Combined cooling, heating and power: A review

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Progress in Energy and Combustion Science 32 (2006) 459–495 www.elsevier.com/locate/pecs

Combined cooling, heating and power: A review D.W. Wu, R.Z. Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Dongchuan Road 800, Shanghai 200240, China Received 25 July 2005; accepted 28 February 2006 Available online 21 August 2006

Abstract Combined cooling, heating and power (CCHP) systems, including various technologies, provide an alternative for the world to meet and solve energy-related problems, such as energy shortages, energy supply security, emission control, the economy and conservation of energy, etc. In the first part of this paper, the definition and benefits of CCHP systems are clarified; then the characteristics of CCHP technologies—especially technical performances—are presented, as well as the status of utilization and developments. In the third part, diverse CCHP configurations of existing technologies are presented, particularly four typical systems of various size ranges. The worldwide status quo of CCHP development is briefly introduced by dividing the world into four main sections: the US, Europe, Asia and the Pacific and rest of the world. It is concluded that, within decades, promising CCHP technologies can flourish with the cooperative efforts of governments, energy-related enterprises and professional associations. r 2006 Elsevier Ltd. All rights reserved. Keywords: Combined cooling, heating and power; Technologies; Developments worldwide

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460 Status and developments of CCHP technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 2.1. Prime movers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 2.1.1. Steam turbines [1,18,19] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 2.1.2. Reciprocating internal combustion engines [1,6,7,18,20–22]. . . . . . . . . . . . . . . . . . . . . . . . . . . 464 2.1.3. Combustion turbines [1,6,18,21–24] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 2.1.4. Micro-turbines [1,6,7,15,18,20,24,25]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 2.1.5. Stirling engines [1,6,18,20] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 2.1.6. Fuel cells [1,6,7,15,18,20,22,24,26,27] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 2.2. Thermally activated technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 2.2.1. Absorption chillers [13,22,28–31] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 2.2.2. Adsorption chillers [32–38] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 2.2.3. Desiccant dehumidifiers [3,13,31] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 2.2.4. Other options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472

Corresponding author. Tel.: +86 21 34206776; fax: +86 21 34206056.

E-mail address: [email protected] (R.Z. Wang). 0360-1285/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pecs.2006.02.001

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3.

4.

5.

Typical CCHP systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 3.1. Diverse configurations of CCHP systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 3.2. Representative CCHP systems in use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 3.2.1. Micro systems (under 20 kW). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476 3.2.2. Small-scale systems (20 kW–1 MW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 3.2.3. Medium systems (1–10 MW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 3.2.4. Large-scale systems (above 10 MW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 Development of CCHP around the world . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 4.1. United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 4.2. Europe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483 4.2.1. Austria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 4.2.2. Denmark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 4.2.3. Finland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 4.2.4. The Netherlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 4.2.5. France . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 4.2.6. Germany . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484 4.2.7. Hungary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 4.2.8. Italy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 4.2.9. Poland. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 4.2.10. Spain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 4.2.11. Sweden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 4.2.12. UK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 4.3. Asia and the Pacific . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 4.3.1. China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 4.3.2. Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 4.3.3. India . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 4.3.4. Association of South East Asian Nations (ASEAN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490 4.4. Other countries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 Discussions and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

1. Introduction Combined cooling, heating and power (CCHP), is derived from combined heat and power (CHP, also called cogeneration1)—a proven and reliable technology with a history of more than 100 years, which was utilized mainly in large-scale centralized power plants and industrial applications. The conventional way to provide electricity and heat is to purchase electricity from the local grid and generate heat by burning fuel in a boiler. But in a CHP system, by-product heat, which can be as much as 60–80% of total primary energy in combustion-based electricity generation, is recycled for different uses. Typically, CHP is defined as the combined production of electrical (or mechanical), 1

Most literature lists statistics of CHP/cogeneration instead of CCHP. In fact, data of CHP/cogeneration with cooling options count these statistics in most cases, though these applications only take a small fraction of the gross. For a better description of these systems, the term CHP/cogeneration is substituted by CCHP in most part of this article.

and useful thermal energy from the same primary energy source [1]. A slight difference between CCHP and CHP is that thermal or electrical/mechanical energy is further utilized to provide space or process cooling capacity in a CCHP application. In some literature, CCHP systems are also referred to as trigeneration and building cooling heating and power (BCHP) systems [2–4]. CCHP can be defined as a more extensive concept than CHP is. In winter, many CCHP systems can be seen as CHP units, when there is no cooling demand of building air-conditioning. In other words, CHP system is CCHP without any thermally activated equipments for generating cooling power, though this difference will change the structure of systems to some extent. In general, recent development of CCHP systems is related to the emergence of DER2 (distributed/ 2 Other similar abbreviations in the literature are DP (distributed/decentralized power) and DG (distributed generation/ decentralized generation), which is slightly different from DER.

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Nomenclature Tri-generation CCHP combined cooling heating and power

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BCHP building cooling heating and power DER distributed/decentralized energy resources DP distributed/decentralized power DG distributed generation/ decentralized generation

Cogeneration CHP

combined heating and power

decentralized energy resources)—a novel technical concept in energy supply. DER is defined as an electricity-generation system located in or near user facilities, which provides electrical and thermal energy simultaneously to meet local users in top-priority. Certain factors, such as various rated capacities, ownership of systems, technologies employed and types of connection with utility grids, are not critical in a consensus definition of DER. According to some reports, DER can be divided into two major sections [5,6]. The first section is high-efficiency CHP or CCHP systems in industry and buildings throughout the world, using prime mover technologies as reciprocating engines, gas turbines, micro-turbines, steam turbines, Stirling engines and fuel cells. The second major area of DER is on-site renewable energy systems with energy recycling technologies, including photovoltaic and biomass systems, on-site wind and water turbine generators, plus systems powered by gas pressure reduction, exhaust heat from industrial processes, and other lowenergy content combustibles from various processes. Due to the relationship between traditional CHP and novel DER (Fig. 1), CCHP systems are classified into two categories: 1. Traditional large-scale CCHP applications (predominantly CHP systems without cooling options) in centralized power plants or large industries; 2. Relatively small capacity distributed CCHP units with advanced prime mover and thermally activated technologies3 to meet multiple energy demands in commercial, institutional, residential and small industrial sections. 3 Thermally activated technologies: technologies that are able to use waste heat as a fuel and offer the chance to replace electric air conditioning and/or dehumidification loads with thermal loads, such as absorption chiller, adsorption chiller and desiccant dehumidifiers.

Fig. 1. Categories of CCHP and DER.

There is no clear borderline between two categories. CCHP systems can cover a wide range of capacity from 1 kW to 500 MW. Most centralized power plants and industries applying cogeneration exceed 1 MW. The capacity of distributed CCHP systems ranges from less than 1 kW in domestic dwellings to more than 10 MW in hospitals or university campuses, and as much as 300 MW to supply energy to a district of a city [7,8]. One report defines ‘‘everything under 1 MW’’ as ‘‘small-scale’’. ‘‘Mini’’ usage is under 500 kW and ‘‘micro’’ use is under 20 kW’’ [9]. A typical CCHP system is showed in Fig. 2. It is comprised of a gas engine, a generator and an absorption chiller. The engine is driven by natural gas and the mechanical energy is further changed into electricity power by the generator. At the same time, the absorption chiller to generate cooling power in summer and heating power in winter utilizes exhaust gas and jacket water derived from the engine. If waste heat from engine is not enough for users, a combustor in absorption chiller can burn natural gas as a supplement. Thus, the energy demands of cooling, heating and electrical power in a building or a district can be met by this system simultaneously. Compared with the energy supply mode of large centralized power plant and local air-conditioning

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Fig. 2. Typical CCHP system.

system, distributed CCHP systems will receive more attention, because—along with the developing tendency and promising prospects—they possess some advantages, which traditional energy supplies do not share [1,9–13]. First, overall fuel energy utilization has dramatically improved, ranging from 70% to more than 90% compared with 30–45% of typical centralized power plants. In general, less primary energy is needed to obtain the same amount of electricity and thermal energy. In addition to the saving in primary energy, vast reductions in net fuel costs, transmission and distribution savings can be achieved. A theoretical calculation of prime energy utilization based on traditional energy supply mode and typical CCHP system as Fig. 2 can be seen in Figs. 3 and 4. If end user needs 33 units of electrical power, 40 units of cooling power and 15 units of heating power in a summer day, 148 units of prime energy are consumed in a traditional way. Centralized power plant runs at the efficiency of 33% and 100 units of prime energy are used to generate 33 units of electrical power. Traditional boiler burns 18 units fuel to heat 15 units of domestic hot water at the efficiency of 85%. Electrical air-conditioner driven by 10 units of electrical power can generate 40 units of cooling power at COP of 4. However, consider the efficiency of electricity generation in power plant, 30 units of prime energy is needed in all for space cooling. Based on a typical CCHP system shown in Fig. 2, only 100 units of prime energy are needed for 33 units of electrical power, 40 units of cooling power and 15 units of heating power in a summer day. The electricity generation efficiency of CCHP system is similar to centralized power plant, because electricity is consumed locally without loss on distribution lines, though small-scale prime mover is less efficient than large prime mover in power plant. The keystone of full energy utilization of CCHP system lies on the recovery of waste heat from prime mover.

Fig. 3. Energy flow of traditional supply mode.

Fig. 4. Energy flow of typical CCHP system.

Thirty four units of waste heat in the form of exhaust gas and machine coolant are used to drive an absorption chiller at COP of 1.2, thus 40 units of cooling power can be obtained. And another 18 units of waste heat can be recovered to heat 15 units domestic water at the efficiency of 85% similar to the efficiency of a boiler. Compared with traditional

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energy supply mode, CCHP system can save 48 units of prime energy to meet the same demand of cooling, heating and power. The second benefit of distributed CCHP systems is emission reduction; viewed from two aspects and sorted by different prime movers. Some prime movers with new technologies, like fuel cells and micro-turbines exhaust much less emissions (including NOx, CO2), than do traditional technologies from centralized power plants. Other prime movers, equipped in CCHP systems with smaller capacity than their larger counterparts in centralized power plants, emit somewhat more NOx and CO2 per kW electricity generated. Nevertheless, the promotion of energy efficiency—CCHP systems should be encouraged at this time: burning significantly less fuel to meet the same demand results in significant emission reduction, which surely overrides the additional emissions caused by the slight decrease in converting efficiency in small-scale prime movers. Last, but of equal importance, CCHP systems increase the reliability of the energy supply network. Obviously, generation/distribution systems can malfunction: weather and terrorism are fatal threats to centralized power plants. A smaller, more flexible and dispersed system, CCHP might prevent these threats from becoming reality, and controlled repercussions and fast recovery could be achieved if these situations occurred. A study following the 11 September attacks suggested that a system based more on distributed generation plants may be five times less sensitive to systematic attack than a centralized power system [14]. A typical CCHP system consists of five basic elements: the prime mover; electricity generator; heat recovery system; thermally activated equipment and the management and control system. According to current technologies, options in prime movers can be steam turbines, reciprocating internal combustion engines, combustion turbines, microturbines, Stirling engines and fuel cells; the last three prime movers are relatively new technologies developed in last decade. Any of these options can be selected to meet diverse demands and limitations from site-to-site, especially local heat and electricity profiles, regional emissions and noise regulations and installation restrictions. Thermally activated equipment is another part of CCHP systems, to provide cooling or dehumidification. Commercialized thermally activated technologies include absorption chillers and desiccant dehumidifiers;

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moreover, novel adsorption chillers—currently almost entirely for commercial use—can be another choice for small CCHP systems. Some existing systems also apply electric chillers, or engine-driven chillers integrated with prime movers, to fulfill cooling demands, which, combined with thermally activated technologies, are the cooling or dehumidification options of CCHP systems in some of the literature. Different prime movers, connecting with different cooling or dehumidification options, can result in various kinds of CCHP systems in theory, but only several modes of combination are widely adopted in commercial markets; other promising possibilities are being investigated to overcome technological or economic problems. In the next two parts of this paper, brief reviews of prime mover technologies, cooling and dehumidification options and various CCHP system modes, with four typical examples are presented in sequence, to present a clear picture of current CCHP technologies. Although governments worldwide, experts, manufacturers and users have acknowledged that CCHP systems are the current development trend in energy supply, the share of decentralized power generation (including CCHP systems) in the world market remains at around 7%—unchanged between 2001 and 2003 [14]. The distributed CCHP market of the US grew significantly until 2002, but since then it has slowed sharply in the face of high natural gas prices and persistent regulatory barriers. The European distributed CCHP market was flat in last 4 years. Although some developing country markets are beginning to emerge, including China, Brazil and India, it is presumed that the boom in these burgeoning markets will take much more time and effort than markets in developed countries. The obstacles come from every direction: technology performance, costs, policies, regulations and market demands. The year 2004 can be viewed as a turning point of low growth in CCHP market worldwide. A WADE survey, forecasts that growth will be reinforced by the probable introduction of the European Union Emissions Trading Scheme in January 2005, which is expected to further increase power prices. In the fourth chapter of this paper, the status of CCHP system development worldwide is presented; the world is divided into the US, Europe, Asia and the Pacific and other countries, for a review of existing or potential markets, and to present a forecast and analysis.

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2. Status and developments of CCHP technologies CCHP technologies include components relating to energy conversion, recovery and management. Among these technologies, prime movers obviously play a critical role; they are the keystones of CCHP systems and, to some extent, they determine possibilities and availability of other related technologies. As for the importance of thermal activated options, these alternative technologies dramatically shift the energy utilization of energy conversion systems compared to conventional electrical power systems. Therefore, the following paragraphs focus on these two major aspects of CCHP systems, especially the advantages, drawbacks and developing trends of these technologies. 2.1. Prime movers There are several ways to classify prime mover technologies, based on fuel used, technical maturity, market shares or capacity range. Although quite a few newly emerging technologies appear to be promising, reciprocating internal combustion engines, steam turbines and combustion turbines that can be considered conventional prime movers still make up most of the gross capacity being installed [9,15]. In addition, fuel cells, Stirling engines and micro-turbines, mainly gas driven, present a promising future for prime movers [9,15–17]. Brief introductions follow and major parameters and performance of these prime movers can be referenced in Table 2, at the end of this section. 2.1.1. Steam turbines [1,18,19] Steam turbines are the most common technology used in power plants and industries. Depending upon the exit pressure of the steam, steam turbines fall into two types: backpressure turbines and condensing turbines. Backpressure turbines operate with an exit pressure at least equal to atmospheric pressure, and are suitable for some sites with a steam demand of intermediate pressure. Condensing turbines have the advantage of changing electrical and thermal power independently and they work with an exit pressure lower than atmospheric pressure. In theory, steam turbines equipped with a suitable boiler can be run on any kind of fuel. As a mature technology, steam turbines have an extremely long life and, with proper operating and maintenance, are very reliable. However, several problems limit their further application, which

include low electrical efficiency, slow start-up time, and poor partial load performance. As a result, steam turbines are more popular in large central plant utilities or industrial cogenerations than in distributed energy applications, although some claims are made that future ‘‘plug and play’’ turbines will operate with fractional kW outputs, wherever steam pressure is reduced [6]. 2.1.2. Reciprocating internal combustion engines [1,6,7,18,20– 22] Two types of internal combustion engines are currently in use; spark ignition engines, which are operated mainly with natural gas (although biogas or landfill gas can also be used); and compression ignition engines, which can use diesel fuel, as well as other petroleum products, such as heavy fuel oil or biodiesel. Reciprocating engines are a proven technology with a range of size and the lowest first capital costs of all CCHP systems. In addition to fast start-up capability and good operating reliability, high efficiency at partial load operation give users a flexible power source, allowing for a range of different energy applications—especially emergency or standby power supplies. Reciprocating engines are by far the most commonly used power generation equipment under 1 MW. Although they are a mature technology, reciprocating engines have obvious drawbacks. Relatively high vibrations require shock absorption and shielding measures to reduce acoustic noise. A large number of moving parts, and frequent maintenance intervals, increase maintenance costs, strongly offsetting fuel efficiency advantages. In addition, full utilization of the various heat sources with diverse temperature levels in CCHP applications is difficult. Moreover, high emissions—particularly nitrogen oxides—are the underlying aspect of this technology, which need improvement. Major manufacturers around the world continuously develop new engines with lower emissions; at the same time, emissions control options, such as selective catalytic reduction (SCR), have been utilized to reduce emissions. 2.1.3. Combustion turbines [1,6,18,21– 24] Combustion turbines are frequently used prime movers in larger-scale cogenerations due to their high reliability and large range of power. Sets smaller than 1 MW have so far been generally uneconomical because of their low electrical efficiency and consequent high cost per kWe output.

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Combustion turbines are easier to install than steam turbines and they have the added benefit of being less area intensive, with lower capital costs; maintenance costs are slightly lower than reciprocating engines, but so is their electrical efficiency. Emissions are somewhat lower than that of reciprocating engines, and cost-effective NOx emissions-control technology is commercially available. Combustion turbine exhaust—typically around 540 1C—can be used to support the combustion of additional fuel. This technology is called supplementary firing, and it can raise the temperature of exhaust gas more than 1000 1C and increase the amount of high-pressure steam produced. Using produced steam to power a steam turbine is known as a combined-cycle gas turbine (CCGT), with higher net electrical efficiency (35–55%), which is appropriate for public utility companies and industrial plants. The major disadvantages of combustion turbine are described below. Combustion turbines require premium fuels, especially natural gas, which historically has high price volatility. The high temperatures involved lead to demanding standard of materials with higher production costs. Additionally, turbine performance is significantly reduced at higher altitudes or during periods of high ambient temperatures. 2.1.4. Micro-turbines [1,6,7,15,18,20,24,25] Micro-turbines extend combustion turbine technology to smaller scales. They are primarily fuelled with natural gas, but they can also operate with diesel, gasoline or other similar high-energy fuels. Research on biogas is ongoing. Micro-turbines have only one moving part; they use air bearings and they do not need lubricating oil, although they have extremely high rotational speed, up to 120,000 rpm. A striking characteristic is their flexibility that small-scale individual units can be combined readily into large systems of multiple units. Additionally, there are environmental advantages, such as lower combustion temperatures assuring low NOx emissions levels and less noise than an engine of comparable size. This technology has been commercialized only recently and is offered by a small number of suppliers. The main disadvantages at this stage are its short track record and high first costs compared with reciprocating engines. Other issues include relatively low electrical efficiency and sensitivity of efficiency to changes in ambient conditions.

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Micro-turbines can be used as a distributed energy resource for power producers and consumers, including industrial, institutional, commercial and even residential users of electricity in the future. Moreover, the heat produced by a micro-turbine can be used to produce low-pressure steam or hot water for on-site requirements. 2.1.5. Stirling engines [1,6,18,20] Compared to conventional internal combustion engine, Stirling engine is an external combustion device. The cycle medium—generally helium or hydrogen—is not exchanged during each cycle, but within the device, while the energy driving the cycle is applied externally. Stirling engines can operate on almost any fuel (gasoline, alcohol, natural gas or butane), with external combustion that facilitates the control of the combustion process and results in low air emissions, low noise and more efficient process. In addition, best in class machines fewer moving parts compared to conventional engines limit wear on components and reduce vibration levels. Stirling engine technology is still in its development; no statistical data on availability is therefore available. High cost also prevents popularization of this technology. Nevertheless, the promising prospects of Stirling engines stimulate further research, especially for CCHP applications. Small size and quiet operation mean that they will integrate well into residential or portable applications. Some literature indicates the possibility of using a solar dish to heat the Stirling engine, thus potentially eliminating the need for combustion of a fuel. 2.1.6. Fuel cells [1,6,7,15,18,20,22,24,26,27] Fuel cells are quiet, compact power generators without moving parts, which use hydrogen and oxygen to make electricity and; at the same time, can provide heat for a wide range of applications. In general, fuel cells show high electrical efficiencies under varying load and; thus, result in low emissions. The transportation sector is the major potential market for fuel cells. Power generation, however, seems to be another promising market in which fuel cells could be quickly commercialized. Five major fuel cell technologies listed below have the most attractive prospects. In reality, with the exception of PAFC, no fuel cells are yet completely commercially viable; a total capacity of over 40 MW PAFC having been installed worldwide. A detail

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Table 1 Characteristics of fuel cells [6,20,18,26] PEMFC

AFC

PAFC

MCFC

SOFC

Charge carrier Type of electrolyte

H+ ions Polymeric membrane

H+ ions Phosphoric acid solutions

CO3¼ ions Phosphoric acid (immobilized liquid)

O ¼ ions Stabilized zirconia ceramic matrix with free oxide ions

Typical construction

Plastic, metal or carbon

OH
ions Aqueous potassium hydroxide soaked in a matrix Plastic, metal

Carbon, porous ceramics

Ceramic, high temp metals

Catalyst Oxidant

Platinum Air or O2

Platinum Purified air or O2

Fuel

Hydrocarbons or methanol

Clean hydrogen or hydrazine

Platinum Air or O2enriched air Hydrocarbons or alcohols

High temp metals, porous ceramic Nickel Air

Operational temperature Size range Electrical efficiencya Primary contaminants

50–100 1C

60–80 1C

3–250 kW 30–50% CO, sulfur and NH3

10–200 kW 32–70% CO, CO2 and sulfur

a

Parasites Air Natural gas or propane

100–200 1C

Clean hydrogen, nature gas, propane, diesel 600–700 1C

100–200 kW 40–55% CO41%, sulfur

250 kW–5 MW 55–57% Sulfur

1–10 MW 50–60% Sulfur

600–1000 1C

Electrical efficiencies are based on values for hydrogen fuel and do not include electricity required for hydrogen reforming.

comparison of the characteristics of these fuel cells appears in Table 1. 2.1.6.1. Proton exchange membrane fuel cell (PEMFC). PEMFCs are quite simple and can be made very small to adjust to variable power demands. They are easier to start up and they apply solid electrolyte that reduces corrosion. At the same time, the low operating temperature requires the use of an expensive platinum catalyst, and limits cogeneration potential. As for the fuel sources, this fuel cell technology is highly sensitive to fuel impurities and hydrogen storage; delivery and reforming technology has yet to evolve. PEMFCs appear to be the choice for automotive applications. The advantage of being small allows application for laptops, mobile phones and other portable appliances. With relatively low-quality waste heat, the PEMFC is unlikely to be widely used for high voltage stationary power generation; but small-scale domestic CCHP applications—the simplest thermal load of which is hot water—would be considerable. 2.1.6.2. Alkaline fuel cell (AFC). AFCs were the first fuel cells used on spacecrafts and space shuttles. The technology has obvious merits, such as low operating temperature, rapid start-up time, readily

available non-precious metal electrodes, and high efficiency, up to 70%. However, the primary disadvantage is the tendency to absorb carbon dioxide, converting the alkaline electrolyte to an aqueous carbonate electrolyte that is less conductive. Thus, the fuel input must be restricted to pure hydrogen, which limits applications to those in which pure hydrogen are available. If the CO2 is removed from fuel and oxygen streams, the operating costs are much greater. Although the attractiveness of AFC has declined substantially with the pursuit of improved PEMFC technology, recent developers still believe that AFC can be used for many applications, such as stationary power generation, and mobile applications including both marine and road vehicles. 2.1.6.3. Phosphoric acid fuel cell (PAFC). PAFCs are the most mature of the technologies in commercial production, although its costs remain uncompetitive with other non-fuel cell technologies. Hydrogen is still the ultimate fuel for the reaction in the PAFC, but various fuels, including natural gas, LPG and methanol, can be used as raw input converted by a reformer. Other advantages are resistance to fuel impurities, and the ability to use a less expensive catalyst. The drawbacks of this fuel

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cell include a lower efficiency than other fuel cell technologies and corrosive liquid electrolyte. In 2002, over 200 commercial units were manufactured, delivered and operated in the US, Europe and Japan. In the near future, with lower operating temperatures, PAFC would be ideal for small and mid-size power plants, replacing large electrical generators and other types of CCHP utilities in hospitals, hotels and airports. 2.1.6.4. Molten carbonate fuel cells (MCFC). A MCFC uses a molten carbonate salt mixture as its electrolyte. The composition of the electrolyte varies, but usually consists of lithium carbonate and potassium carbonate, which is chemically aggressive and puts strain on the stability and wear of the cell components. As a result, MCFC is more expensive than either SOFC or PEMFC in terms of capital cost. Fuel reforming of MCFC occurs inside the stack and tolerates impurities; therefore, this technology may use a variety of fuels. In addition, the high operating temperature allows for combined heat and power generation and high fuel-to-electricity efficiency. Nevertheless, the long start-up time to reach operating temperatures, and poorer flexibility in output, make MCFC ideally suited to base load power generation where continuous operation is necessary, such as heavy industries and national electrical grid networks. 2.1.6.5. Solid oxide fuel cell (SOFC). Due to allsolid-state ceramic construction, SOFCs share important characteristics, such as stability and reliability. A variety of hydrocarbon fuels can be used, like gasoline, methanol and natural gas. As another asset, the high operating temperature makes internal reforming possible and removes the need for a catalyst, and also produces highgrade waste heat suited well to CCHP applications. But the high temperature also creates some difficulties: expensive alloys for components are required, a very long time is needed for the electrolyte to heat and flexible small applications are difficult. Start-up time is less of an issue for stationary and continuous applications. SOFCs generally achieve around 60% efficiency in an average 5 MW plant, compared to around 30% for a traditional gas turbine. The last critical problem that prevents its commercialization is the comparatively high costs of SOFC.

467

2.2. Thermally activated technologies An important difference between CCHP systems and conventional cogenerations is that CCHP systems—including some cooling or dehumidification components—provide not only electricity and heating but also cooling capacity for space airconditioning or process. These cooling or dehumidification options can employ advanced thermally activated technologies as well as traditional technologies. But recent research indicates that thermally activated technologies are favored, as the overall efficiency of CCHP systems is enhanced by their application. In addition to high primary fuel efficiency, other benefits such as low emissions and net cost reduction are also achieved with thermally activated technologies (Table 2). Major thermally activated technologies include absorption chillers, adsorption chillers and desiccant dehumidifiers. These cooling and dehumidification systems can be driven by steam, hot water or hot exhaust gas derived from prime movers. However, waste heat from various prime movers falls into different temperature ranges; at the same time, cooling and dehumidification systems have their own suitable working temperature. As a result, best pairing of recoverable energy streams with thermally driven technologies is shown in Table 3. 2.2.1. Absorption chillers [13,22,28– 31] Absorption chillers are one of the commercialized thermally activated technologies widely applied in existing CCHP systems; they are similar to vapor compression chillers, with a few key differences. The basic difference is that a vapor compression chiller uses a rotating device (electric motor, engine, combustion turbine or steam turbine), to raise the pressure of refrigerant vapors, while an absorption chiller uses heat to compress the refrigerant vapors to a high pressure. Therefore, this ‘‘thermal compressor’’ has no moving parts. Basic absorption cycle is illustrated in Fig. 5. After the evaporator of absorption chiller generates cooling power, vapor generated in the evaporator is absorbed into a liquid absorbent in the absorber. The absorbent that has taken up refrigerant with spent or weak absorbent is pumped to the generator. The refrigerant is released again as a vapor by waste heat from steam, hot water or hot exhaust gas, and vapor is to be condensed in the condenser. The regenerated or strong absorbent is then led back to the absorber to pick up refrigerant vapor

20 340–1000 0.0075–0.015

25–35 1000–2000

0.004

0.0075–0.015

20 800–1600

95 Good

0.2–1.0

Loud 500–620

0.0045–0.0105

20 450–950

96–98 Fair

0.3–0.5

Loud 580–680

65–87 0.2–0.8 Up to 540

250 kW –50 MW Gas, propane, distillate oils, biogas 25–42

Combustion turbines

0.01–0.02

10 900–1500

98 Fair

0.1

Fair 720

60–85 1.2–1.7 200–350b

15–300 kW Gas, propane, distillate oils, biogas 15–30

Micro-turbines

N/A

10 1300–2000

N/A Good

0.23d

Fair 672d

65–85 1.2–1.7 60–200

40

1 kW–1.5M W Any (gas, alcohol, butane, biogas)

Stirling engines

0.007–0.05

10–20 2500–3500

90–95 Good

0.005–0.01

Quiet 430–490

85–90 0.8–1.1 260–370

5 kW–2 MW Hydrogen and fuels containing hydrocarbons 37–60

Fuel cells

a Up to a third of the fuel energy is available in the exhaust at temperatures from 370 to 540 1C; other rejected heat is low temperature, often too low for most processes. (Jacket cooling water at 80–95 1C, lube oil cooling at 70 1C and intercooler heat rejection at 60 1C, all difficult to use in CHP.) b 650 1C without recuperator. c Emissions associated with a steam turbine are dependent on the source of the steam. Steam turbines can be used with a boiler firing any one or a combination of a large variety of fuel sources, or they can be used with a gas turbine in a combined cycle configuration. Boiler emissions vary depending on fuel type and environmental conditions. d Stirling engine emission characteristics / STM 4–260. Gas-fired distributed energy resource technology characterizations.

95 Good

10

90–95 Poor

c

c

Loud 650

a

a

60–80 0.1–0.5 Up to 540

Loud

70–92 0.5–0.7

65–90 0.8–2.4

7–20

Efficiency electrical (%) Efficiency overall (%) Power to heat ratio Output heat temperature ( 1C) Noise CO2 emissions (kg/ MWh) NOx emissions (kg/ MWh) Availability (%) Part load performance Life cycle (year) Average cost investment ($/kW) Operating and maintenances costs ($/ kWh)

3 kW–6 MW Gas, biogas, liquid fuels, propane 25–43

5 kW–20 MW Gas, propane, distillate oils, biogas 35–45

Spark ignition engines

50 kW–500 MW Any

Diesel engines

Capacity range Fuel used

Steam turbines

Table 2 Characteristics and parameters of prime movers in CCHP systems [1,6,11,15,16,18,20]

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Table 3 Recoverable energy qualities with matching technologies [28] Power source

Temp. (1C)

Matching technology

Gas turbine Solid oxide fuel cell Micro-turbine Phosphoric acid fuel cell Stirling engine IC engine PEM fuel cell

540 480 320 120 90 80 60

Triple-effect/ double-effect absorption Triple-effect/ double-effect absorption Triple-effect/ double-effect absorption Double-effect/ single-effect absorption Single-effect absorption, adsorption or dehumidification Single-effect absorption, adsorption or dehumidification Single-effect absorption, adsorption or dehumidification

Fig. 5. A single-effect absorption refrigeration system [30].

anew. Heat is supplied to the generator at a comparatively high temperature and rejected from the absorber at a comparatively low level, analogously to a heat engine. The most common working fluids for absorption chillers are water/NH3 and LiBr/water, although there are 40 refrigerant compounds and 200 absorbent compounds available in the literature [30]. Lithium–bromide/water absorption chillers play a predominant role in the absorption chiller market in Asia-Pacific countries like China, Japan, Korea and in the US. In contrast, ammonia/water absorptions chillers are more popular in Europe. Depending on how many times the heat supply is utilized within the chiller; absorption chillers can be divided into single-effect, double-effect and tripleeffect. A single-effect absorption refrigeration system is the simplest and most commonly used design. Fig. 5 shows a single-effect system using nonvolatility absorbent such as LiBr/water. When volatility absorbent such as water/NH3 is used, the system requires an extra component called ‘‘a rectifier’’, which will purify the refrigerant before entering the condenser. The parameters and characteristics of different absorption chillers can be viewed in Table 4.

Absorption chillers can also be used in chilled water storage systems to produce chilled water during off-peak electric load periods when the cost of electricity is low and the demand for cooling is low. The stored chilled water is then drawn upon during the peak cooling periods when electricity costs are high, to supplement the chiller operation. The storage system helps to reduce the chiller capacity requirement and total installed cost of chillers. The installed cost of absorption chillers varies from 140 to 290 US$/kW, with the decrease of overall capacity. The O&M cost is in the range of 4.5–9 US$/kW/yr [31]. 2.2.2. Adsorption chillers [32– 38] Adsorption-cooling technology is a novel, environmentally friendly and effective means of using low-grade heat sources. An adsorption refrigeration system is similar to vapor compression systems except that heat—instead of work—provides the energy needed for compression. Unlike conventional vapor compression systems which require a mechanical compressor assembly, this new technology uses a thermally driven static sorption bed, saving as much as 90% of the required input power typically used to drive a mechanical compressor. The functioning of the basic cycle of adsorption cooling can be presented as comprising four phases as shown in the schematic Fig. 6 and described as follow: 1. A heating-pressurization 1–2, during which the adsorber is isolated from both the condenser and the evaporator. The pressure inside the adsorber then increases until reaching the condensation pressure, thanks to the heat supplied by an external heat source. 2. An isobaric condensation 2–3, during which the adsorber is connected to the condenser, allowing

80–110

120–150

120–150

120–150

200–230

Single effect cycle

Double effect (series flow)

Double effect (parallel flow)

Triple effect cycle

Heat source

LiBr/water

Water/NH3

o0

5–10

LiBr/water

Water/NH3

o0

5–10

LiBr/water

Working fluid

5–10

Cooling

Operating temp. ( 1C)

Single effect cycle

System

Table 4 Characteristics of absorption technologies [30]

N/A

Up to 1000

200–1500

3–1000+

10–1500

Cooling capacity (ton)

1.4–1.5

0.8–1.2

More than 1.2

0.5

More than 0.7

COP

Computer model and experimental unit

Experimental unit

Large water chiller

Commercial

Large water chiller

Current status

Rectification of refrigerant required Working solution is environmental friendly Operating pressure as high as with NH3 No crystallization problem Suitable for use as heat pump due to wide operating range

1. High complexity control system 2. Likely to be direct-fired, as input temp is very high 3. Requires more maintenance as a result of high corrosion due to high operating temperature

1. Heat release from first stage absorber used for second stage generator

1. High performance cycle, commercially available 2. Heat of condensation from first effect used as heat input for second stage

1. 2. 3. 4. 5.

1. Simplest and widely used 2. Using water as a refrigerant, cooling temperature is above 0 1C 3. Negative system pressure 4. Water cooled absorber required to prevent crystallization at high concentration

Remark

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Fig. 6. Standard Clapeyron’s lnp-1/T diagram of basic cycle.

the refrigerant vapor to flow the adsorber to the condenser and condenses therein as the heating is still continuing. This simultaneity of heating and vapor flow and condensation makes the process isobaric. The condensation heat is absorbed by the cooling medium or could be used to provide heating if the purpose of the system is heat pumping. The condensate is then expanded and drained into evaporator at lower pressure. 3. A cooling-depressurisation 3–4, during which the adsorber is isolated from both the condenser and the evaporator. The adsorber is cooled down and the pressure decreases back to the value of adsorption condition. 4. An isobaric adsorption, during which the adsorber is connected to the evaporator and isolated from condenser. The low-pressure liquid water contained in the evaporator is evaporated by extracting latent heat of evaporation from the space when being cooled down, and, simultaneously, the evaporated vapor is adsorbed anew by the reactivated adsorbent contained in the adsorber. The system takes advantage of the ability of certain adsorbent material, stored in an adsorber, to soak up a relatively large quantity of refrigerant vapor at some low temperature and pressure. At this stage, cooling capacity is achieved in the evaporator because of the evaporation of the refrigerant. The refrigerant is subsequently released to the condenser at a higher pressure simply by applying heat to the sorption bed. The basic cycle is the cycle without neither heat nor mass recovery. When operated with a single bed, the cold production of this cycle is intermittent. One step forward in the path of

Fig. 7. Schematic of two-bed adsorption refrigeration systems [37].

improvement of this cycle has been the invention of the two beds quasi-continuous cooling production system, shown in Fig. 7. In addition to the quasi-continuity of the cold production, the system offers the possibility for heat recovery and mass recuperation from one bed to anther, thereby helping to improve cycle’s efficiency. A heat regeneration fluid also can be used to increase system efficiency by transferring heat from a hot to a cold bed. As a critical part of this technology, the characteristics of various adsorbent–adsorbate working pairs are listed in the Table 5. Since there are no moving parts, except for valves, the sorption system is considerably simpler, requiring no lubrication and thus, little maintenance. Other advantages include quiet operation and modularity so it is readily scalable for increased heating and cooling capacity by additional beds. Furthermore, any heat source, such as waste heat or renewable energy resources, can be used, so energy saving can be potentially significant. Based on these merits of the adsorption system, active research in China, Europe, Japan and the US has resulted in the breakthrough of this technology. The adsorption refrigerators first appeared on the market in 1986, which were produced by the Nishiyodo Kuchouki, Co. Ltd. The silica gel–water adsorption chillers produced by this company are sold in the American market by the HIJC USA Inc. This company estimated the payback of this chiller to about 2–3 years. The chiller is driven by hot water from 50 to 90 1C, and the temperature of chilled water is close to 3 1C. The COP can reach 0.7

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Table 5 Characteristics of adsorption working pairs [32,35,36] Adsorbent

Adsorbate

Heat of adsorption (kJ/kg)

Toxicity

Vacuum level

Release temp. ( 1C)

Heat sources

Applications

Silica gel

H2O CH3OH

2800 1000–1500

No Yes

High High

70–100

Space cooling, refrigeration

Zeolite

H2O NH3 C2H5OH CH3OH

3300–4200 4000–6000 1200–1400 1800–2000

No Yes No Yes

High Low Moderate High

4150 100 110

Solar energy, lowtemperature waste heat High-temperature waste heat Solar energy, lowtemperature waste heat

42000

Yes

High

120

1368 N/A

Yes Yes

Low Low

95

Activated charcoal Charcoal fiber CaCl2

NH3 CH3OH

Space cooling, refrigeration Low temperature, ice making

Solar energy, low- Low temperature, temperature waste ice making heat

when the chiller is power by hot water at 90 1C. Another company producing silica gel–water adsorption is Mayekawa Co. The chillers from this company can be powered by hot water at 75 1C and yield chilled water at 14 1C with a reported COP of 0.6. In China, a series of adsorption chillers are commercially available with the cooperation of Shanghai Jiao Tong University and Jiangsu Shuangliang Air Conditioner Equipment Company. The products are rated at 10, 20, 50, 100 kW, etc. and the costs in US dollars could be $10,000, $15,000, $30,000 and $50,000, respectively. There are two typical CCHP applications with adsorption chillers. The CCHP system installed at the beginning of 2000, in the St. Johannes hospital is composed by a fuel cell, solar collectors, a heat storage vessel, a mechanical compression chiller, an adsorption chiller, an ice-storage tank and cooling ceilings. The hot water derived from solar collector and waste heat of the fuel cell drives a 105 kW Mycom ADR 30 adsorption chiller, manufactured by the Japanese company, Mayekawa. Another example is the CCHP systems set up at Shanghai Jiao Tong University, which will be specified in later section.

the desiccant material (such as silica gel, activated alumina, lithium chloride salt, or molecular sieves) to a moisture-laden process air stream, retaining the moisture of the air in desiccant and regenerating desiccant material via a heated air stream. System capacity is often expressed in volume of airflow or in moisture removal rate. Table 6 shows some specifications and costs of desiccant dehumidification systems. Dehumidification technology is divided into two major types, solid desiccant dehumidifiers and liquid desiccant dehumidifiers; both are useful for the mitigation of indoor environmental quality and health problems and for humidity control in buildings. Liquid desiccant technologies—particularly those with air washing and biocidal capabilities—are viewed as a critical path toward ensuring indoor environmental security under extraordinary circumstances and reducing indoor air pollution in general. Dehumidification technology in the commercial sector remains a young technology with a premium price. To date, commercial desiccant technologies have not been designed for integration into CCHP systems.

2.2.3. Desiccant dehumidifiers [3,13,31] Desiccant dehumidifiers can work in concert with sorption chillers or conventional air-conditioning systems to significantly increase overall system energy efficiency by avoiding overcooling air and precluding oversized capacity to meet dehumidification loads. The desiccant process involves exposing

2.2.4. Other options Although thermally activated technologies indicate the trend in cooling and dehumidification options in CCHP systems, electric vapor-compression refrigeration systems still play an important role for their maturity and reliability. Therefore, quite a few CCHP systems in research and practical

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Table 6 Costs and performance of desiccant dehumidification systems [31] Flux (m3/min)

Cost (US$/m3/min)

Thermal input (W/m3/min)

Maximum latent removal (W/m3/min)

40–140 140–280 280+

280–630 210–390 210–320

300–1000 300–1000 300–1000

300–600 300–600 300–600

Table 7 Costs and performance of engine-driven chillers [31] Capacity (kW)

Electric use (kWe/kW)

Cost (US$/kW)

Maintenance cost (US$/kW/yr)

35–350 350–1760 1760–7030

0.014–0.020 0.003–0.014 0.001–0.003

230–300 180–270 130–210

12.8–28.4 10.0–21.3 7.1–17.0

utilization still employ these conventional technologies as their cooling options. Nonetheless, it is unwise for a CCHP system to drive chillers using electricity generated by prime movers, since smaller prime movers have lower efficiency than larger types used in power plants. Engine-driven chillers have emerged as a substitute for electric chillers in CCHP units, avoiding the losses in energy conversion. Engine-driven chillers, including reciprocating, centrifugal and screw types, are conventional chillers driven by an engine, in lieu of an electric motor. They employ the same thermodynamic cycle and compressor technology used in electric chillers, but an engine or other prime mover drives the compressor directly. In engine-driven chillers smaller than 700 kW, reciprocating compressors are typically packaged with the engine. In applications ranging from over 700 kW to less than about 4220 kW, both screw and centrifugal compressors are used. In the largest, over 4500 kW, centrifugal compressors are the only option [22]. An advantage of engine-driven chillers is better variable speed performance, which improves partial-load efficiency. Engine-driven chillers can also operate in a CCHP system for hot water loads when the waste heat produced by the engine is recovered. Table 7 shows the costs and performances of various engine-driven chillers (Table 8). In general, mechanical vapor compression (typically by electric compression chillers and enginedriven chillers) is not a characteristic part of CCHP systems. It can be added to increase redundancy, diversity, reliability and economics of CCHP systems.

Table 8 Natural gas demand forecast (10 million m3) [84] Sector

Year 2005

Year 2010

Power generation Chemicals Industrial material Domestic fuel

174 120 168 106

484 180 257 230

Total

568

1151

3. Typical CCHP systems 3.1. Diverse configurations of CCHP systems CCHP systems, including both existing units and experimental models in laboratories, vary from site to site, with diverse prime movers, cooling options, connecting forms, rated size ranges, heat-to-power rates, user demand limitations and similar characteristics. Based on CCHP technologies and their characteristics described earlier, this section will discuss practical and potential CCHP systems and their development. Regarding the classification of CCHP systems in the Introduction, traditional large-scale systems, predominantly CHP systems without cooling options in centralized power plants or large industries, account for large portions in installed CHP capacity in many countries. The technologies used in this type of CHP approach have developed for several decades and these systems are relatively mature. For these CHP systems, there are two common configurations [39]: one is based on boiler and steam

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turbine, shown as Fig. 8; the other system is based on combustion turbines, shown as Fig. 9. The steam boiler/turbine approach has always been the most widely used CHP system. In this approach, a boiler produces high-pressure steam that is fed to a turbine to produce electricity. However, the turbine is designed so that there is steam left over to feed an industrial process. Thus, one fuel input to the boiler supplies electrical and thermal energy by recovering waste heat from the steam turbine electric generator. Typically, two thirds of the energy in a conventional power plant is lost when waste steam is condensed in the cooling tower. This type of system typically generates about five times as much thermal energy as electrical energy [39]. Thus, this kind of system is suit for heat plants in which electricity power is generated as byproduct. In newer, large, centralized CHP systems, a combustion turbine (a diesel reciprocating engine can also be used) is used to generate electricity, and thermal energy is recovered from the exhaust stream to make steam for other thermal uses. In these

Fig. 8. CHP system with backpressure steam turbine [1].

Fig. 9. CHP system with combustion turbine [1].

systems (Fig. 9), the thermal energy is typically one to two times the electric energy generated [39]. An improved system model called combined cycle gas turbine system (CCGT) combined combustion turbine with steam turbine in one configuration (Fig. 10), which is the most widely used model in large central power plants today. The reliability of combined cycle systems is 80–85%, the annual average availability is 77–85% and the economic life cycle is 15–25 years. The electrical efficiency is in the range 35–45%, the total efficiency is 70–88% and the power to heat ratio is 0.6–2.0. The electrical efficiency can be increased further. As for other categories of CCHP systems, relatively small-capacity-distributed CCHP units are the trend in future applications. In this category, novel technologies such as fuel cells, micro-turbines, Stirling engines, adsorption chillers and dehumidifiers are emerging in some research models and practical applications, which possess some promising characteristics, including low emission, high efficiency and low-grade thermal energy recovery. Reciprocating engines, combustion turbines, electrical chillers and absorption chillers are predominant in the recent distributed CCHP market, for the maturity and stability of these technologies. Reciprocating engines plus absorption or electrical (engine driven) chillers are popular for small utilizations. Jacket cooling fluids, lubricating oil systems, and engine exhaust are three heat recovery options which can produce hot water using exchangers, for heat demands and other cooling and dehumidification usages. This configuration, shown in Fig. 11, represents a large percentage of CCHP systems with reciprocating engines as prime mover. Reciprocating engines with engine driven chillers have fewer applications with low on-site electricity demands. This small-scale engine-based CCHP (CHP) system was a research issue addressed in

Fig. 10. Combined cycle CHP system with backpressure steam turbine [1].

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much of the literature. Maidment [40,41], mentioned energy management of an engine-based CCHP system with the application of an absorption chiller. Riley [42], examined the emission from this type of small CCHP system. Talbi [43], examined the interfacing of the turbocharged diesel engine with an absorption refrigeration unit in a CCHP system and estimated the performance enhancement. Miguez et al. [44,45], illustrated design and performance of a CCHP system with engine and heat pump equipment. Smith [46], also analyzed a similar system in his articles. Reciprocating engines, as the most mature prime mover technology used in distributed CCHP systems, made new improvements in some recent research. Moss et al. [47], attempted to combine the Joule-cycle used in gas turbines with an internal-combustion engine and formed a reciprocating Joule-cycle engine-based CHP system. A project at Shanghai Jiao Tong University [48], experimented with novel adsorption chillers, using heat recovered from engines, to generate cooling capacity because of the relatively low byproduct heat temperature of small engines in CCHP systems. Research on engine-based CCHP systems is active and the literature is extensive. Micro-turbines became available commercially in 2001 and 2002, and they immediately became an ideal prime mover for small-scale distributed CCHP systems. Absorption chillers and desiccant dehumidifiers driven by byproduct heat of micro-turbines are employed to meet cooling demands of users. A number of the same micro-turbine units can be connected to fulfill any electricity range in practice. This configuration of CCHP systems is applied in many locations, especially in the US, where micro-

turbine-based units have become serious competitors with engine-based units in the small-scale CCHP market. With rising awareness of microturbine-based systems, more research has been focus on this method in recent years [49,50]. Fig. 12 illustrates a typical schematic diagram of a micro-turbine CHP system. In this category of CCHP systems, the Stirling engine is viewed as a promising prime mover in small commercial and residential applications for their low emissions, fewer moving parts, low noise, small-scale availability and relatively low byproduct heat. Only a few commercial Stirling engine units can be found on the market, but research on Stirling engines in some companies and laboratories has advanced to a near-commercial stage, both in the US and in Europe. There has also been research on the feasibility of CCHP driven by Stirling engines [51]. Some possible cooling and dehumidification options for Stirling engines are absorption chillers, dehumidifiers and adsorption chillers. Fig. 13 shows an STM 4-120 Stirling engine system [18], which is the first commercialized Stirling engine in the world; until now it has had limited applications. It is envisioned that fuel cell systems will serve a variety of CCHP applications in the future, but there is limited experience to validate potential applications. Since most fuel cells are still in an early stage of development and commercial use, fuel cells CCHP systems carry high capital costs and higher project risk due to unproven durability and reliability. Simpler CHP systems based on PAFC systems have been deployed in commercial practice. Although difficulties remain, some fuel cell CCHP systems have already emerged in the US; Fig. 14 demonstrates a solid polymer fuel cell plant [52]. Tokyo Gas Co., Ltd. will market the first domestic polymer electrolyte fuel cell in 2005 [53]. In addition, Hamada et al. [54] field-tested the

Fig. 11. Schematic of reciprocating engine heat recovery [7,22].

Fig. 12. Schematic diagram of micro-turbine [7,22].

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performance of a polymer electrolyte fuel cell for a residential energy system. According to the fuel cell characteristics illustrated earlier, different fuel cells can produce various temperature levels of byproduct heat to drive certain cooling and dehumidification equipment. One major challenge of CCHP systems is the lack of integrated systems [22]. In the US, seven industrial teams have announced research, development and testing of ‘‘first generation’’ integrated CHP and absorption chillers with controls—some with desiccant units as well. This program holds promise for the building market for CHP, offering multiple benefits, such as lower integration costs

Fig. 13. STM 4-120 power unit packaged DG system [18].

and risks. In addition, it is a positive step forward for the use of thermal cooling with CHP in the industrial sector. 3.2. Representative CCHP systems in use In terms of rated sizes, CCHP applications are categorized into micro, small-scale, medium and large-scale systems, while the size range of these categories are under 20 kW, from 20 kW to 1 MW, from 1 to 10 MW and above 10 MW, respectively. In the following sections, four typical CCHP applications, selected to represent these four categories, are discussed in detail for a close look at various size CCHP systems currently in use. 3.2.1. Micro systems (under 20 kW) In this category, there are limited examples in the current market for a relatively small capacity, although micro systems show great potential for commercial, institutional and residential utilization. As regards their technological feasibility, reciprocating engines, fuel cells and Stirling engines are regarded as prospective prime movers. At Shanghai Jiao Tong University, a micro CCHP system, comprised of a 12 kW gas-fired reciprocating engine, a 10 kW adsorption chiller, a floor radiate heating system, a waste heat recovery, a hot water tank and a cooling water tower, has been set up

Fig. 14. Typical configuration of a solid polymer fuel cell plant [52].

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(Fig. 15), which is one of the smallest CCHP applications currently in use [48]. Fig. 16 shows the configuration of the microCCHP system at SJTU. Natural gas or LPG is used to drive the engine. Engine jacket cooling water passes through the heat exchanger and is reheated by exhaust gas that is up to 580 1C. Reheated water then passes through an adsorption chiller to produce chilled water for space cooling in summer; or through the heat exchanger to produce hot water for a floor radiate heating system in winter. After that, the jacket water enters water tank to produce domestic hot water and finally returns to the engine jacket. The generator at rated power (electricity efficiency is about 21.4%) recovers 13.6 and 14.4 kW heat from exhaust gas and cylinder jacket cooling water, respectively. The highlight of this micro CCHP system is its practical utilization of an adsorption chiller developed by SJTU with the cooperation of Jiangsu Shuangliang Air Conditioner Equipment Company (Fig. 17), which makes possible the recovery of low-

Fig. 15. Test facility view of the micro CCHP.

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grade thermal energy [55,56]. In the tests, the COP of silica/gel–water adsorption chiller reaches 0.3–0.4 with a heat source of 60–95 1C. With the help of this thermal-activated technology, the overall thermal and electrical efficiency of the micro CCHP system is more than 70%. After an analysis was executed based on this micro CCHP system [48,55], it was concluded that the payback period is between 2.1 and 3.2 years for commercial buildings, or between 1.7 and 2.4 years for hotels, while the natural gas costs from 0.193 to 0.230 US$/Nm3. In recent years, many other new developments have been achieved to commercialize water chillers with small cooling capacities. Examples of these are: 1. Water–LiBr absorption chillers  EAW in Westenfeld, Germany (lowest available cooling capacity 15 kW)  Pho¨nix Sonnenwa¨rme in Berlin, Germany (10 kW)  University de Catalunya in Terrassa, Spain: air-cooled system (10 kW)  Rotartica in Spain: air-cooled system with rotating absorber/generator (10 kW) 2. Ammonia water systems with mechanical solution pump  Joanneum Research in Graz, Austria (10 kW, operation temperature
20–10 1C)  AOSOL in Portugal: air-cooled machine (6 kW) 3. Ammonia water systems without mechanical solution pump  University of Applied Research in Stuttgart, Germany (approximately 2–5 kW)  SolarFrost in Graz, Austria

Fig. 16. Schematic diagram of the micro-CCHP [48].

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These products are not yet well established in the market, but promise to open new market segments for CCHP in small commercial buildings (e.g., offices, small hotels, etc.) and even residential buildings.

College Park. Fig. 18 shows the CCHP package at the University of Maryland. The small-scale CCHP system is located in Chesapeake Building of the University of Maryland. This 4900 m2 building is representative of the medium-sized commercial buildings accounting for 23% of all buildings in the US [58]. The prime mover is a Capstone C60 micro-turbine that generates 60 kW electricity at 90,000 rpm and exhausts flue gas at 310 1C after recuperation [59]. The average efficiency of the micro-turbine is 26.9%, when operating at full output capacity and an average air inlet temperature of 7 1C. However, the partial-load efficiency drops to a low of 11% at 9 kW output power. Waste heat from the microturbine exhaust powers a Broad BD6.4NF-15 single effect absorption chiller, which achieves 65 kW of cooling power at the COP of 0.65. The absorption chiller assists the RTU (316 kW direct expansion electric rooftop cooling units) in providing air conditioning for cooling zone two, seen in Fig. 19. Chilled water produced by the absorption chiller is supplied at 7 1C and returns at 12 1C [49]. Flue gas from the chiller powers the ATS solid desiccant dehumidifier. The dehumidifier dries the supply air for the building, a function normally performed by the roof top unit and the absorption chiller, reducing the need for grid-based electrical power. Together, these interactive components efficiently supply air conditioning for cooling zone two and supplement the power requirement for the entire building. The overall fuel utilization of this CCHP system is as much as 72%. Compared with the conventional energy supply for this building, annual savings of applying the new CCHP system are forecasted to be $25,000, with a 40% reduction in CO2 [49].

3.2.2. Small-scale systems (20 kW– 1 MW) In this group of size ranges, a large number of applications are constructed for different uses, such as retail stores, supermarkets, hospitals, offices, schools, small industry, etc. This sector is the most active and mature market for CCHP, since almost every prime mover and cooling/dehumidification technology above can find their particular market. Micro-turbines are strong competitors of internal combustion reciprocating engines in this stage. Following the example of small-scale CCHP systems, one of the first CCHP applications with micro-turbines is located at University of Maryland,

3.2.3. Medium systems (1– 10 MW) Generally, existing applications ranging from 1 to 10 MW are set up for industrial sites, where no cooling demand is needed. As an example, the system in the Domain Plant of Austin is equipped with a combustion turbine for electricity demands as well as an absorption chiller for cooling and heating. This plant is powered by a 4.6 MW Solar Turbine Centaur 50 gas turbine (Fig. 20), which generates 4.3 MW net outputs for full-load continuous duty with 28.6% electrical efficiency and 510 1C exhaust. The exhaust from the gas turbine is ducted into a two-stage indirect-fired Broad Co. absorption chil-

Fig. 17. Adsorption chiller prototype.

4. Solid sorption  Sortech in Halle, Germany: adsorption heat pump (10 kW, working pair water/silica gel)  ClimateWell AB in Ha¨gersten, Sweden (10 kW, working pair lithium chloride/water; includes thermo-chemical storage)  SWEAT b.v., in the Netherlands (working pair sodium sulfide/water; includes thermochemical storage)

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ler (Fig. 21) via a diverter valve, which produces 8918 kW of cooling power at chiller water volumetric flow rate of 1390 m3/h. The chilled water is supplied to users at 6.7 1C and returns to the chiller at 12.2 1C [60–63]. The overall schematic layout of the CCHP system is illustrated in Fig. 22. The CCHP plant was constructed by Burns & McDonnell partnered with the municipal utility Austin Energy, in an existing building that is the right size to house the modular package layout. A remarkable characteristic of this system is its modularization, which enables ease of construction. Austin Energy owns and operates the CCHP system as part of an existing central utility station that generates power for the grid and sells chilled water to industrial tenants and a downtown district cooling system. Overall system integration is controlled by Allen Bradley software, which provides programmable logic remote monitoring capacity for

Fig. 18. The CCHP package at the University of Maryland [57].

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the complete system. The system is intended to run in continuous duty operation at full base load output 24 h a day [62,63]. After beginning commercial service, the CCHP system operated at overall fuel efficiency of 76.8% with less than 15 ppm NOx and no catalyst exhaust treatment. The system is expected to cut equipment and installation costs by 15–30% and achieve 3 million m3 in natural gas saving annually, depending on the amount of infrastructure available at the site [62,63]. 3.2.4. Large-scale systems (above 10 MW) Large-scale CCHP systems with capacity above 10 MW are the ideal energy supply scheme for large industries or institutional/commercial/residential districts. Although large cogenerations can be found everywhere, large systems that provide vast cooling capacity simultaneously have limited applications similar to micro CCHP systems. The 57.4 MW CCHP plant at the University of Illinois at Chicago is a successful model for large-scale CCHP applications. This CCHP system provides service to the entire campus of about 744,000 m2 and a student population of over 27,000 [64]. This application consists of two sections: the East Campus system and the West Campus system, which were established from 1993 to 2002. The CCHP plant on the campus is shown in Fig. 23. Equipment utilized in the East Campus system includes: two 6.3 MW Cooper-Bessemer dual-fuel reciprocating engine generators; two 3.8 MW Wa¨rtsila¨ 18V-28SG gas reciprocating engine generators;

Fig. 19. Schematic diagram of the CCHP application at the University of Maryland [49,58].

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Fig. 20. Solar gas turbine package [60].

Fig. 21. Broad absorption chiller.

a 3.5 MW Trane two-stage absorption chiller; two 7 MW York International electrical centrifugal chillers; and several remote building absorption chillers activated by the hot water loop (4.7 MW maximum cooling capacity). The system generates 20.2 MW of electricity total, to cover nearly 100% of the entire electrical demand of the East Campus. The recovered heat from the CCHP system offsets the heating and cooling requirements of 29 east campus buildings, more than 353,400 m2. The configuration of this complex system is illustrated in Fig. 24. The overall installed cost is 25.7 million US dollars and estimated payback is less than 10 years. It is estimated that the CCHP application provides an overall source energy reduction of 14.2%, an estimated 28.5% reduction in CO2, a 52.8% reduction in NOx, and an 89.1% reduction in SO2, along with approximately three million US dollars in annual operating saving [65].

Fig. 22. Schematic layout of the CCHP system in Austin [62].

After the success of CCHP system in the East Campus, a second system began operation on the West Campus, with an additional 37.2 MW electricity power to offset the heating and cooling demands of the several hospitals and other buildings on the West Campus. At the heart of the West Campus CCHP system are three 5.4 MW Wa¨rtsila¨ gas engine generators and three 7.0 MW Solar Taurus turbines. The cooling components of the system are several Carrier absorption chillers, totaling 7 MW cooling capacity. The schematic configuration of the West Campus CCHP system is illustrated in Fig. 25. The installation cost of the system was 36 million US dollars; annual savings of 7 million US dollars are expected, with an estimated simple payback of 5.1 years [66].

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4. Development of CCHP around the world 4.1. United States The beginning of CCHP development in the US dates back to 1978, when Public Utility Regulatory Policy Act of 1978 (PURPA) was enacted to require utilities to purchase electricity generated by independent suppliers and thus, stimulate the development of renewable energy and CCHP (CHP or cogeneration). In 1995, the installed capacity of CCHP systems in the US was 45 GW compared to

Fig. 23. CCHP plant at the University of Illinois at Chicago [66].

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12 GW in 1980 and; in this period, the average increased capacity annually was about 2.2 GW [67–69]. However, in the mid 1990s; a liberated market concept was introduced into the electricity generation industry by government; during this time, intense competition and instability in the electricity market blocked the rapid development of CCHP applications. The installed capacity of CCHP increased very slightly from 45 GW in 1995 to 46 GW in 1998. Subsequently, the US government took a series of measures to promote CCHP development again. First, the US Department of Energy (DOE), with the cooperation of the Environmental Protection Agency (EPA) and the Combined Heat & Power Association (CHPA), put a ‘‘CHP Challenge’’ into effect in 1998. The aim of this ‘‘challenge’’ was to boost the installed capacity of CCHP from 46 GW in 1998 to 92 GW in 2010. Then, in 1999, ‘‘Combined Cooling Heating & Power for Buildings 2020 Vision’’ was published by the DOE, which presented a timetable of CCHP development. It was recommended that obstacles to connect distributed CCHP applications with utility grids be eliminated, and that parameters be established to achieve change before 2005. By 2010, CCHP is to be applied in 25% of new constructions and 10% of existing commercial and institutional buildings; CCHP will substitute in 50% of CHP buildings. By 2020, 50% of new construction and 25% of existing commercial/ institutional buildings will be equipped with CCHP [67,69].

Fig. 24. System configuration of the East Campus [64,65].

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Fig. 25. System configuration of the West Campus [64,66].

In 2001, President George W. Bush established the National Energy Policy Development (NEPD) Group, directing it to ‘‘develop a national energy policy designed to help the private sector, and, as necessary and appropriate, State and local governments, promote dependable, affordable, and environmentally sound production and distribution of energy for the future.’’ CHP policy recommendations contained in The National Energy Policy of 2001, set forth by the NEPD Group included [70–74]: encouraging increased the use of these cleaner, more efficient technologies CHP projects by shortening the depreciation life for CHP projects or providing an investment tax credit. As mentioned in the Introduction, CCHP is divided into traditional large-scale CCHP applications (CHP non-DG) and relatively small capacity distributed CCHP (CHP DG). These two parts of recent CCHP capacity additions can be seen in Fig. 26. The overall electricity capacity addition (2001–2003) in the US presented on the left below, including both utility and non-utility, interconnected and non-interconnected, capacity additions of all sizes. The 15.5 GW change in CCHP reflects incentives to build after the California crisis and other market changes. It should be noted that 87% of new CCHP is non-DG, which is traditional largescale CCHP applications. The installed capacity of CCHP in 2001 was as large as 56 GW and about seven percent of overall installed capacity that year. Examined from the aspect of electricity generation, 310 billion kWh was generated by CCHP systems that year—up to nine percent of the overall electricity generated in the US.

The US CCHP market grew significantly through 2002 but has since slowed sharply in the face of high natural gas prices and persistent regulatory barriers. The major blackout of 2003 in North America brought about major review of options to minimize such disruption in future. CCHP, especially DG CCHP, can reduce vulnerability to such outages, and to the threat of terrorist attack on power systems [14,75,76]. Fig. 27 shows 35.2 GW non-DG CCHP capacity was added from 1990 to 2003, including many merchant plants and 7.2 GW DG CCHP capacity was added the same period, creating a total CHP DG capacity of 22 GW [69]. The overall electricity capacity of CCHP in the US reached 80 GW in 2004. There were 1540 existing commercial CCHP applications, with 9024 MW and 1189 industrial CCHP sites with 65,621 MW in the US [77]. Capacities of various application sectors are compared by years (1995, 2000 and 2004), shown in Figs. 28 and 29. The future of CCHP markets in the US is promising; though there are still certain factors that influence the potential outcome. Key motivators and barriers to CCHP development are listed as follows [14]: Key motivators

   

Need for higher quality power supply. Congested transmission and distribution lines. Concerns about system vulnerability. State/national energy policy support for cogeneration and renewable energy. (States currently represent a more important policy leader than does national government.)

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Fig. 26. Capacity additions 2001–2003 (GW) [69]. Fig. 27. CCHP capacities in all sizes by 1st year installed [69].

Key barriers

    

High gas prices (with delayed impact on power prices) and energy price volatility. Non-standardized grid access and interconnection requirements across the USA. Continuing monopoly of energy utilities. Emissions standards that do not reflect the efficiency of cogeneration and other DE. Continued ban on private wires and prohibitions against third party sales in 15 states.

It is estimated that the potential US CCHP market could be as large as 209.9 GW, based on the analysis of overall capacity data in 1999. The industrial CCHP potential could be another 88 and 75 GW of the commercial sector. Moreover, the existing 22 GW of CCHP/DG could double to 42 GW, even under high gas price conditions [14,77]. CCHP applications would dominate overall DG industrial and commercial market potential, comprising over two-thirds of all DG base case market potential and over half of the future case market potential. Also, CCHP is currently underutilized in the commercial building sector, where there is great potential. 4.2. Europe In the European Union, the most important legislative initiatives of CCHP development are the Cogeneration Directive, the Emissions Trading Directive, the New Electricity and Gas Directives, and the Energy Performance of Buildings and Taxation of Energy Products Directives. EU policies both recognize the importance of CCHP for

Fig. 28. Industrial CCHP applications [77].

achieving climate change commitments and define possible instruments to promote the technology at the EU level. When the strategy was issued in 1997, the share of electricity produced from CCHP in the EU was about 9%. The strategy sets a target of 18% by 2010 [67,78,79]. However, possible measures and instruments to achieve this target have so far not been defined in depth. The development of CCHP in the EU is characterized by a wide diversity, both in the scale and nature of the development. This diversity reflects differences in policy priorities, natural resources, history, culture and climate and has it close links to the structure and activity of electricity markets. Obviously, the main reason for this diversity has been the different political choices made by governments in energy matters. Fig. 30 portrays, as nearly as possible, the situation of CCHP development in EU countries and the projected scenario in 2010 [1,9,16].

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cular supportive policies are undertaken. Nevertheless, CCHP was recognized as the economic means of generating electricity. Other reasons for the healthy development of CCHP have been the high demand for heating, subsidies for new technologies and an absence of barriers [9]. 4.2.4. The Netherlands Success in the Netherlands has been achieved through strong promotional activities and a clear positive policy framework:

  Fig. 29. Commercial CCHP applications [77].

 In the diagram, it can be concluded that Austria, Denmark, Finland and the Netherlands are the four leading countries in the popularization of CCHP utilization. 4.2.1. Austria Austria has strong environmental policies, and CCHP technology has always been encouraged. Industrial and district heating sectors have developed relatively well; the former through the benefits that the technology brings to high-energy intensity users and the latter as a response to energy price rises in the 1970s and central state support [9]. 4.2.2. Denmark At the time of the oil crisis, in the beginning of the 1970s, Denmark was 90% dependent on foreign oil. Today, Denmark is self sufficient in oil and gas, one of several factors, which led the government to promote CHP technology. The popular use of wind energy in Denmark is also a contributing factor. The existence of district heating networks and the environmental concerns of the society also propelled CHP development. The success of CHP development in Denmark has been largely due to government policy resolved to ensure that the technology can flourish, and has been achieved through significant subsidy and grant provisions [1,16,78]. 4.2.3. Finland Finland has always been one of the most liberal energy markets in Europe. The development of CCHP in Finland has not been largely a consequence of specific political action, since no parti-

fuel (gas) tax exemption for fuel used to generate CHP electricity; investment in highly-efficient CHP units, partly fiscally deductible; eco-tax exemption for heat supplied by CHP.

The unofficial national long-term target for CHP is to achieve CCHP capacity of 15 GW by 2010. The development in other countries in the EU follows [1,9,14,16,80–83]. 4.2.5. France The major electricity demands of France are fulfilled by nuclear energy, which comprises more than 70 percent of overall capacity. Thus, the capacity of CCHP is responsible for a small fraction of electricity. As shown in Fig. 31, after the boom in CCHP installation in 1998, a sharp decline occurred the next year, which remains unchanged in recent years [16,80]. 4.2.6. Germany In Germany, liberalization has had negative effects on CCHP due to price wars between the utilities that have caused electricity to be sold below its production cost. Although the price of electricity has begun to rise again, about 20 GW CCHP capacity was closed down before 2001. Thus, the government has taken several measures [1,14,16] listed below. First, ecological tax reform was undertaken: cogeneration with a global efficiency of 70% or more is exempt from electricity and gas taxes. The second measure was the Emergency Plan making it compulsory to buy electricity from cogeneration with an extra subsidy of 0.03 DM/ MWh [1]. In addition, a quota model was introduced, mandating that every company supplying electricity to a final consumer must supply a certain percentage from cogeneration. New projects under

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Fig. 30. Percentage of CCHP in 1999 and 2010 [80].

the new CCHP law in Germany can be seen in Fig. 32.

4.2.7. Hungary As one of the European Union’s accession countries, the statistics for CCHP in Hungary are incomplete. However, the government and electricity company each take measures to promote CCHP development. Cogenerated electricity of 2004 in Hungary is 5600 GWh, about 15.6% of total generation, and cogenerated heat is 46,335 TJ, about 71% of total generation. It is hoped cogenerated electricity will reach 9.0–9.5 TWh— about 20–22% of total generation—by 2010 [81].

4.2.8. Italy The annual electricity demand supplied by CHP is about 15%. Industrial sector applications are more important than district heating or smaller public or private utilization. A series of policies have been set forth by the government to establish low tax rates on gas used for district heating; tax reductions on gas for industrial CHP schemes proportional to their electrical efficiency; carbon tax exemptions for CHP; dispatch priorities for CHP in the transmission network and more [16,78].

4.2.9. Poland There are over 1000 CCHP installations in Poland, but no specific legal framework is established. General provisions in the 1997 Energy Law apply. Although there is no obligation upon distribution companies to purchase electricity from CCHP, in general, they do purchase it. The generator-producing electricity from cogeneration must be licensed if the installed capacity of the unit exceeds 50 MW. For the promotion of CCHP, KOGEN Polska, has been created, which is the national member for COGEN Europe in Poland [1,16]. 4.2.10. Spain Approximately 12% of electricity production is from CHP, generated mostly in the industrial sector and with no district heating. Natural gas fuels half of the existing CHP installations. A special regime for new CHP units meeting certain criteria was introduced in 1998; it included mutual obligations by CHP producers and distribution companies. Some funding for small-scale CHP installations was made available from the Institute for Energy Saving and Diversification [16,78]. 4.2.11. Sweden CHP represents about 6% of the total electricity production, mostly in district heating and

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Fig. 31. CCHP projects in France 1991–2002 [80].

CCHP from the renewable obligation base. During the 1990s, installed CHP capacity in the UK more than doubled [83]. However, this buoyant trend has been interrupted by recent market conditions. Fig. 33 shows the trend in installed CCHP capacity and how it relates to the 10 GW target [1,16]. Attempts have been made in many European Union countries to remove the barriers and promote cogeneration. Various incentives have been used, such as relatively high prices for exported electricity sold to the grid, and grants on investments. Other measures have included spreading related information, energy auditing and analysis of data, and support of research and development. Most of these measures were designed at a time when many of the barriers to the development of CCHP derived from the existence of monopolistic electricity and gas markets. The most frequently mentioned barriers to CCHP in the EU were:

   

Fig. 32. New projects under the new CCHP law in Germany (February 2002) [80].

industrial use. The low CHP share is not the result of regulatory obstacles, but rather of the abundance of low-priced electricity. In Sweden, more than 90% of electricity is hydro or nuclear power [16,78]. 4.2.12. UK Since the year 2000, the government has introduced a wide range of measures to support the growth of CCHP capacity. These measures fall into several categories: fiscal incentives, grant support, regulatory framework, promotion of innovation and government leadership and partnership. The main support measures favored by the CCHP industry in their response to the strategy consultation were a CCHP obligation, and exempting

low price paid for the surplus of electricity exported to the grid; high fees for top-up and back-up supplies; no possibility of third party access; predatory pricing against possible competition.

The share of CCHP in the electricity production in Europe is currently about 10%. This is far from its full potential, which COGEN Europe estimates to be at least 30%. This can be supported by the fact that three countries have achieved this share [1,9,16]. As previously mentioned, in its 1997 Strategy to Promote Combined Heat and Power, the European Commission (EC), set a target of 18% by 2010. In the current situation, uncertainties caused by incomplete liberalization of electricity markets in Europe make it unlikely that this target will be reached without a reorientation of the policy framework. Political support for CCHP, and energy saving technologies from various national governments is a proven necessity. A possible estimate of future CCHP capacity by sectors in Europe is shown in Fig. 34. 4.3. Asia and the Pacific 4.3.1. China In the 1980s, China became concerned about CCHP (CHP/cogeneration) development for the first time. The government emphasized that size and type of these systems should be determined by heat demands of users, (called as ‘‘heat-match mode’’).

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During that period, many single systems with 3– 12 MW steam turbines were constructed. In the late 1980s and early 1990s, when China experienced electricity supply problems, the government set forth a series of policies which included tax exemption, investment tax credit and direct subsidy for energy saving projects—especially CCHP systems. The rapid development of CCHP systems slowed at the end of the 1990s, as preferential policies were abolished with several factors, such as an abundance of temporary electricity, reform of the national accounting system and monopolization in the electricity market [67,84,85]. After the National Energy Conservation Law took effect in 1998, China encouraged the development of general energy-saving technologies and projects; energy grade utilization and overall efficiency was promoted. Through the end of 1999, there were as many as 1402 cogeneration units with individual capacity over 6 MW in China. The overall capacity of these units was 28,153 MW, consisting of 12.6% fuel-combustion electricity generation [14,84,86]. Fig. 35 shows the capacity of different size ranges CCHP systems in 1999. However, most systems were fueled with coal and applied boilers and steam turbines as prime movers. In 2001, the government enacted the regulation of CHP for better management of CHP (including CCHP) projects; it stressed the heat-match mode and prescribed the lowest efficiency limitation and heat-to-power ratio of different systems. This

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measure sparked development not only of centralized cogeneration plants, but also small-scale distributed CCHP systems. In recent years, CHP units with cooling capacity developed rapidly, and several cities have coal-combustion CHP plants with cooling capacity supply. Jinan has 49.6 MW cooling supply CCHP system, and in Hangzhou there are two systems of more than 120 MW cooling capacity each. CCHP systems based on gas-combustion turbines or engines also emerged; typical examples are Shanghai Huangpu Central Hospital, Pudong International Airport, the Beijing Gas Company building and the system used in Tsinghua University [67,84,85]. Current CCHP development in China has some unique characteristics. Following the heat-match mode, users select the system size based on practical on-site heat demands; as a result, there are many more small-scale units than large ones. In small and middle size cities of north China, cogeneration plants supply steam for both industrial processes and domestic space heating, while the heating connections and distributions are quite complex and require a large investment. In big cities of the north, cogeneration plants consist of large steam turbines—more than 100 MW per unit—which can supply space heating of 10 million m2. Most CHP systems in large industries are set up solely for the power and heat demands of that industry. Although these systems connect with utility grids, they sell very little of the electricity

Fig. 33. CCHP installed capacity and targets in UK [83].

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Fig. 34. Future possible scenario for CCHP capacity [80].

generated. Most CCHP systems supported by the government generate cooling capacity for process utilization in textile mills, chemical industries or large institutional buildings. Domestic space cooling by CCHP systems cannot be addressed, as the problem of metering and charge remains. In general, more than 95% of CCHP or CHP systems are fueled with coal and there are limited combined-cycle projects in China, since the production capacity of nature gas in China is low and the price of natural gas is relatively expensive [84]. To further CCHP applications in China, several measures should be taken [86]:

1. Distributed CCHP generators should be permitted grid access on transparent and nondiscriminatory terms. 2. Emerging industry structures should not maintain market control in the hands of incumbent utilities. 3. The transmission and distribution costs associated with central generation should be fully accounted for in any system planning. 4. Fuel and power pricing should be determined by markets as much as possible. 5. Private and foreign investors should face no undue commercial, legal or regulatory barriers in carrying out their business. 6. The overall output efficiency (including usable heat), of utility plants should be rewarded. 7. The clean development mechanism should be encouraged to contribute significantly to China’s power demand requirements.

Fig. 35. The capacity (GW) of different size range CCHP systems in 1999 [84].

Fuel diversification for future CCHP development is likely to be significant with biomass, biogas and natural gas providing new opportunities for developers. Natural gas-driven combined cycle CCHP systems will play an important role in future markets. Although gas-driven systems cannot compete with coal-driven cogenerations in the north, they can become a strong competitor with 600 MW units using coal in the south. It is believed that about 1 GW combined cycle CCHP systems using gas will be put into production in 2005 and even more in the next several years [84]. The capacity of CCHP applications in China is predicted to grow at a high rate in coming years, with an estimated potential increase of 3.1 GW annually, comprising 620 MW for industries, 2000 MW for cities in north China and 500 MW for new industrial area in south China [84]. At the same time, the increase in annual capacity is about 4.5 GW. By 2006, CCHP capacity could reach 45 GW. If some, or all of measures listed above, can be achieved, the scale of DER (most are CCHP) development in China could exceed that of central power and go beyond 100 GW by 2010 [14]. 4.3.2. Japan At the end of March 2003, there were 2915 CCHP (including cogeneration) units, totaling 1429 MW installed for commercial applications and 1600 units totaling 5074 MW for industrial usages [87]. The accumulated data of installation numbers and total capacity are illustrated in Fig. 36. The number of installations as well as the capacity has been steadily increasing over the last decade, which can be seen in Fig. 37. After a sharp rise in 1990, the growth rate

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slowed in 1992, due to recession and decline in energy prices. There has been a renewed interest in CCHP, proven by the fact that over 850 MW was added between March 1996 and 1998 [88]. In recent years, development of CCHP application in Japan restarted with the emergence of new CCHP technologies. Data of commercial and industrial cogeneration by type of activity, including number of installations and generating capacities, are summarized in Table 9. Among commercial applications, commercial stores rank first in terms of number and total capacity, followed by hospitals, hotels and offices. The main features of these sites include long and continuous operating hours, constant demand on thermal energy for hot water, steam and chilled water. Although few in number, the district heating and cooling network projects, with much larger average sizes, represent an important application in Japan. Among industrial uses, gas and oil industries have the largest share in terms of capacity. Other sectors having large capacities are pulp and paper, chemical pharmaceutical, iron and metals, and glass, soda and ceramics industries. In contrast, the food industry uses many smaller systems. Since the 1980s, the support extended by government for promoting CCHP may be classified into four categories: special taxation, low interest loans, investment subsidies, and subsidies for new technology development. The ‘‘Law Concerning Promotion of the Use of New Energies’’ was enacted in June 1997, as a framework for encouraging the introduction of renewable and non-conventional fuels (including CCHP/cogeneration). The budget in 1998 allocated 74.8 billion JPf (up from 56 billion JPf in 1997) for new energy promotion [89]. Other detailed measures taken by the government are listed as follows: CCHP system investors may choose either 30% depreciation on the installation cost or 7% of tax exemption in the 1st year of acquisition of cogeneration plant; low interest loans (2.3% per year) can be obtained for 40–70% of the total investment cost. Additionally, electricity market reform also has an obvious effect on CCHP development. Under the former Electricity Supply Law, nine regional electric utilities had the monopoly to supply electricity in the whole country. This law was revised in 1995, which now helps in further propagation of cogeneration. The law allows the private sector to sell self-generated electricity to the electric utilities or supply self-generated electricity

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to third parties. Such action, or even the credible possibility of such action, would put competitive pressure on the utility to change its prices and reduce its costs to those customers who can credibly self-generate. In addition to this encouragement for CCHP development, the obligation of environmental protection also plays a critical role. Following the Third Conference of Parties held at Kyoto in December 1997, Japan set itself a target of reducing greenhouse gas emission by 6% by the year 2010, taking 1990 as the base year. An Environmental White Book was released in June 1998 wherein CCHP appears as one of the important measures to reduce CO2 emission [89,93]. In the ‘‘Energy Policies of IEA Countries: Japan 1999 Review’’ [89], the target of cogeneration set by the Japanese government concludes that total installed cogeneration capacity is expected to increase from 3.85 GW in 1996 to 10 GW in 2010 (cogeneration is regarded as a demand side new energy in Japan). 4.3.3. India With continuing economic growth, the Indian electricity system is in need of urgent investment and development. DER (mainly CCHP systems) capacity is only 4.1 GW—about 3.6% of total electricity capacity in India. High priced and unreliable electricity supply, government capital grants and soft loans are the key drivers for CCHP development. At the same time, some barriers exist, such as lack of adequate policy framework, lack of technical knowledge and support services, shortage

Fig. 36. The cumulative capacity of CCHP in each fiscal year [87].

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Fig. 37. The number and generating capacity of CCHP in each fiscal year [87].

Table 9 CCHP commercial and industrial applications at the end of March 2003 [87,93] Commercial sectors

Number of sites

Generation capacity (MW)

Store Hospital Hotel Office Sports facility Welfare facility Public bath Training center/ sanatorium

497 460 440 289 236 214 169 124

264 213 219 193 94 11 23 43

Gasoline station School District heating and cooling

86 77 21

5 42 81

Industrial sectors

Number of sites

Generation capacity (MW)

Food Chemical pharmaceutical

294 279

1333 4344

Machinery Electric equipment

223 158

2865 2981

Iron and metal Textile Pulp and paper Gas, oil and other energy

141 90 73 66

4078 2433 4657 9500

Glass, soda and ceramics

44

4091

Other

302

242

Other

232

1806

Total

2915

1429

Total

1600

3171

of investment finance and limited natural gas network for cogeneration [14]. In the CCHP market, there is tremendous potential in industrial sugar cane. Bagasse-based cogenerations in sugar mills are the main form of CCHP development in India [90,91]. A distributed generation revolution began in India with 87 new local power projects, producing 710 MW from sugar cane waste. In September 2001, the Ministry of Power estimated that there was a

total potential for some 15 GW of cogeneration capacity, of which 2 GW had been implemented to date. 4.3.4. Association of South East Asian Nations (ASEAN) There is huge potential for CCHP systems in ASEAN but market conditions differ from one country to another. The driving force for industry to invest in CCHP is lower energy cost, which is

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independent of overcapacity present in some countries. Financing is the largest obstacle to investment, despite market liquidity. Most countries have an indirect biomass cogeneration policy through biomass power and energy efficiency policy, legislation and support programs. However, none of the ASEAN countries have any particular policy, legislation or support program for coal and natural gas cogeneration presently [92,93]. The huge difference between developments in the electricity supply industry in ASEAN Countries is illustrated in Table 10. Typically, CCHP (cogeneration) policy is part of national energy policy, which is often scattered between different agencies. The EC-ASEAN COGEN Program is an economic cooperation program between the EC and ASEAN; about 15 million EURO is funded by the EC. The program lasted for 3 years from January 2002 to December 2004. As a result, 24 full-scale demonstration projects (FSDP) candidates selected ranges from 0.3 to 41 MW, and the total capacity is 174 MW. With assistance from developed countries and organizations, the potential will become a realistic market of CCHP applications in ASEAN [92,94,95]. 4.4. Other countries In addition to the above three sections of the world, many other countries develop their own CCHP applications by different means. Russia leads in the development of CCHP (most are cogeneration) around world. About 30% of electricity generation is from cogeneration, mostly in association with municipal use, which generates 65 GW annually. The very cold temperature holds great potential for district heating as a whole. Widespread supply of natural gas and its low cost compared to Europe are additional drivers for development of cogeneration applications. However, lack of financial support and a strong monopoly-based market structure block further increase in cogeneration. Once this situation changes, there could be rapid market growth based on growing demand and abundant natural resources [14]. In the Middle East, profuse crude oil resources seem to offer no need for developing an efficient power supply, such as CCHP systems. But environment and economy make CCHP applications valuable. Jaber [96] proposed a commercial-sized

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oil shale integrated tri-generation system (OSITGS) in his article. The proposed plant will probably be located close to the vast naturally occurring oilshale deposits, which will be financially attractive compared with conventional utilization methods, as well as an environmentally acceptable technique for producing synthetic (liquid and gaseous) fuels and electricity from oil-shale. Other literature indicates that countries with abundant energy resources are well aware of CCHP [97]. Large-scale hydropower plays an important role in Brazil’s electricity structure, and the overall DER capacity of this big country is 2.8 GW—about 3.8% of electricity capacity [14]. Several articles reveal that Brazil hopes to join the trend of CCHP development around the world. In his article, Szklo [98,99] applied a COGEN model to two cases in Brazil—a chemical plant and a shopping mall— showing the highest economic potential for gas-fired cogeneration in Brazil. Another article, by Silveira and Gomes [100], presents a study of technical and economic feasibility for the installation of cogeneration systems utilizing fuel cells, connected to an absorption refrigeration system for a building of the tertiary sector, subject to conditions in Brazil. Furthermore, a recent discovery of natural gas near the State of San Paulo has at least tripled Brazil’s reserves, although it will take a few years to develop. Brazilian gas companies have announced a major move towards increasing distribution—the CCHP market being their main target [14]. The potential CCHP market is also significant in Mexico, where petrochemical refinery sites can operate with onsite power generation. Although state-owned companies dominate the power market, changes are being made to electricity regulations, opening areas of the market to the private sector. The government is promoting investment in DER/ cogeneration. In January 2004, the Energy Secretary announced possible additional investments of 1000 MW by 2010. DER is expected to account for 20% of growth in the power market from 2004 to 2010, according to a survey of WADE [14]. In Africa, CCHP development remains in a primary stage. The foremost problem of many countries is the promotion of electricity supply infrastructures nation-wide. To some extent, smallscale distributed CCHP systems provide another power supply method to remote areas, rather than large centralized power plants. Joseph and RoyAikins [101] investigated the potential economic benefits that can be accrued by installing gas turbine

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Table 10 Present differences in ASEAN electricity supply [94] Country

Present situation

Installed capacity (MW)

Forecasted annual growth of power demand (%)

Policy on cogeneration

Cambodia

No national grid

160

10

Preparing phase

Indonesia

Govt.—56% IPP—4% Captive power —40%

23,425

Malaysia

Govt.—85% Private—15%

13,760

6–10

SREPb, cogeneration

Philippines

Govt.—55% Private—45%

14,700

9

Renewable energy

Singapore Thailand

Power pool Govt.—60% Private—40%

8140 24,500

10

SPPc, VSPPd, renewable

Vietnam

Govt.—90% Private—10%

3296

13

Preparing phase

IPPa, conservation captive power

a

IPP—independent power producers. SREP—small renewable energy power. c SPP—small power producers. d VSPP—very small renewable power. b

cogeneration sets along an oil pipeline in a rural area of one sub-Saharan country. South Africa predominantly fueled by coal (93% of overall electricity capacity) and the capacity of DER is only 0.5 GW—about 1.4% of total capacity [14]. In summary, prospects for CCHP and renewable DER in Africa are hopeful, inspite of many barriers. 5. Discussions and conclusions 1. Combined cooling, heating and power systems are derived from the CHP category, which shares some merits with CHP—especially energy conservation. Small-scale distributed CCHP applications, an important part of novel DER technologies, are the issue of CCHP recently. Generally, CCHP indicates large-scale technologies and applications that appear complicated to some government officials, investors and end users. Thus, the definition, benefits and classification of CCHP systems should be made known universally, since the lack of education and awareness about CCHP remains the foremost barrier to progress. Lack of understanding about CCHP concepts, benefits and technologies have halted its further popularization; ‘‘wait and see’’ is the attitude of both investors and users.

2. Existing and potential technologies of CCHP are available. These technologies contain both improved conventional approaches, like steam turbines, reciprocating engines, combustion turbines and electric chillers, as well as relatively new technologies such as fuel cells, microturbines, Stirling engines, sorption chillers and dehumidifiers. Most prime mover technologies are still based on fossil fuel combustion, since renewable energy technologies cannot totally and economically replace traditional technologies in the near future. Therefore, CCHP technologies provide the world with a transitional system of reliable and stable energy supply. There may be grounds for the argument that there are too many alternative technologies and modes of configurations existing, confusing potential users about a particular CCHP unit. However, it is believed that the more choices available, the more possibilities—exist for CCHP utilization in diverse circumstances. Better understanding of user demands, careful selection of technologies and full consideration of revenue are the keystones to a successful CCHP application. 3. The CCHP world market has grown rapidly in the last decade, despite the fact that development levels differ from country to country. CCHP development in the US and Europe restarted recently, after a

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short period of slow growth. Nonetheless, barriers to development still exist in these countries, such as limited liberated electricity market, gas price volatility, high initial cost, etc. Development in Japan seems to be steady; the number of sites and the total capacity are gradually increasing. The two newly emerging markets, China and India still are a long way from a boom in CCHP applications. Other developing countries have begun to encourage development of CCHP in their domestic energy supply market, and the governments of these countries follow various strategies according to the unique characteristics of their countries. From analysis of the world market, section by section, it is apparent that government policies, liberation of the electricity market and price of electricity and fuels are critical in the development of CCHP. Many countries have set a short-term target for CCHP applications, so the capacity share of DER is planned to double from 7% currently to 14% in 2012, with the combined efforts of governments, entrepreneurs, energy professionals and end users. Acknowledgements This work was supported by the Research Fund for the Doctoral Program of Higher Education under contract no. 20040248055 and the National Science Fund for Distinguished Young Scholars of China under contract no. 50225621. The support from the Key Research Program of MOE China regarding Distributed Energy Systems is also appreciated. The authors thank Elsevier for the kind permission to use the Figs. 5, 7 and the Tables 1, 2, 4, and 5, from the references [20,30,35,37].

[6]

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[13]

[14] [15] [16]

[17]

[18]

[19] [20]

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