conditioning demand is by using a vapor compression air conditioning plant. A simplified schematic of a vapor-compression air conditioning system that is used to meet the air-conditioning load of a large hotel is shown in Figure 2.2.

Figure 2.2: Conventional Air Conditioner (Vapour Compression System)

A compressor is used to raise the pressure of a working fluid (refrigerant), in a gaseous state. This compressed refrigerant gas is then condensed in the condenser by exchanging heat with the ambient air, or through the use condenser water, circulated through a cooling tower (Qcond). The condensed gas is then throttled in an expansion valve, after which the throttled liquid is evaporated in the evaporator. The evaporation process absorbs heat from the environment (Qevap), providing a cooling effect. The refrigerant gas returns to the compressor to then repeat the cycle.

Typically, for large buildings, including hotels, chilled water systems are used to distribute cold water to various air-water cooling coils in the HVAC (Heating Ventilating and Air Conditioning) systems air ducting. The lines connecting the evaporator and the air-conditioning load represent the chilled water system. The electrical demand for this type of air-conditioning system is for the refrigerant compressor. Power is also required for pumps that circulate the chilled water from the evaporator throughout the hotel. Also, power is required to either blow air across the condenser or to circulate condensed water from the condenser to a cooling tower or other means of dumping heat to the environment.

2.3.2 Direct Cold Seawater Cooling System

A seawater air conditioning (SWAC) system is shown in Figure 2.3. The buildings to the far right are identical internally to buildings cooled with conventional A/C. Chilled fresh water moves through these buildings with the same temperatures and flows of conventional systems. A conventional chiller, however, does not cool the chilled water loop in this system. The low temperatures in the chilled water loop are maintained by passing this fresh water through a counter-flow heat exchanger with the primary fluid being deep cold seawater. The two fluids are on either side of a titanium plate heat exchanger that transfers the heat from one fluid to the other or do not mix.

Figure 2.3: Sea Water Air Conditioning

The seawater intake brings in water at a temperature lower than the temperature maintained in the chilled water loop. Once the seawater passes through the heat exchanger(s), it is returned to the ocean through another pipeline.

The operation of the system could be briefed as follows:

Water is pumped from a deep cold water source

The cooling station contain heat exchangers

A fresh water closed loop distribution system is pumped through the heat exchangers cooling the water.

Cooled water is pumped to buildings and used in air conditioning through fan coil units, eliminating the use of expensive compressors.

The main components of a basic seawater air conditioning system are the seawater supply system, the heat exchanger or cooling station and the fresh water distribution system. These basic components can be optimized for each specific location, climate and building. For a large building using conventional air conditioning system, a constant flow of cold fresh chilled water is circulated throughout the building for heat removal. As this chilled water moves throughout the building and absorbs heat, its temperature rises from an incoming value of approximately 7-8°C to an outflow value approximately 5°C higher. This chilled water then enters the chiller, a refrigeration system that cools the recirculating fresh water. Water enters the chiller at a nominal 12-13°C and exits at 7-8°C. The water flow through the building varies with demand and the temperature of the water leaving the chiller is constant.

Seawater air conditioning can be operated easily. It is an established technology being applied in an innovative way. All the components necessary exist and have been operated under the required conditions.

The two loops present in the SWAC system are shown in Figure 2.4.

Figure 2.4: Heat exchanger operation in Sea Water Air Conditioning

There are two loops in operation. One is an open loop system pumping deep cold water from the ocean through a heat exchanger and second is a closed chilled water distribution pumped between buildings and the heat exchanger. As the deep cold water is pumped through the heat exchanger, the warm water returning from the building air conditioning supply lines is cooled, thereby providing cold water for the fan coil units present in the building.

Environmental Aspects

A SWAC system has significant environmental benefits; these include significant reductions in electricity consumption which reduces air pollution and greenhouse gas production, and substitution of simple heat exchangers for chiller machinery which often use ozone-depleting chlorofluorocarbons (CFCs).

The existence of the deep water ocean heat sink results from natural climatic processes where water is cooled at the poles becomes dense and sinks to deeper water. Figure 2.5 illustrates a temperature profile in the tropics typical for the world's deep oceans. 7°C or colder can be reached at 700 m deep and 5°C or colder at 1000m. The deep-water portion of this profile changes little seasonally and therefore cold water is available on a year round basis.

The variation of seawater temperature with depth is shown in Figure 2.5. It could be seen that temperature decreases deeper in the ocean.

Figure 2.5: Depth versus Temperature of Sea Water [Source: The Wetware Crisis, 2008]

2.3.3 Cold Seawater Cooling System including use of an Auxiliary Chiller

In some cases, it is either too costly or impractical to supply seawater at the necessary low temperatures to maintain minimum temperatures in the chilled water loop. The distance offshore to reach sufficiently cold water might be prohibitive or the ocean depth may simply not be available. It is sometimes economically possible to use chillers to supplement the cooling provided by the seawater exposure. The typical arrangement of this system is shown in Figure 2.6.

Figure 2.6: Sea Water Air Conditioning assisted by chiller unit

The fresh chilled water is first cooled by seawater through a heat exchanger and then secondarily cooled with an auxiliary chiller. The auxiliary chiller is basically a refrigeration system with its condenser cooled by the returning flow of cool seawater. With the condenser kept cool, the auxiliary chiller can operate at an extremely high efficiency - as high as double that of a conventional chiller.

Chilled Water Storage Tank

A SWAC system has a high capital cost and a low operating cost. The peak capacity of the system must match the peak demand of the buildings that it serves. These demands are not constant throughout the day and the total system is frequently not being used to its maximum capacity. Therefore, capital is spent on a system that may not always be used to its maximum potential. A means of minimizing the capital cost is to use cold-water storage. The seawater air conditioning system would be operated 100 percent of the time and when the building demands are low; the excess capacity is directed into a storage system of cold fresh water. When A/C demand is at its peak, the cold water is drained from its storage to meet the demand. Cold water storage tanks are commercially available that are constant volume. These tanks are now used in conjunction with A/C systems to take advantage of low, off-peak electrical rates.

2.4 Fan Coil Unit

A fan coil unit (FCU) is a simple device consisting of a heating or cooling coil and fan. It is part of an HVAC system found in residential, commercial, and industrial buildings. A fan coil unit is not connected to ductwork, and is used to control the temperature in the space where it is installed, or serve multiple spaces. It is controlled either by a manual on/off switch or by thermostat.

Due to their simplicity, fan coil units are more economic to install than ducted or central heating systems with air handling units. However, they can be noisy because the fan is within the same space. Unit configurations are numerous including horizontal (ceiling mounted) or vertical (floor mounted).

A fan coil unit may be concealed or exposed within the room or area that it serves. An exposed fan coil unit may be wall mounted, freestanding or ceiling mounted, and will typically include an appropriate enclosure to protect and conceal the fan coil unit itself, with return air grille and supply air diffuser set into that enclosure to distribute the air.

A concealed fan coil unit will typically be installed within an accessible ceiling void or services zone. The return air grille and supply air diffuser, typically set flush into the ceiling, will be ducted to and from the fan coil unit and thus allows a great degree of flexibility for locating the grilles to suit the ceiling layout and/or the partition layout within a space. It is quite common for the return air not to be ducted and to use the ceiling void as a return air plenum.

The coil receives hot or cold water from a central plant, and removes heat from or adds heat to the air through heat transfer. Traditionally fan coil units can contain their own internal thermostat, or can be wired to operate with a remote thermostat. However, and as is common in most modern buildings with a Building Energy Management System (BEMS), the control of the fan coil unit will be by a local digital controller or outstation (along with associated room temperature sensor and control valve actuators) linked to the BEMS via a communication network, and therefore adjustable and controllable from a central point, such as a supervisors head end computer.

Fan coil units circulate hot or cold water through a coil in order to condition a space. The unit gets its hot or cold water from a central plant, or mechanical room containing equipment for removing heat from the central building's closed-loop. The equipment used can consist of machines used to remove heat such as a chiller or a cooling tower and equipment for adding heat to the building's water such as a boiler or a commercial water heater.

Fan coil units are divided into two types: Two-pipe fan coil units or four-pipe fan coil units. Two-pipe fan coil units have one supply and one return pipe. The supply pipe supplies either cold or hot water to the unit depending on the time of year. Four-pipe fan coil units have two supply pipes and two return pipes. This allows either hot or cold water to enter the unit at any given time. Since it is often necessary to heat and cool different areas of a building at the same time, due to differences in internal heat loss or heat gains, the four-pipe fan coil unit is most commonly used.

Depending upon the selected operating conditions, it is very likely that the cooling coil will be designed to dehumidify the entering air stream, and as a by-product of this process, it will at times produce a condensate which will need to be carried to drain. The fan coil unit will contain a purpose designed drip tray with drain connection for this purpose. The simplest means to drain the condensate from multiple fan coil units will be by a network of pipe work laid to falls to a suitable point. Alternatively a condensate pump may be employed where space for such gravity pipe work is limited.

Speed control of the fan motors within a fan coil unit is effectively used to control the heating and cooling output desired from the unit. This is normally achieved by manually adjusting the taps on an AC transformer supplying the power to the fan motor. Typically this is adjusted at the commissioning stage of the building construction process and is therefore set for life. Fan motors are typically AC type motors but more recently DC motors have been made available by some manufacturers and can potentially offer significant energy savings.

Figure 2.7: Fan Coil Unit [Source: Alpha Systems and Service, 2010]

2.5 Air Conditioning Cooling Load

The cooling load is an important parameter in designing sea water air conditioning systems. This will determine the required sea water flow rate and other important factors. To estimate the actual cooling load for a representative hotel in a humid climate, weather data for such climates was investigated. By investigating the outdoor air conditions, in conjunction with the desired indoor air conditions, a cooling load for a hotel of known volume could be estimated.

There is a range of conditions that constitute a comfortable environment. These are based on dry bulb temperature as well as wet bulb temperature or relative humidity. For this study the indoor conditions, representing a comfortable indoor environment, are taken to be 24oC dry bulb temperature, with a relative humidity of 50%. An air-water mixture at this condition has an enthalpy of 65.6 kJ/kg - dry air.

Infiltration of outside air through doors, windows, and so on has a sensible component. If the outdoor air is more humid than the indoor desired conditions, it also has a latent component. People inside the building contribute to sensible and latent heat to the building air. Transmission of sensible heat through the building exterior surfaces due to the temperature difference between inside and outside conditions also affect the cooling load. Solar loading also provides sensible heat to the building. However this is not an instantaneous load like a transmission load. Internal equipment also contributes to sensible load and depending on the type of equipment, it can also contribute to latent load to the environment. An indoor pool is an example of an item that would contribute a sizable latent load by continuously evaporating water into the interior of the building. Replenishment air provides a sensible and latent load to the environment. This is outside air brought into the environment by the ventilation system to meet fresh air requirements. This air must be cooled and dehumidified as necessary.

It was understandable that determining this load is an extremely involved task due to numerous factors affecting it. For the purpose of this study, an alternative method of estimating this load was used instead of determining each component of the cooling load separately.

2.6 Operation of Auxiliary Chiller

The chiller works according to a typical refrigeration cycle. In the refrigeration cycle, a heat pump transfers heat from a lower-temperature heat source into a higher-temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerator works in much the same way, as it removes heat out of the interior and into the room in which it stands.

This cycle takes advantage of the way phase changes work, where latent heat is released at a constant temperature during a liquid/gas phase change, and where varying the pressure of a pure substance also varies its condensation/boiling point.

Figure 2.8: Refrigeration Cycle

The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile, the compressor is driven by a belt over a pulley, the belt being driven by the engine's crankshaft (similar to the driving of the pulleys for the alternator, power steering and so on). Whether in a car or building, both use electric fan motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners use a compressor to cause pressure changes between two compartments, and actively condense and pump a refrigerant around. A refrigerant is pumped into the evaporator coil, located in the compartment to be cooled, where the low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. At the opposite side of the cycle is the condenser, which is located outside of the cooled compartment, where the refrigerant vapor is compressed and forced through another heat exchange coil, condensing the refrigerant into a liquid, thus rejecting the heat previously absorbed from the cooled space. The components of the refrigeration cycle have been shown in Figure 2.8.

By placing the condenser, where the heat is rejected, inside a compartment and the evaporator, which absorbs heat, in the ambient environment or merely running a normal air conditioner refrigerant in the opposite direction, the overall effect is the opposite, and the compartment is heated. This is usually called a heat pump, and is capable of heating a home to comfortable temperatures of 25° C, even when the outside air is below the freezing point of water.

Cylinder unloaders are a method of load control used mainly in commercial air conditioning systems. On a semi-hermetic (or open) compressor, the heads can be fitted with unloaders which remove a portion of the load from the compressor so that it can run better when full cooling is not needed. Unloaders can be electrical or mechanical.

Air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold (below the dew point) evaporator coil condenses water vapor from the processed air, much as a cold drink will condense water on the outside of a glass. The water is drained, removing water vapor from the cooled space and thereby lowering its relative humidity. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space. In food retail establishments, large, open chiller cabinets act as highly effective dehumidifiers.

Some air conditioning units dry the air without cooling it. These work like a normal air conditioner, except that a heat exchanger is placed between the intake and exhaust. In combination with convection fans, they achieve a similar level of comfort as an air cooler in humid tropical climates, but only consume about one-third the energy. They are also preferred by those who find the draft created by air coolers uncomfortable.

2.7 Refrigerant used in Chiller

A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a gas to a liquid. Traditionally, fluorocarbons, especially chlorofluorocarbons were used as refrigerants, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications are ammonia, sulfur dioxide, and non-halogenated hydrocarbons such as methane.

Refrigerants were commonly used due to their superior stability and safety properties. However, the chlorine-bearing refrigerants reach the upper atmosphere when they escape. Once the refrigerant reaches the stratosphere, UV radiation from the Sun cleaves the chlorine-carbon bond, yielding a chlorine radical. These chlorine atoms catalyze the breakdown of ozone into diatomic oxygen, depleting the ozone layer that shields the Earth's surface from strong UV radiation. Each chlorine radical remains active as a catalyst unless it binds with another chlorine radical, forming a stable molecule and breaking the chain reaction. In most countries the manufacture and use of CFCs has been banned or severely restricted due to concerns about ozone depletion. Newer and more environmentally-safe refrigerants such as HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydro fluorocarbons (HFCs) such as R-410A, which lack chlorine. Carbon dioxide (R-744) is being rapidly adopted as a refrigerant in Europe and Japan. R-744 is an effective refrigerant with a global warming potential of 1. It must use higher compression to produce an equivalent cooling effect.

For the chiller unit considered in the study, the refrigerant R-410A is used. Below are some properties of R-410A.

Table 2.1: Properties of R-410A [Source: Adapted from HVAC Basics, 1998]

Property

Value

Formula

50% CH2F2 / 50% CHF2CF3

Molecular Weight

72.6

Melting Point (oC)

-155.0

Boiling Point (oC)

-48.5

Liquid Density at 30 oC (kg/m3)

1040.0

Vapour Density at 30 oC

3.0

Vapour Pressure at 21 oC (MPa)

1.4

Critical Temperature (oC)

72.8

Critical Pressure (MPa)

4.9

Gas Heat Capacity at 1 atm & 30 oC (kJ/kg oC)

0.9

Liquid Heat Capacity at 1 atm & 30 oC (kJ/kg oC)

1.8

2.8 Economic Benefits

A SWAC system has significant environmental benefits: These include significant reductions in electricity consumption which reduces air pollution and greenhouse gas production, and substitution of simple heat exchangers for chiller machinery which often use ozone-depleting chlorofluorocarbons (CFCs).

There are significant secondary applications for this seawater. Secondary cooling, aquaculture, desalination and even agriculture can benefit from the cold seawater. Aqua culturists value the water because it is clean and disease free. When used in conjunction with a warm source of water, they can have any temperature seawater their product needs. Secondary cooling can be used in greenhouses and other locations where humidity control is not a major factor. Finally, research in Hawaii has shown that even an arid land can be made highly productive with low fresh water consumption by cooling the soil and the roots of many tropical and non-tropical plants. Deep seawater is also desalinated and sold as a premium drinking water in the orient.

The economic viability of a SWAC system is site specific. Each location has unique advantages as well as disadvantages. The main factors influencing the economic viability of a specific location include:

• The distance offshore to cold water; shorter pipelines are more economical than long pipelines.

• The size of the air conditioning load; there is an economy of scale associated with SWAC - systems less than 1000 tons A/C (3.5 GW) capacity are more difficult to justify economically,

• The percent utilization of the air conditioning system; the higher the utilization throughout the year, the higher the direct benefits.

• The local cost of electricity; a high cost of electricity makes conventional AC more costly and SWAC, in comparison, more attractive. Any cost analysis should include current and future costs of electricity.

• The complexity of the distribution system on shore; SWAC works best with a district cooling arrangement, where many buildings are cooled taking advantage of the economy of scale. SWAC is even more economical if this distribution system is compact.

Energy

Operation & Maintenance Costs

Capital

Figure 2.9: Cost for Conventional and SWAC system [Source: Adapted from Sea Water Air Conditioning, 2004]

Figure 2.9 shows the difference in lifetime costs for a conventional AC system and a typical SWAC system. The costs could be broken down into capital, operating (energy) and maintenance. The primary cost of a SWAC system is in the initial capital cost. The operating and maintenance costs are small. For a conventional AC system, the primary cost is in the power consumed over its lifetime. Hence, SWAC systems are ideal for base load A/C that has high utilization and conventional AC may be better for situations of infrequent use.

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