THE ROLE OF THERMAL ENERGY STORAGE SYSTEMS IN SUSTAINABLE DEVELOPMENT

Ibrahim Dincer

Faculty of Engineering and Applied Science University of Ontario Institute of Technology (UOIT)

2000 Simcoe Street North, Oshawa, ON L1H 7K4, Canada Tel: 1-905-721-8668/2573

Fax: 1-905-721-3370 E-mail: Ibrahim.Dincer@uoit.ca

ABSTRACT Thermal energy storage (TES) systems are examined from the perspectives of energy, exergy, environmental impact, sustainability and economics. Reductions possible through TES in energy use and environmental pollution levels are discussed in detail and highlighted with an illustrative example. The importance of using exergy analysis to obtain more realistic and meaningful assessments than the 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, sustainable 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. 1. INTRODUCTION There are a number of environmental problems that we face today. These problems span a continuously growing range of pollutants, hazards and ecosystem degradation over ever wider areas. The most significant problems are acid precipitation, stratospheric ozone depletion, and global climate change. The latter is potentially the most important environmental problem relating to energy utilization. Increasing atmospheric concentrations of greenhouse gases are increasing the manner in which these gases trap heat radiated from the earth's surface, thereby raising the surface temperature of the earth and as a consequence sea levels. Recently, a variety of potential solutions to the current environmental problems associated with the harmful pollutant emissions has evolved. TES appears to be the one of the most effective solutions and plays a significant role in environment policies.

Sustainable development demands a sustainable supply of energy resources that, in the long term, is readily and sustainably available at reasonable cost and can be utilized for all required tasks without causing negative societal impacts. TES systems can contribute significantly to meeting societys desire for more efficient, environmentally benign energy use and for sustainable development of the society, particularly in the areas of building heating and cooling and electric power generation. By reducing energy consumption, the utilization of TES systems results in two significant environmental benefits: (i) the conservation of fossil fuels through efficiency increases and/or fuel substitution, and (ii) reductions in emissions of such pollutants as CO2, SO2, NOx and CFCs.

The primary objective of this paper is to examine how thermal energy systems may play a significant role in helping and contributing to the local and global sustainable development, and hence how exergy in this regard can be a potential tool for better efficiency, better cost effectiveness, better environment and hence better sustainability. 2. ECONOMIC ASPECTS OF TES SYSTEMS Economical aspects behind the design and operation of energy conversion systems has brought TES forefront. In conjunction with this, provisions must be included in an energy conversion system when the supply of and demand for thermal energy do not coincide in time. The past research has revealed that there is a wide range of practical opportunities for employing TES systems in industrial applications. Such TES systems are of great practical potential for more effective use of thermal energy equipment and for facilitating large-scale energy substitutions from the point of the economic perspective. In principal, a coordinated set of actions has to be taken in several sectors of the energy system for the maximum potential benefits of storage to be realized.

TES-based systems are usually economically justifiable when the annualized capital and operating costs are less than those costs for primary generating equipment supplying the same service loads and periods. TES is mainly

installed to lower initial costs of the other plant components and operating costs. Lower initial equipment costs are usually obtained when large durations occur between periods of energy demand. Secondary capital costs may also be lower for TES-based systems. For example, the electrical service equipment size can sometimes be reduced when energy demand is lowered.

In complete economic analyses of systems including and not including TES, the initial equipment and installation costs must be determined, usually from manufacturers, or estimated. Operating cost savings and the net overall costs should be assessed using life cycle costing or other suitable methods to determine which system is the most beneficial.

Utilizing TES can enhance the economic competitiveness of both energy suppliers and building owners. For example, one study (CEC, 1996) for California indicates that, assuming 20% statewide market penetration of TES, the following financial benefits can be achieved in the state:

For energy suppliers, TES leads to lower generating equipment costs (30% to 50% lower to serve air conditioning loads), reduced financing requirements (US$1-2 billion), and improved customer retention.

For building owners statewide, TES leads to lower energy costs (over one half billion US dollars annually), increased property values (US$5 billion), increased financing capability (US$3-4 billion), and increased revenues. Since there are many factors that influence the selection, implementation, and operation of a TES system, it is

necessary that comprehensive feasibility study be developed. This study should take into consideration all variables which impact evaluation of the true cost benefits of a candidate TES implementation. However, sometimes, it is practically impossible to conduct all. In practice, concerned people prefer a checklist on how to evaluate a TES system. In such cases, at least the following significant issues should be clarified and addressed before its implementation (for details, see Dincer and Rosen, 2002; Bejan et al., 2004):

short- or long-term management objectives, environmental impact analysis, energy conservation targets, economical aims, financial parameters of the project, available utility incentives, new or existing TES system (of course, existing plant would reduce its implementation cost), net heating or cooling storage capacity (especially for peak-day requirements), utility rate schedules and associated energy charges, full or partial operating strategies, TES system options best suited for the specific application, operating strategies for each of the TES options, space availability (e.g., tank), type of the TES period (short- or long-term), and type of the TES system (open or closed). TES may be economical if one or more of the following conditions exist:

high utility demand costs, high utility rates during peak hours, high daily load variations, short duration loads, infrequent or cyclical loads, insufficient capacity of cooling equipment to handle peak loads, and rebates available for load shifting to avoid peak demand. Some effective applications of TES include:

Electrical power use management by shifting the cooling load to off-peak hours and reducing peak load Reducing required capacity of building and process cooling systems, or helping existing cooling equipment to handle an increased load

3. ENERGY CONSERVATION ASPECTS OF TES SYSTEMS TES appears to be the only solution to correcting the mismatch between the supply and demand of energy. TES is a key component of any successful thermal system and a good TES should allow minimum thermal energy losses, leading to energy savings, while permitting the highest possible extraction efficiency of the stored thermal energy.

TES is an important element of energy conservation programs in industry, particularly in commercial buildings and in solar energy utilization. For many years TES systems have been investigated, which show that although many technically and economically successful TES systems have been in operation, no broadly valid basis for comparing the achieved performance of one storage with that of another operating under different conditions has found general acceptance. The development of such a basis for comparison has been received increasing attention, especially using exergy analysis technique, which is identified as one of the most powerful ways in evaluating the thermal performance of TES systems is based primarily on the second law of thermodynamics, as compared to energy analysis which is based on the first law, and takes into account the quality of the energy transferred. These aspects will be discussed later. Energy conservation from TES systems can be achieved in several manners (Dincer, 2002), including:

The consumption of purchased energy can be reduced by storing waste or surplus thermal energy available at certain times for use at other times. For example, solar energy can be stored during the day for heating at night.

The demand of purchased electrical energy can be reduced by storing electrically produced thermal energy during off-peak periods to meet the thermal loads that occur during high demand periods. For example, an electric chiller can be used to charge a chilled water TES at night for reducing the electrical demand peaks usually experienced during the day. The purchase of additional equipment for heating, cooling or air-conditioning applications can be deferred and the equipment sizing in new facilities can be reduced. The equipment can be operated when thermal loads are low to charge TES systems. Energy can be