ارزیابی صرفه جویی در انرژی اولیه سه نسل برای برنامه ریزی انرژی و سیاستگذاری

Trigeneration or combined heat, cooling and power (CHCP) is becoming an increasingly important energy option, particularly on a small-scale basis (below 1 MWe), with several alternatives nowadays available for the cooling power production and the coupling to cogeneration systems. This paper deals with the introduction of a suitable framework for assessing the energy saving performance of trigeneration alternatives, orientated towards energy planning studies and the development of regulatory policies. In particular, a new generalized performance indicator—the trigeneration primary energy saving (TPES)—is introduced and discussed, with the aim of effectively evaluating the primary energy savings from different CHCP alternatives. The potential of the TPES indicator is illustrated through specific analyses run over different combinations of trigeneration equipment, providing numerical examples based on time-domain simulations to illustrate the dependence of the energy saving characteristics on the CHCP system configurations and equipment, as well as on the loading levels. In addition, the key aspect of adequately establishing the reference efficiencies for the conventional separate production of electrical, thermal and cooling power is addressed in detail. This aspect affects both equipment selection and potential profitability of the considered solutions under the outlook of receiving financial incentives.

In recent years, the operators of the energy sector have put an increasingly high focus on issues concerning energy saving and implementation of high-efficiency energy systems, both from the technical and from the regulatory point of view (Cardona and Piacentino, 2005). In particular, the latest concerns in the energy sector are mainly related to the worldwide increase of energy consumption, the attempts to reduce the energy dependence from some regions of the world holding a relevant share of fossil primary sources and the emergence of binding environmental constraints aimed at limiting the production of greenhouse gases (GHGs). In addition, the development of liberalized energy markets in many countries has created new interests for analyzing the possibility of exploiting the equipment available for electricity production in a more profitable way. Cogeneration (Horlock, 1997) is being extensively used as an efficient technique to produce heat and electricity, leading to a substantial energy saving with respect to the “conventional” separate production (SP) of the same energy vectors, respectively, in heat generators and in the power system. In particular, in the past, mostly because of economy-of-scale reasons, cogeneration was limited to large-sized (industrial and district heating) plants. Yet, the recent development of “thermal” distributed generation (DG) technologies, such as microturbines (MTs) and internal combustion engines (ICEs) (Willis and Scott, 2000; Borbely and Kreider, 2001) has enabled the deployment of various small-scale (below 1 MWe) applications. In addition, DG technologies are being encouraged in several countries owing to their high potential for emission reduction of CO2 and other hazardous pollutants, as, for instance, discussed by Strachan and Dowlatabadi (2002) and Strachan and Farrell (2006). As a further point, fuel cells (FCs) (Willis and Scott, 2000; Borbely and Kreider, 2001) could play an important role in the future, within alternative high-efficiency energy scenarios based on a potential hydrogen economy (Clark and Rifkin, 2006; McDowall and Eames, 2006). Several small-scale cogeneration applications, besides heat and electricity, require cooling power (e.g., for air conditioning purposes). In order to supply this threefold energy need, it is possible to set up the so-called trigeneration or combined heat, cooling and power (CHCP) plants ( EcoGeneration Solutions LLC Companies, 1999; Resource Dynamics Corporation, 2003). Trigeneration can be seen as the simultaneous production of electricity, heat and cooling power from the same source of energy (typically gas). From this point of view, a trigeneration plant can be considered as the extension of a cogeneration or combined heat and power (CHP) plant. The literature typically refers to trigeneration as the combination of a traditional CHP prime mover (i.e., a thermal machine such as an ICE, a MT or a FC that cogenerates electricity and heat) with an absorption group, fed by hot water or steam produced by the cogeneration group (Colonna and Gabrielli, 2003; Bassols et al., 2002; Maidment and Prosser, 2000; Hwang, 2004). The rationale of this approach is based on the potential efficiency of using the thermal power cogenerated also in the summertime to fire the absorption machine for cooling production, enabling better and longer exploitation of the prime mover, as shown, for instance, by Havelsky (1999), Heteu and Bolle (2002), and Cardona and Piacentino (2003). This kind of application may be referred to as “seasonal” trigeneration. However, an array of other applications (for instance, hospitals, department stores, hotels and so forth) require an actual trigeneration production throughout the whole year, so that the optimal setup of the plant, also accounting for the economic issues, could be different from the cases of seasonal trigeneration. Thus, in previous works (Chicco and Mancarella, 2005 and Chicco and Mancarella, 2006; Mancarella, 2006), the authors have considered a generalized concept of trigeneration, considering a set of different optional technologies and sizes for the cooling side coupled to the CHP side. As a consequence of the increasing diffusion of various types of plants, the evaluation of a trigeneration system is becoming a crucial issue and requires the adoption of adequate performance indicators. From this perspective, the energy savings attributable to adopting one plant configuration compared with another could be a suitable indicator for evaluating and comparing the effectiveness of each alternative. However, the definition of “energy saving” in a trigeneration system also needs to be discussed and clarified. In fact, as pointed out by Chicco and Mancarella (2006), classical tools for evaluating CHP plants, such as the fuel energy saving ratio (FESR) indicator (Horlock, 1997), are not always adequate for CHCP plant assessment. Thus, other approaches may be necessary, such as the ones taken up by Havelsky (1999) and Heteu and Bolle (2002), that assess trigeneration systems by explicitly accounting for the SP of cooling power, besides heat and electricity. In addition, it is not always clear how to evaluate specific energy savings and what reference situation to apply ( Boonekamp, 2006). As a further fundamental point, to date and to the authors’ knowledge, there is no official regulatory framework dealing with the issue of evaluating the performance of CHCP systems. Differently, CHP plants, whose energy saving are officially recognized and expressed through suitable indicators, receive financial incentives in many countries. The details are discussed by Cardona and Piacentino (2005), with practical applications provided, for instance, in Italy by Deliberation no. 42/02 of the Italian AEEG (2002), and in the European Directive 2004/8/EC (2004) on the establishment of a common framework for regulating cogeneration at a continental level. On these premises, it is clear how the proper evaluation of the performance of a trigeneration system could represent a key point for promoting, through regulated incentives, the diffusion of high-efficiency CHCP plants on a wider basis. In this light, in analogy with the cogeneration FESR, and according to the lines drawn by Havelsky (1999) and Heteu and Bolle (2002) for evaluating the performance of “classical” CHCP systems with absorption chillers, this paper introduces a generalized indicator called trigeneration primary energy saving (TPES), aimed at assessing the potential energy saving from any kind of trigeneration plant. After discussing the main issues relevant to the formulation of the new indicator, the effectiveness of adopting the TPES is shown in a comprehensive case study that highlights the manifold key variables and parameters for plant performance assessment. The benefit of this kind of modeling is that it is possible, starting from the threefold user's energy needs, to track back the energy inputs to the whole system, typically gas from a gas distribution network and electricity from the electrical grid. In this way, seeing the whole plant as a black box with fuel and electricity as input and electricity, heat and cooling as output, it is easy to calculate and compare the TPES for every different trigeneration solution. The paper is structured as follows: Section 2 contains a general introduction to the main characteristics and components of trigeneration plants; Section 3 describes the CHCP equipment performance models and defines and discusses the TPES indicator; Section 4 presents a comprehensive case study, based on time-domain simulations of different types of CHCP plants and loads, pointing out the potential of the proposed indicator; and Section 5 contains the concluding remarks.

In this paper, the trigeneration primary energy saving (TPES) indicator has been introduced to synthetically assess, from the most general point of view, the primary energy savings brought about by the adoption of any type of trigeneration system. In particular, after deriving and illustrating the general model, a more specific formulation has been considered for evaluating the trigenerated energy vectors with respect to the conventional separate production (SP) of electricity, heat and cooling power. Through a comprehensive case study based on time-domain simulations, the proposed indicator has proven to be effective at highlighting the role of the different relevant variables and parameters involved in determining the efficiency of trigeneration. Of course, the numerical results obtained in the case study cannot be generalized to other trigeneration plants. However, the conceptual exploration of the results has provided a useful indication of the type of trigeneration solution relevant to different loading levels, operation strategies and reference efficiencies for SP. In particular, the numerical results emphasize the importance of properly selecting the models and the numerical characteristics for the SP references in order to accurately determine the most effective trigeneration solution. In the future, regulation will have to explicitly take into account trigeneration because of the increasing importance that this technique, in different forms and with different plant schemes, is assuming worldwide. In particular, owing to the enhanced energy generation efficiency of combined heat, cooling and power (CHCP) plants, proper performance evaluation of trigeneration will become more relevant within potential regulatory frameworks envisaging financial incentives aimed at improving the energy sector efficiency, in line with what already occurs for cogeneration systems (Directive 2004/8/EC, 2004). In this respect, the proposed TPES indicator, as a straightforward extension of the fuel energy saving ratio (FESR) widely adopted for cogeneration technical and policy assessment in several countries, could represent an effective tool for evaluating the efficiency characteristics of different types of combined systems for electrical, thermal and cooling power generation. For instance, using the TPES indicator in regulatory analyses, potential financial incentives for trigeneration energy savings could be devised. Potentially, this could boost the spread of high-efficiency energy systems, with benefits in terms of primary energy savings as well as consequent CO2 emission reduction. In addition, the diffusion of absorption technologies for cooling generation would positively impact the electrical power grid management and structure, by decreasing the electrical load required of the share needed for conventional electrical air conditioners in the summertime peak periods. However, besides the energy saving evaluation, economic analysis will play a fundamental role in the final selection of a CHCP plant. Hence, assuming roughly the same fixed cost for electric and absorption chillers, comparative electricity and gas prices would determine the selection of CERG or GARG for the cooling generation (Foley et al., 2000; Mancarella, 2006). In general, as shown by Mancarella (2006) and Chicco and Mancarella (2006), different trigeneration solutions and the relevant energy savings could bring about important economic benefits with respect to the conventional SP (with pay-back times even lower than 3 years), which however strongly depend on the specific market framework and on electricity and gas prices. In particular, if the TPES had regulatory relevance for delivering financial incentives, as in the case of the FESR for cogeneration, the economic balance could be moved towards solutions that, although slightly more expensive, would ensure higher energy and environmental benefits. However, from this perspective, and as illustrated in this paper, a suitable discussion on the selection of the SP reference values is necessary, and could mark the turning point for encouraging the diffusion of higher-efficiency and lower-emission trigeneration systems. In this respect, setting average reference values for the SP evaluation, to be determined within the specific regulatory framework, could represent an effective approach to boost the diffusion of trigeneration systems. This is in line with considering that switching the bulk of current SP technologies to the state of the art requires time (of the order of one decade); meanwhile, trigeneration can contribute to improve energy efficiency and reduce GHG emissions from energy generation. Further work is in progress to set up a comprehensive energy, environmental and economic evaluation framework for trigeneration plants, including the assessment of the potential reduction of CO2 emissions attributable to different types of trigeneration systems, as well as of the economic benefits relevant to different TPES values for different CHCP solutions within different energy market scenarios.