Thermal Energy Storage for Sustainable Energy Consumption

This chapter discusses the history of thermal energy storage focusing on natural energy sources. Links are made to recent trends of using renewable energy to achieve greater energy efficiencies in heating, cooling and ventilating buildings. The Deep Lake Water Cooling development in Toronto is presented as a typical modern interpretation of past practices with an integration of municipal services of water supply and district cooling. Environmental concerns and restrictions have also stimulated thermal energy storage developments. Cold storage in aquifers originated in China where excessive groundwater extraction related to industrial cooling had resulted in significant land subsidence.

Thermal energy storage (TES) systems and their applications are examined from the perspectives of energy, exergy, environmental impact, sustainability and economics. Reductions possible through TES in energy use and pollution levels are discussed in detail and highlighted with illustrative examples of actual systems. The importance of using exergy analysis to obtain more realistic and meaningful assessments, than provided by the more conventional energy analysis, of the efficiency and performance of TES systems is demonstrated. The results indicate that cold TES can play a significant role in meeting society’s preferences for more efficient, environmentally benign and economic energy use in various sectors, and appears to be an appropriate technology for addressing the mismatches that often occur between the times of energy supply and demand.

Climate change is increasingly apparent. Regional impacts of climate change are being observed. Those commonly cited include extended growing seasons; shifts of plant and animal ranges; earlier flowering of trees, emergence of insects and egg-laying in birds; and local temperature, humidity and wind-speed anomalies. Air temperatures in Alaska and western Canada have increased as much as 3–4 °C in the past 50 years. Engineers who design infrastructure for predicted future conditions face challenges due to these shifts in climate. Building codes already specify minimum health and safety requirements for some key climate variables such as heating and cooling design temperatures; heating and cooling degree days; rainfall and snow loads; and wind pressures. Predicted changes in these variables at specific locations are not usually available. Regional scenarios give a general trend but lack precision and verification.

The purpose of this paper is to present a controversial and CO₂ free explanation to global warming and to show that global warming means that large-scale thermal energy storage. Global warming is here explained by dissipation of heat from the global use of non-renewable energy sources (fossil fuels and nuclear power). Resulting net heat is thus released into the atmosphere. A minor part of this heat is emitted to space as outgoing longwave radiation while the remaining is heating the Earth. Some of this heat is accumulated as sensible, i.e., by heating air, ground, and water. The rest is also stored as latent heat, i.e., in the form of vapor in the air and in the melting of the large ice fields of the planet.

The most dramatic challengewe are facing today is climate change induced to a considerable degree by human originated greenhouse gas emissions, especially the correlation between CO₂ concentrations and temperatures. The reduction of energy consumption and greenhouse emissions is among the issues of greatest interest for the prevention of global warming and climate change. Hence, developing and deploying more energy efficient and environmentally friendly energy technology is critical to achieving the objectives of Energy security, Environmental protection, Economic growth and social development known as three E’s. Energy Storage systems—a tool in forming the structure of sustainable energy for sustainable future—can play a key role in decreasing emissions that lead to global warming.

This chapter discusses the potential for cost-effectively reducing the energy intensity of office buildings by applying proven technologies, especially the use of ground source systems with thermal energy storage. It is shown that significant energy use reductions are possible without increases in capital costs and that reductions of more than 50% are possible within normal investment criteria. Energy storage techniques need to be adapted to these reduced energy levels. Energy efficient buildings are better suited to energy storage. Low-energy building design can contribute to dramatically reduced energy usage and can be applied to all new building projects. The example of a small office building located in Canada is used to illustrate this potential.

Heat storage systems by phase changing materials (PCM) need to identify the performance limits and optimize processes and cycles with thermodynamic analysis. Such an analysis consists of concepts like thermoeconomics, entropy generation minimization, and the extended exergy accounting, which calculates resource-based value of a commodity by establishing a comprehensive relation between exergy and economic values. Some other concepts are the cumulative exergy consumption method and exergy cost theory. Thermoeconomics connects mainly the second law of thermodynamics and economics to estimate the cost of exergy by using the cost 0accounting and structural theory. This study presents a thermodynamic analysis of latent heat storage by PCM. It also presents a case study for a seasonal solar heat storage system by paraffin wax by using a system of 18 solar air heaters with 27 m² of total absorber area in order to heat a 180 m² greenhouse.

Storage of renewable energy in the underground will reduce the usage of fossil fuels and electricity. Hence, these systems will benefit to CO₂ reduction as well as the reduction of other environmentally harmful gas emissions, like SO_X and NO_X. ATES, BTES and CTES are three options of Underground Thermal Energy Storage (UTES) systems. ATES and BTES are widely used in some countries. Relevant properties and different aspects of design and construction of ATES systems is discussed in this article.

A Geothermal Response Test measures the temperature response to a thermal energy forcing of a borehole heat exchanger. The temperature response is related to the ground and borehole thermal parameters such as thermal conductivity, heat capacity and the conductivity of the borehole material and is therefore used to obtain estimates on these important parameters. Generally such data are analysed using a line source method. Although quick and, when certain conditions are met, accurate, the line source method has several drawbacks. First of all it is only valid for the constant heat flux case, secondly it only allows estimates of ground thermal conductivity and borehole resistance, other parameters, like heat capacity, cannot be estimated. Thirdly effects of ground water flow cannot be quantified. Moreover, selecting test parameters a-priory is not always easy and it is difficult to adjust test conditions due to the necessity of constant heat flux. Parameter estimation techniques have been developed to allow analysis of test results under varying heat flux conditions and to allow simultaneous estimates of different parameters to be obtained. In this paper we develop such a method based on the TRNSYS simulation package with the DST borehole model, using a generic optimisation package—GenOpt—to perform the calibration. Moreover we extend the GRT test protocol to use different heat extraction and injection energy levels within the same experiment. Apart from improving estimates of ground and borehole thermal parameters, it is our final goal to allow the characterisation of ground water flow using the in situ thermal response test.

There are approximately 300,000 borehole systems in Sweden with an annual growth rate of 30,000 systems. Such borehole systems are mainly used for heating of single family houses but in recent years several hundred larger systems, for both heating and cooling, have been constructed. The systems delivered 16% of all space heating of Sweden in 2000 and are estimated to cover 27% in 2010. This strong development indicates that borehole systems are reliable and supply heat at a lower cost than more conventional alternatives. This paper concerns a rare freezing problem that occurs in 1 of 10,000 systems. In such cases freezing that occurs in the boreholes as a result of heat extraction creates an over pressure that flattens the pipe system, thereby stopping the circulating of the heat carrier fluid. Even if this is a rare problem it means big problems for the unlucky individuals and also for the industry since one such problem reduces the market in that region.

In the autumn of 2000, the British Engineering Council awarded an Environmental Engineering award to the groundsource heatpump project at Commerce way, Croydon Surrey. This, one of the larger UK groundsource projects, is a speculative built industrial building of about 3,000 m² with both offices andwarehouse facilities. The building, that is leased by Ascom Hassler Ltd. (a Swiss based IT company), is expected to have an annual cooling load of 100—125 MWh and a heating load of 90—100 MWh. Peak loads under hot summer conditions are anticipated to reach up to 130 kW. During the normal life span of a building (25 years) the surplus of heat rejection would lead to increasing ground temperatures. This results in a less efficient heat pump operation and may even result in insufficient capacity during cooling peak demands. As a solution a hybrid system, incorporating a dry-cooler, was developed. The principal idea is to use the dry-cooler to store cold in the wellfield during early spring, when the required summer peak load cool can be generated very efficiently and cheaply. The operation and efficiency of the wellfield, the installed heat pump system and dry-cooler is controlled and monitored under a Building Management System (BMS). The results of the first three years of operation of the system are presented. Using the monitoring data an evaluation of the original design will be made.

The borehole thermal energy storage system (BTES) at University of Ontario Institute of Technology (UOIT) is described and its technical details are presented from the energy conservation point of view. An illustrative example is given to demonstrate performance aspects of the of the heat pump system integrated with it. The BTES system forms an important component of the drive for an efficient campus, and is also used in UOIT’s energy-related engineering programs and research.

Borehole Thermal Energy Storage (BTES) system is used for heating and cooling of the new Institution for Astronomy at the University of Lund in Sweden. The system has been in operation since autumn 2001. 20 boreholes each with a depth of 200 m are placed under the parking area. The system delivers free cooling of 150 MWh with a approximate COP of 50 in summer. The building is heated by the heat pump using the storage as a source of heat and providing 300 MWh of heat.

Newly established residential area in Västra Hamnen (West Harbour) in the city of Malmö uses Aquifer Thermal Energy Storage (ATES) as part of the district heating and cooling system. ATES system has 5 warm and 5 cold wells that are 70–80 m deep. Cold from the nearby sea (Öresund) and water cold from heat pump is stored from winter to summer. The purpose of the system is to deliver free cooling to district cooling system at a temperature level below +6.8 °C. Operational experiences and economic aspects are discussed.

Aquifer Thermal Energy Storage (ATES) system is used to serve the district cooling of the inner city of Stockholm in Brunkebergs Torg with natural cold from a lake, which is stored during night and recovered during peak hours at daytime. The wells are situated at two narrow streets, six wells each street. The system is designed for a capacity of approx. 25 MW cooling power at a storage working temperature of +4 to +14 °C and at a flow rate of 600 l/s. The operational problems, solutions and economic aspects are discussed.

Energy Pile System uses building foundation piles as ground heat exchangers. For the new building at Sapporo City University steel foundation pile is decided to be used with the HVAC system. The construction work started in January 2005. Total of 51 steel pipes have been screwed. The paper gives the results from performance analysis and construction phases of the project.

This section is an introduction into materials that can be used as Phase Change Materials (PCM) for heat and cold storage and their basic properties. At the beginning, the basic thermodynamics of the use of PCM and general physical and technical requirements on perspective materials are presented. Following that, the most important classes of materials that have been investigated and typical examples of materials to be used as PCM are discussed. These materials usually do not fulfill all requirements. Therefore, solution strategies and ways to improve certain material properties have been developed. The section closes with an up to date market review of commercial PCM, PCM composites and encapsulation methods.

Temperature control is a very suitable application because there you can take advantage of the high capacity of PCMs in a small temperature range. In the case of transport boxes, PCM modules to keep the internal temperature constant within a few degrees for a long time have already penetrated the market. Further applications that are currently under development or in a first stage of market introduction are textiles and clothes, electronic equipment, medical applications, cooling of newborns, catering and even a laptop that includes PCM!

Research for the application of PCM in buildings has been focused in recent years on three fields. The first one is the reduction of temperature swings of lightweight buildings by increasing their thermal mass. This is done by incorporation of PCM into building materials. The second one is the cooling of buildings through intermediate storage of cold from the night or other cheap cold sources. If the cold is for free, as with cold from night air, this is also called free cooling and very promising with respect to energy saving. A third field of application is for heat storage in space heating systems. Here the main advantage is a reduction of storage volume by a factor of three or more. This section describes the basic strategies for different systems for heating and cooling of buildings as well as the state of the art in R&D.

Ice storage for cooling is an ancient technology which was common until thebeginning of the 20th century, when chillers were introduced. During the past fewdecades new techniques using both snow and ice for comfort cooling and food storage have been developed. Cold is extracted from snow or ice by re-circulation of water or air between the cooling load and the snow/ice. The snow cooling plant in Sundsvall, Sweden, is used for cooling of the regional hospital. The stored natural and artificial snow is used for comfort cooling from May to August. It was taken into operation in June 2000 and is the first of its kind. Here the plant is described and the experience of its first six years of operation is presented.

Energy Pile System uses building foundation piles as ground heat exchangers. For the new building at Sapporo City University steel foundation pile is decided to be used with the HVAC system. The construction work started in January 2005. Total of 51 steel pipes have been screwed. The paper gives the results from performance analysis and construction phases of the project.

Energy conversion technologies using chemical reaction are introduced. Thermal energy conversion by chemical heat pumps and a hydrogen production system is shown mainly as efficient energy utilization technology utilizing chemical reaction ability. Chemical reaction would be useful for thermal energy management, because heat density of chemical changes is relatively higher than one of physical changes, which are used in conventional conversion system. Then, reversible chemical reaction is expected to have potential for thermal energy conversion, storage and utilization process in the next generation. The know-how of development of energy conversion system utilizing reversible chemical reactions is explained using chemical reaction equilibrium analysis. Chemical heat pump for thermal energy storage and conversion, and hydrogen production utilizing separation process are reviewed as practical example. Possibility of chemical energy conversion methodology would be understood from this section.

The theory of sorption processes and its relevance for thermal energy storage (TES) concepts shall be introduced. Starting from the thermodynamics of TES systems a motivation for sorption storage systems will be developed. The adsorption theory is based on the adsorption equilibrium. The equilibrium can be described by isotherms (curves of equal temperature), isobars (curves of equal water vapour pressure) and isosteres (curves of equal water concentration within the adsorbent). From the adsorption equilibrium the adsorption enthalpy or heat of adsorption can be calculated. The heat of adsorption describes the amount of energy involved in the process. The ratio of discharged to charged thermal energy and the possible storage capacity of different applications derived from the adsorption equilibrium and the heat of adsorption will be defined. A similar method—from the equilibrium to the storage capacity—will be shown for liquid sorbents.

Adsorption systems for thermal energy storage can be designed as closed or open systems. The two possibilities are described in chapter V.2. In this chapter some examples of complete systems will be given. There will be two examples for closed systems. One is a commercially available self cooling beer keg (ZeoTech Zeolite Technology, http://www.zeo-tech.de) and the other one is a seasonal storage for solar heat. For open systems one adsorption storage is described, which is installed in the district heating net and is able to provide heat for heating purposes in winter and air conditioning in summer.

Open absorption systems for thermal energy storage have been investigated over the last years. Open sorption systems using liquid desiccants like Lithium chloride are able to dehumidify an air stream. By adiabatic humidification this dry air can be cooled down and used for air conditioning of buildings. These systems provide cool and dry air to the rooms. At the same time these systems are able to store thermal energy very efficiently. The thermal energy can be stored within the difference of salt concentration between the diluted solution (after absorption) and the concentrated solution (after regeneration). Examples of demonstration projects will be given. Asolar application for the dehumidification of an office building and an application connected to a district heating net for the cooling of a jazz club will be presented in detail.