Thermal Energy Storage (TES) Engineering
TES DESIGN: THERMAL ENERGY STORAGE OVERVIEW
TES is a way of producing cooling (or heating) at one point in time and using that heat or cold at another. Common TES systems include the storage of chilled water or hot water in a tank.
With a properly designed thermal energy storage system, Goss Engineering can help you:
TES systems are generally either full storage or partial storage systems. TES systems gain their major economic advantage from the difference between on-peak (daytime) and off-peak (night-time) electric energy rates and demand charges. Since electricity is the primary energy source often used for cooling, chilled water TES is more common than hot water TES (heating is generally fueled by natural gas, which is not subject to time-of-use price differentials).
Full storage systems provide all the cooling from the TES system during a certain part of the day (generally on-peak hours) with cooling equipment shut off. During other hours, the cooling equipment is operating at higher load than required to meet the instantaneous load, with the cooling surplus stored.
Partial storage systems typically operate chillers at a constant rate over the full day. Peak loads are met utilizing a mix of stored cooling plus instantaneous chiller produced cooling. During lighter loads, the excess capacity from the chiller is stored.
Though full storage systems can achieve greater energy cost savings than partial storage systems, the amount of cooling equipment and TES capacity (and thus the capital cost) is generally greater with full storage systems than with a partial storage system.
TEXTBOOK ON TES SYSTEMS BY LUCAS B. HYMAN, P.E., LEED – AP
Sustainable Thermal Storage Systems: Planning Design and Operations
A practical guide on how to plan, design, and construct sustainable thermal storage systems.
Use of thermal storage—also called thermal energy storage (TES)—can result in: reduced on-peak electric demand; reduced energy costs; smaller required chiller capacity to meet peak cooling demand; lower capital costs; lower life cycle costs; improved operational flexibility; less air pollution. This book covers all of these aspects.
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ABOUT THE AUTHOR:
Lucas B. Hyman, P.E., LEED-AP, is one of the co-founders of Goss Engineering, Inc. A professional mechanical engineer with nearly 30 years’ experience, he has planned and designed thermal energy storage (TES) systems, central heating and cooling plants, cogeneration plants, and other mechanical systems. Hyman has participated as a mechanical engineer member in value engineering project reviews, conducted and led forensic engineering studies, and served as an expert witness. He is coauthor of Sustainable On-Site CHP Systems: Design, Construction, and Operations (2009).
COMPLETED TES SYSTEM PROJECTS
Goss Engineering is an active participant in the evolution of Thermal Energy Storage design practice in the United States. Goss Engineering staff members have co-authored the following articles on TES:
REFERENCES ON TES / THERMAL ENERGY STORAGE
Sustainable Thermal Energy Storage Systems: Planning, Design and Operations A practical guide on how to plan, design, and construct sustainable thermal storage systems, by Lucas B. Hyman, P.E., LEED-AP, President of Goss Engineering, Inc. (Published by McGraw-Hill.)
Thermal Energy Storage for Space Cooling: Technology for reducing on-peak electricity demand and cost [includes TES design and applications] U.S. Department of Energy — Federal Energy Management Program — December 2000 — 36 pages
[Excerpt] Thermal energy storage for space cooling, also known as cool storage, chill storage, or cool thermal storage, is a relatively mature technology that continues to improve through evolutionary design advances. Cool storage technology can be used to significantly reduce energy costs by allowing energy-intensive, electrically driven cooling equipment to be predominantly operated during off-peak hours when electricity rates are lower. In addition, some system configurations result in lower first costs and/or lower operating costs compared to non-storage systems. more
Energy Storage: A Critical Path to Sustainability Mark M. MacCracken, PE, LEED AP, President, CALMAC [manufacturer of thermal storage equipment]
[Excerpt] To understand the importance of storage, it is imperative that one understands the electric power grid. If you have ever lived in a warm environment, you have probably experienced a brown out. Brown outs typically happen in the heat of day, when the temperatures are high and buildings across the area are turning up the air-conditioning and creating an enormous need for energy. Because of this, in the middle of any day, the demand on the power grid is the highest. In addition to the air-conditioning running at full power, more lights are on and multiple appliances are in use. Because of the strain on the grid, the costs for electricity are highest during those “on-peak” hours and the generation is often the dirtiest since all the old plants are turned on to help meet the demand. On the flip side—at night—when the majority of people are sleeping, there is a very low demand on the grid, and sometimes, even over-capacity. This is called “off-peak.”
Storage is the Answer: In its present configuration, our electric grid has almost no “storage” capability so that electricity must be produced exactly when it is needed. This is possible when your source of energy is fossil fuel (stored energy) but is very difficult and expensive when it is renewable energy (wind or solar). Adding energy storage to the grid will be critical in our quest to lower societies’ carbon emissions. more
Thermal Energy Storage Myths , by Mark M. MacCracken, P.E., offers systematic data to refute popular misconceptions about TES (too big, too complicated, too expensive, etc). From ASHRAE Journal.
How to Use Solar Energy at Night
Thermal Energy Storage at a Federal Facility The Dallas Veterans Administration Medical Center and Texas Utilities Electric Company join in an unprecedented partnership to lower energy costs. U.S. Department of Energy — Federal Energy Management Program — July 2000 — 2 pages
2009 Energy Storage Research Portfolio from the Electric Power Research Institute
Source Energy and Environmental Impacts of Thermal Energy Storage — a 1996 report from the California Energy Commission Thermal Energy Storage Systems Collaborative
Thermal Energy Storage: Systems and Applications by Ibrahim Dincer and Marc A. Rosen (Wiley Engineering Textbook, 2002)
[Book description from Amazon] During the last two decades many research and development activities related to energy have concentrated on efficient energy use and energy savings and conservation. In this regard, Thermal Energy Storage (TES) systems can play an important role, as they provide great potential for facilitating energy savings and reducing environmental impact. Thermal storage has received increasing interest in recent years in terms of its applications, and the enormous potential it offers both for more effective use of thermal equipment and for economic, large-scale energy substitutions. Indeed, TES appears to provide one of the most advantageous solutions for correcting the mismatch that often occurs between the supply and demand of energy. Despite this increase in attention, no book is currently available which comprehensively covers TES.
Presenting contributions from prominent researchers and scientists, this book is primarily concerned with TES systems and their applications. It begins with a brief summary of general aspects of thermodynamics, fluid mechanics and heat transfer, and then goes on to discuss energy storage technologies, environmental aspects of TES, energy and exergy analyses, and practical applications. Furthermore, this book provides coverage of the theoretical, experimental and numerical techniques employed in the field of thermal storage. Numerous case studies and illustrative examples are included throughout.
Some of the unique features of this book include:
In short, this book forms a valuable reference resource for practicing engineers and researchers, and a research-oriented text book for advanced undergraduate and graduate students of various engineering disciplines. Instructors will find that its breadth and structure make it an ideal core text for TES and related courses. more
Thermal Energy Storage for Sustainable Energy Consumption: Fundamentals, Case Studies and Design NATO Science Series II: Mathematics, Physics and Chemistry (Springer, 2007)
[Book description from Amazon] We all share a small planet. Our growing thirst for energy already threatens the future of our earth. Fossil fuels – energy resources of today – are not evenly distributed on the earth. 10% of the world’s population exploits 90% of its resources. Today’s energy systems rely heavily on fossil fuel resources which are diminishing ever faster. The world must prepare for a future without fossil fuels.
Thermal energy storage provides us with a flexible heating and/or cooling tool to combat climate change through conserving energy and increasing energy while utilizing natural renewable energy resources. Thermal storage applications have been proven to be efficient and financially viable, yet they have not been exploited sufficiently.
Çukurova University, Turkey in collaboration with Ljubljana University, Slovenia and the International Energy Agency Implementing Agreement on Energy Conservation Through Energy Storage (IEA ECES IA) has organized this NATO Advanced Study Institute on Thermal Energy Storage for Sustainable Energy Consumption – Fundamentals, Case Studies and Design (NATO ASI TESSEC), in Cesme, Izmir, Turkey on June 6-17, 2005.
Eminent experts who have worked in a number of Annexes of IEA ECES IA were among the lecturers of this Advanced Study Institute. 24 lecturers from Canada, Germany, Japan, The Netherlands, Slovenia, Spain, Sweden, Turkey, and USA have all enthusiastically contributed to the scientific programme. In Çesme, Turkey, 65 students from 17 countries participated in this 2 week summer school.
This book contains the manuscripts prepared based on the lectures included in the scientific programme of the NATO ASI TESSEC. You can also find the design example assignments from the computer workshops. more
Chapter 1 — Overview ( Download Chapter 1 as PDF file )
Sustainable Thermal Storage Systems: Planning Design and Operations
TEXTBOOK ON TES SYSTEMS BY LUCAS B. HYMAN, P.E., LEED – AP
Modern thermal storage is a proven technology with numerous benefits for multiple stakeholders. In fact, thermal storage has been saving money for knowledgeable facility managers and building owners for several decades. Utility companies have also benefited from the implementation of thermal storage over the years. The savings to utility companies, by eliminating the need for additional power generation and transmission construction simply to meet on-peak electric load, is so valuable to utilities that many utilities have provided financial incentives to facility owners to reduce the first cost of thermal storage installation. In addition, utilities often offer electric rates that provide incentive to transfer consumption of power from on-peak to off-peak periods, when utility-wide electric demand is much lower.
This book provides proven techniques for reducing energy use and cost and even potentially reducing capital costs with the use of thermal energy storage. It also provides a practical guide to the planning, design, basic construction, and operation of sustainable thermal storage systems.
There are several different types of energy storage methods including mechanical energy storage, chemical energy storage, magnetic field energy storage (an up-andcoming field), and thermal energy storage. Each energy storage method has numerous storage technologies such as, for example, mechanical energy storage—pumped hydro, compressed air, and flywheels; chemical energy storage—fuel oil storage; and many different types of battery storage technologies. However, this book discusses thermal storage, which relates to storing sensible or latent thermal energy; it focuses primarily on common applications of thermal storage in use today for building or facility heating and cooling.
The addition or removal of sensible energy causes a change in a material or substance’s temperature, whereas the addition or removal of latent energy causes a material or substance to change phase (e.g., turn from water to ice). These processes are discussed in detail in later chapters.
Thermal storage is a low-cost form of energy storage that is often highly efficient. In fact, some thermal storage systems provide heating or cooling with less input energy than a nonstorage conventional production at time of use system. When combined with less expensive energy input, thermal storage provides a powerful “green” tool for the building or facility owner.
There are both passive and active thermal storage systems. Active thermal storage systems use mechanical production and transportation heating or cooling equipment and prime movers such as fans or pumps with ducts or piping; passive thermal storage systems do not. An example of passive thermal storage is the sun shining on a high mass wall. However, this book focuses primarily on active thermal storage systems.
At its most basic, thermal storage provides methods to separate the time when heating or cooling is produced or generated from the time when that heating or cooling is needed or used. Thermal storage is sometimes defined as a way of producing an energy sink or source at one point in time and using it at another point in time. Thermal storage, therefore, provides methods and systems that allow storage of either cooling or heating produced at one period in time for later use at another period in time. For example, cooling can be produced at night in a more efficient manner than during the day at much less costly energy rates for use during the next day. Likewise, for example, solar heating can be produced during the peak sun output during the day and stored for space heating or domestic hot water use later in the evening or during morning warm-up. Cooling or heating can even be stored and used over our seasonal periods, for example, storing cooling in winter for use during summer, or storing heat in summer for use during winter.
Air-conditioning is often the largest single contributor to a building’s energy costs, and when combined with other building energy use, the time of peak cooling energy needs produces the highest building electrical demand by far. That peak electrical demand is not only costly to the building owner in purchased energy, but it also creates challenges for the electric utility serving the building or plant. The utility may need to own and operate peak electric production and transportation equipment that is needed only for a few hours a year. To the utility that means a very costly plant investment for the revenue received.
Thermal storage is also sometimes called thermal energy storage or TES for short. Cooling thermal storage is sometimes called “cool storage” or “cooling storage.” These terms are shortened versions of the original terms but understood by designers and users of such system even though not technically accurate, as it is not technically possible to store “cool.” What the term means is to store an energy sink or a place that accepts heat removed from a building or structure.
As discussed further in this chapter, there are many benefits to the use of thermal storage including the following:
Adding thermal storage can make the existing production equipment serve a far greater load than the installed equipment capacity. When thermal storage is combined with measures to increase the temperature rise or drop (i.e., delta-T) in existing air or water systems, the thermal storage solution often means greater loads can be served with the same pipes and pumps or fans and ducts.
Society also benefits with the installation of thermal storage due to less pollution as a result of more efficient facility and utility operations as well as less polluting utility operations at night (depending upon the fuel–power mix used by the utility to produce electric power during thermal storage charging).
As discussed in Chap. 2, thermal storage technology can be and has been used by a variety of facilities and buildings including residential, commercial, and institutional for both the public and private sectors.
One typical highly desirable condition to use thermal storage is to have a building(s) or facility where the loads are cyclical or otherwise variable over time. The time period is often daily but can be weekly, seasonal, or even short term, for example, 10 to 20 minutes for emergency cooling. If the facility or building has a very flat load profile, thermal storage may not be suitable for load shifting, unless there is a significant difference in energy costs at one part of the day over other periods of the day. Another case for thermal storage with a constant load is to serve as an emergency backup in case of loss of heating or cooling production equipment. However, most facilities or buildings have a cyclical load profile.
Thermal storage systems are often most economical in locations that have time-dependent energy costs. For example, many utility companies charge more for on-peak power consumption (power used during the day) than for off-peak power consumption (power used during the night). In addition to energy consumption charges, most utilities also charge for peak power use. Thermal storage offers a powerful tool to reduce peak power demand. Some utilities provide economic incentives such as super off-peak pricing to encourage off-peak cooling production as well as utility rebates for the installation of thermal storage. One recent design will receive more than $4 million from the local utility company to assist with their thermal storage installation. Another client buys power from the utility, as well as generating much of their own electric power on site, and dispatches thermal storage to store produced chilled water using very low-price power. The serving utility has a large nuclear plant base and during some nights actually pays the project to use electricity. At other times, the combined real-time-pricing demand costs for electricity may approach $1000 per megawatt (MW). During those expensive utility time periods, the stored chilled water is used to meet campus cooling needs. The use of thermal storage with smart meters and real-timepricing offers the opportunity for enhanced energy cost savings through specifically targeted peak demand reduction in those hours with the highest overall energy cost rates.
Thermal storage is often the most economically attractive:
Life-cycle cost analysis often shows that employing thermal storage is the lowest life-cycle cost option among the many alternatives analyzed.
Thermal storage can also provide important benefits to utility electric generating and combined heat and power (CHP) plants when used for combustion turbine inlet-air cooling (CTIC).
There are a number of materials that can be used as a storage medium in thermal storage systems. The most common material is water, either as a liquid or as a solid. The most typical water thermal storage systems include stratified chilled water thermal storage systems, different types of ice storage systems for cooling, and hot water storage systems for heating. As discussed in this book, each type of thermal storage material and thermal storage system has its own thermal storage, equipment, and system requirements. Each system also has its own advantages and disadvantages. The past two to three decades have refined the thermal storage systems commonly used so that over time many of the thermal storage systems that were once tried and proved less successful are no longer used.
Thermal storage is becoming ever more important as energy costs rise and capital resources are ever scarcer. In addition, thermal storage is an important piece of the “green” solution puzzle to successfully incorporate renewable energy resources such as solar power and wind power into the power grid.
Benefits of Thermal Storage
As outlined above, there are many benefits to the use of thermal storage. The following paragraphs provide an overview of some of the many benefits of thermal storage.
Reduced Utility-Bill Peak Demand
As will be shown and explained in this book, the use of thermal storage can allow a facility or building to reduce or virtually eliminate the on-peak electrical demand associated with facility cooling production. Demand is an engineering name for the instantaneous rate of consumption of the item being measured. Examples include flow, cooling load, heating load, or most common electric power. Utility companies, in particular, measure a facility’s electric power use both by the amount of total electrical energy consumption (often measured in kilowatthours, kWh) over a particular time period (e.g., monthly) and also by the peak power consumption rate, that is, the maximum demand (often measured in kilowatts, kW) during certain utility rate time periods; the utility company measures, records, and charges for the maximum peak power demand during each utility rate time period. In deregulated or re-regulated utility environments, there may be real-time pricing (RTP) or spot market rates, which will also reflect time-of-use variations in electricity costs.
Fortunately, the facility on-peak (and even mid-peak) power demand, typically, the most expensive utility rate period, can be reduced through the use of thermal storage. For example, as shown in Chap. 7, and known as full storage, cooling production can occur at night (with subsequent recorded energy consumption and demand costs accruing to the facility during that nighttime period), with only the thermal storage system operating the next day, during the peak cooling load period, to supply all of the facility’s cooling needs with all of the cooling production equipment turned off (except for chilled water distribution pumps and facility air-handling units, which supply conditioned air to building occupants, and are, of course, turned on whenever a facility is occupied, whatever the cooling or heating method employed). As cooling production equipment is turned off during the on-peak period in this example, no electric power is, of course, required to operate the cooling production equipment, and the facility electric power demand is, therefore, reduced accordingly. Note that the facility cooling or heating load or demand itself is not reduced, just the cooling production equipment demand required to meet the facility load during the on-peak period, as operations required to meet the facility cooling load are modified, and, as will be seen, smaller production equipment can be used to meet the facilities cooling load.
Improved Facility Plant Efficiencies
As noted above, with cool thermal storage, cooling production equipment (e.g., chillers) are often operated more at night, when air temperatures (dry-bulb and wet-bulb) are typically lower than during the day. The amount of work and hence energy required by cooling production equipment per unit of cooling produced is, in part, a function of the condensing temperature, which is a function of the outside air temperature. In fact, for installed cooling systems, at any given time, the condensing temperature is often a major factor in cooling system efficiency performance, since many of the other efficiency factors are fixed for an installed cooling system. Lowering the condensing temperature can improve cooling production equipment energy efficiencies by up to approximately
1.5 percent for each degree Fahrenheit that the condensing temperature is lowered below the design condensing temperature.
Another contributor to improved chiller efficiency, with thermal storage systems, is that chiller equipment runs at full load most of the time, resulting in better overall plant kW-per-ton performance than when chiller equipment runs at low part load. Further, many ancillary components such as condenser pumps, cooling towers and similar appurtenances are based on full chiller load and do not unload linearly (if at all) with cooling load reductions. Therefore, operating mechanical equipment at or close to full load is generally more efficient than part load operations, and plant fixed parasitic losses are proportionally minimized at full load. Additionally, a fair amount of mechanical equipment often operates more inefficiently at low part load than at full load. As a result, conventional nonstorage chiller plants often have very poor annualized performance due to excessive part load operations. However, there are some all-variable-speed systems that do try to minimize power consumption at part load by adjusting individual equipment operating speeds.
Reduced Energy Consumption
As suggested, operating cooling production equipment more fully loaded at night, when dry-bulb and wet-bulb air temperatures are lower, often results in higher facility cooling plant energy efficiency. This is due, in part, to lower condensing temperatures at night but is also due to greater efficiency of chiller operating at design point. Another factor is that when pumps and cooling towers are operating with a full design temperature difference, the energy consumption per ton of cooling produced is less. Many nonstorage conventional plants, which must operate most of the year at part load, can use more energy in pumping energy than in cooling production equipment such as chillers or ice-makers. Another factor is that a well-designed thermal storage often uses much higher delta-T and, therefore, less chilled water flow than conventional systems, which must match delta-T to the chiller. Most thermal storage systems use variable flow, and this also reduces pump energy use during part-load operations. Therefore, a well-planned, designed, and constructed thermal storage plant has lower overall cooling plant energy consumption than conventionally operated nonstorage cooling plants.
Lower Energy Costs
Lower facility electric demand results in lower utility demand costs, which are often a significant portion of the lower energy costs experienced by facilities with installed thermal storage systems. However, reduced energy consumption as well as shifting production from more expensive utility energy rate periods to less expensive utility energy rate periods also provide for lower facility energy costs. Based on experience, annual energy cost savings can be as much as 20 percent (and sometimes even more) depending upon the thermal storage system installed. Installations with real-timepriced power have shown even greater savings.
More Flexible Facility Plant Operations
Thermal storage provides for more flexible plant operations. The thermal storage system can act as a “buffer,” and the rate of discharge from a thermal storage system is typically variable from near zero discharge up to a maximum discharge flow rate as designed. Thermal storage systems are often designed for facility peak load conditions, and, therefore, during non-peak load conditions (which are most of the hours of the year), plant operations can easily be adjusted so that, for example, more cooling production occurs during off-peak hours with less cooling production occurring during mid-peak hours and no cooling production occurring during on-peak hours (even if the thermal storage was designed as a partial storage system on the peak day). Further, during lower load periods of the year, it may likely be possible, for example, for a cooling production unit (e.g., a chiller) to be out of commission and for the cooling plant using thermal storage still to be able to meet the facility’s cooling load. In addition, during the case of an unforeseen plant shutdown, it may still be possible to serve critical loads for a period of time from thermal storage (if the thermal storage was charged or partially charged when the unforeseen plant shutdown occurred).
Added Backup Capacity
A daily thermal storage system is typically sized for the peak design day load of the facility or building. On non-peak load days (which are the majority of the days throughout the year), the facility or building load can be met with less equipment capacity. Since less than full cooling generation equipment capacity is required during periods with lower loads, the additional generation equipment available is in a sense added backup capacity. In some cases, one or two chillers or similar equipment units can be nonoperational (e.g., down for maintenance), and the facility plant can still meet the facility load. In a real-life example, a chilled water storage system allowed for emergency cooling when the entire chilled water plant was down, by dumping ice into the storage tank.
Smaller Facility Equipment Requirements
As shown in Chap. 7, using thermal storage, it is possible to meet a facility’s peak heating or cooling load with installed equipment smaller than the facility’s peak load. For example, a thermal storage system can be sized so that on a peak day, the cooling production equipment operates at full load for the entire 24 hours (known as partial storage), and, therefore, the cooling production equipment size required with partial thermal storage to meet the facilities load is equal to the average load on the peak day (plus any desired spare capacity). With a conventional cooling plant (no thermal storage), for example, the cooling production equipment needs to be sized equal to the facility’s peak cooling load (plus any desired spare capacity). With a partial thermal storage system, depending upon the facility load profile, it is often possible to use cooling equipment only half the size of the facility’s peak load and still meet the facility peak load.
Lower Facility Maintenance Costs
Maintenance costs are, in part, proportional to the number and capacity of the installed equipment; and, therefore, less equipment capacity requires less maintenance and results in lower maintenance costs. Additionally, operating mechanical equipment most of the time at full load with less starts and stops reduces maintenance requirements and subsequent maintenance costs. Therefore, some thermal storage systems can require less maintenance costs per unit of thermal energy delivered.
Potential Lower Capital Costs
A properly planned and designed thermal storage system often provides lower capital costs in addition to lower energy use and lower energy costs. This is due to smaller equipment requirements and smaller related infrastructure requirements, which is possible even with the added cost of the thermal storage tank. For example, a thermal storage system may only require 1000 tons of cooling equipment versus 2000 to 3000 tons of cooling equipment required for a conventional cooling plant with no thermal storage. Not only does the cooling production equipment cost much less, but the electrical and building infrastructure costs to support the cooling equipment will also be lower, potentially more than offsetting the capital cost of the thermal storage system, and, thus, producing a net capital cost savings versus a nonstorage system. Depending on the tank location and space demands, it is possible that there is also land cost savings.
Ongoing lower utility energy costs as well as potentially lower construction capital costs obtained when a facility uses thermal storage compared to a conventional nonstorage facility plant provide initial and ongoing life cycle direct economic benefits to a facility or building owner. Therefore, thermal storage systems can provide initial capital cost savings and ongoing energy cost savings for a combined economic benefit.
As many utilities presently use fossil fuels to generate power, and the burning of fossil fuels harms the environment due to stack emissions (NOx, SOx, CO, particulates, etc.) as well as CO 2 emissions [considered by the majority of scientists today (2011) as a likely factor in global warming], using less energy with a well-designed and constructed thermal storage system typically results in less pollution. Additionally, at night, utilities typically operate their most efficient or most cost effective power production equipment to meet their base nighttime electric load. This base electric load often includes nuclear, hydroelectric, wind, and other green-energy power production systems as well as coal-fired systems. In contrast, many peak energy plants burn fossil fuel such as natural gas or oil, which typically increases the carbon burned over the base-load power plant (except for base-load coal power plants). Utility plants are also often more efficient at night than during the day, also due to the colder nighttime temperatures. Additionally, power line (transportation and distribution) losses (I 2 R losses) are much lower at night due to lower amperage levels in the conductors and transformers at night than during the day. Taken together, nighttime power is generally less polluting than daytime power; however, this depends on the utility power generation fuel mix.
For example, some utilities use base-loaded nuclear power plants, and, therefore, with respect to stack emissions, nighttime power is very clean (nuclear power plants, of course, have other serious environmental issues beyond stack emissions). However, some nighttime power is actually dirtier than daytime power; for example, with utilities whose nighttime base electric load is supplied by coal-fired power plants but who use natural gas fired power plants to meet customer peak demand during the day. Studies have shown that for every kilowatthour of energy shifted to off-peak from on-peak, there is a reduction of 8 to 30 percent in fuel consumption, air-pollutant emissions, and greenhouse-gas emissions produced by the power plant. 1
Finally, as less cooling equipment capacity, which often use refrigerants, is needed with thermal storage systems, less refrigerant leakage occurs, which may be harmful for the environment (destruction of the ozone layer, for example) depending on the refrigerant used.
Benefits to the Utility
During peak periods of the year, it can be an ongoing challenge for some utilities to meet customer peak electrical demand when they do not have enough of their own power generation resources and must buy on-peak electric power from other sources, which is often relatively very expensive. In addition, some cities have highly loaded or constrained electrical grid infrastructures (in addition to the constrained power transmission lines noted previously) that are at or near capacity during peak demand periods. The capacity shortfall challenge is aggravated at high ambient air temperatures, which reduce the transmission capacity of the electrical distribution system. Thermal storage offers a way to shift peak cooling production, which can represent as much as 40 percent of a facility’s electric demand, 2 from the constrained on-peak periods to off-peak hours, when excess utility capacity exists in both power generation resources and in the infrastructure grid (the wires could handle more electrons). The use of thermal storage can delay the need for additional utility power plants and additional transmission network capacity. Utilities may even be willing to negotiate better electric rates due to the thermal storage facility’s improved load factor.
Society benefits from the use of thermal storage for some of the reasons listed above, and the current (2010) U.S. Congress is presently considering incentivizing the installation of thermal storage. Society benefits from less pollution and CO 2 emissions from power plants due to lower facility energy use, less transmission loss, and possible more efficient power plant operations for nighttime produced power. Modern society uses tremendous amounts of electricity and all projections show a continued increase in peak electricity demand. Utilities and governments are looking at investing billions of dollars into upgrading and expanding existing electrical infrastructure grids, creating smart grids, and developing additional power generation resources, increasingly including those from alternative energy sources such as wind, ocean, geothermal, biofuels, and solar power. Thermal storage can help reduce peak period electric demands, and, therefore, reduce the strain on utility generation resources, electric transmission lines, and city electric infrastructure grids. In addition, as wind power (which is by nature intermittent and often has its greatest output during off-peak nighttime periods) is employed in an increasing share of the electric generation mix, energy storage (thermal or otherwise) must play a correspondingly increasing role in the electric grid. Thermal storage is perhaps the most technologically ready and most economical means of energy storage to contribute to this role.
Thermal Storage Basics
The term “thermal,” as discussed in this book, means relating to or caused by heat. One definition of heat is the flow of energy due to a temperature difference; however, heat can also be transferred by radiation (as from the sun) and by direct mixing of fluids (known as convection). From a study of physics, as well as one’s own experience, one learns that it takes energy to raise a material’s temperature or to cause any material to change phase from a solid to a liquid or from a liquid to a gas. Conversely, energy is released or removed when a material or substance’s temperature is lowered or when a substance changes phase from a gas to a liquid or from a liquid to a solid. A given amount of a material with a higher temperature contains more energy than the same amount of material at a lower temperature. Likewise, a given amount of material at a higher state of matter (e.g., as a gas versus as a liquid, or as a liquid versus as a solid) contains more energy than the same amount of material at a lower state of matter.
Therefore, thermal energy can be stored (charged) or removed (discharged) in a material by raising or lowering a material or substance’s temperature or by causing a material or substance to change phase. When energy is stored and discharged by a difference in temperature of a material or substance, the process is known as “sensible” thermal storage, and when energy is stored and discharged due to a change in phase (e.g., liquid to ice and ice to liquid, respectively), it is known as “latent” thermal storage. When a material or substance changes phase, usually, there is no change in a material’s temperature.
The amount of energy per unit of mass that must be removed from liquid to cause it to turn into a solid is called the latent heat of fusion, and, of course, is the same amount of energy that must be added to a solid to cause a solid to turn into a liquid. In English units, the latent heat of fusion for water is approximately 144 British thermal units (Btu) per pound mass, and occurs at 32°F at 1 atmosphere of pressure.
A common thermal energy storage term is tons or ton-hours. This term is derived from the fact that the earliest space cooling was achieved by melting ice. A ton of refrigeration is the amount of cooling that can be achieved by melting 2000 lb (1 ton) of ice in 1 day. As a rate of cooling, the “ton” unit of cooling capacity is defined by the rate of melting a ton of ice over 24 hours. With the heat of fusion of 144 Btu per pound (lb) of ice and 2000 lb of ice, this equates to 288,000 Btu over 24 hours, which is equal to an hourly rate of 12,000 Btu/h. Thermal storage is measured in ton-hours and 1 ton-hour is equal to 12,000 Btu.
Thermal Storage Media
There are various materials or substances that have been successfully used in thermal storage systems, with sensible water and latent water or ice being the most common thermal storage media. Successful thermal storage media will likely have many of the following material properties:
Of course, the more common a material, the more readily available it is. And it is likely that the material will be of relatively low cost. Ideally, thermal storage media should not harm the environment if it were to leak to the surroundings. Nor would one want thermal storage media to be dangerous or toxic to plant operators, or to be corrosive to plant systems. The ideal thermal storage media should be relatively inert and stable over time and require little or no attention. A relatively high thermal density (mass density times specific heat) is a positive attribute for thermal storage media because it allows relatively more heat transfer per unit volume, all else being equal. Likewise, a high specific heat is a positive attribute for sensible thermal storage media, because it also allows relatively more heat transfer per unit volume, all else being equal. For latent systems, such as ice thermal storage, it is a positive attribute for thermal storage media to have a high latent heat of fusion because it allows more heat to be stored per unit volume. It is apparent from a review of the ideal thermal storage media properties outlined above that water exhibits all of these properties and that is why water (or water-ice) is the most common thermal storage media in use today.
The following are typical materials used for thermal storage media and thermal storage systems:
Ice (the solid form of water) is one of the more common thermal storage materials used today. Ice provides one of the highest theoretical thermal storage densities (and, therefore, the lowest storage volumes) of any thermal storage system and uses one of the least expensive storage materials, water. Ice systems have been shown to be excellent for smaller and even for some larger packaged installations. The primary disadvantages in ice-based thermal storage systems involve the often special ice-making equipment required for the systems, the often greater energy consumption inherent in operating the lower temperature equipment, and a typically shorter equipment life than competing sensible chilled water solutions.
PCM include the true PCM such as paraffin waxes, organic waxes, and salt hydrates (water or ice is, of course, a PCM but is typically considered as a separate category). PCM also include materials that combine to change phase at a given temperature point, such as eutectic solutions, and materials that form high energy bonding structures. Research is presently under way to develop advanced compact phase change thermal storage materials and to investigate thermo-chemical materials and methods (e.g., sorption).
Thermal storage can be obtained in the temperature changes of almost any material. Many thermal storage systems have been built using gravel. Gravel has the advantage of being able to produce air temperatures very close to the storage temperature because of the large surface area of thermal contact. Gravel thermal storage systems in some cases can be relatively inexpensive. Systems using gravel use cooler night air or evaporative cooling for production of cooling. In some cases, air quality problems may result, including damp and musty odors. In certain cases, tests have shown that the gravel systems require more energy than conventional cooling solutions.
Concrete or other building materials are sometimes used to provide at least part of the building cooling during on-peak energy periods by sub-cooling a building during the night or outside the peak electric demand period. The idea is as old as buildings themselves. In areas with substantial daily temperature swings, the windows are often opened. In other cases, buildings are sub-cooled to reduce the load at a peak period. When buildings are conditioned around the clock, the only variable is the outdoor heat gain factors. The mass of the buildings averages the effect of changes in outside air temperature and solar gains.
The earth or rock strata underground are sometimes used for thermal storage. Most of the work in this area is aimed at long-term or seasonal thermal storage. For example, a supply well and a recharge well combined with a closed-loop cooling tower might cool water during winter and, over a long period, reduce the temperature of a confined underground gravel strata or contained aquifer. During summer, circulation of water through the strata could exchange energy to provide chilled water. There are many variables with this system and much research is needed to develop optimum applications. Several projects using seasonal storage in aquifers have been designed in the U.S. Pacific Northwest, Canada, Sweden, and the People’s Republic of China. One of the oldest in operation was installed in Portland, Oregon, in 1947.
Types of Thermal Storage Systems
As discussed in Chap. 3, and in other chapters of this book, there are many different types of sensible and latent thermal storage systems including the following:
Additionally, there are many different types of arrangements, and thermal storage has even been achieved by collecting and piling up snow in winter and covering it up with reflective tarps to be used for cooling throughout summer.
Chilled water thermal storage systems are sensible thermal storage systems, and thermal storage is, therefore, achieved by a change, or the difference, in the chilled water temperature, that is, the temperature difference between the “warm” chilled water return temperature and the “cool” chilled water supply temperature. As the volume of chilled water thermal storage to meet a given ton-hour capacity is a function of the temperature difference (delta-T), chilled water systems with a higher delta-T require smaller thermal storage volumes and are subsequently more economical than thermal storage systems with relatively lower chilled water system delta-T. System chilled water delta-Ts are typically in the range of 15 to 30°F; however, some chilled water systems have been designed with lower delta-T (higher delta-Ts are better).
Chapter 5 describes different types of ice thermal storage systems including the following:
An ice harvester is similar to the icemaker seen in motels and restaurants and is one of the oldest types of active thermal storage systems. The most common type of ice harvester systems produce ice on refrigerated plates or tubes, which, through the process of periodic defrosting, is allowed to fall off the plates or tubes and drop into a water-ice mix in a tank below.
An ice-on-coil thermal storage system typically has heat exchange tubes immersed in a water storage tank with refrigerant or water-glycol or brine flowing through the inside of the tubes. Ice forms on the tubes during the charge cycle and is then melted during the discharge cycle. One disadvantage with ice-on-coil systems is that the ice acts as an insulator and reduces heat transfer the thicker the ice becomes. Encapsulated ice uses small dimpled containers filled with water and surrounded by a flowing water-glycol or brine solution; the encapsulated water freezes during the charging or storage process and thaws during the discharge process.
ATES has been used successfully by a number of countries, including Sweden and China who have a large number of successful projects that provide cooling to business and industry and heating to residences. In fact, China demonstrated an ATES system in Shanghai in 1960 when cold water was injected into the ground to stop subsidence, after which it was discovered that the cold water could actually be stored in the aquifer for later use.
Brick storage heaters (or electric storage heaters) use electric resistance heat dispersed in an arrangement of a dense mass of the bricks to store thermal energy. Likewise, gravel can be used for an under-floor (actually under one’s house or building) thermal storage system.
Thermal Storage Operating Strategies
As shown in Chap. 7, thermal storage systems are usually sized to be either full storage or partial storage systems. A full storage system provides all the cooling or heating required in a specific period with all the production equipment shut off. This time period is usually the on-peak hours when electrical costs are highest. During nonpeak hours, the production equipment is operated at a higher capacity than that required to meet the instantaneous thermal loads. The surplus heating or cooling is stored in the thermal storage system.
Partial storage systems typically use equipment operating at a constant rate over the entire day (24 hours), when serving peak design day load profiles. For example, facility peak cooling demand can be met with partial storage systems provided from a mix of stored cooling plus chiller-produced instantaneous cooling. During periods of lighter loads, a portion of the chiller output is stored in the thermal storage system. The minimum production capacity is equal to the sum of the loads over a design period such as a day or a week divided by the number of hours in the design period. The production capacity then is the average load for that period.
Full storage systems gain their major advantage from the difference between on-peak and off-peak electric demand charges and energy rates. Transferring demand and energy consumption out of the periods of the day when it is most costly can achieve large energy cost savings, but the amount of cooling equipment and thermal storage capacity needed is generally greater with full storage system than it is with a partial storage system. As noted, a thermal storage system designed to operate as a partial storage system on a peak design day can often operate as a full storage system on days with sufficiently lower loads than peak design.
Thermal Storage Economics
Favorable economics for any endeavor means that a favorable return on investment (ROI) is realized or is projected to be realized based on careful study and planning. What is considered a favorable return, of course, varies from person to person, from company to company, and varies over the short and long term depending upon current economic conditions. The bottom line is that capital (i.e., money) is or will be invested with a minimum expected financial payback (a certain amount of money returned over a certain amount of time). However, in some cases, thermal storage systems represent both the lowest first cost alternative as well as the alternative with the lowest energy costs (or greatest energy cost savings).
As discussed in Chap. 8, there are a number of key economic factors that affect any financial analysis such as the following: the discount rate; the length of the time period used in the analysis; and estimated escalation rates for utilities (electricity, natural gas, other fuels, and water), labor, and maintenance. Further, thermal storage economics depend on a number of important factors including both technical and economic factors. Initial capital required for construction (known as first costs) directly impacts the bottom line of any life-cycle cost analysis and is a major factor whether or not a proposed thermal storage system is more economical than proposed alternatives. The construction costs, of course, depend upon a myriad of factors too numerous to enumerate here, and there is a wide range of construction costs even normalized to a per ton-hour basis. The cost required to install a piping distribution system can be significant, especially if there are challenges such as numerous existing underground utilities (known and unknown).
Key technical factors that affect thermal storage economics include the following: the size of the delta-T for sensible thermal storage systems (the larger the delta-T, the more economical the system—this is true for all hydronic systems, not just thermal storage systems); the availability of space for a thermal storage system; the type of space available for a thermal storage system (e.g., open land above ground, open land below ground, space in mechanical rooms, space in or under parking garages); the location of the thermal storage system relative to the central plant; the type of thermal storage system installed; and the thermal storage operating strategy (full or partial shift) employed. Full storage systems provide the maximum operating cost savings; however, partial shift systems utilize smaller thermal storage and smaller equipment, and thus often provide the lowest capital cost, coupled with some operating cost savings, often yielding the best life-cycle economics. However, actual conditions such as hourly load profiles and utility rate structures must be analyzed to determine the optimum design solution for any given application.
Thermal storage occurs whenever a heating or cooling process is performed at one time while energy is stored to accomplish heating or cooling at a different time. Of course, some thermal storage occurs naturally, and some of the earliest forms of thermal storage were designs that built on those natural processes. As an example, people found that massive stone structures created a thermal flywheel effect. A stone fortress, with its thick walls, did not get as hot in the day as the outside, or as cold at night, even without any artificial heat or cooling added. This fact led to buildings that purposely used mass to accomplish passive thermal storage.
Some 4000 years ago in Iraq, it is reported that communities used natural ice for cooling, 3 and even today in northern Iraq, Kurdish communities still continue to use natural ice, which is stored in earth pits for use during summer. Later, annual thermal storage was achieved when the Roman emperors and wealthy Romans sent slaves into the Alps to gather ice blocks from glaciers and bring them to Rome for use during summer for cooling buildings and cooling some desserts. Victorian era houses used blocks of ice cut from frozen lakes during winter for cooling during hot summer months. The process of using naturally produced ice was expanded, especially in the United States in the later 1800s using ice from the Great Lakes region. During winter when the lakes froze over, men took ice saws and cut blocks of ice, which were then stacked in large piles covered with sawdust to prevent melting. This pile of ice was thermal storage because it was stored for later use. The ice was removed from the pile during summer and delivered to customers. Some ice was used for space cooling, but it was primarily used for food preservation keeping food cool in specially constructed ice boxes, beverage chilling, and hospital use.
When mechanical freezing processes were developed on a commercial scale in the early 20th century, which largely put the natural ice harvesting companies out of business, refrigeration systems were typically limited to large central plants and large ice plants that produced block ice, which was delivered to homes even up to the 1940s in the United States.
Another very early form of thermal storage was hot water storage. Some of the earliest forms of thermal storage were large tanks that were heated from a fire located under them. With the development of more sophisticated plumbing systems, hot water systems were designed with storage tanks. Hot water storage tanks were included for one or more of several reasons. Usually, the reason to add hot water storage tanks was either to provided large quantities of hot water for a short duration using smaller heating systems than would be needed for instantaneous heat, or, in the case of homes, to use heat from the kitchen stove to heat and store hot water when the stove was being used. The stored hot water could then be used later when the stove was not in use. Most of the early systems took advantage of the fact that hot water compared to cold water is less dense and, therefore, lighter than the same volume of cold water. With a properly designed piping system, the difference in density between the water at different temperatures would cause a natural flow of hot water from the stove back to the top of the tank, with the denser colder water in the bottom of the storage tank flowing back to the stove. Hot water storage tanks are almost universally used today in residential buildings. In some cases, electric hot water tanks are controlled by the serving utility to hold off the electric heating element during peak electrical demands such as morning warm-up. During that period, hot water was provided from stored hot water, which had been heated previously.
In 1940, MIT constructed a house with solar heating that had a thermal storage tank in the basement, which was able to keep the occupants comfortable even in winter.
Gravel has been used as a storage medium for both heating and cooling storage. One example is a state office building for California in Sacramento. Another is a church in Springfield, Oregon, and a solar home near Cottage Grove, Oregon. In 1959, a system was installed in Denver, Colorado, that used glass plate solar collectors and a pea gravel bed for thermal energy storage.
Additionally, a few buildings have been designed using the building structure itself as a mass storage. One such building is an Oregon State University science building constructed in the late 1970s. The floors are constructed with hollow cores and cold outside air is drawn through them at night for cooling storage or exhaust air discharged through them for heating storage. Several such buildings have been designed in Europe.
Some early air-cooling systems used cold river or lake water pumped through coils or sprayed into the air stream to provide space cooling. With the invention of refrigeration systems, mechanically generated chilled water replaced cold river or lake water for the same use. One of the first notable uses of mechanically cooled air was in movie theaters. Movie theaters would often announce the presences of air-conditioned space even more predominately than the movies they were showing. The large difference in cooling load between the hours when a theater was in use and the other hours when a theater was not in use led to an operational technique to reduce the size of the cooling equipment needed. When not in use, the space was precooled colder than the desired comfort temperature. This meant some discomfort when people first arrived and, by the end of the day’s use, the space temperatures were higher than comfortable. This process of using the building’s mass for thermal storage is essentially a flywheel cooling process. However, the size of cooling equipment needed is smaller than that needed for the peak instantaneous loads. A similar technique is used today in some large sports arenas.
By the 1920s, Willis Carrier had developed a centrifugal chiller to chill water for distribution. The first centrifugal chiller is on display in the Smithsonian Museum in Washington, D.C. Chiller equipment proved to be reliable long-life compression refrigeration-cycle machines. A chiller machine with a serial number in the 300s was still in use in the Los Angeles Times building in the 1980s. Of course, many chiller design advancements and improvements were made in the 60 years that this chiller had been in service.
Early chilled water systems had to meet the cooling load they served at the time the load occurred. For a system serving a high peak load, this meant large costly equipment, heavy electrical loads at peak times, and much lower or no load at other times.
Some of the earliest space cooling thermal storage systems ever designed were for churches. A church might have a relatively huge cooling load Sunday morning for a couple of hours and little or no cooling load for the rest of the week. By the 1930s, a few church systems were designed using chillers operating all week long producing and storing chilled water for use during the peak hours Sunday morning. This thermal storage solution only required a chiller perhaps 1/80 of the size of a chiller sized to meet the peak load when it occurred. However, the thermal storage process also needed large volumes of chilled water since, at that time, nearly all chilled water cooling systems used 8 to 10°F temperature differences (delta-T). This meant that very large chilled water storage volumes were needed, and also the small temperature difference meant that the resultant small difference in density made it very difficult to circulate water in a thermal storage tank, without a mixing of the colder supply water and the returning warmer chilled water, which degrades the thermal storage capacity.
The fairly poor performance of early chilled water thermal storage systems and the high cost of large storage volumes needed discouraged much development of chilled water thermal storage systems for buildings until some factors changed later on. However, thermal storage continued to be used in dairy farms, churches, and movie theaters, all applications with traditionally short-duration high peak loads.
One factor that encouraged the development of thermal storage for buildings was related to building heating needs. In the U.S. Pacific Northwest, Bonneville dam was built in the 1930s, and that was followed by a series of other hydroelectric dams on the Columbia River. Those dams provided large amounts of relatively low-cost electrical energy. The Northwest tends to be cool most of the year, and the typical heating and cooling system for large buildings use central air-handing units. The large central systems typically mixed return-air with colder outside-air to a usual blend of 55°F supply air. That mixture was cool enough to provide the cooling needed by any space without mechanical cooling. Spaces that required less cooling or required heating had heat added. The concept was called “free outside-air cooling,” since one did not have to pay to operate cooling equipment, but perhaps a better name was free heat rejection. The center of any large building, without exposure such as a roof, ground floor, or outside wall can only have heat gains, such as that from people, lights, and equipment. On the other hand, spaces with external exposures can require either heating or cooling depending on outside conditions. It is typical for older large buildings in cool climates to require heating on the perimeter most of the year, and, at the same time, have a heat surplus in the central building core. This fact led to a widespread application of the so-called “bootstrap heat pump.” J. Donald Kroker was one of the leaders of its use, dating back to the years following World War II, and his designs were based on systems he had seen in Europe while he served in the military. A bootstrap heat pump eliminated the so-called free cooling cycle and instead provided mechanical cooling with chilled water to the spaces that required cooling such as in the interior core. Outside air was kept to the minimum needed for ventilation. The chilled water was produced by a central centrifugal chiller with a double-bundle water-cooled condenser. A water chiller uses the refrigeration cycle to take heat from the chilled water (and thus cools the chilled water) and raises the refrigerant in temperature to a point high enough to be able to reject the heat to the ambient, such as with condenser water in a cooling tower. A typical standard chiller might have chilled water from 55 to 45°F and rejected the condenser heat to cooling tower water at 95°F. Water from the cooling tower is cooled by evaporation and returned to the chiller at perhaps 85°F. The typical bootstrap heat pump would provide greater temperature lift with perhaps 120°F water temperature leaving the condenser and perhaps 100°F entering water temperature. This warmer condenser water was used in specially designed coils to provide the building heat. The heat provided to the building included the heat removed from the building core and the additional equivalent heat energy work output from operating the chiller.
This higher condenser temperature resulted in higher energy consumption per ton removed than in a typical cooling process. A typical process might require up to 2 percent more energy per °F higher temperature of the condenser water. Still, even with that penalty, the process from a heating perspective is very efficient with a coefficient of performance (COP) of 3 or better. That means 1 kWh of electric energy provides heating equivalent to 3 kWh of electric heating. Moreover, that combined with very low-cost electric power resulted in a very cost-effective combined heating and cooling solution. As a consequence, many larger buildings had those heat pump systems installed.
One drawback of the bootstrap heat pump solution, however, is the fact that heating and cooling cycles for the buildings do not typically coincide. In the early morning, buildings tend to have high demands to heat up the structure and meet other heat loss, and, at that time, have little if any cooling demands on the interior. Later in the day, with warmer outside temperatures, solar gains, lights, people, and processes, there tends to be more heat available than needed by the building. The issue of dealing with noncoincident heating and cooling loads when using a heat pump solution led to the use of thermal storage by the 1970s.
The systems simply added chilled water thermal storage to the same heat pump systems just described. The heat pumps then were used primarily as needed to meet the heating load drawing heat from the chilled water in the storage tank. The stored chilled water was then used later in the day to meet surplus cooling loads and in the process store heat in the chilled water tank to meet the next day’s heating loads. A serendipitous effect was the fact that the chiller did not have to be as large as the peak cooling load. In fact, the design typically spread the cooling load evenly over a 24-hour period. The reduced cooling plant size often resulted in a system that cost less than a conventional nonstorage system even with the additional thermal storage tank cost. Lower capital costs occurred because of much lower chilled water equipment cost and the lower related building, piping, cooling tower, electrical service, costs, and so on.
In the latter 1960s, Mt. Hood Community College was built around a central campus heating and cooling process using that approach and included thermal energy storage. A number of other larger schools and office buildings were designed with such heat pump or thermal storage systems. Because the volume of stored water is large, pressure tanks would be prohibitively expensive. The thermal storage tanks systems, therefore, tend to be atmospheric pressure type tanks (i.e., nonpressurized).
In the mid-1970s, a somewhat different approach was designed for Weyerhaeuser near Tacoma, Washington. A pressurized tank of warm condenser water was designed into the system. During the afternoon surplus gain period, some of the condenser water was stored in the tank. If more heat was needed by the building, an electric heater could heat the water even more with low-cost off-peak electric energy. During startup and high heating periods, the heat from the stored water was introduced into the chilled water system to increase the cooling load and thus the heat was available from the system for building heating. The primary differences were the thermal storage tank was smaller, but more costly being a pressure tank, and the chiller was larger because it had to meet the peak cooling load.
During the 1980s, similar solutions were designed in Wisconsin using ice as a storage medium. The ice was used for peak cooling, and heat was produced and utilized at night as ice was produced.
Many of the systems during the 1960s to 1980s used large concrete underground structures that resembled basements but were filled with chilled water. Those systems tended to have poor separation between warmer and colder chilled water. This was due, in part, to the fact that stratified systems tend to have a 2- to 3-ft zone of mixed water separating the colder (and denser) chilled water supply at the bottom of the TES tank from the warmer (and less dense) chilled water return on top of the thermal storage tank. The underground thermal storage tank design tended to be 15- to 25-feet (ft) deep, and 2 to 3 ft is a significant percentage of the volume if the tank is only 20 ft deep. Some smaller systems were designed using fiberglass storage tanks similar to gasoline underground storage tanks. Most such systems used several tanks in series to provide separation between the chilled water supply and chilled water return.
During the 1920s, a number of large buildings in Toronto, Ontario, Canada were designed with large underground concrete storage substructures. In an effort to improve stratification, a patented system was developed to introduce a membrane diaphragm between the chilled water supply and return within the TES tank.
A different approach was used in the Lane County, Oregon, Courthouse in 1978. There the tank stratification was improved by using a tall vertical steel storage tank. The tank was 65 ft tall, and as a consequence, the 2 to 3 ft stratification zone was a much smaller part of the total tank volume. The thermal storage tank performance was also enhanced by providing a greater temperature difference in the chilled water system than the typical 10 to 14°F used commonly at that time. Using a 40°F chilled water supply temperature and a 64°F chilled water return temperature (and therefore a 24°F delta-T), the thermal storage system only required half the volume of chilled water that would have been needed with a then standard 12°F delta-T system.
By the mid 1980s, thermal storage systems were being designed as purely chilled water storage without any use of the heat rejected by the chillers. The driving force for this change from previous designs was largely cost incentives offered by utilities for systems that would transfer the maximum amount of electric load from on-peak periods to off-peak periods. Many of the thermal storage systems designed were full storage systems. As previously noted, that means that the chillers were held off entirely during a certain on-peak period of highest demand, and cooling was provided entirely from chilled water previously produced and stored during the off-peak hours. To encourage design of such systems, utilities provided capital toward the first costs (generally proportional to the on-peak electric load demand reduction), and developed special electric rate structures. Those incentives often changed the economic advantage from partial storage solutions to full storage solutions. Many full storage systems of that type have been designed and have proven to be cost-effective especially on large central systems such as those with colleges, universities, or district cooling systems.
A different type of utility incentive for Lane Community College resulted in the campus being designed to be heated with two 3000-kW electric boilers in the mid-1960s. At that time, the local electric utility offered a rate of $0.003/kWh, with the agreement that the utility could require one boiler to be turned off at the time of peak utility demand. For the utility at that time, their peak electric demand was largely heating driven, with a peak load from 6 to 8AM. As the electric load increased over time, the utility modified that agreement in a way that encouraged other solutions. A bootstrap heat pump was installed along with a large heating thermal storage system. The heating thermal storage system used 200,000-gal underground pressurized stainless steel insulated storage tanks. Water in the thermal storage tanks was heated to 240°F by the boilers at night and used to provide additional warm up heat the next morning. The tanks were obtained from the government as surplus from an early missile development program.
From the late 1980s forward, stratified chilled water storage has been the dominant form of sensible cool storage, supplanting the earlier forms, which included the multiple tank (or “empty tank”) designs, the diaphragm or membrane tanks, and labyrinth tanks. Additionally, from the late 1980s forward, most chilled water tanks are specified and procured as “turnkey” installations complete with the internal flow diffuser systems for proper thermal stratification and inclusive of guaranteed thermal performance, versus earlier installations with separately designed and installed diffusers.
Stratified chilled water storage systems in the past have been limited to chilled water applications of 39°F or greater chilled water supply temperature, and typically have had about a 20° temperature rise (delta-T) at best. A couple of developments have increased the range of temperatures. 39°F has been the limit on the low side due to the fact that 39.4°F is the maximum density point for pure water (pure water becomes less dense at temperatures below 39.4°F). A patented solution is available to add chemicals that lower both the temperature of maximum density and the freezing point below that of pure water. Low-temperature fluid (LTF) thermal storage systems with 30° or lower chilled fluid supply temperature have been installed and are discussed in Chaps. 4 and 9.
By the 1970s, some systems were being designed using ice and other PCM. Early ice storage systems (ice harvesters) tended to be designed around some method of freezing ice on plates or on tubes with direct expansion refrigerant on one side of the plates or tubes and water freezing on the other side. As the water freezes, it adheres to the plate or tube and forms an insulating layer, which slows down subsequent heat transfer. Most systems had some way of removing ice from the plates or tubes such as defrosting. Defrosting tended to introduce more energy inefficiency into the process. Some alternative ice thermal storage systems had large piping systems with pipes in a water bath and refrigerant in the pipes. Such systems tend to require large volumes of refrigerant and lose ice production capacity with the increase of ice coating on the pipes. A trend today is that most ice systems use chillers and a water-antifreeze solution to freeze water inside capsules or on the outside of pipes in a water bath storage vessel.
Early thermal storage work also developed several eutectic storage solutions. These were used on both heating and cooling storage systems. Eutectic storage systems are latent storage systems and as with ice tend to provide most of their thermal storage within a single temperature point or at a very narrow range of temperatures. Most typically, the eutectic solution was in some capsules similar to encapsulated ice. The major difference was that the circulating solution used was water, and standard chillers were used producing chilled water at about the same temperature and cost as conventional nonthermal storage systems. There have been many problems with those solutions, including failure to function properly over time and high circulation volumes in comparison with stratified chilled water solutions.
There also have been several systems that drew on natural processes to provide longer term storage, typically annual. One such solution drew freezing outside air in winter into a building size insulated structure, and water was sprayed into the air producing ice crystals inside the structure. The ice crystals were stored in the building for cooling during summer. Similar systems have circulated water between wells, and the water cooled using cold winter air, which produced lower well water temperatures to use in summer for cooling.
A fairly recent technology development generates ice slurry in a vacuum vessel at the triple-point of water (where solid, liquid, and vapor phases are all in equilibrium). The refrigerant is the water vapor; and as there is no heat exchanger surface between the refrigerant and the water-ice, it has low energy consumption compared to conventional ice-making systems. The technology has been applied for thermal storage, as well as in district energy systems (as a heat pump), deep mine cooling, and warm weather snow-making applications.
Some systems today have naturally produced annual storage. Cold water generated by winter temperatures in the Great Lakes or other lakes is taken off the bottom of the lake in summer and pumped through plate-and-frame heat exchangers and then returned to the top of the lake. This lake cooling can produce chilled water for very low costs. Similar solutions are under design in some very hot regions such as Hawaii and parts of China, where, despite the hot climate, brine water temperatures below 40°F are available at great ocean depths near the shore. Although these systems appear to be very cost-effective over time, they may not be properly thought of as thermal storage in the sense that man did not construct the process that provides the cooling or the thermal storage.
Recent developments include concentrated and parabolic trough solar power systems, which use molten salt, hot oil, or hot rocks in their process that can be used as a high-temperature thermal storage system for use in the evenings and overcast periods.
Key Issues Facing Industry Today
Some of the key issues facing the thermal storage industry today, as challenges or opportunities include the following: the ever-increasing demand for electricity; the need for energy storage in conjunction with alternative energy technology including especially wind energy; the need to better understand utility emissions at night versus utility emissions during the day depending on one’s particular location and one’s utility company; the debate about utility incentives as to whether incentives help or hurt the thermal storage industry; proposed thermal storage legislation in the United States; and educating the public and policymakers regarding the many important benefits of thermal storage to facility or building owners, utility companies, and to people and their nations.
Total electrical energy consumption has trended up throughout most of the world for more than half-century a century. In the United States, per capita electrical energy use has increased from approximately 4000 kWh per year in 1960 to approximately 14,000 kWh per year today. 4 The on-peak electric load growth is increasing faster than the off-peak load growth; and much of the peak growth (past and future) is related to increasing air-conditioning or cooling loads—loads that can be addressed via cool thermal storage. It seems as if every day there is a new device that needs to be plugged into a power outlet. With an increasing population and a tripling of the per capita electrical energy use over the past 50 years, increased electric power demands beyond those envisioned have caused challenges for utility companies as they work to meet customer demand, with sometimes insufficient generation resources and highly loaded infrastructure grids that are at or near the limit of their current carrying capacity. As discussed in this chapter, thermal storage benefits utility companies by helping to reduce on-peak demand growth by shifting production to off-peak periods, thereby deferring or delaying costly electric infrastructure improvements and investments.
Thermal storage has an important role to play in the success of alternative energy sources. For example, wind and some solar energy are often not constant or predictable. In fact, peak wind turbine output does not coincide with peak electrical demand; as one can imagine, a utility experiences peak electrical demand due to building air-conditioning on a hot, windless day. Cool thermal storage can allow for air-conditioning production at night, when wind power is often in excess of demand, for use during the day when wind power availability may be less. One might imagine a situation where the local utility could have the ability (via the “smart grid”) to dispatch (discharge) regional thermal storage systems from individual buildings and facilities as needed to reduce peak power demand; thermal storage that was produced the night before using inexpensive nighttime wind power to generate cooling. Additionally, as discussed in this chapter and further in Chaps. 3 and 6, hot water thermal storage is an integral part of many solar thermal systems, as it is with most domestic hot water systems. Utility companies and the thermal storage industry sometimes travel in different circles, and one mission is to build more bridges between the respective organizations.
As noted in this chapter, calculating the expected emissions credit from the use of thermal storage is presently not straightforward, as the amount of utility emissions at night versus the day depends upon location, utility company provider, and the mix of power generation resources (e.g., fuel oil, natural gas, hydropower, nuclear power). For example, a utility that uses nuclear power plants to meet their base electric load will obviously have much less stack emissions at night than a utility company that obtains its base electric load from coal-fired power plants. Presently, only average emissions data are available for a particular utility company or utility region but not how those utility emissions change from day to night or from season to season. Those in the thermal storage industry need utility company hourly emissions data so that facility or building owners, planners, sustainability professionals, and thermal storage engineers can quantify for stakeholders the environmental benefits of installing thermal storage.
There seems to have been a debate over the many years as to whether utility financial incentives help or hurt the thermal storage industry. Basic economics seems to indicate that if more dollars are supplied (e.g., by providing utility financial incentives) then, all else being equal, demand should increase; utility incentives have helped fund many thermal storage projects, some of which would definitely not have proceeded without the utility incentive. However, the skeptical nature of human beings, being what it is, leads people to sometimes say or think that if utility needs to provide an incentive, then thermal storage cannot stand on its own, which, unfortunately, has been a barrier to the installation of thermal storage. Thermal storage provides financial benefits to utility companies, and, therefore, they also have an incentive to support the installation of thermal storage. Nevertheless, a great many thermal storage installations have proven economically very attractive even in the absence of utility incentives, though generally these are partial storage systems and are typically built only at times of new construction, major load growth, or chiller plant rehabilitations. A much more widespread use of full shift storage systems, including those constructed as pure retrofits (i.e., not associated with cooling load growth or chiller retirements), can be foreseen only with utility system investment or cost sharing. Continuous education of various stakeholders is always needed.
The Storage Technology of Renewable Green Energy Act of 2009—in the year 2009, legislation was introduced in the U.S. House of Representatives that would provide a 30-percent tax credit to individuals and business when they install thermal storage systems. As discussed, thermal storage provides benefits to the public, including less pollution and CO 2 emissions, and less strain on our utilities and electrical infrastructures in the face of ever-increasing electric demand. Companion legislation has also been introduced into the U.S. Senate. Unfortunately, progress on the bill has been limited due to the present (2011) political climate in Washington, D.C.
As already discussed in this chapter, implementing thermal storage has many benefits for multiple stakeholders and often helps to reduce facility energy and to reduce power plant fuel consumption, pollutant emissions, and CO 2 emissions. The use of thermal storage allows for smaller refrigeration equipment, and, therefore, has lower refrigerant leakage rates, which may damage the atmosphere. Therefore, thermal storage is a “green” energy solution that also saves facility or building owners energy costs over time. Unfortunately, at the present time, most of the public does not even know what thermal storage is, let alone the benefits of thermal storage as a green technology. It is necessary for those in the thermal energy storage industry to educate and advocate for the installation of thermal storage so that its many benefits are understood by and distributed to all those for whom the benefits of this proven technology can and do accrue.