Chilled water storage for effective energy management in smart buildings

Transcription

1 The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 214 Chilled water storage for effective energy management in smart buildings Wanyun Zhong University of Toledo Follow this and additional works at: Recommended Citation Zhong, Wanyun, "Chilled water storage for effective energy management in smart buildings" (214). Theses and Dissertations This Thesis is brought to you for free and open access by The University of Toledo Digital Repository. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page.

2 A Thesis entitled Chilled Water Storage for Effective Energy Management in Smart Buildings by Wanyun Zhong Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Electrical Engineering Dr. Lingfeng Wang, Committee Chair Dr. Weiqing Sun, Committee Member Dr. Hong Wang, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo May 214

3 Copyright 214, Wanyun Zhong This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

4 An Abstract of Chilled Water Storage for Effective Energy Management in Smart Buildings by Wanyun Zhong Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Electrical Engineering The University of Toledo May 214 A smart building takes advantage of new highly energy-efficient technologies, which reduce the operating cost of the building. One of the key features of a smart building is some type of thermal energy storage (TES) technology. TES allows for shifting of the electrical load from peak periods to off peak periods when electrical rates are lower. One of the most common types of thermal energy storage is a chilled water storage (CWS) system. Chilled water storage systems work by cooling water overnight or during the off peak period and then using that chilled water during the day or peak period to cool the building. This results in less electrical usage throughout the day when electrical rates are the highest. This study proposed a two-agent management structure that may efficiently control the CWS system. The study further investigated and modeled the use of several strategies and scenarios of implementing a CWS system in a smart building. Each scenario takes into account different peak periods, off peak periods, and regular periods. Depending on the commercial goals and needs of the business, different strategies may be suitable for that particular application. iii

5 This work is for my dear family members.

6 Acknowledgements I would like to thank my advisor Dr. Lingfeng Wang for his guidance, mentorship and useful critiques of my research work. I also want to express my thanks to Dr. Weiqing Sun and Dr. Hong Wang for serving on my committee. Thanks for their time in this work. I would also like to thank my friends who have always been there to answer my questions. Finally, I would also like to thank my family for their support throughout my studies. v

7 Table of Contents Abstract... iii Acknowledgements...v Table of Contents... vi List of Tables... viii List of Figures... ix List of Abbreviations... xii 1 Introduction Background Organization Literature Review Smart Building Time of Use Energy Storage Technologies Available for On-Demand Energy Technologies for Shifting Load Demand Optimization of System for Cost Savings Chilled Water Storage System Operation System design and description Structure of System Operational Strategies in Local Agent...21 vi

8 4 Simulation Results and Analysis Simulation result of each strategy Comparisons of each strategy Concluding remarks and future work Concluding remarks Future work References...56 vii

9 List of Tables 4.1 The WBT profile The HVAC load profile Electrical cost, chiller capacity, and energy consumption of each strategy viii

10 List of Figures 1-1: Shifting the load from peak to off peak hours : Stratified chilled system during peak load and off peak load : The proposed structure of the designed system : HVAC load without a CWS system : HVAC load shifting in strategy : HVAC load shifting in strategy : HVAC load shifting in strategy : HVAC load shifting in strategy : HVAC load shifting in strategy : HVAC load shifting in strategy : The COP profile under the different wet bulb temperature : Daily WBT and COP data : Four electrical pricing methods proposed in the project : Electricity consumption in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy ix

11 4-1: Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : Thermal energy usage in strategy : Electricity consumption in strategy : Cooling production by chillers in strategy : The electricity cost for different strategies of the CWS system...52 x

12 4-33: The chiller capacity for different strategies of the CWS system : The electricity consumption for different strategies of the CWS system...53 xi

13 List of Abbreviations BEMS...Building Energy Management System COP...Coefficient of Performance CWS...Chilled Water Storage DOE...Department of Energy EIA...Energy Information Administration HVAC...Heating, Ventilation, and Air Conditioning S #...Strategy # TES...Thermal Energy Storage TOU...Time of Use US...United States WBT...Wet Bulb Temperature xii

14 Chapter 1 Introduction The smart building technology, which optimizes the energy usage between a building and the power grid, has become an important way to improve energy efficiency in the last few years. The power grid optimization results in a highly energy-efficient building which uses less energy, costs less to operate and produces less environmental impact than traditional buildings [1]. Smart building technology involves a wholebuilding design approach that integrates the use of the most advanced energy efficient technologies while still meeting the comfort and demand of the occupants [2]. In large commercial buildings, there is usually a considerable difference in load demand throughout the day. This is especially true for offices and schools, which only operate throughout the day. As such, in order for power supply companies to meet the energy demands during peak hours, power companies often need to increase their power generation. However, smart building technology can utilize various new technologies (such as solar energy and thermal energy storage) to transfer the load from peak hours to off peak hours. This leads to a more efficient use of electricity and less demand on power companies to expand their power generation capabilities. Thermal energy storage (TES) technology, one of the cost-effective energy saving 1

15 technologies, is now being implemented in a number of smart buildings [1]. TES is used for shifting the load of the HVAC in the building from peak hours to off peak hours in order to reduce demand and cost during peak hours. It can help the building optimize the coordination of energy load and smoothen the load curve. Chilled water storage (CWS) is one kind of thermal energy storage technology, which uses a water storage tank to transfer energy usage from peak periods (daytime) to off load periods (overnight). The use of TES technologies, such as a chilled water storage system, in combination with a smart building management system results in better energy efficiency and lower operating costs for air conditioning. In this thesis, I investigate the use of chilled water storage (CWS) as a means of smoothing peak and off-peak demands caused by uneven HVAC loads in a smart building system so as to meet comfort and commercial goals. 1.1 Background Over the last several decades, there has been a dramatic increase in demand for energy driven by economic growth, industrial growth, technology advances, and the world s continually expanding population. Figures from the United States Energy Information Administration (U.S. EIA) project that over the next 3 years there will be a 56% increase in global energy consumption [3]. The estimates are an increase from 524 quadrillion Btu in 21 to about 82 quadrillion Btu in 24 [3]. According to the DOE, up to 39% of the energy usage in commercial building is consumed by space heating, cooling, and air conditioning [9]. Since these commercial buildings only operate throughout the day, it results in a large difference in load demand between the day and 2

16 night. Due to this situation, companies are trying to find ways of being more energy efficient with the current energy that is being produced. The smart building s architectural design is heavily considered with the energy design. For example, the size and capacity of electrical and mechanical systems can be minimized by considering the energy produced by solar technologies and natural lighting loads [1]. Throughout the design process, building simulation software guides decisions to achieve the most efficient building possible. Over the last 1 years, technology and efficiency has greatly advanced allowing for new state of the art buildings to utilize a variety of strategies to optimize their energy usage. Passive solar technologies take advantage of solar heat and light to offset the need for electric heating, air conditioning, and lighting. Common passive solar techniques include placing building windows in a south-facing orientation so that solar energy can be absorbed or reflected as needed [1]. This works by changing the shading of the window to affect absorption or reflection during winter and summer seasons. Additionally, another form of passive solar energy is daylighting, in which walls are made entirely of glass to allow natural lighting and minimize the need for electric artificial lighting [1]. Active solar technologies include photovoltaic solar panels which convert solar energy into electricity. High performance insulation is another technology used in smart buildings to further maximize the energy efficiency. The insulation is made up of panels consisting of a sandwich of rigid foam plastic insulation and plywood [1]. The cost is nearly the same as buildings with a wood frame construction, but the insulation can better keep the smart building s temperature constant. 3

17 What is Thermal Energy Storage (TES)? Thermal energy storage (TES) is a technology that allows thermal energy to be stored for later use. It is mainly used for heating and cooling thermal applications. For example, excess heat during the day time can be stored and later accessed at night when temperatures outside drop. Other examples include using stored chilled water as a way of air conditioning and cooling in buildings or homes. There are multiple mediums for storage of thermal energy: water or ice tanks, bedrock or other solid materials, and phasechange materials are just a few. There are two main types of thermal energy storage systems: sensible and latent. If the storage medium remains in a single phase during the storing cycle, the TES is using sensible heat. Sensible thermal energy storage generally involves the use of heated or chilled water tanks. If the medium undergoes a phase change (for example, ice to water) it is known as a latent TES system [4, pg. 233]. Therefore, in sensible TES systems, the medium exhibits changes in temperature as heat is added or removed (as the system is charged or discharged). On the other hand, in latent TES systems the temperature remains constant because when heat is added or removed the energy is used to change the phase (liquid to solid, for example) of the medium. Most TES systems use sensible single phase storing cycles due to simplicity and their high efficiency. Their drawbacks include requiring a larger volume of medium than latent systems, but the simplicity of the system often overcomes this. Advantages of using a latent system, such as ice, are the compact storage volume, but the drawbacks are that they require very low temperatures for freezing water into ice and more insulation is required. 4

18 TES systems can be full storage or partial storage systems. This means that the TES system may be completely capable of using thermal energy storage for cooling or heating or it may only be able to partially reduce the load [5]. The selection and usage of a TES system depends upon many factors such as storage period (daily versus seasonal, for example), operating conditions (heating or cooling), and economic viability [4, pg 222]. When is using a TES system appropriate? Once electricity is generated, it cannot be stored easily and efficiently in large amounts. Therefore, power plants and companies must be able to generate enough capacity to cover the highest peak of consumption which is usually the hottest days of the year around noon time. Since thermal energy is easily stored, a TES system can exploit the off peak hours of the day by using the electrical power to chill or heat water that can then be used in air conditioning applications [4, pg 458]. Therefore, implementing TES systems can significantly help meet modern needs for more efficient and environmentally friendly energy use in a building s heating and cooling. The use of TES systems has been shown to be able to reduce energy consumption, energy costs, and conserve limited fossil fuels by facilitating more efficient energy use. Economic justification for TES systems usually requires that the annual capital and operating costs be less than annualized costs of equipment supplying the same service. TES systems are usually economically compelling when a building is in need of expanding their current cooling or heating system or the building is undergoing new construction. This cuts the initial cost of implementing the TES system and allows for the initial investment to be recuperated faster through decreased annual costs [4, pg 212]. 5

19 Implementation TES can be implemented in two fashions: to reduce energy consumption or to transfer an energy load from one period to another. The consumption reduction is achieved through storing unused waste energy, such as solar energy or heat produced by equipment or appliances. Energy load transfer is achieved by storing energy at a given time for later use during peak hours. Electrical energy is normally high in demand during the day time and for that reason costs more. There is much interest in using TES systems to reduce peak demand by transferring the day time energy load to low consumption periods during the night. The most common example is an electrical chiller that chills water during the night and then uses the thermal energy to cool the building during the day [4, pg 213]. A major application of TES is to lower electrical demand and reduce the associated charges. Reduction of the charges is accomplished by limiting electrical heating or cooling during peak electrical demand periods. Cooling and heating are then set to operate during off peak hours and charge the TES. During the peak demand period, the TES is used to either fully or partially cool the building leading to a reduction in operating costs. Figure 1-1 provides an example of shifting the load of a building from peak hours to off peak hours in order to reduce demand and cost during peak hours. Building Load(KW) cooling Building Load(KW) Noncooling cooling Noncooling cooling Time Time Figure 1-1: Shifting the load from peak to off peak hours 6

20 TES systems like the one shown above can be implemented in a retrofit application or as a new installation. For example, consider a building s air conditioning system that cannot maintain the building adequately cool during high cooling load hours but does well during average cooling. Instead of expanding the current air cooling system, a partial TES system could be added to the current system to reduce the high load. During the average or low load hours the current air conditioning system could charge the TES, and then during peak hours the TES would help reduce load. Overall, this would be an effective retrofit application because the current air conditioning system would not need to be upgraded to a larger system that would have more energy costs. The best places to implement TES systems include locations where electric rates have demand charges (for peak usage), buildings where most of the cooling load occurs when there is peak electrical load and rates, and climates with higher temperatures during the day [5]. The building must also have available physical space to house the storage medium and equipment. The best candidates for TES systems are buildings that do not need around the clock cooling. For example, a hospital would not be a primary candidate for TES system because the hospital is always running and there would be no time (like overnight) to charge the TES. The best candidates are office buildings, schools, and other day time facilities [5]. TES: A Water Chilled Storage Design Early refrigeration systems and cooling systems used blocks of ice as a means of cooling. The invention of mechanical refrigeration led to a decrease in interest of using cool storage systems. During the 198s, electric companies realized that the peak demand on their systems was exceedingly high and it needed to be reduced [5]. These companies 7

21 recognized that it was costing them more to produce electricity during peak hours and began to offer financial incentives for customers to shift from using high peak energy periods to low peak periods. Chilled water thermal storage can be used with standard air conditioning chillers without special equipment. The use of such a system is ideal for increasing the capacity of an existing system or for the use in a new building. These systems are increasingly economical as the tank size increase; for example, systems with million gallon tanks have significantly lowered capital costs than non-storage chilled buildings [5]. As described earlier, chilled water storage systems are taking advantage of the sensible heat capacity of water. A well-designed chilled water storage system maximizes the cooling capacity by minimizing the storage temperature and preventing the mix of return (warm) water with the chilled water. Chilled water storage systems typically use water temperatures between 4 and 7 degrees Celsius. This temperature range is compatible with most conventional cooling systems and allows for the use of chilled water storage in older buildings. Chilled water storage is best for applications requiring the storage of 7,kWh or more of energy (approximately a 2, gallon tank). Many large chilled water storage systems store between 1 and 5 million gallons of water [5]. Chilled water systems must maintain thermal separation between cool water and warm return water. There are multiple methods of achieving this: stratification, multiple tanks, and membrane designs [7]. Stratified chilled water storage is generally accepted as the simplest and most efficient method of chilled water storage separation. Stratified chilled water tanks use the 8

22 tendency of water to form layers based on temperature and density [5]. As water gets colder it becomes denser (until 4 C); therefore the cold water will collect and stabilize in the bottom of the tank while the warm water is near the top. During the charging, warm water is taken from the top of the container and is chilled. This water is then added to the bottom of the tank. During discharge, water is taken from the lowest portion of the tank and return warm water goes to the top. It is important that a diffuser distributes the flow of water into and out of the tank smoothly to avoid turbulence and mixing of the water [5]. A basic diagram of a stratified chilled system is shown below: Building Building ON Chiller(OFF) OFF ON Chiller(ON) ON Warm Water (typical 17 C) Warm Water (typical 17 C) Transition Layer Transition Layer Cold Water (typical 4 C) Cold Water (typical 4 C) Figure 1-2: Stratified chilled system during peak load and off peak load On the left side of Figure 1-2, a CWS system is shown during peak load; the chilled water is leaving the storage tank through the bottom and cooling the buildings air conditioning coils. The figure on the right shows the system during low peak load; and the warm water is being chilled overnight using low cost electricity rates [6]. 9

23 1.2 Organization The remainder of this thesis is organized as follows: Chapter 2 provides a literature review of this research field, Chapter 3 describes the system design and mathematical models, Chapter 4 presents the simulation results and analysis of results, and Chapter 5 provides conclusions and future work. 1

24 Chapter 2 Literature review 2.1 Smart Building Smart buildings often use multiple energy sources for power, both in-house supplied and grid supplied energy, with an eye on reducing the overall amount of energy that the building requires from the power grid. Multiple sources are used to not only assure continuous and reliable power, but also to help reduce the cost of energy for the building [18]. The benefits of a smart building are many fold for organizations with the most sought after benefits being return on investment of building projects, reduction of operating expenses for organizations, good will and good image, improvement of social responsibility and sustainability, and energy savings [12]. While an organization can reap all of these benefits by creating energy self-sustaining buildings, one often underestimated fact is the ability to save money. A smart building is shown to reduce costs through various mechanisms, including optimization of climate control, matching power to usage patterns, through dynamic power consumption, and through proactive measures to reduce the overall energy required [12]. Money savings can be achieved, despite the large capital investments to build a smart building through not only the conservation (reduction in use of overall energy by a building) but also through more 11

25 effective use of energy; namely using stored and captured energy during peak energy times when the cost of grid-supplied energy is the greatest. Through the reduction of peak-cost energy, relying only on grid energy during low cost periods of the day, organizations can save on energy expenses. Fox-Penner (21) finds that a smart building, when designed and harnessed properly, can add up to tremendous energy savings as buildings can reduce their reliance on the energy grid when costs are high. Reducing peak demand has the ability to help the organization to avoid paying top-dollar for energy. This is known as time of use [13]. 2.2 Time of Use Smart building can reduce the overall energy use of a building, yet real cost savings can be achieved by using energy in a wise manner, namely purchasing energy when it is at low cost and using stored energy when the cost of supplied energy is at its highest. Therefore, it is important to understand when energy costs are high and then rely strictly upon stored energy during these times, switching from the grid to stored energy. Energy switching during peak times is known as time of use or the ability to use signals and feedback from the grid to plan when to use stored energy and when to rely upon the grid for energy. While traditional non-grid power relies on the sources for energy flow continuously and uses grid when stored energy is exhausted, such as at night when the sun is no longer providing energy, time of use scheme uses grid energy when costs are lowest (e.g., at night) and utilizes the stored energy during peak hours [18]. Time of use is also known as dynamic power consumption which takes signals from the open electricity market and alters usage of grid-supplied power to be high during non-peak times and low or none during peak usage and cost periods. Power 12

26 companies supply power at different rates depending upon demand and during high-use times. Additionally, the power rate is higher for consumption, but also higher for power buy back. This means that power used off the grid will cost more at peak times than at non-peak times; however, any excess energy returned to the grid will also reap higher paybacks. Therefore, companies that can store power for high peak times can alter usage by using in-house power only during peak rate times and can also return any excess energy back to offset grid supplied power costs [15]. Time of use looks at ways to reduce the overall cost of energy and how the use of storage devices, such as batteries and water tanks, can help the organization use grid energy during low-cost hours and rely upon smart energy during high-cost hours. Therefore, not only is energy use reduced, but also the expense of the energy used is reduced due to lower electrical rates during off-peak hours. Having multiple energy sources within a structure allows more optimal power management and therefore a smart building must incorporate real-time calculations and demands into planning and not just efficiency measures to assure optimal payback of the system [18]. In order for time of use energy to work, the building must have sufficient and reliable energy storage to assure that the building can switch off-grid when needed to save money. 2.3 Energy Storage Technologies Available for On-Demand Energy On-demand power requires saved and stored power to provide energy. Traditionally renewable or green energy is both produced for immediate use and also stored for on-demand use in a variety of energy storage capacity devices. While batteries are the best known and most utilized source of on-demand power, smart buildings must use numerous technologies to assure that needed power is both available and reliable and 13

27 can be sourced when the grid-supplied energy is at its peak. While smart buildings do not suffer power fluctuations, on-demand power shifting requires adequate on-hand power to supply power during peak-cost power times, thus allowing the conversion from grid to stored power [19]. The most utilized storage devices used in smart buildings are batteries, ice/heat storage units, water tanks, kinetic energy, super-capacitors and flywheels. Each technology, while different in design and ability, works by taking produced energy from sources, such as solar or wind power, and storing the energy in-house until the energy is required to power the building. Storing energy has two main advantages: reliable and continuous power and the ability to assure that produced energy is neither lost nor sent into the grid during low-rate periods. By having energy stored, the building can not only use the energy anytime, such as when power demand is highest and most expensive, but can also be used to release excess energy back into the grid when the buyback rates are highest [16]. Batteries come in many different forms and are the most utilized source of energy storage in smart buildings. Batteries can be lead-acid, nickel-based, lithium-ion, sodiumsulfur, or flow. Batteries store and produce energy through chemical reactions that produce electrons; this reaction is stored within the cell and creates flows when demanded. The energy can be depleted and then reversed by sending electrons back into the cells. The storage is long-term and can be expanded through the use of multiple batteries [19]. Flywheels store energy in a rotating disk and as energy is added, the flywheel speed increases, storing energy as rotational energy. As energy is extracted from the flywheel it reduces speed and thus loses rotation [19]. Super-capacitors store energy by the capacitance effect. The energy is stored between plates and then can be released 14

28 when needed [19]. Water tanks store energy as heat by heating or cooling stored water and holding the water to be released to either run turbines for power or to flow through radiators to produce heat. Ice/heat storage systems work by storing the energy as either ice or hot water and then releasing this energy to regulate the climate within a building, reducing the need for electricity-controlled units [16]. 2.4 Technologies for Shifting Load Demand Properly regulating energy use and switching from the grid to internally stored energy is known as demand-side management where the customer controls the flow of energy to and from the grid. Demand-side management of power requires the customer to control power use and to regulate the flow of power from the grid into the structure. As such, the customer can control not only the amount of energy from the grid, but also the times when energy from the grid can flow. Smart switching allows customers to make informed choices about energy consumption, adjusting both the timing and quantity of energy use [11]. Shifting of load demand from the grid to internal energy requires the ability to monitor the grid and to understand when peak energy times are, thus assuring that energy use is not only reduced when the peak demand occurs, but also that the energy comes from the storage and not the grid. Demand switches work by shutting off power from a source and then opening a switch to allow power to flow from another source. As such, the flow of energy is controlled thus assuring that the right power is being used only at the right time [11]. Load shifting requires the use of controls to not only understand when the load should be switched, but also to control what sources of energy are utilized to assure that the in-house power is used effectively and efficiently. Through verification of demand 15

29 and continual monitoring of the grid, signals activate the load shifting and give the customer complete control of energy within the building. Load shifting controls include DG/S control devices that optimize usage and storage of energy and building energy controllers which fully monitor and regulate the use of energy based upon time, conditions, and available energy [11]. 2.5 Optimization of System for Cost Savings Integration of energy storage systems into power supplies and the grid is not new, yet the advanced power electronics require special designs and controls which can require costly designs and debugging of systems to assure the on-demand power supplies operate effectively and that switching of power is controlled in real-time. When operating ondemand power systems in a smart building, it is a requirement of the storage system to operate and to control power flow and demand, thus creating issues within the system in terms of demand, supply, switching and storage. Without an optimized system, cost savings are offset or not realized [14]. Smart grid technology and smart building work only when optimized around reduced demand and also around assuring that the demand is decreased during times of prime-cost energy. Therefore, system optimization requires the use of intelligent system architecture to assure ongoing monitoring of environments for use and to optimize power usage based upon records of time, persons, demand, and temperature. The information about relevant occupancy and setting conditions, as well as the final values of environmental variables is used to train a multi-layer neural network whose outcomes will provide ideal environmental values in case of absence of occupants or of provided preference information. These systems control environments and provide optimized 16

30 energy, lowering usage at key times and offering substantial cost savings through automation of environments [17]. 2.6 Chilled Water Storage System Operation Thermal storage units, including chilled water storage units, are usually in place due to the economical advantage that they provide by utilizing energy during the off peak period and their decreased energy use during the peak period. The two most common models for chilled water storage would be a partial storage system and a full storage system. One offers the capability of storing enough energy so that the chillers do not have to run at all during the peak period, and the partial storage system can store enough to lessen the demand during the peak period [5], [1]. In a full storage system, enough water is chilled during the off peak period to satisfy the demand during the peak hours. During the peak hours, the chiller is not operating, and all the cooling is done from the chilled water storage. The advantage of this system is to maximize the savings by only using electricity during the off peak period (which is the cheapest), and using none during the peak hours. However, since enough water needs to be chilled to meet the total demand, the storage capacity needs to be very large, and the cost of equipment is considerably more than the partial storage unit. Full storage system is best utilized when there is a big gap in pricing between peak and off peak usage, or if the peak period demand is short [5], [1]. The main type of partial storage system is the load leveling system. In this system, the chiller is running at full capacity at all times. The chiller is on 24 hours, so the chiller capacity is maximized. During the off peak period, the water is chilled. During the peak 17

31 period, the chiller is still on while complemented by the chilled water, utilizing all of the stored chilled water during the peak hours. This is usually called load leveling. The chilled water storage system is charged when the demand is less than the output of the chiller, and the system is discharged when the demand is more than the output of the chiller. The advantage of partial storage system is to minimize the cost of building a larger water tank, as well as a more powerful chiller. The disadvantage is the electricity usage is balanced between peak and off peak period, and the savings of using the off peak price is not maximized. The system is usually used when there isn t a large pricing difference between peak and off peak or when the peak usage or period is very long [5], [1]. Another type of partial storage is demand limiting. It is similar to the partial storage in that the chiller is always on, but only is at the maximum capacity during the off peak period, and during the peak period the chiller is running below maximum capacity, while being assisted from the chilled water storage. This system is essentially a hybrid between the load leveling and full load system. The cost is in between the full load and load leveling systems, as is the savings between those two. The main reason for using this system is when there is a limit on how much energy can be used at one time during the peak period. In this situation, the options left are the full load and demand limiting systems. The capital cost of installing the demand limiting load is less than the full load, but the savings are not as great [5], [1]. 18

32 Chapter 3 System design 3.1 Structure of System The proposed agent controlled CWS system in a smart building comprises a twolayer agent management structure, which communicates between the utility power grid, a CWS system and a smart building, as seen in Figure 3-1. Utility Power Grid Primary Agent (Multiple pricing methods) Local Agent (7 Different Strategies) Smart Building (HVAC Load) CWS System Figure 3-1: The proposed structure of the designed system 19

33 The primary agent communicates with the local agent and controls the electric supply from the utility power grid into the smart building. The main function of the primary agent is to set the electricity price based on the current electricity load on the utility power grid as a whole. The primary agent often has multiple pricing methods. These prices usually vary based on the amount of electricity purchased or used as well as the time of day and month of the usage. The prices can be adjusted in real-time. This means that the price varies every day and time based on the current load of the power grid. Alternatively, other pricing methods exist which charge by average load for that particular season (e.g., winter versus summer). On the other hand, the local agent is responsible for communicating with the smart building, the CWS system and the primary agent, so as to control the electric supply into the smart building. In particular, the local agent collects and analyzes information from the primary agent, the CWS system and the smart building, such as the HVAC loads of the smart building, the pricing methods (determined by the primary agent), the CWS capacity, the CWS current charge, as well as user defined input. Accordingly the local agent chooses a suitable strategy that would optimize the charging and discharging of the CWS in order to satisfy the user s goal and needs. After an operational strategy is chosen, the local agent applies a pricing method from the primary agent and operates the HVAC and the CWS system (e.g., when to charge and discharge the CWS and when to turn off or turn on the HVAC). This management system allows for easy and optimal energy usage and reduction of operating costs for the smart building. 2

34 3.2 Operational Strategies in Local Agent The local agent operates under various strategies to meet different consumer s demands. The strategies may include, but are not limited to, those described as follows: Strategy : The smart building works without a CWS system. Refer to Fig. 3-2, there is no energy shifting for the HVAC load. QLoad(KW) 24 Time (hours) Figure 3-2: HVAC load without a CWS system The electric energy consumption on the chillers is Q (t) Energy Consumption= COP(temp(t)) dt (3.1) Strategy 1: Scope: Consequently, the electricity expense is Q (t) Electricity Cost= Price(t)dt (3.2) COP(temp(t)) The HVAC loads have one peak per day; the rest is off peak periods. Description of strategies: Full storage; 21

35 During the peak period, the chillers stop working; only the CWS tanks supply the HVAC loads of the smart building; During the off peak period, the chillers supply the HVAC loads and charge the CWS tanks at the same time. Pictorial Illustration: QLoad(KW) W chiller h b h e 24 Figure 3-3: HVAC load shifting in strategy 1 Time(hours) Mathematical Models: ( ) ( (( )) ( )) Wherein, + ( ) ( ) (3.3) hb: peak period begin he: peak period end QLoad(t): the HVAC load of the smart building WChiller: power rating of the chillers Q (t)dt : the peak loads shifted to the off peak period 22

36 (W COP(temp(t)) Q (t))dt + (W COP(temp(t)) Q (t))dt: The rest capability of the chillers after supplying the HVAC loads during the off peak period k 2% COP(temp(t)): coefficient of performance regarding the chillers temp(t): wet bulb temperatures (1 ) = ( ) (3.4) k : coefficient of thermal energy loss of the CWS tanks per day; typically N: number of the CWS tanks Etank: the maximum thermal energy stored per tank. N= 1, (he hb 4) and (24 lb+le) 4 2, (he hb<4) or (24 lb+le)<4 (3.5) A tank needs to be charged or discharged more than 4 hours in order to eliminate turbulence and provide a stable, sharply defined transition layer, or thermocline. If the charging or discharging time is less than 4 hours, two parallel tanks need to be used. Strategy 2: Scopes: The HVAC loads have one peak, one regular period, and one off peak period. Basic strategies: Full storage; During the peak period, the chillers stop working; only the CWS tanks supply the HVAC loads of the smart building; 23

37 During the regular period, the chillers supply the HVAC loads During the off peak period, the chillers supply the HVAC loads and charge the CWS tanks. Pictorial Illustration: QLoad(KW) W chiller l e h b h e l b 24 Time(hours) Mathematical Models: ( ) Wherein, Figure 3-4: HVAC load shifting in strategy 2 ( (( )) ( )) + ( (( )) ( )) (3.6) lb: off peak period begin le: off peak period end In order to maximize the shifting loads, the chillers may supply HVAC loads and charge the CWS tanks during the regular period. 24

38 ( ) ( (( )) ( )) Strategy 3-1: Scopes: + ( (( )) ( )) (3.7) The HVAC load has multiple peak periods, regular periods, and off-peak periods. Basic strategies: Full storage; Pictorial Illustration: During the peak period, the chillers stop working; only the CWS tanks supply the HVAC load of the smart building; During the regular periods, the chillers supply the HVAC loads; During the off peak periods, chillers supply the HVAC load and charge the CWS tanks. QLoad(KW) W chiller le1 hb1 he1 lb1 le1 hb1 he2 lb1 24 Figure 3-5: HVAC load shifting in strategy 3 Time(hours) Mathematical Models: 25

39 (Q ( )) ( (( )) Q ( )) + ( (( )) Q ( )) + ( (( )) Q ( )) (3.8) Strategy 3-2: In order to maximize the shifting loads, the chillers may supply HVAC loads and charge the CWS tanks during the regular period. (Q (t))dt (W COP(temp(t)) Q (t))dt + ( ) (W COP(temp(t)) Q (t))dt + (W COP(temp(t)) Q (t))dt (3.9) 26

40 Strategy 4: Scopes: The HVAC load has one peak period per day; the rest are off-peak periods. Basic strategies: Partial storage; During the peak period, both the chillers and the CWS tanks supply the HVAC loads of the smart building; During the off peak period, chillers supply the HVAC loads and charge the CWS tanks. Pictorial Illustration: QLoad(KW) W chiller h b h e 24 Time(hours) Mathematical Models: Figure 3-6: HVAC load shifting in strategy 4 ( ( ) (( ))) ( (( )) ( )) + ( (( )) ( )) (3.1) 27

41 Strategy 5: Scopes: The HVAC loads have one peak, one regular period, and one off peak period. Basic strategies: Partial storage; During the peak period, both the chillers and the CWS tanks supply the HVAC loads of the smart building; During the regular period, the chillers supply the HVAC loads; During the off peak period, the chillers supply the HVAC loads and charge the CWS tanks. Pictorial Illustration: QLoad(KW) W chiller Mathematical Models: Figure 3-7: HVAC load shifting in strategy 5 ( ( ) (( ))) l e h b ( (( )) ( )) h e + ( (( )) ( )) (3.11) l b 24 Time(hours) 28

42 In order to maximize the shifting loads, the chillers may supply HVAC loads and charge the CWS tanks during the regular period. ( ( ) (( ))) ( (( )) ( ))+ ( (( )) ( )) (3.12) 29

43 Strategy 6-1: Scopes: The HVAC load has multiple peak periods, regular periods, and off-peak periods. Basic strategies: Partial storage; During the peak periods, the chillers stop working; only the CWS tanks supply the HVAC load of the smart building; During the regular periods, the chillers supply the HVAC loads; During the off peak periods, chillers supply the AC load and charge the CWS tanks. Pictorial Illustration: QLoad(KW) W chiller le2 hb1 he1 lb1 le1 hb2 he2 lb2 24 Figure 3-8: HVAC load shifting in strategy 6 Time(hours) Mathematical Models: ( ( ) (( ))) ( (( )) ( )) + ( (( )) ( )) + ( (( )) ( )) (3.13) 3

44 Strategy 6-2: In order to maximize the shifting loads, the chillers may supply HVAC loads and charge the CWS tanks during the regular period. ( ( ) (( ))) ( (( )) ( )) + ( ) ( (( )) ( )) + ( (( )) ( )) (3.14) 31

45 Strategy 7: Scopes: Real-time pricing Basic strategies: When EPrice is lower than or equal to a predetermined buy price and the CWS tanks are not full, charge the tanks; When EPrice is higher than or equal to the cost per unit multiplied by a profit margin and the tanks are not empty, discharge the tanks; The buy price is adjusted by an average rolling method. Mathematical Models: CWSPERC: energy storage percentage; detected by temperature sensors in CWS tanks EPRICE: real-time price, reflect the demand degree in local grid BUYPRICE: buy price CWSCOST: average cost per unit in CWS tanks PROFITMARGIN: target profit=sell price/buy price; typically PROFITMARGIN=3 Determining Buy Price: BUYPRICE = ( ) 36 (+1) (3.15) Charging Procedure: WHILE EPRICE<=BUYPRICE AND CWSPERC<1 CHARGE; 32

46 SET Timer t; = + ( ) (( )) (3.16) = ENDWHILE + ( ) (3.17) Discharging Procedure: WHILE EPRICE>=CWSCOST*PROFITMARGIN and CWSPERC> DISCARGE; SET a timer t; = (3.18) ENDWHILE 33

47 Chapter 4 Simulation and Analysis 4.1 Simulation result of each strategy An Expo Center project in Shanghai is illustrated here as an exemplary simulation. Since July is the hottest month in Shanghai, the weather data of a typical July day is used to test the capacity of the system. The wet bulb temperatures of the same day are shown in table 4.1: Table 4.1 The WBT profile HOURS : 1: 2: 3: 4: 5: 6: 7: WBT( ) HOURS 8: 9: 1: 11: 12: 13: 14: 15: WBT( ) HOURS 16: 17: 18: 19: 2: 21: 22: 23: WBT( ) COP is the ratio of cooling production to the actual energy consumption [23]. It measures the efficiency of a cooling pump. A higher COP means higher efficiency and lower operation cost. According to the models used in other projects, we assume COP and wet bulb temperate has a linear relationship [22], [24]. Regarding the chillers used in the project, COP is 5.6 during 2 C, refer to Equation 4-1. =(1 ( 2 )) 5.6 (4.1) 34

48 In this project, K is assumed as 2%, refer to Figure COP Wet Bulb Temperature( C) Figure 4-1: The COP profile under the different wet bulb temperature wet bulb temperature( C) Time (h) COP Time (h) Figure 4-2: Daily WBT and COP data The estimated HVAC load in a typical day is shown in table 4.2. Table 4.2 The HVAC load profile HOURS : 1: 2: 3: 4: 5: 6: 7: LOAD (KW) 137 HOURS 8: 9: 1: 11: 12: 13: 14: 15: LOAD (KW) HOURS 16: 17: 18: 19: 2: 21: 22: 23: LOAD (KW)

49 The primary agent offers different pricing methods, including constant pricing, two-period TOU pricing, three-period TOU pricing, and multiple-period TOU pricing, and real-time pricing. The first four pricing methods are shown in Figure 4-3. Constant Pricing Two-Period TOU Pricing Electricity Pricing($) Time (hours) Three-Period TOU Pricing Electricity Pricing($) Time (hours) Multi-Period TOU Pricing Electricity Pricing($) Time (hours) Figure 4-3: Four electrical pricing methods proposed in the project Electricity Pricing($) Time (hours) According to these assumptions, the strategies in the local agent are simulated and analyzed as follows: Strategy : 8 x Q (KW) Time (hours) Figure 4-4: Electricity consumption in strategy The figure above illustrates the energy usage without a CWS system. This is a foundation of the HVAC system that is used to compare with the following strategies. 36

50 Strategy 1 1 x 14 8 CHILLER TO BUILDING CHILLER TO TANK TANK TO BUILDING Q (KW) Time (hours) Figure 4-5: Thermal energy usage in strategy 1 The figure above illustrates the thermal energy usage in strategy 1. Strategy 1 employs a system with charging the CWS tanks during off peak periods (22:-6:) and utilizing the stored energy during peak periods (6:-22:). Using this strategy, there is full storage which allows the CWS tanks to completely cool the building during peak hours without the need of running the chillers. x 1 4 Electricity consumption W (KW) Time (hours) Figure 4-6: Electricity consumption in strategy 1 The figure above illustrates the electrical consumption of strategy 1. As seen above, electricity is only used during the off peak hours of (22:-6:). 37

51 x 1 4 Cooling production 1 8 Q (KW) Time (hours) Figure 4-7: Cooling production by chillers in strategy 1 The figure above illustrates the cooling production by chillers in strategy 1. As seen above, the CWS is only cooled during the off peak hours of 22:-6:. This aligns itself with the same time frame of electricity consumption seen in Figure 4-6. It can be noted that although electricity consumption is constant over this period, the cooling production slightly decreases due to changing COP. Strategy 2 1 x CHILLER TO BUILDING CHILLER TO TANK TANK TO BUILDING 7 6 Q (KW) Time (hours) Figure 4-8: Thermal energy usage in strategy 2 The figure above illustrates thermal energy usage in strategy 2. Strategy 2 employs a system with charging the CWS tanks during off peak periods (22:-6:), utilizing the stored energy during the peak periods (8:-21:) and uses the chillers to 38