Cooling Systems and Thermal Energy Storage

Transcription

1 Cooling Systems and Thermal Energy Storage Kent Peterson, PE, LEED AP Sponsored By Cooling Systems and Thermal Energy Storage Page 1 Copyright APPA 2009

2 Published by APPA: APPA is the association of choice serving educational facilities professionals. APPA's mission is to support educational excellence with quality leadership and professional management through education, research, and recognition. Reprint Statement: Except as permitted under copyright law, no part of this chapter may be reproduced, stored in a retrieval system, distributed, or transmitted in any form or by any means - electronic, mechanical, photocopying, recording, or otherwise - without the prior written permission of APPA. From APPA Body of Knowledge APPA: Leadership in Educational Facilities, Alexandria, Virginia, 2009 This BOK is constantly being updated. For the latest version of this chapter, please visit This chapter is made possible by APPA 1643 Prince Street Alexandria, Virginia Copyright 2009 by APPA. All rights reserved. Cooling Systems and Thermal Energy Storage Page 2 Copyright APPA 2009

3 Cooling Systems and Thermal Energy Storage Introduction Central cooling systems are an attractive method for cooling individual buildings by connecting them onto a common cooling loop. Central cooling systems can displace several small localized chillers with central systems that have options that are not feasible at individual sites. Facilities managers should consider a number of factors when evaluating the merits of central cooling systems. These factors include annual operating costs, annual use of energy and domestic water, the age and condition of the existing system, and availability of capital. Advantages of Central Cooling Systems Central chiller plants offer several advantages over individual building air conditioning systems. Central chiller plants can take advantage of the diversity factor in the sizing of equipment and operation of the plant, as all connected loads will not peak at the same time. Less capital is required for central cooling equipment than for equipment in many individual buildings. Operations and maintenance (O&M) staffing costs are minimized and easier to control due to the centralized equipment location. Increased efficiencies are possible with large heating and cooling equipment, which reduces operating cost per unit of energy output. Part-load performance efficiencies are substantially improved by the ability to meet the system load with the most efficient equipment. Continuous and accurate monitoring of operating efficiencies is practical when the equipment is centralized. Single-point delivery of purchased utilities allows for favorable rates to a large-volume customer. Multiple fuel sources can also be a practical alternative. Disadvantages of Central Cooling Systems Central systems present some disadvantages over cooling equipment distributed in individual buildings. Thermal and hydraulic losses occur in large-distribution networks. These losses must be evaluated against the increased generating efficiencies of a central plant. The initial construction cost requires a large capital investment. Therefore, the most cost-effective options may have to be deferred if capital cannot be secured. Central Plant Design Major equipment such as chillers, pumps, and cooling towers should not be purchased without a set of detailed specifications on the items being purchased. These specifications establish the minimum quality and the maximum energy consumption that will be acceptable for each item. The equipment should not be accepted until it is tested to demonstrate compliance for capacity and performance. It is important to remember that the efficiency of a central plant is measured by dividing the energy supplied by all the energy inputs (chillers, pumps, and cooling towers). The goal is to deliver the required cooling throughout the year with the least amount of energy input. It is therefore necessary to understand how the central cooling plant will perform at all anticipated load conditions. A thorough understanding of the major equipment used in these plants is required to maximize the performance and economic benefits of upgraded or new chilled water plants. This chapter provides an in-depth look at this equipment, as well as essential information on how the components relate to Cooling Systems and Thermal Energy Storage Page 3 Copyright APPA 2009

4 one another, how they are controlled, and their physical and operational limitations. This chapter discusses the basic refrigeration cycle; the components commonly used in commercially available packaged water chillers; methods of heat rejection, with an emphasis on cooling towers and air-cooled refrigerant condensers; the characteristics of different types of pumps; pump and system curves, with an emphasis on understanding the nature of variable-speed pumping. Refrigeration Cycle The refrigeration cycle, also known as the Carnot cycle, is the fundamental thermodynamic basis for removing heat from buildings and rejecting it to the outdoors. The cycle s four basic components are the compressor, evaporator, condenser, and throttling device. The refrigeration cycle diagram shows the relationship of these components, as does the pressure-enthalpy chart, also known as a P-H or Mollier chart. The following is a description of the refrigeration cycle: Starting at point 3, the refrigerant is a liquid at high pressure. As it passes through the throttling device (also called the thermal expansion valve and orifice) to point 4, the pressure drops. At point 4, the refrigerant is a mixture of liquid and gas. At this point the gas is called flash gas. At point 3, the liquid refrigerant upstream of the throttling device has been cooled to a temperature below saturation. This effect is called subcooling and has the effect of reducing the amount of flash gas, as shown by point 4. From point 4 to point 1 the liquid is converted to a gas by absorbing heat (refrigeration effect). From point 1 the refrigerant is drawn into the suction of the compressor where the gas is compressed, as shown by point 2. At point 2, the temperature and pressure of the gas have been increased. The refrigerant is now called hot gas. Notice that this point is to the right of the saturation curve, which also represents a superheated state. The hot gas, point 2, moves into the condenser where the condensing medium (either air or water) absorbs heat and changes the refrigerant from a gas back to a liquid as shown by point 3. At point 3, the liquid is at an elevated temperature and pressure. The liquid is forced through the liquid line to the throttling device and the cycle is repeated. Cooling Systems and Thermal Energy Storage Page 4 Copyright APPA 2009

5 Figure 1. The Refrigeration Cycle and Enthalpy Chart Chillers Many chiller types are available to match the specific needs of a given chiller plant. Factors affecting chiller selection include energy sources, prime movers, physical size, load requirements, anticipated load profile throughout the year, and refrigerant selections. Energy sources, prime movers, and refrigerants are discussed separately later in this subchapter. Types The three types of chillers most commonly used in central chiller plants are rotary, centrifugal, and absorption. A wide variety of chiller combinations can occur, especially in plants that have undergone multiple phases of expansion. Rotary Screw Type Chillers There are a number of types of rotary compressors used in the heating, ventilation, and air Cooling Systems and Thermal Energy Storage Page 5 Copyright APPA 2009

6 conditioning (HVAC) industry, including scroll, single blade (fixed vane), rotating vane, and screw (helical-rotary). Single blade and rotating vane compressors are generally used in smaller applications and will not be discussed further here. Scroll compressors are largely replacing reciprocating compressors for the smaller chiller sizes (although there are scroll machines up to 400 tons in capacity). In packaged water chillers the most commonly used compressor is the screw. There are two types in use today: the single screw and the multiple screw. Single Screw The single screw consists of a single cylindrical main rotor that works with a pair of gaterotors. The compressor is driven through the main rotor shaft and the gaterotors follow by direct meshing action. As the main rotor turns, the teeth of the gaterotor, the sides of the screw, and the casing trap refrigerant. As rotation continues, the groove volume decreases and compression occurs. Since there are two gaterotors, each side of the screw acts independently. Single-screw compressors are noted for long bearing life as the bearing loads are inherently balanced. Some single-screw compressors have a centrifugal economizer built into them. This economizer has an intermediate pressure chamber that takes the flash gas (via a centrifugal separator) from the liquid and injects it into a closed groove in the compression cycle, with the result of increased efficiency. The single screw is controlled from a slide valve in the compressor casing that changes the location where the refrigerant is introduced into the compression zone. This causes a reduction in groove volume and hence the volume of gas varies (variable compressor displacement). The machines are fully modulating. The single screw has slide valves on each side that can be operated independently. This allows the machine to have a very low turndown with good part-load energy performance. Twin Screw The twin screw is the most common of the multiple screw compressors. The twin screw is the common designation for double helical rotary screw compressor. The twin screw consists of two mating helically grooved rotors, one male and the other female. Either the male or female rotor can be driven. The other rotor either follows the driven rotor on a light oil film or is driven with synchronized timing gears. At the suction side of the compressor, the gas fills a void between the male and female rotors. As the rotors turn, the male and female rotors mesh and work with the casing to trap the gas. Continued rotation decreases the space between lobes and the gas is compressed. The gas is discharged at the end of the rotors. The twin screw has a slide valve for capacity control, located near the discharge side of the rotors, which bypasses a portion of the trapped gas back to the suction side of the compressor. VSD Controls In recent years manufacturers have introduced screw chillers with variable-speed drive (VSD) controls. In addition to great part-load performance, these chillers offer significantly reduced noise and wear at off-design conditions. Centrifugal Chillers Centrifugal chillers are by far the most popular chiller type in central cooling systems. They are available in sizes ranging from 90 to 10,000 tons. They are simple to operate, reliable, compact, and relatively quiet; have low vibration; and are designed for long life with low maintenance. Centrifugal compressors are dynamic compression devices (as opposed to positive displacement) that on a continuous basis exchange angular momentum between a rotating mechanical element and Cooling Systems and Thermal Energy Storage Page 6 Copyright APPA 2009

7 a steadily flowing fluid. Like centrifugal pumps, centrifugal chillers have an impeller that rotates at high speed. The molecules of refrigerant enter the rotating impeller in the axial direction and are discharged radially at a higher velocity. The dynamic pressure of the refrigerant obtained by the higher velocity is converted to static pressure through a diffusion process that occurs in the stationary discharge or diffuser portion of the compressor just outside the impeller. A centrifugal compressor can be single stage (having only one impeller) or multistage (having two or more impellers). On a multistage centrifugal compressor, the discharge gas from the first impeller is directed to the suction of the second impeller, and so on for as many stages as there are. Like the rotary compressor, multistage centrifugals can incorporate economizers, which take flash gas from the liquid line at intermediate pressures and feed this into the suction at various stages of compression. The result is a significant increase in energy efficiency. Like reciprocating compressors, centrifugal compressors can be either open or hermetic. Open centrifugal compressors have the motors located outside the casings with the shaft penetrating the casing through a seal. Hermetic centrifugal compressors have the motor and compressor fully contained within the same housing, with the motor in direct contact with the refrigerant. Because the discharge pressure developed by the compressor is a function of the velocity of the tip of the impeller, for a given pressure, the faster impeller speed, the smaller the diameter needs to be. Similarly, for a given pressure, the more stages of compression there are, the smaller the impeller diameter needs to be. With these variables in mind, some manufacturers have chosen to use gear drives to increase the speed of a smaller impeller, while other manufacturers use direct drives with larger impellers and/or multiple stages. There are pros and cons to both systems, but direct drive machines have fewer moving parts and fewer bearings and are generally simpler machines. The capacity of centrifugal compressors is controlled by three methods. The most common is to use inlet guide vanes or prerotation vanes. The adjustable vanes are located in the suction line at the eye of the impeller and swirl the entering refrigerant in the direction of rotation. This changes the volumetric flow characteristics of the impeller, providing the basis for unloading. A second control method is to vary the speed of the impeller in conjunction with using inlet guide vanes. Not unlike a variable-speed fan or pump, reducing the impeller speed produces extremely good part-load energy characteristics. The impeller must produce an adequate pressure differential (lift) to move the refrigerant from the low-pressure side (evaporator) to the high-pressure side (condenser). It is this lift that determines the minimum speed of the impeller. The lower the lift, that is, the closer the temperature difference between the evaporator and condenser, the more slowly the impeller can rotate. When the impeller is at the slowest possible speed, further reductions in capacity are obtained by using the inlet guide vanes. With VSDs and aggressive water temperature reset schedules, centrifugal compressors can produce the most energy-efficient part-load performance of any refrigerant compressor. It is important to note that centrifugal compressors with VSDs use both the VSD and inlet vanes for control. The inlet vanes are used to prevent the chiller from getting into surge. For efficient operation the controls must either dynamically measure or model surge so that the bulk of the unloading can be done by the VSD. This is particularly important with primary-only variable-flow plants as some manufacturers' systems use load as an input to the surge map and they measure only temperature and not flow. There have been cases where VSD minimums have to be set at 40 percent or higher to prevent the chiller from tripping from surge at low flows. As described for the screw chillers above, centrifugal chillers with VSDs have both lower noise and reduced wear at off-design conditions. A third method of capacity control for the centrifugal chiller is hot gas bypass (HGBP). Like other types of compressors, HGBP can be used to unload a machine to zero load by directing the hot gas from the compressor discharge back into the suction. There are no part-load energy savings with HGBP. It is used only as a last resort when very low turndown is required and cycling the machine on/off would not produce acceptable results. Cooling Systems and Thermal Energy Storage Page 7 Copyright APPA 2009

8 One of the characteristics of the centrifugal compressor is that it can surge. Surge is a condition that occurs when the compressor is required to produce high lift at low volumetric flow. Centrifugal compressors must be controlled to prevent surge, and this is a limit on part-load performance. During a surge condition, the refrigerant alternately moves backward and forward through the compressor, creating a great deal of noise, vibration, and heat. Prolonged operation of the machine in surge condition can lead to failure. Surge is relatively easy to detect in that the electrical current to the compressor will alternately increase and decrease with the changing refrigerant flow. Just before going into surge, the compressor may exhibit a property called incipient surge in which the machine gurgles and churns. This is not harmful to the compressor but may create unwanted vibration. The electrical current does not vary during incipient surge. Absorption Chillers Absorption chillers are used in many central plant applications. They are available in sizes ranging from 100 to 1,500 tons and are usually operated by low-pressure steam or hot water or are directly fired with natural gas. The higher O&M costs associated with these chillers, compared with those of compressor-type units, usually make them uneconomical to use except where electric power is expensive, where fuel costs are low, or when balancing electrical and thermal loads in cogeneration facilities. The absorption process, while appearing quite complex, is in reality the same refrigeration process discussed previously except the compressor has been replaced with an absorber, generator, pump, and recuperative heat exchanger. The following description is based on lithium-bromide and water, which is the most common process among several possibilities. In the absorption refrigeration cycle, the low-pressure (high vacuum) refrigerant (water) in the evaporator migrates to the lower pressure absorber where it is soaked up by a solution of lithium-bromide. While mixed with the lithium-bromide the vapor condenses and releases the heat of vaporization picked up in the evaporator. This heat is transferred to condenser water and rejected out the cooling tower. The lithium-bromide and refrigerant solution (weak solution) are pumped to a heat exchanger (generator) where the refrigerant is boiled off and the lithium-bromide (strong solution) returns to the evaporator. As the hot lithium-bromide (strong solution) returns to the evaporator, a heat exchanger cools the liquid with the cool mixture of lithium-bromide and refrigerant (weak solution). The boiled-off refrigerant migrates to the cooler condenser, where it is condensed back into a liquid and returned to the evaporator to start the cycle again. Absorption machines can be direct-fired or indirect-fired. The direct-fired absorber has an integral combustion heat source that is used in the primary generator. An indirect-fired absorber uses steam or hot water from a remote source. Cooling Systems and Thermal Energy Storage Page 8 Copyright APPA 2009

9 Figure 2. Absorption Refrigeration Cycle A double-effect absorption process is similar to that described above except that a generator, condenser, and heat exchanger are added. The refrigerant vapor from the primary generator runs through a heat exchanger (secondary generator) before entering the condenser. The secondary generator with the hot vapor on one side of the heat exchanger boils some of the lithium-bromide and refrigerant solution (weak solution), creating the double effect. The double-effect absorption process is significantly more energy efficient than the single-effect absorption process. Cooling Systems and Thermal Energy Storage Page 9 Copyright APPA 2009

10 Figure 3. Double-Effect Absorption The lithium-bromide is a salt with a crystalline structure that is soluble in water. If the saturation point of the solution is exceeded, the salt will precipitate out and form a slush-like mixture that can plug pipes and make the machine inoperable. Crystallization does not harm the equipment but is a nuisance. Usually air leakage or improper (too cold) temperature settings cause crystallization. However, crystallization is generally not a problem in modern equipment that uses microprocessor-based controls. The microprocessor continuously monitors solution concentration and automatically purges the system. Absorption machines are controlled by modulating the firing rate of the direct-fired machine or modulating the flow of steam or hot water in the indirect-fired machine. Variable-speed refrigerant and solution pumps greatly enhance the controllability of the absorption machine. Cooling Systems and Thermal Energy Storage Page 10 Copyright APPA 2009

11 Refrigerants To work properly, refrigerants must have low toxicity, low flammability, and a long atmospheric life. Recently, refrigerants have come under increased scrutiny by scientific, environmental, and regulatory communities because of the environmental impacts attributed to their use. Some refrigerants particularly chlorofluorocarbons (CFCs) are believed to destroy stratospheric ozone. The relative ability of a refrigerant to destroy stratospheric ozone is called its ozone depletion potential (ODP). CFCs are being phased out according to the 1987 Montreal Protocol. The production of CFCs in developed countries ceased in 1995, and their most common replacement, halogenated chlorofluorocarbons (HCFCs), are due for phase-out in the 21st century. Replacements are currently being developed for HCFC R-123 and HCFC R-22, which are commonly used in the industry. For all practical purposes, however, HCFCs should be available well into the middle of the 21st century and certainly within the lifetimes of machines currently being manufactured. The global warming potential (GWP) of refrigerants is another significant environmental issue. Gases that absorb infrared energy enhance the greenhouse effect in the atmosphere, leading to the warming of the earth. Refrigerants have been identified as greenhouse gases. Theoretically, the best refrigerants would have zero ODP and zero GWP, like R-717 (ammonia). Although some refrigerants used in a particular system may have a direct effect on global warming, there will also be an indirect effect on global warming as a result of that system s energy consumption. The indirect effect is caused by the burning of fossil fuels and subsequent release of carbon dioxide. To reduce greenhouse gases to the greatest extent possible, it is critical to focus on the system s overall energy efficiency, not just to consider the refrigerant s GWP. When comparing the theoretical and practical efficiencies of different refrigerants, it becomes apparent that there are only slight differences among various refrigerants, with R-123 being somewhat better than the refrigerants it is designed to replace. Calm et al. (1997) report that efficiency is not an inherent property of the refrigerant, but rather achieving the highest efficiencies depends on optimization of the system and individual components for the refrigerant. Prime Movers Several different types of prime movers are used to drive chillers. Electric motors and steam turbines are the most common, but reciprocating engines and gas turbines have also been used, with varying degrees of success. Commercial cooling places the highest summertime demand on the electrical systems and the second highest annualized energy consumption on electrical utilities. Growth in cooling is expected to increase significantly in upcoming years, and electrical utilities will have difficulty meeting these demands if the chilling equipment is driven primarily by electric sources without thermal energy storage. That is why the utility companies have provided, and will continue to provide, incentives to encourage their customers to look at thermal energy storage systems and district cooling alternatives. Steam Turbines The steam turbine drive is excellent for larger capacity chillers, because it is a smoothly rotating power source, is available in all horsepower ranges, and can usually match the compressor's design speed without using a speed-increasing gear. Steam turbine drives are sometimes selected to make use of existing boilers in a central heating plant. Using the existing boilers saves on the capital cost, improves the year-round load factor on the steam-generating equipment, and takes advantage of possible reductions in off-season fuel rates. Cooling Systems and Thermal Energy Storage Page 11 Copyright APPA 2009

12 Electric Motors Electric motors are the most common drives on centrifugal chillers, especially with hermetics. However, hermetics are available in sizes only up to about 1,850 tons, so conventional-type motors are used on open-drive centrifugal chillers in the larger sizes. Electric motor drives offer several advantages but also have limitations. Synchronous electric motors run at exact speeds: 3,600 rpm, 1,800 rpm, 1,200 rpm, 900 rpm, and so on. Induction-type motors run at slightly less than synchronous speeds, depending on the slip. However, centrifugal compressors operate most efficiently at speeds much higher than the available motor speeds, so it is necessary to provide a speed-increasing gear between the motor and the compressor. The speed-increasing gear imposes additional frictional losses and additional equipment that must be maintained. Gear losses may amount to 1 to 2 percent of the required compressor horsepower. It is sometimes possible to eliminate the speed-increasing gear by selecting a two-stage compressor that operates at 3,600 rpm. This speed may be less than optimum for the compressor but will not result in as much loss as a gear would. Also, 3,600-rpm motors in 2,000-plus horsepower sizes are not off-the-shelf items; each is custom designed and manufactured to meet the requirements of the application. Motors in the large horsepower sizes can be manufactured to operate at practically any voltage. However, economics will be a key factor in the selection of the voltage. The voltage is usually related to the horsepower: the higher the horsepower, the higher the voltage. Normal voltage ranges are as follows: Horsepower Range Voltage , ,000 2,400 5,000 5,000 10,000 5,000 12,000 Existing electrical service must be considered when selecting the voltage for a new large-capacity chiller. It may be necessary to bring in a new feeder or to change transformers. In many cases the central chiller plant will be the largest electrical load on campus, and the plant may become the focal point for the incoming utilities. Thermal Energy Storage As global economies strive to reduce dependence on fossil fuels renewable energy and the smart grid will have an impact on the way electricity is generated, delivered, consumed, and purchased. This transformation from fossil fuels to renewable energy such as wind and solar energy will require some effort. The current electric grid has no energy storage component. The electric grid works today because the storage component is from the fuel itself. Fossil fuels are a form of stored energy. The sun and Mother Nature took millions of years to create and ultimately store energy in the form of fossil fuels. The stored energy is not released until the fuel is ignited and burned. Replacing fossil fuels with renewable energy for electricity generation will require a storage component to make the grid reliable. Energy storage will also help the economic viability of renewable energy projects by increasing the usable output of renewable energy generation facilities. There are many types of energy storage technologies. Some technologies can be applied on a large utility grade scale. Pumped hydro, long duration flywheels, compressed air storage, and sodium sulfur (NaS) battery storage are some of the technologies being pursued at a grid scale level. There are also technologies on the customer side of the meter that can be used to store energy as well. Lithium ion batteries, lead acid batteries, and thermal energy storage (TES) can store energy on a building level scale very effectively. Cooling Systems and Thermal Energy Storage Page 12 Copyright APPA 2009

13 Thermal energy storage is an effective solution for storing energy because its cycle effectiveness is better than most of the grid scale storage options and the stored energy form is designed for a specific purpose: peak load shifting of inductive motor loads used to provide cooling. These motor loads are the very loads contributing to the peak demand issues most utilities are experiencing in the summer. Another benefit of TES is that it is dramatically cheaper to store cooling than it is to store an electron that will be used to create cooling. Black and Veatch, in their energy newsletter last year, showed estimated installed costs of $4,000/kW for pumped hydro and $4,500/kW for NaA battery storage on grid scale applications. Flywheels can be installed for about $2,000/kW. Thermal energy storage can be installed for about $1,500/kW. Today s thermal energy storage market consists of two major types, chilled water storage and ice storage. Either technology is a great energy storage choice for university facilities that need air conditioning. Both technologies have a large installed base with proven reliability and performance. Each technology has is strengths and weaknesses relative to each other. The technology chosen will depend upon specific job conditions and economics. Thermal energy storage systems in their simplest form consist of a mechanical cooling device, a storage tank, a cooling load, and a few more cooling system controls. At night the mechanical cooling device will use nighttime electricity to charge the storage tank, either with ice or chilled water. During the day, the stored ice or chilled water will cool the building completely (full storage) or augment a smaller sized cooling system (partial storage) to cool the building. Water storage systems are generally applied at larger facilities where the economies of scale can decrease the cost of the water storage tank on a per unit basis. Additionally, facilities that have existing chillers and facilities like data centers, which need large amounts of cooling quickly, are great water storage candidates. The water storage tanks can be above ground or below ground and can be specifically designed for a certain footprint, height, and architectural look. Some of the storage systems can also be used to meet fire safety requirements. Some typical applications for water storage TES systems are college campuses, distribution centers, and manufacturing facilities. The economics of commercial ice storage systems allow these TES systems to be applied to a broader range of applications. Because of the heat of fusion the volume for storing energy is smaller with ice storage TES. Churches, K-12 schools, office buildings, community colleges and universities can benefit from ice storage systems. The ice storage tanks are generally factory assembled while site work is taking place at the project location so ice applications projects are up and running faster. Modular ice storage systems can offer a great deal of safety factors because there are more tanks, and some tanks, depending upon the manufacturer and type, can be redeployed for use at another location or sold to another user. Ice storage systems do require a mechanical cooling device that can make ice. This is a disadvantage in an existing facility with new cooling systems as they may not be able to create the cold temperature to freeze water. Commercial TES systems are affordable with paybacks in less than five years in new construction applications and about seven years or less in retrofit applications. Thermal energy storage systems have been around for some time. Today, TES equipment is better than ever. Application, design, and control best practices have been redefined and developed to provide reliable and affordable energy storage cooling systems. Owners and operators of iconic urban skyscrapers and college campus are using energy storage systems to lower cooling costs by 20 40% and reduce peak demand. Today s TES systems require little maintenance because they have few moving parts, if any, and can last twenty-five years or more. Utility and merchant generators are promoting TES to reduce peak demand while helping renewable energy generation to become more viable. TES offers affordable energy storage on the customer side of the meter. TES can be operational quickly with great cycle efficiencies and continual performance throughout its lifetime. As the infrastructure moves to more renewable energy and a smart grid, energy storage will play an enormous role helping this major transition is electricity generation and delivery. Storage, both grid scale and building scale, will be needed to make this transition reliable and fiscally sound. Often utilities will jump to a grid scale only solution when other more viable solutions are available. Cooling Systems and Thermal Energy Storage Page 13 Copyright APPA 2009

14 Pumps In the chilled water plant, centrifugal pumps are the prime movers that create the differential pressure necessary to circulate water through the chilled and condenser water distribution system. In the centrifugal pump a motor rotates an impeller that adds energy to the water after it enters the center (eye). The centrifugal force coupled with rotational (tip speed) force imparts velocity to the water molecules. The pump casing is designed to maximize the conversion of the velocity energy into pressure energy. In the HVAC industry most pumps are single-stage (one impeller) volute-type pumps that have either a single inlet or a double inlet (double suction). Axial-type pumps have bowls with rotating vanes that move the water parallel to the pump shaft. These pumps are likely to have more than one stage (bowls). The vertical turbine pump is an example of an axial-type pump and is sometimes used in a cooling tower sump application. Double-suction pumps are more likely to be used in high-volume applications, but either a single-inlet or double-inlet pump is available with similar performance characteristics and efficiencies. Variable-speed motors should be considered for the chilled water system. Pump outputs can be adjusted to match required system flows without overpressurizing the system, which improves the overall operating efficiencies. Like chilled water pumps, condensing water pumps can be end-suction, horizontal double-suction, or vertical turbine pumps. If horizontal double-suction or end-suction pumps are used, then the cooling tower basin must be at an elevation that is sufficient to provide a positive head on the suction side of the pumps. Vertical turbine pumps tend to be the preferred type for the larger tonnage cooling towers. The vertical turbine pumps allow most of the basin to be located below grade, which in turn improves accessibility to the pump motors and cooling tower screens. The sump pits associated with the vertical turbine pumps should be designed in strict accordance with the recommendations of the Hydraulic Institute. If end-suction or horizontal double-suction pumps are selected for the condensing water system, it will be mandatory to elevate the cooling tower basin or locate the pumps in a pit to provide sufficient head to the suction of the pumps. Because the head is critical, it is not advisable to use Y-type strainers on the suction of these pumps. It is best to use a rough screen inside the tower basin and then pump through a Y-type strainer to remove the small debris. Pump seals are either mechanical or packing gland-type seals. Mechanical seals are adequate, provided the water is clean and the water treatment is compatible with the seal material. Replacing a mechanical seal is more difficult and time-consuming than repacking a gland-type seal. The packing gland seals depend on friction between the shaft and the packing material to prevent leakage. To prevent damage to the shaft, the packing should not be too tight. It should allow one or two drops of water to leak each minute, which will provide cooling and lubrication to the shaft at this point. Pump Performance Curves For a given impeller size and rotational speed, the performance of a pump can be represented on a head-capacity curve of total developed head in feet of water versus flow in gallons per minute. Total dynamic head (TDH) is the difference between suction and discharge pressure and includes the difference between the velocity head at the suction and discharge connection. Starting from zero flow, as the pump delivers more water, the mechanical efficiency of the pump increases until a best efficiency point (BEP) is reached. Increasing the flow further decreases the efficiency until a point where the manufacturer no longer publishes the performance (end of curve). Pump performance curves are a family of curves for different size impellers. Notice that as the impellers get smaller, the pump efficiency decreases. The power (horsepower) requirements are also shown on the performance curve; notice that the power lines cross the pump curve until one value does not cross. This value is called nonoverloading horsepower because operation at any point on the published pump curve will not overload the motor. Finally, information on the net positive suction head Cooling Systems and Thermal Energy Storage Page 14 Copyright APPA 2009

15 required (NPSHR) is shown on the pump curve. This will be discussed in greater detail below. Pump curves are also rated as steep or flat. The definition of a flat curve pump is when the pressure from shut-off head (head at zero flow) to the pressure at the BEP does not vary more than 1.1 to 1.2 times the pressure. Parallel and Series Pumping When two or more pumps are operated in parallel, a combined parallel pump curve that holds the head constant and adds the flow can be drawn. Similarly, a series pump curve that holds the flow constant and adds the head can be drawn. Variable-speed booster pumps, when required at a building, are typically placed in series with the central plant distribution pumps. Variable-Speed Pumping For a given impeller size, a family of curves can be drawn to represent the variable-speed performance of a pump. Notice that the BEP follows along a parabolic curve that looks surprisingly like a system curve (this will be discussed in greater detail below). Also notice that the NPSHR lines follow fairly closely with the published end-of-curve lines for the various speeds. The power lines decrease rapidly as the speed decreases, which graphically demonstrates the potential power savings of variable-speed operation in variable-flow systems. Some designers have placed variable-speed pumps in parallel with constant-speed pumps with unexpected results. The constant-speed pump will always overpower the variable-speed pump until the variable speed is increased sufficiently to meet the pressure created by the constant-speed pump. One unexpected result is that as the flow and pressure in the system decrease, the flow in the constant-speed pump increases and the operating point moves steadily down the pump curve. This can result in the constant-speed pump operating beyond the end of its published curve with resultant increase in radial thrust forces and potential cavitation. Cooling Systems and Thermal Energy Storage Page 15 Copyright APPA 2009

16 Piping Systems Figure 4. Variable-Speed Performance Curve The major considerations in the design of piping systems are pressure and temperature within the system, velocities in the pipes, pipe material and its compatibility with the contents, expansion and contraction, supports, and insulation. The velocity of the fluid in the pipe is directly related to the pressure loss in the system: the higher the velocity, the higher the losses. Velocity is therefore inversely proportional to the pipe size: the smaller the pipe, the higher the velocity for a given flow. Smaller pipes equate to lower first cost, but the higher losses mean higher pumping cost in perpetuity. There must be an optimum balance between the two in design. Thermal expansion and contraction do not present much of a problem in chilled water systems because the differential temperatures are relatively low, but they cannot be ignored. Chiller Plant Piping The most common piping material and method of fabrication for a central chiller is standard-weight, Cooling Systems and Thermal Energy Storage Page 16 Copyright APPA 2009

17 black steel pipe with welded fittings. In addition to welding, methods used to join the pipe include the use of grooved pipe with bolted couplings. The method selected is usually based on personal preference of the owner, the engineer, or perhaps the contractor, if two methods are allowed. Distribution Piping A central chilling plant system requires supply and return piping to deliver chilled water to the various buildings. The distribution system may be an intricate network of pipes with hydraulic loops and cross-connections serving many buildings or loads. A large network distribution system can also be served by several plants simultaneously. The piping system may be direct-buried in the earth, located in a shallow trench, or routed in a utility tunnel. The construction materials used will depend to a large degree on the environment in which the piping will be located. The primary distribution system must be designed as carefully as any part of the chilled water system. Size and location of the pipes should be determined from a thermal utility master plan. Pipes should be sized and located to accommodate future loads, if applicable. Pumping cost must also be considered. Materials Four of the most common piping materials for direct-buried chilled water distribution systems are polyvinyl chloride (PVC), polyethylene, ductile iron, and black steel pipe. Material selection depends on the initial material cost, corrosion requirements, operating pressures, joining methods, and expected life. Listed below are some of the advantages and disadvantages of each piping material. Polyvinyl Chloride Advantages: corrosion resistant, low thermal conductivity, joints are simply pushed together. Disadvantages: higher leak potential, requires thrust restraint, difficult to find with utility location equipment without a tracer wire. Polyethylene Advantages: corrosion resistant, high-quality weld joints, low thermal conductivity, can tolerate freezing of chilled water. Disadvantages: pressure is normally limited to 100 psig, requires specialized equipment and contractor, difficult to find with utility location equipment without a tracer wire. Ductile Iron Advantages: joints are simply pushed together. Disadvantages: higher material cost, high thermal conductivity, higher leak potential, difficult to find with utility location equipment without a tracer wire. Black Iron Advantages: higher pressure rating, high-quality weld joints. Disadvantages: requires a protective coating for corrosion control, requires cathodic protection for corrosion control, requires electrical isolation from other systems to reduce corrosion potential, Cooling Systems and Thermal Energy Storage Page 17 Copyright APPA 2009

18 time-consuming to install, high installation cost. Terminal Components Performance and Design Maximizing cooling coil performance is crucial for the entire chilled water system operation. Chilled water temperature differential will be determined by how well the terminal devices perform. Each coil should be selected to achieve the desired chilled water temperature differential while meeting the airside performance requirements. Listed below is a specification example that can provide some guidelines when specifying coils at a specific location. Entering water temperatures shall be 40 F. Coil shall perform with a minimum of a 20 F temperature rise at design conditions. Fin spacing shall be maximum 10 fins per inch. Coil shall be drainable, with a vent at the highest location and a drain in the lowest location. Aluminum fins shall be in. thick. Tubes shall be in. copper with in. walled U-bends. Casings shall be galvanized steel. Maximum air velocity shall be 450 ft./min. No water carryover shall occur at rated airflow. Minimum tube velocity at rated capacity shall be 4 ft./sec. Turbulators are not allowed. Maximum water pressure drop shall be 10 psig. Coils shall be Air Conditioning and Refrigerant Institute rated, with a fouling factor. Coils shall be rated for a pressure of 200 psig. Coils shall be sized to the stated entering air condition, airflow rate, discharge air temperature, and entering water conditions. Performance testing is required. Control Valves Control valves are a necessity for all cooling coils. Control valves should be selected to match the cooling coil they will control. Pressure drop at the rated flow rate can vary depending on where the cooling coil is located in the pumping hydraulic gradient curve. The control valve will modulate to maintain the desired coil discharge air temperature. Two-way control valves rather than three-way valves should be installed in large campus district cooling systems. Three-way valves cause re-circulation of chilled water, which increases the pumping flow rate and decreases the overall chilled water temperature differential. Care should be taken to provide minimum flow required in the evaporator of the chillers under low flow load conditions when utilizing variable flow chilled water distribution. When necessary this can be provided by the use of a minimum flow bypass valve at the chiller plant. Cooling Systems and Thermal Energy Storage Page 18 Copyright APPA 2009

19 Heat Rejection Heat Rejection Simply put, evaporation is a cooling process. More specifically, the conversion of liquid water to a gaseous phase requires the latent heat of vaporization. Cooling towers use the internal heat from water to vaporize the water in an adiabatic saturation process. A cooling tower s purpose is to expose as much water surface area to air as possible to promote the evaporation of the water. In a cooling tower, approximately 1 percent of the total flow is evaporated for each 12.5 F temperature change. Two important terms are used in the discussion of cooling towers: Range: The temperature difference between the water entering the cooling tower and the temperature leaving the tower. Approach: The temperature difference between the water leaving the cooling tower and the ambient wet-bulb temperature. The performance of a cooling tower is a function of the ambient wet-bulb temperature, entering water temperature, airflow, and water flow. The dry-bulb temperature has an insignificant effect on the performance of a cooling tower. Nominal cooling tower tons are the capacity based on a 3 gpm flow, 95 F entering water temperature, 85 F leaving water temperature, and 78 F entering wet-bulb (EWB) temperature. For these conditions the range is 10 F (95-85 F) and the approach is 7 F (85-78 F). Types of Cooling Towers Cooling towers come in a variety of shapes and configurations. A direct tower is one in which the fluid being cooled is in direct contact with the air. This is also known as an open tower. An indirect tower is one in which the fluid being cooled is contained within a heat exchanger or coil and the evaporating water cascades over the outside of the tubes. This is also known as a closed-circuit fluid cooler. The tower airflow can be driven by a fan (mechanical draft) or can be induced by a high-pressure water spray. The mechanical draft units can blow the air through the tower (forced draft) or can pull the air through the tower (induced draft). The water invariably flows vertically from the top down, but the air can be moved horizontally through the water (crossflow) or can be drawn vertically upward against the flow (counterflow). Water surface area is increased by using fill. Fill can be splash-type or film-type. Film-type fill is most commonly used and consists of closely spaced sheets of PVC arranged vertically. Splash-type fill uses bars to break up the water as it cascades through staggered rows. Typically, in the HVAC industry, cooling towers are packaged towers that are factory fabricated and shipped intact to a site. Field-erected towers mostly serve very large chiller plants and industrial/utility projects. When aesthetics play a role in the selection of the type of tower, custom-designed field-erected cooling towers are sometimes used. In these towers, the splash-type fill is often made of ceramic or concrete blocks. The following is a discussion of the most common types of cooling towers encountered in the HVAC chilled water plant. Spray Towers Cooling Systems and Thermal Energy Storage Page 19 Copyright APPA 2009

20 Spray towers distribute high-pressure water through nozzles into a chamber where air is induced to flow with the water spray. There are no fans. The air exits out the side of the tower after going through mist eliminators. Spray towers are seldom used. One problem is that the nozzles are easily plugged by the precipitation of mineral deposits and by airborne particulates that foul the water. Capacity is controlled by varying the water flow through the tower. This can be accomplished by using multiple-speed pumps or VSDs on the pumps, or by passing water around the tower. Varying the water flow through the condenser of a chiller is not always recommended, as will be discussed under system design considerations. Because air velocities are very low, spray towers are susceptible to adverse effects from the wind. Spray towers are very quiet and can have a very low first cost. Forced Draft Cooling Towers Figure 5. Spray Tower Forced draft towers can be of the crossflow or counterflow type, with axial or centrifugal fans. The forward curved centrifugal fan is commonly used in forced draft cooling towers. The primary advantage of the centrifugal fan is that it has the capability to overcome high static pressures that might be encountered if the tower were located within a building or if sound traps were located on the inlet and/or outlet of the tower. Crossflow towers with centrifugal fans are also used where low-profile towers are needed. These towers are quieter than other types of towers. Forced draft towers with centrifugal fans are not energy efficient. The energy to operate this tower is more than twice that required for a tower with an axial fan. Another disadvantage of forced draft towers is that, because of low-discharge air velocities, they are more susceptible to recirculation than induced draft towers. This is discussed in further detail below. Cooling Systems and Thermal Energy Storage Page 20 Copyright APPA 2009